Resting memory CD8+ T cells are hyperreactive to antigenic challenge in vitro

BCG immunization induces CX3CR1hi effector memory T cells to provide cross-protection via IFN-γ-mediated trained immunity

After a century of using the Bacillus Calmette-Guérin (BCG) vaccine, our understanding of its ability to provide protection against homologous (Mycobacterium tuberculosis) or heterologous (for example, influenza virus) infections remains limited. Here we show that systemic (intravenous) BCG vaccination provides significant protection against subsequent influenza A virus infection in mice. We further demonstrate that the BCG-mediated cross-protection against influenza A virus is largely due to the enrichment of conventional CD4+ effector CX3CR1hi memory αβ T cells in the circulation and lung parenchyma. Importantly, pulmonary CX3CR1hi T cells limit early viral infection in an antigen-independent manner via potent interferon-γ production, which subsequently enhances long-term antimicrobial activity of alveolar macrophages. These results offer insight into the unknown mechanism by which BCG has persistently displayed broad protection against non-tuberculosis infections via cross-talk between adaptive and innate memory responses.

Survival and division fate programs are preserved but retuned during the naïve to memory CD8+ T-cell transition

Memory T cells are generated from naïve precursors undergoing proliferation during the initial immune response. Both naïve and memory T cells are maintained in a resting, quiescent state and respond to activation with a controlled proliferative burst and differentiation into effector cells. This similarity in the maintenance and response dynamics points to the preservation of key cellular fate programs; however, whether memory T cells have acquired intrinsic changes in these programs that may contribute to the enhanced immune protection in a recall response is not fully understood. Here we used a quantitative model-based analysis of proliferation and survival kinetics of in vitro-stimulated murine naïve and memory CD8+ T cells in response to homeostatic and activating signals to establish intrinsic similarities or differences within these cell types. We show that resting memory T cells display heightened sensitivity to homeostatic cytokines, responding to interleukin (IL)-2 in addition to IL-7 and IL-15. The proliferative response to αCD3 was equal in size and kinetics, demonstrating that memory T cells undergo the same controlled division burst and automated return to quiescence as naïve T cells. However, perhaps surprisingly, we observed reduced expansion of αCD3-stimulated memory T cells in response to activating signals αCD28 and IL-2 compared with naïve T cells. Overall, we demonstrate that although sensitivities to cytokine and costimulatory signals have shifted, fate programs regulating the scale of the division burst are conserved in memory T cells.

CD100 boosts the inflammatory response in the challenge phase of allergic contact dermatitis in mice

BACKGROUND

Allergic contact dermatitis (ACD) is an inflammatory disease with a complex pathophysiology in which epidermal-resident memory CD8+ T (TRM ) cells play a key role. The mechanisms involved in the activation of CD8+ TRM cells during allergic flare-up responses are not understood.

METHODS

The expression of CD100 and its ligand Plexin B2 on CD8+ TRM cells and keratinocytes before and after allergen exposure was determined by flow cytometry and RT-qPCR. The role of CD100 in the inflammatory response during the challenge phase of ACD was determined in a model of ACD in CD100 knockout and wild-type mice.

RESULTS

We show that CD8+ TRM cells express CD100 during homeostatic conditions and up-regulate it following re-exposure of allergen-experienced skin to the experimental contact allergen 1-fluoro-2,4-dinitrobenzene (DNFB). Furthermore, Plexin B2 is up-regulated on keratinocytes following exposure to some contact allergens. We show that loss of CD100 results in a reduced inflammatory response to DNFB with impaired production of IFNγ, IL-17A, CXCL1, CXCL2, CXCL5, and IL-1β and decreased recruitment of neutrophils to the epidermis.

CONCLUSION

Our study demonstrates that CD100 is expressed on CD8+ TRM cells and is required for full activation of CD8+ TRM cells and the flare-up response of ACD.

Pre-existing T Cell Memory to Novel Pathogens

Immunological experiences lead to the development of specific T and B cell memory, which readies the host for a later pathogen rechallenge. Currently, immunological memory is best understood as a linear process whereby memory responses are generated by and directed against the same pathogen. However, numerous studies have identified memory cells that target pathogens in unexposed individuals. How "pre-existing memory" forms and impacts the outcome of infection remains unclear. In this review, we discuss differences in the composition of baseline T cell repertoire in mice and humans, factors that influence pre-existing immune states, and recent literature on their functional significance. We summarize current knowledge on the roles of pre-existing T cells in homeostasis and perturbation and their impacts on health and disease.

Impact of chronic alcohol exposure on conventional and regulatory murine T cell subsets

Introduction

Chronic alcohol use poses significant negative consequences to public health and, among its many biologic effects, is associated with significant T cell dysregulation within the adaptive immune system that has yet to be fully characterized. Novel, automated strategies for high dimensional flow cytometric analysis of the immune system are rapidly improving researchers' ability to detect and characterize rare cell types.

Methods

Using a murine model of chronic alcohol ingestion in conjunction with viSNE and CITRUS analysis tools, we performed a machine-driven, exploratory analysis comparing rare splenic subpopulations within the conventional CD4+, regulatory CD4+ and CD8+ T cell compartments between alcohol- and water-fed animals.

Results

While there were no differences in the absolute numbers of bulk CD3+ T cells, bulk CD4+ T cells, bulk CD8+ T cells, Foxp3- CD4+ conventional T cells (Tconv) or Foxp3+ CD4+ regulatory T cells (Treg), we identified populations of naïve Helios+ CD4+Tconv and naïve CD103+ CD8+ splenic T cells that were decreased in chronically alcohol exposed mice versus water-fed controls. In addition, we identified increased CD69+ Treg and decreased CD103+ effector regulatory T cell (eTreg) subsets in conjunction with increased frequency of a population that may represent a transitional phenotype between central regulatory T cell (cTreg) and eTreg.

Discussion

These data provide further resolution into the character of decreased naïve T cell populations known to be present in alcohol exposed mice, as well as describe alterations in effector regulatory T cell phenotypes associated with the pathogenesis of chronic alcohol-induced immune dysfunction.

Calreticulin mutant myeloproliferative neoplasms induce MHC-I skewing, which can be overcome by an optimized peptide cancer vaccine

The majority of JAK2V617F-negative myeloproliferative neoplasms (MPNs) have disease-initiating frameshift mutations in calreticulin (CALR), resulting in a common carboxyl-terminal mutant fragment (CALRMUT), representing an attractive source of neoantigens for cancer vaccines. However, studies have shown that CALRMUT-specific T cells are rare in patients with CALRMUT MPN for unknown reasons. We examined class I major histocompatibility complex (MHC-I) allele frequencies in patients with CALRMUT MPN from two independent cohorts. We observed that MHC-I alleles that present CALRMUT neoepitopes with high affinity are underrepresented in patients with CALRMUT MPN. We speculated that this was due to an increased chance of immune-mediated tumor rejection by individuals expressing one of these MHC-I alleles such that the disease never clinically manifested. As a consequence of this MHC-I allele restriction, we reasoned that patients with CALRMUT MPN would not efficiently respond to a CALRMUT fragment cancer vaccine but would when immunized with a modified CALRMUT heteroclitic peptide vaccine approach. We found that heteroclitic CALRMUT peptides specifically designed for the MHC-I alleles of patients with CALRMUT MPN efficiently elicited a CALRMUT cross-reactive CD8+ T cell response in human peripheral blood samples but not to the matched weakly immunogenic CALRMUT native peptides. We corroborated this effect in vivo in mice and observed that C57BL/6J mice can mount a CD8+ T cell response to the CALRMUT fragment upon immunization with a CALRMUT heteroclitic, but not native, peptide. Together, our data emphasize the therapeutic potential of heteroclitic peptide-based cancer vaccines in patients with CALRMUT MPN.

