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

•We investigate the effect of bacterial flagellin and diphtheria toxin on thymocytes.•Both molecules do not induce cytokine-producing cells by themselves.•However, both flagellin and diphtheria toxin co-activate IL-17-producing thymocytes.•There is no additive effect of flagellin and diphtheria toxin combined.•Our results suggest a new mechanism of diphtheria toxin to act on immune cells.

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.