IgM(high)CD21(high) lymphocytes enriched in the splenic marginal zone generate effector cells more rapidly than the bulk of follicular B cells

Ag encounter will-recruit-Ag-specific cells from the pool of mature B lymph ocytes;in the spleen and activate them to perform effector functions: gener ation of Ah-forming cells(plasma cells) and presentation of Ag to T cells. We have compared the ability of mature follicular and marginal zone cells t o develop into effector B Cells. The generation of marginal zone B cells an d their localization in the marginal sinus area are T cell and CD40 ligand independent, suggesting that they do not represent a postgerminal center po pulation. Compared with mature recirculating follicular B cells; they expre ss several characteristics of previous antigenic experience, including high er levels of B7.1 (CD80) and B7.2 (CD86) when freshly isolated and followin g in vitro stimulation. After a brief 6- to 8-h in vitro stimulation with L PS or anti-CD40 Abs, marginal zone B cells become potent APCs. In addition, their ability to proliferate and differentiate into plasma cells in respon se to low doses of T-independent polyclonal stimuli (LPS) is far greater th an that of follicular:B cells. These findings indicate a functional heterog eneity within splenic mature B lymphocytes, with marginal zone B cells havi ng the capacity to generate effector cells in early stages of the immune re sponse against particulate Ags scavenged efficiently in this special anatom ical site.