The effects of exogenous superantigens on human natural killer cells

This thesis examined the effects of two superantigens (sAgs), staphylococcal enterotoxin B (SEB) and Mycoplasma arthritidis mitogen (MAM) on human natural killer (NK) cells. Incubation of peripheral blood mononuclear cells (PBMC) with either sAg augmented NK cytotoxicity against NK-sensitive targets in a dose-dependent manner with the greatest augmentation occurring within the first 24 hours. Additionally, incubation of PBMC with SEB generated lymphokine-activated killer (LAK) activity. Although both sAgs consistently up-regulated NK activity in PBMC, MAM but not SEB increased lytic activity of sorted CD56$\sp+$ (i.e., NK) cells in a statistically significant manner (p $\leq$ 0.05). To understand which PBMC subpopulations were required for SEB-mediated killer induction of CD56$\sp+$ cells, cell mixing experiments were performed. When co-cultured with NK cells, purified T lymphocytes, but neither enriched monocytes nor the monocytic THP-1 cell line, supported the development of sAg-activated killer (SAK) activity. Further experiments revealed that in PBMC, generation of SAK activity was mediated by IL-2, IFN$\gamma$ and IL-12, and in co-cultures of NK and T cells, SAK induction was mediated by IL-2 and IFN$\gamma$. In addition, antibodies to major histocompatibility complex (MHC) class II HLA-D scR molecules blocked SAK induction in co-cultures of NK cells and T cells. When tested for their ability to present sAg, NK cells promoted SEB-mediated up-regulation of CD25 on T cells, and this presentation of SEB was dependent on MHC class II since depletion of class II-positive cells from the NK population prevented them from supporting SEB-induced T cell activation. NK cells also promoted cytokine production in response to sAg. Besides increasing the amount of IFN$\gamma$ in cultures of T cells and monocytes, NK cells produced IFN$\gamma$ in response to SEB by 72 h in the absence of other immune cells. Together, these data showed that exposure of PBMC to sAgs produced by certain bacteria and mycoplasma resulted in cytokine-dependent up-regulation of NK cytotoxicity, that NK cells presented sAg to T cells, and that NK cells produced IFN$\gamma$ in response to sAg