following the molecular pathways toward an … the molecular pathways toward an understanding of the...

Following the Molecular Pathways toward an Understanding of thePathogenesis of Systemic SclerosisSergio A. Jimenez, MD, and Chris T. Derk, MD

Systemic sclerosis is a disease of unknown origin charac-terized by excessive deposition of collagen and other

connective tissue macromolecules in skin and multiple in-ternal organs, prominent and often severe alterations in themicrovasculature, and humoral and cellular immunologicabnormalities (see Glossary). Systemic sclerosis is a com-plex and heterogeneous disease. Clinical forms range from

limited skin involvement with minimal systemic alterations(limited cutaneous systemic sclerosis) to forms with diffuseskin sclerosis and severe internal organ disease (diffuse cu-taneous systemic sclerosis) (1), and occasionally a fulmi-nant course (fulminant systemic sclerosis) (2).

The most apparent and almost universal clinical fea-tures of systemic sclerosis are related to the progressive

Ann Intern Med. 2004;140:37-50.

For author affiliations, see end of text.For definition of terms used, see Glossary.

Clinical Principles Physiologic Principles

Systemic sclerosis is an autoimmune connective tissuedisorder of unknown cause.

It is clinically heterogeneous, ranging from limited skininvolvement to diffuse skin sclerosis with severe internalorgan involvement.

The Raynaud phenomenon, swelling of the extremities, anddiffuse arthralgias often precede the onset of skin tightnessand induration.

Visceral involvement with fibrosis, microvascular alterations,and mononuclear cell infiltration of the gastrointestinaltract, lungs, heart, and kidneys is present to a variableextent in most patients.

Autoantibodies, some with very high specificity, are presentalmost universally.

Mortality and morbidity are substantial and are directlyrelated to the extent of the fibrotic and microvascularalterations.

A better understanding of the pathogenesis of this incurabledisorder will help to design effective therapies in thefuture.

The pathogenesis of systemic sclerosis is extremely complex;at present, no single unifying hypothesis explains allaspects.

Fundamental abnormalities in at least 3 types of cells areintimately involved in the development of the clinical andpathologic manifestations of the disease: 1) fibroblasts; 2)endothelial cells; and 3) cells of the immune system, inparticular T and B lymphocytes.

The profound functional alterations in these cells result in thecharacteristic triad of pathologic changes: severe and oftenprogressive cutaneous and visceral fibrosis; obliteration ofthe lumen of small arteries and arterioles; and humoral andcellular immunologic abnormalities, which include theproduction of autoantibodies, the chronic mononuclear cellinfiltration of affected tissues, and the dysregulation oflymphokine and growth factor production.

A crucial component in systemic sclerosis pathogenesis is thepersistent and unregulated activation of genes encodingvarious collagens and other extracellular matrix proteins.

Abnormal regulation of transcription of genes encodingvarious collagens is responsible for tissue and vascularfibrosis.

Alterations in the production of cytokines and growth factors,which are capable of modulating the function of fibroblastsand other target cells, appear to play a relevant role.

Transforming growth factor-� is one of the growth factorsintimately involved in the fibrosis that accompaniessystemic sclerosis.

ReviewPHYSIOLOGY IN MEDICINE: A SERIES OF ARTICLES LINKING MEDICINE WITH SCIENCEPhysiology in MedicineDennis A. Ausiello, MD, Editor; Dale J. Benos, PhD, Deputy Editor; Francois Abboud, MD, Associate Editor;William Koopman, MD, Associate Editor

Annals of Internal MedicinePaul Epstein, MD, Series Editor

© 2004 American College of Physicians 37

Glossary

Allele: One of 2 or more different genes containing specific inheritable characteristics that occupy corresponding positions (loci) on paired chromosomes.

Allogenic: Having a different genetic constitution but belonging to the same species.

Apoptosis: Disintegration of cells into membrane-bound particles that are then phagocytosed by other cells.

Calcitonin gene-related peptide: A second product transcribed from the calcitonin gene. It is found in a number of tissues, including the nervous system. It can act as avasodilator.

Cellular immunity: Involves the production of lymphocytes by the thymus (T cells) in response to exposure to an antigen.

Codon: A sequence of 3 nucleotides in a strand of DNA that provides the genetic code for a specific amino acid.

COL1A1: Gene encoding the �1 chain of type I collagen.

COL1A2: Gene encoding the �2 chain of type I collagen.

Connective tissue growth factor (CTGF): Growth factor that plays a critical role in tissue fibrosis as well as angiogenesis, axial development of the musculoskeletal system,structural organization of connective tissue, and embryo implantation.

Cytokines: Intercellular messenger proteins. Hormone-like products of many different cell types that are usually active within a small radius of the cells producing them.

Dinucleotide: The product of cleaving a polynucleotide (nucleic acid composed of �2 nucleotides).

Epitope: Any component of an antigen molecule that functions as an antigenic determinant by permitting the recognition and attachment of specific antibodies.

Extracellular matrix: The ground substance occupying the space outside cells in a specific tissue.

Exon: A portion of the DNA that is transcribed to RNA, which is then expressed.

Fibroblasts: Mesenchymal cells responsible for the production of fibrous tissue.

Fibronectin: A cell-binding glycoprotein.

Growth factors: Polypeptides produced by various cells of the body that promote growth and development by directing cell division, maturation, and differentiation and bymediating maintenance and repair of tissues.

Haplotypes: The genetic constitution of an individual with respect to 1 member of a pair of allele genes.

HLAs: Human leukocyte antigens. The cell surface molecules encoded by genes in the MHC on chromosome 6. These molecules are divided into 2 classes (class I and class II)on the basis of their structure. Class I and II molecules attach and present antigens to CD8� (cytotoxic and suppressor cells) T cells and CD4� (helper) T cells, respectively.

Humoral immunity: Immunologic system involved in the production of plasma lymphocytes (B cells) in response to antigen exposure with subsequent antibody production.

Intercellular adhesion molecule-1 (ICAM-1): A glycoprotein that is expressed on activated endothelial cells because of stimulation by local production of cytokines, which inturn causes adherence of leukocytes.

Interferons: A class of small glycoprotein cytokines produced by T cells, fibroblasts, and other cells in response to viral infection and other biological and synthetic stimuli. Theireffects include inducing enzymes, suppressing cell proliferation, inhibiting viral proliferation, enhancing the phagocytic activity of macrophages, and augmenting thecytotoxic activity of T lymphocytes.

Interleukins: A large family of hormone-like messenger proteins produced by immune cells that act on leukocytes and other cells.

Intron: A portion of DNA that lies between two exons, is transcribed into RNA, but does not appear in mature messenger RNA because the intron is removed and the exonsare spliced together, and so it is not expressed.

Lymphokine: A hormone-like intercellular messenger protein produced by lymphocytes.

Matrix metalloproteinases: A family of protein-hydrolyzing endopeptidases that hydrolyze extracellular proteins, especially collagens and elastin.

MHC: Major histocompatibility complex. The pair of genes on human chromosome 6 that encode the cell surface molecules known as HLAs.

Nucleotide: A combination of a nucleic acid (purine or pyrimidine), 1 sugar (ribose or deoxyribose), and a phosphoric group.

Phenotype: The observable characteristics at the physical, morphologic, or biochemical level of an individual, which are determined by the genetic sequence of the individual.

Platelet-derived growth factor (PDGF): A factor present in platelets that is mitogenic for cells at the site of a wound, causing proliferation of endothelial cells, fibroblasts,smooth-muscle cells, and glial cells.

Polymerase: Any enzyme catalyzing a polymerization, as of nucleotides to polynucleotides.

Polymorphism: The presence of alleles in a population at a frequency that is higher than expected and that cannot be explained by mutation, suggesting a positive selectionmechanism.

Prostacyclin: A prostanoid produced by endothelial cells that is a potent natural inhibitor of platelet aggregation and a powerful vasodilator.

Protease: Descriptive term for proteolytic enzymes, both endopeptidases and exopeptidases, which hydrolize polypeptide chains.