Persistent CAD activity in memory CD8+ T cells supports rRNA synthesis and ribosomal biogenesis required at rechallenge

Memory CD8⁺ T cells are characterized by their ability to persist long after the initial antigen encounter and their capacity to generate a rapid recall response. Recent studies have identified a role for metabolic reprogramming and mitochondrial function in promoting the longevity of memory T cells. However, detailed mechanisms involved in promoting their rapid recall response are incompletely understood. Here, we identify a role for the initial and continued activation of the trifunctional rate-limiting enzyme of the de novo pyrimidine synthesis pathway CAD (carbamoyl-phosphate synthetase 2, aspartate transcarbamylase, and dihydroorotase) as critical in promoting the rapid recall response of previously activated CD8⁺ T cells. We found that CAD was rapidly phosphorylated upon naïve T cell activation in an mTORC1-dependent manner, yet remained phosphorylated long after initial activation. Previously activated CD8⁺ T cells displayed continued de novo pyrimidine synthesis in the absence of mitogenic signals, and interfering with this pathway diminished the speed and magnitude of cytokine production upon rechallenge. Inhibition of CAD did not affect cytokine transcript levels but diminished available pre-rRNA (ribosomal RNA), the polycistronic rRNA precursor whose synthesis is the rate-limiting step in ribosomal biogenesis. CAD inhibition additionally decreased levels of detectable ribosomal proteins in previously activated CD8⁺ T cells. Conversely, overexpression of CAD improved both the cytokine response and proliferation of memory T cells. Overall, our studies reveal a critical role for CAD-induced pyrimidine synthesis and ribosomal biogenesis in promoting the rapid recall response characteristic of memory T cells.

Identification of neoantigens for individualized therapeutic cancer vaccines

Somatic mutations in cancer cells can generate tumour-specific neoepitopes, which are recognized by autologous T cells in the host. As neoepitopes are not subject to central immune tolerance and are not expressed in healthy tissues, they are attractive targets for therapeutic cancer vaccines. Because the vast majority of cancer mutations are unique to the individual patient, harnessing the full potential of this rich source of targets requires individualized treatment approaches. Many computational algorithms and machine-learning tools have been developed to identify mutations in sequence data, to prioritize those that are more likely to be recognized by T cells and to design tailored vaccines for every patient. In this Review, we fill the gaps between the understanding of basic mechanisms of T cell recognition of neoantigens and the computational approaches for discovery of somatic mutations and neoantigen prediction for cancer immunotherapy. We present a new classification of neoantigens, distinguishing between guarding, restrained and ignored neoantigens, based on how they confer proficient antitumour immunity in a given clinical context. Such context-based differentiation will contribute to a framework that connects neoantigen biology to the clinical setting and medical peculiarities of cancer, and will enable future neoantigen-based therapies to provide greater clinical benefit.

Targeting intra-viral conserved nucleocapsid (N) proteins as novel vaccines against SARS-CoVs

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has caused the global pandemic of the Coronavirus disease in late 2019 (COVID-19). Vaccine development efforts have predominantly been aimed at 'Extra-viral' Spike (S) protein as vaccine vehicles, but there are concerns regarding 'viral immune escape' since multiple mutations may enable the mutated virus strains to escape from immunity against S protein. The 'Intra-viral' Nucleocapsid (N-protein) is relatively conserved among mutant strains of coronaviruses during spread and evolution. Herein, we demonstrate novel vaccine candidates against SARS-CoV-2 by using the whole conserved N-protein or its fragment/peptides. Using ELISA assay, we showed that high titers of specific anti-N antibodies (IgG, IgG1, IgG2a, IgM) were maintained for a reasonably long duration (> 5 months), suggesting that N-protein is an excellent immunogen to stimulate host immune system and robust B-cell activation. We synthesized three peptides located at the conserved regions of N-protein among CoVs. One peptide showed as a good immunogen for vaccination as well. Cytokine arrays on post-vaccination mouse sera showed progressive up-regulation of various cytokines such as IFN-γ and CCL5, suggesting that TH1 associated responses are also stimulated. Furthermore, vaccinated mice exhibited an elevated memory T cells population. Here, we propose an unconventional vaccine strategy targeting the conserved N-protein as an alternative vaccine target for coronaviruses. Moreover, we generated a mouse monoclonal antibody specifically against an epitope shared between SARS-CoV and SARS-CoV-2, and we are currently developing the First-in-Class humanized anti-N-protein antibody to potentially treat patients infected by various CoVs in the future.

Vaccination reshapes the virus-specific T cell repertoire in unexposed adults

We examined how baseline CD4+ T cell repertoire and precursor states impact responses to pathogen infection in humans using primary immunization with yellow fever virus (YFV) vaccine. YFV-specific T cells in unexposed individuals were identified by peptide-MHC tetramer staining and tracked pre- and post-vaccination by tetramers and TCR sequencing. A substantial number of YFV-reactive T cells expressed memory phenotype markers and contained expanded clones in the absence of exposure to YFV. After vaccination, pre-existing YFV-specific T cell populations with low clonal diversity underwent limited expansion, but rare populations with a reservoir of unexpanded TCRs generated robust responses. These altered dynamics reorganized the immunodominance hierarchy and resulted in an overall increase in higher avidity T cells. Thus, instead of further increasing the representation of dominant clones, YFV vaccination recruits rare and more responsive T cells. Our findings illustrate the impact of vaccines in prioritizing T cell responses and reveal repertoire reorganization as a key component of effective vaccination.

The molecular and epigenetic mechanisms of innate lymphoid cell (ILC) memory and its relevance for asthma

Repetitive exposure of Rag1^−/− mice to the Alternaria allergen extract generated a form of memory that elicited an asthma-like response upon a subthreshold recall challenge 3–15 wk later. This memory was associated with lung ICOS⁺ST2⁺ ILC2s. Genetic, pharmacologic, and antibody-mediated inhibition and adoptive transfer established an essential role for ILC2s in memory-driven asthma. ATAC-seq demonstrated a distinct epigenetic landscape of memory ILC2s and identified Bach2 and AP1 (JunD and Fosl2) motifs as major drivers of altered gene accessibility. scRNA-seq, gene knockout, and signaling studies suggest that repetitive allergenic stress induces a gene repression program involving Nr4a2, Zeb1, Bach2, and JunD and a preparedness program involving Fhl2, FosB, Stat6, Srebf2, and MPP7 in memory ILC2s. A mutually regulated balance between these two programs establishes and maintains memory. The preparedness program (e.g., Fhl2) can be activated with a subthreshold cognate stimulation, which down-regulates repressors and activates effector pathways to elicit the memory-driven phenotype.

