IL-4 treatment of small splenic B cells induces costimulatory molecules B7-1 and B7-2.

IL-4 has been shown to be involved in the early stages of B cell maturation. Changes induced by IL-4 include cell enlargement, increased viability, and increased MHC class II expression. However IL-4 alone does not induce B cell activation as defined by proliferation, lymphokine production, or Ig class switching. In this study, we demonstrate that incubation with IL-4 enhances the ability of small splenic murine B cells, normally poor stimulators of murine Th1 clones, to stimulate lymphokine production and proliferation by Th1 clones. Moreover, small resting B cells induce anergy, whereas IL-4-treated B cells do not. IL-4-treated B cells were found to express both B7 (B7-1) and a second ligand for CTLA4Ig (B7-2). Although IL-4 induces both B7-1 and B7-2, the kinetics of expression of these molecules are different: B7-2 is detected by 6 h, whereas B7-1 is not detectable until 48 h. In addition, only CTLA4Ig fully blocks IL-4 induced costimulatory activity; a mAb to B7-1 does not. Thus, these results suggest that IL-4 may function indirectly as a costimulatory factor by inducing costimulatory molecules on resting B cells. Additionally, these findings support our previous findings that an alternative ligand for CD28 and CTLA4 is important in providing costimulation.