“Self”-reflection by KIR

Natural killer (NK)–cell activity is regulated by the balance of interactions between several families of activating and inhibitory NK-cell receptors and their cognate ligands on target cells. Importantly, loss of expression of class I major histocompatibility complex (MHC; “missing self”) renders targets more susceptible to NK cell–mediated killing due to the loss of inhibitory signals via self-MHC–recognizing receptors.²  The process by which NK cells acquire functional competence (referred to as education or licensing) is not fully understood. It is generally agreed that the expression of NK-cell receptors correlates temporally with the acquisition of NK-cell function, that self-tolerance is mediated via inhibitory signals from self-MHC, and that interactions between self-MHC–recognizing receptors and self-MHC modulate the functional competence of NK cells.³  The belief that NK cells must express “at least one” inhibitory receptor to maintain self-tolerance was disproven with the identification of human and murine populations of NK cells lacking inhibitory receptors that were not autoreactive.^4,5  Such cells lack function either because they are developmentally immature and have not undergone licensing or because a lack of appropriate inhibitory signals results in a dampened response to stimuli (“disarming”) and a hyporesponsive phenotype.⁶  Understanding the mechanisms of NK-cell education might allow us to manipulate this process for therapeutic benefit for patients with cancer undergoing allogeneic transplantation.

The stochastic expression of killer cell immunoglobulin-like receptors (KIRs) has been explained by the product rule, which predicts that the probability of coexpression of 2 (or more) KIRs is the product of the individual expression frequencies for those KIRs, which are assumed to be independent values.⁷  Using specific monoclonal antibodies and multicolor flow cytometry, Andersson et al characterize the expression of inhibitory KIRs and NKG2A in 44 normal individuals to determine the influence of cognate HLA class I ligand and selection processes on KIR repertoire formation. In contrast to the frequencies predicted by the product rule, their data show that coexpression of multiple KIRs is more frequent than expected, with the expression of one or more KIRs increasing the probability of expression of subsequent KIRs. This demonstrates that KIR acquisition is a random, sequential event, with gradually increasing conditional probabilities of expression dependent on previous KIR acquisition. More importantly, the KIR expression frequencies are not affected by the presence or absence of self–class I MHC.

NKG2A, an inhibitory receptor from the lectin family that recognizes HLA-E, is also important in sensing loss of self and mediating tolerance. The authors find that coexpression of NKG2A with inhibitory KIR was less frequent than expected based on the product rule. These data suggest that, while self-MHC does not have a direct effect on NKG2A expression, selective pressures may be at play to increase the expression of NKG2A on cells that lack the self-MHC–recognizing KIR. Thus, NKG2A provides another inhibitory receptor by which those cells, which would otherwise remain uneducated, can interact with self-MHC and acquire function.

Taken together, the data by Anderssson et al suggest that the mechanisms by which self-MHC influences NK-cell tolerance and education do not involve repertoire selection (ie, alterations in the frequencies of cells expressing given receptors after encountering self), and that the expression of NKG2A can titrate the function of individual or overall populations of NK cells. It is important to note that this study tested NK cells at steady state in healthy donors. The implications of these data may not apply to NK cells developing under stress either during infection or after hematopoietic cell transplantation, after which KIR expression is known to be dysregulated^8,9  and where different mechanisms may control self-tolerance.⁹  Furthermore, other self-MHC–recognizing receptor families such as LIR may be involved in self-tolerance and education. An additional caveat is that, to avoid the confounding effects of activating KIR, only persons homozygous for the KIR A haplotype were tested. It is possible different results would be seen in two-thirds of the population with KIR B haplotypes. That said, this thorough study by Andersson et al provides convincing evidence that the KIR repertoire is formed by a process by which the probability of sequential expression of random KIR is dependent on prior acquisition of KIR. This suggests that transcriptional regulation of KIR, regulated by epigenetics and/or bidirectional promoters, may be influenced by a common KIR locus control element during development. Furthermore, the presence of cognate KIR ligand has no effect on this process, suggesting that “self”-reflection by KIR is more important in determining the responsiveness of individual NK cells.