The B cell surface protein CD72 recruits the tyrosine phosphatase SHP-1 upon tyrosine phosphorylation.

Activation signals of lymphocytes are negatively regulated by the membrane molecules carrying the immunoreceptor tyrosine-based inhibition motif (ITIM). Upon tyrosine phosphorylation, ITIMs recruit SH2-containing phosphatases such as SHP-1, resulting in down-modulation of cell activation. We showed that the cytoplasmic domain of the CD72 molecule carries an ITIM and is associated in vitro with SHP-1 upon tyrosine phosphorylation. Moreover, cross-linking of B cell Ag receptor (BCR) enhances both tyrosine phosphorylation of CD72 and association of CD72 with SHP-1 in B cell line WEHI-231. These results indicate that CD72 recruits SHP-1 upon tyrosine phosphorylation induced by BCR signaling, suggesting that CD72 is a negative regulator of BCR signaling.

In vivo and in vitro specificity of protein tyrosine kinases for immunoglobulin G receptor (FcgammaRII) phosphorylation.

Human B cells express four immunoglobulin G receptors, FcgammaRIIa, FcgammaRIIb1, FcgammaRIIb2, and FcgammaRIIc. Coligation of either FcgammaRII isoform with the B-cell antigen receptor (BCR) results in the abrogation of B-cell activation, but only the FcgammaRIIa/c and FcgammaIIb1 isoforms become phosphorylated. To identify the FcgammaRII-phosphorylating protein tyrosine kinase (PTK), we used the combination of an in vitro and an in vivo approach. In an in vitro assay using recombinant cytoplasmic tails of the different FcgammaRII isoforms as well as tyrosine exchange mutants, we show that each of the BCR-associated PTKs (Lyn, Blk, Fyn, and Syk) shows different phosphorylation patterns with regard to the different FcgammaR isoforms and point mutants. While each PTK phosphorylated FcgammaRIIa/c, FcgammaRIIb1 was phosphorylated by Lyn and Blk whereas FcgammaRIIb2 became phosphorylated only by Blk. Mutants lacking both tyrosine residues of the immune receptor tyrosine-based activation motif (ITAM) of FcgammaRIIa/c were not phosphorylated by Blk and Fyn, while Lyn-mediated phosphorylation was dependent on the presence of the C-terminal tyrosine of the ITAM. Results obtained in assays using an FcgammaR- B-cell line transfected with wild-type or mutated FcgammaRIIa demonstrated that exchange of the C-terminal tyrosine of the ITAM of FcgammaRIIa/c was sufficient to abolish FcgammaRIIa/c phosphorylation in B cells. Additionally, we could show that Lyn and Fyn bind to FcgammaRIIa/c, with the ITAM being necessary for association. Comparison of the phosphorylation pattern of each PTK observed in vitro with the phosphorylation pattern observed in vivo suggests that Lyn is the most likely candidate for FcgammaRIIa/c and FcgammaRIIb1 phosphorylation in vivo.

Aberrant B cell development and immune response in mice with a compromised BCR complex.

The immunoglobulin alpha (Ig-alpha)-Ig-beta heterodimer is the signaling component of the antigen receptor complex on B cells (BCR) and B cell progenitors (pre-BCR). A mouse mutant that lacks most of the Ig-alpha cytoplasmic tail exhibits only a small impairment in early B cell development but a severe block in the generation of the peripheral B cell pool, revealing a checkpoint in B cell maturation that ensures the expression of a functional BCR on mature B cells. B cells that do develop demonstrate a differential dependence on Ig-alpha signaling in antibody responses such that a signaling-competent Ig-alpha appears to be critical for the response to T-independent, but not T-dependent, antigens.

Immunoreceptor tyrosine-based activation motif is required to signal pathways of receptor-mediated growth arrest and apoptosis in murine B lymphoma cells.

