CD72-mediated suppression of human naive B cell differentiation by down-regulating X-box binding protein 1

B cells can differentiate into antibody-secreting plasma cells, however the signals that control the entry into this pathway are not clearly understood. We have investigated the role of human CD72 in mature B cell differentiation. Human CD72 is preferentially expressed in naive B cells, but marginal levels of expression can be found in switched memory B cells. CD72 cross-linking promoted an increase in B cell activation and proliferation. Interestingly, expression of CD27, whose signal induces the differentiation of B cells into plasma cells, was down-modulated by CD72 stimulation. This CD72 signaling also induced tyrosine phosphorylation of various proteins such as Blk. Plasma cell differentiation and Ig syntheses were diminished by CD72 ligation in the presence of Staphylococcus aureus Cowan strain (SAC) plus IL-2 but not in the presence of CD40 signaling or CpG oligodeoxynucleotide. Our results show that CD72 signaling reduces the expression of X-box binding protein I in B cells stimulated with SAC plus IL-2, but the expression of PRDI-BF1 was unaffected. Taken together, these data demonstrate that CD72 is a key molecule in regulating mature B cell differentiation, particularly in preventing the differentiation of naive B cells into plasma cells, thus blocking the production of low-affinity antibodies.