Ig alpha and Ig beta are functionally homologous to the signaling proteins of the T-cell receptor

Immune complexes and late complement proteins trigger activation of Syk tyrosine kinase in human CD4(+) T cells

In systemic lupus erythematosus (SLE), the autoantibodies that form immune complexes (ICs) trigger activation of the complement system. This results in the formation of membrane attack complex (MAC) on cell membrane and the soluble terminal complement complex (TCC). Hyperactive T cell responses are hallmark of SLE pathogenesis. How complement activation influences the T cell responses in SLE is not fully understood. We observed that aggregated human γ-globulin (AHG) bound to a subset of CD4(+) T cells in peripheral blood mononuclear cells and this population increased in the SLE patients. Human naive CD4(+) T cells, when treated with purified ICs and TCC, triggered recruitment of the FcRγ chain with the membrane receptor and co-localized with phosphorylated Syk. These events were also associated with aggregation of membrane rafts. Thus, results presented suggest a role for ICs and complement in the activation of Syk in CD4(+) T cells. Thus, we propose that the shift in signalling from ζ-chain-ZAP70 to FcRγ chain-Syk observed in T cells of SLE patients is triggered by ICs and complement. These results demonstrate a link among ICs, complement activation and phosphorylation of Syk in CD4(+) T cells.

Ig beta tyrosine residues contribute to the control of B cell receptor signaling by regulating receptor internalization

Immunoglobulin (Ig)α and Igβ initiate B cell receptor (BCR) signaling through immune receptor tyrosine activation motifs (ITAMs) that are targets of SH2 domain–containing kinases. To examine the function of Igβ ITAM tyrosine resides in mature B cells in vivo, we exchanged these residues for alanine by gene targeting (Igβ_AA). Mutant mice showed normal development of all B cell subtypes with the exception of B1 cells that were reduced by fivefold. However, primary B cells purified from Igβ_AA mice showed significantly decreased steady-state and ligand-mediated BCR internalization and higher levels of cell surface IgM and IgD. BCR cross-linking resulted in decreased Src and Syk activation but paradoxically enhanced and prolonged BCR signaling, as measured by cellular tyrosine phosphorylation, Ca⁺⁺ flux, AKT, and ERK activation. In addition, B cells with the ITAM mutant receptor showed an enhanced response to a T-independent antigen. Thus, Igβ ITAM tyrosines help set BCR signaling threshold by regulating receptor internalization.

A B cell receptor with two Igalpha cytoplasmic domains supports development of mature but anergic B cells

B cell receptor (BCR) signaling is mediated through immunoglobulin (Ig)α and Igβ a membrane-bound heterodimer. Igα and Igβ are redundant in their ability to support early B cell development, but their roles in mature B cells have not been defined. To examine the function of Igα–Igβ in mature B cells in vivo we exchanged the cytoplasmic domain of Igα for the cytoplasmic domain of Igβ by gene targeting (Igβ_c→α_c mice). Igβ_c→α_c B cells had lower levels of surface IgM and higher levels of BCR internalization than wild-type B cells. The mutant B cells were able to complete all stages of development and were long lived, but failed to differentiate into B1a cells. In addition, Igβ_c→α_c B cells showed decreased proliferative and Ca²⁺ responses to BCR stimulation in vitro, and were anergic to T-independent and -dependent antigens in vivo.

Use of isolated immature-stage B cells to understand negative selection and tolerance induction at the molecular level

Encounter with antigen by newly developing antigen receptor-positive B cells leads to negative selection. This process positions the B cell antigen receptor (BCR) in a central role for initiating the process of negative selection and suggests developmental regulation of its signaling. The observation that immature B cells are more susceptible to negative selection than are mature B cells has been demonstrated in a number of in vitro and in vivo model systems and support the idea of developmental regulation of BCR-initiated responses. Since identical antigen receptors are expressed on immature and mature B cells, the critical fate-determining distinction between these developmental stages must lie downstream of the receptor-ligand interaction itself, in the form of different BCR-linked signaling processes or with different secondary events occurring subsequent to BCR cross-linking. To address the first possibility, our laboratory and others have sought to define the differences in BCR-mediated signal transduction in immature and mature B lymphocytes. In this review article we will discuss current in vitro systems to study this question in primary, nontransformed murine B lymphocytes. In addition, we will discuss our previously published work in order to illustrate how these model systems have been useful in beginning to unravel the molecular basis for immune B cell negative selection and tolerance.

