Immunological tolerance

In this sheet ,, I put everything in the slide so you don’t have to go back to the slide,, I hope everything in this sheet is clear . Sorry for any mistake.

Much of data on immunological tolerance is based on hypotheses which are unproven . although these hypotheses are unproven a lot of these are glimmer of hope( there is a lot of mechanisms were discovered help and benefit us now and and other mechanisms are possible to make evolution later because immunology is rapidly evolving and many of the experiments are performed on mice)until now Immunological tolerance is developing so it’s possible to make evolution later)).

Immunological tolerance: Unresponsiveness to an antigen that is induced ( meaning recognition )by exposure to that antigen => antigens that enter our body are recognized by immune system ( How? By binding to specific receptor )and this binding will induce specific ( specific? receptor specific to certain antigen) Unresponsiveness instead of immune response . • Tolerogen = tolerogenic antigen = antigen that induces tolerance *Are every antigen tolerogen ? No ,sometimes we will induce manipulation of antigens by if we give antigens to the body in certain conditions such as( give antigen to our body in different quantities and different routes ) this will make tolerance while the same antigen in another condition will be immunogenic (immune response) >> In another word , maybe the same antigen in certain conditions will make tolerance (tolerogenic) and in another condition will make immune response . Immunological tolerance is Important for several reasons :

• a) Self-tolerance >>in lecture #8 we mentioned that during maturation of lymphocytes >> if exposed to self-antigen and binds them with strong affinity it will undergo 1) receptor editing(e.g., kappa becomes lambda) or 2) negative selection >>A) unresponsive(anergy) >> in lecture #8 we talked about central tolerance but The most mechanisms (anergy)happen in peripheral tolerance>>(( peripheral tolerance)>> not during maturation … after Lymphocytes leave thymus and bone marrow and go to peripheral tissue ))and we are going to talk about peripheral anergy in details in this lecture . >> B) die by apoptosis (deletion) Self- tolerance … the most important benefits of tolerance is Self-tolerance because if we react to self antigen >> autoimmune disease will happen. **There are a lot of things that can help us by using tolerance, some of this has applied and another hasn’t. • May be used for: -B)autoimmune diseases >> in autoimmune disease our body identify self antigen as a foreign so to treat autoimmune disease I will stimulate tolerance in lymphocytes in our body ( so don’t attack self antigen). - c)allergic diseases (hypersensitivity reaction) >> our body responds to allergen abnormally , so I can stimulate tolerance in our body and therefore I will treat the allergy . -c)transplantation >> • Peter Medawar et al. …1950s ,, (father of transplantation and was the first one who explained the tolerance and discovered this concept.>> in transplantation the problem is in rejection so if I gave the recipient graft from the donor and stimulated the tolerance in the recipient ,the problem would solve. -d)gene therapy>>in gene therapy changes occur in our body so our body identify products of new genes as a foreign so I will stimulate the tolerance and our body will accept these new products.

• Peter Medawar et al. …1950s ,, father of transplantation and was the first one who explained the tolerance and discovered this concept. ( Lebanese in origin) >> He brings mouse from strain A and mouse from strain B and mouse from strain C ,,, ** first : He brought new born mouse from strain A and brought cells from adult mouse from strain B and injected these cell in new born mouse A ,, after a while( after mouse A was growing ) he brought skin graft from mouse B to mouse A( the same mouse that injected when he was new born ) >> The recipient would accept the skin graft because the first injection of the recipient ( new born strain A) was during maturation ,,, in the first injection ,,specific antigens from the donor would make the recipient identify the cell and therefore would accept skin graft later .if we injected adult A by cells from adult B strain >> rejection would happen… the reasons and mechanisms are unknown. Another experience … Bring skin graft from mouse C ,,the recipient would reject these graft,, because the recipient didn’t inject by these cell during maturation. ***Self-tolerance : * Central >> generative lymphoid organs,,in bone marrow and thymus during maturation * Peripheral >> in Peripheral lymphoid tissue ( spleen and lymph node) after recombination of the gene of T- cell receptor and B Cell receptor.

