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International Immunology, Vol. 12, No. 10, 1417-1423, October 2000
© 2000 Japanese Society for Immunology

Negative regulation of B cell receptor-mediated signaling in B-1 cells through CD5 and Ly49 co-receptors via Lyn kinase activity

Hirofumi Ochi and Takeshi Watanabe

Department of Molecular Immunology, Medical Institute of Bioregulation, Kyushu University, Fukuoka 812-8582, Japan

Correspondence to: T. Watanabe


   Abstract
Top
 Abstract
Introduction
 References
 
CD5+ B-1 cells are known to be unresponsive to B cell receptor (BCR)-mediated growth signals but instead undergo apoptosis. However, the B-1 cells from Lyn kinase-deficient (Lyn–/–) mice exhibited an enhanced proliferative response upon BCR cross-linking. It has been reported that BCR-mediated signaling in B-1 cells is negatively regulated by signals from CD22, CD5 and CD72 co-receptors, and that Lyn kinase plays a crucial role in tyrosine phosphorylation of immunoreceptor tyrosine-based inhibitory motifs on the CD22 and CD72, which recruits SHP-1 to the BCR complex. We found that Lyn kinase is also essential for the tyrosine phosphorylation of CD5 and subsequent recruitment of SHP-1 in B-1 cells upon cross-linking of BCR. Moreover, a distinct subpopulation of B-1 cells was found to express cell surface Ly49, which is known as a MHC class I-binding negative regulatory receptor on NK cells. Ly49 was rapidly tyrosine phosphorylated upon cross-linking of BCR and SHP-1 was found to recruit to the phosphorylated Ly49. Addition of F(ab')2 fragments of anti-Ly49 antibodies partially blocked negative signals in B-1 cells. Thus two co-receptors, CD5 and Ly49, which are unique to B-1 cells, play a role in the regulation of B-1 cell activation. These results indicate that BCR-mediated signals in B-1 cells are strictly and negatively regulated through multiple pathways, that are dependent on Lyn kinase activity.

Keywords: autoimmunity, hyper-proliferation, immunoreceptor tyrosine-based inhibitory motif, phosphatase


   Introduction
Top
 Abstract
 Introduction
References
 
The development and immune response of B cells are controlled by the strength and quality of signals transmitted through the B cell receptor (BCR), and also by the balance of positive and negative signals delivered by additional cell surface and intracellular molecules. The balance of tyrosine phosphorylation induced by protein tyrosine kinases and dephosphorylation by protein tyrosine phosphatases, such as SH2-containing protein tyrosine phosphatase-1 (SHP-1) (1) and SH2-containing inositol 5-phosphatase (SHIP) (2), is known to be essential for this regulation.

BCR signal transduction can be abolished by cross-linking with Fc{gamma}RIIB, through the phosphorylation of Fc{gamma}RIIB on its immunoreceptor tyrosine-based inhibitory motif (ITIM) in the cytoplasmic domain and recruitment of SHIP (2). In addition, Lyn-deficient B cells are defective in Fc{gamma}RIIB-mediated suppression of BCR signaling (3). These results indicate a critical role for Lyn in the tyrosine phosphorylation of ITIM and SHIP-mediated suppression of BCR signaling. Similarly, it has been reported that CD22 (4), paired Ig-like receptor B (PIR-B) (5) and CD72 (6) negatively regulate BCR-mediated signal transduction through Lyn kinase-mediated tyrosine phosphorylation of the ITIM motif and recruitment of SHP-1. These observations indicate that, although Lyn kinase is first activated upon cross-linking of BCR and gives rise to the positive signals for B cell activation, it also negatively regulates BCR-mediated signaling through tyrosine phosphorylation of the ITIM motif on co-receptors such as Fc{gamma}RIIB, CD22, PIR-B and CD72.

B-1 cells are present primarily in the peritoneal cavity and pleural cavity, and numbers of B-1 cells are often elevated in autoimmune disease (7). B-1 cells are defined by the surface expression of CD5, a monomeric 67 kDa class I glycoprotein (8). In contrast to B-2 cells, CD5+ B-1 cells are unresponsive to BCR-mediated growth signals but instead undergo apoptosis upon BCR cross-linking (9–11). The B-1 cells from CD5–/– mice, on the other hand, proliferated and apoptotic cell death was blocked in response to BCR cross-linking, indicating that BCR-mediated signaling in B-1 cells is negatively regulated by the signals from CD5 (11). Furthermore, it has been reported in Jurkat T cells that CD5 is associated with SHP-1 and negatively regulates antigen receptor-mediated signaling (12).

