Ian A. Parish

The Transcriptional Repressor Bcl-6 Directs T Follicular Helper Cell Lineage Commitment

Follicular helper T (Tfh) cells provide selection signals to germinal center B cells, which is essential for long-lived antibody responses. High CXCR5 and low CCR7 expression facilitates their homing to B cell follicles and distinguishes them from T helper 1 (Th1), Th2, and Th17 cells. Here, we showed that Bcl-6 directs Tfh cell differentiation: Bcl-6-deficient T cells failed to develop into Tfh cells and could not sustain germinal center responses, whereas forced expression of Bcl-6 in CD4(+) T cells promoted expression of the hallmark Tfh cell molecules CXCR5, CXCR4, and PD-1. Bcl-6 bound to the promoters of the Th1 and Th17 cell transcriptional regulators T-bet and RORgammat and repressed IFN-gamma and IL-17 production. Bcl-6 also repressed expression of many microRNAs (miRNAs) predicted to control the Tfh cell signature, including miR-17-92, which repressed CXCR5 expression. Thus, Bcl-6 positively directs Tfh cell differentiation, through combined repression of miRNAs and transcription factors.

Cross-presentation, dendritic cell subsets, and the generation of immunity to cellular antigens

Summary:  Cross‐presentation involves the uptake and processing of exogenous antigens within the major histocompatibility complex (MHC) class I pathway. This process is primarily performed by dendritic cells (DCs), which are not a single cell type but may be divided into several distinct subsets. Those expressing CD8α together with CD205, found primarily in the T‐cell areas of the spleen and lymph nodes, are the major subset responsible for cross‐presenting cellular antigens. This ability is likely to be important for the generation of cytotoxic T‐cell immunity to a variety of antigens, particularly those associated with viral infection, tumorigenesis, and DNA vaccination. At present, it is unclear whether the CD8α‐expressing DC subset captures antigen directly from target cells or obtains it indirectly from intermediary DCs that traffic from peripheral sites. In this review, we examine the molecular basis for cross‐presentation, discuss the role of DC subsets, and examine the contribution of this process to immunity, with some emphasis on DNA vaccination.

Natural killer cell activation enhances immune pathology and promotes chronic infection by limiting CD8+ T-cell immunity

Infections with HIV, hepatitis B virus, and hepatitis C virus can turn into chronic infections, which currently affect more than 500 million patients worldwide. It is generally thought that virus-mediated T-cell exhaustion limits T-cell function, thus promoting chronic disease. Here we demonstrate that natural killer (NK) cells have a negative impact on the development of T-cell immunity by using the murine lymphocytic choriomeningitis virus. NK cell-deficient (Nfil3−/−, E4BP4−/−) mice exhibited a higher virus-specific T-cell response. In addition, NK cell depletion caused enhanced T-cell immunity in WT mice, which led to rapid virus control and prevented chronic infection in lymphocytic choriomeningitis virus clone 13- and reduced viral load in DOCILE-infected animals. Further experiments showed that NKG2D triggered regulatory NK cell functions, which were mediated by perforin, and limited T-cell responses. Therefore, we identified an important role of regulatory NK cells in limiting T-cell immunity during virus infection.

Diversity in CD8+ T cell differentiation

CD8(+) T cells are key effector cells of the adaptive immune system, however their activity must be tightly regulated to allow pathogen clearance whilst preventing immunopathology and autoimmunity. In this review, we summarise the diversity of responses that CD8(+) T cells make to antigenic stimulation with a focus on how CD8(+) T cell responses are regulated to achieve different immune outcomes. In particular, we discuss phenotypic diversity during tolerance induction as well as signals that drive effector and memory cell differentiation in response to infection.

Dynamic Histone Variant Exchange Accompanies Gene Induction in T Cells

ABSTRACT Changes in chromatin composition are often a prerequisite for gene induction. Nonallelic histone variants have recently emerged as key players in transcriptional control and chromatin modulation. While the changes in chromatin accessibility and histone posttranslational modification (PTM) distribution that accompany gene induction are well documented, the dynamics of histone variant exchange that parallel these events are still poorly defined. In this study, we have examined the changes in histone variant distribution that accompany activation of the inducible CD69 and heparanase genes in T cells. We demonstrate that the chromatin accessibility increases that accompany the induction of both of these genes are not associated with nucleosome loss but instead are paralleled by changes in histone variant distribution. Specifically, induction of these genes was paralleled by depletion of the H2A.Z histone variant and concomitant deposition of H3.3. Furthermore, H3.3 deposition was accompanied by changes in PTM patterns consistent with H3.3 enriching or depleting different PTMs upon incorporation into chromatin. Nevertheless, we present evidence that these H3.3-borne PTMs can be negated by recruited enzymatic activities. From these observations, we propose that H3.3 deposition may both facilitate chromatin accessibility increases by destabilizing nucleosomes and compete with recruited histone modifiers to alter PTM patterns upon gene induction.

