Molecular Mechanisms in Cell-Mediated Cytotoxicity

Abstract

Intense research has been devoted over the past two decades to the elucidation of the precise molecular mechanisms by which cytotoxic T lymphocytes (CTLs) and natural killer (NK) cells recognize and kill target cells. Both issues have been recently discussed at the EMBO Workshop on Cell-Mediated Cytotoxicity held in Kerkrade, The Netherlands, on April 5–9, 1997, organized by R. L. H. Bolhuis and C. J. M. Melief. Recent studies have demonstrated that two independent mechanisms account for the cytotoxicity mediated by CTLs (18Kagi D Vignaux F Ledermann B Burki K Depraetere V Nagata S Hengartner H Golstein P Fas and perforin pathways as major mechanisms of T cell-mediated cytotoxicity.Science. 1994; 265: 528-530Crossref PubMed Scopus (1433) Google Scholar, 26Lowin . Hahne M Mattmann C Tschopp J Cytolytic T-cell cytotoxicity is mediated through perforin and Fas lytic pathways.Nature. 1994; 370: 650-652Crossref PubMed Scopus (966) Google Scholar): the first is consequent to the secretion of their characteristic electron-dense cytoplasmic granules whereas the second is represented by a nonsecretory pathway based on the interaction of CD95L (Fas ligand) with the apoptosis-inducer CD95 (Fas) molecule expressed on target cells. So far, the mechanism by which NK cells mediate lysis of NK-susceptible target cells appears to be mostly, if not exclusively, based on granule exocytosis. The initiation of apoptosis by granule exocytosis is the result of action of two types of molecules released into the extracellular space after contact between effector and target cells: the pore-forming perforin and the lymphocyte-specific granule serine esterase granzyme B (GraB). These molecules interact in the target cells to reproduce all of the features of CTL or NK-induced apoptosis. These include nuclear changes such as chromatin condensation and finally DNA fragmentation. The mechanism of entry into target cells of GraB and the mechanism by which it induces nuclear damage has been investigated by several groups. Thus, C. Froelich (Northwestern University, Evanston, Illinois) C. Bleackley (University of Alberta, Edmonton, Canada), J. Trapani (Austin Research Institute, Victoria, Australia), and P. Henkart (National Cancer Institute, NIH, Bethesda, Maryland) studied in detail the respective roles of GraB and perforin (14Froelich C.J Orth K Turbov J Seth P Gottlieb R Babior B Shah G.M Bleackley R.C Dixit V.M Hanna W New paradigm for lymphocyte granule-mediated cytotoxicity.Target cells bind and internalize granzyme B, but an endosomolytic agent is necessary for cytosolic delivery and subsequent apoptosis. J. Biol. Chem. 1996; 271: 29073-29079Google Scholar, 22Jans D.A Jans P Briggs L.J Sutton V Trapani J.A Nuclear transport of granzyme B (fragmentin-2). Dependence on perforin in vivo and cytosolic factors in vitro.J. Biol. Chem. 1996; 271: 30781-30789Crossref Scopus (91) Google Scholar, 38Shi L Mai S Israels S Browne K Trapani J.A Greenberg A.H Granzyme B (GraB) autonomously crosses the cell membrane and perforin initiates apoptosis and GraB nuclear localization.J. Exp. Med. 1997; 185: 855-866Crossref Scopus (194) Google Scholar). It has been shown that GraB is endocytosed by target cells independently of perforin, possibly through saturable high affinity cell surface binding sites. In the absence of added perforin, GraB shows a cytoplasmic localization. On the other hand, when perforin is added (and presumably enters cells, through an as yet undefined mechanism) in about half of the cells, GraB relocalizes to the nucleus. These cells that show GraB relocalization undergo apoptotic death. The activation of members of cystein protease interleukin-1β converting enzyme (ICE) family, now called the caspase family (2Alnemri E.S Livingston D.J Nicholson D.W Salvesen G Thornberry N.A Wong W.W Yuan J.Y Human ICE/CED-3 protease nomenclature.Cell. 1996; 87: 171Abstract Full Text Full Text PDF PubMed Scopus (2078) Google Scholar), is believed to be required for cell death, at least as to its nuclear manifestations. GraB can cleave and thus activate caspases, leading to cell death. This pathway was examined by C. Bleackley and colleagues, who showed GraB–induced caspase 3 activation and its further processing in target cells (10Darmon A.J Nicholson D.W Bleackley R.C Activation of the apoptotic protease CPP32 by cytotoxic T-cell derived granzyme B.Nature. 