Immune Escape in Hepatocellular Cancer: Is a Good Offense theBest Defense?

SEE ARTICLE ON PAGE 413

Our immune system, charged with the function of cancersurveillance, has the complex but efficient capability to cleargenetically altered cells that have undergone malignanttransformation. Tumor cells must develop strategies to avoidclearance by the immune system to survive, expand theirpopulations, and metastasize. The process by which un-wanted tumor cells are cleared involves recognition of thealtered nature of the cell by the immune system followed byits effective killing and elimination. Thus, tumor cells mayescape immune clearance by altering immune recognition orby modulation of the cytotoxic response.

The recent identification of the Fas receptor (Fas, APO-1/CD95) and its ligand (FasL, CD95L) as a major regulator ofboth apoptosis and immune function has provided insightinto an attractive mechanism of tumor escape from immuneclearance. Fas and FasL are transmembrane proteins of thetumor necrosis factor family of receptors and ligands.1

Engagement of Fas by FasL triggers a cascade of well-characterized intracellular signaling events involving homo-philic associations of the intracellular death domain, forma-tion of the death-inducing signaling complex, and therecruitment and activation of a cascade of effector caspaseproteases. These signaling events culminate in cell death byapoptosis. One of the main mechanisms by which immuneeffector cells kill is by induction of apoptosis by Fas/FasLinteractions. The Fas/FasL system also plays an importantphysiological role in the termination of the immune response.Fas is induced on activated T lymphocytes and targets themfor subsequent elimination by FasL-expressing cells. Thislimits clonal expansion of activated lymphocytes. In addition,the elimination of Fas-expressing leukocytes also serves toprevent inflammation at sites of immune privilege such as theeye, brain, and testis, which constitutively express FasL.2

Given the importance of the Fas/FasL system as a majormechanism by which T lymphocytes kill tumor cells, it is notsurprising that tumor cells can use this system to evadeimmune recognition. The strategies include both defensivemeasures such as acquiring resistance to Fas–mediated apop-

tosis, and offensive measures such as targeted, FasL-mediated, destruction of activated lymphocytes. Indeed, awide range of tumor-derived cell lines or freshly isolatedtumor cells exhibit decreased expression of Fas. Tumor cellsmay acquire resistance to Fas-mediated apoptosis by down-regulating Fas expression, secreting decoy receptors to blockFasL on activated T lymphocytes, or by altering intracellularmechanisms mediating Fas-induced apoptosis. Another strat-egy that has been used by some tumors involves de novoexpression of FasL on the cell surface. When these tumorcells encounter activated T cells bearing Fas, FasL/Fasinteractions induce apoptosis of the T cell thereby terminat-ing the immune response and providing immune privilege totumors.3 This strategy, eloquently termed the Fas counterat-tack, is also of great potential importance in the maintenanceof immune privilege, acquisition of tolerance, and allograftprotection after transplantation.4 However, direct evidence ordefinitive proof of the role played by the Fas counterattack invivo as a mechanism of tumor immune escape is lacking.Observations from some in vivo experimental models suggestantitumoral or proinflammatory properties of FasL expres-sion, thereby undermining the potential role of this strategyin tumor immune escape. Under certain circumstances,expression of FasL results in tumor removal, and overexpres-sion of FasL results in an intense inflammatory response inseveral experimental models.5-7

In this issue of HEPATOLOGY, Nagao et al. report a detailedassessment of alterations in membrane-bound and solubleFas and FasL in human hepatocellular carcinoma and corre-late these changes with clinical and pathological features ofthe tumors.8 Although nonmalignant hepatocytes constitu-tively express Fas, a loss of expression has been identified inmalignant hepatocytes by several investigators.9-13 Further-more, in a recent study, Ito et al. reported an associationbetween loss of Fas expression and the degree of differentia-tion in hepatocellular cancer.13 These observations wereconfirmed in the current study, which also showed anassociation between loss of Fas expression and disease-freesurvival after resection. Although the expression of FasL waspresent in some tumors, a multivariate analysis identified theloss of Fas but not gain of FasL expression as a critical factorin determining intrahepatic tumor spread. The observationsby Nagao et al. suggest that immune escape in hepatocellularcancer is mediated predominantly via the loss of Fas expres-sion, not expression of FasL. Although Strand et al.9 haveshown that expression of FasL in hepatocellular cell lines caninduce Fas-mediated apoptosis in cocultured lymphocytes invitro, the relevance of this as a mechanism of tumor immuneescape in vivo is unknown. Fas-positive tumor-infiltratinglymphocytes, for example, are rarely observed in histopatho-logical analysis of human hepatocellular cancer. Further-more, the expression of FasL by tumor cells might result inautocrine or juxtacrine cell apoptosis unless the tumor and

Abbreviations: FasL, Fas ligand; sFas, soluble Fas; TRAIL, tumor necrosis factor–related apoptosis-inducing ligand.

From the Division of Gastroenterology, Scott and White Clinic and Hospital, TexasA&M University System Health Science Center College of Medicine, Temple, TX.

