Dual regulation of BCR‐mediated growth inhibition signaling by CD72

Abstract

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.

Novel Therapeutic Targets at the Platelet Vascular Interface

“…36,37 CD72 is a 45-kDa immunoreceptor tyrosine-based inhibitory motif (ITIM) domaincontaining type II transmembrane protein that has been shown to affect intracellular signaling in B cells through its ability to associate with the tyrosine phosphatase SHP-1 and the adaptor protein Grb2. 38,39 Plexin-B1 has a type-I PDZ ligand domain at its C terminus, enabling it to interact with cytosolic proteins that have 1 or more PDZ domains. In endothelial cells, plexin-B1 is coupled to Rho-and Aktdependent signaling pathways, 36,40 and there is some evidence that this is the case in platelets as well.…”

CD72 Down-Modulates BCR-Induced Signal Transduction and Diminishes Survival in Primary Mature B Lymphocytes

“…Single-cell suspensions from adult spleens were stained with different mixtures of fluorochrome-conjugated Abs (prepared at Stanford or obtained from BD Pharmingen) containing IgM (DS-1), IgD (11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22)(23)(24)(25)(26), B220 (RA3-6B2), CD5 (53-7.3), CD21 (7G6), CD23 (B3B4), CD16/32 (2.4G2), CD72 (K10.6), CD22 (Cy34.1), and CD19 (1D3). These stain sets also contained Cascade Blue-conjugated CD3⑀ (145-2C11), CD4 (GK1.5), CD8␣ (53-6.7), Gr-1 (RB6-8C5), and F4/80 Abs that are used to gate out irrelevant, non-B cells.…”

“…In contrast, Ogimoto et al (17) recently reported that CD72-deficient BAL-17 cells, a murine mature B lymphoma line, had impaired activation of ERK and JNK, but not p38, after anti-IgM stimulation, even though the CD72-deficient cells exhibited enhanced DNA synthesis. Similarly, Baba et al (12) recently indicated that an immature B cell line lacking CD72, although hyperproliferative to BCR stimulation, had less NF-B activity, less ERK activation, and no difference in p38 activation after BCR cross-linking with anti-IgM compared with its wild-type (WT) counterpart. These discrepancies may result from the different reagents (Ag vs anti-IgM) used to stimulate the BCR (10) or from the use of B cell lines at different developmental stages (immature vs mature) (18 -20).…”

“…Besides the ITIM, the cytoplasmic tail of CD72 also carries an ITIM-like motif that binds Grb-2 (7). The formation of the CD72/SHP-1/Grb-2 complex after BCR ligation is linked to a decrease in cellular proliferation in a retrovirus-transformed immature B cell line (12). The inhibitory effect of CD72 on BCR activation can also be seen in studies showing that both mature B cells and an immature B cell line lacking CD72 are hyperproliferative in response to anti-IgM stimulation (12,13).…”

“…The formation of the CD72/SHP-1/Grb-2 complex after BCR ligation is linked to a decrease in cellular proliferation in a retrovirus-transformed immature B cell line (12). The inhibitory effect of CD72 on BCR activation can also be seen in studies showing that both mature B cells and an immature B cell line lacking CD72 are hyperproliferative in response to anti-IgM stimulation (12,13). Furthermore, the ligand for CD72, CD100, can turn off the negative signaling mediated by CD72 in both mature B cells and the immature B cell line WEHI231 (14,15).…”