The B cell Ag receptor is a multimeric protein complex consisting of the ligand binding mlg and the Ig alpha/lg beta heterodimer. The cytoplasmic tails of Ig alpha and Ig beta both contain a consensus sequence termed the immunoreceptor tyrosine-based activation motif (ITAM). This motif is believed to play a critical role in the receptor-mediated signal transduction. To explore the role of ITAM in signaling for B cell death (apoptosis), we transfected CH31 cells, an immature B lymphoma cell line, with expression vectors encoding for the CD8 extracellular/transmembrane domains and the cytoplasmic signal-transducing domain (ITAM) of Ig alpha or Ig beta, respectively. Here, we demonstrate that cross-linking of CD8:Ig alpha or CD:Ig beta with anti-CD8 mAb effectively induced cell growth arrest and apoptosis characterized by [3H]thymidine release and DNA fragmentation; in contrast, CD8:gamma 2a or truncated CD8:Ig alpha lacking the ITAM could not do so. Moreover, selective point mutation of either of the two conserved tyrosine residues within the ITAM, but not the nonconserved tyrosine, completely abrogated the ability of this motif to mediate cell death signals. These findings clearly indicate that ITAM is a critical component required for transmitting growth arrest and apoptotic signals, and that these functions of ITAM are positively regulated by tyrosine phosphorylation.

The tyrosine activation motif as a target of protein tyrosine kinases and SH2 domains.

The signaling subunits of antigen receptor and Fc receptor complexes carry a tyrosin-based activation motif (ITAM). Work of the recent years showed that this motif is required for the activation of protein tyrosine kinases (PTK) via these receptors. We discuss here two models of how ITAM either in its phosphorylated or unphosphorylated state may interact with PTKs. After receptor cross-linking the activated PTKs will also phosphorylate the tyrosines of ITAM. We have found that different members of the src-family of kinases can phosphorylate either both tyrosines or only the first tyrosine of ITAM. We further discuss how this alternative phosphorylation of ITAM can result in the interaction of the BCR with different SH2-containing proteins and thus influence the signal transduction from the receptor.

Dual role of the tyrosine activation motif of the Ig-alpha protein during signal transduction via the B cell antigen receptor.

The B cell antigen receptor (BCR) is a multimeric protein complex consisting of the ligand binding immunoglobulin molecule and the Ig-alpha/beta heterodimer that mediates intracellular signalling by coupling the receptor to protein tyrosine kinases (PTKs). Transfection of the Ig-alpha deficient myeloma cell line J558L microns with expression vectors coding for mutated Ig-alpha allowed us to test the function of the tyrosines in the cytoplasmic region of Ig-alpha in the context of the BCR. Furthermore we expressed Ig-alpha mutations as chimeric CD8-Ig-alpha molecules on K46 B lymphoma cells and tested their signalling capacity in terms of PTK activation and release of calcium. We show here that the conserved tyrosine residues in the cytoplasmic portion of Ig-alpha have a dual role. First, they are required for efficient activation of PTKs during signal induction and second, one of them is subject to phosphorylation by activated src-related PTKs. Phosphorylation on tyrosine in the cytoplasmic portion of Ig-alpha is discussed as a possible mechanism to couple the BCR to SH2 domain-carrying molecules.

Early B cell development requires mu signaling.

In vitro studies with Abelson murine leukemia virus (AMuLV)-transformed murine pre-B cell lines demonstrated that wild-type mu but not mutant mu chains lacking the first constant domain (mu delta 1) can efficiently induce Ig light (L) chain gene rearrangement. Using antibodies against the cytoplasmic tail of the immunoglobulin co-receptor beta (Ig beta) chain we find mu, but not mu delta 1 chains associated with Ig beta. Since a heterodimer of surface-labeled proteins was co-precipitated with mu we conclude that only wild-type mu is associated with the Ig alpha/Ig beta co-receptor on the surface of pre-B cell lines. Mutant mu delta 1 chains achieve their surface expression by utilizing a glycophospholipid anchor. In vivo analysis of transgenic mice expressing either mu or mu delta 1 transgenes revealed the expected "normal" B cell development in the case of wild-type mu transgenic lymphocytes, but a block in differentiation of mu delta 1 transgenic lymphocytes. The maturation block occurs at the developmental transition of pre-B lymphocytes from the CD43/S7+, CD45R/B220low stage to the CD43/S7-, B220low/high stage in which the majority of L chain gene rearrangements occur. These results, together with the observed inability of the mu delta 1 chains to signal activation of L chain gene joining and to associate Ig alpha/Ig beta in pre-B cell lines suggests that signals mediated by the protein complex composed to mu/Ig alpha/Ig beta are crucial during differentiation of pre-B lymphocytes.