B cells mediate multiple functions that influence immune and inflammatory responses. In this study, T cell-mediated inflammation was exaggerated in CD19-deficient (Cd19(-/-)) mice and wild-type mice depleted of CD20(+) B cells, whereas inflammation was substantially reduced in mice with hyperactive B cells as a result of CD19 overexpression (hCD19Tg). These inflammatory responses were negatively regulated by a unique CD1d(hi)CD5(+) B cell subset that was absent in Cd19(-/-) mice, represented only 1%-2% of spleen B220(+) cells in wild-type mice, but was expanded to approximately 10% of spleen B220(+) cells in hCD19Tg mice. Adoptive transfer of these CD1d(hi)CD5(+) B cells normalized inflammation in wild-type mice depleted of CD20(+) B cells and in Cd19(-/-) mice. Remarkably, IL-10 production was restricted to this CD1d(hi)CD5(+) B cell subset, with IL-10 production diminished in Cd19(-/-) mice, yet increased in hCD19Tg mice. Thereby, CD1d(hi)CD5(+) B cells represent a unique subset of potent regulatory B cells.

Anti-CD20 antibody immunotherapy effectively treats non-Hodgkin's lymphoma and autoimmune disease. However, the cellular and molecular pathways for B cell depletion remain undefined because human mechanistic studies are limited. Proposed mechanisms include antibody-, effector cell–, and complement-dependent cytotoxicity, the disruption of CD20 signaling pathways, and the induction of apoptosis. To identify the mechanisms for B cell depletion in vivo, a new mouse model for anti-CD20 immunotherapy was developed using a panel of twelve mouse anti–mouse CD20 monoclonal antibodies representing all four immunoglobulin G isotypes. Anti-CD20 antibodies rapidly depleted the vast majority of circulating and tissue B cells in an isotype-restricted manner that was completely dependent on effector cell Fc receptor expression. B cell depletion used both FcγRI- and FcγRIII-dependent pathways, whereas B cells were not eliminated in FcR common γ chain–deficient mice. Monocytes were the dominant effector cells for B cell depletion, with no demonstrable role for T or natural killer cells. Although most anti-CD20 antibodies activated complement in vitro, B cell depletion was completely effective in mice with genetic deficiencies in C3, C4, or C1q complement components. That the innate monocyte network depletes B cells through FcγR-dependent pathways during anti-CD20 immunotherapy has important clinical implications for anti-CD20 and other antibody-based therapies.

B-1a and B-1b lymphocytes were found to exhibit specialized roles in providing immunity to Streptococcus pneumoniae and differ dramatically in their developmental requirements. Transgenic mice overexpressing CD19 (hCD19Tg) generated B-1a cells and natural antibodies that provided protection during infection, while CD19-deficient (CD19(-/-)) mice lacked B-1a cells, lacked natural antibodies, and were more susceptible to infection. By contrast, pneumococcal polysaccharide (PPS) immunization protected CD19(-/-) mice during lethal challenge, whereas hCD19Tg mice remained unprotected. This resulted from differences in the B-1b subset: the key population found to produce protective PPS-specific antibody in both wild-type and CD19(-/-) mice. Thus, CD19(-/-) mice generated B-1b cells and protective adaptive PPS-specific antibody responses, whereas hCD19Tg mice lacked B-1b cells and adaptive PPS-specific antibody responses. This reciprocal contribution of B-1a and B-1b subsets to innate and acquired immunity reveals an unexpected division of labor within the B-1 compartment that is normally balanced by their coordinated development.