Computational investigations of the immune response to repeated influenza infections and vaccinations

Previous studies have shown that repeated influenza vaccination can enhance susceptibility to subsequent infection with a drifted influenza virus strain. This paper seeks to further understanding of the interactions between influenza viruses and specific immune cells that accompany this phenomenon. The paper argues that repeated vaccination increases susceptibility to infection only in the context of a residual immunity induced by prior vaccination or infection. The results of computational analysis indicate that this is a dynamic consequence of interactions between vaccines, influenza viruses and specific immune cells. In particular, mathematical modelling was used to show that in the presence of residual immunity conferred by a vaccine administered in Canada in the 2013-2014 influenza season, the 2014-2015 season vaccine enhanced susceptibility to infection. Such infection enhancement occurs when the 2014-2015 vaccine boosts suppressive T-regulatory cells induced by the 2013-2014 vaccine, decreasing the strength of antibody responses to the infecting strain. Overall, the study suggests probable characteristics of infecting viruses and vaccines that make repeated influenza infections and vaccinations detrimental.

Stable Epigenetic Programming of Effector and Central Memory CD4 T Cells Occurs Within 7 Days of Antigen Exposure In Vivo

T cell immunological memory is established within days of an infection, but little is known about the in vivo changes in gene regulatory networks accounting for their ability to respond more efficiently to secondary infections. To decipher the timing and nature of immunological memory we performed genome-wide analyses of epigenetic and transcriptional changes in a mouse model generating antigen-specific T cells. Epigenetic reprogramming for Th differentiation and memory T cell formation was already established by the peak of the T cell response after 7 days. The Th memory T cell program was associated with a gain of open chromatin regions, enriched for RUNX, ETS and T-bet motifs, which remained stable for 56 days. The epigenetic programs for both effector memory, associated with T-bet, and central memory, associated with TCF-1, were established in parallel. Memory T cell-specific regulatory elements were associated with greatly enhanced inducible Th1-biased responses during secondary exposures to antigen. Furthermore, memory T cells responded in vivo to re-exposure to antigen by rapidly reprograming the entire ETS factor gene regulatory network, by suppressing Ets1 and activating Etv6 expression. These data show that gene regulatory networks are epigenetically reprogrammed towards memory during infection, and undergo substantial changes upon re-stimulation.

Reference Genes for Expression Studies in Human CD8+ Naïve and Effector Memory T Cells under Resting and Activating Conditions

Reverse-transcription quantitative real-time polymerase chain reaction (RT-qPCR) is widely used for mRNA quantification. To accurately measure changing gene transcript levels under different experimental conditions, the use of appropriate reference gene transcripts is instrumental. In T cell immunology, suitable reference genes have been reported for bulk CD4⁺ and CD8⁺ T cells. However, many CD4⁺ and CD8⁺ T cell subsets have been described in the past. Although they respond differently to given activation stimuli, proper validation of suitable reference genes in these subsets is lacking. In this study, we evaluated twelve commonly used reference gene products in human naïve (NV) and effector memory (EM) CD8⁺ T cells under non-activated and activated (2 h, 10 h and 20 h) conditions. We used five different statistical approaches for data analysis. Our results show that a number of widely used reference transcripts become differentially expressed under activating conditions. Using them as references markedly alters results as exemplified with IFNG mRNA expression. The only candidate reference gene products that remained stable during the activation process were 18S rRNA and SDHA mRNA, encouraging their usage as reference gene products for RT-qPCR experiments, when quantifying mRNA levels in human NV and EM CD8⁺ T cells.

Deciphering the transcriptomic landscape of tumor-infiltrating CD8 lymphocytes in B16 melanoma tumors with single-cell RNA-Seq

Recent studies have proposed that tumor-specific tumor-infiltrating CD8⁺ T lymphocytes (CD8 TIL) can be classified into two main groups: "exhausted" TILs, characterized by high expression of the inhibitory receptors PD-1 and TIM-3 and lack of transcription factor 1 (Tcf1); and "memory-like" TILs, with self-renewal capacity and co-expressing Tcf1 and PD-1. However, a comprehensive definition of the heterogeneity existing within CD8 TILs has yet to be clearly established. To investigate this heterogeneity at the transcriptomic level, we performed paired single-cell RNA and TCR sequencing of CD8 T cells infiltrating B16 murine melanoma tumors, including cells of known tumor specificity. Unsupervised clustering and gene-signature analysis revealed four distinct CD8 TIL states - exhausted, memory-like, naïve and effector memory-like (EM-like) - and predicted novel markers, including Ly6C for the EM-like cells, that were validated by flow cytometry. Tumor-specific PMEL T cells were predominantly found within the exhausted and memory-like states but also within the EM-like state. Further, T cell receptor sequencing revealed a large clonal expansion of exhausted, memory-like and EM-like cells with partial clonal relatedness between them. Finally, meta-analyses of public bulk and single-cell RNA-seq data suggested that anti-PD-1 treatment induces the expansion of EM-like cells. Our reference map of the transcriptomic landscape of murine CD8 TILs will help interpreting future bulk and single-cell transcriptomic studies and may guide the analysis of CD8IL subpopulations in response to therapeutic interventions.

CD49b, CD87, and CD95 Are Markers for Activated Cancer-Associated Fibroblasts Whereas CD39 Marks Quiescent Normal Fibroblasts in Murine Tumor Models

Fibroblasts are thought to be key players in the tumor microenvironment. Means to identify and isolate fibroblasts as well as an understanding of their cancer-specific features are essential to dissect their role in tumor biology. To date, the identification of cancer-associated fibroblasts is widely based on generic markers for activated fibroblasts in combination with their origin in tumor tissue. This study was focused on a deep characterization of the cell surface marker profile of cancer-associated fibroblasts in widely used mouse tumor models and defining aberrant expression profiles by comparing them to their healthy counterparts. We established a generic workflow to isolate healthy and cancer-associated fibroblasts from solid tissues, thereby reducing bias, and background noise introduced by non-target cells. We identified CD87, CD44, CD49b, CD95, and Ly-6C as cancer-associated fibroblast cell surface markers, while CD39 was identified to mark normal fibroblasts from healthy tissues. In addition, we found a functional association of most cancer-related fibroblast markers to proliferation and a systemic upregulation of CD87, and CD49b in tumor-bearing mice, even in non-affected tissues. These novel markers will facilitate the characterization of fibroblasts and shed further light in their functions and implication in cancer progression.

A non-functional neoepitope specific CD8+ T-cell response induced by tumor derived antigen exposure in vivo

Cancer-associated mutations, mostly single nucleotide variations, can act as neoepitopes and prime targets for effective anti-cancer T-cell immunity. T cells recognizing cancer mutations are critical for the clinical activity of immune checkpoint blockade (ICB) and they are potent vaccine antigens. High frequencies of mutation-specific T cells are rarely spontaneously induced. Hence, therapies that broaden the tumor specific T-cell response are of interest. Here, we analyzed neoepitope-specific CD8⁺ T-cell responses mounted either spontaneously or after immunotherapy regimens, which induce local tumor inflammation and cell death, in mice bearing tumors of the widely used colon carcinoma cell line CT26. A comprehensive immune reactivity screening of 2474 peptides covering 628 transcribed CT26 point mutations was conducted. All tested treatment regimens were found to induce a single significant CD8⁺ T-cell response against a non-synonymous D733A point mutation in the Smc3 gene. Surprisingly, even though Smc3 D733A turned out to be the immune-dominant neoepitope in CT26 tumor bearing mice, neither T cells specific for this neoepitope nor their T cell receptors (TCRs) were able to recognize or lyse tumor cells. Moreover, vaccination with the D733A neoepitope did not result in anti-tumoral activity despite induction of specific T cells. This is to our knowledge the first report that neoepitope specific CD8⁺ T cells primed by tumor-released antigen exposure in vivo can be functionally irrelevant.

