Cell surface Fc receptor for IgM antibody (FcμR) is the most recently identified member among FcRs. We determined the cellular distribution of mouse FcμR and the functional consequences of Fcmr disruption. Surface FcμR expression was restricted to B-lineage cells, from immature B to plasma cells, except for a transient downmodulation during germinal center reactions. Fcmr ablation had no significant effect on overall B-and T-cell development, but led to a reduction of marginal zone B cells and an increase in splenic B1 B cells. Preimmune serum IgM in mutant mice was significantly elevated as were natural autoantibodies. When immunized with live attenuated pneumococci, mutant mice mounted robust antibody responses against phosphorylcholine, but not protein, determinants compared with wild-type mice. By contrast, upon immunization with a hapten-carrier conjugate, nitrophenyl-coupled chicken γ-globulin (NP-CGG), the mutant mice had a diminished primary IgG1 response to both NP and CGG. These findings suggest that FcμR has an important role in IgM homeostasis and regulation of humoral immune responses.natural antibody | B-cell tolerance | B-cell subset | autoimmunity

Innate-like splenic marginal zone (MZ) and peritoneal cavity B1 B lymphocytes share critical responsibilities in humoral responses but have divergent B-cell receptor (BCR) signaling features. A discrete marker of these subsets with tyrosine-based dual regulatory potential termed "Fc receptor-like 5" (FCRL5) was investigated to explore this discrepancy. Although FCRL5 repressed the robust BCR activity that is characteristic of MZ B cells, it had no influence on antigen receptor stimulation that is blunted in peritoneal cavityderived B1 B cells. The molecular basis for the receptor's inhibitory function derived from recruitment of the Src homology-2 domaincontaining tyrosine phosphatase 1 (SHP-1) to a cytoplasmic immunoreceptor tyrosine-based inhibitory motif. Surprisingly, mutagenesis of this docking site unearthed coactivation properties for FCRL5 that were orchestrated by independent association of the Lyn Src-family kinase with an intracellular immunoreceptor tyrosine-based activation motif-like sequence. FCRL5's unique binary regulation directly correlated with SHP-1 and Lyn activity, which, like BCR function, differed between MZ and B1 B cells. These findings collectively imply a specialized counterregulatory role for FCRL molecules at the intersection of innate and adaptive immunity.

Marginal zone macrophages (MZMs) act as a barrier to entry of circulating apoptotic debris into the follicles of secondary lymphoid organs. In autoimmune BXD2 mice, there is a progressive reduction in the function and numbers of MZMs. Absence of MZMs results in retention of apoptotic cell debris within the MZ and increased loading of apoptotic cell antigens on MZ B cells and MZ-precursor (MZ-P) B cells. The MZ-P B cells are capable of translocating the apoptotic cell antigens to the follicular zone and stimulating T cells. Both MZMs and MZ-P B cells from BXD2 mice express low levels of tolerogenic signals and high levels of inflammatory signals. Thus, the present study suggests a multifaceted mechanism in which MZMs maintain tolerance to apoptotic autoantigens and suppress their translocation to follicles. Lack of clearance of apoptotic debris by MZMs drives follicular Ag–transportation by MZ-P B cells to stimulate an autoimmune response.

Successful B cell differentiation and prevention of cell transformation depends on balanced and fine-tuned activation of cellular signaling pathways. The phosphatidyl inositol-3 kinase (PI3K) signaling pathway has emerged as a major regulator of B lymphocyte homeostasis and function. Phosphoinositide-dependent protein kinase-1 (PDK1) is the pivotal node in the PI3K pathway, regulating the stability and activity of downstream AGC kinases (including Akt, RSK, S6K, SGK, and PKC). Although the importance of PI3K activity in B cell differentiation is well documented, the role of PDK1 and other downstream effectors is underexplored. Here we used inducible and stage-specific gene targeting approaches to elucidate the role of PDK1 in early and peripheral B cell differentiation. PDK1 ablation enhanced cell cycle entry and apoptosis of IL-7-dependent pro-B cells, blocking Ig synthesis and B cell maturation. PDK1 also was essential for the survival and activation of peripheral B cells via regulation of PKC and Akt-dependent downstream effectors, such as GSK3α/β and Foxo1. We found that PDK1 deletion strongly impaired B cell receptor (BCR) signaling, but IL-4 costimulation was sufficient to restore BCR-induced proliferation. IL-4 also normalized PKCβ activation and hexokinase II expression in BCR-stimulated cells, suggesting that this signaling pathway can act independent of PDK1 to support B cell growth. In summary, our results demonstrate that PDK1 is indispensable for B cell survival, proliferation, and growth regulation.A ctivation of the phosphatidyl inositol-3 kinase (PI3K) signaling pathway is critical to early B cell development as well as peripheral B cell survival and activation (1). Although the catalytic p110 subunits of class I PI3K molecules are partially redundant, the combined loss of the p110α and p110δ isoforms results in impaired IL-7R-driven proliferation (2). Conversely, it has been suggested that attenuation of PI3K signaling via IL-7R signaling is required for pre-B cell differentiation into IgM-expressing cells to cease proliferation and promote RAG expression (3).In peripheral B cells, continued survival requires "tonic" signaling via the B cell receptor (BCR), which can be replaced by constitutive PI3K activity (4). Moreover, generation of the marginal zone (MZ) and B-1 B cell subsets, as well as antigendriven differentiation into antibody-producing cells, are dependent on PI3K (1). PI3K activity generates PtdIns(3,4,5)P 3 , which acts as a secondary messenger by binding the pleckstrin homology domains of downstream effector molecules. PtdIns (3,4,5)P 3 is also the substrate for the phosphatases PTEN and SHIP, generating PtdIns(4,5)P 2 and PtdIns(3,4)P 2 , respectively. Unrestrained activation of PI3K signaling in B cells lacking PTEN and SHIP results in lethal B cell lymphoma (5).Phosphoinositide-dependent kinase 1 (PDK1) represents a pivotal downstream effector of PI3K signaling, regulating cellular responses to growth factors, insulin, and numerous other agonists by activating a number of AGC pr...

Highlights d Dark zone germinal center (GC) B cells are diminished upon cyclin D3 loss d Hyperstabilized cyclin D3 T283A mutant drives dark zone GC B cell expansion d B cell receptor signaling in GC B cells suppresses cyclin D3 d c-Myc requires cyclin D3 for GC B cell expansion