Central tolerance • Immature self-reactive T or B cells • In generative lymphoid organs • It is not known why high-affinity binding to self antigens induces apoptosis rather than activation/proliferation **B – cell :In bone marrow if exposed to self antigen and binds them with strong affinity it will undergo 1)receptor editing or 2) negative selection >> deletions ( apoptosis) Note: Anergy mainly happened in peripheral tolerance,, not in central tolerance,,, so we are going to talk about anergy in peripheral tolerance ( in lecture #8 we talked about tolerance in general ( not in details) so we mentioned anergy) T- cell:

In thymus if exposed to self antigen and binds them with strong affinity it will undergo Negative selection whether its happened in medulla or cortex . negative selection >>1) deletions or the cell will turn to T regulatory cell (If CD4+doesn’t undergo apoptosis ,it will turn to T regulatory cell).,, so the source of T regulatory cell is CD4+ cell; if CD4+ reactive to self antigen it will convert to T regulatory cell.,, (the reason of why some CD4+ undergo apoptosis and another turn to T regulatory cell is unknown) ** T regulatory cell can leave thymus and go to peripheral tissue .

Note from lecture #8 :in cortex ,,,when cd4 and cd8 are expressed we call it double positive -->now we need a single positive by presenting MHC1 and MHC2 with peptide in thymus on (thymic epithelial cells)-->if combine to mhc1=cd8 if mhc2=cd4 and the affinity should be weak (positive selection )-->cells now go to medulla where dendritic cells present MHC with self-antigen-->if binds with strong affinity the t-cell will be eliminated (apoptosis) (negative selection)so:cortex -->positive selection medulla-->negative selection

** in central tolerance,,not every cell has been checked so there is peripheral tolerance .

Peripheral tolerance : *Not all self antigens may be present in thymus or bone marrow ,and hence T and B cells bearing

receptors for such auto-antigens escape into the periphery . *These mechanisms are best defined for T cells, especially CD4+. * Undergo ( mechanisms of tolerance): -Anergy >> unresponsiveness of lymphocytes upon recognition of the antigen ,, lymphocytes bind to antigen ( there is recognition)and therefor unresponsiveness. -Suppression by Tregs )T regulatory cell)>> inhibition of immune response and inhibition self reactive lymphocytes ,, one of mechanisms of tolerance >>suppression by regulatory T cell),, Regulatory T cells have function of contraction(homeostasis or regulation) of normal immune responses when they end -Deletion by apoptosis In peripheral tissue,,these cell will be exposed to specific self antigen( according to the tissue,, lymph node ,, pancreatic cell) and will undergo:

1) Apoptosis or 2) Anergy >> unresponsive to these antigen. 3) Turn to T regulatory cell (if CD4+).

Summary: In central tolerance A) B- cell >> undergo

1) apoptosis or 2) receptor editing

B) T -cell >> undergo 1) apoptosis or 2)become T- regulatory cell ( if it was CD4+)

In peripheral tolerance: If it’s B cell:

1) Apoptosis or 2) Anergy >> unresponsive to these antigen. If it’s T cell:

1)Apoptosis or 2)Anergy >> unresponsive to these antigen. 3) Turn to T regulatory cell ( if it was CD4+).

The most of mechanisms that’s the doctor will mention is about CD4+ cell ..because most studies is about T cell when We talked about tolerance. ** CD4+ are the basic of tolerance in our body because CD4+ cell is cell mediated immunity and B lymphocytes depend on them ..and it’s control the response of peptides antigens in general . ** ( CD4+ has 2 arms ,, one it’s cellular immunity and the other is humeral immunity depending on them) so the most of studies about it . Anergy = functional unresponsiveness • Activation of antigen-specific T cells requires two signals: recognition of peptide antigen in association with self MHC molecules on the surface of APCs and a set of costimulatory signals (“second signals”) from APCs. By binding of molecules on T cells to their ligands (the costimulators B7-1 and B7-2) on APCs. **There is receptor another than T cell receptor ( T cell bind to peptide antigen on MHC) ,,there is receptor on T cell will bind to costimulator on antigen presenting cell. **To activate T cell >>( against foreign body)we require 1) T cell bind to peptide antigen on MHC on the surface of APCs , This first signal is necessary but not sufficient to stimulate a T cell to proliferate and differentiate. ) 2) second signal provided By binding of molecules on T cells to their ligands (the costimulators B7-1 and B7-2 on APCs )is also required for T cell activation .

