Adaptive NK cell response to human cytomegalovirus: Facts and open issues

Human cytomegalovirus (HCMV) infection exerts broad effects on the immune system. These include the differentiation and persistent expansion of a mature NK cell subset which displays a characteristic phenotypic and functional profile hallmarked by expression of the HLA-E-specific CD94/NKG2C activating receptor. Based on our experience and recent advances in the field, we overview the adaptive features of the NKG2C+ NK cell response, discussing observations and open questions on: (a) the mechanisms and influence of viral and host factors; (b) the existence of other NKG2C- NK cell subsets sharing adaptive features; (c) the development and role of adaptive NKG2C+ NK cells in the response to HCMV in hematopoietic and solid organ transplant patients; (d) their relation with other viral infections, mainly HIV-1; and (e) current perspectives for their use in adoptive immunotherapy of cancer.     

Adaptive reconfiguration of the human NK‐cell compartment in response to cytomegalovirus: A different perspective of the host‐pathogen interaction

As discussed in this review, human cytomegalovirus (HCMV) infection in healthy individuals is associated with a variable and persistent increase of NK cells expressing the CD94/NKG2C activating receptor. The expansion of NKG2C⁺ NK cells reported in other infectious diseases is systematically associated with HCMV co‐infection. The functionally mature NKG2C^bright NK‐cell subset expanding in HCMV⁺ individuals displays inhibitory Ig‐like receptors (KIR and LILRB1) specific for self HLA class I, and low levels of NKp46 and NKp30 activating receptors. Such reconfiguration of the NK‐cell compartment appears particularly marked in immunocompromised patients and in children with symptomatic congenital infection, thus suggesting that its magnitude may be inversely related with the efficiency of the T‐cell‐mediated response. This effect of HCMV infection is reminiscent of the pattern of response of murine Ly49H⁺ NK cells against murine CMV (MCMV), and it has been hypothesized that a cognate interaction of the CD94/NKG2C receptor with HCMV‐infected cells may drive the expansion of the corresponding NK‐cell subset. Yet, the precise role of NKG2C⁺ cells in the control of HCMV infection, the molecular mechanisms underlying the NK‐cell compartment redistribution, as well as its putative influence in the response to other pathogens and tumors remain open issues.     

The natural killer cell-mediated killing of autologous dendritic cells is confined to a cell subset expressing CD94/NKG2A,but lacking inhibitory killer Ig-like receptors

The cognate NK-DC interaction in inflamed tissues results in NK cell activation and acquisition of cytotoxicity against immature DC (iDC). This may represent a mechanism of DC selection required for the control of downstream adaptive immune responses. Here we show that killing of monocyte-derived iDC is confined to the NK cell subset that expresses CD94/NKG2A, but not killer Ig-like receptors (KIR). Consistent with these data, the expression of HLA-E (i.e. the cellular ligand of CD94/NKG2A) was down-regulated in iDC. On the other hand, HLA-B and HLA-C down-regulation in iDC was not sufficient to induce cytotoxicity in NK cells expressing KIR3DL1 or KIR2DL. Remarkably, CD94/NKG2A(+)KIR(-) NK cells were heterogeneous in their ability to kill iDC and an inverse correlation existed between their CD94/NKG2A surface density and the magnitude of their cytolytic activity. It is conceivable that the reduced CD94/NKG2A surface density enables these cells to efficiently sense the decrease of HLA-E surface expression in iDC. Finally, most NK cells that lysed iDC did not kill mature DC that express higher amounts of HLA class I molecules (including HLA-E)as compared with iDC. However, a small NK cell subset was capable of killing not only iDC but also mature DC.     

The inhibitory NK cell receptor CD94/NKG2A and the activating receptor CD94/NKG2C bind the top of HLA-E through mostly shared but partly distinct sets of HLA-E residues

The human non-classical MHC class I molecule HLA-E is a ligand for both an inhibitory NK cell receptor (CD94/NKG2A) and an activating receptor (CD94/NKG2C). To identify HLA-E surface recognized by both receptors, especially to determine if both receptors recognize the same epitope, we made a series of individually Ala-substituted HLA-E proteins and analyzed their binding to CD94/NKG2A orCD94/NKG2C. Eight HLA-E mutations that significantly impaired HLA-E binding to CD94/NKG2A are all found in the top of alpha1/alpha2 domain of HLA-E. These results suggest that CD94/NKG2A binds a HLA-E surface equivalent to a NKG2D binding site on MICA. Of the eight mutations that impaired HLA-E binding to CD94/NKG2A, six significantly impaired HLA-E binding to CD94/NKG2C suggesting that CD94/NKG2C also binds a similar surface of HLA-E. Unexpectedly, the two HLA-E mutations (D69A and H155A) selectively abrogated HLA-E binding to CD94/NKG2A, not largely affected CD94/NKG2C. These results indicate that a mostly shared, but partly distinct set of HLA-E residues is discriminated by the two receptors.     

