Characterization of long-term mixed donor-donor chimerism after double cord blood transplantation

Double cord blood transplantation (DCBT) with two matched or partially matched cord blood units has been implemented successfully to circumvent the limitations of graft cell dose associated with single CBT. After DCBT, sustained haematopoiesis is derived almost exclusively from only one of the donated units. None the less, we previously observed two of six evaluable DCBT patients still having mixed donor-donor chimerism at 28 and 45 months post-transplantation, respectively. In the present study we utilize flow cytometry techniques to perform the first thorough analysis of phenotype and functionality of cord blood units in patients with mixed donor-donor chimerism. Our results suggest that the two stable cord blood units are different phenotypically and functionally: one unit shows more naive T cells, lower T cell cytokine production and higher frequencies of natural killer cells, the other shows higher frequencies of well-differentiated and functional lymphocytes. Additionally, in comparison with control patients having a single prevailing cord blood unit, the patients with donor-donor chimerism exhibit less overall T cell cytokine production and a smaller fraction of memory T cells. Furthermore, our results indicate that human leucocyte antigen-C match of donor units may partly explain the development of a donor-donor mixed chimerism.

Herpes simplex virus infection downmodulates NKG2D ligand expression

Herpes simplex virus (HSV) type 1 infection may cause orofacial infections in humans. The virus resides in a latent form in neural ganglia and occasionally reactivates and infects epithelial cells. Natural killer (NK) cells have been implicated in immune control of herpes virus infections, possibly by downmodulating major histocompatibility complex (MHC) class I and by other, as yet unidentified, mechanisms. Upon HSV-1 infection of cell lines, surface levels of NKG2D ligands MHC class I related proteins (MIC) A and UL16 binding protein 2 were downmodulated due to late viral gene product(s). As also MHC class I levels were reduced by HSV-1, NK cell recognition of HeLa cells was not affected by infection. Total cellular MICA contents remained unchanged, suggesting masking, internalization or intracellular retention of MICA as possible mechanisms of viral downregualtion of MICA surface levels. Furthermore, NK cells from patients with active HSV-1 infection had a tendency towards increased expression level of the activating receptor NKG2D. These data support a role for NKG2D-MICA interactions in immune responses to HSV-1 reactivation.

The expression of human natural killer cell receptors in early life

Natural killer (NK) cells play an important role in tumour immunosurveillance and the early defence against viral infections. Recognition of altered cells (i.e. infected- or tumour-cells) is achieved through a multiple receptor recognition strategy which gives the NK cells inhibitory or activating signals depending on the ligands present on the target cell. NK cells originate from the bone marrow where they develop and proliferate. However, further maturation processes and homeostasis of NK cells in peripheral blood are not well understood. To determine the proportions of cells and the expression of NK cell receptors, mononuclear cells from children at three time points during early childhood were compared, i.e. cord blood (CB), 2 and 5 years of age. The proportion of NK cells was high in CB, but the interferon-gamma (IFN-gamma) production low compared to later in life. In contrast, the proportion of T cells was low in CB. This may indicate a deviation of the regulatory function of NK cells in CB compared to later in life, implying an importance of innate immunity in early life before the adaptive immune system matures. Additionally, we found that the proportion of LIR-1(+) NK cells increased with increasing age while CD94(+)NKG2C(-) (NKG2A(+)) NK cells and the level of expression of NKG2D, NKp30 and NKp46 decreased with age. These age related changes in NK cell populations defined by the expression of activating and inhibitory receptors may be the result of pathogen exposure and/or a continuation of the maturation process that begins in the bone marrow.

Proliferative and cytotoxic response of human natural killer cells exposed to transporter associated with antigen-processing-deficient cells

In transporter associated with antigen-processing (TAP)-deficient patients affected by a severe downmodulation of human leucocyte antigen class I (HLA-I) molecules, natural killer (NK) cells have an increased expression of the inhibitory receptor CD94/NKG2A. Focusing our attention on NK cells, we have investigated the phenotype, function and proliferative response of peripheral blood lymphocytes (PBLs) derived from healthy donors after coculturing with TAP (T2)- or HLA-I-deficient (721.221) cell lines and their related HLA-I-expressing transfectants (T3 and DT360, respectively). After 4 days, NK cells cocultured with T2 cells had a threefold increased CD94 expression compared to NK cells cocultured with T3. This increase was due to proliferation of the CD56brightCD94bright subset. In contrast, expression of other inhibitory receptors [killer cell immunoglobulin (Ig)-like receptors] was variable during time and was not related to HLA-I molecules expressed by stimulating cells. Similar results were obtained using HLA-I-deficient cells (721.221). The PBLs cocultured for 4 days with T2 cells displayed enhanced cytotoxic responses. The results suggest that CD56brightCD94bright NK cells are induced to proliferate and kill in response to a TAP-deficient environment. The changes seen in the NK-cell compartment were partially contributed by T lymphocytes present in the coculture. These data could explain the increased CD94 expression and autoimmune manifestations observed in TAP-deficient patients.

NK cells, MHC class I molecules and the missing self

This article is based on a lecture presented at the Novartis Prize ceremony at the International Congress of Immunology in July 2001. It gives a personal and historical perspective on the research performed by the author and his colleagues during the development and pursuit of the model of 'missing-self recognition' for natural killer (NK) cells. This model is based on the idea that one important function of NK cells is to detect and eliminate cells because they fail to express normal self markers. Further mechanistic models predicted the existence of inhibitory major histocompatibility complex (MHC) class I specific receptors, later identified by the fellow Novartis laureates contributing in this issue. The article covers the first decade (1980-1990) of research on this concept. It discusses factors contributing to the formulation of a hypothesis, the use of predictions and experimental test models, the importance of international collaborations and reagent exchange, and several other aspects that allowed the progression of this research project. Finally, the perspective of today's knowledge is used to discuss some surprising findings where the missing-self hypothesis made the wrong predictions, or at least failed to make the correct ones.

