Scandiatransplant acceptable mismatch program (STAMP) a bridge to transplanting highly immunized patients.

BACKGROUND: Highly immunized patients are a challenge for organ transplantation programs. One way of increasing the likelihood of transplantation in this group of patients is to expand the possible donations by defining acceptable HLA mismatches. In the Scandiatransplant Acceptable Mismatch Program (STAMP), a de-centralized approach has been implemented in 2009. AIMS: The program has been improved during the years from utilizing HLA-A, -B, -DR matching only to include typing of all deceased donors for HLA-A, -B, -C, -DRB1 and -DQB1. The calculation of a transplantability score (TS) has been introduced in order to take both HLA and AB0 into consideration resulting in a more realistic picture of the transplantability chance. MATERIALS AND METHODS: Patients were selected for eligibility and results of immunisation status were prepared in each of the 9 tissue typing laboratories, while access to the program is finally governed by a common steering group of immunologists and clinicians. RESULTS: In the period from March 2009 until February 2015, 96 patients were transplanted within this program. The mean recipient age was 49 years and 57% were females, 30% of the patients were first transplants and of these 93% were females. The majority of the patients had 2-5 HLA-A, -B. -DR mismatches. The allograft survival at 60 months was 79.1%. Applying the TS to the cohort confirmed that patients with a low TS score had longer waiting times. CONCLUSION: The program has matured during the years and now proves to be a valid approach for transplanting highly immunized patients.

Natural killer cell subsets in man and rodents.

NK cells are important contributors to the early immune defence against infected or transformed cells. They are rapidly activated in response to cytokines, whereby they exert their effector functions. NK cell responses are controlled by a multitude of receptors, which are expressed by subpopulations of NK cells with distinct phenotypes and functionalities. Direct comparisons between species are often difficult because of differences in the expression of NK cell receptors and other markers. In addition, NK cells change their phenotype and effector functions during differentiation, by tissue-specific factors, or upon activation, complicating interpretations. We will here review the similarities and differences between the major NK cell subsets in man and two well-characterized rodent models.

The role of natural killer cells in resistance to the intracellular bacterium Listeria monocytogenes in rats.

We have investigated the influence of early innate immune resistance mechanisms on infection with the intracellular bacterium Listeria monocytogenes in rats. Rats were injected i.v. with various amounts of Listeria and the number of bacterial colonies in the spleen was determined at different time points after infection. A bacterial dose as low as 2 x 10(4) cells gave reproducible infection within the spleen. Athymic nude rats lacking normal T cells but with a robust NK cell repertoire for MHC antigens were more resistant to bacterial replication within the spleen than were normal littermate rats and eliminated the infection within 3 days. In vivo depletion of NK cells, or NK subpopulations expressing Ly49 receptors, increased the bacterial load in the spleen, indicating that these cells were important in the initial control of Listeria infection. An increased frequency of Ly49 expressing NK cells in Listeria-infected rats further supported this notion. As several rat strains, unlike mice, display a large repertoire of MHC-recognizing activating Ly49 receptors, these observations raise the interesting possibility that NK cells may recognize alterations in the MHC-I molecules on Listeria-infected cells leading to their elimination before the adaptive immune system comes into play.

Investigation of lipopolysaccharide receptor expression on human monocytes after major orthopaedic surgery.

BACKGROUND: The innate immune system is suppressed after major orthopaedic surgery, implicating a risk of septic complications. Whole-blood ex vivo testing with lipopolysaccharide (LPS) has shown a depression of the tumour necrosis factor alpha (TNF-alpha) production until 12 days postoperatively. As part of the innate immune system, the Toll-like receptors TLR2 and TLR4 recognize antigens from Gram-positive and Gram-negative bacteria, respectively. The receptors CD14 and CD11b are involved in the LPS receptor complex, whereas human lymphocyte antigen (HLA)-DR binds endotoxin peptides. It is still uncertain whether the expression of all these receptors changes after major surgery. METHODS: In 6 patients undergoing hip arthroplasty, we investigated three times the display of TLR4, TLR2, CD14, CD11b, and HLA-DR on monocytes by fluorescence-activated cell sorting and white blood cell counts during 12 days postoperatively. At the same time, the plasma levels of interleukin (IL)-1beta, IL-4, IL-6, IL-10, IL-13, and TNF-alpha were measured. RESULTS: There was no significant change in the expression of TLR4, CD14, CD11b, HLA-DR, and TLR2. Monocyte count and cytokine analysis did not differ from the ones pre-operatively taken. CONCLUSIONS: After aseptic orthopaedic surgery, there is no change in the display of the LPS receptor complex on monocytes. Other mechanisms have to be investigated to gain insight into the decrease of the TNF-alpha production capacity postoperatively.

