Helper T cell differentiation and the problem of cellular inheritance

Human CD4+ memory phenotype T cells use mitochondrial metabolism to generate sensitive IFN-γ responses

The transition of naive T lymphocytes into antigenically activated effector cells is associated with a metabolic shift from oxidative phosphorylation to aerobic glycolysis. This shift facilitates production of the key anti-tumor cytokine interferon (IFN)-γ; however, an associated loss of mitochondrial efficiency in effector T cells ultimately limits anti-tumor immunity. Memory phenotype (MP) T cells are a newly recognized subset that arises through homeostatic activation signals following hematopoietic transplantation. We show here that human CD4+ MP cell differentiation is associated with increased glycolytic and oxidative metabolic activity, but MP cells retain less compromised mitochondria compared to effector CD4+ T cells, and their IFN-γ response is less dependent on glucose and more reliant on glutamine. MP cells also produced IFN-γ more efficiently in response to weak T cell receptor (TCR) agonism than effectors and mediated stronger responses to transformed B cells. MP cells may thus be particularly well suited to carry out sustained immunosurveillance against neoplastic cells.

Human invariant natural killer T cells acquire transient innate responsiveness via histone H4 acetylation induced by weak TCR stimulation

Weak TCR stimulation of iNKT cells, such as that resulting from self-antigen recognition, induces histone modifications at the IFNG locus that allow the iNKT cells to subsequently produce IFN-γ in response to proinflammatory cytokines alone.

Invariant NKT cells (iNKT cells) are innate T lymphocytes that are thought to play an important role in producing an early burst of IFN-γ that promotes successful tumor immunosurveillance and antimicrobial immunity. The cellular activation processes underlying innate IFN-γ production remain poorly understood. We show here that weak T cell receptor (TCR) stimulation that does not directly activate iNKT cell IFN-γ messenger RNA transcription nevertheless induces histone H4 acetylation at specific regions near the IFNG gene locus. This renders the iNKT cells able to produce IFN-γ in an innate manner (i.e., not requiring concurrent TCR stimulation) upon exposure to IL-12 and IL-18. The iNKT cells retain the capacity for innate activation for hours to days after the initial weak TCR stimulation, although their innate responsiveness gradually declines as a function of histone deacetylation. These results explain how iNKT cells are able to mediate rapid innate IFN-γ secretion in a manner that does not require them to undergo permanent T_H1 differentiation. Moreover, our results also indicate that iNKT cell motility is maintained during activation by IL-12 and IL-18. Therefore, iNKT cells activated through this pathway can continue to migrate and may thus disseminate the IFN-γ that they produce, which may amplify its impact.

T-cell identity and epigenetic memory

T-cell development endows cells with a flexible range of effector differentiation options, superimposed on a stable core of lineage-specific gene expression that is maintained while access to alternative hematopoietic lineages is permanently renounced. This combination of features could be explained by environmentally responsive transcription factor mobilization overlaying an epigenetically stabilized base gene expression state. For example, "poising" of promoters could offer preferential access to T-cell genes, while repressive histone modifications and DNA methylation of non-T regulatory genes could be responsible for keeping non-T developmental options closed. Here, we critically review the evidence for the actual deployment of epigenetic marking to support the stable aspects of T-cell identity. Much of epigenetic marking is dynamically maintained or subject to rapid modification by local action of transcription factors. Repressive histone marks are used in gene-specific ways that do not fit a simple, developmental lineage-exclusion hierarchy. We argue that epigenetic analysis may achieve its greatest impact for illuminating regulatory biology when it is used to locate cis-regulatory elements by catching them in the act of mediating regulatory change.

A distal region in the interferon-gamma gene is a site of epigenetic remodeling and transcriptional regulation by interleukin-2

Interferon-gamma (IFN-gamma) is a multifunctional cytokine that defines the development of Th1 cells and is critical for host defense against intracellular pathogens. IL-2 is another key immunoregulatory cytokine that is involved in T helper differentiation and is known to induce IFN-gamma expression in natural killer (NK) and T cells. Despite concerted efforts to identify the one or more transcriptional control mechanisms by which IL-2 induces IFN-gamma mRNA expression, no such genomic regulatory regions have been described. We have identified a DNase I hypersensitivity site approximately 3.5-4.0 kb upstream of the transcriptional start site. Using chromatin immunoprecipitation assays we found constitutive histone H3 acetylation in this distal region in primary human NK cells, which is enhanced by IL-2 treatment. This distal region is also preferentially acetylated on histones H3 and H4 in primary Th1 cells as compared with Th2 cells. Within this distal region we found a Stat5-like motif, and in vitro DNA binding assays as well as in vivo chromosomal immunoprecipitation assays showed IL-2-induced binding of both Stat5a and Stat5b to this distal element in the IFNG gene. We examined the function of this Stat5-binding motif by transfecting human peripheral blood mononuclear cells with -3.6 kb of IFNG-luciferase constructs and found that phorbol 12-myristate 13-acetate/ionomycin-induced transcription was augmented by IL-2 treatment. The effect of IL-2 was lost when the Stat5 motif was disrupted. These data led us to conclude that this distal region serves as both a target of chromatin remodeling in the IFNG locus as well as an IL-2-induced transcriptional enhancer that binds Stat5 proteins.

T cytotoxic-1 CD8+ T cells are effector cells against pneumocystis in mice

Host defenses are profoundly compromised in HIV-infected hosts due to progressive depletion of CD4+ T lymphocytes. A hallmark of HIV infection is Pneumocystis carinii (PC) pneumonia. Recently, CD8+ T cells, which are recruited to the lung in large numbers in response to PC infection, have been associated with some level of host defense as well as contributing to lung injury in BALB/c mice. In this study, we show that CD8+ T cells that have a T cytotoxic-1 response to PC in BALB/c mice, as determined by secretion of IFN-gamma, have in vitro killing activity against PC and effect clearance of the organism in adoptive transfer studies. Moreover, non-T cytotoxic-1 CD8+ T cells lacked in vitro effector activity and contributed to lung injury upon adoptive transfer. This dichotomous response in CD8+ T cell response may in part explain the clinical heterogeneity in the severity of PC pneumonia.