The protein tyrosine phosphatase SHP-1 is highly expressed in hematopoietic cells (1). SHP-1-deficient mice [motheaten (me) or viable motheaten (mev)] have numerous immunological disorders (13). The B cell populations are markedly disproportionate; in the absence of CD5– conventional B-2 cells, B cells in me mice are predominantly B-1 cells. Furthermore, these B-1 cells are activated and produce autoantibodies. The phenotypes of me and mev mice suggest that SHP-1 negatively regulates BCR-mediated signaling in B-1 cells and is required for normal B-2 cell development.

In Lyn kinase-deficient (Lyn–/–) mice, tyrosine phosphorylation of various molecules, such as Vav, phospholipase C{gamma}2, PI3K, Cbl, Shc, HS1, CD22, CD72, PIR-B and Fc{gamma}RIIB, upon cross-linking of BCR is largely missing or decreased (4–6,14–16). Numbers of B-2 cells are decreased to ~50% in peripheral lymphoid tissues, but numbers of B-1 cells in peritoneal cavity are rather increased. Serum levels of all Ig isotypes are elevated, especially IgM and IgA, and include a high titer of autoantibodies. Splenomegaly due to massive proliferation of IgM-producing Mac1+ B-1 cells was evident. Splenic B cells showed an enhanced proliferative responses upon BCR cross-linking. Thus the B-1 cell phenotype in Lyn–/– mice is mirrored in me mice, suggesting that Lyn and SHP-1 may lie along common signaling pathways for down-regulation of BCR-mediated signals in B-1 cells. However, unlike me mice, which die shortly after birth and contain predominantly B-1 cells, Lyn–/– mice are viable and contain B-2 cells with increased numbers of B-1 cells (13–16). The less severe phenotype of Lyn–/– mice may suggest that Lyn and SHP-1 also regulate distinct signaling pathways.

B-1 cells also accumulate in CD22– and, to a lesser extent, in CD72– mice (17–21). Given that the ITIM of CD22 and CD72 are tyrosine phosphorylated by Lyn kinase and SHP-1 is recruited to the phosphorylated ITIM, it is likely that negative regulation by CD22 and CD72 is mediated by Lyn kinase and SHP-1. CD22 plays a role in regulating the BCR-mediated growth signaling, as CD22–/– B cells exhibit an enhanced proliferative response upon low-level BCR cross-linking (18). CD22 is expressed at a low density on immature B cells and fully expressed on mature B cells including B-1 cells (22). This increased expression of CD22 may serve to raise the threshold required for B cell triggering. CD72 is also a negative regulator of BCR-mediated signaling, but is expressed at a much higher level on immature B cells than at other stages (21). Thus, CD72 may play an important role in early B cell development. However, compared with Lyn–/– mice or me mice, the phenotype of CD22–/– or CD72–/– mice was less severe. These observations suggest synergistic effects by other co-receptors or signaling molecules that negatively regulate B-1 cell activation through Lyn kinase-mediated tyrosine phosphorylation of the ITIM motif and recruitment of SH2-containing phosphatases.

In the present study, we demonstrated that Lyn kinase has an essential and non-redundant role in regulating the ability of CD5 to recruit SHP-1 for the suppression of BCR-mediated signaling in B-1 cells. In addition, we found that a distinct subpopulation of B-1 cells expressed Ly49 on the surface where it may also play a role in the negative regulation of BCR signaling in B-1 cells.

In previous studies, we demonstrated that Lyn–/– mice had increased numbers of B-1 cells in the peritoneal cavity and that lymphoblast-like cells of the B-1 lineage accumulated in spleen (15,23). In order to determine whether the absence of Lyn kinase affects the ability of B-1 cells to respond to BCR cross-linking, purified splenic B-2 cells and peritoneal B-1 cells were stimulated in vitro with goat F(ab')2 anti-mouse IgM. As previously described, Lyn–/– B-2 cells responded vigorously to even a low dose of F(ab')2 anti-IgM, whereas wild-type B-2 cells responded poorly (3). In contrast to B-2 cells, wild-type B-1 cells did not proliferate upon cross-linking of BCR, but rather died by apoptosis (Fig. 1AGo) (9–11). Under the same conditions, Lyn–/– B-1 cells had a significant proliferative response, indicating that the BCR-mediated proliferative response in peritoneal B-1 cells is restored in the absence of Lyn kinase (Fig. 1AGo). Furthermore, proliferative response of Lyn–/– B-1 cells occurred even with low-level IgM cross-linking, similar to that of Lyn–/– B-2 cells. These results indicate that Lyn kinase acts as a negative regulator of BCR-mediated signaling not only in B-2 cells but also in B-1 cells.



