IMMUNOBIOLOGY Tracking CD40 signaling during germinal center development

Substantial evidence indicates that signal-ing through the CD40 receptor (CD40) is required for germinal center (GC) and memory B-cell formation. However, it is not fully understood at which stages of B-cell development the CD40 pathway is activated in vivo. To address this ques-tion, we induced CD40 signaling in hu-man transformed GC B cells in vitro and identified a CD40 gene expression signa-ture by DNA microarray analysis. This signature was then investigated in the gene expression profiles of normal B cells and found in pre- and post-GC B cells (naive and memory) but, surpris-ingly, not in GC B cells. This finding was validated in lymphoid tissues by showing that the nuclear factor–B (NF-B) tran-scription factors, which translocate to the nucleus upon CD40 stimulation, are re-tained in the cytoplasm in most GC B cells, indicating the absence of CD40 signaling. Nevertheless, a subset of cen-trocytes and B cells in the subepithelium showed nuclear staining of multiple NF-B subunits, suggesting that a fraction of naive and memory B cells may be subject to CD40 signaling or to other signals that activate NF-B. Together, these results show that GC expansion occurs in the absence of CD40 signaling, which may act only in the initial and final stages of the GC reaction. (Blood. 2004;104