Ganglioside GD3 expression on target cells can modulate NK cell cytotoxicity via siglec-7-dependent and -independent mechanisms

Siglec-7 is a sialic acid binding receptor with inhibitory potential, expressed on human NK cells and monocytes. It has an unusual binding preference for alpha2,8-linked disialic acids, such as those displayed by ganglioside GD3. Here we have investigated whether siglec-7-GD3 interactions are able to modulate NK cell cytotoxicity. Using synthetic polyacrylamide glycoprobes, siglec-7 was found to be masked at the NK cell surface but it could be unmasked by sialidase treatment of NK cells. GD3 synthase-transfected P815 target cells expressed high levels of GD3 and bound strongly to recombinant siglec-7-Fc protein. Surprisingly, GD3 synthase-transfected P815 cells were killed more effectively by untreated cells in a siglec-7-independent manner. However, following sialidase treatment of NK cells, a siglec-7-dependent inhibition of killing was observed. These findings have important implications for NK cell cytotoxicity against tumor cells like melanoma that express high levels of GD3 ganglioside.     

Amino acid composition of alpha1/alpha2 domains and cytoplasmic tail of MHC class I molecules determine their susceptibility to human cytomegalovirus US11-mediated down-regulation

During co-evolution with its host, human cytomegalovirus has acquired multiple defense mechanisms to escape from immune recognition. In this study, we focused on US11, which binds to MHC class I heavy chains and mediates their dislocation to the cytosol and subsequent degradation by proteasomes. To examine which domains of class I heavy chains are involved in this process, we constructed chimeric HLA molecules of US11-sensitive and -insensitive class I molecules (HLA-A2 and HLA-G, respectively). Pulse-chase experiments were performed to evaluate protein stability and interactions between class I heavy chains and US11. Flow cytometry was employed to assess the effect of US11 on surface expression of the different chimeras. Our results indicate that the alpha1 and alpha2 domains of HLA molecules are important for the affinity of US11 association. However, the degradation efficiency seems to rely mostly on cytosolic tail residues. We found that the nonclassical HLA-G molecule is insensitive to US11-mediated degradation solely because it lacks essential tail residues. A deletion of the last two tail residues in full-length MHC class I molecules already caused a severe reduction in degradation efficiency. Altogether, our data provide new insights into the mechanism by which US11 down-regulates MHC class I molecules.     

Aberrant expression of alpha-Gal on primary human endothelium does not confer susceptibility to NK cell cytotoxicity or increased NK cell adhesion

The contribution of Gal alpha 1,3Gal (alpha-Gal) to cell-mediated organ xenograft rejection is controversial. We have used recombinant lentiviruses encoding a porcine alpha 1,3 galactosyltransferase (alpha 1,3GalT) to obtain alpha-Gal-expressing primary human aortic endothelial cells (HAEC) at a frequency of 70-90%. These cells were compared to non-transduced and mock-transduced HAEC with regard to their susceptibility to human NK cell-mediated lysis, ability to stimulate IFN-gamma production by NK cells, and support of NK cell adhesion under static and dynamic conditions. Using green fluorescent protein (GFP) as a reporter gene, it was shown that the frequency of green fluorescent HAEC increased until day 5 post-transduction, and at a multiplicity of infection of 2.5, it reached 98%. Lentiviral transduction did not result in activation of HAEC, and transduced HAEC responded as expected to TNF-alpha and IFN-gamma stimulation. No differences were detected between non-alpha-Gal- and alpha-Gal-expressing HAEC in terms of their susceptibility to NK cell-mediated lysis, ability to stimulate IFN-gamma production by NK cells, or ability to support NK cell adhesion under static and dynamic conditions. In conclusion, these data argue against an important role for the alpha-Gal epitope in the direct interaction between endothelium and NK cells and prove that recombinant lentiviruses are efficient gene carriers for primary human endothelial cells.     