1995; 377: 446-448Crossref PubMed Scopus (629) Google Scholar). However, the possibility that this occurs via an upstream caspase was suggested by J. P. Medema (German Cancer Center, Heidelberg, Germany) and P. Krammer (Inst. fur Immunologie und Genetik, Heidelberg, Germany), who demonstrated that caspase inhibitors that do not block caspase 3 do block its processing induced by CTL. Further results led them to conclude that the caspase FLICE (see below) is a likely substrate for GraB. P. Henkart and colleagues presented similar data showing that GraB can process procaspases leading to their proteolytic activation and to apoptotic nuclear damage (37Sarin A Williams M.S Alexander-Miller M.A Berzofsky J.A Zacharchuk C.M Henkart P Target cell lysis by CTL granule exocytosis is independent of ICE/Ced-3 family proteases.Immunity. 1997; 6: 206-215Abstract Full Text Full Text PDF Scopus (188) Google Scholar). However, they also showed that caspase inhibitors block nuclear relocalization and DNA fragmentation but not 51Cr release, strongly suggesting that GraB can also cleave a noncaspase substrate thereby leading to membrane damage. There might well be additional functional molecules in the granules beyond just perforin and GraB. Indeed, Trapani and colleagues showed that the overexpression of Bcl-2 protects cells subjected to perforin and GraB from DNA fragmentation but not cells subjected to cytotoxic cells or to purified granules. Importantly, M. Simon and colleagues (Max-Planck-Institut, Freiburg, Germany) showed that although in vitro– or ex vivo–derived CTL and NK cells (11Ebnet K Hausmann M Lehmann-Grube F Mullbacher A Kptf M Lamers R Simon M.M Granzyme-A-deficient mice retain potent cell-mediated cytotoxicity.EMBO J. 1995; 14: 4230-4239Crossref Scopus (167) Google Scholar) from double knock out GraA−/− GraB−/− mice displayed reduced granule-mediated cytotoxicity in terms of nuclear lesions, they could still induce 51Cr release. This strongly suggests that release of 51Cr can either be mediated by perforin alone, or involves some other granule component as well, possibly another granzyme. As to the major Fas-based mechanism of cytotoxicity, it is now clear that oligomerization, probably trimerization, of CD95 is necessary to attract intracellularly preformed signaling molecules. The cytoplasmic tail of Fas contains a motif called the “death domain.” This domain can interact with the corresponding death domain of the adaptor molecule Fas-associated protein with death domain (FADD), thus recruiting FADD to the plasma membrane. FADD, in turn, interacts with the caspase FLICE through “death effector domains.” These domains are located at the amino terminus of FLICE and at the carboxyl terminus of FADD. This interaction leads to the assembly of the receptor-associated death- inducing signaling complex (DISC). The DISC-bound FLICE induces the proteolytic activation of other members of the caspase family resulting ultimately in apoptosis (5Boldin M.P Goncharov T.M Golstev Y.V Wallach D Involvement of MACH, a novel MORT/FADD-interacting protease, in Fas/APO-1 and TNF-receptor-induced cell death.Cell. 1996; 85: 803-815Abstract Full Text Full Text PDF PubMed Scopus (2057) Google Scholar, 30Muzio M Chinnaiyan A.M Kischkel F.C O'Rourke K Schevchenko A Ni J Scaffidi C Bretz J.D Zhang M Gentz R et al.FLICE, a novel FADD-homologous ICE/CED-3-like protease, is recruited to the CD95 (Fas/APO-1) death-inducing signaling complex.Cell. 1996; 85: 817-827Abstract Full Text Full Text PDF PubMed Scopus (2657) Google Scholar). A new family of virus-encoded inhibitors termed viral FLICE-inhibitory proteins (vFLIPs) that interferes with apoptosis signaled via the CD95 death receptor was described (J. Tschopp, University of Lausanne, Switzerland). These inhibitors, which are found in particular in a series of γ-herpes viruses, allow the virus to undergo replication before the host cells finally succumb. The vFLIPs are characterized by two death-effector domains which, similar to FLICE, are able to interact with FADD, thus competing for the recruitment of FLICE induced by the oligomerization of the CD95 receptor (44Thome M Schneider P Hofmann K Fickenscher H Meinl E Neipel F Mattmann C Burns K Bodmer J.L Schroter M et al.Viral FLICE-inhibitor proteins (FLIPs) prevent apoptosis induced by death receptors.Nature. 