The persistence of T cell memory

In Vitro Analysis of Thymocyte Signaling

When a developing thymocyte expresses a TCR, it is subjected to numerous interactions with self-peptide/MHC complexes that determine its fate. These include death by neglect, negative selection (apoptosis and lineage deviation), positive selection, and lineage commitment. Identifying signals that govern these unique cell fates requires the ability to assess the activity, level of expression, subcellular location, and molecular associations between numerous proteins within the developing T cell. Given the unique, temporal, and developmental changes that occur during development, isolating and analyzing small populations of thymocytes are necessary to get a complete picture of the development process. Thus, this chapter describes methods designed to analyze thymocyte signaling under various types of peptide-based stimulation in vitro.

In Vitro Analysis of Thymocyte Signaling

From the moment a developing thymocyte expresses a TCR, it is subjected to numerous interactions with self-peptide/MHC complexes that determine its ultimate fate. These include death by neglect, negative selection (apoptosis and lineage deviation), positive selection, and lineage commitment. The identification of signals that govern these unique cell fates requires the ability to assess the activity, level of expression, subcellular location, and the molecular associations of numerous proteins within the developing T cell. Thus, this chapter describes methods designed to analyze thymocyte signaling under various types of peptide-based stimulation in vitro.

FTOC-Based Analysis of Negative Selection

Potentially harmful T cell precursors are removed from the conventional T cell pool by negative selection. This process can involve the induction of apoptosis, anergy, receptor editing or deviation into a regulatory T cell lineage. As such this process is essential for the health of an organism through its contribution to central and peripheral tolerance. While a great deal is known about the process, the precise mechanisms that regulate negative selection are not clear. Furthermore, the signals that distinguish the different forms of negative selection are not fully understood. Numerous models exist with the potential to address these questions in vitro and in vivo. This chapter describes methods of fetal thymic organ culture designed to analyze the signals that determine these unique cell fates.

Inhibition of accelerated rejection mediated by alloreactive CD4⁺ memory T cells and prolonged allograft survival by arsenic trioxide

The aim of this study was to evaluate and determine the potential mechanisms of As₂O₃ in accelerated rejection mediated by alloreactive CD4⁺ memory T cells. Vascularized heterotopic cardiac transplantation from C57BL/6 mice to nude mice (pre-transferred CD4⁺ memory T cells) was performed on Day 0, and As₂O₃ was administered to recipient mice from Day 0 to 10. As a result, As₂O₃ could reduce the proliferation of allo-primed CD4⁺ memory T cells in vitro in MLR and the baseline rate of proliferation was restored by the addition of exogenous IL-2. In vivo, compared with the control[+] group, the mean survival time of cardiac allografts in the As₂O₃ group was prolonged from 5.8 ± 0.7 to 14.2 ± 2.5 days. Five days after transplantation, the relative gene expression of IL-2, IFN-γ and Foxp3 was reduced in the grafts by As₂O₃ treatment, but the expression of IL-10 and TGF-β was increased. Correspondingly, the proportions of CD4⁺ T cells, CD4⁺ memory T cells and regulatory T cells (Tregs), both in recipient spleens and lymph nodes, were lowered. These results indicate the potential of As2O3 as a novel immunosuppressant targeting CD4⁺ memory T cells.

Inflammation and TCR signal strength determine the breadth of the T cell response in a bim-dependent manner

Generating a diverse T cell memory population through vaccination is a promising strategy to overcome pathogen epitope variability and tolerance to tumor Ags. The effector and memory pool becomes broad in TCR diversity by recruiting high- and low-affinity T cells. We wanted to determine which factors dictate whether a memory T cell pool has a broad versus focused repertoire. We find that inflammation increases the magnitude of low- and high-affinity T cell responses equally well, arguing against a synergistic effect of TCR and inflammatory signals on T cell expansion. We dissect the differential effects of TCR signal strength and inflammation and demonstrate that they control effector T cell survival in a bim-dependent manner. Importantly, bim-dependent cell death is overcome with a high Ag dose in the context of an inflammatory environment. Our data define the framework for the generation of a broad T cell memory pool to inform future vaccine design.

Programmed death-1 shapes memory phenotype CD8 T cell subsets in a cell-intrinsic manner

Memory phenotype T cells, found in unimmunized mice, display phenotypic and functional traits of memory cells and provide essential protection against infections, playing a role in both innate and adaptive immune responses. Mechanisms governing homeostasis of these memory phenotype T cells remain ill-defined. In this study, we reveal a crucial role of the negative costimulator programmed death-1 (PD-1) in regulating developmental fates of memory phenotype cells. Thus, in lymphoid organs and tissues of PD-1 knockout (KO) mice a marked accumulation of functional effector memory (T(EM)) phenotype CD8 T cells was observed. T(EM) phenotype cells from PD-1 KO mice exhibit decreased proliferation but increased survival potential. These cells could produce effector molecules constitutively, in response to phorbol esters or through bystander activation by innate stimuli. Similarly, in lymphopenia-induced proliferating CD8 T cells, whereby normally naive T cells acquire a memory phenotype, skewing toward a T(EM) phenotype was prominent in the absence of PD-1. Acquisition of the T(EM) phenotype was a CD8 T cell-intrinsic phenomenon as demonstrated by mixed bone marrow transfer experiments. Importantly, adoptively transferred PD-1 KO CD8 central memory T (T(CM)) cells converted into the T(EM) phenotype, indicating that PD-1 sets a major checkpoint in the T(CM) to T(EM) phenotype differentiation process. This was reflected by distinct patterns of gene expression of PD-1 KO T(CM) phenotype cells revealed by global transcriptional analysis. Additionally, adoptively transferred PD-1 KO T(EM) phenotype cells converted to a lesser degree to a T(CM) phenotype. Collectively, these data suggest that PD-1 shapes memory phenotype CD8 T cell subsets.