Human cytomegalovirus (HCMV)-infected endothelial cells and macrophages are less susceptible to natural killer lysis independent of the downregulation of classical HLA class I molecules or expression of the HCMV class I homologue, UL18

A number of reports have suggested that human cytomegalovirus (HCMV)-infected fibroblasts are resistant to natural killer (NK) lysis, and that the HCMV-encoded human leucocyte antigen (HLA) class I homologue UL18 may be responsible for this effect. While fibroblasts are easy to infect in vitro, their role in HCMV pathogenesis in vivo is unclear. Here, we have established systems to address NK recognition of infected endothelial cells and macrophages, two important HCMV cellular reservoirs in vivo. The HCMV-infected endothelial cells exhibited increased resistance to NK killing, and, in most experiments, infected macrophages demonstrated a decreased susceptibility to NK lysis. Infection with the mutant HCMV strain RV670, lacking the genes US1-9 and US11 that are responsible for downregulation of HLA class I molecules, also led to decreased NK susceptibility. Furthermore, reduced NK susceptibility was independent of the expression of the HLA class I homologue UL18, since cells infected with the UL18Delta HCMV strain were also less susceptible to NK killing. These results suggest that HCMV-induced resistance to NK cytotoxicity in endothelial cells and macrophages is independent of known pathways that interfere with the expression of cellular HLA class I A, B and C surface antigens and the HCMV encoded class I homologue UL18.

The role of an exposed loop in the alpha2 domain in the mouse MHC class I H-2Dd molecule for recognition by the monoclonal antibody 34-5-8S and the NK-cell receptor Ly49A

Natural killer (NK) cells express major histocompatibility complex (MHC) class I-specific inhibitory receptors. The region mediating the protective effect of the MHC class I molecule H-2Dd (Dd), recognized by the inhibitory receptor Ly49A, has been mapped to the alpha1/alpha2 domains. Here we have focused on an exposed loop in the N-terminal part of the alpha2 domain, which constitutes a major structural motif that differs between Dd (strong binding to Ly49A) and Db (weak binding to Ly49A at best). We mutated the residues 103, 104 and 107 in Dd to the corresponding amino acids in Db. The Dd mutant molecule retained the ability to be stabilized by a Dd-binding peptide. However, the mutation totally abolished the recognition by the conformational dependent monoclonal antibody (MoAb) 34-5-8S, known to inhibit the interaction between Dd and Ly49A. In addition, there was a marked impairment of the binding to Ly49A as evaluated by the ability of tetramers of the Dd mutant molecule to bind to Ly49A-transfected reporter cells and spleen cells. These results demonstrate that the introduced changes at positions 103, 104 and 107 directly or indirectly affect the epitopes for the MoAb 34-5-8S and the Ly49A receptor.

Acquisition of external major histocompatibility complex class I molecules by natural killer cells expressing inhibitory Ly49 receptors

Murine natural killer (NK) cells express inhibitory Ly49 receptors specific for major histocompatibility complex (MHC) class I molecules. We report that during interactions with cells in the environment, NK cells acquired MHC class I ligands from surrounding cells in a Ly49-specific fashion and displayed them at the cell surface. Ligand acquisition sometimes reached 20% of the MHC class I expression on surrounding cells, involved transfer of the entire MHC class I protein to the NK cell, and was independent of whether or not the NK cell expressed the MHC class I ligand itself. We also present indirect evidence for spontaneous MHC class I acquisition in vivo, as well as describe an in vitro coculture system with transfected cells in which the same phenomenon occurred. Functional studies in the latter model showed that uptake of H-2D(d) by Ly49A+ NK cells was accompanied by a partial inactivation of cytotoxic activity in the NK cell, as tested against H-2D(d)-negative target cells. In addition, ligand acquisition did not abrogate the ability of Ly49A+ NK cells to receive inhibitory signals from external H-2D(d) molecules. This study is the first to describe ligand acquisition by NK cells, which parallels recently described phenomena in T and B cells.

Synergistic effect of IFN-gamma and human cytomegalovirus protein UL40 in the HLA-E-dependent protection from NK cell-mediated cytotoxicity

Human CMV (HCMV) has evolved several strategies to evade the immune system of the infected host. Here, we investigated the role of the HCMV-encoded protein UL40 in the modulation of NK cell lysis. UL40 carries in its leader sequence a nonameric peptide similar to that found in many HLA class I molecules leader sequences. This peptide up-regulates the expression of HLA-E, the ligand for the NK cell inhibitory receptor CD94/NKG2A. The UL40-encoded HLA-E-binding peptide was present in all HCMV clinical (4636, 13B, 109B, 3C) and laboratory (AD169) strains analyzed. However, transfection of UL40 in different cell lines (293T, 721.221, K562) did not consistently confer protection from NK lysis (as measured using NKL and the newly generated NK line Nishi), despite a moderate up-regulation of HLA-E. Interestingly, combined transfection and treatment with IFN-gamma increased the inhibitory effect, via an HLA-E- and CD94/NKG2A-dependent mechanism. Although cells transfected with UL40 derived from either AD169 or 3C showed protection from NK cell lysis, infection of fibroblasts with the viruses resulted in a strong inhibition only with the clinical strain 3C. Our results suggest that UL40 and IFN-gamma-dependent up-regulation of HLA-E is only one possible mechanism to avoid NK cell recognition of HCMV infected cells.

MHC class I recognition by NK receptors in the Ly49 family is strongly influenced by the beta 2-microglobulin subunit

NK cell recognition of targets is strongly affected by MHC class I specific receptors. The recently published structure of the inhibitory receptor Ly49A in complex with H-2Dd revealed two distinct sites of interaction in the crystal. One of these involves the alpha1, alpha2, alpha3, and beta2-microglobulin (beta2m) domains of the MHC class I complex. The data from the structure, together with discrepancies in earlier studies using MHC class I tetramers, prompted us to study the role of the beta2m subunit in MHC class I-Ly49 interactions. Here we provide, to our knowledge, the first direct evidence that residues in the beta2m subunit affect binding of MHC class I molecules to Ly49 receptors. A change from murine beta2m to human beta2m in three different MHC class I molecules, H-2Db, H-2Kb, and H-2Dd, resulted in a loss of binding to the receptors Ly49A and Ly49C. Analysis of the amino acids involved in the binding of Ly49A to H-2Dd in the published crystal structure, and differing between the mouse and the human beta2m, suggests the cluster formed by residues Lys3, Thr4, Thr28, and Gln29, as a potentially important domain for the Ly49A-H-2Dd interaction. Another possibility is that the change of beta2m indirectly affects the conformation of distal parts of the MHC class I molecule, including the alpha1 and alpha2 domains of the heavy chain.

