An extracatalytic function of CD45 in B cells is mediated by CD22

Abstract

The receptor-like tyrosine phosphatase CD45 regulates antigen receptor signaling by dephosphorylating the C-terminal inhibitory tyrosine of the src family kinases. However, despite its abundance, the function of the large, alternatively spliced extracellular domain of CD45 has remained elusive. We used normally spliced CD45 transgenes either incorporating a phosphatase-inactivating point mutation or lacking the cytoplasmic domain to uncouple the enzymatic and noncatalytic functions of CD45 in lymphocytes. Although these transgenes did not alter T-cell signaling or development irrespective of endogenous CD45 expression, both partially rescued the phenotype of CD45-deficient B cells. We identify a noncatalytic role for CD45 in regulating tonic, but not antigen-mediated, B-cell antigen receptor (BCR) signaling through modulation of the function of the inhibitory coreceptor CD22. This finding has important implications for understanding how naïve B cells maintain tonic BCR signaling while restraining inappropriate antigen-dependent activation to preserve clonal "ignorance."lymphocytes face the daunting tasks of distinguishing between benign self-antigens and pathogenic foreign epitopes and of mounting a robust immune response to the latter, but not the former. Although extremely autoreactive B-cell antigen receptors (BCRs) are removed from the repertoire during B-cell development, a large fraction of mature naïve B cells express mildly autoreactive BCRs (1). That these specificities pose a threat to immune homeostasis is evidenced by the extensive network of inhibitory coreceptors and effector phosphatases expressed in naïve B cells that serve to suppress BCR signaling (2). Defects in these pathways result in autoantibody production and frank autoimmune disease in mice and humans (2, 3). However, "tonic" signaling downstream of the BCR is essential for B-cell survival (4). How B cells balance the requirement for tonic BCR signaling with the demands of so-called "clonal ignorance" is not well understood.The receptor-like tyrosine phosphatase CD45 serves as a critical positive regulator of antigen receptor (AgR) signaling by constitutively dephosphorylating the C-terminal inhibitory tyrosine of the src family kinases (SFKs) (5, 6). This function is counteracted by the kinase Csk, which stabilizes a "closed" autoinhibited conformation of the SFKs by phosphorylating this site. Active SFKs in turn phosphorylate tyrosines within cytoplasmic immunoreceptor tyrosine-based activation motifs (ITAMs) of the AgRs that are essential for recruitment of the Syk family kinases and downstream signaling (7). Consistent with this model, the inhibitory tyrosine (Y505) of the T-cell-specific SFK Lck is hyperphosphorylated in CD45-deficient primary T cells (6,8). Consequently, T-cell receptor (TCR) signaling and T-cell development are severely impaired in mice deficient for CD45 (8-11). BCR signaling and B-cell development are also disrupted in CD45-deficient mice, but this phenotype is much milder than that of T cells because of the exp...

Blood-Borne ST6GAL1 Regulates Immunoglobulin Production in B Cells

“…Despite reported interactions with IgM and CD45, the majority of membrane CD22 homo-multimerizes in clathrin rafts by lectin-dependent interactions, sequestered away from the activated BCR complexes in lipid rafts (59,60). Indeed, recent studies suggest that both CD22-CD22 and CD22-CD45 complexes improve BCR signaling (38,61).…”

CD22: A Regulator of Innate and Adaptive B Cell Responses and Autoimmunity

“…A reduction or absence of CD45 most likely leads to more CD22-CD22 homotypic interactions and thus larger clusters. Couglin et al ( 32 ) also implicated extracellular CD45 in the regulation of CD22. They found that expression of transgenes encoding either extracellular CD45 without its cytoplasmic domain or CD45 with a catalytically inactive form of CD45 in CD45 −/− mice rescued B cell defects seen in these mice such as elevated basal Ca 2+ levels but not T cell defects.…”

CD45 pre-exclusion from the tips of microvilli establishes a phosphatase-free zone for early TCR triggering

“…Artificially induced CD45 exclusion can initiate TCR triggering in a nonimmune cell reconstitution system (James and Vale, 2012). CD45 exclusion also plays a key role in B cell receptor (BCR) signaling and high-affinity immunoglobulin E receptor (FcεRI) signaling in mast cells and basophils (Coughlin et al, 2015;Depoil et al, 2008;Felce et al, 2018).…”