Disordered expression of inhibitory receptors on the NK1-type natural killer (NK) leukaemic cells from patients with hypersensitivity to mosquito bites

Recent studies have revealed the existence of a distinct type of NK cell leukaemia of the juvenile type, which presents with hypersensitivity to mosquito bites (HMB) as an essential clinical manifestation and is infected with clonal Epstein-Barr virus (EBV). This disorder is thus called HMB-EBV-NK disease and has been reported in Orientals, mostly from Japan. We investigated the profile of cytokine production and the expression of both types of NK inhibitory receptors, i.e. CD94 lectin-like dimers and killer-cell immunoglobulin-like receptors, in NK leukaemic cells from three patients with HMB-EBV-NK disease. It was found that freshly isolated NK leukaemic cells expressed mRNA for interferon-gamma (IFN-gamma) and additionally produced IL-10 upon stimulation with IL-2, indicating that the NK cells were of NK1 type. More than 98% of NK cells from the patients bore CD94 at a higher level than did normal NK cells, whereas p70 or NKAT2, belonging to immunoglobulin-like receptor, was not expressed in those NK cells. Freshly isolated leukaemic NK cells transcribed mRNA for CD94-associated molecule NKG2C at an abnormally high level, and upon stimulation with IL-2 and/or IL-12 they expressed NKG2A as well. The disordered expression of these inhibitory receptors not only provides some insights into the pathogenesis of HMB-EBV-NK disease but also can be used as phenotypic markers for the diagnosis of this type of NK cell leukaemia.     

KIR2DL3 and KIR2DL1 show similar impact on licensing of human NK cells

Killer cell immunoglobulin‐like receptor/HLA class I (KIR/HLA‐I) combinations are associated with disease risk, implicating functional roles for NK cells (NKCs) or KIR⁺ T cells. KIR/HLA‐I interactions can act through inhibition of NKC activation by target cells and NKC licensing for greater intrinsic responsiveness. We compared licensing conferred by the weaker, HLA‐C group 1/KIR2DL3, and the stronger, HLA‐C group 2/KIR2DL1, inhibitory combinations. The “rheostat model” predicts weaker licensing by HLA‐C1/KIR2DL3 interactions than HLA‐C2/KIR2DL1. We analyzed degranulation in NKC subsets expressing single and multiple receptors for HLA‐I. NKG2A had the strongest licensing impact, while KIR2DL3, KIR2DL1, and KIR3DL1 were weaker, and not significantly different to each other. Presence of one or two matched HLA‐C allotypes did not alter licensing of KIR2DL3⁺ and KIR2DL1⁺ NKC. Coexpression of activating KIR2DS1 disarmed KIR2DL3⁺ and KIR2DL1⁺ NKC to a similar extent. KIR3DL1 and NKG2A combined for more enhanced licensing of double‐positive NKC than the combination of KIR2DL3 and KIR2DL1. Thus, KIR2DL3 and KIR2DL1 have similar capacity to license NKC, suggesting that inhibitory signal strength and amount of available HLA‐C ligands do not correlate with NKC licensing. Altogether, our results show that the basis for disease associations of HLA‐C and KIR2DL likely encompasses factors other than licensing.     

Effects of Chlamydia trachomatis infection on the expression of natural killer (NK) cell ligands and susceptibility to NK cell lysis

Natural killer (NK) cells are an important component of the immediate immune response to infections, including infection by intracellular bacteria. We have investigated recognition of Chlamydia trachomatis (CT) by NK cells and show that these cells are activated to produce interferon (IFN)-gamma when peripheral blood mononuclear cells (PBMC) are stimulated with CT organisms. Furthermore, infection of epithelial cell lines with CT renders them susceptible to lysis by human NK cells. Susceptibility was observed 18-24 h following infection and required protein synthesis by the infecting chlamydiae, but not by the host cell; heat or UV inactivated chlamydiae did not induce susceptibility to NK cell lysis. CT infection was also shown to decrease the expression of classical and non-classical major histocompatibility complex (MHC) molecules on infected cells, thus allowing recognition by NK cells when combined with an activating signal. A candidate activating signal is MICA/B, which was shown to be expressed constitutively on epithelial cells.     

