The CD94 and NKG2-A C-type lectins covalently assemble to form a natural killer cell inhibitory receptor for HLA class I molecules

CD94, a type II membrane protein containing a C-type lectin domain, has been shown to be involved in natural killer (NK) cell-mediated recognition of different HLA allotypes. The inhibitory form of the CD94 receptor has recently been identified by the specific monoclonal antibody (mAb) Z199. Herein, we demonstrate that the inhibitory receptor is in fact a complex formed by the covalent association of CD94 with the NKG2-A molecule (Mr ～ 43 kDa), another member of the C-type lectin superfamily, and that Z199 mAb specifically recognize NKG2-A molecules. Although the NKG2-A-encoding cDNA has been known for several years, the corresponding protein and its possible function remained undefined. Moreover, we show that the NKG2-B protein, an alternatively spliced product of the NKG2-A gene, can also assemble with CD94. Remarkably, both NKG2-A and NKG2-B proteins contain cytoplasmic immunoreceptor tyrosine-based inhibitory motifs (ITIM). This may provide the molecular basis of the inhibitory function mediated by the CD94/NKG2-A receptor complexes.     

Negative regulation of rat natural killer cell activity by major histocompatibility complex class I recognition

The cytolytic activity of human and mouse natural killer (NK) cells is negatively regulated by self major histocompatibility complex (MHC) class I molecules on potential target cells. In the rat, protection by RT1 class I gene products has so far not been formally shown although the complex effects of foreign and self RT1 genes on polyclonal NK cell activity suggest that MHC recognition can have both stimulatory and inhibitory effects. Here we report that the expression of self-MHC class I molecules on target cells strongly inhibits lysis by a long term NK cell line derived from LEW (RT1^l) rats and by LEW NK cells activated by short-term culture in the presence of interleukin-2. This was demonstrated with mouse-rat hybridoma target cells expressing different rat MHC alleles and with mouse tumor target cells transfected with classical (RT1.A^l) and nonclassical (RT1.C^l) rat MHC class I genes. With hybridoma target cells, the strongest reduction in lysis as compared to the parental mouse myeloma line was observed when “self” (LEW) MHC was expressed, while hybridomas expressing other MHC alleles showed less and variable reduction. Transfection of RT1.A^l protected both L-929 fibroblasts and P815 mastocytoma cells from lysis by the NK cell line, while RT1.C^l only protected P815 cells, indicating that additional target cell properties regulate rat NK cell activity.     

Self class I molecules protect normal cells from lysis mediated by autologous natural killer cells

The surface expression of given HLA class I alleles protects target cells from lysis mediated by natural killer (NK) clones specific for these (or related) alleles. We could define two groups of NK clones specifically recognizing either Cw4 and related C alleles (“group 1”) or Cw3 and related C alleles (“group 2”), respectively. Monoclonal antibodies (mAb) to class I molecules should interfere with the interaction between NK receptors and class I molecules, thus resulting in lysis of protected target cells. However, none of the numerous available mAb to class I molecules had this effect. Therefore, we attempted to select new mAb on the basis of their ability to induce lysis of Cw4- or Cw3-protected lymphoblastoid cell lines by “group 1” or “group 2” NK clones, respectively. From mice immunized with phytohemagglutinin (PHA)-activated lymphocytes expressing either Cw3 or Cw4 alleles, two mAb were selected, the 6A4 (IgG1) and the A6-136 (IgM), on the basis of their ability to induce lysis of protected target cell. Both mAb immunoprecipitated molecules which, in sodium dodecyl sulfate-polyacrylamide gel electrophoresis, gave two bands of 45 and 12 kDa, typical of the class I heavy chain and β2 microglobulin, respectively. It has been proposed (but not proven), that self major histocompatibility complex class I molecules protect normal cells from autologous NK cell lysis. Thus, we used the 6A4 and A6-136 mAb to assess this possibility directly. Cw4-specific (“group 1”) and Cw3-specific (“group 2”) NK clones were isolated from donors expressing the corresponding (or related) protective C alleles. None of these clones lysed autologous PHA-induced blasts, used as target cells. However, addition of the F(ab′)₂ of 6A4 mAb or the A6-136 mAb resulted in lysis of autologous target cells by “group 1” or “group 2” NK clones, respectively. These data provide direct evidence that the expression of class I molecules protects normal cells from lysis by autologous NK cells.     