Purified MHC class I molecules inhibit activated NK cells in a cell-free system in vitro

Natural killer cells have been shown to interact with MHC class I molecules via inhibitory receptors. However, it is not known whether the inhibition induced by MHC class I molecules requires other NK cell-target cell interactions. Thus, we examined whether purified MHC class I molecules alone were able to inhibit NK cell function. Purified H-2K(b) and H-2D(b) molecules inhibited the release of IFN-gamma from spleen (H-2(b))-derived lymphokine-activated killer (LAK) cell cultures stimulated by anti-NK1.1 antibody in a concentration-dependent manner. LAK cells generated from newborn mice that express low levels of MHC class I binding Ly49 inhibitory receptors were significantly less sensitive to inhibition by H-2K(b) compared to LAK cells from adult mice. Furthermore, LAK cells generated from spleen cells of Ly49C-transgenic mice were significantly more sensitive to inhibition by H-2K(b) compared to non-transgenic littermates. Taken together, the data indicate that MHC class I induced inhibition of NK cell mediated effector functions, as assessed by IFN-gamma release after NK1.1 triggering, does not require additional cell surface molecules other than MHC class I.

Human immunodeficiency virus type 1 Nef epitopes recognized in HLA-A2 transgenic mice in response to DNA and peptide immunization

We investigated the immune response against a human immunodeficiency virus type 1 (HIV-1) nef DNA sequence administered epidermally in mice transgenic for the human major histocompatibility complex (MHC) class I molecule HLA-A201. Ten potential HLA-A2 binding 9-mer Nef peptides were identified by a computer-based search algorithm. By a cell surface MHC class I stabilization assay, four peptides were scored as good binders, whereas two peptides bound weakly to HLA-A2. After DNA immunization, cytotoxic T lymphocyte (CTL) responses were predominantly directed against the Nef 44-52, 81-89, and 85-93 peptides. Interestingly, the 44-52 epitope resides outside the regions of Nef where previously described CTL epitopes are clustered. Dominance among Nef-derived peptides did not strictly correlate with HLA-A2 binding, in that only one of the high-affinity binding peptides was targeted in the CTL response. The 44-52, 85-93, and 139-147 peptides also generated specific CTLs in response to peptide immunization. T helper cell proliferation was detected after stimulation with 20-mer peptides in vitro. Three Nef regions (16-35, 106-125, and 166-185) dominated the T helper cell proliferation. The implications of these results for the development of DNA-based vaccines against HIV is discussed.

T cell tolerance based on avidity thresholds rather than complete deletion allows maintenance of maximal repertoire diversity

Given the flexible nature of TCR specificity, deletion or permanent disabling of all T cells with the capacity to recognize self peptides would severely limit the diversity of the repertoire and the capacity to recognize foreign Ags. To address this, we have investigated the patterns of CD8+ CTL reactivity to a naturally H-2Kb-presented self peptide derived from the elongation factor 1alpha (EF1alpha). EF1alpha occurs as two differentially expressed isoforms differing at one position of the relevant peptide. Low avidity CTLs could be raised against both variants of the EF1alpha peptide. These CTLs required 100-fold more peptide-H-2Kb complexes on the target cell compared with CTLs against a viral peptide, and did not recognize the naturally expressed levels of EF1alpha peptides. Thus, low avidity T cells specific for these self peptides escape tolerance by deletion, despite expression of both EF1alpha isoforms in dendritic cells known to mediate negative selection in the thymus. The low avidity in CTL recognition of these peptides correlated with low TCR affinity. However, self peptide-specific CTLs expressed elevated levels of CD8. Furthermore, CTLs generated against altered self peptide variants displayed intermediate avidity, indicating cross-reactivity in induction of tolerance. We interpret these data, together with results previously published by others, in an avidity pit model based on avidity thresholds for maintenance of both maximal diversity and optimal self tolerance in the CD8+ T cell repertoire.

Inhibition of human NK cell-mediated killing by CD1 molecules

It is now well established that NK cells recognize classical and nonclassical MHC class I molecules and that such recognition typically results in the inhibition of target cell lysis. Given the known structural similarities between MHC class I and non-MHC-encoded CD1 molecules, we investigated the possibility that human CD1a, -b, and -c proteins might also function as specific target structures for NK cell receptors. Here we report that expression of CD1a, -b, or -c can partially inhibits target cell lysis by freshly isolated human NK cells and cultured NK lines. The inhibitory effects of CD1 molecules on NK cell could be shown upon expression of individual CD1 proteins in transfected NK-sensitive target cells, and these effects could be reversed by incubation of the target cells with mAbs specific for the expressed form of CD1. Inhibitory effects of CD1 expression on NK-mediated lysis could also be shown for cultured human dendritic cells, which represent a cell type that prominently expresses the various CD1 proteins in vivo. In addition, the bacterial glycolipid Ags known to be bound and presented by CD1 proteins could significantly augment the observed inhibitory effects on target cell lysis by NK cells.

Human cytomegalovirus strain-dependent changes in NK cell recognition of infected fibroblasts

NK cells play a key role in the control of CMV infection in mice, but the mechanism by which NK cells can recognize and kill CMV-infected cells is unclear. In this study, the modulation of NK cell susceptibility of human CMV (hCMV)-infected cells was examined. We used a human lung and a human foreskin fibroblast cell line infected with clinical isolates (4636, 13B, or 109B) or with laboratory strains (AD169, Towne). The results indicate that all three hCMV clinical isolates confer a strong NK resistance, whereas only marginal or variable effects in the NK recognition were found when the laboratory strains were used. The same results were obtained regardless of the conditions of infection, effector cell activation status, cell culture conditions, and/or donor-target cell combinations. The NK cell inhibition did not correlate with HLA class I expression levels on the surface of the target cell and was independent of the leukocyte Ig-like receptor-1, as evaluated in Ab blocking experiments. No relevant changes were detected in the adhesion molecules ICAM-I and LFA-3 expressed on the cell surface of cells infected with hCMV clinical and laboratory strains. We conclude that hCMV possesses other mechanisms, related neither to target cell expression of HLA-I or adhesion molecules nor to NK cell expression of leukocyte Ig-like receptor-1, that confer resistance to NK cell recognition. Such mechanisms may be lost during in vitro passage of the virus. These results emphasize the differences between clinical hCMV isolates compared with laboratory strains.

