B1 lymphocytes may be subdivided by CD5 and CD11b/Mac1 expression into B1a, with the CD5 and CD11b/Mac1 phenotype, and B1b, which present as CD19hiCD5loCD11bhi. B1b cells share many surface and functional characteristics with marginal zone B cells but differ in distribution and B cell receptor (BCR) signalling pathways. They are normally concentrated in the peritoneum, pleural cavities, spleen and bone marrow and function as efficient phagocytes and antigen-presenting cells (APCs). While peritoneal B1b cells are relatively anergic, they may be activated by high cytokine levels, notably IL-10, IL-5 and IL-21, CD40 signalling and high doses of Toll-like receptor (TLR) ligands in the context of pathogen invasion; TLR ligation is also necessary. Their anti-inflammatory effects include: secretion of natural IgM by splenic and bone marrow B1b cell subsets as an early response to pathogen invasion; tissue homeostasis and enabling the immunologically silent clearance of neoplastic and apoptotic cells; inhibition of pro-inflammatory cytokines and increased production of TGF-β1, PGE2 and GcMAF by activated macrophages and dendritic cells; and, in the case of peritoneal B1 lymphocytes, acting as ultimate Breg precurors. Pro-inflammatory B1b properties may result from: abnormal trafficking; acting as APCs; and acting as a source of innate-response activator cells. Functional impairment or deficits in Bregs occur in multiple sclerosis, systemic lupus erythematosus and rheumatoid arthritis. Details are given of potential pathogenic roles of IgM and B1b lymphocytes in these autoimmune disorders and in deficit-schizophrenia, and how these changes relate to inflammatory and oxidative and nitrosative stress.
Keywords: Antigent presenting cells; Autoimmunity; B1 cells; Neuroprogression; Psychiatry; Schizophrenia.
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