1997; 386: 517-521Crossref PubMed Scopus (1088) Google Scholar). Cell death signaling pathways related to the Fas and TNF. R1 pathways also exist in invertebrates. A molecule called “reaper” was shown by H. Steller and his colleagues (46White K Grether M.E Abrams J.M Young L Farrell K Steller H Genetic control of programmed cell death in Drosophila.Science. 1994; 264: 677-683Crossref PubMed Scopus (872) Google Scholar) to be necessary and sufficient to induce cell death in Drosophila during development. Homology was found between the death domains and reaper (16Golstein P Marguet D Depraetere V Homology between Reaper and the cell death domains of Fas and TNFR1.Cell. 1995; 81: 185-186Abstract Full Text PDF Scopus (88) Google Scholar). Some periodicity, such as several successive leucine residues, separated by 7 residues, was detected both in death domains of TNF-R1 and in reaper. Amino acid alignment showed that the middle third of reaper was more homologous to the death domains of TNF-R1 and DR3 (7Chinnaiyan A.M O'Rourke K Yu G.L Lyons R.H Garg M Duan D.R Xing L Gentz R Ni J Dixit V.M Signal transduction by DR3, a death domain-containing receptor related to TNFR and CD95.Science. 1996; 274: 990-992Crossref PubMed Scopus (523) Google Scholar) than to the death domains of Fas. Interestingly, the former are bound by TNF-R1-associated protein with death domain (TRADD), the latter by FADD. Thus, both the degree of homology and the connecting molecules are consistent with the existence of subfamilies of death domains. Reaper would be evolutionarily closer to one of these subfamilies (P. Golstein, INSERM-CNRS, Marseille-Luminy, France). Studies using gene knockout animals (E. R. Podack, University of Miami, Florida) indicated that mice deficient for both perforin and Fas ligand expression died of severe pancreatitis, with atrophic uterus and ovary infiltrated with cells from a particular T cell subpopulation. This might be interpreted as a requirement for both or either the Fas or the perforin pathway for the elimination of antigen-presenting cells; thus when the latter are not eliminated, persistent antigen presentation may lead to continuous T cell activation and tissue damage (autoreactive T cells?). A novel class of Fas-activated serine/threonine kinases (FASTs) that phosphorylate the RNA binding protein TIA-1 during stress-induced apoptosis has been described by P. Anderson (Dana Farber Cancer Institute, Boston, Massachusetts). TIA-1 is a component of a signaling cascade that may regulate entry into apoptosis (42Taupin J.L Tian Q Kedersha N Robertson M Anderson P The RNA-binding protein TIAR is translocated from the nucleus to the cytoplasm during Fas-mediated apoptotic cell death.Proc. Natl. Acad. Sci. USA. 1995; 92: 1629-1633Crossref PubMed Scopus (113) Google Scholar). TIA-1 binds to oligopyrimidine tracts found in the 5′ untranslated regions of mRNAs subject to stress-induced translational arrest. Some of these mRNAs encode ribosomal subunits and survival factors such as IGF-II, hence proteins that are essential for cell growth. In response to stress, TIA-1 accumulates in the cytoplasm where it aggregates with free and ER-associated ribosomes at “stress bodies.” The recruitment of selected Bcl-2 family members to these aggregates has been postulated to regulate entry into apoptosis. In recent years, the molecular mechanisms by which NK cells lyse or fail to lyse target cells have been elucidated. NK cells express receptors (NKR) that recognize MHC class I molecules on target cells. The interaction between NKR and MHC class I leads to inhibition of cytolytic activity thus preventing lysis of normal, MHC class I positive, target cells. This mechanism was predicted by the “missing self hypothesis” (24Ljunggren H.-G Karre K In search of the “missing self”. MHC molecules and NK cell recognition.Immunol. Today. 