Phenotype and natural killer cell sensitivity of a radiation-induced acute T-cell leukaemia (Roser leukaemia) in PVG rats.

A radiation-induced T-cell leukaemia [Roser leukaemia (RL)] in the rat was conditioned for growth in vitro by repeated in vivo-in vitro passages. This in vitro cell line, termed RL-T, maintained its leukaemia-inducing property when transferred to syngeneic PVG rats. It expresses several T-cell markers and the T-cell alpha/beta receptor-CD3 complex. RL-T, furthermore, expresses major histocompatibility complex (MHC) I antigens, both classical (RT1.A) and nonclassical (RT1.C), which makes it susceptible to killing by alloreactive natural killer cells in vitro.

The effect of in vivo depletion of NKR-P1+ or CD8+ lymphocytes on the acute rejection of allogeneic lymphocytes (ALC) in the rat.

We have depleted lymphocyte subsets in PVG and AO rats with MoAbs 3.2.3 (against NKR-P1 on NK and NK/T cells) and OX-8 (against CD8 on CTL and NK cells), and examined the effect on the killing of YAC-1 target cells in vitro and the effect on the acute rejection of small allogeneic lymphocytes in vivo (allogeneic lymphocyte cytotoxicity, ALC). While 3.2.3 treatment led to only a partial depletion of 3.2.3-positive cells in PVG rats, this treatment drastically reduced the number of NKR-P1+ cells in AO rats, abolished splenic NK activity against the NK-sensitive tumour target YAC-1, and markedly diminished the ALC response. Rats treated with OX-8 for 1 day showed a similar loss of NK cell function in vivo and in vitro. However, in rats treated with OX-8 for 3 days a 3.2.3+ and OX-8- population consisting of NK cells appeared, restoring ALC. The results demonstrate that NK cell responses can be greatly diminished after in vivo treatment with these MoAbs. Furthermore, they demonstrate that ALC is not necessarily linked to expression of the CD8 molecule.

Physiologic functions of activating natural killer (NK) complex-encoded receptors on NK cells.

Natural killer (NK) cells express a superfamily of surface proteins that share common structural features: dimeric type II integral membrane proteins with extracellular domains resembling C-type lectins. These receptors are encoded by a single genetic region called the NK complex (NKC). The NKC encompasses several families of genes including NKR-PI, Ly-49, CD94/NKG2, and NKG2D. Different NKC-encoded receptors have been shown to activate or to inhibit NK-cell function, and different receptors within the same family can have opposing functions. Within an individual NK cell, inhibitory receptors typically predominate over stimulatory receptors, calling into question the teleologic requirement or physiologic significance of lectin-like activating receptors in NK cells. Despite the widespread expression of inhibitory receptors, however, subtle features of activating receptor biology enable them to stimulate effector functions in vivo and in vitro. Activating receptors and inhibitory receptors differ in their subset expression, in their structural constraints for binding to common ligands, in their ligand repertoires, and in that divergent families of activating receptors utilize different signaling pathways. These subset, binding, repertoire, and signaling diversities may allow activating receptors to manifest their effects in spite of inhibitory receptor functions during pathologic conditions in vivo. In this review, we will present a detailed analysis of the data supporting this hypothesis with particular relevance toward physiologic NK-cell functions.

Rat natural killer cell receptor systems and recognition of MHC class I molecules.

