Mice deficient in the interleukin-1 receptor (IL-1R2/2) are protected from weight loss after immunization with VSV vaccine vectors Mice immunized with recombinant VSV (rVSV) vaccine vectors lose weight rapidly after immunization. Weight loss is usually maximal by the second day after immunization but most animals recover, with weight returning to normal by five to seven days after immunization. Interleukin-1b (IL-1b) is a pro-inflammatory cytokine that causes acute weight loss and fever in mice and humans [16, 17, 18] , and which stimulates the production of other pro-inflammatory mediators such as IL-6 [17, 19] and prostaglandin E2 (PGE 2 ) [20, 21] . To determine whether IL-1 can contribute to the induction of acute pathology after administration of rVSV vaccine vectors, we immunized groups of C57BL/6 wild type (WT, n = 4) or IL-1 receptor deficient (IL-1R2/2, n = 3) mice with a single intramuscular injection of 5610 8 PFU live replication-competent rVSV. All animals survived the immunization, but wild type mice lost approximately 10% of their preimmunization body weight by the first day after immunization ( Figure 1 ) and did not fully regain their pre-immunization weight until eight days after immunization. In contrast, IL-1R2/2 mice had minimal pathology, losing only a small amount (,5%) of weight on the first day after immunization and returning to their starting weight by the second day after immunization. IL-1R2/2 mice lost significantly less weight than wild type control animals after challenge (P = 0.03 from day 1-day 4 after challenge, Mann-Whitney test). Because mice deficient in the IL-1R cannot respond to IL-1, this result demonstrated that either IL-1a or IL-1b, both of which bind to the IL-1R, contributed to acute pathology after rVSV immunization.