SnapShot: T Cell Exhaustion

Exhaustion is a state of T cell differentiation common during chronic infection and cancer that results in T cell dysfunction. Exhausted T cells (TEX) progressively lose effector functions and develop defects in memory T cell properties due to exposure to persistent antigen and inflammation. Exhaustion can be reversed, at least partially, by blocking inhibitory pathways such as PD-1, resulting in improved pathogen or tumor control. To view this SnapShot, open or download the PDF

A Mouse Model of Vitiligo with Focused Epidermal Depigmentation Requires IFN-γ for Autoreactive CD8+CD8^+ T Cell Accumulation in the Skin

Vitiligo is an autoimmune disease of the skin causing disfiguring patchy depigmentation of the epidermis and, less commonly, hair. Therapeutic options for vitiligo are limited, reflecting in part limited knowledge of disease pathogenesis. Existing mouse models of vitiligo consist of hair depigmentation but lack prominent epidermal involvement, which is the hallmark of human disease. They are thus unable to provide a platform to fully investigate disease mechanisms and treatment. $CD8^+$ T cells have been implicated in the pathogenesis of vitiligo and expression of interferon-gamma (IFN-γ) is increased in the lesional skin of patients, however it is currently unknown what role IFN-γ plays in disease. Here, we have developed an adoptive transfer mouse model of vitiligo using melanocyte-specific $CD8^+$ T cells, which recapitulates the human condition by inducing epidermal depigmentation while sparing the hair. Like active lesions in human vitiligo, histology of depigmenting skin reveals a patchy mononuclear infiltrate and single-cell infiltration of the epidermis. Depigmentation is accompanied by accumulation of autoreactive $CD8^+$ T cells in the skin, quantifiable loss of tyrosinase transcript, and local IFN-γ production. Neutralization of IFN-γ with antibody prevents $CD8^+$ T cell accumulation and depigmentation, suggesting a therapeutic potential for this approach

Interleukin 15 Is Required for Proliferative Renewal of Virus-specific Memory CD8 T Cells

The generation and efficient maintenance of antigen-specific memory T cells is essential for long-lasting immunological protection. In this study, we examined the role of interleukin (IL)-15 in the generation and maintenance of virus-specific memory CD8 T cells using mice deficient in either IL-15 or the IL-15 receptor α chain. Both cytokine- and receptor-deficient mice made potent primary CD8 T cell responses to infection with lymphocytic choriomeningitis virus (LCMV), effectively cleared the virus and generated a pool of antigen-specific memory CD8 T cells that were phenotypically and functionally similar to memory CD8 T cells present in IL-15+/+ mice. However, longitudinal analysis revealed a slow attrition of virus-specific memory CD8 T cells in the absence of IL-15 signals.This loss of CD8 T cells was due to a severe defect in the proliferative renewal of antigen-specific memory CD8 T cells in IL-15−/− mice. Taken together, these results show that IL-15 is not essential for the generation of memory CD8 T cells, but is required for homeostatic proliferation to maintain populations of memory cells over long periods of time

Type I interferon receptor deficiency in dendritic cells facilitates systemic murine norovirus persistence despite enhanced adaptive immunity

In order for a virus to persist, there must be a balance between viral replication and immune clearance. It is commonly believed that adaptive immunity drives clearance of viral infections and, thus, dysfunction or viral evasion of adaptive immunity is required for a virus to persist. Type I interferons (IFNs) play pleiotropic roles in the antiviral response, including through innate control of viral replication. Murine norovirus (MNoV) replicates in dendritic cells (DCs) and type I IFN signaling in DCs is important for early control of MNoV replication. We show here that the non-persistent MNoV strain CW3 persists systemically when CD11c positive DCs are unable to respond to type I IFN. Persistence in this setting is associated with increased early viral titers, maintenance of DC numbers, increased expression of DC activation markers and an increase in CD8 T cell and antibody responses. Furthermore, CD8 T cell function is maintained during the persistent phase of infection and adaptive immune cells from persistently infected mice are functional when transferred to Rag1-/- recipients. Finally, increased early replication and persistence are also observed in mixed bone marrow chimeras where only half of the CD11c positive DCs are unable to respond to type I IFN. These findings demonstrate that increased early viral replication due to a cell-intrinsic innate immune deficiency is sufficient for persistence and a functional adaptive immune response is not sufficient for viral clearance