The protein product of the c-cbl protooncogene is phosphorylated after B cell receptor stimulation and binds the SH3 domain of Bruton's tyrosine kinase

X-linked agammaglobulinemia, a B cell immunodeficiency, is caused by mutations in the Bruton's tyrosine kinase (Btk) gene. The absence of a functional Btk protein leads to a failure of B cell differentiation and antibody production. B cell receptor stimulation leads to the phosphorylation of the Btk protein and it is, therefore, likely that Btk is involved in B cell receptor signaling. As a nonreceptor tyrosine kinase, Btk is likely to interact with several proteins within the context of a signal transduction pathway. To understand such interactions, we have generated glutathione S-transferase fusion proteins corresponding to different domains of the human Btk protein. We have identified a 120-kD protein present in human B cells as being bound by the SH3 domain of Btk and which, after B cell receptor stimulation, is one of the major substrates of tyrosine phosphorylation. We have shown that this 120-kD protein is the protein product of c-cbl, a protooncogene, which is known to be phosphorylated in response to T cell receptor stimulation and to interact with several other tyrosine kinases. Association of the SH3 domain of Btk with p120cbl provides evidence for an analogous role for p120cbl in B cell signaling pathways. The p120cbl protein is the first identified ligand of the Btk SH3 domain.

Interaction of p72syk with the gamma and beta subunits of the high-affinity receptor for immunoglobulin E, Fc epsilon RI

Activation of protein tyrosine kinases is one of the initial events following aggregation of the high-affinity receptor for immunoglobulin E (Fc epsilon RI) on RBL-2H3 cells, a model mast cell line. The protein tyrosine kinase p72syk (Syk), which contains two Src homology 2 (SH2) domains, is activated and associates with phosphorylated Fc epsilon RI subunits after receptor aggregation. In this report, we used Syk SH2 domains, expressed in tandem or individually, as fusion proteins to identify Syk-binding proteins in RBL-2H3 lysates. We show that the tandem Syk SH2 domains selectively associate with tyrosine-phosphorylated forms of the gamma and beta subunits of Fc epsilon RI. The isolated carboxy-proximal SH2 domain exhibited a significantly higher affinity for the Fc epsilon RI subunits than did the amino-proximal domain. When in tandem, the Syk SH2 domains showed enhanced binding to phosphorylated gamma and beta subunits. The conserved tyrosine-based activation motifs contained in the cytoplasmic domains of the gamma and beta subunits, characterized by two YXXL/I sequences in tandem, represent potential high-affinity binding sites for the dual SH2 domains of Syk. Peptide competition studies indicated that Syk exhibits a higher affinity for the phosphorylated tyrosine activation motif of the gamma subunit than for that of the beta subunit. In addition, we show that Syk is the major protein in RBL-2H3 cells that is affinity isolated with phosphorylated peptides corresponding to the phosphorylated gamma subunit motif. These data suggest that Syk associates with the gamma subunit of the high-affinity receptor for immunoglobulin E through an interaction between the tandem SH2 domains of SH2 domains of Syk and the phosphorylated tyrosine activation motif of the gamma subunit and that Syk may be the major signaling protein that binds to Fc epsilon RI tyrosine activation motif of the gamma subunit and that Syk may be the major signaling protein that binds to Dc epsilon tyrosine activation motifs in RBL-2H3 cells.

Temporal differences in the activation of three classes of non-transmembrane protein tyrosine kinases following B-cell antigen receptor surface engagement

We evaluated in WEHI 231 B cells the time-dependent responses of Lyn, Blk, Btk, Syk, and three members of the Jak family of protein tyrosine kinases following antibody-mediated surface engagement of the B-cell antigen receptor. Our results show that the enzyme activities of Lyn and Blk were stimulated within seconds of antigen receptor engagement and correlated with the initial tyrosine phosphorylation of the Ig alpha and Ig beta subunits of the B-cell antigen receptor. Btk enzyme activity was also transiently stimulated and was maximal at approximately 5 min after B-cell receptor surface binding. Syk activity gradually increased to a maximum at 10-30 min following receptor ligation and was found to parallel the association of Syk with the tyrosine phosphorylated Ig alpha and Ig beta subunits of the receptor. While the specific activities of the Jak1, Jak2, and Tyk2 protein tyrosine kinases were unaltered following B-cell receptor ligation, the abundance of Jak1 and Jak2 were increased 3- to 4-fold within 10 min of receptor engagement. These results demonstrate that multiple families of non-transmembrane protein tyrosine kinases are temporally regulated during the process of B-cell antigen receptor-initiated intracellular signal transduction.