Innate and adaptive immunity to tumors: IL-12 is required for optimal responses

We have investigated the importance of endogenously produced IL-12 in innate and adaptive immunity to tumor transplants. The immunogenic lymphoma RMA and its TAP-deficient variant RMA-S were tested for rejection responses by normal and IL-12-deficient mice. IL-12 was crucial for the immunity induced by one immunization with irradiated RMA cells, as well as for in vivo priming of a CTL response in mixed lymphocyte tumor cultures against this MHC class I-expressing tumor. The defective in vivo response could be overcome by multiple immunizations. In further studies of in vitro CTL responses, we found that IL-12 production from either the antigen-pulsed dendritic cells or from host cells was necessary to obtain strong CTL responses. In the complete absence of IL-12, no or only very weak responses could be detected. NK cell-mediated innate resistance, as assessed in non-immunized mice inoculated with a threshold dose of RMA-S cells, also required IL-12. However, NK cells with reduced activity were present in IL-12-deficient mice and contributed to innate resistance, as demonstrated with lower cell dose challenges. In conclusion, IL-12 is required for optimal adaptive and innate responses against tumors.

Differential effects on T cell and NK cell development by tissue-specific expression of H-2D(d) transgene

The effect of tissue-specific expression of the MHC class I molecule H-2D(d) on T cell and NK cell specificity was studied in transgenic mice expressing the H-2D(d) gene under the control of the mouse metallothionein-I promoter. MTD mice expressed high amounts of H-2D(d) in the liver, intestine and testis, but only minute amounts in the thymus, spleen and kidney. Zinc administration resulted in a 1.5- and 8.5-fold increase in H-2D(d) expression in the liver and the intestine, respectively, but did not affect expression in the other organs tested. T cell tolerance developed towards H-2D(d) in MTD mice, even in the absence of zinc. In contrast, NK cell-mediated natural resistance against lymphoma grafts was not seen in MTD mice, despite zinc administration. NK cells in MTD mice also failed to develop self tolerance to H-2D(d). The lack of functional effects did not result from inability of NK cells in MTD mice to interact with H-2D(d), as down-regulation of Ly49A receptor expression was observed on liver NK cells in MTD mice. Our data reveal a difference between T cells and NK cells in their requirements for MHC class I molecules in specificity development.

Recruitment and activation of natural killer (NK) cells in vivo determined by the target cell phenotype. An adaptive component of NK cell-mediated responses

Natural killer (NK) cells can spontaneously lyse certain virally infected and transformed cells. However, early in immune responses NK cells are further activated and recruited to tissue sites where they perform effector functions. This process is dependent on cytokines, but it is unclear if it is regulated by NK cell recognition of susceptible target cells. We show here that infiltration of activated NK cells into the peritoneal cavity in response to tumor cells is controlled by the tumor major histocompatibility complex (MHC) class I phenotype. Tumor cells lacking appropriate MHC class I expression induced NK cell infiltration, cytotoxic activation, and induction of transcription of interferon gamma in NK cells. The induction of these responses was inhibited by restoration of tumor cell MHC class I expression. The NK cells responding to MHC class I-deficient tumor cells were approximately 10 times as active as endogenous NK cells on a per cell basis. Although these effector cells showed a typical NK specificity in that they preferentially killed MHC class I-deficient cells, this specificity was even more distinct during induction of the intraperitoneal response. Observations are discussed in relation to a possible adaptive component of the NK response, i.e., recruitment/activation in response to challenges that only NK cells are able to neutralize.

Visualization of inhibitory Ly49 receptor specificity with soluble major histocompatibility complex class I tetramers

Murine natural killer (NK) cells are inhibited from killing their targets by the interaction between inhibitory, C-type lectin like Ly49 receptors and major histocompatibility complex (MHC) class I molecules. The receptors have overlapping specificity, and it has been difficult to analyze specific aspects of the interaction between different Ly49 receptors and their respective ligands. We have addressed this problem using tetramers of bacterially expressed, non-glycosylated, MHC class I molecules refolded with different peptides. Our results indicate that this technology is useful for analysis of Ly49 receptor specificity as well as for monitoring of NK cell subsets, with the following major conclusions emerging from this study: (1) tetramers of H-2D(d) bound the Ly49A receptor; the MHC associated glycan, previously suggested to be involved in recognition by this receptor, is thus not required for Ly49A receptor binding; (2) in support and extension of a recent report indicating peptide selectivity in the recognition of H-2K(b) by Ly49C(+) cells, H-2K(b) tetramer binding to Ly49C receptors was strongly influenced by the peptide presented by the MHC class I molecule; (3) tetramer binding allowed visualization of interactions that have not previously been detected in functional studies, such as the recognition of H-2D(b) by Ly49A and Ly49C.

Recognition of autologous dendritic cells by human NK cells

NK cells can recognize and kill tumor as well as certain normal cells. The outcome of the NK-target interaction is determined by a balance of positive and negative signals initiated by different target cell ligands. We have previously shown that human NK cells kill CD40-transfected tumor targets efficiently, but the physiological significance of this is unclear. We now demonstrate that human NK cells can kill dendritic cells (DC), known to express CD40 and other co-stimulatory molecules. The killing was observed with polyclonal NK cells cultured short term in IL-2 as well as with NK cell clones as effectors, and with allogeneic as well as autologous DC as targets. NK cell recognition could be inhibited, but only partially, by preincubation of target cells with monoclonal antibodies against CD40, suggesting that this molecule may be one of several ligands involved. Addition of TNF-alpha of the cultures stimulated the development of a more mature DC phenotype, while addition of IL-10 resulted in a less mature phenotype, with lower expression of CD40 and other co-stimulatory molecules. Nevertheless, such DC were more NK susceptible than the differentiated DC. This may be partly explained by a reduced MHC class I expression observed on such cells, since blocking of MHC class I molecules on differentiated DC or CD94 receptors of NK cells led to increased NK susceptibility. The results show that NK cells may interact with DC, and suggest that the outcome of such interactions depend on the cytokine milieu.

Peptide dependency and selectivity of the NK cell inhibitory receptor Ly-49C

MHC class I molecules can prevent NK cell-mediated cytotoxicity by interacting with inhibitory receptors on the effector cells. Different conclusions have been reached regarding possible peptide selectivity of these receptors. To address whether peptide selectivity is an exclusive feature of human or immunoglobulin-superfamily receptors, we have studied a system based on the murine NK receptor Ly-49C in the lectin-superfamily. Loading of TAP-deficient RMA-S cells with the H-2Kb-restricted, ovalbumin-derived peptide OVA(257 - 264) (pOVA) induced their ability to bind Ly-49C-transfected reporter cells, and also protected them from killing by Ly-49C+ NK cells. Other peptides that bound and stabilized H-2Kb equally well differed in their NK protective capacity. Comparison of the MHC class I peptide complexes (crystal structures and molecular models) revealed a conformational motif encompassing the C-terminal parts of the alpha1 helix (73 - 77) and the bound peptide that was common for the protective complexes. Substitution analysis of pOVA suggested that position 7 in the peptide may be critical for optimal protection as well as for the conformational motif at position 73 - 77. In conclusion, protection mediated by the murine C-type lectin receptor Ly-49C is peptide dependent and selective.