Histone acetyltransferase CBP is critical for conventional effector and memory T-cell differentiation in mice

Compared with naïve T cells, memory CD8⁺ T cells have a transcriptional landscape and proteome that are optimized to generate a more rapid and robust response to secondary infection. Additionally, rewired kinase signal transduction pathways likely contribute to the superior recall response of memory CD8⁺ T cells, but this idea has not been experimentally confirmed. Herein, we utilized an MS approach to identify proteins that are phosphorylated on tyrosine residues in response to Listeria-induced T-cell receptor (TCR) stimulation in both naïve and memory CD8⁺ T cells from mice and separated by fluorescence- and flow cytometry-based cell sorting. This analysis identified substantial differences in tyrosine kinase signaling networks between naïve and memory CD8⁺ T cells. We also observed that an important axis in memory CD8⁺ T cells couples Janus kinase 2 (JAK2) hyperactivation to the phosphorylation of CREB-binding protein (CBP). Functionally, JAK2-catalyzed phosphorylation enabled CBP to bind with higher affinity to acetylated histone peptides, indicating a potential epigenetic mechanism that could contribute to rapid initiation of transcriptional programs in memory CD8⁺ T cells. Moreover, we found that CBP itself is essential for conventional effector and memory CD8⁺ T-cell formation. These results indicate how signaling pathways are altered to promote CD8⁺ memory cell formation and rapid responses to and protection from repeat infections.

Fibrinogen-like Protein 1 Is a Major Immune Inhibitory Ligand of LAG-3

Lymphocyte-activation gene 3 (LAG-3) is an immune inhibitory receptor, with major histocompatibility complex class II (MHC-II) as a canonical ligand. However, it remains controversial whether MHC-II is solely responsible for the inhibitory function of LAG-3. Here, we demonstrate that fibrinogen-like protein 1 (FGL1), a liver-secreted protein, is a major LAG-3 functional ligand independent from MHC-II. FGL1 inhibits antigen-specific T cell activation, and ablation of FGL1 in mice promotes T cell immunity. Blockade of the FGL1-LAG-3 interaction by monoclonal antibodies stimulates tumor immunity and is therapeutic against established mouse tumors in a receptor-ligand inter-dependent manner. FGL1 is highly produced by human cancer cells, and elevated FGL1 in the plasma of cancer patients is associated with a poor prognosis and resistance to anti-PD-1/B7-H1 therapy. Our findings reveal an immune evasion mechanism and have implications for the design of cancer immunotherapy.

Strong homeostatic TCR signals induce formation of self-tolerant virtual memory CD8 T cells

Virtual memory T cells are foreign antigen‐inexperienced T cells that have acquired memory‐like phenotype and constitute 10–20% of all peripheral CD8⁺ T cells in mice. Their origin, biological roles, and relationship to naïve and foreign antigen‐experienced memory T cells are incompletely understood. By analyzing T‐cell receptor repertoires and using retrogenic monoclonal T‐cell populations, we demonstrate that the virtual memory T‐cell formation is a so far unappreciated cell fate decision checkpoint. We describe two molecular mechanisms driving the formation of virtual memory T cells. First, virtual memory T cells originate exclusively from strongly self‐reactive T cells. Second, the stoichiometry of the CD8 interaction with Lck regulates the size of the virtual memory T‐cell compartment via modulating the self‐reactivity of individual T cells. Although virtual memory T cells descend from the highly self‐reactive clones and acquire a partial memory program, they are not more potent in inducing experimental autoimmune diabetes than naïve T cells. These data underline the importance of the variable level of self‐reactivity in polyclonal T cells for the generation of functional T‐cell diversity.

Antigen-Induced but Not Innate Memory CD8 T Cells Express NKG2D and Are Recruited to the Lung Parenchyma upon Viral Infection

The pool of memory-phenotype CD8 T cells is composed of Ag-induced (AI) and cytokine-induced innate (IN) cells. IN cells have been described as having properties similar to those of AI memory cells. However, we found that pathogen-induced AI memory cells can be distinguished in mice from naturally generated IN memory cells by surface expression of NKG2D. Using this marker, we described the increased functionalities of AI and IN memory CD8 T cells compared with naive cells, as shown by comprehensive analysis of cytokine secretion and gene expression. However, AI differed from IN memory CD8 T cells by their capacity to migrate to the lung parenchyma upon inflammation or infection, a process dependent on their expression of ITGA1/CD49a and ITGA4/CD49d integrins.

TCR stimulation strength is inversely associated with establishment of functional brain-resident memory CD8 T cells during persistent viral infection

Establishing functional tissue-resident memory (T_RM) cells at sites of infection is a newfound objective of T cell vaccine design. To directly assess the impact of antigen stimulation strength on memory CD8 T cell formation and function during a persistent viral infection, we created a library of mouse polyomavirus (MuPyV) variants with substitutions in a subdominant CD8 T cell epitope that exhibit a broad range of efficiency in stimulating TCR transgenic CD8 T cells. By altering a subdominant epitope in a nonstructural viral protein and monitoring memory differentiation of donor monoclonal CD8 T cells in immunocompetent mice, we circumvented potentially confounding changes in viral infection levels, virus-associated inflammation, size of the immunodominant virus-specific CD8 T cell response, and shifts in TCR affinity that may accompany temporal recruitment of endogenous polyclonal cells. Using this strategy, we found that antigen stimulation strength was inversely associated with the function of memory CD8 T cells during a persistent viral infection. We further show that CD8 T_RM cells recruited to the brain following systemic infection with viruses expressing epitopes with suboptimal stimulation strength respond more efficiently to challenge CNS infection with virus expressing cognate antigen. These data demonstrate that the strength of antigenic stimulation during recruitment of CD8 T cells influences the functional integrity of T_RM cells in a persistent viral infection.

Tissue-resident memory (T_RM) cells are a subset of memory T cells that primarily reside in non-lymphoid tissues and serve as sentinels and effectors against secondary infections. T_RM cells have been extensively characterized in mucosal barriers, but much less is known about this population in non-barrier sites such as the brain. In this study, we designed a novel strategy to evaluate the impact of T cell stimulation strength on the generation and functionality of memory CD8 T cells in both lymphoid and nonlymphoid tissues. Using a mouse polyomavirus (MuPyV) library expressing variants of a subdominant epitope recognized by TCR transgenic CD8 T cells, we found that systemic infection producing weaker responses during T cell priming was sufficient for recruitment of effector cells to the brain. Furthermore, lower stimulation conferred greater functionality to memory T cells in the spleen and to brain T_RM cells. Our findings demonstrate that the strength of antigenic stimulation experienced by a naïve T cell early in infection is a determinant of memory functional integrity during viral persistence in a non-barrier organ.