** one signal not enough to activate antigen-specific T cells. ** The first and second signals initiate intracellular signaling cascades activating one or more transcription factors leading to specific gene transcription. Without Costimulation( second signal), T cells either become selectively unresponsive, a condition known as anergy or undergo apoptosis………()

• On resting dendritic cells in normal tissues…costimulatory molecules

are weakly expressed This encounter with autoreactive T cells induces anergy These APCs present self antigens continuously in the tissue they reside in but they are resting: (absence of microbial infection or innate reactions) and when they are resting they under-express costimulators on them ** normally … if there is lymphocytes in Peripheral tissue and these lymphocytes were self reactive and there is antigen presenting cell on this tissue ( For example in skin ( epidermis) there is langerhans (dendritic cell) ) present on its surface self antigen of this tissue ..what will happen? **Note: Resting antigen presenting cell( base line):meaning 1)antigen presenting cell in these tissue present in it’s surface self antigen ( self specific form theses tissue ,, skin an so on )permanently and normally 2) innate system is absence ( No injury and no infection,,no microbe to activate innate system ), 3) little number of( costimulator( B-7 onAPCs.>> resting. **B-7 >> will increase when innate system activated by injury,,necrosis ,,inflammation and infection….. Answer; ** in resting >> in peripheral tissue there is 1) antigen presenting cells present self antigen on there surface normally and permanently ,,2) self reactive lymphocytes reach these tissue and because antigen presenting cell in rest >>( innate system absence ,the number of B-7 is little) lymphocytes (autoreactive T cells ) will bind to antigen presenting cell on MHC in rest situation>> anergy( peripheral tolerance) because there is no costimulation. ** ( in resting state the second signal doesn’t happen( weakly expressed )because the number of B-7 is little( little number of B-7 isn’t enough to bind with T cell and activate second signal) because innate system doesn’t active and the first signals isn’t enough to activate and proliferation of lymphocytes so there is anergy . ** if the innate system activated by injury or trauma in this tissue ( one of the mechanisms that will activated innate system and therefore autoimmune And its one of the mechanisms that’s explain autoimmune disease ) ,,B-7 on APCs will increase in number and lymphocytes (autoreactive T cells )will bind to them and auto immune will happen >> this happen in situations of autoimmune disease,,there is genetic susceptibility and another factor ,, this can help us in explain autoimmune disease..

**CTLA-4 & PD-1 are inhibitory receptors on T cells…they are more expressed if the cell is self-reactive >> this is in normal condition because we have self-reactive lymphocytes but their expression of PD-1 & CTLA-4 is of the mechanisms that make them anergic which helps our body to have tolerance. We mentioned previously that the deficiency of costimulator (B-7(on APCs will lead to ( peripheral tolerance) anergy because in the deficiency of costimulator (B-7( ,,second signal didn’t be activated and this lead to anergy . there is another things that will lead to anergy >> self reactive T lymphocytes. Self reactive T lymphocytes >> when APCs present self antigens (low levels of B7),, Self reactive T lymphocytes will express CTLA-4 & PD-1 instead of CD28 on T cell( which will bind to costimulator on APCs). But we need CD28 for activation of lymphocyte if it is reactive against foreign antigen (here we want the action of CD28 in activating T cell against non-self)

• CTLA-4 binds to B7 with higher affinity than CD28 …so when APCs present self antigens (low levels of B7)…CTLA-4 is preferentially engaged …microbial products elicit innate immune reactions >>B7 levels on APCs increase and low-affinity receptors(CD28) on T cells are engaged more .>> immune response ( activation). • Polymorphisms in the CTLA4 gene are associated with some autoimmune endocrine diseases in humans.>> changes in CTLA4 gene( polymorphism ) >> there is no tolerance>>will lead to autoimmune

endocrine diseases in humans and mice ,, because CTL4 is important to tolerance (unresponsiveness),, ** many of autoimmune disease it has no explanation ( multifactorial) ,,and autoimmune diseases with known genetic abnormality are rare ,, but understanding such mechanisms is very important in understanding mechanisms of tolerance

** The affinity of CTLA-4 to B-7 is higher than the affinity of CD28.,, so if B-7 is low in concentration ,, B- 7 will bind to CTLA-4 instead of CD28.>> in biochemistry ,, if the concentration of any molecule is low( like B-7) ,,this molecule will bind to the receptor which has high affinity( like CTLA-4). ** Regulatory T cells may also use CTLA-4 to suppress immune responses >> CTLA4 will bind to APCs so this binding will inhibit the binding of CD28 to APCs and therefore there is no response ,,no activation of T cell. • Some tumors and viruses may use the same pathways of immune regulation to evade immune attack >> some tumors and viruses will express CTLA-4 and PD-1 on the surface of infection cell>> CTLA4 will bind to APCs so this binding will inhibit the binding of CD28 to APCs and therefore there is no response ,,no activation of T cell ,, this will help us in immunotherapy by >> directed antibody toward CTLA-4 and PD-1 >> block CTLA-4 and PD-1>> stimulate immune response>> stimulate our body to attack the virus and tumors.