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  		Fig. 1. (A) Proliferative response of B cells to BCR cross-linking. Peritoneal B-1 cells from C57BL/6 wild-type and Lyn–/– mice (backcrossed to C57BL/6, eighth generation) were prepared by negative selection using previously described methods (36). Splenic B-2 cells from wild-type and Lyn–/– mice were purified similarly. B cells were cultured with F(ab')2 goat anti-mouse IgM (1 or 5 µg/ml) (Southern Biotechnology Associates, Birmingham, AL). After 24 h of incubation, cells were pulsed for the last 6 h with [3H]thymidine. The mean [3H]thymidine incorporation and SD were calculated for triplicate cultures. (B) Proliferative response of B-1 cells to BCR cross-linking and CD5 pre-cross-linking. Purified B-1 cells or CD5 pre-cross-linked B-1 cells were stimulated with F(ab')2 goat anti-mouse IgM (5 µg/ml) (Southern Biotechnology Associates). For sequestration of CD5, B-1 cells were treated for 1 h with biotinylated anti-CD5 (clone 53.7) (50 µg/ml) plus 250 µg/ml avidin (Sigma, St Louis, MO) at 37°C in 5% CO2 before the addition of F(ab')2 goat anti-mouse IgM (5 µg/ml) (Southern Biotechnology Associates). The cells were cultured for 48 h and pulsed for the last 6 h with [3H]thymidine. The mean [3H]thymidine incorporation and SD were calculated for triplicate cultures.

 
It has been demonstrated that cross-linking of either BCR with anti-IgM antibody or CD5 with biotinylated anti-CD5 antibody and avidin did not induce a proliferative response in normal B-1 cells (11). However, when CD5 was cross-linked first, such that CD5 molecules could not interact with the BCR complex in the membrane, the subsequent addition of F(ab')2 anti-IgM antibody initiated a proliferative response in wild-type B-1 cells (Fig. 1BGo) (11). By contrast, the response in Lyn–/– B-1 cells after BCR cross-linking was unaffected by the same treatment. These data indicate that CD5 on B-1 cells contributes negatively to BCR-mediated signaling through pathways involving Lyn kinase.

We next examined the phosphorylated proteins in B-1 cells from wild-type C57BL/6 and Lyn–/– mice. Whole-cell lysates of freshly purified B-1 cells from the peritoneal cavity of wild-type and Lyn–/– mice were immunoblotted with phosphotyrosine-specific antibody. As shown in Fig. 2Go(A), many cellular proteins were phosphorylated in wild-type B-1 cells even without in vitro BCR stimulation. On the other hand, several cellular proteins were less phosphorylated in Lyn–/– B-1 cells. To our surprise, Syk kinase was tyrosine phosphorylated in Lyn–/– B-1 cells to almost the same level as in wild-type B-1 cells (Fig. 2BGo), indicating that Syk could be activated and give positive signals in B-1 cells even in the absence of Lyn kinase. The protein band at 65–68 kDa that was less tyrosine phosphorylated in Lyn–/– B-1 cells was identified as CD5 (Fig. 2CGo). Purified B-1 cells from Lyn–/– and wild-type mice were incubated with or without F(ab')2 anti-IgM for 3 min. Then the cells were lysed and CD5 was immunoprecipitated (Fig. 2CGo). The immunoprecipitates were assessed both by immunoblotting with antibody to phosphotyrosine and antibody to CD5. The CD5 molecules in wild-type B-1 cells were constitutively tyrosine phosphorylated and ligation of BCR increased phosphorylation by almost 3-fold. However, the CD5 was neither constitutively nor inducibly phosphorylated in B-1 cells from Lyn–/– mice. SHP-1 was co-precipitated with CD5 in untreated and BCR-cross-linked B-1 cells from wild-type mice, and the amount of CD5-associated SHP-1 correlated with the extent of phosphorylation of CD5. In contrast, in Lyn–/– B-1 cells, SHP-1 was neither constitutively nor inducibly associated with CD5. Thus, these results indicate that Lyn–/– B-1 cells are defective in tyrosine phosphorylation of CD5 and subsequent recruitment of SHP-1.