Sequence homology: The degree of similarity between the nucleotide sequences of strands of DNA.

Smad: A family of messenger/transcription factor proteins that are involved in signal transduction by transforming growth factor-�.

Topoisomerases: A group of enzymes capable of converting 1 topologic version of DNA into another. They act by catalyzing the breakage and reformation of DNAphosphodiester linkages.

Transcript: Same as messenger RNA.

Transcription: Transfer of genetic code information from one kind of nucleic acid to another. Commonly used to refer to transfer of genetic information from DNA to RNA.

Transcription factors: Protein factors that interact with promoters and upstream regulatory elements on the DNA of the protein that will be expressed.

Transduction: Transfer of genetic material from one cell to another. Also, transmission of cell surface signals into the nucleus.

Transforming growth factor-� (TGF-�): A regulatory cytokine that has multifunctional properties and can enhance or inhibit many cellular functions, including interfering withthe production of other cytokines and enhancing collagen deposition.

Tumor necrosis factor (TNF): A polypeptide cytokine produced by endotoxin-activated macrophages that has the ability to modulate adipocyte metabolism, lyse tumor cells invitro, and induce hemorrhagic necrosis of certain transplantable tumors in vivo.

Review Pathogenesis of Systemic Sclerosis

38 6 January 2004 Annals of Internal Medicine Volume 140 • Number 1 www.annals.org

thickening and fibrosis of the skin (3). However, multipleinternal organs are always involved in some degree evenwhen it is not clinically apparent. The affected skin is tight,indurated, and firmly bound to the subcutaneous tissue.Hair follicles and sweat and sebaceous glands atrophy. Theskin over the hands and face is most frequently involved; asthe disease progresses, the sclerotic changes extend andmay affect the entire body. In many cases, the cutaneousinvolvement is confined to the digits and the dorsum of thehands and feet (acrosclerosis), and progression of thesclerotic process is relatively slow. This form of disease waspreviously known as the CREST syndrome (Calcinosis,long-standing Raynaud phenomenon, Esophageal dysmo-tility, Sclerodactyly, and Telangiectases) (4). Skin ulcer-ations, usually localized to fingertips or knuckles, and pe-culiar pigmentary changes are frequent. Calcinosis is mostcommonly found in fingertips and periarticular tissues.

The Raynaud phenomenon is the second most com-mon manifestation of systemic sclerosis and is present inmore than 85% of patients (5). It usually appears simulta-neously with other manifestations but may antedate themby several years. Often, the initial vasoconstriction andblanching are followed by a dusky cyanosis and are accom-panied by paresthesias and numbness. With return ofblood flow, reactive erythema occurs. In some cases, largerblood vessels are affected and luminal narrowing and oc-clusion can result in ischemic necrosis.

Musculoskeletal symptoms are often the initial mani-festations and may vary from mild polyarthralgias to frankarthritis (6). Synovitis with a moderate effusion occurs oc-casionally; however, the synovial fluid is only mildly in-flammatory. In more advanced stages, thickening and fi-brosis of periarticular tissues result in severe flexioncontractures and distal phalangeal resorption. Muscle in-volvement can reveal either an inflammatory myopathy ora more indolent noninflammatory form due to muscle in-filtration with fibrotic tissue.

The gastrointestinal tract is the most common internalorgan system involved (7). Esophageal symptoms resultingfrom reduced lower esophageal sphincter pressure and dys-motility of the lower two thirds of the esophagus includereflux, heartburn, and dysphagia to solid foods. In severecases, chronic esophagitis can lead to stricture andodynophagia. Poor gastric emptying and small intestineinvolvement may cause abdominal distention, bloating,nausea, and pain, and, in many instances, bacterial over-growth with secondary malabsorption, diarrhea, andweight loss.

Pulmonary involvement has emerged as potentially themost serious visceral lesion in systemic sclerosis. Becauseno reliable ameliorating treatment is available for such in-volvement, it frequently leads to severe respiratory disabil-ity and often death (8). The most prominent symptoms aretachypnea and exertional dyspnea, secondary to either pul-monary fibrosis or pulmonary hypertension. Many patientsremain asymptomatic despite evidence of fibrotic involve-

ment of the lung parenchyma. In some cases, fulminantpulmonary hypertension may lead to rapid death (9).

Cardiac involvement is not uncommon (9, 10). Chestpain, arrhythmias, various degrees of heart block, and myo-cardiopathy with left ventricular or biventricular failuremay result from myocardial fibrosis. These alterations oc-cur almost exclusively in patients with diffuse systemic scle-rosis. Cor pulmonale can develop in patients with pulmo-nary hypertension. Pericardial involvement is usuallyasymptomatic; however, pericardial effusions are found byechocardiography or autopsy examination in approxi-mately one half of patients.

Until recently, renal disease was the most rapidly fatalvisceral involvement of systemic sclerosis. It is known asscleroderma renal crisis and is typically characterized by theabrupt onset of malignant hypertension and rapidly pro-gressive renal insufficiency (11). It is often heralded bysevere headache; visual symptoms from hypertensive reti-nopathy; seizures and other central nervous system symp-toms; or myocardial ischemia, infarction, or left ventricularfailure. Prompt aggressive treatment now can usually re-verse this process, which otherwise is fatal.

Functional thyroid abnormalities, including elevatedlevels of antithyroid autoantibodies and clinical and sub-clinical hypothyroidism, are common (12). Impotencecaused by erectile failure may be an early feature of sys-temic sclerosis (13), and some degree of erectile dysfunc-tion ultimately develops in many affected patients. Patientsmay develop the sicca syndrome (keratoconjunctivitis siccaand xerostomia) caused by fibrosis and lymphocytic infil-tration of the salivary and lacrimal glands (14). Unusualclinical manifestations include urinary bladder involve-ment, pneumatosis cystoides intestinalis with pneumoperi-toneum, trigeminal neuralgia, and hoarseness from vocalchord involvement.

The pathologic changes in systemic sclerosis encom-pass a spectrum reflecting variable stages of developmentand progression of 3 major processes in the affected tissues:1) severe tissue fibrosis with exaggerated deposition of col-lagen and other connective tissue components in the extra-cellular matrix (see Glossary); 2) chronic inflammation, oc-curring predominantly in the early stages of disease andcharacterized by infiltration with mononuclear cells,mostly of the macrophage and T-cell lineages; and 3) mi-crovascular disease, characterized by intimal proliferation,concentric subendothelial deposition of collagen and mu-cinous material, and narrowing and thrombosis of the ves-sel lumen. Progression of the vascular and fibrotic changesand a decrease in the inflammatory component lead toend-stage fibrosis and atrophy of the affected organs.

The histopathologic findings in the skin includemarked thickening of the dermis with massive accumula-tion of dense collagen, causing epidermal atrophy, flatten-ing of the rete pegs, and replacement of sebaceous andsweat glands as well as hair follicles. A prominent inflam-matory infiltrate is often present at the dermal–adipose

ReviewPathogenesis of Systemic Sclerosis

www.annals.org 6 January 2004 Annals of Internal Medicine Volume 140 • Number 1 39

tissue interphase, especially in early lesions (15, 16). Thesmall vessels of the lower dermis show fibrous thickeningbut no evidence of vasculitis. In the lungs, the architectureof the alveolocapillary membrane and the parenchymal in-terstitium are markedly disrupted by fibrosis and severemononuclear-cell infiltration. Prominent vascular abnor-malities with intimal proliferation causing narrowing orcomplete obliteration of small vessels are frequent. Therenal lesions display severe narrowing and obliteration ofthe medium-size arterioles because of subintimal accumu-lation of loose connective tissue and intimal and perivas-cular fibrosis. The glomeruli frequently appear ischemic,and there is no evidence of glomerulitis. Severe interstitial,perivascular, and periglomerular fibrosis may be present incases of long duration. Other affected organs display vari-able degrees of fibrosis, mononuclear-cell infiltration, andmicrovascular obliteration and fibrosis.

