Editorial: Role of HLA and KIR in viral infections

Abstract

The immune system continuously protects its host against pathogens. During viral infections, both innate and adaptive immune cells contribute to an effective immune response. Natural killer (NK) cells can respond quickly to eliminate pathogens and infected cells and suppress dissemination to other tissues. Subsequently, activation of virus-specific CD8 + cytotoxic T cells results in specific killing of infected cells and activation of virus-specific CD4 + T cells further supports the immune response. Human leukocyte antigens (HLA) play an essential role in activation of both NK cells and T cells. This research topic contains eight articles highlighting the latest insights into the various effects of HLA molecules on both NK cell and T-cell reactivity upon viral infection. Natural Killer cell activation is regulated by a variety of activating and inhibitory receptors, including killer-cell immunoglobulin-like receptors (KIRs). KIRs bind to HLA class I molecules, which are expressed on all nucleated cells. As HLA and KIR molecules are highly polymorphic, each individual expresses a unique set of these molecules. The wide range of combinations of HLA and KIR expression results in differences in binding strengths and variation in NK cell activation (1). In this research topic, a review by Walter and Ansari illuminates the associations of HLA and KIR polymorphisms with the outcome of experimental simian immunodeficiency virus infection in rhesus macaques, a model used to study human immunodeficiency virus (HIV) infection. Not only do these associations show which interactions contribute to disease resistance, they also pinpoint combinations that increase susceptibility to disease. Such associations of HLA and KIR with disease progression have also been found in hepatitis C virus (HCV) infection. In her review, Gardiner summarizes current insights into these associations, and further illustrates how NK cells modulate disease outcome in HCV infection. Furthermore, this review highlights the ongoing search for the NK cell subsets required for protective host responses. In addition to KIRs, NK cells express C-type lectin receptors, including NKG2C, which regulate activation via ligation to non-classical HLA-E molecules. Della Chiesa et al. summarize the role of activating KIRs and NKG2C in several virus infections, including human cytomegalovirus (HCMV) and HIV. In addition, they discuss the induction of memory-like NK cells, which show enhanced responses upon reinfection and may play a role in controlling recurrent or chronic infections. Expression of NKG2C has also been associated with expansion of NK cells during viral infection. This expansion of NKG2C + NK cells with expression of self-HLA class I-specific KIRs has been observed in HCMV infection (2). The force driving these NK cell expansions is largely unclear. In this collection, Beziat et al. investigate the importance of HLA class I expression levels in host defense using data derived from transporter associated with antigen processing (TAP)-deficient individuals who express less than 10% of normal HLA class I levels. They demonstrate that self-HLA class I molecules shape the KIR repertoire of NKG2C +