Semi-invariant natural killer T (iNKT) cells are critical components of the host immune response in peripheral tissues such as the lung, liver, and gut, and they play important roles in cancer, bacterial infections, autoimmunity, wound repair, and atherosclerosis. Tissue-resident iNKT cells exert their effects early in the developing immune response by rapidly producing a wide variety of cytokines and chemokines, and it was recently discovered that different tissues possess iNKT cell subsets that preferentially produce IFN-γ (NKT1), IL-4 (NKT2), or IL-17 (NKT17). Despite their critical role in the immune response, the mechanisms that regulate iNKT cell function in the periphery remain unclear. Signaling lymphocyte activation marker (SLAM) proteins are cell surface-expressed molecular switches that are expressed on all hematopoietic cells. The nine SLAM family receptors serve a variety of functions including promotion of cell-cell adhesion, regulation of cytokine production, co-stimulation, and inhibition. Importantly, SLAM family receptors are critical for the development of iNKT cells. Yet, numerous efforts to ascribe discrete roles of SLAM family receptors in iNKT cell function has proven difficult. We conducted a comprehensive analysis of SLAM family receptor co-expression on iNKT cell subsets in the lung, spleen, liver, and thymus and identified co-expression profiles that varied in a tissue and strain-dependent manner. Interestingly, we found that SLAM family receptor expression profiles varied among different iNKT cell subsets. In particular, we noted a close association of SLAMf6 expression with the NKT2 and NKT17 subsets in both the periphery and in the thymus. Further investigation using SLAMf6-deficient mice revealed a critical role for SLAMf6 in NKT2 and NKT17 subset development, and in iNKT IL-4 and IL-17 cytokine production in the periphery. This investigation also revealed that the SLAMf6high NKT2 and NKT17 subsets exhibited significantly higher proliferative capacity than the NKT1 subset and the NKT2 and NKT17 proliferation was dependent, in part, on SLAMf6 expression. Since Slam family genes are highly polymorphic, we next investigated whether these polymorphisms regulated iNKT function. We employed a B6.129 congenic mouse exhibiting impaired NKT cell function, in which a 6.6 Mbp 129/SvJ locus encompassing Slam genes was introgressed onto the C57BL/6 background. To test the hypothesis that Slam gene polymorphisms regulate iNKT cell function, we refined this genetic interval by generating B6.129 subcongenic lines and assessing iNKT cell function. Unexpectedly, we found that while Slam gene polymorphisms in this model do regulate iNKT cell function, the dominant regulator was in a 0.14 Mbp interval centromeric to the Slam genes. Further experimentation revealed that impaired iNKT cell development and function was associated with changes in the expression of Fcgr3 (Fc gamma receptor III) on iNKT cells, suggesting it as a novel candidate gene regulating iNKT cell function. Taken together, these data reveal for the first time a specific role for SLAMf6 on NKT2 and NKT17 subset development and function. In addition, these data identify Fcgr3 as a novel candidate gene that regulates iNKT cell subset development and cytokine production. Cumulatively, these data reveal the presence of discrete regulatory mechanisms at work in different iNKT subsets, a finding that has broad implications for our understanding of iNKT-cell mediated immunity.

Systemic lupus erythematosus (SLE) is an autoimmune disease characterized by autoantibody production, which can eventually lead to immune complex (IC)-mediated organ damage. Due to the stimulation of plasmacytoid dendritic cells (pDC) by nucleic acid-containing ICs (DNA- or RNA-IC), patients with SLE have an ongoing interferon (IFN)-α production. IFN-α induces a general activation of the immune system that may initiate or propagate an autoimmune process if not properly regulated. Previous studies have shown that natural killer (NK) cells potently enhance the IFN-α production by pDCs. In study I, the mechanisms behind the NK cell-mediated increased IFN-α production by RNA-IC-stimulated pDCs were investigated. ICs triggered CD56dim NK cells via FcγRIIIA to the secretion of cytokines (e.g. MIP-1β) that promoted IFN-α production. Additionally, an LFA-1-dependent cell-cell interaction between pDCs and NK cells strongly contributed to the increased production of IFN-α. In study II, the RNA-IC-induced regulation of surface molecules on pDCs and NK cells was investigated. The expression of CD319 and CD229, which are two SLAM family receptors genetically associated with SLE, was induced on pDCs and NK cells by RNA-IC. IFN-α-producing pDCs displayed an increased expression of CD319 and CD229, whereas pDCs from patients with SLE had a decreased expression of CD319. In study III, we serendipitously identified an SLE patient harboring autoantibodies to the NK cell receptor CD94/NKG2A. In study IV, sera from 203 patients with SLE were analyzed for autoantibodies to the CD94/NKG2A, CD94/NKG2C and NKG2D receptors. Seven patients harbored anti-CD94/NKG2A autoantibodies, and two of these patient’s autoantibodies also reacted with CD94/NKG2C. Anti-CD94/NKG2A and anti-CD94/NKG2C autoantibodies both interfered with the HLA-E-mediated regulation of NK cell cytotoxicity, and facilitated the elimination of target cells expressing these receptors. Furthermore, these autoantibodies were found in a group of severely diseased SLE patients and their titers closely followed disease activity. In conclusion, this thesis provides insights to molecular mechanisms whereby NK cells regulate the IFN-α production, it further links the SLAM receptors to SLE, and it describes novel autoantibodies to receptors regulating NK cell cytotoxicity. Together these findings strengthen the assumption that NK cells are involved in the pathogenesis of SLE.