PATHOGENESIS

The pathogenesis of systemic sclerosis is extremelycomplex. At present, no single unifying hypothesis explainsall aspects of its pathogenesis. However, fundamental ab-normalities in at least 3 types of cells are intimately in-volved in the development of the clinical and pathologicmanifestations of the disease: 1) fibroblasts (see Glossary);2) endothelial cells; and 3) cells of the immune system,

particularly T and B lymphocytes. The profound func-tional alterations in these cells result in the characteristictriad of pathologic changes in systemic sclerosis: severe andoften progressive cutaneous and visceral fibrosis; oblitera-tion of the lumen of small arteries and arterioles; and hu-moral and cellular immunologic abnormalities, which in-clude the production of numerous autoantibodies (somevery highly specific for the disease), the chronic mononu-clear-cell infiltration of affected tissues, and the dysregula-tion of lymphokine and growth factor production (seeGlossary). These dysfunctional cellular processes and theirresulting effects on the affected tissues are illustrated inFigure 1. At present, it is not clear which of these alter-ations is of primary importance or how they interrelate tocause the progressive fibrotic process in systemic sclerosis.However, a crucial component in the pathogenesis of thedisorder is the persistent and unregulated activation ofgenes encoding various collagens and other extracellularmatrix proteins in systemic sclerosis fibroblasts. This is themost important difference between normal fibroblasts,which promote normal wound healing, and systemic scle-rosis fibroblasts, which demonstrate an uncontrolled pro-duction and tissue deposition of collagen resulting inpathologic organ fibrosis. This paper reviews some of thenumerous components of the complex puzzle of systemicsclerosis pathogenesis, as illustrated in Figure 1, and at-

Figure 1. General overview of the pathogenesis of systemic sclerosis.

The illustrations on the bottom row show examples of, from left to right, the fibrotic process (biopsy of skin), microvascular alterations in pulmonaryarterioles, autoantibodies detected by immunofluorescence, and mononuclear inflammatory cell infiltrates in affected skin.



tempts to form hypotheses that may provide frameworksfor the development of novel and effective therapies. Be-cause we were not able to provide a comprehensive discus-sion of the abundant literature published, we refer readersto recent reviews for a more complete and detailed descrip-tion.

Possible Causative AgentsThe cause of systemic sclerosis has remained elusive

despite intense investigations. Although the disease is notinherited in a classical Mendelian pattern, there is strongevidence that genetic factors contribute to its developmentand clinical manifestations, as discussed in more detailshortly. However, it has become apparent that environ-mental agents play a crucial and more important role thangenetic influences. One study reported a remarkably lowconcordance in the development of systemic sclerosisamong homozygous twins, indicating that the heritabilitycomponent of the disease was very low and that the mostimportant factor was of an environmental or acquired ori-gin (17). Many infectious, chemical, and physical agentshave been postulated as being involved in the cause of thedisease. The hypothesis that infectious agents may causesystemic sclerosis has been studied extensively. Some re-searchers have suggested that the production of specificautoantibodies in systemic sclerosis is the result of an anti-gen-driven response caused by “molecular mimicry.” Theconcept of “molecular mimicry” proposes that antibodiesagainst self-antigens are produced because these antigenscontain epitopes (see Glossary) that share structural simi-larities with viral or bacterial proteins. In the immuno-pathogenesis of systemic sclerosis, herpesviruses, retrovi-ruses, and human cytomegalovirus infections, amongothers, have been suggested as possible causative agents.Evidence supporting the role of retroviruses includes thedemonstration of sequence homologies (see Glossary) be-tween certain retroviral proteins and the topoisomerase Iantigen (see Glossary), which is the target of anti–Scl-70antibodies in patients with systemic sclerosis (18). In addi-tion, it has been shown that the induced expression ofretroviral proteins in normal human dermal fibroblasts re-sults in the acquisition of a systemic sclerosis–like pheno-type (see Glossary) in the production of extracellular ma-trix proteins (18). Furthermore, antibodies to retroviralproteins have been detected in serum specimens from pa-tients with systemic sclerosis (19). Another hypothesis hassuggested that human cytomegalovirus may be involved inthe initial events of systemic sclerosis. This hypothesis issupported by the observations of a higher prevalence of IgAantihuman cytomegalovirus antibodies in patients with sys-temic sclerosis, which are capable of inducing apoptosis(see Glossary) in human endothelial cells; the increasedprevalence of anticytomegalovirus IgA antibodies in pa-tients positive for Scl-70 autoantibodies; and the severefibroproliferative vascular changes and the increased occur-rence of antinuclear antibodies with an immunofluores-

cence pattern similar to that present in serum specimensfrom patients with systemic sclerosis in human cytomega-lovirus infections (20, 21). Despite intensive study, how-ever, there is no definitive evidence to conclude that sys-temic sclerosis has a viral origin.

Environmental agents have also been implicated in thedevelopment of systemic sclerosis (22, 23). Silica and metaldust exposure had been shown in case studies to be relatedto systemic sclerosis (22–24), although some studies havefailed to confirm an association. On the other hand, or-ganic solvent exposure may eventually be proven to be animportant environmental factor in triggering this disease.Indeed, persons exposed to vinyl chloride have an increasedrisk for skin thickening, the Raynaud phenomenon, anddigital ulcers (23), and recent epidemiologic studies havefound a higher frequency of organic solvent exposure inpatients with systemic sclerosis than in normal controls(24). Several other environmental exposures have been as-sociated with the development of systemic sclerosis, includ-ing certain pesticides, hair dyes, and fuel-derived or indus-trial fumes (22, 23).

Role of Genetic FactorsThe contribution of genetic factors in the develop-

ment and expression of systemic sclerosis is strongly sup-ported by the observation of familial clustering of the dis-ease, the high frequency of autoimmune disorders andautoantibodies in family members of patients with systemicsclerosis, differences in prevalence and clinical manifesta-tions among different ethnic groups, and the increasedprevalence of certain HLAs and MHC alleles (see Glossary)among different ethnic groups and among patients withdifferent clinical subsets of the disease or with differentpatterns of autoantibodies (25). Strong evidence indicatesthat genetic factors largely determine the production ofspecific autoantibodies in systemic sclerosis. Although theconcordance of systemic sclerosis among identical twins isonly 4.2% and is not significantly different from the con-cordance of disease in dizygotic twins (5.9%), the concor-dance for the presence of specific autoantibodies is substan-tially higher (17). These observations indicate thatinherited genetic factors are important for the productionof autoantibodies but are not sufficient for development ofdisease.

Prevalence of the disease varies by geographic regionand by ethnic background; the prevalence in the UnitedStates is 242 to 286 cases per million persons in the pop-ulation. However, in Native American persons of theChoctaw tribe in Oklahoma, one of the best-studiedgroups for the role of genetic factors in systemic sclerosis,the prevalence is 469 cases per million persons in the pop-ulation (26). The differences of MHC and HLA allele ex-pression are evident when comparing the different haplo-types (see Glossary) identified as being linked to diseaseexpression, in particular to the pattern of autoantibodyresponse, among ethnic groups (27). Disease expression



also appears to differ among ethnic groups. African-Amer-ican persons are more likely to have anti-topoisomerase Iantibodies and more severe visceral manifestations, includ-ing a higher frequency of pulmonary fibrosis. In contrast,anticentromere antibodies are more common in white per-sons, who are also more likely to have limited disease withless severe systemic manifestations (27).

In addition to these genetic characteristics, which maypredispose persons to systemic sclerosis, other genetic fac-tors may influence the expression of the disease. Recentstudies have examined mutations or polymorphisms (seeGlossary) in relevant genes that may result in their in-creased expression. For example, in patients with systemicsclerosis, the upstream region of COL1A2 (see Glossary),the region of the gene preceding the initiation of genetranscription, harbors numerous dinucleotide repeats thatmay increase gene activity (see Glossary) (28). Similarly, 2single nucleotide polymorphisms at codon 10 of the trans-forming growth factor (TGF)-�1 gene were found in sta-tistically significant higher frequency in patients with sys-temic sclerosis (see Glossary) (29). This suggests a geneticpredisposition to higher production of TGF-�1 (29). Fur-thermore, patients who had the TGF-�1 polymorphismhad a higher frequency of pulmonary fibrosis and graftfibrosis following lung transplantation (30). As discussedlater in more detail, TGF-�1 is a potent profibrotic cyto-kine (see Glossary) secreted from activated macrophagesand T lymphocytes that may play a central role in thepathogenesis of tissue fibrosis in systemic sclerosis.