1990; 11: 237-244Abstract Full Text PDF PubMed Scopus (391) Google Scholar). Different types of inhibitory receptors specific for groups of HLA-C (p58.1 and p58.2), HLA-B (p70), or HLA-A (p70–140) alleles have been molecularly identified and cloned (reviewed by27Moretta A Bottino C Vitale M Pende D Biassoni R Mingari M.C Moretta L Receptors for HLA class I molecules in human Natural Killer cells.Annu. Rev. Immunol. 1996; 14: 619-648Crossref PubMed Scopus (743) Google Scholar). They belong to the Ig superfamily and are characterized by two or three extracellular domains. These receptors are distributed in a clonal fashion and each NK cell expresses at least one inhibitory NKR for self HLA–class I alleles. In addition, activating forms of HLA-C-specific receptors have been described (p50.1, p50.2, and p50.3) that differ from the corresponding inhibitory receptors in their transmembrane and cytoplasmic portions. More recently, a second class of MHC class I–specific receptors has been demonstrated in human NK cells (28Moretta A Vitale M Sivori S Bottino C Morelli L Augugliaro R Barbaresi M Pende D Ciccone E Lopez-Botet M Moretta L Human Natural Killer cell receptors for HLA-class I molecules.Evidence that the Kp. 1994; 43: 545-555Google Scholar, 39Sivori S Vitale M Bottino C Marcenaro E Sanseverino L Parolini S Moretta L Moretta A CD94 functions as a natural killer cell inhibitory receptor for different HLA-CLASS-I alleles. Identification of the inhibitory form of CD94 by the use of novel monoclonal antibodies.Eur. J. Immunol. 1996; 26: 2487-2492Crossref Scopus (121) Google Scholar, 23Lazetic S Chang C Houchins J.P Lanier L.L Phillips J.H Human Natural Killer cell receptors involved in MHC class I recognition are disulphide-linked heterodimers of CD94 and NKG2 subunits.J. Immunol. 1996; 157: 4741-4745Google Scholar). These NKR display a broad specificity for different HLA-class I molecules although they appear to bind preferentially to certain groups of HLA-alleles (those belonging to the Bw6 supertypic specificity) and poorly to others (Bw4 alleles). They are formed by CD94 molecules that associate with different members of the NKG2 molecular family to form either inhibitory (CD94/NKG2A) or activating NKR (CD94/NKG2C) (A. Moretta, University of Brescia, Italy). Both CD94 and NKG2A molecules are type 2 transmembrane proteins belonging to the C-type lectin superfamily (reviewed by25Lopez Botet M Pérez-Villar J.J Carretero M Rodriguez A Melero I Bellon T Llano M Navarro F Structure and function of CD94 C-type lectin receptor complex involved in recognition of HLA-class I molecules.Immunol. Rev. 1997; 155: 165-174Crossref Scopus (122) Google Scholar). Remarkably, both NKG2A and inhibitory NKR of the Ig superfamily are characterized by intracytoplasmic immunoreceptor tyrosine based inhibition motifs (ITIM), which are essential for SHP-1 phosphatase activation and induction of inhibitory signals (reviewed by34Rénard V Cambiaggi A Vely F Blery M Olcese L Olivero S Bouchet M Vivier E Transduction of cytotoxic signals in Natural Killer cells a general model of fine tuning between activatory and inhibitory pathways in lymphocytes.Immunol. Rev. 1997; 155: 205-221Crossref Scopus (103) Google Scholar). Because of the molecular structure, the broad specificity and the functional property, CD94/NKG2 complexes could represent the human counterpart of the murine Ly49 family of MHC-class I-specific NK receptors. In view of the recent evolution of the HLA-class I system, it is likely that those NKR belonging to the Ig superfamily have evolved more recently than C-type lectin NKR to adapt to the polymorphism emerging in the HLA class I system (reviewed by45Valiante N.M Lienert K Shilling H.G Smits B.J Parham P Killer cell receptors keeping pace with MHC class I evolution.Immunol. Rev. 1997; 155: 155-164Crossref Scopus (91) Google Scholar). This receptor strategy allows the NK cell system to sense even single allelic loss as it may occur in tumor or virally infected cells. In this context, CD94/NKG2 complexes would represent an ancestral type of MHC-class I recognition strategy. Inhibitory NKR are also expressed on a small subset of human T lymphocytes (reviewed by27Moretta A Bottino C Vitale M Pende D Biassoni R Mingari M.C Moretta L Receptors for HLA class I molecules in human Natural Killer cells.Annu. Rev. Immunol. 