Immunization with dendritic cells breaks immunodominance in CTL responses against minor histocompatibility and synthetic peptide antigens

We have examined the mechanisms involved in immunodominance in two different experimental models: the cytotoxic T lymphocyte (CTL) response in B6 mice against minor histocompatibility antigens of BALB.B mice, and the response of B6 mice against a mixture of five synthetic peptides corresponding to well-defined immunogenic epitopes. The CTL responses in these models focus on a few dominant epitopes, whereas no or only weak responses can be detected against other subdominant epitopes. Neither of these immunodominance phenomena can be explained by insufficient presentation of subdominant epitopes in the presence of the dominant ones. Immunodominance could also be demonstrated in an in vitro system, in which B6 splenocytes primed with BALB.B could interfere with the CTL response against subdominant antigens. This interference was dependent on CD8+ T cells and on the simultaneous presentation of dominant and subdominant antigens on the same antigen-presenting cell, suggesting T cell competition around the antigen-presenting cell as a potential explanation. The immunodominance in both systems could be broken by immunization with dendritic cells (from BALB.B or from B6 loaded with peptides). This procedure allowed detection of CTL responses against both dominant and previously subdominant antigens.

T cell competition for the antigen-presenting cell as a model for immunodominance in the cytotoxic T lymphocyte response against minor histocompatibility antigens

We recently demonstrated that spleen cells primed against dominant BALB.B antigens can inhibit the cytotoxic T lymphocyte (CTL) response against subdominant antigens in vitro. In this study, we show that this interference is dependent on CD8+, but not CD4+, T cells directed against dominant antigens. Similar to immunodominance in vivo, T cell interference in vitro required presentation of dominant and subdominant antigens by the same antigen-presenting cell. In vivo priming with cells expressing dominant and subdominant antigens did not induce long-lasting unresponsiveness against the latter. These results support a model in which immunodominance is mediated by T cell competition. In line with this, we found that the immunodominance effects in the CTL response against these minor histocompatibility antigens could be broken by immunization with live bone marrow-derived dendritic cells.

The MHC class I molecule H-2Dp inhibits murine NK cells via the inhibitory receptor Ly49A

MHC class I molecules strongly influence the phenotype and function of mouse NK cells. NK cell-mediated lysis is prevented through the interaction of Ly49 receptors on the effector cell with appropriate MHC class I ligands on the target cell. In addition, host MHC class I molecules have been shown to modulate the in vivo expression of Ly49 receptors. We have previously reported that H-2Dd and H-2Dp MHC class I molecules are able to protect (at the target cell level) from NK cell-mediated lysis and alter the NK cell specificity (at the host level) in a similar manner, although the mechanism behind this was not clear. In this study, we demonstrate that the expression of both H-2Dd and H-2Dp class I molecules in target cells leads to inhibition of B6 (H-2b)-derived Ly49A+ NK cells. This inhibition could in both cases be reversed by anti-Ly49A Abs. Cellular conjugate assays showed that Ly49A-expressing cells indeed bind to cells expressing H-2Dp. The expression of Ly49A and Ly49G2 receptors on NK cells was down-regulated in H-2Dp-transgenic (B6DP) mice compared with nontransgenic B6 mice. However, B6DP mice expressed significantly higher levels of Ly49A compared with H-2Dd-transgenic (D8) mice. We propose that both H-2Dd and H-2Dp MHC class I molecules can act as ligands for Ly49A.

Triggering of murine NK cells by CD40 and CD86 (B7-2)

NK cell-mediated cytotoxicity is regulated by both triggering and inhibitory signals. The interaction between MHC class I molecules expressed on target cells and specific MHC class I-binding receptors expressed by NK cells generally leads to inhibition of lysis. We have shown recently that CD80 (B7-1) in mice and CD40 in humans trigger NK cell-mediated cytotoxicity in vitro. In the present study, we show that murine CD40 and CD86 (B7-2) trigger murine NK cell-mediated cytotoxicity in vitro when expressed on tumor cells. Preincubation of the transfected cell lines with anti-CD40 F(ab')2 fragments or cytolytic T lymphocyte-associated Ag-4-Ig (CTLA-4-Ig) before the cytotoxic assay abolished the triggering effect. Furthermore, radiolabeled CD40- and B7-2-expressing cells were rapidly eliminated in vivo in an NK cell-dependent manner. NK cells from CD40 ligand (CD40L)-/- or CD28-/- mice were triggered by tumor cells transfected with CD40 and B7-2, respectively, and these transfectants were rapidly eliminated in vivo when inoculated into CD40L-/- and CD28-/- mice. This suggests that the CD40 and B7-2 molecules can interact with receptors on NK cells other than CD40L and CD28, respectively, and that these may account for some of the reactivities observed in the present study. Collectively, these data demonstrate that 1) costimulatory molecules, other than B7-1, can modulate NK cell responses in vitro, 2) they can also affect NK cell-dependent responses in vivo, and 3) parts of these reactions are independent of CD28 and CD40L.

Recognition of the major histocompatibility complex restriction element modulates CD8(+) T cell specificity and compensates for loss of T cell receptor contacts with the specific peptide

Triggering of a T cell requires interaction between its specific receptor (TCR) and a peptide antigen presented by a self-major histocompatibility complex (MHC) molecule. TCR recognition of self-MHC by itself falls below the threshold of detection in most systems due to low affinity. To study this interaction, we have used a read-out system in which antigen-specific effector T cells are confronted with targets expressing high levels of MHC compared with the selecting and priming environment. More specifically, the system is based on CD8(+) T cells selected in an environment with subnormal levels of MHC class I in the absence of beta2-microglobulin. We observe that the MHC restriction element can trigger viral peptide-specific T cells independently of the peptide ligand, provided there is an increase in self-MHC density. Peptide-independent triggering required at least four times the natural in vivo level of MHC expression. Furthermore, recognition of the restriction element at expression levels below this threshold was still enough to compensate for lack of affinity to peptides carrying alanine substitutions in major TCR contact residues. Thus, the specificity in TCR recognition and T cell activation is fine tuned by the avidity for self-MHC, and TCR avidities for peptide and MHC may substitute for each other. These results demonstrate a functional role for TCR avidity for self-MHC in tuning of T cell specificity, and support a role for cross-reactivity on "self" during T cell selection and activation.