Another genetic polymorphism that may influence ex-pression of systemic sclerosis has been described in the geneencoding fibrillin-1, a large extracellular matrix proteinthat is a structural component of connective tissue micro-fibrils. The polymorphism is located in the upstream re-gion (vide supra) of the gene, and its presence is stronglyassociated with the occurrence of systemic sclerosis in Na-tive American persons from the Choctaw tribe and in Jap-anese persons (31). These collective studies on the influ-ence of genetic polymorphisms on the development orexpression of systemic sclerosis, however, must be per-formed in larger cohorts to corroborate their relevance.

MicrochimerismMicrochimerism is a novel and provocative hypothesis

of the cause of systemic sclerosis (32–34). The hypothesissuggests that, during pregnancy, allogenic (see Glossary)fetal or maternal cells cross the placenta in bidirectionaltraffic and persist in the circulation and tissues of themother or child, respectively, as a result of HLA II (DRB1)compatibility between the mother and the fetus. Theseengrafted foreign cells may become activated by a secondevent and may mount a graft-versus-host reaction, whichmanifests as systemic sclerosis. The remarkable similaritiesin clinical, histopathologic, and serologic features betweengraft-versus-host disease and systemic sclerosis, includingesophageal, lung, and skin involvement; lymphocytic infil-

tration and fibrosis of affected organs; and production ofautoantibodies, strongly support this hypothesis. Research-ers suggested this theory when fetal cells were found in thecirculation of normal women several decades after the birthof their children and fetal DNA and fetal cells were subse-quently identified in affected skin samples from womenwith systemic sclerosis who had previously been pregnantwith a male fetus (32). Microchimeric cells of maternalorigin have also been identified in the circulation of off-spring (35). These findings might explain the occurrence ofsystemic sclerosis in nulliparous women and men.

Although some studies found that the frequency ofmale chromosome sequences in the skin of women withsystemic sclerosis who had male offspring was similar tothat of normal controls, quantification of microchimericfetal cells showed a significant difference in the amount ofmale DNA in affected skin between the 2 groups (36).This indicates that it is the quantity and not only the merepresence of fetal cells that might contribute to the patho-genesis of systemic sclerosis. A more recent study con-firmed the quantitative difference in the number of micro-chimeric cells between patients with systemic sclerosis andhealthy individuals and also showed that immunologicstimulation expected to trigger antigen-specific T cellscaused a potent amplification of the microchimeric cells inpatients with systemic sclerosis but not in controls (37).Despite these observations, at present, the role of micro-chimerism in the pathogenesis of systemic sclerosis remainsundetermined.

Humoral Immune System AlterationsThe presence of specific autoantibodies is one of the

most common manifestations of systemic sclerosis. Morethan 90% of patients with systemic sclerosis harbor anti-nuclear antibodies in their serum. Numerous autoantibod-ies, some of which are extremely specific for systemic scle-rosis, have been described in patients with the disease;other autoantibodies are associated with different clinicalmanifestations (38).

Anti–Scl-70 antibodies have been shown to react withDNA topoisomerase I and are almost exclusively present inserum specimens from patients with the diffuse form ofsystemic sclerosis, although only about 30% to 40% ofthese patients harbor these autoantibodies. Anticentromereantibodies are present in 80% to 90% of patients with thelimited form of systemic sclerosis but are found in fewerthan 10% of patients with diffuse systemic sclerosis. These2 autoantibodies rarely coexist in the same patient.

Other autoantibodies are less common in patients withsystemic sclerosis. They include anti-RNA polymerases Iand III antibodies in patients with rapidly progressive dis-ease and severe internal organ involvement; antifibrillarinantibodies, which are commonly found in diffuse systemicsclerosis; and anti–PM-Scl antibodies, which are usuallyfound in patients with systemic sclerosis who develop aninflammatory myopathy. Although autoantibodies are



common in systemic sclerosis, they are not directly in-volved in the clinical manifestations of the disease. How-ever, owing to their high frequency and their specificity forcertain clinical disease subsets, their presence is very helpfulin establishing the diagnosis and predicting a probable pat-tern of organ involvement, severity, and disease progression(38).

Tissue Inflammation: An Early Event in the Cascade?Some of the earliest evidence suggesting that chronic

and persistent inflammation may play a role in the patho-genesis of systemic sclerosis was provided by the demon-stration of lymphocytic infiltrates in affected skin from pa-tients with disease of recent onset (15, 16). Subsequently,researchers found that the extent of lymphocytic infiltra-tion correlated with the severity and the progression of skinsclerosis (39). The mononuclear cells within the skin infil-trates are predominantly CD4� T cells and express theactivation marker class II MHC antigen DR (40, 41). Sub-sequent expansion of these cells within the tissue appears tobe oligoclonal, as shown in studies of T-cell receptor tran-scripts (see Glossary) in the skin of patients with systemicsclerosis (42). These results suggest an antigen-driven re-sponse, although at present there is no information on theputative antigen or antigens that may be involved. Theexpanded populations of inflammatory cells in affected tis-sues release cytokines and growth factors that initiate orperpetuate the fibrotic process as well as the endothelialand vascular alterations. Indeed, clones of T cells estab-lished from infiltrating lymphocytes isolated from affectedskin of patients with systemic sclerosis produce cytokinesthat can stimulate fibroblast collagen production (43).

The mechanisms responsible for the migration of dis-tinct populations of T cells from peripheral blood to skinand other affected tissues in patients with systemic sclerosisare not completely understood. However, it is likely thattheir retention and accumulation in such tissues resultfrom specific interactions between certain T-cell subsetsand fibroblast membrane proteins or extracellular matrix,which are mediated by integrins and cell adhesion mole-cules such as the adhesive protein intercellular adhesionmolecule-1 (see Glossary) (44).

The inflammatory changes in affected tissues are alsoreflected in quantitative and functional alterations in pe-ripheral blood cells (45). For example, the proportions andabsolute numbers of total CD4� CD45RA� (suppressor-inducer) T cells and CD8�/CDIIb� suppressor T cells aredecreased. Thus, the balance between immunoregulatoryT-cell populations appears to be substantially impaired. Pe-ripheral blood T cells in systemic sclerosis manifest evi-dence of previous activation, indicated by the spontaneousexpression of high affinity interleukin-2 receptor (IL-2R)on their membranes (see Glossary). Several observationsfurther support the evidence for in vivo T-cell activation insystemic sclerosis. First, serum specimens from patientswith systemic sclerosis contain approximately 3-fold higher

levels of soluble IL-2R than specimens from normal con-trols. In addition, mononuclear cells obtained from bron-choalveolar lavage fluid in patients with systemic sclerosisdisplay ongoing activation. Finally, elevated levels of seruminterleukin-2 and increased production of this cytokine byT cells in vitro have also been demonstrated in patientswith systemic sclerosis. Abnormalities in numerous othercytokines have also been described (45). However, whetherthe alterations in the proportions of regulatory T-cell sub-sets and the various cytokines are secondary to disease ac-tivity or reflect a more fundamental pathogenic event isunknown.