Received May 26, 1999; accepted June 4, 1999.Supported by the Scott Sherwood and Brindley Foundation, and grant DK02678 from

the National Institutes of Health.Address reprint requests to: Tushar Patel, M.D., Division of Gastroenterology, Scott

and White Clinic, 2401 South 31st St., Temple, TX 76508. E-mail: tpatel@swmail.sw.org;fax: 254-724-8276.

Copyright r 1999 by the American Association for the Study of Liver Diseases.0270-9139/99/3002-0033$3.00/0

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adjacent nonmalignant cells had previously acquired resis-tance to Fas. The acquisition of Fas resistance, such as bydecreased expression of Fas, would thus appear to be animportant early feature in hepatocarcinogenesis, correlatingwith degree of differentiation and with intrahepatic spread.These observations are at odds with reports from other solidtumors such as colon and esophageal cancer in which FasLexpression (in the presence or absence of Fas expression)appears to play a more prominent role in tumor immuneescape by presumably inducing lymphocyte apoptosis.3,14-16

Liver metastases from colorectal tumors show an increasedFasL expression (when compared with their primary tumors)along with a decrease in the number of tumor infiltratinglymphocytes suggesting that expression of FasL provides agrowth advantage to metastasizing colorectal tumor cells andfacilitates growth in the liver.17-19 Whether a similar mecha-nism holds for other tumors that metastasize in the liver isnot yet known. The differences between primary and malig-nant liver tumors may be due to a unique role played by theFas system in the liver. The liver constitutively expresses Fasand is exquisitely sensitive to Fas-mediated apoptosis. Thus,Fas may serve an undetermined physiological function otherthan merely targeting cells for apoptosis. Up-regulation ofFasL is seen in several liver diseases such as alcohol-inducedhepatitis, Wilson’s disease, and hepatitis B, and may beassociated with hepatic inflammation.20,21 FasL expression bymalignant hepatocytes may potentially be counterproductive,enhancing inflammation and facilitating tumor cell destruc-tion.

An alternative mechanism of Fas resistance involves theuse of an antagonistic decoy protein such as soluble Fas(sFas), which is similar to Fas, except that it lacks thetransmembrane domain. Elaboration of sFas by tumor cellsmay contribute to resistance to Fas-mediated apoptosis.Elevated sFas levels in serum have been observed in patientswith hepatocellular cancer, indicating that the use of decoyreceptors may also play a role in liver cancer.8,22 Several othermechanisms for acquiring Fas resistance have been postu-lated. Disruption of intracellular signaling by structuralalterations or functional inactivation of signaling proteinsmay alter susceptibility to Fas-mediated apoptosis.23 Somecancer cells, including hepatocellular cancer cells, expresshigh levels of proteins such as Fas-associated phosphatase-1,which interact negatively with the Fas receptor.12 The Fassignal may be abrogated by the recruitment of some proteinssuch as FLIP (FLICE/caspase 8 inhibitory protein) inhibitorprotein. Members of the Bcl-2 family of apoptosis regulatorsmay also modulate Fas-mediated apoptosis in some circum-stances. Finally, mutations in oncogenes and tumor suppres-sor genes such as p53 may also interfere with Fas signaling.

Several recently identified members of the tumor necrosisfactor receptor family contain a death domain with homologyto that of Fas. These death receptors and their ligands mayresemble Fas, although their involvement in immune escapeor tumor formation remains unknown.24 Of these, tumornecrosis factor–related apoptosis-inducing ligand (TRAIL)receptors share the greatest homology to Fas. Many tumorcell lines are sensitive to TRAIL, whereas normal tissues arenot. Although TRAIL receptors 1-4 are expressed on bothHep 3B and Hep G2 cells, only the latter are sensitive toTRAIL-induced apoptosis.25 Thus, resistance to apoptosis ismore likely to be mediated by intracellular signaling eventsthan by alterations in receptor expression or the presence of

decoy receptors. The relevance of TRAIL and other death-receptor systems to hepatocellular cancer has yet to beelucidated.

The emerging experimental and clinicopathological evi-dence indicates an important role of defensive strategiesaimed at avoiding Fas-mediated apoptosis as a means ofimmune escape in hepatocellular cancer. Nevertheless, muchremains to be learned. Immune-mediated tumor killing canoccur by secretion of perforin and granzymes, and thismechanism is also important in tumor immune surveil-lance.26 As apoptosis can be triggered in several ways,including Fas, perforin/granzyme, or death receptors such asTRAIL, a more efficient approach to immune escape would befor the cell to acquire defects in intracellular apoptoticpathways. Our knowledge of Fas (and other death receptors)is expanding. This will hopefully prompt study of the roleand relevance of perturbations in these pathways duringimmune escape and tumor formation in the liver. Translatingthis knowledge to clinical practice may eventually provevaluable in developing rational immune-based therapies forhepatocellular cancer.

TUSHAR PATEL, M.D.Division of GastroenterologyScott and White Clinic and HospitalTexas A&M University System Health Science Center

College of MedicineTemple, TX

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