CD28 individual signaling up-regulates IL-22 expression and IL-22-mediated effector functions in human T lymphocytes

IL‐22 is a member of the IL‐10 cytokine family involved in host protection against extracellular pathogens, by promoting epithelial cell regeneration and barrier functions. Dysregulation of IL‐22 production has also frequently been observed in acute respiratory distress syndrome (ARDS) and several chronic inflammatory and autoimmune diseases. We have previously described that human CD28, a crucial co‐stimulatory receptor necessary for full T cell activation, is also able to act as a TCR independent signalling receptor and to induce the expression of IL‐17A and inflammatory cytokines related to Th17 cells, which together with Th22 cells represent the main cellular source of IL‐22. Here we characterized the role of CD28 autonomous signalling in regulating IL‐22 expression in human CD4+ T cells. We show that CD28 stimulation in the absence of TCR strongly up‐regulates IL‐22 gene expression and secretion. As recently observed for IL‐17A, we also found that CD28‐mediated regulation of IL‐22 transcription requires the cooperative activities of both IL‐6‐activated STAT3 and RelA/NF‐κ transcription factors. CD28‐mediated IL‐22 production also promotes the barrier functions of epithelial cells by inducing mucin and metalloproteases expression. Finally, by using specific inhibitory drugs, we also identified CD28‐associated class 1A phosphatidylinositol 3‐kinase (PI3K) as a pivotal mediator of CD28‐mediated IL‐22 expression and IL‐ 22‐dependent epithelial cell barrier functions.