Regulatory role of TLR ligands on the activation of autoreactive T cells by retinal astrocytes.

Reactivation of autoreactive T cells within the target organ, such as the CNS or eye, is one of the critical pathogenic events in the development of autoimmunity.1,2 Tissue-invading autoreactive T cells require major histocompatibility complex (MHC) class II antigens for their reactivation in the organ.3–5 It is generally thought that such molecules are provided by macrophages and dendritic cells (DCs) that have infiltrated the inflamed organ. However, residential glial cells in the CNS and eye, such as microglia and astrocytes, have the ability to express MHC molecules6,7 and can initiate cross-talk between effector T cells and parenchymal cells of the organ. We have previously reported that activated RACs have the potential to stimulate T cells in vitro.7 However, their T-cell stimulatory effect varies, depending on whether the RACs are preactivated and whether they are derived from disease-prone or disease-resistant strains of mice. For example, RACs from B10RIII mice, which are highly susceptible to the induction of experimental autoimmune uveitis (EAU), are effective in promoting interphotoreceptor retinoid-binding protein (IRBP)-specific T-cell expansion and cytokine production, whereas RACs from C57BL/6 (B6) mice, which are less susceptible to EAU induction, are not as effective. Recent studies have shown that microbial proteins can act as ligands for Toll-like receptors (TLRs), a family of well-characterized, pattern-recognition receptors recognizing pathogen-associated molecular motifs.8,9 Different TLRs recognize distinct classes of ligands, and this results in changes in innate and adaptive immune responses. Thirteen TLRs have been described in humans. Some of these, such as TLRs 1, 2, 4, 5, and 6, preferentially bind bacterial products (e.g., lipopolysaccharide [LPS] or flagellin), whereas others, such as TLRs 3, 7, 8, and 9, mainly bind nucleic acids (e.g., single- and double-stranded RNA).10 TLR3 mediates responses to double-stranded (ds) RNA or its synthetic analogue polyinosinic:polycytidylic acid (polyI:C). dsRNA makes up the genome of one class of viruses (e.g., Reoviridae) and is also a replication intermediate of most other viruses, such as West Nile, respiratory syncytial, and influenza11,12; thus, TLR3 plays a key role in antiviral defense. TLR triggering of antigen-presenting cells (APCs) results in activation of these cells, as evidenced by the upregulation of maturation markers (e.g., CD83) and costimulatory molecules (e.g., CD86, CD80, and CD40).13 Functional differences in APCs and in the subsequent APC-T-cell responses may result from differences in the TLR triggered. CD4+ T cells, both Th1 and Th17, are involved in the pathogenesis of animal models of uveitis. The polarization of infiltrating T cells into Th1 and Th17 subsets is contingent on the availability of proinflammatory cytokines, such as IL-12 for Th1 and TGF-β and IL-6 and IL-23 for Th17.14,15 Thus, the ability of RACs to sense ocular environmental cues and to translate them to T lymphocytes is important in inducing not only the expansion of cell populations but for CD4 T-cell polarization to phenotypes with either pathogenic or non-pathogenic potential. In this study, we showed that RACs derived from B6 mice were able to recognize and respond to TLR ligands. Depending on the type of TLR ligand to which they were exposed, RACs expressed different levels of MHC II and costimulatory molecules, produced different amount of cytokines, and had different effects in promoting the polarization of CD4+ IRBP-specific T cells. Of the TLR ligands examined, a TLR3 ligand (namely, polyI:C) was most effective in activating RACs, leading to the production of cytokines of the Th1 and Th17 types that induce uveitis in naive mice.