Subsequently, mice were infected with LCMV (106 pfu). On day 8 and 11 after sellckchem infection CD4+ T cell function was analyzed in spleen. The transfer of purified LCMV-immune CD4+ T cells to CD8-depleted mice largely restored the relative and absolute number of LCMV-specific CD4+ T cells producing IFN�� at day 8 and 11 after infection (Fig. 1A,B). Furthermore, it increased the relative and absolute numbers of LCMV-specific CD4+ T cells producing TNF�� on day 11, but not on day 8 after infection (Fig. 1A,B). In analogy, the adoptive transfer of naive LCMV-GP61-specific CD4+ T cells (transgenic SMARTA [15] cells) to CD8-depleted BL/6 mice increased the number of IFN��-producing and to a lesser extent of TNF��-producing LCMV-specific CD4+ T cells on day 11 after LCMV infection (Fig. 1C).

Importantly, CD4+ T cell function after adoptive transfer to CD8-depleted BL/6 mice was comparable to that observed in CD8+ T cell competent BL/6 mice. Therefore, this experimental set-up allows us to analyze the role of CD4+ T cells in the induction of immunopathology at a physiological frequency in the absence of CD8+ T cells. Adoptive transfer of LCMV-immune CD4+ T cells to CD8-depleted BL/6 mice did not reduce the viral load in spleen or liver on day 11 after infection (Fig. 2A). In contrast, LCMV elimination was dependent on CD8+ T cells. Similarly, adoptive transfer of naive SMARTA cells did not influence viral elimination in spleen (data not shown). Figure 2 Virus control by CD8+ and CD4+ T cells.

Reduced neutralizing antibody titers after transfer of activated CD4+ T cells To test the consequence of adoptively transferred CD4+ T cells on antibody production, we measured LCMV-specific neutralizing antibodies under the same experimental conditions as described for Figure 1A and B. LCMV-specific neutralizing antibodies in BL/6 mice evolved late, 60�C80 days after infection, and were of low magnitude (Fig. 2B). In contrast, BL/6 mice depleted of CD8+ T cells produced LCMV-specific neutralizing antibodies earlier and at higher titers (Fig. 2B). Interestingly, the adoptive transfer of LCMV-immune CD4+ T cells to CD8-depleted BL/6 mice significantly reduced neutralizing antibody titers. Therefore, CD4+ T cells impede the production of LCMV-specific neutralizing antibodies.

Intact splenic T and B cell regions after transfer of activated CD4+ T cells Reduced production of LCMV-specific neutralizing antibodies has been linked to the destruction of splenic architecture [1], [16]. Therefore, we performed immunofluorescence microscopy analysis on histological slides 11 days after LCMV infection from spleens of BL/6, CD8-depleted BL/6 and CD8-depleted GSK-3 BL/6 mice receiving LCMV-immune CD4+ T cells (Fig. 3A). LCMV-infected BL/6 mice had disrupted white pulp structures, as shown by staining for CD3+ T cells and B220+ B cells.