Ly-49-independent inhibition of natural killer cell-mediated cytotoxicity by a soluble major histocompatibility complex class I molecule

Natural killer (NK) cells lyse their targets in a non-major histocompatibility complex (MHC)-restricted manner, and the cytotoxicity can be inhibited by a number of MHC class I allele products, suggesting that NK cells may have a “positive receptor” that recognizes the target and a “negative receptort” that receives an inhibitory signal from class I. Since negative receptors could also exert their effect by masking a positive ligand, we have determined whether there may be a direct interaction between class I and an NK surface receptor by measuring cytotoxicity in the presence of a soluble class I molecule, K^d. Soluble K^d at micromolar concentrations could efficiently block NK cell cytotoxicity, suggesting that class I has a direct effect on cytotoxicity, rather than masking another target cell ligand. Inhibition required that K^d be at least divalent, probably because of its higher affinity or its ability to cross-link the NK surface receptor. In addition, the effect was independent of the peptide used to load K^d, and there was inhibition of cytotoxicity of NK cells derived from either H-2^d or H-2^b mice. Finally, depletion of NK cells expressing Ly-49 had no effect on the specific inhibition by K^d, raising the possibility that NK cells are endowed with additional negative receptors besides Ly-49. Taken together, these results suggest that there may be a family of NK receptors recognizing different class I alleles, which can receive negative signals by directly binding to class I on the target cell surface.     

Major histocompatibility complex genes determine natural killer cell tolerance

Murine natural killer (NK) cell subsets, as defined by expression of members of the Ly49 gene family, discriminate target cells expressing different major histocompatibility complex (MHC) class I alleles. For example, Ly49A⁺ NK cells lyse H-2^b but not H-2^d tumor target cells. The specificity arises because D^d on target cells binds to Ly49A, transducing an inhibitory signal into the Ly49A⁺ NK cells. The capacity of NK cells to discriminate allelic class I determinants raises a key issue: are NK cells self-tolerant, and if so what are the mechanisms that lead to self-tolerance? As previously reported, potentially autoaggressive Ly49A⁺ NK cells are not clonally deleted in H-2^b mice. However, IL-2- cultured Ly49A⁺ effector cells from H-2^b mice exhibit reduced lysis of H-2^b (self) concanavalin A blast target cells, compared to Ly49A⁺ effector cells from H-2^d mice. Possible mechanisms accounting for this self-tolerance are addressed in this report. Self-tolerance was not due to anergy of the cells, because the Ly49A⁺ effector cells from both types of mice lysed β2-microglobulin-deficient target cells efficiently and equivalently. These results also suggest that tolerance results from inhibition mediated by β2m-dependent H-2^b class I molecules. Significantly, blockade of Ly49A on Ly49A⁺ effector cells from H-2^b mice did not restore lysis of H-2^b target cells, suggesting that inhibition is not mediated through the Ly49A receptor. Additional experiments suggest that inhibition is also not mediated primarily through the Ly49C receptor. These results suggest that Ly49A⁺ effector cells from H-2^b mice, unlike those from H-2^d mice, express inhibitory receptors specific for H-2^b molecules that are distinct from Ly49A and Ly49C.     

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

CD94 molecules have been suggested to function as inhibitory natural killer cell (NK) receptors involved in the recognition of HLA-B alleles sharing the Bw6 supertypic specificity. In this study, we show that CD94 molecules may play a more general role: they are also involved in the recognition of other HLA class I molecules, including HLA-C and at least some HLA-A alleles. The inhibitory effect mediated by CD94 molecules on NK cytolytic activity is lower in magnitude than that of bona fide inhibitory receptors such as p58 or p70. Distinct from the other human NK receptors involved in HLA class I recognition, CD94 is expressed on virtually all NK cells. In addition, it has been shown to be functionally heterogeneous since, in different clones, CD94 mediated either cell triggering or inhibition. Although NK cells expressing inhibitory CD94 molecules are usually characterized by a CD94^bright phenotype, there is no precise correlation between fluorescence intensity and inhibitory or activating function. Here, we describe two novel monoclonal antibodies (mAb) which selectively recognize inhibitory CD94 molecules and bind to a subset (variable in size among different donors) of CD94^bright cells. The use of these mAb allows the direct assessment of NK cells expressing inhibitory CD94 receptors both at the population and at the clonal level.     

