Cell to Induce X-Linked Immunodeficiency B and Synergizes with B Cell Receptor Signals Mitogen-Activated Protein Kinase Activation Positive Signaling Through CD72 Induces

CD72 is a 45-kDa B cell transmembrane glycoprotein that has been shown to be important for B cell activation. However, whether CD72 ligation induces B cell activation by delivering positive signals or sequestering negative signals away from B cell receptor (BCR) signals remains unclear. Here, by comparing the late signaling events associated with the mitogen-activated protein kinase pathway, we identified many similarities and some differenes between CD72 and BCR signaling. Thus, CD72 and BCR activated the extracellular signal-regulated kinase (ERK) and the c-Jun N-terminal kinase (JNK) but not p38 mitogen-activated protein kinase. Both CD72- and BCR-mediated ERK and JNK activation required protein kinase C activity, which was equally important for CD72- and BCR-induced B cell proliferation. However, CD72 induced stronger JNK activation compared with BCR. Sur-prisingly, the JNK activation induced by both BCR and CD72 is Btk independent. Although both CD72 and BCR induced Btk-dependent ERK activation, CD72-mediated proliferation is more resistent to blocking of ERK activity than that of BCR, as shown by the proliferation response of B cells treated with PD98059 and dibutyryl cAMP, agents that inhibit ERK activity. Most importantly, CD72 signaling compensated for defective BCR signaling in X-linked immunodeficiency B cells and partially restored the proliferation response of X-linked immunodeficiency B cells to anti-IgM ligation. These results suggest that CD72 signals B cells by inducing BCR-independent positive signaling pathways. The Journal of Immunology, 2001, 167: 1263–1273. ERK pathway, we show a differential requirement of ERK for CD72- and BCR-induced B cell proliferation. CD72 and BCR acted in synergy to induce proliferation of X-linked immunodeficiency (Xid) B cells, suggesting that they have independent roles in B cell activation.