antibodies are developed that block CTLA-4 and PD-1 for tumor immunotherapy

Regulatory T cells may also use CTLA-4 to suppress immune responses Suppression by regulatory T cellsSuppression by regulatory T cells They develop mainly in the thymus, as a result of recognition of self antigens • They may also be induced in peripheral lymphoid tissues • The best-defined regulatory T cells: -CD4+ & expressing high levels of -CD25 (the α chain of the IL-2 receptor) -FOXP3…a transcription factor of the forkhead family Regulatory T cells made in central tolerance in thymus ,,later ,go to peripheral tissue ( also, can make in peripheral tissue). ** Regulatory T cells expression on there surface CD25 (the α chain of the IL-2 receptor) and expression of transcription factor FOXP3 . • IL-2 and FOXP3 are required for the development and maintenance of functional CD4+ regulatory T cells *IPEX (an acronym for immune dysregulation, polyendocrinopathy,enteropathy, X-linked) a severe systemic autoimmune disorder…FOXP3 mutations *Recent studies: polymorphisms in the CD25 gene are associated with multiple sclerosis. () (CD25) on regulatory T cell >> indicate that IL-2 receptor important in regulatory T cell.,, when IL-2 bind to these receptor on regulatory T and FOXP3>> important in development and maintenance of regulatory T cell. IPEX: autoimmune disease result from mutation ( dysfunction ) of FoxP3>> destruction of regulatory T cell which is important in () tolerance >> autoimmune disease will happen Polymorphisms in the genes encoding the IL-2 receptor (CD25) and IL-7 receptor α chains are associated with multiple sclerosis and other autoimmune diseases …These cytokines may control the maintenance of regulatory T cells. >> one of the mechanisms associated with multiple sclerosis ,,not all patient with multiple sclerosis have a problem in these genes . Of Treg inhibitory activity: release of immunosuppressive cytokines >>( inhibitory cytokines )IL-10 >> ( # remember IL-10 ,, inhibit the inflammation , TGF-beta, etc. …Tregs also express CTLA-4

Apoptosis: mechanism of tolerance. 2 mechanisms of inducing death in mature self-reactive T cells…based on animal studies: BcL family have 1) pro apoptotic members and 2) anti – apoptotic members.

() 1)Expression of pro-apoptotic member of the Bcl family without anti-apoptotic members >> stimulate mitochondrial pathway 2) Fas (CD95)-Fas ligand system …for deleting B or T cells But here we are talking aboutT cell ( self reactive T lymphocytes) in apoptosis >> stimulate expression fas on self reactive T lymphocytes or expression fas ligand on self reactive T lymphocytes >> fas will bind to fas ligand ( stimulating them self) ( self reactive T lymphocytes will kill another self reactive T lymphocytes by << fas of self reactive T lymphocytes will bind to fas ligand of self reactive T lymphocytes >> death receptor pathway.,, …in mice, mutation in Fas or Fas ligand: autoimmune disease resembling SLE …in human, mutations in Fas gene cause ALPS (autoimmune lymphoproliferative syndrome)>> mutation in fas >> no tolerance( no apoptosis) >> autoimmune disease.

Ignorance Antigen in our body and there is no recognition,,later maybe our body will recognize this antigen and activated immune response .>> like 1) normal flora in colon hidden (sequestered) from the immune system ( lymphocytes and there is inhibitory mechanisms like T helper 2 like environmental ( IL-10 ) >>inhibition of immune ,, in normal condition . In trauma and infection,,maybe our body will recognize this antigen and activated immune response .oral tolerance >> inhibitory cytokines,,our body recognize it but the present of inhibitory cytokines, ,, inhibition of immune response >> so oral tolerance not considered ignorance because immune system recognize it but don’ attack them….we are talking mainly about foods here What is the difference between tolerance and ignorance? Tolerance: recognition of antigen ,,there is binding ,,and the response of immune system is very weak >> unresponsiveness. Ignorance : the antigens are isolated( hidden )from immune system >> ( don’t recognize by immune system)>> no binding.,, Ignored antigens maybe tolerogenic or immunogenic. • Some antigens in certain tissues are hidden (sequestered) from the immune system …no communication with the blood and lymph …self antigens in these tissues are ignored by the immune system …like the testis, eye and brain >> Immune-privileged sites” because it is difficult to induce Immune responses to antigens introduced into these sites . …post-traumatic orchitis( inflammation of testis because of truma)and uveitis ( inflammation of uvia because of truma)>> in Truman… destruction of tissue…these tissue will face with immune cells ( APCs and lymphocytes) . ** Antigen in our body and there is no recognition,,later maybe our body will recognize this antigen and activated immune response .