Inhibition of natural killer cell-mediated bone marrow graft rejection by allogeneic major histocompatibility complex class I,but not class II molecules

The role of major histocompatibility complex (MHC) class I and class II molecules in natural killer (NK) cell-mediated rejection of allogeneic, semi-syngeneic and MHC-matched bone marrow grafts was investigated. The use of β₂-microglobulin (β₂m) -/- and β₂m +/- mice as bone marrow donors to MHC-mismatched recipients allowed an analysis of whether the presence of semi-syngeneic and allogeneic MHC class I gene products would be triggering, protective or neutral, in relation to NK cell-mediated rejection. Loss of β₂m did not allow H-2^b bone marrow cells to escape from NK cell-mediated rejection in allogeneic (BALB/c) or semi-allogeneic (H-2D^d transgenic C57BL/6) mice. On the contrary, it led to stronger rejection, as reflected by the inability of a larger bone marrow cell inoculum to overcome rejection by the H-2-mismatched recipients. In H-2-matched recipients, loss of β₂m in the graft led to a switch from engraftment to rejection. At the recipient level, loss of β₂m led to loss of the capability to reject H-2-matched β₂m-deficient as well as allogeneic grafts. When MHC class II-deficient mice were used as donors, the response was the same as that against donors of normal MHC phenotype: allogeneic and semi-syngeneic grafts were rejected by NK cells, while syngeneic grafts were accepted. These data suggest a model in which allogeneic class I molecules on the target cell offer partial protection, while certain syngeneic class I molecules give full protection from NK cell-mediated rejection of bone marrow cells. There was no evidence for a role of MHC class II molecules in this system.     

Natural killer clones recognize specific soluble HLA class I molecules

Enhancement of major histocompatibility complex (MHC) class I expression leads to protection from natural killer (NK) cell recognition in several systems. MHC class I gene products are released from the cell surface and can be found in sera as soluble forms. To investigate the possible immunoregulatory role of soluble HLA (sHLA) in NK cell-target recognition, several sHLA antigens were studied for their ability to induce NK cell cytotoxicity modulation. NK cell-target recognition was inhibited by the addition of sHLA during the cytotoxicity assay. Our results indicate that sHLA molecules can down-regulate NK killing at the effector level. Moreover, different NK clones are able to specifically recognize different sHLA antigens. Kp43 molecules seem to be involved in the NK recognition of sHLA-B7.     

Expression of killer cell inhibitory receptors on human uterine natural killer cells

The establishment of the human placenta in early pregnancy is characterized by the presence of large numbers of natural killer (NK) cells within the maternal decidua in close proximity to the fetally-derived invading extravillous trophoblast which expresses at least two HLA class I molecules, HLA-G and HLA-C. These NK cells have an unusual phenotype, CD56^bright CD16, distinguishing them from adult peripheral blood NK cells. They may control key events in trophoblast migration and therefore placentation. Human NK cells in peripheral blood express receptors for polymorphic HLA class I molecules. This family of receptors, known as killer cell inhibitory receptors (KIR), are expressed on overlapping subsets of NK cells to give an NK cell repertoire which differs between individuals. Using a panel of monoclonal antibodies to several members of the KIR family and analysis by flow cytometry, we have found that KIR are expressed by decidual NK cells. There is variation in both the percentage of cells expressing a particular receptor and the density of receptor expression between decidual NK cells from different individuals. Comparison of NK cells from decidua and peripheral blood of the same individual showed that NK cells from these two different locations express different repertoires of KIR. Receptors are present in individuals who do not possess the relevant class I ligand, raising the possibility that these NK receptors may be involved in recognition of the allogeneic fetus by the mother at the implantation site.     

Multiple natural killer cell-activating signals are inhibited by major histocompatibility complex class I expression in target cells

Several lines of evidence indicate that major histocompatibility complex class I molecules expressed by target cells can prevent natural killer cell (NK) lysis, possibly by engaging inhibitory receptors expressed by NK cells. On the other hand it is likely that NK cells must be activated to lysis by the recognition of unidentified NK target structures on target cells. To investigate the relationship between positive activation of NK cells by NK target structures versus inhibition by target cell class I molecules, we have examined various NK/target cell interactions for which the expression of inhibitory class I molecules by the target cells is known. The results suggest that specific properties of the target cell other than the absence of class I expression are necessary to activate NK-mediated lysis. Furthermore, different effector cell populations, i.e. freshly isolated versus interleukin-2 activated NK cells, differ in their capacity to kill class I-deficient lymphoblast target cells. In general, class I-deficient target cells that are resistant to direct lysis by a given NK population can be lysed by the NK cells when the reaction is mediated by antibody-dependent cellular cytotoxicity (ADCC). Most significantly, all types of NK-mediated lysis of lymphoblasts, of tumor cells and of almost any target by ADCC can be inhibited by appropriate class I gene expression in the target cell. These results suggest a model in which lysis by NK cells must be triggered by any one of a set of distinct target cell ligands, but that all of these signals can be overruled by class I-mediated inhibition.     