1996; 14: 619-648Crossref PubMed Scopus (743) Google Scholar). NKR+ T cells have been detected in peripheral blood, spleen, tonsil, and lymph nodes but not in thymus and cord blood. Surface marker analysis has revealed that NKR+ T cells always express a memory phenotype; moreover, in several normal donors analyzed, they are characterized by a skewed TCRVβ repertoire. Crosslinking of NKR leads to inhibition of different TCR-dependent and TCR-independent (e.g., NK-like cytotoxicity) T cell functions. The inhibitory effect exerted on TCR-mediated functions has important implications. Thus, the expression of inhibitory NKR may result in functional impairment of specific CTLs. Preliminary data would suggest that this phenomenon may occur in tumor (21Ikeda H Lethé B Lehmann F Van Baren N Baurain J.F De Smet C Chambost H Vitale M Moretta A Boon T Coulie P.G Characterization of an antigen that is recognized on a melanoma showing partial HLA loss by CTL expressing an NK inhibitory receptor.Immunity. 1997; 6: 199-208Abstract Full Text Full Text PDF PubMed Scopus (565) Google Scholar) or virally infected patients (L. Moretta, University of Genoa, Italy). The fact that antigen-specific CTLs simultaneously express TCR and NKR, both recognizing HLA-class I molecules but mediating opposite signals, offers new perspective in our appreciation of the regulation of T cell responses and provides new clues for understanding the immunopathologic events involved in certain diseases. Data were also presented that are compatible with the existence of additional, still unknown, inhibitory receptors in alloantigen-stimulated T cells. These cells were inhibited in their cytolytic function by a set of HLA-C alleles corresponding to those recognized by the p58.2 receptors; however, they did not react with the presently available anti-p58.2 MAbs (K. Falk, University of Munich, Germany). Recent studies indicated that the nonclassical HLA-class I molecule HLA-G, which is the only class I molecule expressed by trophoblast cells, is recognized by inhibitory receptors expressed on human NK cells (31Pazmany L Mandelboim O Valés-Gomez M Davis D.M Reyburn H.T Strominger J.L Protection from natural killer cells—mediated lysis by HLA-G expression on target cells.Science. 1996; 274: 792-795Crossref PubMed Scopus (309) Google Scholar, 29Munz C Holmes N King A Loke Y.W Colonna M Schild H Rammensee H.-G Human histocompatibility leukocyte antigen (HLA-G) molecules inhibit NKAT3 expressing natural killer cells.J. Exp. Med. 1997; 185: 385-391Crossref PubMed Scopus (127) Google Scholar). This issue has been discussed by H. G. Rammensee (University of Tubingen, Germany) who presented data suggesting that the receptors involved in HLA-G recognition may be related to the HLA-Bw4-specific p70 NKR (or NKAT-3). These data are in conflict with those reported by J. Strominger's group (31Pazmany L Mandelboim O Valés-Gomez M Davis D.M Reyburn H.T Strominger J.L Protection from natural killer cells—mediated lysis by HLA-G expression on target cells.Science. 1996; 274: 792-795Crossref PubMed Scopus (309) Google Scholar) showing that HLA-G recognition is mediated by the HLA-C-specific p58 receptors. Further studies are required to elucidate the precise contribution of the various NKR to HLA-G recognition since other groups have recently shown that HLA-G is recognized by the CD94/NKG2A complex and not by p58 or p70 receptors (32Pende D Sivori S Accame L Pareti L Falco M Geraghty D Le Bouteiller P Moretta L Moretta A HLA-G recognition by human natural killer cells. Involvement of CD94 both as inhibitory and as activating receptor complex.Eur. J. Immunol. 1997; 27: 1875-1880Crossref PubMed Scopus (84) Google Scholar, 33Perez-Villar J.J Melero I Navarro F Carretero M Bellon T Llano M Colonna M Geraghty D.E Lopez-Botet M The CD94/NKG2A inhibitory receptor complex is involved in the Natural Killer cell-mediated recognition of cells expressing HLA-G1.J. Immunol. 