Toll-like receptors and B-cell receptors synergize to induce immunoglobulin class-switch DNA recombination: relevance to microbial antibody responses

Abstract

Differentiation of naïve B cells, including immunoglobulin class-switch DNA recombination, is critical for the immune response and depends on the extensive integration of signals from the B-cell receptor (BCR), tumor necrosis factor (TNF) family members, Toll-like receptors (TLRs), and cytokine receptors. TLRs and BCR synergize to induce class-switch DNA recombination in T cell-dependent and T cell-independent antibody responses to microbial pathogens. BCR triggering together with simultaneous endosomal TLR engagement leads to enhanced B-cell differentiation and antibody responses. Te requirement of both BCR and TLR engagement would ensure appropriate antigen-specific activation in an infection. Co-stimulation of TLRs and BCR likely plays a significant role in anti-microbial antibody responses to contain pathogen loads until the T cell-dependent antibody responses peak. Furthermore, the temporal sequence of different signals is also critical for optimal B cell responses, as exemplified by the activation of B cells by initial TLR engagement, leading to the up-regulation of co-stimulatory CD80 and MCH-II receptors, which result in more efficient interactions with T cells, thereby enhancing the germinal center reaction and antibody affinity maturation. Overall, BCR and TLR stimulation and the integration with signals from the pathogen or immune cells and their products determine the ensuing B-cell antibody response.

FIGURE 1

Four main signals for B cell differentiation and antibody responses. During an infection, naïve mature B cells receive several types of stimuli that are then summed up before determining the appropriate response. DCs and T_H cells interact with B cells via surface receptors, such as CD40, and by cytokine receptors, such as IL receptors. A pathogen would directly induce signals by crosslinking the BCR (as shown by the proximity of the O-sccharide of LPS) and by activating innate receptors such as the TLRs (as shown by the interaction of TLR4 with the lipid A component of LPS). TLR ligands are sensed by the extracellular, or endosomal, sickle shaped domains, and signals are initially relayed to the nucleus via homotypic TIR-TIR interactions (orange spheres) with TIR adapters. These signals are integrated and initiate a response by inducing NF-κB and AP-1, inflammatory gene transcription, IFN-inducible gene transcription, and induction of AID activity, leading to CSR and SHM

FIGURE 2

Overview of CSR and SHM. a. Human B cells diversify the variable portion of their antigen receptors during development in the bone marrow through V(D)J recombination, which is mediated by recombination activating gene 1 (RAG1) and RAG2 recombinases. b. During an infection, naïve mature B cells in the spleen and lymph nodes undergo rearrangement of the constant portion of the IgH (i.e., CSR) to endow it with new biological effector functions, as well point mutations in the variable regions (i.e., SHM) to further increase its affinity, both of which depend on AID. Each C_H region is indicated in blue color, beginning with Cμ on the left. The intronic IgH enhancer (iEμ, light green color), which is essential for optimal IgH gene expression, is located upstream of Cμ. The different orange colored thin segments just upstream of each C_H region are the I_H promoters followed by the S regions. I_H promoters are activated in response to particular cytokines, and serve to drive germline transcription through S and C_H regions, possibly opening up local chromatin structure for AID activity, or delivering AID to the S regions by RNA pol II or other trans-acting factors.

FIGURE 3

The interplay of surface TLR4 and endosomal TLR9 in CSR. a. LPS induces CSR to isotypes, as directed by cytokines. b. CpG by itself does not induce significant CSR; however, in the presence of BCR crosslinking (indicated by a virus which is bound by BCR; experimentally it can be mimicked by reagents that crosslink the BCR polyclonally), it behaves similar to LPS in inducing CSR to the isotypes specified by cytokines. c. CpG suppresses LPS-induced CSR; however CpG synergizes with LPS in a dose-dependent way in the presence of BCR crosslinking (d).

FIGURE 4

Model on the role of endosomal TLRs in B cell antibody responses. a. B cells in the secondary lymphoid organs can be directly activated by viruses which crosslink the BCR. This then leads to their phagocytosis, double membrane autophagosome formation, and trafficking of the autophagosomes to TLR-containing endosomes. If any present viral TLR ligands are sensed by TLR3, TLR7/8, or TLR9, an appropriate response is then mounted by the B cells, which includes their differentiation (and in some cells, T-independent CSR), the upregulation of receptors that interact with T_H cells and DCs, thus enhancing the GC reaction (b). The result of T-independent (a) and T-dependent (b) B cell differentiation is the formation of antigen-specific plasma cells, and memory B cells, which can be quickly differentiated to plasma cells upon subsequent re-infection (c).

FIGURE 5

The design features of modular vaccines. a. Virus-mimicking biodegradable particles with favorable pharmacokinetics, which by themselves are not immunogenic, are used as scaffolds for the incorporation of protein antigens from one or several pathogens. Initial screens determine the most immunogenically suitable protein antigens (brown triangles) to use. In addition, combinations of one or more natural or synthetic TLR ligands are included, usually mimicking their occurrence in the actual pathogen(s). b. Particles mimicking bacteria are coated with bacterial protein antigens as well as TLR ligands characteristic of each bacterium. These composite vaccines (a, b) activate both innate immune cells as well as B lymphocytes, which will differentiate to plasma cells that secrete antigen-specific antibodies, as well as to memory B cells which can be activated upon potential future re-infection. c. Legend indicates the protein antigens and the various TLR ligands used in modular vaccines.