In MHC class I–deficient hosts, natural killer (NK) cells are hyporesponsive to cross-linking of activation receptors. Functional competence requires engagement of a self–major histocompatability complex (MHC) class I–specific inhibitory receptor, a process referred to as “licensing.” We previously suggested that licensing is developmentally determined in the bone marrow. In this study, we find that unlicensed mature MHC class I–deficient splenic NK cells show gain-of-function and acquire a licensed phenotype after adoptive transfer into wild-type (WT) hosts. Transferred NK cells produce WT levels of interferon-γ after engagement of multiple activation receptors, and degranulate at levels equivalent to WT NK cells upon coincubation with target cells. Only NK cells expressing an inhibitory Ly49 receptor specific for a cognate host MHC class I molecule show this gain-of-function. Therefore, these findings, which may be relevant to clinical bone marrow transplantation, suggest that neither exposure to MHC class I ligands during NK development in the BM nor endogenous MHC class I expression by NK cells themselves is absolutely required for licensing.

Dysregulated IL-23/IL-17 responses have been linked to psoriatic arthritis and other forms of spondyloarthritides (SpA). RORγt, the key Thelper17 (Th17) cell transcriptional regulator, is also expressed by subsets of innate-like T cells, including invariant natural killer T (iNKT) and γδ-T cells, but their contribution to SpA is still unclear. Here we describe the presence of particular RORγt+T-betloPLZF− iNKT and γδ-hi T cell subsets in healthy peripheral blood. RORγt+ iNKT and γδ-hi T cells show IL-23 mediated Th17-like immune responses and were clearly enriched within inflamed joints of SpA patients where they act as major IL-17 secretors. SpA derived iNKT and γδ-T cells showed unique and Th17-skewed phenotype and gene expression profiles. Strikingly, RORγt inhibition blocked γδ17 and iNKT17 cell function while selectively sparing IL-22+ subsets. Overall, our findings highlight a unique diversity of human RORγt+ T cells and underscore the potential of RORγt antagonism to modulate aberrant type 17 responses.

We have previously demonstrated that the NKR repertoire is profoundly disrupted by SHIP deficiency. This repertoire disruption is characterized by receptor dominance where inhibitory signals from 2B4 repress killing of complex targets expressing MHC class I and activating ligands. In this study, we examine the molecular basis of receptor dominance in SHIP−/− NK cells. In this study, we show that in SHIP−/− NK cells there is a pronounced bias toward the 2B4 long isoform. We have also characterized signaling molecules recruited to 2B4 in SHIP−/− NK cells. Interestingly, we find that ∼10- to 16-fold more Src homology region 2 domain-containing phosphatase 1 (SHP1) is recruited to 2B4 in SHIP−/− NK cells when compared with wild type. Consistent with SHP1 overrecruitment, treatment with sodium orthovanadate or a novel inhibitor with micromolar activity against SHP1 restores the ability of SHIP−/− NK cells to kill Rae1+ RMA and M157+ targets. These findings define the molecular basis for hyporesponsiveness by SHIP-deficient NK cells.

Inhibitory receptors that recognize MHC class I molecules regulate NK cell responses and self-tolerance. Recent evidence indicates that self-ligands not present in the MHC locus also can modulate NK function. In this study, we show that an inhibitory receptor that recognizes an MHC-independent ligand is over expressed in SHIP−/− mice at all stages of NK development and differentiation. Overexpression of this receptor compromises key cytolytic NK functions, including killing of allogeneic, tumor, and viral targets. These results further demonstrate the critical role that SHIP plays in regulation of the NK receptor repertoire and show that regulation of MHC-independent inhibitory receptors is crucial for NK recognition and cytolysis of complex targets.