However, it must be noted that TD and TI responses are not rigidly compartmentalized within the B-2 and MZ/B-1 cell subsets. Pexidartinib chemical structure For instance,
MZ B cells also participate in TD antibody production owing to their ability to shuttle to the follicle and present antigen to T cells [[40, 41]]. Conversely, B-2 cells can initiate TI antibody responses in the intestine [[42]]. Here, we discuss recent advances in our understanding of the mechanisms by which adaptive and innate immune cells provide help to B cells. Protein antigens initiate protective antibody responses in the follicles of secondary lymphoid organs, a microenvironment that favors the interaction of B and T cells with each other as well as with antigen presenting DCs and
antigen exposing follicular dendritic cells (FDCs) (reviewed in [[7]]). After interacting with antigen through the B-cell receptor (BCR), which includes IgM and IgD (Fig. 1), naive B cells migrate Pembrolizumab cost to the boundary between the follicle and the outer T-cell zone [[43]]. At this location, B cells form dynamic conjugates with TFH cells, which deliver cognate B-cell help through a mechanism involving the tumor necrosis factor (TNF) family member CD40L and cytokines such as interferon-γ (IFN-γ, a cytokine also expressed by TH1 cells) and interleukin-4 (IL-4, a cytokine also expressed by TH2 cells) [[13, 14, 43, 44]]. B cells thereafter differentiate
along one of the two pathways. The follicular pathway generates Bcl6-positive germinal center B cells that further differentiate into long-lived memory B cells and plasma cells producing high-affinity antibodies, whereas the extrafollicular pathway generates Bcl6-negative blasts that further differentiate into short-lived plasma cells secreting low-affinity antibodies [[14, 45]]. After receiving activating signals from TFH cells at the border of the follicle with the T-cell zone, B cells upregulate the expression of the DNA-editing enzyme activation-induced cytidine deaminase (AID) and initiate somatic hypermutation (SHM) and class switch recombination (CSR), two Ig gene diversifying processes highly dependent on AID [[46-49]]. SHM introduces point mutations within V(D)J genes, thereby providing the structural find more correlate for selection of high-affinity Ig mutants by antigen (reviewed in [[50]]). By replacing constant (C) μ, and Cδ genes, which encode IgM and IgD, respectively, with Cγ, C, or C genes, which encode IgG, IgA, or IgE, respectively, CSR provides antibodies with novel effector functions without changing antigen specificity (reviewed in [[51]]). In humans, a noncanonical form of CSR from Cμ to Cδ has also been documented in lymphoid structures associated with the upper respiratory tract and generates B cells specialized in IgD production [[52]].