Differential and Site Specific Impact of B Cells in the Protective Immune Response to Mycobacterium tuberculosis in the Mouse

Cell-mediated immune responses are known to be critical for control of mycobacterial infections whereas the role of B cells and humoral immunity is unclear. B cells can modulate immune responses by secretion of immunoglobulin, production of cytokines and antigen-presentation. To define the impact of B cells in the absence of secreted immunoglobulin, we analyzed the progression of Mycobacterium tuberculosis (Mtb) infection in mice that have B cells but which lack secretory immunoglobulin (AID^−/−µS^−/−mice). AID^−/−µS^−/− mice accumulated a population of activated B cells in the lungs when infected and were more susceptible to aerosol Mtb when compared to wild type (C57BL/6) mice or indeed mice that totally lack B cells. The enhanced susceptibility of AID^−/−µS^−/− mice was not associated with defective T cell activation or expression of a type 1 immune response. While delivery of normal serum to AID^−/−µS^−/− mice did not reverse susceptibility, susceptibility in the spleen was dependent upon the presence of B cells and susceptibility in the lungs of AID^−/−µS^−/−mice was associated with elevated expression of the cytokines IL-6, GM-CSF, IL-10 and molecules made by alternatively activated macrophages. Blocking of IL-10 signaling resulted in reversal of susceptibility in the spleens and lungs of AID^−/−µS^−/− mice. These data support the hypothesis that B cells can modulate immunity to Mtb in an organ specific manner via the modulation of cytokine production and macrophage activation.