Signal transduction by the B cell antigen receptor and its coreceptors

The B cell antigen receptor complex: mechanisms and implications of tyrosine kinase activation

The B cell receptor is a multimeric receptor complex whose constituent chains appear to mediate distinct and possibly interrelated functions. In this review we have focused on how one chain, immunoglobulin (Ig)-alpha, may function to activate tyrosine kinases and the consequences of that activation. The cytoplasmic domain of Ig-alpha contains a consensus sequence, the antigen recognition homology 1 (ARH 1) motif, which is found in Ig-beta and other antigen recognition receptor associated chains. We argue that this conserved structure reflects an underlying conserved mechanism of secondary effector activation. Our data also indicates that the specificity of each motif (i.e., the elements which restrict secondary effector binding to particular motifs) is encoded within divergent sequences found in each ARH 1 motif. In the particular case of kinase activation by Ig-alpha, the subsequent phosphorylation of multiple tyrosines on Ig-alpha, Ig-beta, CD19, CD22 and possibly other functionally related chains form recruitment sites for a myriad of secondary signal transducers. In this model, proximal tyrosine kinases and phosphatases do not function so much to mediate the linear transfer of information as to establish and modulate an interrelated network of signal transducers capable of driving complicated cellular responses.

Making agonists of antagonists

Cell-surface receptors can be divided into three classes, depending on whether they transmit information by allosteric conformational change, by receptor dimerization, or by receptor aggregation. So far, only the first class of receptors has proven readily accessible to chemical techniques; this is no accident, but times are changing.

Oct-2 is required early in T cell-independent B cell activation for G1 progression and for proliferation

Oct-2, a POU homeodomain protein expressed primarily in B cells, is a powerful transcriptional activator that binds to DNA at sites appropriately placed for major effects on immunoglobulin gene expression. Our examination of B cell development and function in Oct-2 null mice did not support an essential role for Oct-2 early in B cell development. Rather, Oct-2 was required later, when B cells were induced to differentiate to antibody-secreting cells. We show here that Oct-2 is not required for normal immunoglobulin production by mature B lymphocytes. Instead, it is essential for a normal proliferative response to polyclonal mitogens. Responses to signals from activated T cells are unaffected. The requirement for Oct-2 maps to an early activation step in G1, during which B cells make the commitment to progress through the cell cycle and to divide.

Adhesive signaling in platelets

The anucleate platelet must perform its hemostatic functions in the absence of transcriptional regulation. Central among these functions is cell adhesion, which is mediated by multiple specialized plasma membrane receptors. The adhesive function of one of the key receptors, integrin alpha IIb beta 3, is regulated by intracellular signals triggered by platelet agonists and antagonists. Recent evidence indicates that adhesion receptors can transduce extracellular signals into the platelet to activate intracellular signaling pathways that affect hemostasis.

The B-cell antigen receptor complex: structure and signal transduction

The specificity of the immune response is determined by the interaction between the B-cell receptor (BCR) and its cognate structure, antigen. Recent studies have provided considerable insight into the compartmentalization of function within this extremely versatile hetero-oligomeric receptor complex. In this article, Christopher Pleiman, Daniele D'Ambrosio and John Cambier consolidate new findings regarding BCR structure and signal transduction.

HIV-mediated defects in immune regulation

The Division of AIDS (DAIDS), National Institute of Allergy and Infectious Diseases (NIAID), sponsored a Workshop on HIV-Mediated Defects in Immune Regulation on September 29-30, 1993. Workshop participants included investigators in basic research of immune regulation, animal models of HIV disease, HIV epidemiology, and HIV clinical research and treatment. The purpose of the workshop was to describe and evaluate biological mechanisms of HIV-mediated immune deficiency other than direct killing of infected CD4+ cells. The workshop focused on HIV-mediated dysfunction in signal transduction and in T cell development and maturation. Mechanisms by which HIV has been proposed to influence signal transduction include gp120 ligation to CD4, HIV superantigen(s), and HIV-mediated perturbations in signal pathway components (e.g., receptors, kinases, phosphatases, cytokines, and cyclins). As a result of signal dysfunction, cells may fail to respond to foreign antigens (anergy) or become predisposed to enter suicide pathways, otherwise known as programmed cell death or apoptosis. Programmed cell death is a normal immune regulatory mechanism that is activated to prevent anti-self responses and also to delete expanded but no longer needed cell populations. In the immune system, new cells are constantly produced from stem cells to replace those that die from age, pathological response, or programmed cell death. Dysfunction in these new cells may occur if HIV causes changes in the structural environment of the thymus and lymph nodes, or in cytokine signals.

CELL-SURFACE ANTIGENS

The cell surface is covered with protein molecules that are held in the membrane by hydrophobic transmembrane segments or glycosyl–phosphatidylinositol (GPI) anchors. Antigens found on cell surfaces comprise not only those encoded by the cell itself but also the products of intracellular parasites. Soluble ligands may be bound to receptor structures on the cell membrane or lectin-like molecules bound to cell-surface carbohydrate structures. The molecules that form an integral part of the cell surface may be proteins, glycoproteins, or glycolipids. They subserve one of three major functions: adhesion, antigen recognition, or receptors for soluble mediators. However, many cell-surface molecules possess more than one function—for example, molecules involved in cell–cell or cell–extracellular matrix adhesion can also themselves be involved in signal transduction. The cell-surface antigens of leukocytes have been intensively studied because of ease of access to this cellular compartment.