Modulation of adaptive immunity with Toll-like receptors

Abstract

The discovery of Toll-like receptors (TLRs), and their role in sensing infections represents one of the most seminal advances in immunology in recent years. It is now clear that TLRs play a fundamental role in innate recognition of microbes, and stimulate and tune the quality of the adaptive immune response. However, major knowledge gaps remain in our understanding of how TLRs regulate the development and persistence of T- and B-cell memory. Here, we review our current understanding of how TLR-signaling shapes the adaptive immune response, and highlight unanswered questions, the solution of which will be imperative in the rational exploitation of TLRs in vaccine design and immune therapy.

Figure 1. Modulation of adaptive immune responses by Toll-like receptors (TLRs) and its adaptor proteins

Activation of distinct TLRs in dendritic cells (DCs) results in the induction of distinct DC responses and adaptive immunities. In humans, TLRs 7 and 9 are expressed within the ER/phagolysosomes of pDCs, and signaling via these induces potent IFN-α, and induction of Th1 responses and cross-presentation of exogenous antigens to stimulate cytotoxic T cell (CTL) responses. In contrast, myeloid DCs in humans express TLR3 (in ER/phagolysosomal compartments), or TLR2 (heterodimerized with TLR1 or 6), or TLRs 3,4,5,8 and 11 on the surface. Stimulation via most TLRs induce potent IL-12p70 and Th1 responses. However, activation of TLR2 heterodimers (TLR2/1 or TLR2/6) produce relatively little IL-12 (p70) but robust IL-10, and skews the balance towards the Th0/Th2/T-regulatory phenotype. Interestingly, each TLR is associated with different adaptor proteins, which mediate distinct functions. For example, MyD88 signaling induces pro-inflammatory cytokines (IL-12, IL-6, and tumor necrosis factor) but not type I IFNs, and MyD88 does not upregulate costimulatory molecules. Signaling via TRIF or TRAM induces type I IFNs and upregulates costimulatory molecules.

Figure 2. Regulation of TLR-mediated type I IFN production in plasmacytoid DCs, by mTOR

CpG or viral mediated activation of TLR9 leads to recruitment of MyD88 and TRAF6, and subsequent phosphorylation of IRF7, which promotes its translocation to the nucleus resulting in transcriptional activation of type I IFN genes. PI3K-AKT- signaling pathway activates mTOR which in-turn promotes TLR9-MyD88 complex formation through S6K phosphorylation and activation. Simultaneously, mTOR-mediated phosphorylation of 4E-BP results in dissociation of eukaryotic translation initiation factor (eIF)4E which binds to eIF4G/eIF4A complex and thus initiates more translation of IRF7 mRNA.

Figure 3

Programming DCs to induce Th1, Th2, Th17 or T regulatory responses. Signaling via TLR4 induces potent p38 and JNK1/2 MAPK activation which leads to the induction of interleukin-12 (IL-12) (p70). In contrast, Pam-3-cys, a TLR2/1 ligand, induce enhanced ERK 1/2 activation, which results in the stabilization of the transcription factor c-Fos that potently suppresses IL-12(p70) and enhances IL-10, thus favoring a Th2 bias. Interestingly, triggering DCs through TLR2/6 by zymosan efficiently induces ERK activation, which mediates induction of Raldh2. This results in the conversion of retinal to retinoic acid (RA), which then exerts an autocrine effect on DCs via RAR or RXR to induce SOSC3, which suppresses activation of p38 MAPK and proinflammatory cytokines. In contrast, zymosan mediated triggering of dectin-1in DCs promotes induction of proinflammatory cytokines IL-6 and IL-23 and thus mediates Th17 response.