B cells as antigen-presenting cells: the requirement for B cell activation.

Abstract In this study we have extended our previous work on the capacity of B cells to act as antigen-presenting cells (APC) to investigate the requirements for activation and antigen concentration when antigen uptake is mediated by binding to surface immunoglobulin. We investigated the requirements for B cell activation for stimulation of an antigen-specific (rabbit IgG) T cell proliferative response when as antigen rabbit anti-mouse lgG-Fab’2 (RAMIG-Fab’2) was used, an antigen which is taken up polyclonally by virtue of its binding to B cell surface Ig. The results showed that B cells pulsed for 1 hr with RAMIG-Fab’2 were unable to stimulate the proliferation of normal rabbit IgG-Fab’2- (NRGG-Fab’2) primed T cells, whereas Iipo-polysaccharide- (LPS) activated B cells pulsed with RAMIG-Fab’2 for 1 hr were very effective stimulators of T cell proliferation. Because B cell Ig has been shown to be involved in activation of B cells for proliferation, the capacity of RAMIG-Fab’2 similarly to activate B cells to serve as APC was also investigated. Only after B cells were incubated in RAMIG-Fab’2 for over 8 hr were they able to stimulate detectable proliferation of NRGG-Fab’2-primed T cells with the maximum APC capacity requiring about 16-hr incubation in RAMIG-Fab’2. Thus, B cells appear to require activation to serve as APC when antigen is taken up by surface Ig, and at least in the case of RAMIG-Fab’2 the interaction of “antigen” with surface Ig can itself sufficiently activate B cells for the function of antigen presentation. It is of interest that the time of incubation with RAMIG-Fab’2 required for optimal activation for antigen presentation (16 hr) is an insufficient time to activate B cells for proliferation. The relative efficiency of antigen presentation by B cells when antigen was taken up via membrane Ig compared to antigen taken up nonspecifically was examined using LPS-B cells pulsed with various concentrations of RAMIG-Fab’2 (surface lg-mediated uptake) or NRGG-Fab’2 (non-Ig mediated uptake). The results showed that lg-mediated uptake of antigen allowed antigen presentation using less than 1 microg of RAMIG-Fab’2 to pulse LPS-B cells, while 2 to 6 mg of NRGG-Fab’2 was required to stimulate equivalent T cell proliferation responses, a difference in antigen concentration requirement of 104. In about 15% of the studies, B cells served as effective APC without additional activation by LPS or RAMIG-Fab’2. Percoll density gradient fractionation of such cells showed that the small dense B cells could not serve as APC for RAMIG-Fab’2 without additional activation, whereas B cells recovered in the less dense fractions were already at a stage of differentiation that permitted them to be effective APC without additional in vitro activation. Using this model system the results of these studies suggest that uptake of antigen by immunoglobulin receptors may perform two vital functions with respect to the capacity of B cells to present antigen. First, immunoglobulin receptors serve as a concentrating mechanism so that the small amounts of antigen to which cells are usually exposed in the course of immunization would be sufficient to enable antigen-specific B cells to serve as APC. Second, antigen may, if it acts similar to RAMIG, activate B cells to the state of differentiation required for them to serve as APC.