| Summary: | Natural killer (NK) cells are a potent source of interferon (IFN)-gamma which is required for the protection and clearance of microbial pathogens and tumours. Interleukin (IL)-12 is a cytokine released by dendritic cells (DC), macrophages (M-Phi), and B cells during early pathogenic infection in response to TLR signalling. It is a key inducer of IFN-gamma production and is thought to execute this primarily via the stimulation of NK cells. The focus of this thesis was to elucidate the cells involved in regulating mouse NK cell IFN-gamma release using stimulation with exogenous or endogenously produced IL-12. Our results show that IL-12 is able to stimulate IFN-gamma elaboration primarily from NK cells in cultures of unfractionated splenocytes, yet it is unable to stimulate highly purified NK cells unless exogenous IL-18 is also added. Macrophages, dendritic cells, or NKT cells do not act as partners to co-stimulate NK cell IFN-gamma production. Rather, our work uncovers a necessary and sufficient role for B cells in providing stimulatory help for NK cell IFN-gamma release during IL-12 stimulation. We show that B cells provide secreted IL-18 and contact dependent factors which remain undetermined. We next used the TLR9 ligand CpG-ODN (unmethylated cytokine-guanine dinucleotides) in order to test the ability of B cells to stimulate NK cell IFN-gamma production in the presence of endogenously produced IL-12. While NK cells were again the primary source of IFN-gamma in unfractionated splenocyte cultures, their activation was suppressed rather than enhanced by B cells. This suppressive activity was attributed to a small subset of splenic B cells that express CD5 and secrete IL-10 when directly activated with CpG. IL-10 suppressed the production of IFN-gamma from NK cells by reducing the levels of IL-12 available for NK cell stimulation, and by also acting on the NK cells directly and blunting their release of IFN-gamma. Overall, the research presented herein elucidates a novel role for B cells as both positive and negative regulators of resting NK cell IFN-gamma production in mice.
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