Influence of glycosylphosphatidylinositol-linked H-2Dd molecules on target cell protection and natural killer cell specificity in transgenic mice

The expression of certain major histocompatibility complex (MHC) class I ligands on target cells is one important determinate of their susceptibility to lysis by natural killer (NK) cells. NK cells express receptor molecules that bind to MHC class I. Upon binding to their MHC class I ligand, the NK cell is presumed to receive a signal through its receptor that inhibits lysis. It is unclear what role the MHC class I molecules of the effector and target cells play in signaling to the NK cell. We have investigated the role of the cytoplasmic and transmembrane domains of MHC class I molecules by producing a glycosylphosphatidylinositol (GPI)-linked H-2D^d molecule. The GPI-linked H-2D^d molecule is recognized by H-2D^d-specific antibodies and cytotoxic T lymphocytes. Expression of the GPI-linked H-2D^d molecule on H-2^b tumor cells resulted in protection of the tumor cells after transplantation into D8 mice (H-2^b, H-2D^d) from rejection by NK cells. In addition, NK cells from mice expressing the GPI-linked H-2D^d molecule as a transgene were able to kill nontransgenic H-2^b lymphoblast target cells. The GPI-linked MHC class I molecule was able to alter NK cell specificity at the target and effector cell levels. Thus, the expression of the cytoplasmic and transmembrane domains of MHC class I molecules are not necessary for protection and alteration of NK cell specificity.     

Location-specific regulation of transgenic Ly49A receptors by major histocompatibility complex class I molecules

Inhibitory receptors expressed on natural killer (NK) cells and T cells specific for major histocompatibility complex (MHC) class I are believed to prevent these cells from responding to normal self tissues. To understand the regulation and function of Ly49 receptor molecules in vivo, we used the CD2 promoter to target Ly49A expression to all thymocytes, T cells, and NK cells. In animals expressing its MHC class I ligand, H-2D^d or H-2D^k, there was a large decrease in the expression of Ly49A on thymocytes, peripheral T cells, and NK1.1⁺ cells. The extent of the down-regulation of Ly49A was dependent on the expression of the MHC ligand for Ly49A and on the site where the cells were located. The level of expression of endogenous Ly49A was similarly found to be dependent upon the organ where the cells resided. Data from bone marrow chimeras indicated that most cell types may regulate Ly49A expression, but the efficacy to regulate receptor expression may vary depending on the cell type.     

Role of amino acid position 70 in the binding affinity of p50.1 and p58.1 receptors for HLA-Cw4 molecules

In an attempt to identify the amino acid position(s) of the HLA-C-specific p58.1/p50.1 natural killer cell receptors that determine the binding affinity for their ligand, we used soluble fusion proteins formed by the ectodomain of either receptor and the Fc portion of human IgG1. We show that the soluble p50.1 (activating) receptor binds weakly to 221-Cw4 transfectants. In contrast, the soluble p58.1 (inhibitory) receptor binds with high affinity. A single amino acid mutation at position 70, obtained by site-directed mutagenesis, was found to affect the binding affinity of both the p50.1 and the p58.1 receptors. Thus, sub-stitution in p50.1 of lysine 70 by threonine (typical of the inhibitory p58.1 molecule) resulted in a dramatic increase in binding affinity, comparable to that of the p58.1 molecule. On the other hand, substitution of threonine 70 by lysine in p58.1 almost abolished binding to 221-Cw4 cells. Our present data indicate that a single amino acid difference greatly influences the p58.1/p50.1 affinity for their HLA-C ligand and suggests a possible role of position 70 as a contact site in the natural killer cell receptor/major histocompatibility complex class I interaction.     