Rat natural killer (NK) cells recognize MHC-I molecules encoded by both the classical (RT1-A) and non-classical (RT1-C/E/M) MHC class I (MHC-I) regions. We have identified a receptor, the STOK2 antigen, which belongs to the Ly-49 family of killer cell lectin-like receptors, and we have localized the gene encoding it to the rat natural killer cell gene complex. We have also shown that it inhibits NK cytotoxicity when recognizing its cognate MHC-I ligand RT1-A1c on a target cell. This is the first inhibitory Ly-49-MHC-I interaction identified in the rat and highlights the great similarity between rat and mouse Ly-49 receptors and their MHC ligands. However, the mode of rat NK-cell recognition of target cells indicates that positive recognition of allo-MHC determinants, especially those encoded by the RT1-C/E/M region, is a prevalent feature. NK cells recruited to the peritoneum as a consequence of alloimmunization display positive recognition of allodeterminants. In one case, NK cells activated in this way have been shown to be specific for the immunizing, non-classical class I molecule RT1-Eu. These findings show that allospecific NK cells sometimes show features reminiscent of the adaptive immune response.

Natural killing of xenogeneic cells mediated by the mouse Ly-49D receptor.

NK lymphocytes lyse certain xenogeneic cells without prior sensitization. The receptors by which NK cells recognize xenogeneic targets are largely uncharacterized but have been postulated to possess broad specificity against ubiquitous target ligands. However, previous studies suggest that mouse NK cells recognize xenogeneic targets in a strain-specific manner, implicating finely tuned, complex receptor systems in NK xenorecognition. We speculated that mouse Ly-49D, an activating NK receptor for the MHC I ligand, H2-Dd, might display public specificities for xenogeneic target structures. To test this hypothesis, we examined the lysis of xenogeneic targets by mouse Ly-49D transfectants of the rat NK cell line RNK-16 (RNK. Ly-49D). Of the xenogeneic tumor targets tested, RNK.Ly-49D, but not untransfected RNK-16, preferentially lysed tumor cells derived from Chinese hamsters and lymphoblast targets from rats. Ly-49D-dependent recognition of Chinese hamster cells was independent of target N-linked glycosylation. Mouse Ly-49D also specifically stimulated the natural killing of lymphoblast targets derived from wild-type and MHC-congenic rats of the RT1lv1 and RT1l haplotypes, but not of the RT1c, RT1u, RT1av1, or RT1n haplotypes. These studies demonstrate that Ly-49D can specifically mediate cytotoxicity against xenogeneic cells, and they suggest that Ly-49D may recognize xenogeneic MHC-encoded ligands.

Genes in two major histocompatibility complex class I regions control selection, phenotype, and function of a rat Ly-49 natural killer cell subset.

We have generated a monoclonal antibody (STOK2) which reacts with an inhibitory MHC receptor on a subset of alloreactive NK cells in the rat. This receptor, termed the STOK2 antigen (Ag), belongs to the Ly-49 family of lectin-like molecules and displays specificity for the classical MHC class I molecule RT1-A1c of PVG rats. Here, we have investigated the influence of the MHC on the selection, phenotype and function of the STOK2+ NK subset in a panel of MHC congenic and intra-MHC recombinant strains. STOK2 receptor density was influenced by the presence of its classical MHC I ligand RT1-A1c, as evidenced by a reduction of STOK2 Ag on the surface of NK cells from RT1-A1c+, as compared with RT1-A1c-, strains. In addition, a role for nonclassical MHC I RT1-C/E/M alleles in the selection of the STOK2 Ag was demonstrated. The relative number of STOK2+ NK cells was fivefold higher in rats expressing the RT1-C/E/M(av1) as compared with those expressing the RT1-C/E/M(u) class Ib haplotype. The STOK2 ligand RT1-A1c inhibited cytotoxicity of STOK2+ NK cells regardless of effector cell MHC haplotype. Allospecificity of STOK2+ NK cells varied markedly with effector cell MHC, however, and suggested that inhibitory MHC I receptors apart from STOK2 were variably co-expressed by these cells. These data provide evidence for the MHC-dependent regulation of the allospecific repertoire within a subset of potentially autoreactive Ly-49+ rat NK cells.

Identification of an inhibitory MHC receptor on alloreactive rat natural killer cells.

