Antigen processing of vesicular stomatitis virus in situ. Interdigitating dendritic cells present viral antigens independent of marginal dendritic cells but fail to prime CD4(+) and CD8(+) T cells.

Acute macrophage (M phi) depletion, using a liposome-mediated 'suicide technique', markedly suppressed priming of splenic CD4(+) and CD8(+) T-cell responses to vesicular stomatitis virus (VSV). However, phagocytic marginal dendritic cells (MDC), but not interdigitating dendritic cells (IDC), are now known to be also depleted by this technique. To clarify the role splenic dendritic cell (DC) subsets and M phi play in priming for a virus-specific T-cell-mediated immune response, DC and M phi were purified from VSV-infected mice and assayed for the presence of epitopes recognized by VSV helper T (Th) cells and cytotoxic T lymphocytes (CTL). Antigen pulse experiments performed in situ demonstrated that VSV Th cell and CTL epitopes became transiently associated only with DC, but not M phi or B cells, indicating that DC represent the critical antigen-presenting cell (APC) population in vivo for this virus. The failure of MDC/M phi-deficient mice to become primed was not due to the complete elimination of antigen-presenting DC because VSV peptide/class I and II complexes were detected on IDC following lipsome-mediated elimination of phagocytic cells. However, the VSV-induced chemokine response was dramatically suppressed in these mice. Thus, despite the expression of VSV peptide/class I and II complexes, IDC are not sufficient to prime VSV Th cells in the absence of MDC and/or splenic M phi.