人自然杀伤细胞抑制性受体P58.1基因的克隆与鉴定

目的 克隆及鉴定P58.1基因。方法 采用RT-PCR技术,从正常人外周血单个核细胞中扩增P58.1基因全长cDNA,经酶切后构建重组克隆载体,测序鉴定。结果 RT-PCR获得预期的扩增产物P58.1全长cDNA,成功构建pSPORT1-P58.1重组克隆载体,酶切、酶谱分析与预期结果相符。DNA测序结果与GenBank登记的人P58.1cDNA全长碱基序列一致。结论 pSPORT1-58.1重组克隆载体构建成功;P58.1基因序列与文献报道一致,为下一步构建表达载体和基因转染等工作打下良好的基础。     

Cloning of a mouse homolog of CD94 extends the family of C-type lectins on murine natural killer cells

Two families of major histocompatibility complex (MHC) class I-specific receptors are found on natural killer (NK) cells: immunoglobulin-like receptors and C-type lectin receptors. In mice, the latter category is represented by the Ly49 family of receptors, whereas in humans, NK cells express the distantly related CD94, which forms MHC class I-specific heterodimers with NKG2 family members. Humans also express the MHC class I-specific p50/p58/p70 family of immunoglobulin-like receptors, but these have not been identified in mice. Hence, there is no known instance of an MHC class I-specific receptor that is expressed by both human and murine NK cells. Here we report the cloning of CD94 from the CB.17 and C57BL/6 strains of mice. Mouse CD94 is 54 % identical and 66 % similar to human CD94, and is also a member of the C-type lectin superfamily. Mouse CD94 is expressed efficiently on the cell surface of cells transiently transfected with the corresponding cDNA, but surface CD94 was unable to mediate detectable binding to MHC class I-expressing ConA blasts. Notably, mouse CD94, like human CD94, has a very short cytoplasmic tail, suggesting the existence of partner chains that may play a role in ligand binding and signaling. Like many other C-type lectins expressed by NK cells, mouse CD94 maps to the NK complex on distal chromosome 6, synteneic to human CD94. We also demonstrate that mouse CD94 is highly expressed specifically by mouse NK cells, raising the possibility that mice, like humans, express multiple families of MHC class I-specific receptors on their NK cells. Murine homologs of human NKG2 family members have not yet been identified, but we report here the existence of a murine NKG2D-like sequence that also maps to the murine NK complex near CD94 and Ly49 family members.     

Control of self-reactive cytotoxic T lymphocytes expressing γδ T cell receptors by natural killer inhibitory receptors

The majority of peripheral blood γδ T cells in human adults expresses T cell receptors (TCR) with identical V regions (Vγ9 and Vδ2). These Vγ9Vδ2 T cells recognize the major histocompatibility complex (MHC) class I-deficient B cell line Daudi and broadly distributed nonpeptidic antigens present in bacteria and parasites. Here we show that unlike αβ or Vγ9^? γδ T cells, the majority of Vγ9Vδ2T cells harbor natural killer inhibitory receptors (KIR) (mainly CD94/NKG2A heterodimers), which are known to deliver inhibitory signals upon interaction with MHC class I molecules. Within Vγ9δ2 T cells, KIR were mainly expressed by clones exhibiting a strong lytic activity against Daudi cells. In stark contrast, almost all Vγ9Vδ2 T cell clones devoid of killing activity were KIR^?, thus suggesting a coordinate acquisition of KIR and cytotoxic activity within Vγ9Vδ2 T cells. In functional terms, KIR inhibited lysis of MHC class I-positive tumor B cell lines by Vγ9Vδ2 cytotoxic T lymphocytes (CTL) and raised their threshold of activation by microbial antigens presented by MHC class I-positive cells. Furthermore, masking KIR or MHC class I molecules revealed a TCR-dependent recognition by Vγ9Vδ2 CTL of ligands expressed by activated T lymphocytes, including the effector cells themselves. Taken together, these results suggest a general implication of Vγ9Vδ2 T cells in immune response regulation and a central role of KIR in the control of self-reactive γδ CTL.     

Rat interleukin-2-activated natural killer (A-NK) cell-mediated lysis is determined by the presence of CD18 on A-NK cells and the absence of major histocompatibility complex class I on target cells

The precise mechanism by which target cells are recognized and subsequently lysed by interleukin-2-activated natural killer (A-NK) cells is poorly understood. In this study the role of major histocompatibility complex (MHC) class I and adhesion molecules in the recognition and lysis of tumor cells was investigated in a syngeneic Wag rat model. Preincubation of tumor cells with F(ab′)₂ fragments of anti-MHC class I monoclonal antibody (mAb) OX18 strongly enhanced the A-NK cell-mediated lysis. Also normal syngeneic cells such as T cells and A-NK cells became highly sensitive for lysis by A-NK cells after preincubation with mAb OX18. Two other mAb against MHC class I had no effect on lysis of target cells. These data indicate that masking of MHC class I on syngeneic tumor and normal cells by mAb OX18 is sufficient for A-NK cells to recognize target cells as non-self, resulting in lysis. In addition, we found that the presence of mAb against the β2 (CD18)-integrins blocked the lysis of all tumor cell lines by A-NK cells in ⁵¹Cr-release assays, also when target cells were preincubated with mAb OX18. Because of the absence of CD18 on most tumor cells we concluded that a CD18-associated integrin on A-NK cells is essential for lysis of target cells. These results show that in this syngeneic rat model CD18 on A-NK cells together with MHC class I on tumor cells determine A-NK cell-mediated lysis. Furthermore, we hypothesize that the anti-MHC class I OX18 recognizes an epitope on rat MHC class I which is, or is very close to, the restriction element determining A-NK cell-mediated lysis.     

