B-lymphocyte signaling receptors and the control of class-II antigen processing

Phenotype and functional characteristics of islet-infiltrating B-cells suggest the existence of immune regulatory mechanisms in islet milieu

B-cells participate in the autoimmune response that precedes the onset of type 1 diabetes, but how these cells contribute to disease progression is unclear. In this study, we analyzed the phenotype and functional characteristics of islet-infiltrating B-cells in the diabetes-prone NOD mouse and in the insulitis-prone but diabetes-resistant (NOD x NOR)F1 mouse. The results indicate that B-cells accumulate in the islets of both mice influenced by sex traits. Phenotypically and functionally, these B-cells are highly affected by the islet inflammatory milieu, which may keep them in a silenced status. Moreover, although islet-infiltrating B-cells seem to be antigen experienced, they can only induce islet-infiltrating T-cell proliferation when they act as accessory cells. Thus, these results strongly suggest that islet-infiltrating B-cells do not activate islet-infiltrating T-cells in situ, although they may affect the progression of the disease otherwise.

Abeta-immunotherapy for Alzheimer's disease using mannan-amyloid-Beta peptide immunoconjugates

In Alzheimer's disease (AD) the accumulation of pathological forms of the beta-amyloid (Abeta) peptide are believed to be causal factors in the neurodegeneration that results in the loss of cognitive function in patients. Anti-Abeta antibodies have been shown to reduce Abeta levels in transgenic mouse models of AD and in AN-1792 clinical trial on AD patients; however, the clinical trial was halted when some patients developed meningoencephalitis. Theories on the cause of the adverse events include proinflammatory "primed patients," a Th1-inducing adjuvant, and Abeta autoreactive T cells. New immunotherapy approaches are being developed to eliminate these putative risk factors. Mannan, which is recognized by pattern recognition receptors of the innate immune system, can be utilized as a molecular adjuvant to promote a Th2-mediated immune response to conjugated B cell epitopes. The N-terminus of Abeta was conjugated to mannan, and used to immunize mice with low concentrations of immunoconjugate, without a conventional adjuvant. Mannan induced a significant and highly polarized toward Th2 phenotype anti-Abeta antibody response not only in BALB/c, but also in B6SJL F1 mice. New preclinical trials in AD mouse models may help to develop novel immunogen-adjuvant configurations with the potential to avoid the adverse immune response that occurred in the first clinical trial.

Floating the raft hypothesis: the roles of lipid rafts in B cell antigen receptor function

The initiation of antibody responses to foreign antigens requires that B cells receive and integrate a variety of signals through an array of cell surface receptors including the B cell antigen receptor (BCR) as well as a number of essential coreceptors. Recent evidence indicates that cholesterol-rich plasma membrane microdomains, referred to here as lipid rafts, serve as platforms for BCR signaling and trafficking in B cells. The existence of rafts suggests a previously unappreciated level of organization at the B cell surface that may explain, at least in part, how BCR signaling is coordinated. Here the current evidence that lipid rafts play a key role in B cell responses is reviewed.

A role for lipid rafts in B cell antigen receptor signaling and antigen targeting

The B cell antigen receptor (BCR) serves both to initiate signal transduction cascades and to target antigen for processing and presentation by MHC class II molecules. How these two BCR functions are coordinated is not known. Recently, sphingolipid- and cholesterol-rich plasma membrane lipid microdomains, termed lipid rafts, have been identified and proposed to function as platforms for both receptor signaling and membrane trafficking. Here we show that upon cross-linking, the BCR rapidly translocates into ganglioside G(M1)-enriched lipid rafts that contain the Src family kinase Lyn and exclude the phosphatase CD45R. Both Igalpha and Lyn in the lipid rafts become phosphorylated, and subsequently the BCR and a portion of G(M1) are targeted to the class II peptide loading compartment. Entry into lipid rafts, however, is not sufficient for targeting to the antigen processing compartments, as a mutant surface Ig containing a deletion of the cytoplasmic domain is constitutively present in rafts but when cross-linked does not internalize to the antigen processing compartment. Taken together, these results provide evidence for a role for lipid rafts in the initial steps of BCR signaling and antigen targeting.

Regulation of B Cell Receptor-Mediated MHC Class II Antigen Processing by FcγRIIB1

The processing and presentation of Ag by Ag-specific B cells is highly efficient due to the dual function of the B cell Ag receptor (BCR) in both signaling for enhanced processing and endocytosing bound Ag. The BCR for IgG (FcγRIIB1) is a potent negative coreceptor of the BCR that blocks Ag-induced B cell proliferation. Here we investigate the influence of the FcγRIIB1 on BCR-mediated Ag processing and show that coligating the FcγRIIB1 and the BCR negatively regulates both BCR signaling for enhanced Ag processing and BCR-mediated Ag internalization. Treatment of splenic B cells with F(ab′)2 anti-Ig significantly enhances APC function compared with the effect of whole anti-Ig; however, whole anti-Ig treatment is effective when binding to the FcγRIIB1 was blocked by a FcγRII-specific mAb. Processing and presentation of Ag covalently coupled to anti-Ig were significantly decreased compared with Ag coupled to F(ab′)2anti-Ig; however, the processing of the two Ag-Ab conjugates was similar in cells that did not express FcγRIIB1 and in splenic B cells treated with a FcγRII-specific mAb to block Fc binding. Internalization of monovalent Ag by B cells was reduced in the presence of whole anti-Ig as compared with F(ab′)2 anti-Ig, but the internalized Ag was correctly targeted to the class II peptide loading compartment. Taken together, these results indicate that the FcγRIIB1 is a negative regulator of the BCR-mediated Ag-processing function.