Role of Transforming Growth Factor-�, ConnectiveTissue Growth Factor, and Smad Proteins

At present, it is unknown whether the exaggeratedconnective tissue production by systemic sclerosis fibro-blasts is a response to an unknown injury, thus represent-ing an abnormal regulation of a physiologic process, orwhether the primary event is an alteration in the regulationof expression of the relevant matrix protein genes. It hasbeen postulated that alterations in the regulation of extra-cellular matrix gene expression may be induced by cyto-kines and growth factors released from the tissue-infiltrat-ing inflammatory cells. Numerous alterations in theexpression of cytokines and growth factors with potent ef-fects on fibroblast collagen synthesis, endothelial cell func-tions, and T-cell responses have been demonstrated in pa-tients with systemic sclerosis (45). Many studies haveexamined the effects of various products of inflammatorycells, including TGF-�, connective tissue growth factor,platelet-derived growth factor, interleukin-1, interleukin-2,interleukin-4, interferons, and tumor necrosis factor, onmany different aspects of fibroblast biology (see Glossary)(46). These proteins exert stimulatory or inhibitory effectson fibroblast proliferation and collagen production andsometimes exert bifunctional effects, depending on theirconcentrations and on the context of the other cytokinesand growth factors present simultaneously in the pericellu-lar environment.

One of the growth factors that appears to play a crucialrole in the fibrosis that accompanies systemic sclerosis isTGF-�. The 3 functionally and structurally similar humanisoforms of TGF-� play important roles in embryonic de-velopment, in immune responses, and in the regulation oftissue repair following injury (47). One of the most impor-tant effects of TGF-� is the stimulation of extracellularmatrix synthesis by stimulating the production of variouscollagens and other matrix proteins, including fibronectin(see Glossary) (48). Small amounts of TGF-� appear tosensitize fibroblasts and maintain them in a persistentlyactivated state involving an autocrine signaling mechanismthat causes further production of TGF-�. Systemic sclero-sis fibroblasts also seem to express increased levels ofTGF-� receptors on their surface. This might account forthe increased TGF-�–induced signaling and the resulting



increased collagen production by these fibroblasts (49). Inaddition to its stimulatory effects on extracellular matrixsynthesis, TGF-� also decreases the production of colla-gen-degrading metalloproteinases and stimulates the pro-duction of protease (see Glossary) inhibitors, such as tissueinhibitor of metalloproteinases-1, which prevent break-down of the extracellular matrix (46).

Recent studies have provided a clearer picture of theintricate pathways that mediate the stimulation of collagengene expression by TGF-� (50, 51). These include thebinding of TGF-� to specific cell surface receptors and thetransduction (see Glossary) of the binding signal into thenucleus to influence the activity and expression of TGF-�–responsive genes, such as those encoding extracellularmatrix proteins. Briefly, TGF-� is secreted from activatedlymphocytes or monocytes in an inactive form and requirescomplex events to become activated. Once active TGF-�comes into contact with a given target cell, it binds to aspecific TGF-�II receptor on the target cell surface, whichin turn recruits and phosphorylates a TGF-�I receptor toform an active receptor complex. Subsequent signaling tothe nucleus then occurs through the Smad family of pro-teins (see Glossary) (50, 51). Smad 2 or Smad 3 binds tothe TGF-� receptor complex and then becomes phosphor-ylated and able to form a complex with Smad 4, a cyto-plasmic protein involved in the translocation of the Smadcomplex into the nucleus. Smad 7 is an inhibitory Smadthat can bind to the TGF-� receptor complex and prevent

Smad 2 or Smad 3 phosphorylation. It appears that bothinterferon-� and tumor necrosis factor-�, 2 potent colla-gen synthesis inhibitory cytokines, stimulate increased ex-pression of Smad 7. This results in decreased signaling ofTGF-�–induced collagen gene expression. Following nu-clear translocation, the stimulatory Smad complexes bindto specific promoter sites in target genes and activate theirexpression. The pathways involved in the activation of col-lagen genes following TGF-� binding to target cell surfacereceptors are shown in Figure 2. The potential involve-ment of Smad proteins in the pathogenesis of systemicsclerosis has been reviewed elsewhere (52). Furthermore, arecent study described substantially reduced levels of Smad7 in systemic sclerosis (53). The reduction of Smad 7 in-hibitory effects may be responsible for an exaggerated andunregulated TGF-� signaling cascade resulting in excessiveextracellular matrix tissue accumulation.

Connective tissue growth factor also appears to play acrucial role in fibrosis (54). It participates in angiogenesis,axial development of the musculoskeletal system, structuralorganization of connective tissues, and embryo implanta-tion. Transforming growth factor-� causes potent stimula-tion of connective tissue growth factor synthesis in fibro-blasts, vascular smooth-muscle cells, and endothelial cells.Connective tissue growth factor also seems to be involvedin an autocrine loop stimulating its own production and,thus, maintaining a continuous or prolonged cycle of ex-cessive fibrosis. Although studies on the role of connective

Figure 2. Schematic diagram of the transforming growth factor-� (TGF-�) and Smad pathways involved in stimulation of collagengene expression.

R � receptor.



tissue growth factor in the pathogenesis of fibrotic diseasesare just emerging, it is likely that this growth factor maybecome recognized as an important mediator in the tissueaccumulation of extracellular matrix in systemic sclerosis.

Tissue and Vascular Fibrosis: The Final Step in thePathogenic Pathway

The most prominent clinical manifestations of sys-temic sclerosis are caused by the exaggerated accumulationof collagen and other connective tissue components in theaffected organs. The excessive collagen deposition in sys-temic sclerosis is due to overproduction of this protein byfibroblasts. Regardless of the etiologic event, the resultingalterations in the biosynthetic phenotype of collagen-pro-ducing cells are crucial in the pathogenesis of systemic scle-rosis. Indeed, it is the persistent activation of the genesencoding various collagens in systemic sclerosis fibroblaststhat distinguishes controlled repair, such as that occurringduring normal wound healing, from the uncontrolled fi-brosis that is the hallmark of systemic sclerosis.

Despite recent advances in the understanding of theregulation of collagen gene expression under normal con-ditions or under the effects of various cytokines and growthfactors, the intimate mechanisms responsible for the patho-logic increase in the expression of collagen genes in sys-temic sclerosis remain obscure. The exaggerated produc-tion of extracellular matrix macromolecules by systemicsclerosis fibroblasts largely results from increased transcrip-tion rates of their corresponding genes (55, 56). The mostimportant molecule in the fibrotic processes of systemicsclerosis is type I collagen, the prototype of the interstitialcollagens, which is largely responsible for the functionalfailure of the affected organs. Normal fibroblasts are capa-ble of regulating collagen production according to the re-quirements of the situation during development, differen-tiation, and repair. As for most eukaryotic genes, theregulation of transcription rates of collagen genes involvesprecise interactions between specific nucleotide sequencespresent in the promoters and, often, in their first introns(see Glossary) and various transcription factors (see Glos-sary) capable of recognizing and binding specifically tothese sequences. The entire complement of regulatory ele-ments in collagen genes is not yet known, and the numberof identified transcription factors that interact with theseelements continues to increase. Numerous transcriptionfactors capable of regulating the expression of collagengenes have been identified (57).

One of the most extensively studied transcription fac-tors is Sp1. Sp1 recognizes a specific sequence in the pro-moters of various Sp1-responsive genes. Sp1 appears toplay a pivotal function in the increased expression ofCOL1A1 (see Glossary) in systemic sclerosis. IncreasedSp1 binding activity has been demonstrated in systemicsclerosis fibroblast nuclear extracts in comparison with nu-clear extracts from normal cells (58), and increased Sp1phosphorylation associated with increased expression of

type I collagen genes has also been found in these cells(59). Another study showed that activated hepatic stellatecells, which are the primary cells responsible for increasedtype I collagen production during liver fibrosis, also displayincreased Sp1 binding activity (60). Binding of Sp1 to itsrecognition sites has also been implicated in the regulationof expression of type I collagen genes under the influenceof TGF-�. Furthermore, numerous studies have shownthat specific inhibition of Sp1 binding to its cognate ele-ments within collagen genes caused potent and selectiveinhibition of collagen production in normal fibroblasts andfibroblasts cultured from patients with systemic sclerosis(61, 62).

Another transcription factor that appears to play animportant role in collagen gene regulation is the CCAAT-binding factor (CBF). Both CBF and Sp1 transcriptionfactors are involved in a protein–protein interaction in thecollagen gene promoter, and both appear to be crucial reg-ulatory factors in the expression of collagen genes. The roleof CBF in the regulation of collagen gene expression insystemic sclerosis has also been examined, and it has beenshown that CBF binding activity is increased in systemicsclerosis fibroblasts (63). In contrast with the stimulatoryeffects of Sp1 and CBF, the transcription factor c-Kroxmediates tissue-specific inhibition of collagen gene expres-sion (64), although its role in pathologic fibrosis has notbeen investigated.

