Interleukin 15 (IL-15) is essential for NK cell survival and proliferation. Unlike other cytokines, which are soluble, IL-15 bound to the IL-15 receptor alpha chain (IL-15Ralpha) is presented in trans to cells bearing the IL-2 receptor beta and common gamma chains. As IL-15 transpresentation occurs in the context of an immunological synapse, it opens the possibility that signaling for NK cell proliferation may be subject to regulation by other receptor-ligand interactions. We examined the sensitivity of IL-15 transpresentation to NK inhibitory receptors that bind HLA class I molecules. Human cells expressing HLA class I ligands for inhibitory receptors KIR2DL1, KIR2DL2/3, or CD94-NKG2A were co-transfected with IL-15Ralpha. Proliferation of primary NK cells in response to transpresented IL-15 was reduced by engagement of either KIR2DL1 or KIR2DL2/3 by cognate HLA-C ligands. Inhibitory KIRHLA-C interactions did not reduce the proliferation induced by soluble IL-15. Therefore, transpresentation of IL-15 is subject to down-regulation by MHC class I-specific inhibitory receptors. Similarly, proliferation of the NKG2A+ cell line NKL induced by IL-15 transpresentation was inhibited by HLA-E. Co-engagement of inhibitory receptors, either KIR2DL1 or CD94-NKG2A, did not inhibit phosphorylation of Stat5 but inhibited selectively phosphorylation of Akt and S6 kinase. These findings demonstrate a novel mechanism to attenuate IL-15 dependent NK cell proliferation and suggest that inhibitory NK cell receptors contribute to NK cell homeostasis. To examine whether NK cells mediate ADCC toward Plasmodium-infected RBCs (iRBC), we have developed several sensitive and quantitative assays to measure release of hemoglobin, delivery of granzyme into iRBC, activation of caspase 3 in iRBC, and lysis of iRBC. To test the sensitivity of RBC to the ADCC activity of NK cells we first used polyclonal antibodies of rabbits that had been immunized with human RBCs. Primary human NK cells lysed RBCs coated with rabbit antibodies, indicating that NK cells may be able to lyse infected RBCs in the presence of specific antibodies. We are using this experimental system to test lysis of iRBC by NK cells in the presence of serum from individuals living in malaria endemic regions. Parallel experiments showed that NK cells do not exhibit natural cytotoxicity against iRBC. To examine whether primary NK cells mount an innate response to iRBC we are using multiple approaches, including time-lapse microscopy, and profiling transcriptional and secretion responses. Comparative analysis of NK cell contacts with uninfected or infected RBCs indicated weak interactions with infected RBCs. We found that mouse liver-resident NK cells are activated during the liver-stage of a primary Plasmodium yoelii 17XNL infection. We have also confirmed that liver-resident NK cells can mount antigen-specific memory responses in models of allergic contact dermatitis following adoptive transfer into nave mice. We have now a system in place to study whether liver-resident NK cells exposed to liver-stage Plasmodium infection generate a memory response that could protect nave mice following Plasmodium infection.
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