Development, phenotype, and function of non-conventional B cells.

Three populations of B cells develop in fetal life: B1, B2 and marginal zone B cells. B1 cells play important roles in innate immunity in contrast to B2 cells that perform the conventional roles in adaptive immunity. B1 cells were first identified in mice based on their expression of CD5 and spontaneous secretion of IgM. B1 cells were subsequently found to have unique developmental, phenotype, tissue distribution, and functional characteristics that differ from B2 cells. These phenotypic and functional differences allow B1 cells to play important roles in immunity and homeostasis, but be implicated in autoimmune disease. The presence of B1 cells has been examined in other species including humans, primates, rabbits, guinea pigs, cattle, pigs, and horses. Here we review the known developmental and functional differences between B1 and B2 cells, and discuss the evidence of the presence of homologous population of cells in different species.

Developmental expression of B cell molecules in equine lymphoid tissues.

Identification and classification of B cell subpopulations has been shown to be challenging and inconsistent among different species. Our study tested aspects of ontogeny, phenotype, tissue distribution, and function of equine CD5hi B cells, which represented a greater proportion of B cells early in development and in the peritoneal cavity. CD5hi and CD5lo B cells differentially expressed B cell markers (CD2, CD21, IgM) measured using flow cytometry, but similar mRNA expression of signature genes (DGKA, FGL2, PAX5, IGHM, IL10) measured using quantitative RT-PCR. Sequencing lambda light chain segments revealed that CD5hi B cells generated diverse immunoglobulin repertoires, and more frequently bound to fluorescence-labeled phosphorylcholine. This study shows developmental characteristics and tissue distribution of a newly described subpopulation of B cells in the horse.