Studies of allogeneic lymphocyte cytotoxicity have shown that the rat NK allorecognition repertoire is controlled by genetic elements in both the MHC (RT1) and the NK gene complex (NKC). DA rats, possessing NK cells that are unable to lyse allogeneic lymphoblasts, were immunized with alloreactive NK cells from MHC-matched PVG.1AV1 rats, and two mAb, STOK1 and STOK2, were generated. STOK1 and STOK2 stained identical subsets of NKR-P1+ T and NK cells from certain strains of rats. Relative numbers varied markedly in a panel of MHC congenic strains, however, implicating a role for self MHC genes in their development. Both STOK1 and STOK2 immunoprecipitated a 110-kDa disulfide-linked homodimeric molecule, with extensive N-linked glycosylations, encoded by a gene that mapped to the NKC. NK cells expressing this glycoprotein displayed an increased ability to lyse allogeneic lymphoblasts, while syngeneic targets were spared. However, blockade of the STOK2 Ag with F(ab')2 of STOK2 permitted the NK lysis of syngeneic targets, but did not affect NK allorecognition. These results indicate that mAb STOK1 and STOK2 identify an NKC-encoded MHC receptor in the rat that acts as a negative regulator of cytotoxicity.

Cloning, functional activities and in vivo tissue distribution of rat NKR-P1+ TCR alpha beta + cells.

We have successfully cloned nine NKR-P1+ TCR alpha beta + cells from PVG rat spleens, utilizing murine macrophage inflammatory protein-1 alpha (MIP-1 alpha) and IL-2. These clones are either double negative (DN, CD4-CD8-), which included clones 3.31, 3.71, 4.19, 4.59 and 4.65, or single positive (SP, CD4+CD8-), which included clones 1.64, 3.8, 3.76 and 3.78. No CD8+ clone was recovered. All nine clones are restricted in terms of their expression of the V beta antigens, since they express V beta 8.2 but not V beta 8.5, V beta 10 or V beta 16. These clones are agranular and they fall to generate NK or LAK activity upon incubation with IL-2, IL-12 or their combination. On the basis of their production of intracellular cytokines they can be divided into three categories: (I) SP clones (1.64, 3.8, 3.76 and 3.78) do not produce IL-2 or IL-4, but produce IFN-gamma and IL-12, and they vary in their production of IL-1, RANTES or tumor necrosis factor (TNF)-alpha; (II) DN clones 4.59 and 4.65 produce IL-1 alpha and IFN-gamma only, and fall to produce other cytokines; and (III) DN clones 3.31, 3.71 and 4.19 produce IL-1 alpha, IL-1 beta, IL-2, IL-12, IFN-gamma, RANTES and TNF-alpha. From all the clones examined only DN clones 3.31 and to a lesser degree 4.19 produce IL-4. In vivo tissue localization of clones 3.8, 3.31 and 4.59 shows that these cells distribute into the liver and bone marrow 24 h post i.v. administration. Their accumulation in the liver and bone marrow along with their ability to secrete various cytokines suggest that these cells may influence the generation, differentiation or apoptosis of immune or hematopoietic cells.

Positive and negative MHC class I recognition by rat NK cells.

The prompt rejection of transplanted allogeneic lymphocytes by rat NK cells in non-sensitized recipients (allogeneic lymphocyte cytotoxicity or ALC) is determined by MHC genes as well as by genes located in the NK complex. The same genetic control is found when NK alloreactivity is measured by an in vitro assay, and we have employed this assay to delineate the specificity of NK cells for the MHC. The MHC of the rat, RT1, contains class I genes situated on either side of the class II/class III region. The majority of these class I genes are located in the RT1.C region and expressed class I products usually behave as non-classical (class Ib) molecules. They do not serve as restriction elements for the vast majority of conventional alpha/beta T-cells, in contrast to those class I molecules encoded by one or more loci in the classical (class Ia) region, RT1.A. However, NK cells appear to recognize the products of either class I region. Immunogenetic studies suggest that NK cells are inhibited by RT1. A molecules, whereas RT1.C region molecules may have a dual role in regulating NK cytolytic activity, i.e. they either inhibit or activate natural killing. Based on these premises, a model is proposed in which identification of a target as self or non-self depends on different receptors for class I in single NK cells, interpreting coincident positive and negative signals from the various target class I molecules. The putative role of peptides presented by class I, the biological implications, and the evolution of the NK receptors and their ligands are discussed.