Differentiation and Protective Capacity of Virus-Specific CD8

Noroviruses can establish chronic infections with active viral shedding in healthy humans but whether persistence is associated with adaptive immune dysfunction is unknown. We used genetically engineered strains of mouse norovirus (MNV) to investigate CD8+ T cell differentiation during chronic infection. We found that chronic infection drove MNV-specific tissue-resident memory (Trm) CD8+ T cells to a differentiation state resembling inflationary effector responses against latent cytomegalovirus with only limited evidence of exhaustion. These MNV-specific Trm cells remained highly functional yet appeared ignorant of ongoing viral replication. Pre-existing MNV-specific Trm cells provided partial protection against chronic infection but largely ceased to detect virus within 72 hours of challenge, demonstrating rapid sequestration of viral replication away from T cells. Our studies revealed a strategy of immune evasion by MNV via the induction of a CD8+ T cell program normally reserved for latent pathogens and persistence in an immune-privileged enteric niche. Chronic infections often cause T cell dysfunction, but how noroviruses (NV) evade immunity is unknown. Tomov et al. show that gut-resident T cells against NV remain functional but ignorant of chronic viral replication, suggesting that NV persists in an immune-privileged enteric niche. © 2017 Elsevier Inc

TCR Signal Transduction in Antigen-Specific Memory CD8 T Cells

Memory T cells are more responsive to Ag than naive cells. To determine whether memory T cells also have more efficient TCR signaling, we compared naive, effector, and memory CD8 T cells of the same antigenic specificity. Surprisingly, initial CD3 signaling events are indistinguishable. However, memory T cells have more extensive lipid rafts with higher phosphoprotein content before TCR engagement. Upon activation in vivo, they more efficiently induce phosphorylation of-LAT (linker for activation of T cells), ERK (extracellular signal-regulated kinase), JNK (c-Jun N-terminal kinase), and p38. Thus, memory CD8 T cells do not increase their TCR sensitivity, but are better poised to augment downstream signals. We propose that this regulatory mechanism might increase signal transduction in memory T cells, while limiting TCR cross-reactivity and autoimmunity

Requirement for T-bet in the aberrant differentiation of unhelped memory CD8+ T cells

Immunity to intracellular pathogens requires dynamic balance between terminal differentiation of short-lived, cytotoxic effector CD8+ T cells and self-renewal of central–memory CD8+ T cells. We now show that T-bet represses transcription of IL-7Rα and drives differentiation of effector and effector–memory CD8+ T cells at the expense of central–memory cells. We also found T-bet to be overexpressed in CD8+ T cells that differentiated in the absence of CD4+ T cell help, a condition that is associated with defective central–memory formation. Finally, deletion of T-bet corrected the abnormal phenotypic and functional properties of “unhelped” memory CD8+ T cells. T-bet, thus, appears to function as a molecular switch between central– and effector–memory cell differentiation. Antagonism of T-bet may, therefore, represent a novel strategy to offset dysfunctional programming of memory CD8+ T cells

Enhancing therapeutic vaccination by blocking PD-1–mediated inhibitory signals during chronic infection

Therapeutic vaccination is a potentially promising strategy to enhance T cell immunity and viral control in chronically infected individuals. However, therapeutic vaccination approaches have fallen short of expectations, and effective boosting of antiviral T cell responses has not always been observed. One of the principal reasons for the limited success of therapeutic vaccination is that virus-specific T cells become functionally exhausted during chronic infections. We now provide a novel strategy for enhancing the efficacy of therapeutic vaccines. In this study, we show that blocking programmed death (PD)-1/PD-L1 inhibitory signals on exhausted CD8+ T cells, in combination with therapeutic vaccination, synergistically enhances functional CD8+ T cell responses and improves viral control in mice chronically infected with lymphocytic choriomeningitis virus. This combinatorial therapeutic vaccination was effective even in the absence of CD4+ T cell help. Thus, our study defines a potent new approach to augment the efficacy of therapeutic vaccination by blocking negative signals. Such an approach may have broad applications in developing treatment strategies for chronic infections in general, and perhaps also for tumors