Endothelial CellsVascular dysfunction is one of the earliest alterations

of systemic sclerosis, and it has been suggested that it mayrepresent the initiating event in its pathogenesis (65). Se-vere alterations in small blood vessels of skin and internalorgans, including fibrosis and perivascular cellular infiltra-tion with activated T cells, are almost always present insystemic sclerosis. Cytokines such as TGF-� are secretedby these activated lymphocytes and in turn injure the en-dothelial cells, inducing their expression of MHC class Iand II antigens and adhesion ligand intercellular adhesionmolecule-1. Transforming growth factor-� causes an up-regulation of connective tissue growth factor, which alsoresults in increased production of extracellular matrix com-ponents as well as an upregulation of platelet-derivedgrowth factor. Elevated expression of platelet-derivedgrowth factor causes increased endothelial cell proliferationand a downregulation of vascular endothelial growth fac-tor, the endogenous growth factor that promotes neovas-cularization. Chemoattraction of fibroblasts into the vesselwall also occurs as a result of the effects of the cytokinesreleased locally, leading to increased collagen synthesis anddeposition in the vessel wall. Transforming growth fac-tor-� may also induce transdifferentiation of vessel wallfibroblasts into myofibroblasts, which are phenotypicallydistinct cells with contractile properties and increased ex-tracellular matrix biosynthetic ability.

The endothelial injury initiated by the release of cyto-



kines causes the subendothelium to become exposed tocirculating platelets, which eventually adhere to it and ini-tiate fibrin deposition and intravascular thrombus forma-tion. It has also been suggested that endothelial cell damagein systemic sclerosis may be due to a direct effect of cyto-toxic factors for endothelium, since serum specimens frompatients with systemic sclerosis contain cytotoxic activitydirected against endothelial cells (66). Other proposedmechanisms of endothelial cell injury in systemic sclerosisare those mediated by proteolytic activities present in se-rum (67) or by serum IgG antibodies that result in anti-body-dependent cell-mediated cytotoxicity (68). These an-tibodies occasionally coexist with Clq-binding immunecomplexes and anticardiolipin antibodies, both of whichare associated with thrombosis.

Vasodilation, which is controlled by both endothelialand nonendothelial (neural) mechanisms, also appears tobe impaired in patients with systemic sclerosis. Endothelialcells normally secrete vasoactive substances that control

vascular tone, such as the potent vasodilators prostacyclin,nitric oxide, and calcitonin gene–related peptide (see Glos-sary) (69). On the other hand, they also produce endothe-lin-1, a 21-amino acid polypeptide that has potent vaso-constrictor activities and stimulates extracellular matrixproduction and deposition in the vessel wall (70). Patientswith systemic sclerosis appear to have a relative deficiencyof vasodilators and a remarkable increase in levels of endo-thelin-1. This imbalance in turn causes further vascularhypoxia, which leads to increased collagen gene expressionas well as endothelial injury, thus maintaining the viciouscycle of endothelial injury and fibrosis.

DISCUSSION

Although the exact mechanisms involved in the patho-genesis of systemic sclerosis are not known, there is suffi-cient evidence to allow cogent hypotheses about the possi-ble pathway the disease takes from its causation to organ

Figure 3. Diagram of the pathogenesis of systemic sclerosis according to the hypothesis of microchimerism.

CTGF � connective tissue growth factor; IL � interleukin; NO � nitric oxide; PDGF � platelet-derived growth factor; TGF-� � transforming growthfactor-�; TNF-� � tumor necrosis factor-�.



fibrosis and resulting clinical manifestations. At present,there is no single unifying concept but rather several pos-sible pathways, as shown in Figures 3, 4, and 5. The novelhypothesis of a graft-versus-host reaction secondary to en-grafted microchimeric cells is illustrated in Figure 3. In thispathway, it is necessary to postulate an environmentalagent (for example, physical, chemical, or infectious) actingas a required second event that will trigger this reaction. Incontrast, in Figure 4, it is hypothesized that the environ-mental agent or agents is the inciting factor that acts on agenetically predisposed host and results in the subsequentrecruitment and homing of macrophages and T cells to theaffected tissues. The inflammatory cells would undergo se-

lective proliferation and expansion, perhaps because of anantigen-driven response, and then release cytokines andgrowth factors that initiate the process of tissue and vascu-lar fibrosis. Finally, in Figure 5, it is postulated that theenvironmental factor or factors, most likely of an infectiousorigin acting in a genetically susceptible host, cause a pro-found phenotypic change in various target cells of differentlineages (immune cells, fibroblasts, and endothelial andvascular smooth-muscle cells). This phenotypic changecould be caused by integration of genetic material (for ex-ample, of retroviral origin) within the genetic sequence ofthe target cells that through unknown mechanisms wouldinduce the expression of specific regulatory genes, altering

Figure 4. Diagram of the pathogenesis of systemic sclerosis according to the hypothesis of an environmental agent causing initialtissue inflammation.

CTGF � connective tissue growth factor; HCMV � human cytomegalovirus; NO � nitric oxide; PDGF � platelet-derived growth factor; TGF-� � transforming growth factor-�.



the function and behavior of the target cells. Thesealterations are manifested by increased collagen and extra-cellular matrix production in fibroblasts, generation of au-toantibodies and cellular immune abnormalities in lym-phocytes, and severe fibroproliferative and prothromboticalterations in endothelial cells. The target cell effects ofcytokines and growth factors, particularly TGF-� and con-nective tissue growth factor, and of the various compo-nents of their regulatory cascades are downstream compo-nents common to all 3 pathways.

All 3 hypotheses, and other plausible ones that can beconstrued on the basis of currently known molecular ab-normalities in systemic sclerosis, may prove to be correct,since the disease exhibits a remarkably heterogeneous spec-

trum of clinical manifestations and its pathogenesis may bedifferent in different individuals. Although the ultimategoal of research studies about the pathogenesis of a diseaseis to find a unifying mechanism, in the case of systemicsclerosis, a single common pathogenetic mechanism maynot be the correct answer.

In the past 30 years, the study of systemic sclerosis hasevolved from the concept that the disease was purely due tocollagen overproduction by affected fibroblasts to the cur-rent stage, in which it is possible to postulate various dis-tinct and extremely complex processes during the initialphases. Regardless of whether the answer to the puzzle ofsystemic sclerosis pathogenesis is a single, unified mecha-nism or a constellation of different ones, the knowledge

Figure 5. Diagram of the pathogenesis of systemic sclerosis according to the hypothesis of phenotypic change in target cells.

CTGF � connective tissue growth factor; HCMV � human cytomegalovirus; IFN � interferon-�; IL � interleukin; NO � nitric oxide;PDGF � platelet-derived growth factor; TGF-� � transforming growth factor-�; TNF-� � tumor necrosis factor-�.



acquired in this search will undoubtedly uncover novel tar-gets, including those involving regulatory transcription fac-tors for collagen gene expression, such as Sp1; intracellularmediators, such as Smad proteins; and TGF-��signalingcascades. These targets may lead to the development oflong-awaited and potentially effective therapeutic optionsfor this incurable disease.

From Thomas Jefferson University, Philadelphia, Pennsylvania.

Acknowledgments: The authors thank Kate Salmon for expert assis-tance in the preparation of the manuscript and M. Sonsoles Piera-Velazquez, PhD, for assistance in the preparation of some of the illustra-tions.

Grant Support: By National Institutes of Health Grant AR19616. Dr.Derk was supported by National Institutes of Health Training GrantAR07583.

Potential Financial Conflicts of Interest: None disclosed.

Requests for Single Reprints: Sergio A. Jimenez, MD, Division ofRheumatology, Thomas Jefferson University, 233 South 10th Street,Room 509 BLSB, Philadelphia, PA 19107-5541.