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  		Fig. 2. (A) Tyrosine phosphorylation of total cellular proteins in wild-type and Lyn–/– B-1 cells. Freshly purified peritoneal B-1 cells were lysed at 4°C for 30 min in 1% NP-40 lysis buffer with protease inhibitors. Whole-cell lysates were separated on 10% SDS–PAGE gels (40 µg/lane) and transferred to nitrocellulose membranes. Tyrosine-phosphorylated proteins were detected with anti-P-Tyr mAb, 4G10 (Upstate Biotechnology, Lake Placid, NY) and visualized by the ECL system (Amersham, Arlington Heights, IL). (B) Tyrosine phosphorylation of Syk kinase in B-1 cells. Whole-cell lysates of freshly purified peritoneal B-1 cells were precleared twice with 50 µl of Protein G–Sepharose beads to remove proteins that may bind non-specifically and then subjected to immunoprecipitation with anti-Syk antibody (Santa Cruz Biotechnology), followed by cross-linked Protein G–Sepharose beads. The immunoprecipitates were separated on 10% SDS–PAGE gels, transferred to nitrocellulose membrane and probed with anti-P-Tyr mAb, 4G10 (Upstate Biotechnology), and visualized by the ECL system (Amersham). The same membrane was stripped and reprobed with anti-Syk antibody (Upstate Biotechnology). The blots were visualized by incubating with horseradish peroxidase-conjugated anti-rabbit IgG and developed by the ECL system (Amersham). (C) CD5 is associated with SHP-1 and constitutively tyrosine phosphorylated in wild-type B-1 cells. Purified peritoneal B-1 cells were treated with media alone or with F(ab')2 goat anti-mouse IgM (20 µg/ml) (Southern Biotechnology) at 37°C for 3 min. Whole-cell lysates of treated cells were precleared and immunoprecipitated with biotinylated anti-CD5 (Southern Biotechnology), followed by cross-linking avidin–agarose beads. The immunoprecipitates were separated on 10% SDS–PAGE gels, transferred to nitrocellulose membrane and probed with anti-anti-P-Tyr mAb, 4G10 (Upstate Biotechnology). The same membrane was stripped and reprobed with anti-SHP-1 (Upstate Biotechnology) or anti-CD5 antibody as indicated. The blots were visualized by incubating with horseradish peroxidase-conjugated anti-rabbit IgG and developed by the ECL system (Amersham).

 
Concerning the proteins of 40–45 kDa that were less tyrosine phosphorylated in Lyn–/– B-1 cells than that in wild-type B-1 cells, we assumed that, among the co-receptors with the ITIM or immunoreceptor tyrosine-based activation motif in their cytoplasmic domain, they might be CD72 or Ly49. As shown in Fig. 3Go, the expression level of CD72 on B-1 cells (CD5+B220lo) was lower than that on B-2 cells (CD5–B220hi). There was, however, no difference in expression between B-1 cells of wild-type and Lyn–/– mice.



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  		Fig. 3. Expression of CD72 on B cells. To examine CD72 expression by B-1 and B-2 cells, peritoneal lavage cells from wild-type C57Bl/6 and Lyn–/– mice were stained with CyChrome-labeled rat anti-mouse CD45R/B220 antibody (PharMingen), phycoerythrin-labeled rat anti-mouse CD5 antibody (PharMingen) and biotin-labeled rat anti-mouse CD72 antibody (PharMingen), followed by FITC-labeled streptavidin. FACS profile of peritoneal B-1 (CD5+B220lo) and B-2 (CD5–B220hi) cells (A). CD72 expression by peritoneal B-2 cells (B) and B-1 cells (C). Dotted lines indicated the control fluorescence intensity.

 
CD72, a B cell co-receptor, contains an ITIM in its cytoplasmic domain, and has been shown to be phosphorylated by Lyn kinase and to associate with SHP-1 (6). CD72 is also a negative regulator of BCR-mediated signaling. Immature B cells express the highest level of CD72 compared to the other stages of B cells, suggesting that CD72 plays an important role in early B cell development (21). CD72–/– mice had decreased numbers of B-2 cells and slightly increased numbers of B-1 cells. These observations suggested that the negative regulatory role of CD72 in B-1 cells appeared to be subtle compared to that in B-2 cells.

