This difference became more prominent at day 8 p.i. At this time point, viral titers in spleen, liver, and lungs were 100–1000-fold lower in immune serum-treated mice. Further experiments in CD8+ T-cell-depleted

recipients showed that accelerated virus clearance by immune serum transfer was only effective in the presence of CD8+ T cells. To provide direct evidence that the antiviral activity of the transferred immune serum was mediated by Abs, the experiments were repeated using protein-G-purified IgG Abs. As depicted in Fig. 6, viral titers in mice treated with purified IgG Abs from LCMV immune serum were significantly decreased compared to mice that received the same amounts of IgG from normal serum. Of note, purified IgG from immune serum lacked activity in virus neutralization assays in vitro up to a concentration of 100

μg/mL (data not shown). Hence, ABT-263 nonneutralizing IgG Abs from LCMV LDK378 cell line immune serum possessed antiviral activity in vivo. Virus-specific Abs have been demonstrated to improve antiviral T-cell priming through the formation of immune complexes that enhance antigen presentation [18-20]. We therefore compared the LCMV-specific CD8+ T-cell responses in B6 mice treated with normal or LCMV immune serum. Since viral load is known to inversely affect the magnitude of the LCMV-specific T-cell response [21], virus-specific T-cell responses were analyzed at day 6 p.i. At this time point, viral loads in both groups of mice differed only slightly. As shown in Fig. 7A, LCMV-specific

CD8+ T-cell reactivity as determined by intracellular IFN-γ staining did not differ between the two groups. The same conclusion was reached when NK-cell and LCMV-specific CTL activity was examined in 51Cr release assays (Fig. 7B). Thus, transfer of LCMV immune serum did neither enhance NK-cell reactivity nor the LCMV-specific CTL response in the recipient mice. The observation that the LCMV immune sera Protein kinase N1 used in our experiments predominantly contained Abs specific for LCMV NP prompted us to ask whether NP-specific Abs per se show anti-viral activity. To address this point, LCMV Docile infected B6 mice were treated 1 day after infection with LCMV NP specific mAbs and viral titers were determined at day 8 p.i. Indeed, treatment of mice with these Abs significantly decreased viral titers compared with controls (Fig. 8A). Viral titer reduction was most prominent in liver followed by that in the lungs and spleen. Importantly, reduction of viral titers was observed with two different LCMV NP specific mAbs of mouse (KL53, IgG2a) and rat (VL-4, IgG2b) origin. As expected, both NP-specific mAbs did not exhibit virus neutralizing activity (data not shown) confirming previous findings [13, 22, 23]. LCMV NP represents the most abundant internal viral protein present in both infected cells and virions.