Complementary Roles for CD19 and Bruton’s Tyrosine Kinase in B Lymphocyte Signal Transduction

Abstract

CD19 and Bruton’s tyrosine kinase (Btk) may function along common signaling pathways in regulating intrinsic and B cell Ag receptor (BCR)-induced signals. To identify physical and functional interactions between CD19 and Btk, a CD19-negative variant of the A20 B cell line was isolated, and CD19-deficient (CD19−/−) and CD19-overexpressing mice with the X-linked immunodeficient (Xid; Btk) mutation were generated. In A20 cells, Btk physically associated with CD19 following BCR engagement. CD19 and Btk interactions were not required for initial Btk phosphorylation, but CD19 expression maintained Btk in an activated state following BCR engagement. In primary B cells, CD19 signaling also required downstream Btk function since CD19-induced intracellular Ca2+ ([Ca2+]i) responses were modest in Xid B cells. In addition, CD19 overexpression did not normalize the Xid phenotype and most phenotypic and functional hallmarks of CD19 overexpression were not evident in these mice. However, CD19 and Btk also regulate independent signaling pathways since their combined loss had additive inhibitory effects on BCR-induced [Ca2+]i responses and CD19 deficiency induced a severe immunodeficiency in Xid mice. Thus, CD19 expression amplifies or prolongs Btk-mediated signaling, rather than serving as a required agent for Btk activation. Consistent with this, phosphatidylinositol 3-monophosphate kinase and Akt activation were normal in CD19−/− B cells following IgM engagement, although their kinetics of activation was altered. Thus, these biochemical and compound gene dosage studies indicate that Btk activation and [Ca2+]i responses following BCR engagement are regulated through multiple pathways, including a CD19/Src family kinase-dependent pathway that promotes the longevity of Btk signaling.

CD19 is a major B cell receptor–independent activator of MYC-driven B-lymphomagenesis

“…One known mechanism of its action is the recruitment and activation of PI3 and subsequently Akt kinases (38)(39)(40)(41)(42). This is accomplished through the docking function of 2 tyrosine residues, Y482 and Y513, in the cytoplasmic region of CD19 (60).…”

“…One of the known functions of CD19 signaling is the recruitment and activation of PI3K (38) and ensuing activation of Akt (39)(40)(41)(42), all in the context of BCR signaling (reviewed in ref. 43).…”

B Cell Receptor-Mediated Syk-Independent Activation of Phosphatidylinositol 3-Kinase, Ras, and Mitogen-Activated Protein Kinase Pathways

“…Another group using dominant-negative mutant of Syk did not obtain similar result, suggesting that this mutant did not completely inhibit Syk function in A20 cells (11). During preparation of manuscript, it was reported that BCRinduced phosphorylation of Akt is not reduced in the absence of CD19 (20), suggesting that the Syk-dependent recruitment of PI3K to CD19, as seen in Fig. 5B, might not play a central role in the BCR-induced PI3K activation.…”

“…For example, BCR-mediated activations of ERK, JNK, and p38 MAPKs are almost lost in BLNK-deficient DT40 cells (13), but nearly intact in BLNK-deficient mouse B cells (30). BLNK or Btk deficiency results in complete loss of BCR-induced calcium mobilization in DT40 cells (13,41), but partial or slight impairment in mouse B cells (20,33,(42)(43)(44). It is possible that DT40 cells lack a Syk-independent pathway(s) for BCR signaling.…”

Suppression of innate and adaptive B cell activation pathways by antibody coengagement of FcγRIIb and CD19

“…49 A recent paper claims that TLR9 and BCR cosignaling also depend on BTK, which is essential for the colocalization of BCR and TLR9. 32 Taking all of these results together, we suggest that in the case of BCR-CD19 crosslinking by anti-IgG and XENP6187, BTK is exhausted by CD19 and is not available for BCR-TLR9-induced synergistic signaling.…”