Tyrosine phosphorylation of Blk and Fyn Src homology 2 domain-binding proteins occurs in response to antigen-receptor ligation in B cells and constitutively in pre-B cells.

Abstract

Proteins that bind to discrete domains of the Blk, Fyn, Lyn, and Btk protein tyrosine kinases were emied in pre-B cells that had not been subjected to any external stimulation, as well as in nonstimulated and antigen-receptorligated B cells. Proteins that bind to the Src homology 2 domains of Blk and Fyn were identified in B cells that had been activated with anti-IgM but were not identified in unstimulated B cells. A number of Blk and Fyn Src homology 2 domainbinding phosphoproteins were also observed in pre-B cells that had not been stimulated in viro. The phosphoproteins seen in activated B cells potentially represent substrates that play a role in the pathway of antigen-receptor-mediated signaling. Distinct gnaling pathways involving distinguishable kinase substrates may be relevant in pre-B-cell-receptor-mediated cell survival during ontogeny. These results indirectly support models that predict constitutive ligand-independent signaling by the preantigen receptor during lymphoid ontogeny.During B-lymphocyte ontogeny, pre-B cells that make inframe variable region (V) to diversity-joining region (DJ) rearrangements at the ,u heavy-chain locus receive a survival signal and are able to differentiate further; however cells that make nonproductive rearrangements at this locus fail to be selected and are lost. Membrane immunoglobulin heavy chains associate in pre-B cells with the w and l surrogate immunoglobulin light chains (1, 2), which are the protein products ofthe pre-B-cell-specific A5 and vPreB genes (3-7). The pre-B receptor is believed to provide a feedback signal to cells that have made successful rearrangements. Because in murine bone-marrow pre-B cells the membrane form ofthe immunoglobulin ,u heavy chain (p,J-surrogate light-chain complex appears primarily intracellular (7), we suggested that a signal may well be generated constitutively from this receptor in the absence of a conventional ligand (2,7,8). Genetic experiments in which the membrane exons of the ,u heavy chain (9) or the A5 gene (10) were deleted by homologous recombination led to similar arrests of ontogeny at the late pre-B-cell stage, confirming the hypothesis that signal transduction through this pre-antigen receptor may play a critical role in late pre-B-cell survival. A similar selection mechanism has now been identified in the T-cell lineage, wherein cells that make an in-frame rearrangement at the T-cell-receptor j-chain locus receive a signal that permits further differentiation (11,12). In pre-T cells the ( chain of the T-cell receptor covalently associates with a 33-kDa glycoprotein that is presumed to represent the T-cell-lineage equivalent of the w chain (13).In B cells, surface IgM functions as the antigen receptor. Associated with membrane immunoglobulin is a disulfidelinked heterodimer made up of two glycoproteins whose cytoplasmic tails contain motifs that play a role in linking receptors to cytoplasmic Src-family tyrosine kinases (14,15). One of these proteins is the product of the mb-i gene and is often referred...

Basal Igα/Igβ Signals Trigger the Coordinated Initiation of Pre-B Cell Antigen Receptor-Dependent Processes

“…Receptor oligomerization induced after Ag binding is thought to be a required step for generating signals leading to responses such as negative selection and activation (17,39). However, it is becoming increasingly apparent that both the BCR and the pre-BCR on developing B cells are capable of generating signals independently of ligand (Ag) binding (24,31,32,35,51,52). Despite several reports that implicate ligand-independent BCR signaling in B cell development, neither it's regulation nor its linkage to specific events in B cell biology have been carefully studied.…”

Analysis of the Individual Contributions of Igα (CD79a)- and Igβ (CD79b)-Mediated Tonic Signaling for Bone Marrow B Cell Development and Peripheral B Cell Maturation

“…In contrast, we have found that the cytoplasmic domains of Ig␣ and Ig␤ in the absence of extracellular or transmembrane domains are sufficient to trigger B cell development to transitional stages supporting a model where an unligated receptor is sufficient for this function. Accordingly, fluorescence resonance energy transfer studies support that the BCR is present in a monomeric form in resting B cells (22) and Src tyrosine kinases are found in close association with the unligated receptor (23)(24)(25).…”

Correction of B-cell development in Btk-deficient mice using lentiviral vectors with codon-optimized human BTK

“…[7][8][9][10][11][12] In erythroid cells, Btk is associated with the Epo-receptor 8,13 whereas in (pre) B cells, Btk is coupled to the (pre) B-cell receptor (BCR). [10][11][12]14,15 In XLA patients, BTK mutations can be found throughout the gene and these frequently result in the absence of the Btk protein, 16,17 but amino acid replacements also occur. 18 XLA has a heterogeneous phenotype and this heterogeneity might be related to the nature of the mutation in combination with other genetic factors.…”