Ex vivo activation of naturally occurring IL-17-producing T cells does not require IL-6

Bacteria and their cell wall components uniformly co-activate interleukin-17-producing thymocytes

Interleukin (IL)-17-producing T cells play a critical role in the immune response against microbial pathogens. Traditionally, experimental studies have focused upon understanding the activity of IL-17-producing T cells which differentiate from naive T cells in the peripheral immune system. However, we have demonstrated previously that IL-17-producing T cells are also present in the thymus of naive wild-type mice and can be co-activated there by microbial stimuli. Other studies have supported the concept that IL-17-producing thymocytes have a specific role in the immediate defence against microbial pathogens, which is independent from the development of an adaptive immune response. Given an important role of the thymus in systemic bacterial infection and sepsis, in this study we investigate the effect of a broad spectrum of bacteria and cell wall components on thymocyte cytokine production. Surprisingly, we find that all types of bacteria investigated (including non-pathogenic species) uniformly activate IL-17-producing thymocytes upon α-CD3 stimulation. In contrast, there is a heterogeneous effect on IL-6 and interferon (IFN)-γ-production with Gram-negative bacteria inducing far higher frequencies of IL-6- and IFN-γ-producing thymocytes than Gram-positive bacteria. We conclude that IL-17-producing thymocytes constitute a 'first line of recognition', but not a 'first line of defence' against bacteria in general. Their activity might lead to immune activation, but not necessarily to a pathological inflammatory disease condition. The difference between these two states might be determined by other immunological effector molecules, such as IL-6 and IFN-γ.

Bacterial flagellin and diphtheria toxin co-stimulate IL-17-producing thymocytes

IL-17-producing thymocytes have been recently described and are believed to play a role as an immune cell population which is able to react against microbial components rapidly. For this reason, we here investigated the ability of two microbial stimulants, bacterial flagellin (a ligand for TLR5) and diphtheria toxin from Corynebacterium diphtheriae, to activate or co-activate (together with α-CD3 stimulation) thymocyte cytokine production. We find that both bacterial molecules do not induce cytokine-production by themselves, but co-activate IL-17-producing thymocytes together with α-CD3. Since diphtheria toxin is unlikely to affect mouse cells through the same mechanism as the lethal effect on human cells, our results point to an additional mechanism of diphtheria toxin to act on immune cells. However, there is no additive or super-additive effect after stimulation with diphtheria toxin combined with flagellin and α-CD3 co-activation, which suggests that microbial stimuli used in this study can only activate a limited number of IL-17 producing thymocytes.

Induction of pro-inflammatory cytokine production in thymocytes by the immune response modifiers Imiquimod and Gardiquimod™

An emerging role is postulated for IL-17-producing thymocytes, which in their majority consist of IL-17-producing CD4(+) cells. For these, a specific role in the immediate defense against infectious pathogens is suggested, independent from the development of an adaptive immune response in the immune periphery. Immune response modifiers, like the TLR7 ligands Imiquimod and Gardiquimod™ are effective pharmacological therapeutics applied topically against dermal tumors and virus infections and have been demonstrated to activate immune cells. In this study, we investigated the effect of Imiquimod and Gardiquimod™ on murine thymocyte cytokine production with a particular focus on IL-17. We find that both substances dose-dependently are able to trigger IFN-γ and IL-6 production, but no IL-17 production. Moreover, a strong co-stimulating effect is detected on α-CD3-induced IFN-γ, IL-6 and IL-17 production. We conclude that Imiquimod and Gardiquimod™ are not only modifiers of the adaptive immune response, but might also have additional therapeutic potential by modifying the immune activity in the thymus.

T cell activation status determines the cytokine pattern induced by zymosan and bacterial DNA both in thymocytes and splenocytes

Proinflammatory cytokines are essential mediators of the immunopathology associated with microbial sepsis. The fungal cell wall component zymosan and bacterial DNA are well-studied experimental tools for investigating these processes, simulating the presence of fungal or bacterial infection. Cells of the immune periphery, but also immune cells in the thymus, are affected essentially by the presence of microbes or their immune stimuli in sepsis. For this reason, we investigated the cytokine pattern present in the spleen (containing mature immune cells) and the thymus (containing immature immune cells) upon exposure to zymosan and Escherichia coli DNA. To study the role of T cell activation status, we investigated ex-vivo cultures with and without αCD3 stimulation for changes in their cytokine secretion pattern as measured by cytokine enzyme-linked immunospot (ELISPOT) and flow cytometry analysis. We found that both substances strongly co-stimulate αCD3-induced interferon (IFN)-γ and interleukin (IL)-6 secretion in the thymus and in the spleen, but stimulate IL-17 production only moderately. Moreover, zymosan increases PLP peptide (PLPp)-specific IFN-γ and IL-6 production in experimental autoimmune encephalomyelitis (EAE) induced in Swiss Jim Lambert (SJL)/J mice, confirming that T cell activation status is crucial for the cytokines secreted by an immune cell population encountering a microbial pathogen or immunostimulating parts of it.