Identification of T- and B-Cell Subsets That Expand in the Central and Peripheral Lymphoid Organs during the Establishment of Nut Allergy in an Adjuvant-Free Mouse Model

Nut allergies are potentially fatal and rarely outgrown for reasons that are not well understood. Phenotype of T- and B-cell subsets that expand during the early stages of nut allergy is largely unknown. Here we studied this problem using a novel mouse model of nut allergy. Mice were rendered hazelnut allergic by a transdermal sensitization/oral elicitation protocol. Using flow cytometry, the T- and B-cell phenotype in the bone marrow (BM), spleen, and the mesenteric lymph node (MLN) of allergic and control mice was analyzed. Nut allergic mice exhibited an expansion of CD4+ CD62L− T cells in BM and spleen; a similar trend was noted in the MLN. There was expansion of CD80+ B cells in BM and spleen and MLN and CD62L− cells in BM and spleen. Interestingly, among CD80+ B cells, significant proportion was CD73− particularly in the MLN. These data demonstrate that during the early establishment of hazelnut allergy there is (i) expansion of CD4+CD62L− T-cell subsets in both the BM and the periphery, (ii) expansion of CD80+ and CD62L− B-cell subsets in BM and the periphery, and (iii) a significant downregulation of CD73 on a subset of B cells in MLN.

Dissociating markers of senescence and protective ability in memory T cells

No unique transcription factor or biomarker has been identified to reliably distinguish effector from memory T cells. Instead a set of surface markers including IL-7Rα and KLRG1 is commonly used to predict the potential of CD8 effector T cells to differentiate into memory cells. Similarly, these surface markers together with the tumor necrosis factor family member CD27 are frequently used to predict a memory T cell's ability to mount a recall response. Expression of these markers changes every time a memory cell is stimulated and repeated stimulation can lead to T cell senescence and loss of memory T cell responsiveness. This is a concern for prime–boost vaccine strategies which repeatedly stimulate T cells with the aim of increasing memory T cell frequency. The molecular cues that cause senescence are still unknown, but cell division history is likely to play a major role. We sought to dissect the roles of inflammation and cell division history in developing T cell senescence and their impact on the expression pattern of commonly used markers of senescence. We developed a system that allows priming of CD8 T cells with minimal inflammation and without acquisition of maximal effector function, such as granzyme expression, but a cell division history similar to priming with systemic inflammation. Memory cells derived from minimal effector T cells are fully functional upon rechallenge, have full access to non-lymphoid tissue and appear to be less senescent by phenotype upon rechallenge. However, we report here that these currently used biomarkers to measure senescence do not predict proliferative potential or protective ability, but merely reflect initial priming conditions.

Autoimmune melanocyte destruction is required for robust CD8+ memory T cell responses to mouse melanoma

A link between autoimmunity and improved antitumor immunity has long been recognized, although the exact mechanistic relationship between these two phenomena remains unclear. In the present study we have found that vitiligo, the autoimmune destruction of melanocytes, generates self antigen required for mounting persistent and protective memory CD8+ T cell responses to melanoma. Vitiligo developed in approximately 60% of mice that were depleted of regulatory CD4+ T cells and then subjected to surgical excision of large established B16 melanomas. Mice with vitiligo generated 10-fold larger populations of CD8+ memory T cells specific for shared melanoma/melanocyte antigens. CD8+ T cells in mice with vitiligo acquired phenotypic and functional characteristics of effector memory, suggesting that they were supported by ongoing antigen stimulation. Such responses were not generated in melanocyte-deficient mice, indicating a requirement for melanocyte destruction in maintaining CD8+ T cell immunity to melanoma. Vitiligo-associated memory CD8+ T cells provided durable tumor protection, were capable of mounting a rapid recall response to melanoma, and did not demonstrate phenotypic or functional signs of exhaustion even after many months of exposure to antigen. This work establishes melanocyte destruction as a key determinant of lasting melanoma-reactive immune responses, thus illustrating that immune-mediated destruction of normal tissues can perpetuate adaptive immune responses to cancer.

The burden of obesity on infectious disease

The world is now experiencing an epidemic of obesity. Although the effects of obesity on the development of metabolic and cardiovascular problems are well studied, much less is known about the impact of obesity on immune function and infectious disease. Studies in obese humans and with obese animal models have repeatedly demonstrated impaired immune function, including decreased cytokine production, decreased response to antigen/mitogen stimulation, reduced macrophage and dendritic cell function, and natural killer cell impairment. Recent studies have demonstrated that the impaired immune response in the obese host leads to increased susceptibility to infection with a number of different pathogens such as community-acquired tuberculosis, influenza, Mycobacterium tuberculosis, coxsackievirus, Helicobacter pylori and encephalomyocarditis virus. While no specific mechanism has been defined for the decreased immune response to infectious disease in the obese host, several obesity-associated changes such as excessive inflammation, altered adipokine signaling, metabolic changes and even epigenetic regulation could affect the immune response. This review will discuss what is currently known about the relationship between obesity and infectious disease.