CD5-Negative Regulation of B Cell Receptor Signaling Pathways Originates from Tyrosine Residue Y429 Outside an Immunoreceptor Tyrosine-Based Inhibitory Motif

Abstract

CD5 is a cell surface receptor that negatively regulates B cell function, but whose relationship to the immunoreceptor tyrosine-based inhibitory motif (ITIM) family of B cell inhibitory receptors is unclear. Using Fcγ type IIB receptor-CD5 chimeras encompassing the cytoplasmic domain of CD5, we previously showed that a particular region of the molecule containing two tyrosine residues, Y429 and Y441, in an amino acid stretch similar to the Src autophosphorylation motif and a putative ITIM, respectively, antagonized early signaling events triggered through the B cell receptor (BCR). In this study, we provide evidences that only Y429 is mandatory for the inhibition by CD5 of the calcium response activated via the BCR. This residue also efficiently controls inhibition of the Ras/extracellular signal-related kinase-2 pathway. Analyzing the membrane translocation of the AKT protooncogene using its 3′-phosphoinositide-specific pleckstrin homology domain fused to the green fluorescent protein as a probe, we also show that CD5 strongly impairs its cellular redistribution and demonstrate the role played by Y429 in this process. We finally report that Y429 controls almost exclusively CD5 phosphorylation as well as inhibition of BCR-triggered IL-2 production upon coaggregation of the two receptors. Thus, CD5 uses an ITIM-independent strategy, centered on Y429, the major tyrosine-phosphorylated residue in its cytoplasmic domain, to inhibit BCR activation.

High Mda-7 expression promotes malignant cell survival and p38 MAP kinase activation in chronic lymphocytic leukemia

“…We have however observed that, contrary to the majority of CD5 À B cells, normal CD5 þ blood B cells displayed a low Ca 2 þ response following anti-IgM stimulation, 15 confirming the role of CD5 as a negative regulator of BCR signaling. 16,17 Even though BCR-mediated activation is often deficient in CLL, CD5 may act independently of the BCR by recruiting signaling molecules, 18,19 and eliciting the production of B-cell survival factors such as interleukin (IL)-10. 15 IL-10 is indeed produced by most CLL B-cells, 20 improves the survival of malignant CLL B-cells, 21 and IL-10 serum levels in CLL correlated with the severity of the disease.…”

Rescue of Tumor-Infiltrating Lymphocytes from Activation-Induced Cell Death Enhances the Antitumor CTL Response in CD5-Deficient Mice

“…More recently, it has been demonstrated that CD5 is rapidly recruited and colocalized with the TCR/CD3 complex at the immune synapse (7), and that it inhibits TCR signaling in T lymphocytes interacting with APC without influencing conjugate formation (8). Furthermore, it has been reported that CD5-mediated inhibition of TCR signaling does not require the CD5 extracellular domain, but only its cytoplasmic tail (9), in which a pseudo-ITAM is likely to play a role (8,10). Therefore, interaction of CD5 with its ligand does not seem to be necessary for TCR signaling inhibition.…”

Cutting Edge: B-1 Cells Are Deficient in Lck: Defective B Cell Receptor Signal Transduction in B-1 Cells Occurs in the Absence of Elevated Lck Expression

“…Although we found that B-1 cells lack Lck, we also found that B-1 cells express phosphorylated CD5, and so the latter may, as reported by others (12,13,19), play a key role in blocking BCR signaling in B-1 cells, presumably after phosphorylation by another src kinase. Recently, however, Bondada and colleagues reported that BCR signaling is defective in peritoneal B-1b cells that lack CD5 (30), indicating that abnormal BCR signaling takes place in the absence of CD5 in this B-1 cell subset.…”

“…As noted above, it has been suggested that the phosphorylated form of CD5 is responsible for blocked BCR signaling, and that high levels of Lck are responsible for constitutive CD5 phosphorylation, in naive B-1 cells (13,22). Because B-1 cells express little, if any, Lck, it was of interest to determine whether B-1 cells contain phosphorylated CD5.…”

“…Because BCR-triggered Ca 2ϩ mobilization is affected, but PMA-induced NF-B activation is not (10), it has been suggested that BCR coupling to phospholipase C is impaired in B-1 cells, and direct measurements of BCRinduced phospholipase C activity suggest that this is so (11,17). This in turn is thought to result from the inhibitory effect of CD5, particularly the tyrosine phosphorylated form of CD5, due to its association with SHP-1 phosphatase (12,13,19). Recently, the origin of phosphorylated CD5 and impaired BCR signaling has been reported to lie in an increased level of the canonical T cell src kinase Lck.…”