Major histocompatibility complex class I-independent killing of xenogeneic targets by rat allospecific natural killer cells.

Major histocompatibility complex class I molecules can inhibit mouse as well as human natural killer (NK) cell cytotoxicity. In contrast, antigens encoded in the RT1.C region of the rat MHC gene complex have been suggested to trigger, rather than inhibit, rat NK cells. In an attempt to analyze rat NK cell specificity, with respect to the cross-species difference that may exist in NK cell-mediated cytotoxicity, we investigated the ability of interleukin 2-activated, allospecific rat NK cells to recognize MHC class I-positive and -deficient target cells of mouse and human origins. Recognition of xenogeneic target cells by rat allospecific NK cells was found to be MHC class I independent; target cell MHC class I was not required for killing, and expression of different sets of mouse and human MHC class I molecules did not influence the cytotoxic response. These results indicate that rat NK cells can recognize xenogeneic nontransformed cells by mechanisms not related to target cell MHC class I expression, and that mouse and human MHC class I molecules, at least among those tested in this study, are unable to confer inhibition of rat NK cells.

Preferential involvement of Go and Gz proteins in mediating rat natural killer cell lysis of allogeneic and tumor target cells.

IL-2-activated NK cells from PVG rats potently lyse target cells expressing allo-MHC class I determinants. Here, we investigated the role that G proteins play in mediating this activity. Pretreatment of NK cells with pertussis toxin (PT) or cholera toxin (CT) inhibited NK cell killing of tumor (YAC-1 or P815), and allogeneic target cells. ADP ribosylation assay revealed that PT ADP ribosylates a 39-kDa G protein, whereas CT ADP ribosylates a 45 to 47-kDa G protein in PVG NK cell membranes. Membranes prepared from intoxicated NK cells with either PT or CT lost their ability to incorporate [32P]NAD. These membranes possess Gi, Go, Gs, and Gz as demonstrated by immunoblot analysis. However, Gq was not clearly detected by this method. IL-2-activated NK cells were permeabilized with streptolysin O. Permeabilized cells incorporated Abs to Gi, Go, Gz, Gs, and Gq as determined by flow cytometric analysis. When Abs to Go or Gz, but not to Gi, Gs, or Gq, were incorporated inside permeabilized NK cells, a significant reduction in the lysis of tumor or allo-MHC target cells was observed, suggesting that Go and Gz play important roles in transducing the signals necessary to lyse target cells. Our results show for the first time a role for G proteins in mediating NK cell killing of allo-MHC-encoded target cells, and provide evidence for Gz protein involvement in NK cell recognition of target cells. The effect of Gz is novel and has not been previously described in any other system or cell type.

Genes in two MHC class I regions control recognition of a single rat NK cell allodeterminant.

We have previously presented evidence suggesting that the non-classical class I region of the rat MHC, RT1.C, encodes polymorphic molecules which induce the cytolytic activity of alloreactive NK cells. Those studies used target cells from a panel of MHC congenic rat strains possessing recombined portions of the RT1a, RT1l and RT1u MHC haplotypes. We have now examined in addition a set of rat strains bearing MHC haplotypes recombinant between RT1av1 and RT1c, and a more complex picture of the MHC control of rat NK alloreactivity has emerged. The expression of a major NK allodeterminant [the allogeneic lymphocyte cytotoxicity (ALC) determinant 2 or ALC-2, defined operationally using cold-target inhibition assays], appears to be under the control of both the RT1.C and the classical class I RT1.A region. Similarly, the alloreactive repertoires of NK cells from these recombinant strains are influenced by elements encoded within these two MHC class I regions. We present a model in which the classical class I autoantigen RT1.Ac exhibits dominant inhibition of NK cytotoxicity specific for the stimulatory determinant ALC-2 shared by the nonclassical class I molecules RT1.Cav1, RT1.Ca and RT1.Cc, and also prevents the deletion of NK cells of this specificity during their development.

