We report herein that vesicular stomatitis virus (VSV) induced a concurrent primary Th1 (T helper 1) and Th2 cytokine response detectable ex vivo. Liposome-encapsulated clodronate-mediated elimination of CD8- marginal dendritic cells (DCs) and splenic macrophages (m Phi), but not CD8+ interdigitating DCs, prior to infection resulted in a markedly diminished chemokine and Th1 (IL-2, interferon-gamma) cytokine response, although the Th2 response (IL-4) remained relatively intact. Repopulation with marginal DCs and marginal metallophilic macrophages (MMM) restored Th1 cytokine profiles but did not restore chemokine responsiveness or reduce VSV-induced morbidity/mortality. Chemokine competency returned approximately 4 weeks post-depletion, which correlated temporally with repopulation of the spleen with marginal zone macrophages (MZM) and red pulp macrophages (RPM). Unexpectedly, virus-induced morbidity persisted for over 1 month post-depletion and was associated with virus dissemination and distinctive histological lesions in the liver. Depletion of interferon-producing plasmacytoid dendritic cells did not account for virus-induced morbidity because serum levels of type I interferon were not diminished in Cl2MBP-liposome-treated mice. Thus, distinct m Phi subsets are critical for chemokine production and viral clearance, and, in their absence, VSV disseminates even in the presence of high titers of interferon.

Acute macrophage (Mw) 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 Mw play in priming for a virus-speci®c T-cell-mediated immune response, DC and Mw were puri®ed 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 Mw or B cells, indicating that DC represent the critical antigen-presenting cell (APC) population in vivo for this virus. The failure of MDC/Mw-de®cient 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 VSVinduced chemokine response was dramatically suppressed in these mice. Thus, despite the expression of VSV peptide/class I and II complexes, IDC are not suf®cient to prime VSV Th cells in the absence of MDC and/or splenic Mw.

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.