T Cell Receptor and Cytokine Signaling Can Function at Different Stages to Establish and Maintain Transcriptional Memory and Enable T Helper Cell Differentiation

Experienced T cells exhibit immunological memory via a rapid recall response, responding to restimulation much faster than naïve T cells. The formation of immunological memory starts during an initial slow response, when naïve T cells become transformed to proliferating T blast cells, and inducible immune response genes are reprogrammed as active chromatin domains. We demonstrated that these active domains are supported by thousands of priming elements which cooperate with inducible transcriptional enhancers to enable efficient responses to stimuli. At the conclusion of this response, a small proportion of these cells return to the quiescent state as long-term memory T cells. We proposed that priming elements can be established in a hit-and-run process dependent on the inducible factor AP-1, but then maintained by the constitutive factors RUNX1 and ETS-1. This priming mechanism may also function to render genes receptive to additional differentiation-inducing factors such as GATA3 and TBX21 that are encountered under polarizing conditions. The proliferation of recently activated T cells and the maintenance of immunological memory in quiescent memory T cells are also dependent on various cytokine signaling pathways upstream of AP-1. We suggest that immunological memory is established by T cell receptor signaling, but maintained by cytokine signaling.

Chromatin priming of genes in development: Concepts, mechanisms and consequences

During ontogeny, cells progress through multiple alternate differentiation states by activating distinct gene regulatory networks. In this review, we highlight the important role of chromatin priming in facilitating gene activation during lineage specification and in maintaining an epigenetic memory of previous gene activation. We show that chromatin priming is part of a hugely diverse repertoire of regulatory mechanisms that genes use to ensure that they are expressed at the correct time, in the correct cell type, and at the correct level, but also that they react to signals. We also emphasize how increasing our knowledge of these principles could inform our understanding of developmental failure and disease.

Structure-guided development of a high-affinity human Programmed Cell Death-1: Implications for tumor immunotherapy

Programmed Cell Death-1 (PD-1) is an inhibitory immune receptor, which plays critical roles in T cell co-inhibition and exhaustion upon binding to its ligands PD-L1 and PD-L2. We report the crystal structure of the human PD-1 ectodomain and the mapping of the PD-1 binding interface. Mutagenesis studies confirmed the crystallographic interface, and resulted in mutant PD-1 receptors with altered affinity and ligand-specificity. In particular, a high-affinity mutant PD-1 (HA PD-1) exhibited 45 and 30-fold increase in binding to PD-L1 and PD-L2, respectively, due to slower dissociation rates. This mutant (A132L) was used to engineer a soluble chimeric Ig fusion protein for cell-based and in vivo studies. HA PD-1 Ig showed enhanced binding to human dendritic cells, and increased T cell proliferation and cytokine production in a mixed lymphocyte reaction (MLR) assay. Moreover, in an experimental model of murine Lewis lung carcinoma, HA PD-1 Ig treatment synergized with radiation therapy to decrease local and metastatic tumor burden, as well as in the establishment of immunological memory responses. Our studies highlight the value of structural considerations in guiding the design of a high-affinity chimeric PD-1 Ig fusion protein with robust immune modulatory properties, and underscore the power of combination therapies to selectively manipulate the PD-1 pathway for tumor immunotherapy.

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We report the crystal structure of human PD-1 and the mapping of the PD-1 ligand binding site.

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A high-affinity variant was identified and used to engineer a soluble PD-1 Ig fusion protein (HA PD-1 Ig) for immunotherapy.

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HA PD-1 Ig enhanced T cell activation and strongly synergized with RT to control tumor growth in a lung carcinoma model.

PD-1 is an inhibitory immune receptor that dampens T cell responses. PD-1 blockade is a successful strategy for cancer immunotherapy.

We report the crystal structure of human PD-1 and mapping of its ligand binding site. A high-affinity mutant (HA PD-1) was identified and used to engineer a blocking Ig fusion protein, expected to disrupt the endogenous PD-L/PD-1 pathway.

HA PD-1 Ig increased lymphocyte proliferation and cytokine production. Moreover, in combination with radiation therapy it enhanced control of tumor growth in a Lewis lung carcinoma model. This study offers an alternative strategy to manipulate the PD-1 pathway for tumor immunotherapy.

Critical role of SP thymocyte motility in regulation of thymic output in neonatal Aire-/- mice

Autoimmune regulator (Aire) is essential in the perinatal period to prevent the multiorgan autoimmunity. Here we show that Aire-regulated single positive thymocyte trafficking in neonatal period is critical for thymic egress. Reduced thymic emigration was found in Aire-/- mice during neonatal period, leading to enhanced homeostatic expansion of peripheral T cells as early as 2 weeks of age. In neonatal Aire-/- mice, thymic expression of CCR7 ligands were dramatically reduced, resulting in decreased thymocyte motility and thymocyte emigration. This reduction of thymic egress in Aire-/- mice was alleviated beyond 3 weeks of age by an early upregulation of S1P1 signaling. As the numbers and quality of thymic emigrants are essential for the establishment and maintenance of peripheral tolerance, the reduced thymic emigration during neonatal period may deteriorate autoimmunity caused by the emigration of autoreactive T cells.

Chromatin priming elements establish immunological memory in T cells without activating transcription: T cell memory is maintained by DNA elements which stably prime inducible genes without activating steady state transcription

We have identified a simple epigenetic mechanism underlying the establishment and maintenance of immunological memory in T cells. By studying the transcriptional regulation of inducible genes we found that a single cycle of activation of inducible factors is sufficient to initiate stable binding of pre-existing transcription factors to thousands of newly activated distal regulatory elements within inducible genes. These events lead to the creation of islands of active chromatin encompassing nearby enhancers, thereby supporting the accelerated activation of inducible genes, without changing steady state levels of transcription in memory T cells. These studies also highlighted the need for more sophisticated definitions of gene regulatory elements. The chromatin priming elements defined here are distinct from classical enhancers because they function by maintaining chromatin accessibility rather than directly activating transcription. We propose that these priming elements are members of a wider class of genomic elements that support correct developmentally regulated gene expression.

Immune signatures of protective spleen memory CD8 T cells

Memory CD8 T lymphocyte populations are remarkably heterogeneous and differ in their ability to protect the host. In order to identify the whole range of qualities uniquely associated with protective memory cells we compared the gene expression signatures of two qualities of memory CD8 T cells sharing the same antigenic-specificity: protective (Influenza-induced, Flu-TM) and non-protective (peptide-induced, TIM) spleen memory CD8 T cells. Although Flu-TM and TIM express classical phenotypic memory markers and are polyfunctional, only Flu-TM protects against a lethal viral challenge. Protective memory CD8 T cells express a unique set of genes involved in migration and survival that correlate with their unique capacity to rapidly migrate within the infected lung parenchyma in response to influenza infection. We also enlighten a new set of poised genes expressed by protective cells that is strongly enriched in cytokines and chemokines such as Ccl1, Ccl9 and Gm-csf. CCL1 and GM-CSF genes are also poised in human memory CD8 T cells. These immune signatures are also induced by two other pathogens (vaccinia virus and Listeria monocytogenes). The immune signatures associated with immune protection were identified on circulating cells, i.e. those that are easily accessible for immuno-monitoring and could help predict vaccines efficacy.

