Regulation of innate and adaptive immune responses by Toll-like receptor ligands

Abstract

Toll-like receptors (TLR) recognize conserved molecular patterns of microorganisms and are essential for the activation of host immunity. Whereas the immune-stimulatory effects of TLR ligands are well described, the circumstances of TLR-mediated inhibition of immune responses are still largely unknown. In this thesis, it was investigated how ligands for TLR9 and TLR7 modulated cytotoxic T cell responses in mice infected with recombinant adenovirus or<i> Plasmodium ssp. in vivo</i>. The results of this thesis point out, that the anatomical site, dose, and time-point of TLR ligand application determined whether an immune response to an infection was stimulated or suppressed. In contrast to subcutaneous application of TLRs that stimulate adaptive immune responses, systemic application of TLR ligands in high doses prior to infection suppressed CD8 T cell cytotoxicity. TLR-ligand induced suppression of adaptive immune responses was characterized by reduced numbers of antigen-specific T cells and a (subsequent) lack of cytotoxicity and was crucially dependent on TLR expression. Importantly, by adoptive transfer of activated antigen-specific CD4⁺ T helper cells into TLR ligand-treated mice it was possible to overcome TLR-ligand mediated suppression of CTL responses. This indicated that a block in CD4⁺ T cell activation is likely to account for insufficient CTL induction. Although TLR ligands caused functional impairment of dendritic cells, we doubt that DC paralysis was solely responsible for CTL suppression. An adoptive transfer of exogenously matured and antigen loaded DCs did not overcome CTL suppression in TLR ligand-treated mice, which further indicated that TLR ligand-induced CTL suppression was rather due to the induction of an inhibitory milieu than due to inability of DCs to activate T cells. In TLR7 ligand-mediated CTL suppression, inhibitory effects could be attributed to type I interferons.<br /> A physiological relevance of TLR-mediated suppression of effector immune responses was shown in mouse models of <i>Plasmodium ssp.</i> infection. Cerebral malaria (CM) is a severe immune-mediated complication in <i>P. falciparum</i> infection in humans. IFNγ and T cells are among the few factors that are known to be essential in the pathogenesis of experimental CM (ECM) in mice upon <i>P. berghei</i> ANKA (PbA) infection. Here, we show by use of knock out mice that TLR2, -3 and -9 and IL-12 are crucial for the development of ECM upon PbA infection. <i>PbA</i>-infected mice were also significantly protected against ECM if they were depleted of DCs or were splenectomized. However, a key finding of this thesis was the observation that PbA parasites themselves were able to prevent detrimental effector responses. Wild type mice but not mice deficient in TLR2, TLR9, IL-12p40, IL-10 or iNOS were significantly protected against ECM upon infection with a highly enhanced dose of PbA. Also, systemic application of TLR9 ligand CpG protected PbA-low dose-infected wild type mice but not TLR9 deficient littermates from ECM. CpG blocked the production of pro-inflammatory mediators by DCs and prevented the generation of <i>Plasmodium</i> specific CD8⁺ T cell responses. These results strongly suggest that TLRs fulfill dual functions in regulation of immune responses in <i>Plasmodium</i> infection. <br /> Overall, we conclude that the induction of immune suppression represents a sort of “emergency shutdown” that is induced if circumstances of TLR activation or infection due to excessive inflammation endanger the host’s life. Prevention of immune pathology ensures survival of the host. Pathogens such as <i>Plasmodium ssp.</i> may benefit from these mechanisms of TLR-mediated immune suppression as it offers a possibility of immune evasion or silencing that allows completion of their life cycle.