Responding to infection and apoptosis-a task for T_H17 cells

Two of the critical cytokines required for the differentiation of T helper 17 (T_H17) cells from naive CD4 T cells are transforming growth factor-beta (TGF-β) and interleukin-6 (IL-6). Innate recognition of apoptotic cells in the presence of Toll-like receptor engagement directs the simultaneous synthesis of these cytokines by antigen-presenting cells (APCs), and as such provides a cytokine milieu that favors T_H17 cell induction. In this situation, APCs are activated in response to ligands derived from apoptotic cells, but also to those from the infecting pathogen. Induction of a T_H17 response against Citrobacter rodentium infection was dependent on the ability of Citrobacter to induce apoptosis of intestinal epithelial cells. In this review, we will discuss how simultaneous activation of inflammatory and noninflammatory pattern recognition receptors on APCs impacts T helper cell differentiation, and what relevance this effect has on the immune response generated against bacterial infections that cause host cell apoptosis.