In addition to the robust responses of MHC Class I restricted CD8+ T cells, infection with L. monocytogenes also induces strong activation of MHC Class II restricted CD4+ T cells. Several MHC Class II restricted listerial epitopes, derived primarily from LLO (Safley et al. 1991; Sanderson et al. 1995; Campbell and Shastri 1998) and p60 (Geginat et al., 1998,1999, 2001), have been identified (see Table 11.1.). Typically, MHC Class II restricted antigens are acquired by APCs through phagocytosis of extracellular bacteria; a key component in the control of bacterial spread after intraveinous infection with L. monocytogenes.
However, the rapid LLO-mediated escape of bacteria into the cytosol would limit the accessibility of bacterial antigens to the MHC Class II presentation. Rather, it is likely that naïve CD4+ T cells are stimulated by L. monocytogenes antigens that are cross-presented on the surface of DCs (Skoberne and Geginat 2002).
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