Current author addresses are available at www.annals.org.

References1. LeRoy EC, Black C, Fleischmajer R, Jablonska S, Krieg T, Medsger TA Jr,et al. Scleroderma (systemic sclerosis): classification, subsets and pathogenesis.J Rheumatol. 1988;15:202-5. [PMID: 3361530]2. Lally EV, Jimenez SA, Kaplan SR. Progressive systemic sclerosis: mode ofpresentation, rapidly progressive disease course, and mortality based on an anal-ysis of 91 patients. Semin Arthritis Rheum. 1988;18:1-13. [PMID: 3187542]3. Rodnan GP. Progressive systemic sclerosis: clinical features and pathogenesis ofcutaneous involvement (scleroderma). Clin Rheum Dis. 1979;5:49-79.4. Fritzler MJ, Kinsella TD. The CREST syndrome: a distinct serologic entitywith anticentromere antibodies. Am J Med. 1980;69:520-6. [PMID: 6968511]5. Block JA, Sequeira W. Raynaud’s phenomenon. Lancet. 2001;357:2042-8.[PMID: 11438158]6. Pope JE. Musculoskeletal involvement in scleroderma. Rheum Dis Clin NorthAm. 2003;29:391-408. [PMID: 12841301]7. Rose S, Young MA, Reynolds JC. Gastrointestinal manifestations of sclero-derma. Gastroenterol Clin North Am. 1998;27:563-94. [PMID: 9891698]8. Co HT, Block JA, Sequeira W. Scleroderma lung: pathogenesis, evaluation,and current therapy. Am J Ther. 2000;7:321-4. [PMID: 11317180]9. Coghlan JG, Mukerjee D. The heart and pulmonary vasculature in sclero-derma: clinical features and pathobiology. Curr Opin Rheumatol.2001;13:495-9. [PMID: 11698727]10. Deswal A, Follansbee WP. Cardiac involvement in scleroderma. Rheum DisClin North Am. 1996;22:841-60. [PMID: 8923599]11. Steen VD. Scleroderma renal crisis. Rheum Dis Clin North Am. 2003;29:315-33. [PMID: 12841297]12. Gordon MB, Klein I, Dekker A, Rodnan GP, Medsger TA Jr. Thyroiddisease in progressive systemic sclerosis: increased frequency of glandular fibrosisand hypothyroidism. Ann Intern Med. 1981;95:431-5. [PMID: 7283293]13. Lally EV, Jimenez SA. Impotence in progressively systemic sclerosis. AnnIntern Med. 1981;95:150-3. [PMID: 6789736]14. Cipoletti JF, Buckingham RB, Barnes EL, Peel RL, Mahmood K, CignettiFE, et al. Sjogren’s syndrome in progressive systemic sclerosis. Ann Intern Med.1977;87:535-41. [PMID: 921080]15. Fleischmajer R, Perlish JS, Reeves JR. Cellular infiltrates in scleroderma skin.

Arthritis Rheum. 1977;20:975-84. [PMID: 861067]16. Jimenez SA. Cellular immune dysfunction and the pathogenesis of sclero-derma. Semin Arthritis Rheum. 1983;13:104-13. [PMID: 6353581]17. Feghali-Bostwick C, Medsger TA Jr, Wright TM. Analysis of systemicsclerosis in twins reveals low concordance for disease and high concordance forthe presence of antinuclear antibodies. Arthritis Rheum. 2003;48:1956-63.[PMID: 12847690]18. Jimenez SA, Diaz A, Khalili K. Retroviruses and the pathogenesis of systemicsclerosis. Int Rev Immunol. 1995;12:159-75. [PMID: 7650419]19. Dang H, Dauphinee MJ, Talal N, Garry RF, Seibold JR, Medsger TA Jr,et al. Serum antibody to retroviral gag proteins in systemic sclerosis. ArthritisRheum. 1991;34:1336-7. [PMID: 1930323]20. Pandey JP, LeRoy EC. Human cytomegalovirus and the vasculopathies ofautoimmune diseases (especially scleroderma), allograft rejection, and coronaryrestenosis. Arthritis Rheum. 1998;41:10-5. [PMID: 9433864]21. Neidhart M, Kuchen S, Distler O, Bruhlmann P, Michel BA, Gay RE, etal. Increased serum levels of antibodies against human cytomegalovirus and prev-alence of autoantibodies in systemic sclerosis. Arthritis Rheum. 1999;42:389-92.[PMID: 10025936]22. Mayes MD. Epidemiologic studies of environmental agents and systemicautoimmune diseases. Environ Health Perspect. 1999;107 Suppl 5:743-8.[PMID: 10502540]23. Diot E, Lesire V, Guilmot JL, Metzger MD, Pilore R, Rogier S, et al.Systemic sclerosis and occupational risk factors: a case-control study. OccupEnviron Med. 2002;59:545-9. [PMID: 12151611]24. Garabrant DH, Lacey JV Jr, Laing TJ, Gillespie BW, Mayes MD, CooperBC, et al. Scleroderma and solvent exposure among women. Am J Epidemiol.2003;157:493-500. [PMID: 12631538]25. Johnson RW, Tew MB, Arnett FC. The genetics of systemic sclerosis. CurrRheumatol Rep. 2002;4:99-107. [PMID: 11890874]26. Arnett FC, Howard RF, Tan F, Moulds JM, Bias WB, Durban E, et al.Increased prevalence of systemic sclerosis in a Native American tribe in Okla-homa. Association with an Amerindian HLA haplotype. Arthritis Rheum. 1996;39:1362-70. [PMID: 8702445]27. Reveille JD. Ethnicity and race and systemic sclerosis: how it affects suscep-tibility, severity, antibody genetics, and clinical manifestations. Curr RheumatolRep. 2003;5:160-7. [PMID: 12628048]28. Hata R, Akai J, Kimura A, Ishikawa O, Kuwana M, Shinkai H. Associationof functional microsatellites in the human type I collagen alpha2 chain(COL1A2) gene with systemic sclerosis. Biochem Biophys Res Commun. 2000;272:36-40. [PMID: 10872800]29. Crilly A, Hamilton J, Clark CJ, Jardine A, Madhok R. Analysis of trans-forming growth factor beta1 gene polymorphisms in patients with systemic scle-rosis. Ann Rheum Dis. 2002;61:678-81. [PMID: 12117671]30. Awad MR, El-Gamel A, Hasleton P, Turner DM, Sinnott PJ, HutchinsonIV. Genotypic variation in the transforming growth factor-beta1 gene: associationwith transforming growth factor-beta1 production, fibrotic lung disease, and graftfibrosis after lung transplantation. Transplantation. 1998;66:1014-20. [PMID:9808485]31. Tan FK, Wang N, Kuwana M, Chakraborty R, Bona CA, Milewicz DM,et al. Association of fibrillin 1 single-nucleotide polymorphism haplotypes withsystemic sclerosis in Choctaw and Japanese populations. Arthritis Rheum. 2001;44:893-901. [PMID: 11315929]32. Artlett CM, Smith JB, Jimenez SA. Identification of fetal DNA and cells inskin lesions from women with systemic sclerosis. N Engl J Med. 1998;338:1186-91. [PMID: 9554859]33. Artlett CM. Microchimerism and scleroderma: an update. Curr RheumatolRep. 2003;5:154-9. [PMID: 12628047]34. Nelson JL. Microchimerism and HLA relationships of pregnancy: implica-tions for autoimmune diseases. Curr Rheumatol Rep. 2001;3:222-9. [PMID:11352791]35. Maloney S, Smith A, Furst DE, Myerson D, Rupert K, Evans PC, et al.Microchimerism of maternal origin persists into adult life. J Clin Invest. 1999;104:41-7. [PMID: 10393697]36. Ohtsuka T, Miyamoto Y, Yamakage A, Yamazaki S. Quantitative analysis ofmicrochimerism in systemic sclerosis skin tissue. Arch Dermatol Res. 2001;293:387-91. [PMID: 11686513]



