The Pleckstrin Homology Domain Adaptor Protein Bam32/DAPP1 Is Required for Germinal Center Progression

Abstract

Ab affinity maturation within germinal centers (GCs) requires weeks to complete. Several signaling pathways in B cells have been shown to be required for initiation of the GC response; however, the signaling checkpoints controlling progression and eventual dissolution of the GC reaction are poorly understood. The adaptor protein Bam32/DAPP1 was originally isolated from human GCs and functions downstream of phosphoinositide 3-kinase enzymes, which are known to have critical roles in B cell activation and GC responses. In this study we identify a unique role of Bam32/DAPP1 in promoting GC progression. Bam32-deficient mice show normal GC initiation, but premature GC dissolution after immunization with protein Ag in alum or low doses of sheep red blood cells. Adoptive transfer studies confirmed that Bam32-deficient B cells have an intrinsic impairment in the ability to mount sustained GC responses. Bam32 deficiency was also associated with impaired Ab affinity maturation. Proliferation of Bam32-deficient GC B cells was not compromised; however, these cells show impaired switch to IgG1 and increased apoptosis in situ. GCs formed by Bam32-deficient B cells contain fewer T cells, indicating that Bam32 is required for B cell–dependent T cell accumulation within established GCs. Exogenous CD40 ligand restored GC B cell numbers and switch to IgG1, indicating that Bam32-deficient B cells are competent to respond to CD40 stimulation when ligand is available. These data demonstrate that Bam32 is not required for GC initiation, but rather functions in a late checkpoint of GC progression associated with T cell recruitment and GC B cell survival.

Bam32/DAPP1 Promotes B Cell Adhesion and Formation of Polarized Conjugates with T Cells

“…We have recently found evidence supporting a role for Bam32 in T-dependent Ab responses (36). Upon immunization with Tdependent Ag, GCs initiate normally in Bam32-deficient mice, but subsequently collapse, resulting in impaired affinity maturation.…”

“…All B cell subsets develop normally in Bam32-deficient mice, but they show markedly impaired T-independent Ab responses (32,35). In contrast, Bam32-deficient mice were reported to mount quantitatively normal responses to T-dependent Ag; however, we recently found that Bam32 deficiency leads to premature dissolution of GCs and impaired affinity maturation (36).…”

S1PR2 links germinal center confinement and growth regulation

“…DOCK8 is a member of the DOCK family, whose members contain a DOCK homology region 1 domain (DHR1), which recruits them to PIP3, and have Rho-Rac family GTP-exchange factor activity. Another PIP3-binding protein, Bam32, which is a pleckstrin homology domain adapter protein, was also found to be important for BCR-induced integrin adhesion and spreading as well as the formation of stable conjugates with T cells (107), and Bam32 deficiency resulted in a failure of GCs to persist after formation and a reduction in affinity maturation (108). Bam32-deficient GC B cells were more susceptible to apoptosis.…”

The Adaptor Protein Bam32 in Human Dendritic Cells Participates in the Regulation of MHC Class I-Induced CD8+ T Cell Activation

“…Bam32 was found to be expressed mainly in murine hematopoietic cells but also in human cells, including B and T lymphocytes. It has multiple functions in B cell activation and is required for biological responses including BCR-induced proliferation, Ab responses to type II T-independent Ags, germinal center progression, and formation of polarized conjugates with T cells (11)(12)(13)(14). It functions by regulating F-actin formation, and this Bam32-induced cytoskeletal rearrangement has been shown to be important for receptor endocytosis and membrane ruffling (15,16).…”