Recent progress in our understanding of mechanisms by which the immunosuppressive cytokine interleukin-10 (IL-10) participates in an ever-increasing diversity of T-cell lineages to maintain immune homeostasis has broadened the framework for defining regulatory and effector T cells and has blurred the lines between them. In this review, we highlight established and emerging roles for IL-10 produced by distinct CD4(+) T-cell lineages that underlie its non-redundant role in curbing immune responses to the intestinal microbiota at steady state and its role to limit T-cell-driven inflammation in responses to pathogens.
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