NKRs were first described as surface receptors on NK cells that bind to specific HLA class I molecules (4). Upon binding to their respective ligands, the receptors transmit inhibitory or activating intracellular signals. Many of these inhibitory and activating receptors have been identified. NKG2D, NKG2A, and KIR3DL1 are three of ZD1839 in vivo the most prevalent NKRs and play important roles in a variety of cellular functions (5). NKG2D and NKG2A are both members of the C-type

lectin NKR family. NKG2D is a key member of an array of receptors that can activate or co-stimulate NK cells, while NKG2A recognizes non-classical HLA-E molecules and inhibits the function of NK cells (6–7). Meanwhile, KIR3DL1 is one of the KIRs from the immunoglobulin-like superfamily. This BKM120 receptor binds to HLA-B and HLA-A allotypes bearing the HLA-Bw4 serospecificity and delivers inhibitory signals (8). Since their discovery on NK cells, NKR expression has also been detected on T cells. Although both CD4+ and CD8+αβT cells can express NKRs, expression is much more common on CD8+αβ T cells (9–10). These NKRs have been shown to be functional. Certain NKRs are able to downmodulate cytotoxicity induced by TCR/CD3, and cross-linking of NKRs may inhibit cytolysis by CD8+ T cells (3). Additionally,

TCR-initiated stimulatory signals can be overridden by signals generated by inhibitory NKRs, preventing T cell cytokine release (11). In contrast, NKG2D is an activating receptor that is expressed on CD8+ T cells and some CD4+ T cells Dichloromethane dehalogenase (12). NKG2D is a potent costimulator of TCR-mediated functions that up-regulates antigen-specific, T cell-mediated cytotoxicity directed against cells or tissues expressing stress-induced NKG2D ligands, particularly under conditions of suboptimal TCR engagement (13–14). In addition, NKG2D on T cells can function

independently of the TCR (14). Only a few studies have been published on the expression of NKRs on T cells in HIV infection. One research group found that HIV-specific CTL isolated from infected patients were inhibited in their cytolytic activity against HIV-expressing autologous target cells as a consequence of the surface expression of iNKR. Furthermore, addition of anti-NKR mAbs restored CTL cytolytic activity (15). This finding strongly suggests that iNKRs are involved in the downregulation of HIV-specific CTL activity. Consistent with this, coexpression of multiple iNKRs on CD8+ T cell clones derived from HIV-infected patients has been observed (16). Another study observed low expression of inhibitory NKRs on CD8+ T cells in HIV-infected, long-term non-progressors, indicating that a lack of iNKR-mediated functional inhibition may provide an additional mechanism of efficient control of viral spread in LTNPs (17). Moreover, the expression of NKG2D on NK cells was lower in HIV-infected patients (18).