2 B-CELL SUBPOPULATIONS HAVE DISTINCT RECIRCULATION CHARACTERISTICS

This report describes two subpopulations of B cells in sheep. These su bpopulations have distinct recirculation characteristics and tissue di stributions. Phenotypically the populations are distinguished by their differential expression of the complement receptors, CD21 (CR2) and C D11b/CD18 (CR3). CD11b(+) B cells are surface (s)IgM(hi), co-express C D11c but are L-selectin negative. They populate the splenic marginal z one but are absent from splenic and ileal Peyer's patch (IPP) follicle s and both afferent and efferent lymph compartments. Fluorescent traci ng experiments showed that the CD11b(+) B cells are non-recirculating as they did not appear in lymph after intravenous inoculation but are restricted to the blood and spleen. The CD11b-negative population expr esses a conformational determinant of CD21 that is recognized by the m onoclonal antibody Du 2-74. These cells are sIgM(lo) and coexpress L-s electin. They populate the splenic and IPP follicles, are absent from the splenic marginal zone and are the only B cells in efferent lymph, efferent lymph and all lymph nodes. Fluorescence tracing experiments s howed that the CD21(+) B cells are recirculating cells with their entr y into efferent lymph being detectable by 16 h and peaking at 24-30 h. These data suggest that there are at least two lineages of B cells in the sheep with different phenotypic, functional and recirculation cha racteristics.