A simple mechanistic explanation for original antigenic sin and its alleviation by adjuvants

A large number of published studies have shown that adaptive immunity to a particular antigen, including pathogen-derived, can be boosted by another, cross-reacting antigen while inducing suboptimal immunity to the latter. Although this phenomenon, called original antigenic sin (OAS), was first reported approximately 70 years ago (Francis et al. 1947 Am. J. Public Health 37, 1013-1016 (doi:10.2105/AJPH.37.8.1013)), its underlying biological mechanisms are still inadequately understood (Kim et al. Proc. Natl Acad. Sci. USA 109, 13 751-13 756 (doi:10.1073/pnas.0912458109)). Here, focusing on the humoral aspects of adaptive immunity, I propose a simple and testable mechanism: that OAS occurs when T regulatory cells induced by the first antigen decrease the dose of the second antigen that is loaded by dendritic cells and available to activate naive lymphocytes. I use both a parsimonious mathematical model and experimental data to confirm the deductive validity of this proposal. This model also explains the puzzling experimental observation that administering certain dendritic cell-activating adjuvants during antigen exposure alleviates OAS. Specifically, the model predicts that such adjuvants will attenuate T regulatory suppression of naive lymphocyte activation. Together, these results suggest additional strategies for redeeming adaptive immunity from the destructive consequences of antigenic 'sin'.

XCR1+ dendritic cells promote memory CD8+ T cell recall upon secondary infections with Listeria monocytogenes or certain viruses

Alexandre et al. demonstrate the XCR1⁺ DCs are instrumental in memory CD8⁺ T cell responses to Listeria, VSV or vaccinia virus infection, but not CMV. Depending on the infection, robust memory CTL responses require cytokine- and chemokine-dependent cross-talk between XCR1⁺ DCs and NK cells or other IFN-γ–producing lymphocytes.

Naive CD8⁺ T cell priming during tumor development or many primary infections requires cross-presentation by XCR1⁺ dendritic cells (DCs). Memory CD8⁺ T lymphocytes (mCTLs) harbor a lower activation threshold as compared with naive cells. However, whether their recall responses depend on XCR1⁺ DCs is unknown. By using a new mouse model allowing fluorescent tracking and conditional depletion of XCR1⁺ DCs, we demonstrate a differential requirement of these cells for mCTL recall during secondary infections by different pathogens. XCR1⁺ DCs were instrumental to promote this function upon secondary challenges with Listeria monocytogenes, vesicular stomatitis virus, or Vaccinia virus, but dispensable in the case of mouse cytomegalovirus. We deciphered how XCR1⁺ DCs promote mCTL recall upon secondary infections with Listeria. By visualizing for the first time the in vivo choreography of XCR1⁺ DCs, NK cells and mCTLs during secondary immune responses, and by neutralizing in vivo candidate molecules, we demonstrate that, very early after infection, mCTLs are activated, and attracted in a CXCR3-dependent manner, by NK cell–boosted, IL-12–, and CXCL9-producing XCR1⁺ DCs. Hence, depending on the infectious agent, strong recall of mCTLs during secondary challenges can require cytokine- and chemokine-dependent cross-talk with XCR1⁺ DCs and NK cells.

Regulation of effector and memory CD8(+) T cell function by inflammatory cytokines

Cells communicate with each other through the production and secretion of cytokines, which are integral to the host response to infection. Once recognized by specific cytokine receptors expressed on the cell surface, these exogenous signals direct the biological function of a cell in order to adapt to their microenvironment. CD8(+) T cells are critical immune cells that play an important role in the control and elimination of intracellular pathogens. Current findings have demonstrated that cytokines influence all aspects of the CD8(+) T cell response to infection or immunization. The cytokine milieu induced at the time of activation impacts the overall magnitude and function of the effector CD8(+) T cell response and the generation of functional memory CD8(+) T cells. This review will focus on the impact of inflammatory cytokines on different aspects of CD8(+) T cell biology.

Chitosan hydrogel vaccine generates protective CD8 T cell memory against mouse melanoma

CD8(+) T cells are important in the control of viral infections and cancers because of their cytolytic activity. A vaccine able to generate these cells could be beneficial in the prevention or treatment of these diseases. Chitosan hydrogel is a promising vaccine formulation that has previously been shown to generate effector CD8(+) T cells in a mouse model. This vaccine promotes sustained release of antigen and adjuvant, which generates a robust effector response. For longer lasting immunity, a memory population of these CD8(+) T cells is required to control further disease. We found that vaccination with chitosan hydrogel or dendritic cells using ovalbumin protein as a model antigen and Quil-A adjuvant provided protection in a subcutaneous melanoma challenge 30 days later. Ovalbumin-specific memory CD8(+) T cells were detectable following vaccination with the chitosan hydrogel but not the dendritic cell vaccine and an in vivo cytotoxicity assay demonstrated specific lysis of target cells in chitosan hydrogel vaccinated mice but not those receiving dendritic cell vaccination. These results demonstrate that vaccination with chitosan hydrogel is equally effective as dendritic cell vaccination in tumour protection but has more readily detectable immune correlates of protection. This may be advantageous in predetermining protection in vaccinated individuals.

T-bet and Eomes instruct the development of two distinct natural killer cell lineages in the liver and in the bone marrow

Trail(+)DX5(-)Eomes(-) natural killer (NK) cells arise in the mouse fetal liver and persist in the adult liver. Their relationships with Trail(-)DX5(+) NK cells remain controversial. We generated a novel Eomes-GFP reporter murine model to address this question. We found that Eomes(-) NK cells are not precursors of classical Eomes(+) NK cells but rather constitute a distinct lineage of innate lymphoid cells. Eomes(-) NK cells are strictly dependent on both T-bet and IL-15, similarly to NKT cells. We observed that, in the liver, expression of T-bet in progenitors represses Eomes expression and the development of Eomes(+) NK cells. Reciprocally, the bone marrow (BM) microenvironment restricts T-bet expression in developing NK cells. Ectopic expression of T-bet forces the development of Eomes(-) NK cells, demonstrating that repression of T-bet is essential for the development of Eomes(+) NK cells. Gene profile analyses show that Eomes(-) NK cells share part of their transcriptional program with NKT cells, including genes involved in liver homing and NK cell receptors. Moreover, Eomes(-) NK cells produce a broad range of cytokines, including IL-2 and TNF in vitro and in vivo, during immune responses against vaccinia virus. Thus, mutually exclusive expression of T-bet and Eomes drives the development of different NK cell lineages with complementary functions.

Memory CD8+ T cells exhibit increased antigen threshold requirements for recall proliferation

Memory CD8⁺ T cells require stronger TCR stimulation than naive cells to enter cell cycle due to reduced Zap70 activation and increased levels of protein tyrosine phosphatases.