The differential impact of natural killer (NK) cell education via KIR2DL3 and KIR3DL1 on CCL4 secretion in the context of in‐vitro HIV infection

Carriage of certain inhibitory natural killer (NK) cell receptor (iNKR)/HLA ligand pairs is associated with protection from infection and slow time to AIDS implicating NK cells in HIV control. NK cells acquire functional potential through education, which requires the engagement of iNKRs by their human leucocyte antigen (HLA) ligands. HIV infection down‐regulates cell surface HLA‐A/B, but not HLA‐C/E. We investigated how NK cell populations expressing combinations of the iNKRs NKG2A, KIR2DL3 (2DL3) and KIR3DL1 (3DL1) responded to autologous HIV infected CD4 (iCD4) cells. Purified NK cells from HIV‐uninfected individuals were stimulated with autologous HIV iCD4 or uninfected CD4 T cells. Using flow cytometry we gated on each of the 8 NKG2A^+/–2DL3^+/–3DL1^+/‐ populations and analysed all possible combinations of interferon (IFN)‐γ, CCL4 and CD107a functional subsets responding to iCD4 cells. Infected CD4 cells induced differential frequencies of NKG2A^+/–2DL3^+/–3DL1^+/– populations with total IFN‐γ⁺, CCL4⁺ and CD107a⁺ functional profiles. 2DL3⁺NKG2A⁺ NK cells had a higher frequency of responses to iCD4 than other populations studied. A higher frequency of 2DL3⁺ NK cells responded to iCD4 from individuals that were not HLA‐C1 homozygotes. These results show that 2DL3⁺ NK cells are mediators of HIV‐specific responses. Furthermore, responses of NK cell populations to iCD4 are influenced not only by NK cell education through specific KIR/HLA pairs, but also by differential HIV‐mediated changes in HLA expression.     

膜型/分泌型MICA对NK细胞受体NKG2D的相反调节效应及其对NK细胞受体谱的影响

目的　观察膜型和分泌型MICA对NK细胞受体表达的影响 ,以探讨NK细胞抗肿瘤活化机制及肿瘤细胞表达MICA分子的意义。方法　用MTT法测定人NK细胞系 (NK92 )的细胞毒活性 ;用RT PCR或FACS检测NK细胞受体 (NKG2D ,NKG2A B ,KIR2DL1,KIR2DS1)及NKG2D的识别配体MICA的表达。结果　肿瘤细胞表面的MICA分子可上调NKG2D的表达 ,下调抑制性受体NKG2A B和KIR2DL1的表达 ;而分泌型MICA (sMICA)分子对NKG2D及抑制性受体的表达均有抑制作用。结论　膜型MICA分子可上调NKG2D的表达 ,激发NK细胞对肿瘤细胞的细胞毒效应 ;分泌型MICA分子则通过降低NKG2D的表达下调机体的抗肿瘤免疫效应 ,肿瘤细胞分泌sMICA分子为肿瘤发生免疫逃逸的机制之一。     

The impact of killer cell immunoglobulin-like receptor (KIR) genes and human leukocyte antigen (HLA) class I ligands on predisposition or protection against prostate cancer

