Co-stimulation by anti-immunoglobulin is required for B cell activation by CD40Llow T cells

During cognate B : T interactions, B cells encounter antigen (Ag) through surface immuno-globulin (sIg) and present antigenic peptides to T helper (Th) cells. However, most in vitro systems used to study contact events involved in the delivery of T help for B cells circumvent the requirement for T cell Ag specificity by using anti-CD3/T cell receptor (TcR) monoclonal antibodies (mAb) to activate T cells. To study the role of sIg engagement in the responsiveness of B cells to T help, we pre-treated small resting B cells with soluble anti-χ mAb prior to contact with an activated Th1 clone. By reducing the concentration of anti-TcR mAb we obtained low levels of CD40 ligand (CD40L^low) on Th cells, comparable to those expressed by lymph node T cells activated in vitro (ex vivo T cells). In contrast to untreated B cells, which did not respond to CD40L^low Th, anti-Ig-treated B cells responded strongly. Low buoyant density B cells also responded to CD40L^low Th cells. There was no B cell response to resting Th cells. mAb against CD54/intercellular adhesion molecule-1 or major histocompatibility complex (MHC) class II completely inhibited B cell responses to CD40L^low Th1 cells, equivalent to the effects of blocking CD40 interactions. This contrasts with mAb blocking responses to CD40L^highTh, where CD40 effects predominate. Our data show that sIg engagement is necessary for the induction of B cell response to CD40L^low Th cells. Anti-CD3-activated ex vivo T cells that were also CD40L^low did not provide help to small resting B cells, but did induce responses from sIg-stimulated B cells. Thus, our data support a requirement for sIg signaling in physiological B cell activation, and further confirm previous work showing CD40 ligation to be necessary but not sufficient for delivery of T help to B cells.