Cross‐linking CD40 on B cells preferentially induces stress‐activated protein kinases rather than mitogen‐activated protein kinases

The B cell‐associated surface molecule CD40 plays a key role in T cell‐dependent B cell maturation, as individuals with defects in either CD40 or its ligand are impaired in immunoglobulin isotype class switching and germinal center formation. CD40 signaling activates downstream effectors, including the tyrosine protein kinase, Lyn, the phosphatidylinositol‐3‐kinase (PI‐3 kinase), and the transcription factor, NF‐kappa B. In this study, we demonstrate that stress‐activated protein kinases (SAPK) are activated after CD40 cross‐linking on various B cell lines or human tonsillar B cells. The activation is rapid and transient and is mediated through a cyclosporin A‐insensitive pathway. Furthermore, this signaling pathway appears not to rely on protein kinase C. While CD40 ligation strongly activates the SAPKs (up to 25‐fold), it does not affect members of the mitogen‐activated protein kinase family (MAPK; ERK1 and ERK2). Consistent with these data, CD40 signals up‐regulate c‐jun but not c‐fos mRNA and alter the transcription factor ATF2 but not the Raf‐1 protein. In summary, CD40 signaling preferentially induces SAPK but not MAPK.