Natural killer (NK) cell development largely depends on killer cell immunoglobulin-like receptors (KIRs) and human leukocyte antigen (HLA) class I ligands. In the current study, we investigated the role of KIR genes, HLA ligands, and KIR-HLA combinations in vulnerability or protection against prostate cancer (PC). To analyze the frequency of 16 KIR genes and 5 HLA ligands, polymerase chain reaction with sequence-specific primers (PCR-SSP) was conducted in 150 PC patients and 200 healthy controls (CNs). KIR2DL5 (p = 0.0346, OR = 0.606, CI = 0.3916–0.9336), KIR2DS5 (p = 0.0227, OR = 0.587, CI = 0.3793–0.9139), HLA-B Bw4^Thr80 (p = 0.0401, OR = 0.3552, CI = 0.1466–0.9059), HLA Bw4 (p = 0.0190, OR = 0.4744, CI = 0.2656–0.8521), and T4 gene cluster (including KIR2DS5-2DL5-3DS1-2DS1 genes) (p = 0.0194, OR = 0.5575, CI = 0.3449–0.8938) had a lower frequency in the PC patients compared to the control group. Moreover, a lower frequency of the genotypes contacting activating KIR (aKIR) > inhibitory KIR (iKIR) (p = 0.0298, OR = 0.5291, CI = 0.3056–0.9174) and iKIR + HLA < aKIR + HLA (p = 0.0183, OR = 0.2197, CI = 0.0672–0.7001) in PC patients compared to the CNs implies a protective role for aKIR genes. In the case of KIR-HLA interactions, we detected a significant association between KIR3DS1⁺ + HLA-A Bw4⁺ (p = 0.0113, OR = 0.5093, CI = 0.3124–0.8416) and KIR3DL1^? + HLA-A Bw4⁺ (p = 0.0306, OR = 0.1153, CI = 0.0106–0.6537) combinations and resistance to prostate cancer. In contrast, the presence of KIR3DL1 in the absence of HLA-A Bw4 (p = 0.0040, OR = 2.00, CI = 1.264–3.111), HLA Bw4 (p = 0.0296, OR = 2.066, CI = 1.094–3.906), and HLA-Bw4^Thr80 (p = 0.0071, OR = 2.505, CI = 1.319–4.703) genes probably predisposes to prostate cancer. Carrying the CxT4 genotype in PC patients was positively associated with lower tumor grades (Gleason score ≤ 6) (p = 0.0331, OR = 3.290, and CI = 1.181–8.395). Altogether, our data suggest a protective role for aKIRs, HLA-B Bw4^Thr80, and HLA Bw4 ligands as well as a predisposing role for certain KIR-HLA combinations in prostate cancer. The findings of this study offer new insight into the population's risk assessment for prostate cancer in men. Additionally, predicting immunotherapy response based on KIR-HLA combinations aids in implementing the most effective therapeutic approach in the early stages of the disease.     

Influence of congenital human cytomegalovirus infection and the NKG2C genotype on NK‐cell subset distribution in children

Human cytomegalovirus (HCMV) has been reported to reshape the NK‐cell receptor (NKR) distribution, promoting an expansion of CD94/NKG2C⁺ NK and T cells. The role of NK cells in congenital HCMV infection is ill‐defined. Here we studied the expression of NKR (i.e., NKG2C, NKG2A, LILRB1, CD161) and the frequency of the NKG2C gene deletion in children with past congenital infection, both symptomatic (n = 15) and asymptomatic (n = 11), including as controls children with postnatal infection (n = 11) and noninfected (n = 20). The expansion of NKG2C⁺ NK cells in HCMV‐infected individuals appeared particularly marked and was associated with an increased number of LILRB1⁺ NK cells in cases with symptomatic congenital infection. Increased numbers of NKG2C⁺, NKG2A⁺, and CD161⁺ T cells were also associated to HCMV infection. The NKG2C deletion frequency was comparable in children with congenital HCMV infection and controls. Remarkably, the homozygous NKG2C^+/+ genotype appeared associated with increased absolute numbers of NKG2C⁺ NK cells. Moreover, HCMV‐infected NKG2C^+/+ children displayed higher absolute numbers of NKG2A⁺ and total NK cells than NKG2C^+/? individuals. Our study provides novel insights on the impact of HCMV infection on the homeostasis of the NK‐cell compartment in children, revealing a modulatory influence of NKG2C copy number.     

A distinct subset of human NK cells expressing HLA-DR expand in response to IL-2 and can aid immune responses to BCG

Subsets of NK cells can have distinct functions. Here, we report that >25% of human peripheral blood NK cells express HLA-DR after culture with IL-2. This can be driven by an expansion of a small subset of NK cells expressing HLA-DR, in contrast to previous assumptions that HLA-DR is upregulated on previously negative cells. HLA-DR-expressing NK cells showed enhanced degranulation to susceptible target cells and expressed chemokine receptor CXCR3, which facilitated their enrichment following exposure to CXCL11/I-TAC. Suggesting HLA-DR-expressing NK cells have an important role in an immune response, stimulation of PBMCs with Mycobacterium bovis BCG (BCG) triggered expansion of this subset. Importantly, the magnitude of an individual's NK cell IFN-γ response triggered by BCG was associated with the initial frequency of HLA-DR-expressing NK cells in PBMCs. More directly indicating the importance of HLA-DR-expressing NK cells, enriching the frequency of this subset in PBMCs substantially augmented the IFN-γ response to BCG. Thus, HLA-DR expression marks a distinct subset of NK cells, present at low frequency in circulating blood but readily expanded by IL-2, that can play an important role during immune responses to BCG.     