Murine class I major histocompatibility complex H-2Dd: expression, refolding and crystallization

A truncated soluble form of the murine class I major histocompatibility antigen complex H-2Dd was cloned using an Escherichia coli based system. It was expressed, refolded in vitro and crystallized in a complex with murine beta2 microglobulin and the peptide RGPGRAFVTI from the V3-loop of the gp160 HIV-1 protein. Crystals belonging to the space group P212121 with cell dimensions a = 51.3, b = 92.5, c = 108.8 A were obtained using two different crystallization conditions. The crystals contain one complex per asymmetric unit and diffract to at least 2.4 A resolution.

External and internal calibration of the MHC class I-specific receptor Ly49A on murine natural killer cells

Expression of the H-2Dd-specific inhibitory receptor Ly49A on murine NK cells is subject to MHC class I-dependent modulation in vivo. As a result, NK cells in H-2Dd-transgenic mice express low cell surface levels of Ly49A, whereas NK cells from nontransgenic C57BL/6 (B6) mice express high levels. The purpose of this study was to assess the role of MHC class I molecules on the NK cell itself vs those on surrounding cells in this calibration and to test whether the Ly49A levels are subject to regulation in mature NK cells also. Analysis of transgenic mice with mosaic expression of an H-2Dd/Ld transgene showed that MHC class I molecules on surrounding cells (external ligands) and on the NK cell itself (internal ligands) played distinct roles in the determination of Ly49A levels. External ligands were involved in down-regulation of Ly49A levels in vivo, whereas internal ligands kept the down-regulated levels of Ly49A low upon NK cell activation in vitro. Furthermore, in an experimental system based on adoptive transfer of spleen cells, receptor down-regulation of Ly49A occurred as a rapid adaptation process in mature NK cells after interaction with the H-2Dd ligand in vivo. This suggests that Ly49 levels are not fixed but can be changed in mature NK cells when they are exposed to a changed MHC class I environment.

Immunodominance in the CTL response against minor histocompatibility antigens: interference between responding T cells, rather than with presentation of epitopes

We have investigated mechanisms involved in immunodominance of the CTL response of C57BL/6 (B6) mice against cells of BALB.B origin. This transplantation barrier consists of at least 40 minor histocompatibility (H) Ags. Insufficient presentation of nondominant epitopes in the presence of dominant epitopes was investigated as a possible mechanism for immunodominance. Ag presentation was assessed by recognition of dendritic cells of BALB.B origin, MLC restimulatory capacity, and quantification of cell surface presentation by peptide elution from intact cells. Cells from BALB.B mice, which fail to elicit CTL against nondominant epitopes, presented nondominant epitopes to a similar extent as cells from minor H congenic mice; the latter do elicit CTL against nondominant minor H Ags. Nevertheless, presentation of nondominant and dominant epitopes by the same APC appeared to be an important factor for immunodominance to occur, since simultaneous immunization with the epitopes on separate cells elicited CTL against both types of epitopes. This suggested that immunodominance is determined in the interaction between different responding T cells and the APC. Support for this was obtained in an in vitro model in which the CTL response against a nondominant epitope was inhibited by the concomitant response against a dominant epitope. This study suggests that immunodominance in the CTL response against certain minor H Ags results from interference between T cell responses and not from insufficient presentation of peptide epitopes. The study also provides an in vitro model for further investigations of the immunodominance phenomenon.

Impaired MHC class I (H-2Dd)-mediated protection against Ly-49A+ NK cells after amino acid substitutions in the antigen binding cleft

The MHC class I molecule H-2Dd (Dd) acts as a ligand for the inhibitory NK cell receptor Ly-49A. We have constructed altered Dd molecules by site-directed mutagenesis, replacing residues with the corresponding amino acids from the Db molecule, which fails to inhibit via Ly-49A. Mutations at positions 73 and 156 (DdS73WD156Y) impaired the protective effect of the Dd molecule, as evaluated by testing lymphoma cells transfected with the mutant gene for sensitivity to killing by Ly-49A+ NK cells in vitro and rejection by NK cells in vivo. The altered residues form a hydrophobic ridge across the floor of the antigen binding cleft. A mutation in the alpha helix of the alpha2 domain, facing the solvent and without direct contact with the peptide (DdA150S) had no effect. Dd recognition by Ly-49A+ NK cells is considered to be peptide dependent, but not peptide specific. Our results indicate that alterations of residues buried in the antigen binding cleft can induce changes in peptide binding patterns and/or conformational changes in the Dd molecule that make the trimolecular complex less permissive for inhibition of Ly-49A+ NK cells.

The crystal structure of H-2Dd MHC class I complexed with the HIV-1-derived peptide P18-I10 at 2.4 A resolution: implications for T cell and NK cell recognition

The structure of H-2Dd complexed with the HIV-derived peptide P18-I10 (RGPGRAFVTI) has been determined by X-ray crystallography at 2.4 A resolution. This MHC class I molecule has an unusual binding motif with four anchor residues in the peptide (G2, P3, R/K/H5, and I/L/F9 or 10). The cleft architecture of H-2Dd includes a deep narrow passage accomodating the N-terminal part of the peptide, explaining the obligatory G2P3 anchor motif. Toward the C-terminal half of the peptide, p5R to p8V form a type I' reverse turn; residues p6A to p9T, and in particular p7F, are readily exposed. The structure is discussed in relation to functional data available for T cell and natural killer cell recognition of the H-2Dd molecule.

The alpha2 domain of H-2Dd restricts the allelic specificity of the murine NK cell inhibitory receptor Ly-49A

Mouse NK lymphocytes express Ly-49 receptors, which inhibit cytotoxicity upon ligation by specific MHC I molecules on targets. Different members of the lectin-like mouse Ly-49 receptor family recognize distinct subsets of murine H-2 molecules, but the molecular basis for the allelic specificity of Ly-49 has not been defined. We analyzed inhibition of natural killing by chimeric MHC I molecules in which the alpha1, alpha2, or alpha3 domains of the Ly-49A-binding allele H-2Dd were exchanged for the corresponding domains of the nonbinding allele H-2Db. Using the Ly-49A-transfected rat NK cell line, RNK-mLy-49A.9, we demonstrated that the H-2Dd alpha2 domain alone accounts for allelic specificity in protection of rat YB2/0 targets in vitro. We also showed that the H-2Dd alpha2 domain is sufficient to account for the allele-specific in vivo protection of H-2b mouse RBL-5 tumors from NK cell-mediated rejection in D8 mice. Thus, in striking contrast to the alpha1 specificity of Ig-like killer inhibitory receptors for human HLA, the lectin-like mouse Ly-49A receptor is predominantly restricted by the H-2Dd alpha2 domain in vitro and in vivo.

