IgA production in MHC class II-deficient mice is primarily a function of B-1a cells

Mice deficient in MHC class II expression (C2d mice) do not make antibody to protein antigens administered systemically, but their ability to produce IgA antibody to antigen administered at mucosal sites has not been described. We investigated IgA production by C2d mice and their IgA antibody response to antigen given orally. Young C2d mice had normal amounts of serum IgA, intestinal-secreted IgA and normal numbers of intestinal IgA plasma cells, compared to control C57BL/6 mice. IgA production by C2d mice increased with age. Following oral immunization with cholera toxin, C57BL/6 mice responded with IgA and IgG antibody, and had increased numbers of IgA plasma cells, but C2d mice gave no response. The Peyer's patch and mesenteric lymph node tissues of C2d mice contained very few CD4-expressing T cells. Thus, C2d mice have no typical mucosal CD4 Th cells and cannot respond to a strong oral immunogen, yet they still produced and secreted IgA. We hypothesized that B-1 lymphocytes could provide a source of IgA independent of antigen-specific T cell help. Young C2d mice had normal numbers of peritoneal B-1a cells and their frequency increased with age. To test the role of these B-1a cells, we bred C2d mice to obtain mice that had no MHC class II expression and expressed the Xid gene that confers deficiency in B-1a cells. These double-deficient mice had 10-fold less serum and secreted IgA than all other F2 littermates. We conclude that B-1a cells are essential for the majority of IgA production in C2d mice. Thus, the C2d mouse may provide a useful tool for analysis of the role of intestinal IgA provided by B-1a cells.