Second lecture will begin,,take a breath and continue… In last lecture we mentioned that autoimmune disease is multifactorial (genetic factors + environmental factors that collectively break tolerance)… autoimmune disease will happen ( not that simple) ,,there must be predisposing event lead to autoimmune disease. Autoimmune disease:

1) Genetic susceptibility.,, normally there is immunological tolerance to self antigen and

disruption of immunological tolerance maybe is associated with certain gene.

2) Environmental stimuli ,, ( Infection, truma) of tissue >> increase costimulator on antigen

presenting cell >> innate immune system >>self-reactive lymphocytes activation >> increase exposed antigens from apoptotic cell by infection , injury and death.

** decrease of PD-1or CTL4>>CD28 will bind to costimulator on APCs and activated of lymphocytes >> autoimmune disease. ** the role of PD-1 and CTL4>> tolerance and inhibit CD28 to bind with costimulator on APCs. Role of Susceptibility Genes : polymorphism of some genes will lead to autoimmune disease and the relation between this polymorphism and disease is unknown in most cases. • Most autoimmune diseases are complex multigenic disorders . • Among the genes known to be associated with autoimmunity, a great contribution is that of HLA genes . • Ankylosing spondylitis and HLA-B27 .>> just polymorphism of certain HLA ,,and this will lead to autoimmune disease.are the polymorphism effect the tolerance? Unproven yet • The mechanisms underlying these disease associations remain poorly understood . • Different HLA alleles may contribute to a disease but their presence is not, by itself, the cause of any disease. • Association of Non-MHC Genes with Autoimmune Diseases -PTPN22 association with rheumatoid arthritis, type 1 diabetes, and several other autoimmune diseases -NOD2 and Crohn disease >> sensor in epithelial cell in GI to normal flora ( sensor>> normal flora is isolated)>> if NOD2 is normal and active … normal flora will isolate from immune system and doesn’t attack by immune system ( Ignorance) . # mutation of NOD2 >> normal flora wouldn’t be isolated( will enter and be face with immune system ,,immune system will become active and consider normal flora as foreign body ( microbe)>> increase number of B-7 on APCs >>activate self reactive lymphocytes (due to resemblance of normal flora ags to self ags that self-reactive cells must be anergic to them normally or due to that normal flora ags may activate a receptor on the self-reactive lymphocyte other than the receptor to self antigen and as a collateral damage due to lymphocyte activation by microbe it also will react strongly against the self antigen that it was previously anergic to it)…( genetic susceptibility)>> disruption of the tolerance >> autoimmune disease(Crohn disease). ** PTPN22 is coding phosphatase >> phosphatase inhibit tyrosine kinase>> tyrosine kinase Important in single transduction which activate lymphocytes. ** PTPN22 >> keep the tolerance. ** gene mutation of PTPN22>> decrease phosphatase >> increase tyrosine kinase in signal transduction >> lymphocytes self reactive activation >> destruction of tolerance. All of these we mentioned also need environmental factor >> multifactorial…not just gene susebtability. Association of Non- -MHC Genes with Autoimmune Diseases, cont’dMHC Genes with Autoimmune Diseases, cont’d • Remember: AIRE, CTLA4, PD1, FAS, FASL, and IL2 and its receptor CD25 • B cells express an Fc receptor that recognizes IgG antibodies bound to antigens and switches off further antibody production (a normal negative-feedback mechanism)

…Knockout of this receptor results in autoimmunity, presumably because the B cells can no longer be controlled B cells secret antibody,, and express an Fc receptor for secreted antibodies,,after certain response ,, there is negative – feedback mechanisms.,,, if Fc receptor destruct ,, there is no negative – feedback mechanisms( there is no regulation of immune response)>> autoimmune disease,, No control of T cell,, Note : autoimmune disease are multifactorial so there must be group of mechanisms ( infection,,environmental factor ) ( not only destroy of Fc receptor >> autoimmune disease)

Infection : environmental factor

Figure A: microbial ags may activate the innate response and APC and will bind a receptor on the self-reactive lymphocyte other than the receptor to self antigen and as a collateral damage due to lymphocyte activation by microbe the lymphocyte also will react strongly against the self antigen that it was previously anergic to it) Figure B: a microbe with Ag that resembles self ag will induce innate response and increase B7 on APC so will activate the self-reactive lymphocyte specific to the ag breaking its anergy

Some times all it takes is just one du’a to change every thing.

## Done by : Marah Saeed