T cell exhaustion is a state of T cell dysfunction that arises during many chronic infections and cancer. It is defined by poor effector function, sustained expression of inhibitory receptors and a transcriptional state distinct from that of functional effector or memory T cells. Exhaustion prevents optimal control of infection and tumors. Recently, a clearer picture of the functional and phenotypic profile of exhausted T cells has emerged and T cell exhaustion has been defined in many experimental and clinical settings. Although the pathways involved remain to be fully defined, advances in the molecular delineation of T cell exhaustion are clarifying the underlying causes of this state of differentiation and also suggest promising therapeutic opportunities.

T cell exhaustion

CD4 T cells play critical roles in mediating adaptive immunity to a variety of pathogens. They are also involved in autoimmunity, asthma, and allergic responses as well as in tumor immunity. During TCR activation in a particular cytokine milieu, naive CD4 T cells may differentiate into one of several lineages of T helper (Th) cells, including Th1, Th2, Th17, and iTreg, as defined by their pattern of cytokine production and function. In this review, we summarize the discovery, functions, and relationships among Th cells; the cytokine and signaling requirements for their development; the networks of transcription factors involved in their differentiation; the epigenetic regulation of their key cytokines and transcription factors; and human diseases involving defective CD4 T cell differentiation.

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Differentiation of Effector CD4 T Cell Populations

T cell help to B cells is a fundamental aspect of adaptive immunity and the generation of immunological memory. Follicular helper CD4 T (T(FH)) cells are the specialized providers of B cell help. T(FH) cells depend on expression of the master regulator transcription factor Bcl6. Distinguishing features of T(FH) cells are the expression of CXCR5, PD-1, SAP (SH2D1A), IL-21, and ICOS, among other molecules, and the absence of Blimp-1 (prdm1). T(FH) cells are important for the formation of germinal centers. Once germinal centers are formed, T(FH) cells are needed to maintain them and to regulate germinal center B cell differentiation into plasma cells and memory B cells. This review covers T(FH) differentiation, T(FH) functions, and human T(FH) cells, discussing recent progress and areas of uncertainty or disagreement in the literature, and it debates the developmental relationship between T(FH) cells and other CD4 T cell subsets (Th1, Th2, Th17, iTreg).

Follicular Helper CD4 T Cells (TFH)

The IL-10 family of cytokines consists of nine members: IL-10, IL-19, IL-20, IL-22, IL-24, IL-26, and the more distantly related IL-28A, IL-28B, and IL-29. Evolutionarily, IL-10 family cytokines emerged before the adaptive immune response. These cytokines elicit diverse host defense mechanisms, especially from epithelial cells, during various infections. IL-10 family cytokines are essential for maintaining the integrity and homeostasis of tissue epithelial layers. Members of this family can promote innate immune responses from tissue epithelia to limit the damage caused by viral and bacterial infections. These cytokines can also facilitate the tissue-healing process in injuries caused by infection or inflammation. Finally, IL-10 itself can repress proinflammatory responses and limit unnecessary tissue disruptions caused by inflammation. Thus, IL-10 family cytokines have indispensable functions in many infectious and inflammatory diseases.

Regulation and Functions of the IL-10 Family of Cytokines in Inflammation and Disease

https://www.cell.com/cell/pdf/S0092-8674(11)00847-6.pdf

Control of TH17/Treg Balance by Hypoxia-Inducible Factor 1

AP-1 (activator protein-1) transcription factors, also known as JUN factors, are broadly acting factors regulating many inducible genetic responses. Schraml et al. report a surprising mechanism that expands the biological repertoire of the AP-1 family. They find that the AP-1 transcription factor Batf plays a critical role in the differentiation of IL17-producing T-helper cells. TH17 cells comprise a subset of CD4+ T cells that coordinate the inflammatory response in host defence but are pathogenic in autoimmunity. Here, the AP-1 transcription factor BATF is shown to have a critical role in TH17 cell differentiation, with Batf−/− mice displaying a defect in TH17 differentiation and resistance to experimental autoimmune encephalomyelitis. Activator protein 1 (AP-1, also known as JUN) transcription factors are dimers of JUN, FOS, MAF and activating transcription factor (ATF) family proteins characterized by basic region and leucine zipper domains1. Many AP-1 proteins contain defined transcriptional activation domains, but BATF and the closely related BATF3 (refs 2, 3) contain only a basic region and leucine zipper, and are considered to be inhibitors of AP-1 activity3,4,5,6,7,8. Here we show that Batf is required for the differentiation of IL17-producing T helper (TH17) cells9. TH17 cells comprise a CD4+ T-cell subset that coordinates inflammatory responses in host defence but is pathogenic in autoimmunity10,11,12,13. Batf-/- mice have normal TH1 and TH2 differentiation, but show a defect in TH17 differentiation, and are resistant to experimental autoimmune encephalomyelitis. Batf-/- T cells fail to induce known factors required for TH17 differentiation, such as RORγt11 (encoded by Rorc) and the cytokine IL21 (refs 14–17). Neither the addition of IL21 nor the overexpression of RORγt fully restores IL17 production in Batf-/- T cells. The Il17 promoter is BATF-responsive, and after TH17 differentiation, BATF binds conserved intergenic elements in the Il17a–Il17f locus and to the Il17, Il21 and Il22 (ref. 18) promoters. These results demonstrate that the AP-1 protein BATF has a critical role in TH17 differentiation.

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Ben Solomon

Papers

The AP-1 transcription factor Batf controls TH17 differentiation