Decreased expression of killer cell inhibitory receptors on natural killer cells in eutopic endometrium in women with adenomyosis

BACKGROUND: Increased expression of killer cell inhibitory receptors (KIRs) has been found on natural killer (NK) cells in peritoneal fluid in women with endometriosis. In this study, we tried to measure the expression of KIRs on NK and T cells in women with adenomyosis, in an attempt to find the possible role of KIRs in the development of adenomyosis. METHODS: A total of 10 women with adenomyosis (study group) and 12 women with uterine myoma (control group) were included in this study. The expression of KIRs, including NKB1, GL183, EB6 and CD94, on NK and T cells in myometrium and endometrium was examined by flow cytometry. RESULTS: There was a decreased expression of NKB1 and GL183 on NK cells in the endometrium, but not in the myometrium, in women with adenomyosis. However, the expression of KIRs on T cells, either CD4+ or CD8+, was not different in either myometrium or endometrium between women with and without adenomyosis. CONCLUSIONS: The expression of KIRs on NK cells was decreased in eutopic endometrium in women with adenomyosis. It may be a compensatory effect in which the NK cytotoxicity is activated in order to eradicate the abnormal endometrial cells that might exit of the eutopic site of the endometrium.     

Licensing of natural killer cells by self-major histocompatibility complex class I

Summary:  Natural killer (NK) cells have potent capacities to immediately kill cellular targets and produce cytokines that may potentially damage normal self-tissues unless they are kept in check. Such tolerance mechanisms are incompletely understood. Here we discuss recent studies suggesting that NK cells undergo a host major histocompatibility complex (MHC) class I-dependent functional maturation process, termed 'licensing'. Ironically, licensing directly involves inhibitory receptors that recognize target cell MHC class I molecules and block activation of NK cells in effector responses. This process results in two types of tolerant NK cells: functionally competent (licensed) NK cells, whose effector responses are inhibited by self-MHC class I molecules through the same receptors that conferred licensing, and functionally incompetent (unlicensed) NK cells.     

Alloreactive natural killer cells in the rat: complex genetics of major histocompabitility complex control

A major role for the nonclassical major histocompatibility complex (MHC) class I region, i.e. RTI.C, in controlling rat natural killer (NK) cell alloreactivity has recently been established, and several findings suggested the existence of NK-triggering alloantigens coded for by this region. Here, we have extended our studies on the MHC control of NK cell cytotoxicity against concanavalin A-activated T cell blasts by comparing semi-syngeneic and fully allogeneic combinations, and we show the following: (a) The self MHC exerted a strong influence on the NK allorecognition repertoire. (b) When anti-F₁ hybrid cytolytic activities of parental strain NK cells were measured, both recessively and non-recessively inherited susceptibility patterns emerged. (c) In most combinations parental strain cells were lysed by F₁ hybrid NK cells, thus resembling the hybrid resistance phenomenon described in mice. The cytotoxicity was lower in strain combinations where NK susceptibility was inherited non-recessively, i.e. when parent anti-F₁ reactivity was detected, than in recessive combinations. (d) LEW.1LM1 (RT1^1m1) target cells, with a deletion in the RTI.C region that includes expressed class I genes, were more sensitive to lysis by MHC matched NK cells (PVG.IL(LEW), RT1¹) than were parental LEW (RT1¹) cells. The effect of the deletion was the opposite when MHC allogeneic (RT1^c, RT1^u) as well as semi-syngeneic (RT1^1/c) NK cells were employed, i.e. sensitivity was decreased. We conclude that certain MHC-encoded antigens, depending on the haplotype combination of effector and target cells, may either trigger or inhibit rat NK cell cytotoxicity. Furthermore, the potential role of peptides bound to MHC class I molecules recognized by NK cells is discussed.