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 Ig isotypes, 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.