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Ligands binding to Toll-like receptor (TLR), interleukin 1 receptor (IL-1R), or IFN-γR1 are known to trigger MyD88-mediated signaling, which activates pro-inflammatory cytokine responses. Recently we reported that staphylococcal enterotoxins (SEA or SEB), which bind to MHC class II molecules on APCs and cross link T cell receptors, activate MyD88- mediated proinflammatory cytokine responses. We also reported that MyD88-/- mice were resistant to SE-induced toxic shock and had reduced levels of serum cytokines. In this study, we investigated whether MHC class II-SE interaction by itself is sufficient to activate MyD88 in MHC class II+ cells and induce downstream pro-inflammatory signaling and production of cytokines such as TNF-α and IL-1β. Here we report that human monocytes treated with SEA, SEB, or anti-MHC class II monoclonal antibodies up regulated MyD88 expression, induced activation of NF-kB, and increased expression of IL-1R1 accessory protein, TNF-α and IL-1β. MyD88 immunoprecipitated from cell extracts after SEB stimulation showed a greater proportion of MyD88 phosphorylation compared to unstimulated cells indicating that MyD88 was a component of intracellular signaling. MyD88 downstream proteins such as IRAK4 and TRAF6 were also up regulated in monocytes after SEB stimulation. In addition to monocytes, primary B cells up regulated MyD88 in response to SEA or SEB stimulation. Importantly, in contrast to primary B cells, MHC class II deficient T2 cells had no change of MyD88 after SEA or SEB stimulation, whereas MHC class II-independent activation of MyD88 was elicited by CpG or LPS. Collectively, these results demonstrate that MHC class II utilizes a MyD88-mediated signaling mechanism when in contact with ligands such as SEs to induce pro-inflammatory cytokines. [ABSTRACT FROM AUTHOR]

Staphylococcal enterotoxin (SE) -induced toxic shock is triggered by inflammatory cytokine signal amplification after SE binding to major histocompatibility complex class II molecules on antigen-presenting cells and T-cell receptors. Identifying host cellular elements contributing to this pro-inflammatory signal amplification is critical for developing a strategy for therapeutic intervention. Myeloid differentiation primary-response protein 88 (MyD88) is an intracellular signalling adaptor protein primarily known for mediating pro-inflammatory cytokine responses. We investigated the role of MyD88 in staphylococcal enterotoxin A (SEA) -treated cell cultures and mouse models of toxic shock. Our results demonstrated that elevated levels of tumour necrosis factor-α, interferon-γ, interleukin-1α/β (IL-1α/β), IL-2 and IL-6 production correlated with up-regulation of MyD88 after treatment of spleen cells and mice with SEA alone or in combination with lipopolysaccharide (LPS). The SEA-induced lethality was also observed in (LPS-independent)d-galactosamine-sensitized mice. While LPS potentiated SEA-induced cytokine responses,d-galactosamine treatment had no additive effect. Most importantly, our results demonstrated that MyD88−/− mice were resistant to SEA-induced toxic shock and had reduced pro-inflammatory cytokine responses. These results suggest that SEA-induced lethality is primarily dependent on MyD88. Our findings offer an important insight on potential therapeutic treatment of SEA-induced toxic shock targeting MyD88. [ABSTRACT FROM AUTHOR]

Cell contact by the plague bacterium Yersinia pestis initiates the injection of several virulence factors that target biochemical pathways critical for host clearance of bacteria. Despite this impairment of innate immunity, it is unclear whether antigen recognition by T cells is equally affected. We present evidence that human cytolytic T cells respond to Y. pestis virulence proteins presented by infected monocytes and dendritic cells. These T cell antigens consisted of a panel of proteins encoded by pCD1, a 70-kDa plasmid that harbors virulence factors and transport proteins of the cell contact—dependent, type III secretion system. Infected cells retained the ability to process and present tetanus toxoid to T cells, which indicates that responses to unrelated antigens were also maintained. Our results indicate that T cell immunity remains functional during Y. pestis infection, which thus suggests the potential benefits of therapeutic vaccination and strategies that emphasize the inclusion of cytotoxic T lymphocyte responses. [ABSTRACT FROM AUTHOR]

Using the natural killer (NK) cell-surface marker CD56 to study NK T cells in peripheral blood, we found that their frequency in mononuclear cells among healthy individuals was 1%–20% (average, 7.3%) and sporadically increased 4–5-fold within individuals over the course of 8 months. Infection of mononuclear cells in vitro with Venezuelan equine encephalitis virus replicon particles (VRPs) resulted in a significant increase in CD56+ T cells and in the expression of interferon-α, tumor necrosis factor (TNF)–α, and interferon-γ by CD56+ but not CD56− T cells. NK and CD56+ T cells expressed higher levels of Toll-like receptor (TLR)–3 and TLR4 after infection with VRPs, whereas only NK cells expressed inducible TNF-α and TLR2. Most of these effects were duplicated by activating mononuclear cells with double-stranded RNA. These expression patterns indicate that T cells coexpressing NK markers respond like NK cells to viral infection or double-stranded RNA, potentially fulfilling innate and adaptive immune functions. [ABSTRACT FROM AUTHOR]