A hallmark of immunological memory is the ability of previously primed T cells to undergo rapid recall responses upon antigen reencounter. Classic work has suggested that memory T cells proliferate in response to lower doses of antigen than naive T cells and with reduced requirements for co-stimulation. In contrast to this premise, we observed that naive but not memory T cells proliferate in vivo in response to limited antigen presentation. To reconcile these observations, we tested the antigen threshold requirement for cell cycle entry in naive and central memory CD8⁺ T cells. Although both naive and memory T cells detect low dose antigen, only naive T cells activate cell cycle effectors. Direct comparison of TCR signaling on a single cell basis indicated that central memory T cells do not activate Zap70, induce cMyc expression, or degrade p27 in response to antigen levels that activate these functions in naive T cells. The reduced sensitivity of memory T cells may result from both decreased surface TCR expression and increased expression of protein tyrosine phosphatases as compared with naive T cells. Our data describe a novel aspect of memory T cell antigen threshold sensitivity that may critically regulate recall expansion.

Early Decision: Effector and Effector Memory T Cell Differentiation in Chronic Infection

As effector memory T cells (Tem) are the predominant population elicited by chronic parasitic infections, increasing our knowledge of their function, survival and derivation, as phenotypically and functionally distinct from central memory and effector T cells will be critical to vaccine development for these diseases. In some infections, memory T cells maintain increased effector functions, however; this may require the presence of continued antigen, which can also lead to T cell exhaustion. Alternatively, in the absence of antigen, only the increase in the number of memory cells remains, without enhanced functionality as central memory. In order to understand the requirement for antigen and the potential for longevity or protection, the derivation of each type of memory must be understood. A thorough review of the data establishes the existence of both memory (Tmem) precursors and effector T cells (Teff) from the first hours of an immune response. This suggests a new paradigm of Tmem differentiation distinct from the proposition that Tmem only appear after the contraction of Teff. Several signals have been shown to be important in the generation of memory T cells, such as the integrated strength of “signals 1-3” of antigen presentation (antigen receptor, co-stimulation, cytokines) as perceived by each T cell clone. Given that these signals integrated at antigen presentation cells have been shown to determine the outcome of Teff and Tmem phenotypes and numbers, this decision must be made at a very early stage. It would appear that the overwhelming expansion of effector T cells and the inability to phenotypically distinguish memory T cells at early time points has masked this important decision point. This does not rule out an effect of repeated stimulation or chronic inflammatory milieu on populations generated in these early stages. Recent studies suggest that Tmem are derived from early Teff, and we suggest that this includes Tem as well as Tcm. Therefore, we propose a testable model for the pathway of differentiation from naïve to memory that suggests that Tem are not fully differentiated effector cells, but derived from central memory T cells as originally suggested by Sallusto et al. in 1999, but much debated since.

Activation of the TCR complex by small chemical compounds

Small chemical compounds and certain metal ions can activate T cells, resulting in drug hypersensitivity reactions that are a main problem in pharmacology. Mostly, the drugs generate new antigenic epitopes on peptide-major histocompatibility complex (MHC) molecules that are recognized by the T-cell antigen receptor (TCR). In this review we discuss the molecular mechanisms of how the drugs alter self-peptide-MHC, so that neo-antigens are produced. This includes (1) haptens covalently bound to peptides presented by MHC, (2) metal ions and drugs that non-covalently bridge self-pMHC to the TCR, and (3) drugs that allow self-peptides to be presented by MHCs that otherwise are not presented. We also briefly discuss how a second signal-next to the TCR-that naïve T cells require to become activated is generated in the drug hypersensitivity reactions.

CD27 costimulation contributes substantially to the expansion of functional memory CD8(+) T cells after peptide immunization

Naive T cells require signals from multiple costimulatory receptors to acquire full effector function and differentiate to long-lived memory cells. The costimulatory receptor, CD27, is essential for optimal T-cell priming and memory differentiation in a variety of settings, although whether CD27 is similarly required during memory CD8(+) T-cell reactivation remains controversial. We have used OVA and anti-CD40 to establish a memory CD8(+) T-cell population and report here that their secondary expansion, driven by peptide and anti-CD40, polyI:C, or LPS, requires CD27. Furthermore, antigenic peptide and a soluble form of the CD27 ligand, CD70 (soluble recombinant CD70 (sCD70)), is sufficient for secondary memory CD8(+) T-cell accumulation at multiple anatomical sites, dependent on CD80/86. Prior to boost, resting effector- and central-memory CD8(+) T cells both expressed CD27 with greater expression on central memory cells. Nonetheless, both populations upregulated CD27 after TCR engagement and accumulated in proportion after boosting with Ag and sCD70. Mechanistically, sCD70 increased the frequency of divided and cytolytic memory T cells, conferred resistance to apoptosis and enabled retardation of tumor growth in vivo. These data demonstrate the central role played by CD27/70 during secondary CD8(+) T-cell activation to a peptide Ag, and identify sCD70 as an immunotherapeutic adjuvant for antitumor immunity.

CD8+ memory T cells appear exhausted within hours of acute virus infection

CD8(+) memory T cells are abundant and are activated in a near-synchronous manner by infection, thereby providing a unique opportunity to evaluate the coordinate functional and phenotypic changes that occur in vivo within hours of viral challenge. Using two disparate virus challenges of mice, we show that splenic CD8(+) memory T cells rapidly produced IFN-γ in vivo; however, within 18-24 h, IFN-γ synthesis was terminated and remained undetectable for ≥ 48 h. A similar on/off response was observed in CD8(+) memory T cells in the peritoneal cavity. Cessation of IFN-γ production in vivo occurred despite the continued presence of immunostimulatory viral Ag, indicating that the initial IFN-γ response had been actively downregulated and that the cells had been rendered refractory to subsequent in vivo Ag contact. Downregulation of IFN-γ synthesis was accompanied by the upregulation of inhibitory receptor expression on the T cells, and ex vivo analyses using synthetic peptides revealed a concurrent hierarchical loss of cytokine responsiveness (IL-2, then TNF, then IFN-γ) taking place during the first 24 h following Ag contact. Thus, within hours of virus challenge, CD8(+) memory T cells display the standard hallmarks of T cell exhaustion, a phenotype that previously was associated only with chronic diseases and that is generally viewed as a gradually developing and pathological change in T cell function. Our data suggest that, instead, the "exhaustion" phenotype is a rapid and normal physiological T cell response.

Standardization and cross validation of alloreactive IFNγ ELISPOT assays within the clinical trials in organ transplantation consortium

Emerging evidence indicates memory donor-reactive T cells are detrimental to transplant outcome and that quantifying the frequency of IFNγ-producing, donor-reactive PBMCs by ELISPOT has potential utility as an immune monitoring tool. Nonetheless, differences in assay performance among laboratories limit the ability to compare results. In an effort to standardize assays, we prepared a panel of common cellular reagent standards, developed and cross validated a standard operating procedure (SOP) for alloreactive IFNγ ELISPOT assays in several research laboratories supported by the NIH-funded Clinical Trials in Organ Transplantation (CTOT) Consortium. We demonstrate that strict adherence to the SOP and centralized data analysis results in high reproducibility with a coefficient of variance (CV) of ≈ 30%. This standardization of IFNγ ELISPOT assay will facilitate interpretation of data from multicenter transplantation research studies and provide the foundation for developing clinical laboratory testing strategies to guide therapeutic decision-making in transplant patients.