Superdominance among immunodominant H-2Kb-restricted epitopes and reversal by dendritic cell-mediated antigen delivery

To examine possible interference patterns between immunodominant CTL Ags, we analyzed the response to mixtures of five well-characterized H-2Kb-restricted epitopes, each of which had earlier been described as immunodominant within its antigenic system. Clear patterns of dominance were observed between peptides in the mixture, with the CTL response focusing on the Sendai virus nucleoprotein 324-332 and vesicular stomatitis virus nucleoprotein 52-59 epitopes. The dominance of these epitopes correlated with high CTL availability. Subdominance of the OVA(257-264) and the MCF1233 murine leukemia virus envelope 574-581 peptides could not be explained by inferior ability to bind and stabilize MHC class I molecules. Interestingly, immunodominance was broken if the peptide mixture was pulsed on bone marrow-derived dendritic cells, a mode of immunization allowing efficient recognition of a broader set of specificities. Our results show that immunodominance is neither an absolute feature of a given epitope nor does it apply only in relation to other epitopes within the same protein, micro-organism, or cell. Novel "superdominant" hierarchies emerge in the response against multiple "dominant" epitopes. A T cell competition model to explain the data in terms of a balance influenced by CTL frequencies and available APC capacity is discussed.

Beta2-microglobulin-deficient NK cells show increased sensitivity to MHC class I-mediated inhibition, but self tolerance does not depend upon target cell expression of H-2Kb and Db heavy chains

Mice lacking beta2-microglobulin (beta2m- mice) express greatly reduced levels of MHC class I molecules, and cells from beta2m- mice are therefore highly sensitive to NK cells. However, NK cells from beta2m- mice fail to kill beta2m- normal cells, showing that they are self tolerant. In a first attempt to understand better the basis of this tolerance, we have analyzed more extensively the target cell specificity of beta2m- NK cells. In a comparison between several MHC class I-deficient and positive target cell pairs for sensitivity to beta2m- NK cells, we made the following observations: First, beta2m- NK cells displayed a close to normal ability to kill a panel of MHC class I-deficient tumor cells, despite their nonresponsiveness to beta2m- concanavalin A (Con A)-activated T cell blasts. Secondly, beta2m- NK cells were highly sensitive to MHC class I-mediated inhibition, in fact more so than beta2m+ NK cells. Thirdly beta2m- NK cells were not only tolerant to beta2m- Con A blasts but also to Con A blasts from H-2Kb-/Db- double deficient mice in vitro. We conclude that NK cell tolerance against MHC class I-deficient targets is restricted to nontransformed cells and independent of target cell expression of MHC class I free heavy chains. The enhanced ability of beta2m- NK cells to distinguish between MHC class I-negative and -positive target cells may be explained by increased expression of Ly49 receptors, as described previously. However, the mechanisms for enhanced inhibition by MHC class I molecules appear to be unrelated to self tolerance in beta2m- mice, which may instead operate through mechanisms involving triggering pathways.

Generation of CD8+ T cells specific for transporter associated with antigen processing deficient cells

Cells with impaired transporter associated with antigen processing (TAP) function express low levels of cell surface major histocompatibility complex (MHC) class I molecules, and are generally resistant to lysis by MHC class I restricted cytotoxic T lymphocytes (CTLs). Here we report the generation of MHC class I restricted CD8(+) CTLs that surprisingly require target cell TAP deficiency for efficient recognition. C57BL/6 (B6) mice immunized with syngenic B7-1 (CD80) expressing TAP-deficient cells generated a potent CTL response against both TAP-deficient RMA-S tumor cells and TAP-deficient Con A blasts, whereas the corresponding TAP-expressing target cells were considerably less susceptible or resistant to lysis. The CTL epitopes recognized were expressed also by the human TAP-deficient cell line T2, transfected with appropriate MHC class I molecules. B6 mice immunized with B7-1-transfected TAP-deficient RMA-S cells were protected from outgrowth of a subsequent RMA-S tumor challenge. These findings are discussed in relation to the biochemical nature of MHC class I dependent CTL epitopes associated with impaired TAP function, as well as implications for immunotherapy and autoimmunity.

NK cell receptor calibration: effects of MHC class I induction on killing by Ly49Ahigh and Ly49Alow NK cells

NK cells from C57BL/6 (B6) mice and H-2Dd transgenic B6 (D8) mice express different levels of the inhibitory receptor Ly49A, and they also differ in their target cell specificity. Here, we examined this differential specificity with respect to the role of the Ly49A receptor expression on effector cells and levels of H-2Dd inhibitory ligands on target cells. NK cells from D8 mice express low levels of Ly49A receptor (Ly49Alow), and are able to kill SP2/0 tumor cells in spite of their expression of H-2Dd. H-2Dd is expressed at reduced levels on SP2/0 cells; when these were increased three- to fivefold after IFN-gamma treatment, the killing by Ly49Alow NK cells from D8 mice was markedly reduced. Efficient killing was restored when the effectors were preincubated with anti-Ly49A F(ab')2 Abs. A separate experimental system was based on D8 TAP1-deficient Con A blasts exogenously loaded with H-2Dd-specific peptides. In this system, higher levels of cell surface H-2Dd had to be induced by peptide to inhibit D8 Ly49Alow NK cells to an extent similar to that of B6 Ly49Ahigh NK cells. Ly49A receptors on NK cells from H-2Dd transgenic mice are thus functional, although they require high levels of ligand to inhibit progression of the NK-target cell interaction. The data are in favor of the "receptor-calibration" model, which suggests that down-regulation of inhibitory receptors on NK cells may be useful in order for NK cells to discriminate between normal and reduced levels of MHC class I molecules.

