Epistatic interactions between killer cell immunoglobulin-like receptors (KIRs) and their cognate HLA class I ligands have important implications for reproductive success antiviral immunity susceptibility to autoimmune conditions and cancer as well as for graft-versus-leukemia reactions in settings of allogeneic stem cell transplantation. function was impartial of expression of self-HLA class I ligands. Finally despite similarities in the stochastic regulation of KIRs by the bidirectional proximal promoter the specificity of inhibitory KIRs on CD8 T cells was often distinct from that of natural killer cells in the same individual. The results provide new insight into the formation BMS-536924 of KIR repertoires on human T cells. Introduction Epidemiologic studies have found that combinations of killer cell immunoglobulin-like receptors (KIRs) and their cognate HLA class I ligands influence the outcome of human pregnancy resistance to infections susceptibility to autoimmune diseases and cancer as well as the effects of hematopoietic stem cell transplantation.1-5 Although it is commonly perceived that such correlations indicate a role BMS-536924 for natural killer (NK) cells in these settings the scarcity of mechanistic insight at a cellular level raises the possibility that other KIR-expressing cell BMS-536924 types might also contribute to the effects observed. Indeed as recently shown the presence of a specific inhibitory KIR around the genetic level enhanced HLA class I-restricted antiviral CD8 T-cell immunity suggesting that KIR-expressing CD8 T cells might influence the outcome of chronic viral infections.6 KIRs represent a polygenic and polymorphic family of receptors.7 Inhibitory and activating KIRs signal via immunoreceptor tyrosine-based inhibition/activation motifs (ITIM/ITAM) respectively.8 NK cells are the main KIR-expressing immune cells.9 10 They express inhibitory KIRs in a variegated fashion to ensure a broad specificity and capacity to sense the presence of single HLA class I alleles.11 Although there is a minor influence of HLA class I around the expression of specific KIRs on NK cells 12 13 that seems more prominent in settings of viral infections 14 15 KIR expression is primarily determined by gene and promoter polymorphisms as well the production of antisense RNAs in a random process that is not subject to any positive or unfavorable selection.16 17 To preserve self-tolerance in the absence of repertoire selection NK cells are functionally tuned in an yet not fully understood educational process in which the strength of the interactions between inhibitory KIRs and their cognate HLA class I ligands is one important factor.18 Other than NK cells CD4 and CD8 T cells as well as γδ T BMS-536924 cells also express KIRs.19-24 With respect to VPREB1 CD8 T cells KIR expression starts to appear on effector memory CD8 cells and a substantial fraction of terminally differentiated effector CD8 T cells are KIR+.20 22 25 The function of KIRs on CD8 T cells has been studied to some extent. Although most studies have been performed with KIR+ CD8 T-cell clones it is clear that inhibitory KIRs can modulate T-cell receptor (TCR) signaling and dampen CD8 T-cell responses whereas activating KIRs can enhance functional T-cell responses.26 27 However little is known with respect to the role of cognate HLA class I molecules in shaping the KIR repertoire and function of CD8 T cells. Specifically it is not known whether HLA class I molecules have a KIR-dependent educational effect on human CD8 T-cell function as it has on NK cells. In the present study we performed a high-resolution analysis of KIR expression on human CD8 T cells with the use of newly developed FACS panels allowing also for the assessment of activating KIRs. We show that KIR expression on CD8 T cells is restricted to clonally expanded terminally differentiated CD8 T cells. These cells display a narrow KIR repertoire dominated by a single inhibitory or activating KIR. We show that this specificity of KIR expressed on CD8 T cells is usually random and often distinct compared with that expressed on NK cells within the same individual. Furthermore we have attempted to dissect potential underlying mechanisms behind the diverse KIR repertoires found on CD8 T cells and NK cells by evaluating the level of selection conferred by cognate HLA class I molecules as well as transcriptional regulation of KIRs on CD8 T cells and NK cells. Finally we show that KIR expression down-modulates the functional responses of CD8 T cells in an HLA-independent manner suggesting that human CD8 T cells are not subject to functional education by HLA class I molecules. Taken together our.