Our previous studies have identified a B-cell subset that is induced under inflammatory conditions in T-cell receptor alpha knockout (TCRalphaKO) mice and contributes to the attenuation of colitis by producing interleukin (IL)-10. However, it is unclear whether IL-10-producing B cells directly or indirectly regulate inflammation.
Cytokine production of purified mesenteric lymph node (MLN) B cells was examined by flow cytometric analysis, enzyme-linked immunosorbent assay, quantitative polymerase chain reaction, and RNase protection assay. To investigate the functional role of IL-12p70 in the pathogenesis of colitis in TCRalphaKO mice, IL-12p35-deficient TCRalpha double knockout mice were generated.
In the absence of B cells or IL-10, IL-12p35 expression was significantly down-regulated in the MLN of TCRalphaKO mice. The expression of IL-12p35 was restored in the recipient B-cell-deficient TCRalpha double knockout (alphamicroDKO) mice by the transfer of B cells capable of producing IL-10. Notably, B cells predominantly produced IL-12p35 in the MLN through the help of IL-10-producing B cells. Functionally, IL-12 is involved in the regulation of the T-helper (Th) 2-mediated inflammation as indicated by the development of much more severe colitis in IL-12p35-deficient TCRalpha double knockout (alphap35DKO) mice compared with TCRalphaKO mice. In addition, transfer of MLN B cells from TCRalphaKO mice but not from alphap35DKO mice suppressed colitis in recipient alphamicroDKO mice.
These studies have identified a novel IL-12-producing regulatory B-cell subset that develops under Th2-mediated intestinal inflammatory conditions and in the presence of IL-10 and is involved in the regulation of intestinal inflammation.

Under steady-state conditions, internalization of self-antigens embodied in apoptotic cells by dendritic cells (DCs) resident in peripheral tissue followed by DC migration and presentation of self-peptides to T cells in secondary lymphoid organs are key steps for induction and maintenance of peripheral T-cell tolerance. We show here that, besides this traffic of apoptotic cells mediated by peripheral tissue-resident DCs, splenic marginal zone DCs rapidly ingest circulating apoptotic leukocytes, process apoptotic cell-derived peptides into major histocompatibility complex class II (MHC-II) molecules, and acquire CD8alpha during their mobilization to T-cell areas of splenic follicles. Because apoptotic cells activate complement and some complement factors are opsonins for phagocytosis and play roles in the maintenance of peripheral tolerance, we investigated the role of complement receptors (CRs) in relation to phagocytosis of apoptotic cells by DCs. Apoptotic cell uptake by marginal zone DCs was mediated in part via CR3 (CD11b/CD18) and, to a lesser extent, CR4 (CD11c/CD18) and was reduced significantly in vivo in hypocomplementemic animals. Following phagocytosis of apoptotic cells, DCs exhibited decreased levels of mRNA and secretion of the proinflammatory cytokines interleukin 1alpha (IL-1alpha), IL-1beta, IL-6, IL-12p70, and tumor necrosis factor alpha (TNF-alpha), without effect on the anti-inflammatory mediator transforming growth factor beta1 (TGF-beta1). This selective inhibitory effect was at least partially mediated through C3bi-CD11b/CD18 interaction. Characterization of apoptotic cell/DC interaction and its outcome provides insight into the mechanisms by which apoptotic cells affect DC function without disrupting peripheral tolerance.