Inhibitory 2B4 contributes to NK cell education and immunological derangements in XLP1 patients

X‐linked lymphoproliferative disease 1 (XLP1) is an inherited immunodeficiency, caused by mutations in SH2D1A encoding Signaling Lymphocyte Activation Molecule (SLAM)‐associated protein (SAP). In XLP1, 2B4, upon engagement with CD48, has inhibitory instead of activating function. This causes a selective inability of cytotoxic effectors to kill EBV‐infected cells, with dramatic clinical sequelae. Here, we investigated the NK cell education in XLP1, upon characterization of killer Ig‐like receptor (KIR)/KIR‐L genotype and phenotypic repertoire of self‐HLA class I specific inhibitory NK receptors (self‐iNKRs). We also analyzed NK‐cell cytotoxicity against CD48⁺ or CD48^? KIR‐ligand matched or autologous hematopoietic cells in XLP1 patients and healthy controls. XLP1 NK cells may show a defective phenotypic repertoire with substantial proportion of cells lacking self‐iNKR. These NK cells are cytotoxic and the inhibitory 2B4/CD48 pathway plays a major role to prevent killing of CD48⁺ EBV‐transformed B cells and M1 macrophages. Importantly, self‐iNKR defective NK cells kill CD48^? targets, such as mature DCs. Self‐iNKR^? NK cells in XLP1 patients are functional even in resting conditions, suggesting a role of the inhibitory 2B4/CD48 pathway in the education process during NK‐cell maturation. Killing of autologous mature DC by self‐iNKR defective XLP1 NK cells may impair adaptive responses, further exacerbating the patients’ immune defect.     

Deletion mutant of human cytomegalovirus lacking US2-US6 and US11 maintains MHC class I expression and antigen presentation by infected dendritic cells

A HCMV mutant of endothelial- and DC-tropic strain TB40/E lacking the described MHC downregulating genes US2-6 and US11 (RVTB40/E(4)ΔUS11) was generated. We analyzed the susceptibility of DC to RVTB40/E(4)ΔUS11 and subsequently studied antigen presentation and T-cell stimulation. Wildtype TB40/E- and RVTB40/E(4)ΔUS11 showed no significant difference in the efficiency of infection of DC. Whereas infection with TB40/E induced downregulation of MHC I, no significant MHC I downregulation was observed on RVTB40/E(4)ΔUS11-infected DC, indicating that the US2-6, US11 region encodes for the major genes relevant for MHC I downregulation. However, both viruses induced downregulation of MHC II, as well as CD40, CD80, CD86 and CD83 to the same levels. Stimulation of IFN-γ production by HCMV-specific CD8+ T-cells by infected autologous DC correlated with the modulation of MHC expression. While TB40/E-infected DC did not efficiently stimulate IFN-γ production, RVTB40/E(4)ΔUS11-infected DC efficiently stimulated CD8+ T-cells to produce IFN-γ.     

Markedly decreased expression of TAP1 and LMP2 genes in HLA class I-deficient human tumor cell lines

HLA class I antigens of the human major histocompatibility complex play an important role in immune response. These molecules present foreign antigenic peptides to cytotoxic T lymphocytes and thereby play a role in the immune surveillance of cells infected with virus or other intracellular pathogens or altered by malignant transformation. A marked deficiency or lack of expression of these antigens has been reported in a variety of human neoplasms. In the present study, we examined the expression of class I chain, β₂-microglobulin, TAP (TAP1 and TAP2) and LMP (LMP2 and LMP7) genes in a number of human tumor cell lines including small-cell lung carcinoma, hepatocellular carcinoma, colon adenocarcinoma and basophilic leukaemia. These cell lines were deficient in expression of both class I chain and β₂-microglobulin gene products. In addition, these cell lines lacked the products of MHC-encoded proteasome subunit LMP2 as well as the putative peptide transporter TAP1 genes. In contrast, TAP2 and LMP7 genes were expressed in these cell lines. Treatment of cells with γ-IFN markedly enhanced the expression of class I chain, β₂-microglobulin, TAP1 and LMP2 genes with a concomitant increase in cell-surface expression of class I molecules. The upregulation of TAP1 and LMP2 expression is associated with increased class I expression, suggesting that endogenous antigens, e.g. tumor antigens, could be presented by class I molecules following treatment of tumor cells with γ-IFN.