There has been no report on the expression of Ly49 on B cells. We next examined Ly49 expression on peritoneal B cells of wild-type C57BL/6 mice. Peritoneal B cells were first stained with Ly49A-specific mAb (A1). As shown in Fig. 4Go(A), a distinct subpopulation of B-1 cells (CD5+B220lo) clearly expressed Ly49A on the surface, but peritoneal B-2 cells (CD5–B220hi) did not. Ly49 expression on B-1 cells was examined with various strains of mice by using several antibodies against Ly49 subspecificity. Although no expression of Ly49 was detected on peritoneal B-2 cells (data not shown), a distinct subpopulation expressing Ly49 was observed in B-1 cells from different strains of mice (Fig. 4BGo). Ly49A and Ly49A/D were clearly expressed on B-1 cells of C57BL/6 and Lyn–/– mice. On the contrary, Ly49C/I was expressed at a high intensity and a high percentage on B-1 cells from BALB/c mice. These Ly49 expression patterns on B-1 cells resemble those of NK cells from each strain of mice, although the staining intensity and the percentage of Ly49+ B-1 cells were somewhat lower than on NK cells (24). These observations raised the possibility that MHC class I molecules may affect expression of the Ly49 receptors on B-1 cells similarly to that on NK cells (25–31). Further study is, however, necessary for the clarification of this notion.



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  		Fig. 4. Distribution of Ly49 receptors on peritoneal B-1 cells. Peritoneal lavage cells from various strains of mice were evaluated by three-color analysis using cells stained with appropriated anti-mouse Ly49 antibody (PharMingen) and CyChrome-labeled anti-mouse CD45R/B220 antibody (PharMingen) and phycoerythrin-labeled anti-mouse CD5 antibody (PharMingen). The Fc receptors on cell surface were blocked by incubation with anti-FcRII/III (2.4G2) mAb (PharMingen) at a final concentration of 10 µg/ml for 20 min before incubation with mAb of interest. (A) Peritoneal B cells from wild-type C57Bl/6 mice were stained with Ly49A-specific mAb (A1). Histograms of peritoneal B-1 cells (gated on CD5+B220lo) and peritoneal B-2 cells (gated on CD5–B220hi) are shown. (B) Histograms of peritoneal B-1 cells from various strains of mice. Ly49 receptors were enumerated with the following antibodies: Ly49A (A1), Ly49A/D (12A8) and Ly49C/I (5E6). The data shown are representative of three or more experiments.

 
Peritoneal B-1 cells from wild-type C57BL/6 and Lyn–/– mice of the H-2b haplotype were stimulated with pervanadate. Cell lysates from stimulated and unstimulated cells were precipitated with anti-Ly49C/I antibody (5E6), and immunoprecipitates were blotted with phosphotyrosine-specific antibody. Immunnoprecipitation with mAb 5E6 demonstrated that Ly49C/I was phosphorylated even in the absence of Lyn kinase (Fig. 2DGo). However, Ly49C/I was weakly (one-third) phosphorylated in Lyn–/– B-1 cells in contrast to wild-type B-1 cells from C57BL/6 mice. Since the expression level of Ly49 on wild-type and Lyn–/– mice was not significantly different, the reduced tyrosine phosphorylation suggested that Lyn might play a role in tyrosine phosphorylation of Ly49 in B-1 cells. Residual phosphorylation observed in Lyn–/– B-1 cells may have been induced by other Src family protein tyrosine kinases such as Lck (34). To examine the association between Ly49 and BCR complex in B-1 cells, we next stimulated wild-type B-1 cells from C57BL/6 mice with F(ab')2 anti-IgM antibody for 0–5 min at 37°C (Fig. 5BGo). After stimulation, Ly49 was precipitated from cell lysates by anti-Ly49C/I antibody, transferred to a nitrocellulose membrane and probed with antibody to phosphotyrosine. The tyrosine phosphorylation of Ly49 molecules was rapidly increased within 1 min and returned to almost baseline level after 5 min of stimulation. Furthermore, SHP-1 was co-precipitated with phosphorylated Ly49 at 1 min after BCR cross-linking. These results indicate that Ly49 is functionally associated with the BCR complex in B-1 cells and gets tyrosine phosphorylated upon cross-linking of BCR. Furthermore, the phosphorylated Ly49 recruits SHP-1.