1997; 158: 5736-5743PubMed Google Scholar). The need for inhibitory NKR to prevent NK-mediated lysis of self cells implies the existence of one or more activating receptors that interact with ligand(s) expressed on target cells. However, limited information is available on the surface receptors that are involved in NK cell triggering. Thus the search for new molecules capable of triggering the NK-mediated cytotoxicity is now becoming crucial for defining the molecular mechanisms involved in NK cell activation/inactivation. In this context, a new molecule termed p46, has been identified that induces strong NK cell activation upon cross-linking by specific MAbs (S. Sivori, University of Genoa, Italy). The p46 molecule is selectively expressed on all human NK cells, resting or activated, and the levels of cytolytic activity of different NK clones against HLA-negative targets correlates with the number of p46 molecules expressed at their cell surface. p46 is clearly distinguishable from p50 molecules (i.e., the HLA-C-specific activating receptors) because of its molecular size, cell distribution, and distinct specificity. It may represent a receptor for non-HLA ligands expressed on both HLA+ and HLA− target cells. The precise definition of the p46 ligand will be possible upon cloning of this new receptor. In mice, all the MHC class I-specific receptors characterized so far belong to the Ly49 molecular family (reviewed by48Yokoyama W.M Seaman W.E The Ly49 and NKR-P1 gene families encoding lectin-like receptors on natural killer cells the NK gene complex.Annu. Rev. Immunol. 1993; 11: 613-635Crossref Scopus (439) Google Scholar). Most of them are inhibitory receptors including Ly49A specific for Dd and DK, Ly49C specific for Kb (and some H-2d) and Ly49G2 specific for Dd and Ld. Analysis of the peptide dependency in the NKR-mediated recognition of MHC molecules revealed that Ly49C-mediated recognition is strictly peptide-dependent. Indeed, studies employing TAP-deficient RMA-S target cells showed that “empty” class I molecules do not protect cells from Ly49C+ NK cells. On the other hand, the same cells are protected after incubation in the presence of peptides binding to Kb. This is in line with previous experiments on the Ly49A receptors displaying a similar peptide-dependency (8Correa I Raulet D.H Binding of diverse peptides to MHC class I molecule inhibits target cell lysis by activated natural killer cells.Immunity. 1995; 2: 61-67Abstract Full Text PDF PubMed Scopus (151) Google Scholar). Similar to the human NKR, a dichotomy of function between the various members of the Ly49 family has been demonstrated also in mice. In particular, cross-linking of Ly49A or Ly49C resulted in thyrosine phosphorylation of the receptors while this was not observed after cross-linking of Ly49D (J. Ortaldo, National Cancer Institute, Frederick, Maryland). These results are consistent with both the known structural and functional characteristics of these Ly49 molecules, in that Ly49A, C, and G2 have ITIM motifs containing phosphorylation sites while Ly49D does not contain this inhibitory motif. Therefore, despite significant divergence in their extracellular domains, human p58/p50 receptors and murine Ly49 receptors are likely to share common signaling properties (reviewed by34Rénard V Cambiaggi A Vely F Blery M Olcese L Olivero S Bouchet M Vivier E Transduction of cytotoxic signals in Natural Killer cells a general model of fine tuning between activatory and inhibitory pathways in lymphocytes.Immunol. Rev. 1997; 155: 205-221Crossref Scopus (103) Google Scholar). It is well known that T lymphocytes, primarily MHC class I-restricted CD8+ CTL, play a major role in the control of virus infections. Some DNA viruses, on the other hand, have evolved strategies to interfere with the process of MHC/viral peptide presentation to CTLs (reviewed by40Spriggs M.K One step ahead of the game viral immunomodulatory molecules.Annu. Rev. Immunol. 1996; 1: 101-130Crossref Scopus (217) Google Scholar). The recent understanding of the mechanisms involved in antigen presentation made possible the unraveling of several of the molecular mechanisms by which viruses interfere with MHC class I antigen presentation. However, while downregulation of cell surface expression of MHC class I molecules (the so-called “stealth strategy”) will prevent the CTL-mediated lysis of infected cells, it should render them susceptible to NK-mediated lysis (20Karre K Welsh M Viral decoy vetoes killer cell.Nature. 