The in vivo balance between B cell clonal expansion and elimination is regulated by CD95 both on B cells and in their micro-environment

The expression of CD95 (Fas/APO-1) on B cells has been shown to play a direct role in their fate. B cells that chronically bind antigen due to prolonged antigen exposure, such as self-reactive B cells, are induced to express CD95 by CD40 ligand (CD40L) and are subsequently eliminated by CD95 ligand (CD95L) when they present antigen to CD4⁺ T cells. B cells that bind antigen acutely due to sudden antigen encounter, such as foreign antigen reactive B cells, up-regulate CD95, but are normally protected from CD95L- mediated apoptosis. Here, however, it is shown in vivo that foreign antigen- specific B cells fail to be protected from CD95-dependent elimination in a host that is CD95 deficient, regardless of antigenic challenge. These data indicate that B cell antigen receptor (BCR)-induced protection against CD95L- mediated apoptosis is not absolute but depends upon other micro-environmental factors in vivo. The normal balance between T cell-dependent humoral immunity and tolerance is thus regulated intrinsically by CD95 expression on responding B cells, and extrinsically by CD95-mediated control of CD95L or other molecules in the lymphoid micro-environment.