Activation of T and natural killer (NK) cells leads to the tyrosine phosphorylation of pp36 and to its association with several signaling molecules, including phospholipase Cγ-1 and Grb2. Microsequencing of peptides derived from purified rat pp36 protein led to the cloning, in rat and man, of cDNA encoding a T- and NK cell–specific protein with several putative Src homology 2 domain–binding motifs. A rabbit antiserum directed against a peptide sequence from the cloned rat molecule recognized tyrosine phosphorylated pp36 from pervanadate-treated rat thymocytes. When expressed in 293T human fibroblast cells and tyrosine-phosphorylated, pp36 associated with phospholipase Cγ-1 and Grb2. Studies with GST–Grb2 fusion proteins demonstrated that the association was specific for the Src homology 2 domain of Grb-2. Molecular cloning of the gene encoding pp36 should facilitate studies examining the role of this adaptor protein in proximal signaling events during T and NK cell activation.

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 aUoantigens 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 ofMHC 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 (RT/.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 RT/-controlled NK allospecificities (represented by the u, l, a, and n haplotypes) could be discerned when we used polyclonal NK cells from the PVG (RTI') 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 RTI.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 NKtriggering alloantigens that are coded for by the RT1.C region.

A major subset of non-alloreactive NK cells in PVG strain rats is generally low in Ly49 receptors, but expresses the rat NKR-P1B PVG receptor (previously termed NKR-P1C). The NKR-P1B 1 NK subset is inhibited by a non-polymorphic target cell ligand, which we have shown here to be a C-type lectin-related molecule (Clr). Clr11 ligates two divergent NKR-P1B alleles as judged by an NFAT-driven reporter assay, and inhibits NK-cell cytotoxicity of NKR-P1B 1 NK cells. Clr11 also interacts with the prototypic NKR-P1A receptor and exerts a stimulatory influence on NK lysis. NKR-P1A and B are encoded by adjacent genes in the proximal part of the NK gene complex and show close sequence homology in their extracellular region. They diverge from another pair, NKR-P1F and -G, which is encoded by a second, distal Nkrp1 gene cluster. NKR-P1F and -G bind an overlapping panel of Clr ligands, but not Clr11. Rat Clr molecules appear to be constitutively expressed by hematopoietic cells; expression in tumor cell lines is more variable. The data show the existence of two phylogenetic groups of NKR-P1 molecules, which demonstrate conservation of ligand-binding properties independent of signaling function.Key words: Cytotoxicity . NK cells . Receptor . Rodent Introduction NK cells express several families of killer cell lectin-like receptors (KLR). These are type II transmembrane proteins encoded by the NK gene complex (NKC), which is located on chromosome 4 in the rat [1,2]. The NKR-P1 molecules (KLRB or CD161) were among the first KLR to be characterized, but their function long remained elusive [3][4][5]. Although the NKR-P1 family has expanded to include both activating and inhibitory members in rodents, it consists of only one member in several other mammalian species, including in human [6]. The nature of their ligands has been controversial [7,8], but it has recently been shown that members of another KLR-related receptor family, the C-type lectin-related (Clr) molecules [9], also termed C-type lectin 2D (CLEC2D), can act as ligands for certain NKR-P1 molecules [10,11]. The Clr gene family also shows considerable expansion in rodents and the Clr genes are interspersed with the Nkrp1 genes in the proximal (centromeric) part of the NKC. A recent analysis concluded that there are 11 Clr/Clec2d genes in the rat BN strain genome, which were numbered consecutively from proximal to distal. One of these (Clr8) is most likely only a gene fragment, which could have been excluded, but we will nevertheless adhere to the Clr numbering suggested by Hao et al. [6].In the mouse, the inhibitory NKR-P1B and -D receptors both bind Clrb (also termed Ocil). Clrb is constitutively expressed by hematopoietic cells and inhibits cytolytic activity of NKR-P1B/ D-expressing NK cells [10,11]. The activating NKR-P1F receptor has been shown to bind another Clr molecule, Clrg, by the use of a reporter assay and soluble recombinant proteins [11] functional consequence of this interaction remains obscure. In human, the NKR-P1A receptor binds a Clr orthologue term...