1997; 386: 446-447Crossref Scopus (24) Google Scholar) (see below). The first known example of viral protein interfering with MHC class I pathway is the E19 protein of adenovirus (3Andersson M Paabo S Nilsson T Peterson P.A Impaired intracellular transport of class I MHC antigens as a possible means for adenoviruses to evade immune surveillance.Cell. 1985; 43: 215-222Abstract Full Text PDF PubMed Scopus (234) Google Scholar). Since E19 molecule interacts with MHC class I molecules at the ER level, class I molecules do not reach the cell surface, but they are retrieved from the Golgi together with E19 molecules. Not all the HLA-class I alleles bind equally well to E19, which therefore preferentially inhibits the expression of certain alleles (thus rendering cells susceptible to lysis by NK cell subsets expressing appropriate NKR). Other viruses that inhibit the MHC class I expression include members of the Herpes virus family. For example, the Herpes simplex (HSV) encoded ICP47 molecule (49York I.A Roop C Andrews D.W Riddell S.R Grahm F.L Johnson D.C A cytosolic herpes simplex protein inhibits antigen presentation to CD8+ T lymphocytes.Cell. 1994; 77: 525-535Abstract Full Text PDF PubMed Scopus (456) Google Scholar) binds to TAP1 and TAP2 in the cytosol and prevents the TAP-mediated binding and transport of peptides. The human cytomegalovirus (HCMV) employs different strategies to inhibit HLA class I expression (15Fruh K Ahn K Peterson P.A Inhibition of MHC class I antigen presentation by viral proteins.J. Mol. Med. 1997; 75: 18-27Crossref Scopus (40) Google Scholar). The US6 glycoprotein resides in the ER and interferes with the peptide translocation, but not with their binding to TAP molecules. US2 and US11 (47Wiertz E.J.H.J Jones T.R Sun L Bogyo M Geuze H.J Ploegh H.L The human cytomegalovirus US11 gene product dislocates MHC class I heavy chains from the ER to the cytosol.Cell. 1996; 84: 769-779Abstract Full Text Full Text PDF PubMed Scopus (894) Google Scholar) prevent the HLA class I heavy chains from leaving the ER compartment and shuttle them back into the cytosol where they are degraded by proteasomes. US3 protein (1Ahn K Angulo A Ghazal P Peterson P.A Yang Y Fruh K Human cytomegalovirus inhibits antigen presentation by a sequential multistep process.Proc. Natl. Acad. Sci. USA. 1996; 93: 10990-10995Crossref Scopus (306) Google Scholar) is expressed very early during HCMV infection, as early as virus-encoded transcription factors, binds to class I molecules and prevents their export to the surface. Consequently, in the course of HCMV infection, HLA-class I molecules are first retained and then degraded, suggesting that US3 represents the first line of viral interference, followed by US11 and, possibly, by other US proteins (U. Koszinowski, Max von Pettenkofer Institut, Munchen, Germany). It is interesting to note that NK cells play an important role in controlling both HSV and HCMV infections. However, a potential mechanism to escape the NK cell recognition can be the expression of a β2-binding protein with a structural homology to MHC class I heavy chain (“decoy strategy”). This protein, termed UL18, also binds peptides (4Beck S Barrell B.G Human cytomegalovirus encodes a glycoprotein homologous to MHC class I antigens.Nature. 1988; 331: 269-272Crossref PubMed Scopus (333) Google Scholar, 13Fahnestock M.J Johnson J.L Feldman R.M Neveu J.M Lane W.S Bjorkman P.J The MHC class I homolog encoded by human cytomegalovirus binds endogenous peptides.Immunity. 1995; 3: 583-590Abstract Full Text PDF Scopus (113) Google Scholar). Rather than interfering substantially with antigen presentation to T cells, UL18 has been shown to function by protecting HCMV- infected cells against NK cell lysis (reviewed by35Reyburn H Mandelboim O Valé]\\|s-Gom\éz M Sheu E.G Pazmany L Davis D.M Strominger J.L Human NK cells their ligands, receptors and functions.Immunol. Rev. 1997; 155: 119-125Crossref PubMed Scopus (86) Google Scholar). Recent experiments showed that UL18 transfection into HLA class I negative target cells conferred to these cells resistance to NK-mediated lysis. The broad specificity CD94/NKG2A receptor appears to be responsible for binding UL18. Therefore HCMV has evolved two complementary strategies to avoid identification of infected cells by both CTLs and NK cells. A corollary of the fact that HCMV has developed a mechanism for evading NK-mediated detection is that NK cells must represent an important c