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  		Fig. 5. (A) Tyrosine phosphorylation of Ly49C/I in B-1 cells. Purified peritoneal B-1 cells were incubated with or without 0.03% H2O2 and 0.1 mM sodium orthovanadate (pervanadate) for 10 min at 37°C and disrupted in 1% NP-40 lysis buffer with protease inhibitors. Lysates were immunoprecipitated overnight at 4°C with biotinylated anti-Ly49C/I mAb 5E6 (PharMingen, San Diego, CA), followed by cross-linked avidin–agarose beads. The immunoprecipitates were separated on 10% SDS–PAGE gels, transferred to nitrocellulose membranes and probed with anti-P-Tyr mAb, 4G10 (Upstate Biotechnology), and visualized by the ECL system (Amersham). (B) Tyrosine phosphorylation of Ly49C/I after cross-linking of BCR in B-1 cells. Purified peritoneal B-1 cells were stimulated with F(ab')2 goat anti-mouse IgM (20 µg/ml) (Southern Biotechnology Associates) at 37°C for different periods of time. Whole-cell lysates of treated cells were precleared and immunoprecipitated with biotinylated anti-Ly49C/I mAb 5E6 (PharMingen), followed by cross-linked avidin–agarose beads. The immunoprecipitates were separated on 10% SDS–PAGE gels, transferred to nitrocellulose membrane and probed with anti-anti-P-Tyr mAb, 4G10 (Upstate Biotechnology). The same membrane was stripped and reprobed with anti-SHP-1 (Upstate Biotechnology). The blots were visualized by incubating with horseradish peroxidase-conjugated anti-rabbit IgG and developed by ECL system (Amersham). (C) F(ab')2 anti-Ly49C/I (5E6) mAb restored the IgM-induced proliferation in peritoneal B-1 cells. Freshly purified peritoneal B-1 cells from wild-type C57Bl/6 mice were stimulated with different concentrations of F(ab')2 goat anti-mouse IgM (Southern Biotechnology). The cells were cultured for 48 h and pulsed for the last 6 h with [3H]thymidine. Where indicated, peritoneal B-1 cells had been preincubated with F(ab')2 anti-Ly49C/I (5E6) mAb at a final concentration of 200 µg/ml. The mean [3H]thymidine incorporation and SD were calculated for triplicate cultures.

 
The Ly49 gene family encodes MHC class I binding inhibitory receptors on NK cells that function through the tyrosine phosphorylation of ITIM in the cytoplasmic domain and recruitment of SHP-1 (29–33). A crucial role of SHP-1 in the inhibitory function of Ly49 was demonstrated by the studies of NK cell activity in me mice, in which the Ly49-mediated inhibition was compromised (33).

Finally, the functional activity of Ly49 expressed on B-1 cells was examined. Recent studies indicate that addition of F(ab')2 antibody to Ly49 could block the receptor-mediated recognition of MHC class I complexes on target cells and induce NK cell cytotoxic function in vitro as well as in vivo (35). It has been also shown that the ligation of BCR on B-1 cells with anti-IgM antibody induced growth arrest or apoptosis (9–11). When F(ab')2 antibody to Ly49C/I antibody was included with anti-IgM antibody, so as to block the interaction between Ly49C/I and self-MHC class I molecules, BCR-induced growth arrest/apoptosis was partially attenuated in B-1 cells from wild-type C57BL/6 mice (Fig. 5Go). The same antibody to Ly49C/I alone had little or no effect on B-1 cell proliferation. Thus, Ly49 might negatively regulate BCR signaling in B-1 cells by binding to self-MHC class I molecules. Taken together, the present results provide evidence to support a negative regulatory function of Ly49 in BCR-mediated growth signals by recruiting SHP-1 into the BCR complex in B-1 cells, in which Lyn kinase-dependent phosphorylation might be involved.


   Abbreviations
 
BCR B cell receptor
ITIM immunoreceptor tyrosine-based inhibitory motif
me motheaten
mev viable motheaten
PIR-B paired Ig-like receptor B
SHIP SH2-containing inositol 5-phosphatase
SHP-1 SH2-containing protein tyrosine phosphatase-1

   Notes
 
Transmitting editor: M. Taniguchi

Received 15 May 2000, accepted 21 June 2000.


   References
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