Role of IgM antibodies versus B cells in influenza virus‐specific immunity

We have compared the role of IgM antibodies with the role of B cells in control of primary influenza virus infection. Mice deficient in IgM (IgM‐/‐), but capable of producing other Igisotypes, exhibited increased pulmonary virus titers compared to wild‐type mice. However, IgM‐/‐ mice were less susceptible compared to B cell‐deficient (μMT) mice. CD4+ T cells from spleen and lung draining lymph nodes of infected μMT mice showed reduced proliferation upon virus re‐stimulation in vitro. Furthermore, numbers of IFN‐γ‐producing CD4+ effector T cells were reduced in the alveolar lavage (BAL) of μMT mice but not IgM‐/‐ mice. In contrast, total number of virus‐specific CTL was almost comparable in BAL of μMT and wild‐type mice. Pulmonary recruitment of inflammatory macrophages and neutrophils occurred normally in both μMT and IgM‐/‐ mice. Interestingly, virus‐specific IgG2a and IgG2b antibody responses were affected locally in the BAL and in the serum of IgM‐/‐ mice, while IgG1 responses remained largely normal. Taken together, our data suggest a role for B cells to promote effector T cell responses and a role of both IgM and IgG antibodies in the defense against acute influenza virus infection.