NK cell receptor calibration: effects of MHC class I induction on killing by Ly49Ahigh and Ly49Alow NK cells

NK cells from C57BL/6 (B6) mice and H-2Dd transgenic B6 (D8) mice express different levels of the inhibitory receptor Ly49A, and they also differ in their target cell specificity. Here, we examined this differential specificity with respect to the role of the Ly49A receptor expression on effector cells and levels of H-2Dd inhibitory ligands on target cells. NK cells from D8 mice express low levels of Ly49A receptor (Ly49Alow), and are able to kill SP2/0 tumor cells in spite of their expression of H-2Dd. H-2Dd is expressed at reduced levels on SP2/0 cells; when these were increased three- to fivefold after IFN-gamma treatment, the killing by Ly49Alow NK cells from D8 mice was markedly reduced. Efficient killing was restored when the effectors were preincubated with anti-Ly49A F(ab')2 Abs. A separate experimental system was based on D8 TAP1-deficient Con A blasts exogenously loaded with H-2Dd-specific peptides. In this system, higher levels of cell surface H-2Dd had to be induced by peptide to inhibit D8 Ly49Alow NK cells to an extent similar to that of B6 Ly49Ahigh NK cells. Ly49A receptors on NK cells from H-2Dd transgenic mice are thus functional, although they require high levels of ligand to inhibit progression of the NK-target cell interaction. The data are in favor of the "receptor-calibration" model, which suggests that down-regulation of inhibitory receptors on NK cells may be useful in order for NK cells to discriminate between normal and reduced levels of MHC class I molecules.

Synthetic peptides derived from the melanocyte-stimulating hormone receptor MC1R can stimulate HLA-A2-restricted cytotoxic T lymphocytes that recognize naturally processed peptides on human melanoma cells

Human melanoma-specific HLA-A2 restricted CTLs have recently been shown to recognize antigens expressed by melanoma lines and normal melanocytes, including Melan-A/Mart-1, gp100, gp75, and tyrosinase. Herein, we define HLA-A2-restricted CTL epitopes from a recently cloned melanocortin 1 receptor (MC1R), which belongs to a new subfamily of the G-protein-coupled receptors expressed on melanomas and melanocytes. Thirty-one MC1R-derived peptides were selected on the basis of HLA-A2-specific motifs and tested for their HLA-A2 binding capacity. Of a group of 12 high or intermediate HLA-A2 binding peptides, three nonamers, MC1R244 (TILLGIFFL), MC1R283 (FLALIICNA), and MC1R291 (AIIDPLIYA), were found to induce peptide-specific CTLs from peripheral blood mononuclear cells of healthy HLA-A2+ donors after repeated in vitro stimulation with peptide-pulsed antigen-presenting cells. The CTLs raised against these three HLA-A2+-restricted peptides could recognize naturally processed peptides from HLA-A2+ melanomas and from Cos7 cells cotransfected with MC1R and HLA-A2. CTLs induced by the MC1R291 peptide (but not induced or induced only to a very low extent by the other two MCR1 peptide epitopes) showed cross-reactions with two other members of the melanocortin receptor family, which are more broadly expressed on other tissues. Taken together, our findings have implications in relation both to autoimmunity and immunotherapy of malignant melanomas.

Natural killer cell tolerance in mice with mosaic expression of major histocompatibility complex class I transgene

We have studied natural killer (NK) cell tolerance in a major histocompatibility complex (MHC) class I transgenic line, DL6, in which the transgene product was expressed on only a fraction of blood cells. In contrast with transgenic mice expressing the same transgene in all cells, NK cells from mosaic mice failed to reject transgene-negative bone marrow or lymphoma grafts. However, they retained the capability to reject cells with a total missing-self phenotype, i.e., cells lacking also wild-type MHC class I molecules. Tolerance against transgene-negative cells was demonstrated also in vitro, and could be broken if transgene-positive spleen cells of mosaic mice were separated from negative cells before, or after 4 d of culture in interleukin-2. The results provide support for selective NK cell tolerance to one particular missing-self phenotype but not to another. We suggest that this tolerance is determined by NK cell interactions with multiple cells in the environment, and that it is dominantly controlled by the presence of cells lacking a specific MHC class I ligand. Furthermore, the tolerant NK cells could be reactivated in vitro, which suggests that the tolerance occurs without deletion of the potentially autoreactive NK cell subset(s), and that it may be dependent upon the continuous presence of tolerizing cells.

A new mechanism of NK cell cytotoxicity activation: the CD40-CD40 ligand interaction

NK recognition is regulated by a delicate balance between positive signals initiating their effector functions, and inhibitory signals preventing them from proceeding to cytolysis. Knowledge of the molecules responsible for positive signaling in NK cells is currently limited. We demonstrate that IL-2-activated human NK cells can express CD40 ligand (CD40L) and that recognition of CD40 on target cells can provide an activation pathway for such human NK cells. CD40-transfected P815 cells were killed by NK cell lines expressing CD40L, clones and PBL-derived NK cells cultured for 18 h in the presence of IL-2, but not by CD40L-negative fresh NK cells. Cross-linking of CD40L on IL-2-activated NK cells induced redirected cytolysis of CD40-negative but Fc receptor-expressing P815 cells. The sensitivity of human TAP-deficient T2 cells could be blocked by anti-CD40 antibodies as well as by reconstitution of TAP/MHC class I expression, indicating that the CD40-dependent pathway for NK activation can be downregulated, at least in part, by MHC class I molecules on the target cells. NK cell recognition of CD40 may be important in immunoregulation as well as in immune responses against B cell malignancies.

Altered expression of Ly49 inhibitory receptors on natural killer cells from MHC class I-deficient mice

MHC class I molecules in the host affect the specificity of NK cells. Previous work has suggested that this specificity is conferred by the expression of products encoded by the Ly49 gene family. This gene family encodes receptors that upon specific recognition of MHC class I ligands mediate an inhibitory signal that prevents killing by NK cells. The pattern of expression of the Ly49 MHC class I binding inhibitory receptors on NK cells is thought to be adapted to the host to ensure the generation of a self-tolerant, yet functional, NK cell repertoire. In the present study we have examined the expression of inhibitory receptors (Ly49A, Ly49C, and Ly49G2) on NK1.1+ cells from B6 (H-2b) and D8 (B6 mice transgenic for H-2Dd) mice as well as corresponding TAP1 -/-, beta2m -/-, and TAP1/beta2m -/- mutants of these mice. We demonstrate that receptor expression on NK1.1+ cells can be specifically modulated by host MHC class I molecules in at least two different ways: alteration of numbers of cells expressing a given receptor and modulation of the levels of expression of a given receptor at the cell surface. The degree of this modulation varies significantly among the various receptors studied and may depend upon the nature of their MHC class I ligands. The results are discussed in relation to the influence of MHC class I molecules on the development of an NK cell repertoire.