Synergistic B Cell Activation by CD40 and the B Cell Antigen Receptor

Abstract

Optimal activation of B-lymphocytes depends both upon expression of various cell surface receptors and ad-equate integration of signaling pathways. This requires signals generated upon recognition of antigen by the B lymphocyte antigen receptor (BCR) as well as additional signals provided by cognate interaction with T helper cells, including the CD40-CD154 interaction. Engagement of both the BCR and CD40 results in synergistic activation of B cells. Previous studies identified tumor necrosis fac-tor receptor-associated factor (TRAF)-2 and TRAF3 in the CD40-signaling pathway together with BCR-activated protein kinase D (PKD) as important cooperative factors in this synergy. To better understand the role of these factors in bridging the BCR and CD40 signaling path-ways, BCR signal regulation of TRAF function was exam-ined. Results show that phosphorylation of TRAF2 is in-creased upon BCR but not CD40 engagement and that of the potentially phosphorylated residues of TRAF2, tyro-sine 484 is crucial for BCR-CD40 synergy. Additionally, wild type or constitutively active Bruton's tyrosine kinase (Btk) enhanced, whereas the xid mutant form of Btk pre-vented, BCR-CD40 synergy. These effects were dependent upon TRAF2 and PKD activity. These findings suggest a model in which Btk contributes to the enhancement of the CD40 response by TRAF2 in a PKD-dependent manner.