Direct in vivo evidence of CD4 + T cell requirement for CTL response and memory via pMHC-I targeting and CD40L signaling

摘要

CD4 + T cell help contributes critically to DC-induced CD8 + CTL immunity. However, precisely how these three cell populations interact and how CD4 + T cell signals are delivered to CD8 + T cells in vivo have been unclear. In this study, we developed a novel, two-step approach, wherein CD4 + T cells and antigen-presenting DCs productively engaged one another in vivo in the absence of cognate CD8 + T cells, after which, we selectively depleted the previously engaged CD4 + T cells or DCs before allowing interactions of either population alone with na?ve CD8 + T cells. This protocol thus allows us to clearly document the importance of CD4 + T-li-censed DCs and DC-primed CD4 + T cells in CTL immunity. Here, we provide direct in vivo evidence that primed CD4 + T cells or licensed DCs can stimulate CTL response and memory, independent of DC-CD4 + T cell clusters. Our results suggest that primed CD4 + T cells with acquired pMHC-I from DCs represent crucial "immune intermediates" for rapid induction of CTL responses and for functional memory via CD40L signaling. Importantly, intravital, two-photon microscopy elegantly provide unequivocal in vivo evidence for direct CD4-CD8 T cell interactions via pMHC-I engagement. This study corroborates the coexistence of direct and indirect mechanisms of T cell help for a CTL response in noninflammatory situations. These data suggest a new "dynamic model of three-cell interactions" for CTL immunity derived from stimulation by dissociated, licensed DCs, primed CD4 + T cells, and DC-CD4 + T cell clusters and may have significant implications for auto-immunity and vaccine design.