Dual regulation of BCR‐mediated growth inhibition signaling by CD72

CD72 has been reported to regulate BCR‐mediated signals both positively and negatively. SHP‐1 and Grb2 bind, respectively, to ITIM1 and ITIM2 of CD72. We generated transformed B cell lines with an immature phenotype following J2 virus infection of splenocytes from CD72–/– and wild‐type (Wt) mice. The transformed lines were infected with retroviral vectors carrying Tyr (Y) to Phe (F) substitutions in the ITIM sequences (ITIM1 mutated: Y7/F; ITIM2 mutated: Y39/F; and both ITIM mutated: Y7,39/F). Cross‐linking of the BCR induced growth inhibition in transfectants expressing Wt CD72, but this response was less sensitive in transfectants with Y7,39/F. The Y7/F transfectants demonstrated the least sensitive response. We were not able to obtain transfectants with Y39/F, suggesting that CD72 associated with SHP‐1, but not with Grb2, delivers a strong negative signal. Pre‐ligation of CD72, which induces dephosphorylation of the molecule, partially rescued the Wt transfectants from growth inhibition, leading to a growth response profile similar to that of Y7,39/F transfectants. These results suggest that ITIM1/SHP‐1 delivers a very strong negative signal that is down‐modulated by signals through ITIM2/Grb2, leading to delivery of an attenuated negative signal. Thus, pre‐ligation of CD72 results in the manifestation of an ostensible positive signal.