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

A major role for the nonclassical major histocompatibility complex (MHC) class I region, i.e. RT1.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 (RT1lm1) target cells, with a deletion in the RT1.C region that includes expressed class I genes, were more sensitive to lysis by MHC matched NK cells (PVG.1L(LEW), RT1l) than were parental LEW (RT1l) cells. The effect of the deletion was the opposite when MHC allogeneic (RT1c, RT1u) as well as semi-syngeneic (RT1l/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.

Recruitment of alloreactive natural killer cells to the rat peritoneum by a transfected cell line secreting rat recombinant interleukin-2.

In order to obtain large numbers of natural killer (NK) cells from single rats for functional studies, we have devised a method for the generation of IL-2-activated NK cells in vivo. Rats were implanted intraperitoneally with cell-impermeable diffusion chambers (DC) containing cultures of transfected Chinese hamster ovary (CHO) cells secreting rat recombinant interleukin-2 (rIL-2). This resulted in a dramatic increase in the peritoneal exudate cell (PEC) number with a peak (300-1000 x 10(6)) 1 week after implantation. The majority were mononuclear cells of which a large proportion were CD3-NKR-P1+ NK cells, but with substantial numbers of macrophages (M phi) and CD3+8+NKR-P1+ T cells also. The NK activity against standard tumor target cells was high among PEC from six different inbred rat strains tested. However, the NK cell-mediated reactivity against concanavalin A (ConA)-activated T cell blasts from a panel of major histocompatibility complex (MHC) congenic strains differed widely. PEC from some strains (PVG, LOU/C, and AO) efficiently lysed all the MHC-disparate lymphoblasts. In other strains (BN and LEW) more restricted allorecognition repertoires were observed, whereas PEC from one strain (DA) were unresponsive. The secretion of rat rIL-2 intraperitoneally did not lead to a significant increase in the IL-2 level in the blood or in the total number or activity of NK cells in blood and spleen. The present method represents a most potent technique for generating large numbers of functional rat NK cells and shows the high efficiency with which IL-2 can induce NK cell recruitment in vivo.

Control of rat natural killer cell-mediated allorecognition by a major histocompatibility complex region encoding nonclassical class I antigens.

The ability of natural killer (NK) cells to eliminate normal allogeneic hemic cells is well established in several species including mice, rats, and humans. The controlling elements for NK susceptibility in these species map to the major histocompatibility complex (MHC), but in contrast to findings in mice and humans, the mode of inheritance is not always recessive in rats. This finding is not easily explained by the missing self and hemopoietic histocompatibility (Hh) models for NK recognition, and has led to the idea that certain alloantigens may trigger NK cell reactivity. In our in vitro system for assessing rat NK alloreactivity, we have employed target and inhibitor cells from a large panel of MHC congenic, intra-MHC recombinant and MHC mutant rat strains, as well as appropriate F1 hybrids between them, and we show the following: (a) The nonclassical class I (RT1.C) region was most important in determining the susceptibility of target cells to alloreactive NK cells in vitro. Lymphocyte susceptibility to lysis in vivo also mapped to the C region, which supports the concept that the in vivo and in vitro alloreactivity assays reflect the same recognition process. (b) Four different RT1-controlled NK allospecificities (represented by the u, l, a, and n haplotypes) could be discerned when we used polyclonal NK cells from the PVG (RT1c) strain as effector cells. Three of the target specificities recognized were controlled mainly by the RT1.C region. (c) The expression of RT1.C region-controlled parental strain NK allodeterminants could be demonstrated in F1 hybrids heterozygous for the C region alone and were therefore inherited nonrecessively. (d) Loss of an RT1.C region-controlled NK allospecificity could be shown with the MHC mutant LEW.1LM1 rat strain characterized by a genomic deletion of about 100 kb of the C region. Taken together, these observations have demonstrated a major importance of the nonclassical class I region, i.e., RT1.C, in controlling rat NK allorecognition, and have thereby assigned a hitherto undescribed immunological property to this region. Furthermore, some of the present data are consistent with the existence of polymorphic NK-triggering alloantigens that are coded for by the RT1.C region.