A self-help program for memory CD8+ T cells: positive feedback via CD40-CD40L signaling as a critical determinant of secondary expansion

The ability of memory CD8+ T cells to rapidly proliferate and acquire cytolytic activity is critical for protective immunity against intracellular pathogens. The signals that control this recall response remain unclear. We show that CD40L production by memory CD8+ T cells themselves is an essential catalyst for secondary expansion when systemic inflammation is limited. Secondary immunization accompanied by high levels of systemic inflammation results in CD8+ T cell secondary expansion independent of CD4+ T cells and CD40-CD40L signaling. Conversely, when the inflammatory response is limited, memory CD8+ T cell secondary expansion requires CD40L-producing cells, and memory CD8+ T cells can provide this signal. These results demonstrate that vaccination regimens differ in their dependence on CD40L-expressing CD8+ T cells for secondary expansion, and propose that CD40L-expression by CD8+ T cells is a fail-safe mechanism that can promote memory CD8+ T cell secondary expansion when inflammation is limited.

Differential polarization of C-terminal Src kinase between naive and antigen-experienced CD8+ T cells

In CD8(+) T cells, engagement of the TCR with agonist peptide:MHC molecules causes dynamic redistribution of surface molecules including the CD8 coreceptor to the immunological synapse. CD8 associates with the Src-family kinase (SFK) Lck, which, in turn, initiates the rapid tyrosine phosphorylation events that drive cellular activation. Compared with naive T cells, Ag-experienced CD8(+) T cells make shorter contacts with APC, are less dependent on costimulation, and are triggered by lower concentrations of Ag, yet the molecular basis of this more efficient response of memory T cells is not fully understood. In this article, we show differences between naive and Ag-experienced CD8(+) T cells in colocalization of the SFKs and their negative regulator, C-terminal Src kinase (Csk). In naive CD8(+) T cells, there was pronounced colocalization of SFKs and Csk at the site of TCR triggering, whereas in Ag-experienced cells, Csk displayed a bipolar distribution with a proportion of the molecules sequestered within a cytosolic area in the distal pole of the cell. The data show that there is differential redistribution of a key negative regulator away from the site of TCR engagement in Ag-experienced CD8(+) T cells, which might be associated with the more efficient responses of these cells on re-exposure to Ag.

Viral-specific adoptive immunotherapy after allo-SCT: the role of multimer-based selection strategies

Recipients of hematopoietic SCT undergo a period of profound immunosuppression due to the chemotherapy and/or radiotherapy used for the conditioning and to the graft versus host reaction. SCT patients are highly susceptible to the development of viral infections such as CMV or EBV. The achievement of a competent immunological response, such as viral-specific T cells, is associated with a lower incidence of viral infections. Methods for direct identification of antigen-specific T cells have been based on the functional characteristics of these T cells. Techniques such as proliferation and ELISPOT assays, intracellular cytokine staining and IFN-γ capture have been used to quantitate and obtain viral-specific T cells. Multimers are composed of several MHC molecules loaded with immunodominant peptides joined to a fluorescent molecule, which signal can be quantified by a flow cytometer. Multimer technology together with recent advances in flow cytometry, have facilitated the monitoring and selection of antigen-specific T cells without the need for in vitro cultures and manipulation. This has resulted in a better characterization of the function and phenotype of the different subpopulations of T cells involved in the immune recovery post allogeneic SCT. It is becoming a distinct possibility to isolate individual antigen-specific T cells, without long-term culture techniques, and potentially use them as adoptive immunotherapy in the SCT setting.

Functional avidity: a measure to predict the efficacy of effector T cells?

The functional avidity is determined by exposing T-cell populations in vitro to different amounts of cognate antigen. T-cells with high functional avidity respond to low antigen doses. This in vitro measure is thought to correlate well with the in vivo effector capacity of T-cells. We here present the multifaceted factors determining and influencing the functional avidity of T-cells. We outline how changes in the functional avidity can occur over the course of an infection. This process, known as avidity maturation, can occur despite the fact that T-cells express a fixed TCR. Furthermore, examples are provided illustrating the importance of generating T-cell populations that exhibit a high functional avidity when responding to an infection or tumors. Furthermore, we discuss whether criteria based on which we evaluate an effective T-cell response to acute infections can also be applied to chronic infections such as HIV. Finally, we also focus on observations that high-avidity T-cells show higher signs of exhaustion and facilitate the emergence of virus escape variants. The review summarizes our current understanding of how this may occur as well as how T-cells of different functional avidity contribute to antiviral and anti-tumor immunity. Enhancing our knowledge in this field is relevant for tumor immunotherapy and vaccines design.

Transplant immunology for non-immunologist

Transplantation is the treatment of choice for end-stage kidney, heart, lung, and liver disease. Short-term outcomes in solid-organ transplantation are excellent, but long-term outcomes remain suboptimal. Advances in immune suppression and human leukocyte antigen matching techniques have reduced the acute rejection rate to <10%. Chronic allograft injury remains problematic and is in part immune-mediated. This injury is orchestrated by a complex adaptive and innate immune system that has evolved to protect the organism from infection, but, in the context of transplantation, could result in allograft rejection. Such chronic injury is partially mediated by anti-human leukocyte antigen antibodies. Severe rejections have largely been avoided by the development of tissue-typing techniques and crossmatch testing, which are discussed in detail. Further advances in the understanding of T- and B-cell immunology have led to the development of new immunomodulatory therapies directed at prolonging allograft survival, including those that decrease antibody production as well as those that remove antibodies from circulation. Further application of these immunomodulatory therapies has allowed expansion of the donor pool in some cases by permitting ABO-incompatible transplantation and transplantation in patients with preformed antibodies. Although vast improvements have been made in allograft survival, patients must remain on lifetime immunosuppression. Withdrawal of immunosuppression almost always ultimately leads to allograft rejection. The ultimate dream of transplant biologists is the induction of tolerance, where immune function remains intact but the allograft is not rejected in the face of withdrawn immunosuppression. This, however, has remained a significant challenge in human studies.

Mechanisms behind functional avidity maturation in T cells

During an immune response antigen-primed B-cells increase their antigen responsiveness by affinity maturation mediated by somatic hypermutation of the genes encoding the antigen-specific B-cell receptor (BCR) and by selection of higher-affinity B cell clones. Unlike the BCR, the T-cell receptor (TCR) cannot undergo affinity maturation. Nevertheless, antigen-primed T cells significantly increase their antigen responsiveness compared to antigen-inexperienced (naïve) T cells in a process called functional avidity maturation. This paper covers studies that describe differences in T-cell antigen responsiveness during T-cell differentiation along with examples of the mechanisms behind functional avidity maturation in T cells.

Increased sensitivity of antigen-experienced T cells through the enrichment of oligomeric T cell receptor complexes

Although memory T cells respond more vigorously to stimulation and they are more sensitive to low doses of antigen than naive T cells, the molecular basis of this increased sensitivity remains unclear. We have previously shown that the T cell receptor (TCR) exists as different-sized oligomers on the surface of resting T cells and that large oligomers are preferentially activated in response to low antigen doses. Through biochemistry and electron microscopy, we now showed that previously stimulated and memory T cells have more and larger TCR oligomers at the cell surface than their naive counterparts. Reconstitution of cells and mice with a point mutant of the CD3ζ subunit, which impairs TCR oligomer formation, demonstrated that the increased size of TCR oligomers was directly responsible for the increased sensitivity of antigen-experienced T cells. Thus, we propose that an "avidity maturation" mechanism underlies T cell antigenic memory.