The molecular signature of CD8+ T cells undergoing deletional tolerance

Peripheral tolerance induction is critical for the maintenance of self-tolerance and can be mediated by immunoregulatory T cells or by direct induction of T-cell anergy or deletion. Although the molecular processes underlying anergy have been extensively studied, little is known about the molecular basis for peripheral T-cell deletion. Here, we determined the gene expression signature of peripheral CD8(+) T cells undergoing deletional tolerance, relative to those undergoing immunogenic priming or lymphopenia-induced proliferation. From these data, we report the first detailed molecular signature of cells undergoing deletion. Consistent with defective cytolysis, these cells exhibited deficiencies in granzyme up-regulation. Furthermore, they showed antigen-driven Bcl-2 down-regulation and early up-regulation of the proapoptotic protein Bim, consistent with the requirement of this BH3-only protein for peripheral T-cell deletion. Bim up-regulation was paralleled by defective interleukin-7 receptor alpha (IL-7Ralpha) chain reexpression, suggesting that Bim-dependent death may be triggered by loss of IL-7/IL-7R signaling. Finally, we observed parallels in molecular signatures between deletion and anergy, suggesting that these tolerance pathways may not be as molecularly distinct as previously surmised.

Prostaglandin E2 and programmed cell death 1 signaling coordinately impair CTL function and survival during chronic viral infection

More than 10% of the worlds population is chronically infected with HIV, hepatitis C virus (HCV) or hepatitis B virus (HBV), all of which can cause severe disease and death. These viruses persist in part because continuous antigenic stimulation causes the deterioration of virus-specific cytotoxic T lymphocyte (CTL) function and survival. Additionally, antiviral CTLs autonomously suppress their responses to limit immunopathology by upregulating inhibitory receptors such as programmed cell death 1 (PD-1). Identification and blockade of the pathways that induce CTL dysfunction may facilitate the clearance of chronic viral infections. We found that the prostaglandin E2 (PGE2) receptors EP2 and EP4 were upregulated on virus-specific CTLs during chronic lymphocytic choriomeningitis virus (LCMV) infection and suppressed CTL survival and function. We show that the combined blockade of PGE2 and PD-1 signaling was therapeutic in terms of improving viral control and augmenting the numbers of functional virus-specific CTLs. Thus, PGE2 inhibition is both an independent candidate therapeutic target and a promising adjunct therapy to PD-1 blockade for the treatment of HIV and other chronic viral infections.

Reducing Mitochondrial ROS Improves Disease-related Pathology in a Mouse Model of Ataxia-telangiectasia

The disease ataxia-telangiectasia (A-T) has no cure and few treatment options. It is caused by mutations in the ATM kinase, which functions in the DNA-damage response and redox sensing. In addition to severe cerebellar degeneration, A-T pathology includes cancer predisposition, sterility, immune system dysfunction, and bone marrow abnormalities. These latter phenotypes are recapitulated in the ATM null (ATM(-/-)) mouse model of the disease. Since oxidative stress and mitochondrial dysfunction are implicated in A-T, we determined whether reducing mitochondrial reactive oxygen species (ROS) via overexpression of catalase targeted to mitochondria (mCAT) alleviates A-T-related pathology in ATM(-/-) mice. We found that mCAT has many beneficial effects in this context, including reduced propensity to develop thymic lymphoma, improved bone marrow hematopoiesis and macrophage differentiation in vitro, and partial rescue of memory T-cell developmental defects. Our results suggest that positive effects observed on cancer development may be linked to mCAT reducing mitochondrial ROS, lactate production, and TORC1 signaling in transforming double-positive cells, whereas beneficial effects in memory T cells appear to be TORC1-independent. Altogether, this study provides proof-of-principle that reducing mitochondrial ROS production per se may be therapeutic for the disease, which may have advantages compared with more general antioxidant strategies.

Chronic viral infection promotes sustained Th1-derived immunoregulatory IL-10 via BLIMP-1

During the course of many chronic viral infections, the antiviral T cell response becomes attenuated through a process that is regulated in part by the host. While elevated expression of the immunosuppressive cytokine IL-10 is involved in the suppression of viral-specific T cell responses, the relevant cellular sources of IL-10, as well as the pathways responsible for IL-10 induction, remain unclear. In this study, we traced IL-10 production over the course of chronic lymphocytic choriomeningitis virus (LCMV) infection in an IL-10 reporter mouse line. Using this model, we demonstrated that virus-specific T cells with reduced inflammatory function, particularly Th1 cells, display elevated and sustained IL-10 expression during chronic LCMV infection. Furthermore, ablation of IL-10 from the T cell compartment partially restored T cell function and reduced viral loads in LCMV-infected animals. We found that viral persistence is needed for sustained IL-10 production by Th1 cells and that the transcription factor BLIMP-1 is required for IL-10 expression by Th1 cells. Restimulation of Th1 cells from LCMV-infected mice promoted BLIMP-1 and subsequent IL-10 expression, suggesting that constant antigen exposure likely induces the BLIMP-1/IL-10 pathway during chronic viral infection. Together, these data indicate that effector T cells self-limit their responsiveness during persistent viral infection via an IL-10-dependent negative feedback loop.

Too dangerous to ignore: self-tolerance and the control of ignorant autoreactive T cells.

Self‐reactive T cells often escape thymic negative selection and are released into the periphery. While many of these T cells are tolerized by peripheral deletion or anergy, a proportion persists in a naïve (or ignorant) state. Self‐ignorant T cells are probably one of the greatest threats to the maintenance of self‐tolerance, as inadvertent activation of these cells may provoke autoimmune pathology. Nevertheless, despite the presence of self‐ignorant T cells within most individuals, the majority of people fail to develop autoimmunity. The means by which self‐ignorant T cells are silenced by the immune system remains a major issue within the tolerance field and it has received surprisingly little attention within the literature. In this review, we first summarize the factors that allow such cells to persist in a self‐ignorant state, with a particular focus on the role of self‐antigen dose. We next consider the conditions under which such self‐reactive cells may become activated and speculate on how the immune system is able to prevent such cells from precipitating autoimmune disease.