Increases in IgA+ B cells in Peyer's patches during milk-borne mouse mammary tumor virus infection are influenced by Toll-like receptor 4 and are completely dependent on the superantigen response

Mouse mammary tumor virus (MMTV) is a milk-borne betaretrovirus that has developed strategies to exploit and subvert the host immune system. Although mammary glands are the final target of infection, Peyer's patches (PP) are the entry site of the virus. Herein, we show that the infection induces increases in the number of PP IgA+ B cells and higher expression of the α circular transcript, which is a specific marker of the switch to IgA. In addition, IgA+ B-cell increases correlated with higher levels of cytokines related to IgA class switching, such as interleukin (IL)-5 and IL-6. Of interest, the increases in IgA+ B cells were lower in Toll-like receptor 4-deficient mice and were completely dependent on the presence of superantigen-reactive T cells. Our results point to a novel mechanism involved in MMTV infection and suggest that IgA+ B cells may play an important role in carrying the virus to the mammary glands.