37. Burastero SE, Galbiati S, Vassallo A, Sabbadini MG, Bellone M, Mar-chionni L, et al. Cellular microchimerism as a lifelong physiologic status inparous women: an immunologic basis for its amplification in patients with sys-temic sclerosis. Arthritis Rheum. 2003;48:1109-16. [PMID: 12687555]38. Ho KT, Reveille JD. The clinical relevance of autoantibodies in scleroderma.Arthritis Res Ther. 2003;5:80-93. [PMID: 12718748]39. Roumm AD, Whiteside TL, Medsger TA Jr, Rodnan GP. Lymphocytes inthe skin of patients with progressive systemic sclerosis. Quantification, subtyping,and clinical correlations. Arthritis Rheum. 1984;27:645-53. [PMID: 6375682]40. Kraling BM, Maul GG, Jimenez SA. Mononuclear cellular infiltrates inclinically involved skin from patients with systemic sclerosis of recent onset pre-dominantly consist of monocytes/macrophages. Pathobiology. 1995;63:48-56.[PMID: 7546275]41. Chizzolini C. T lymphocyte and fibroblast interactions: the case of skininvolvement in systemic sclerosis and other examples. Springer Semin Immuno-pathol. 1999;21:431-50. [PMID: 10945035]42. Sakkas LI, Xu B, Artlett CM, Lu S, Jimenez SA, Platsoucas CD. Oligo-clonal T cell expansion in the skin of patients with systemic sclerosis. J Immunol.2002;168:3649-59. [PMID: 11907131]43. Ferrarini M, Steen V, Medsger TA Jr, Whiteside TL. Functional and phe-notypic analysis of T lymphocytes cloned from the skin of patients with systemicsclerosis. Clin Exp Immunol. 1990;79:346-52. [PMID: 1969335]44. Shimizu Y, Van Seventer GA, Horgan KJ, Shaw S. Regulated expressionand binding of three VLA (beta 1) integrin receptors on T cells. Nature. 1990;345:250-3. [PMID: 2139716]45. White B. Immunopathogenesis of systemic sclerosis. Rheum Dis Clin NorthAm. 1996;22:695-708. [PMID: 8923591]46. Postlethwaite AE. Role of T cells and cytokines in effecting fibrosis. Int RevImmunol. 1995;12:247-58. [PMID: 7650423]47. Roberts AB. Molecular and cell biology of TGF-beta. Miner ElectrolyteMetab. 1998;24:111-9. [PMID: 9525693]48. Varga J, Rosenbloom J, Jimenez SA. Transforming growth factor beta (TGFbeta) causes a persistent increase in steady-state amounts of type I and type IIIcollagen and fibronectin mRNAs in normal human dermal fibroblasts. BiochemJ. 1987;247:597-604. [PMID: 3501287]49. Ihn H, Yamane K, Kubo M, Tamaki K. Blockade of endogenous transform-ing growth factor beta signaling prevents up-regulated collagen synthesis in sclero-derma fibroblasts: association with increased expression of transforming growthfactor beta receptors. Arthritis Rheum. 2001;44:474-80. [PMID: 11229480]50. Massague J. How cells read TGF-beta signals. Nat Rev Mol Cell Biol. 2000;1:169-78. [PMID: 11252892]51. Attisano L, Wrana JL. Signal transduction by the TGF-beta superfamily.Science. 2002;296:1646-7. [PMID: 12040180]52. Varga J. Scleroderma and Smads: dysfunctional Smad family dynamics cul-minating in fibrosis. Arthritis Rheum. 2002;46:1703-13. [PMID: 12124852]53. Dong C, Zhu S, Wang T, Yoon W, Li Z, Alvarez RJ, et al. Deficient Smad7expression: a putative molecular defect in scleroderma. Proc Natl Acad Sci U S A.2002;99:3908-13. [PMID: 11904440]54. Leask A, Holmes A, Abraham DJ. Connective tissue growth factor: a new

and important player in the pathogenesis of fibrosis. Curr Rheumatol Rep. 2002;4:136-42. [PMID: 11890879]55. Jimenez SA, Saitta B. Alterations in the regulation of expression of the alpha1(I) collagen gene (COL1A1) in systemic sclerosis (scleroderma). Springer SeminImmunopathol. 1999;21:397-414. [PMID: 10945033]56. Trojanowska M. Molecular aspects of scleroderma. Front Biosci. 2002;7:d608-18. [PMID: 11861221]57. Ghosh AK. Factors involved in the regulation of type I collagen gene expres-sion: implication in fibrosis. Exp Biol Med (Maywood). 2002;227:301-14.[PMID: 11976400]58. Hitraya EG, Varga J, Artlett CM, Jimenez SA. Identification of elements inthe promoter region of the alpha1(I) procollagen gene involved in its up-regulatedexpression in systemic sclerosis. Arthritis Rheum. 1998;41:2048-58. [PMID:9811061]59. Ihn H, Tamaki K. Increased phosphorylation of transcription factor Sp1 inscleroderma fibroblasts: association with increased expression of the type I colla-gen gene. Arthritis Rheum. 2000;43:2240-7. [PMID: 11037883]60. Rippe RA, Almounajed G, Brenner DA. Sp1 binding activity increases inactivated Ito cells. Hepatology. 1995;22:241-51. [PMID: 7601417]61. Nehls MC, Brenner DA, Gruss HJ, Dierbach H, Mertelsmann R, Herr-mann F. Mithramycin selectively inhibits collagen-alpha 1(I) gene expression inhuman fibroblast. J Clin Invest. 1993;92:2916-21. [PMID: 7504695]62. Gaidarova S, Jimenez SA. Inhibition of basal and transforming growth fac-tor-beta-induced stimulation of COL1A1 transcription by the DNA intercala-tors, mitoxantrone and WP631, in cultured human dermal fibroblasts. J BiolChem. 2002;277:38737-45. [PMID: 12138160]63. Saitta B, Gaidarova S, Cicchillitti L, Jimenez SA. CCAAT binding tran-scription factor binds and regulates human COL1A1 promoter activity in humandermal fibroblasts: demonstration of increased binding in systemic sclerosis fibro-blasts. Arthritis Rheum. 2000;43:2219-29. [PMID: 11037881]64. Widom RL, Culic I, Lee JY, Korn JH. Cloning and characterization ofhcKrox, a transcriptional regulator of extracellular matrix gene expression. Gene.1997;198:407-20. [PMID: 9370309]65. Campbell PM, LeRoy EC. Pathogenesis of systemic sclerosis: a vascularhypothesis. Semin Arthritis Rheum. 1975;4:351-68. [PMID: 1135634]66. Drenk F, Deicher HR. Pathophysiological effects of endothelial cytotoxicactivity derived from sera of patients with progressive systemic sclerosis. J Rheu-matol. 1988;15:468-74. [PMID: 3379623]67. Kahaleh MB, LeRoy EC. Endothelial injury in scleroderma. A proteasemechanism. J Lab Clin Med. 1983;101:553-60. [PMID: 6339651]68. Renaudineau Y, Revelen R, Levy Y, Salojin K, Gilburg B, Shoenfeld Y, etal. Anti-endothelial cell antibodies in systemic sclerosis. Clin Diagn Lab Immu-nol. 1999;6:156-60. [PMID: 10066646]69. Herrick AL. Vascular function in systemic sclerosis. Curr Opin Rheumatol.2000;12:527-33. [PMID: 11092203]70. Xu S, Denton CP, Holmes A, Dashwood MR, Abraham DJ, Black CM.Endothelins: effect on matrix biosynthesis and proliferation in normal and sclero-derma fibroblasts. J Cardiovasc Pharmacol. 1998;31 Suppl 1:S360-3. [PMID:9595482]



Current Author Addresses: Dr. Jimenez: Division of Rheumatology,Thomas Jefferson University, 233 South 10th Street, Room 509 BLSB,Philadelphia, PA 19107-5541.

Dr. Derk: Division of Rheumatology, Thomas Jefferson University,1025 Walnut Street, Room 613 Curtis, Philadelphia, PA 19107-5541.

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