Cytotoxic T-cell memory without antigen

A guide to adaptive immune memory

Immune memory - comprising T cells, B cells and plasma cells and their secreted antibodies - is crucial for human survival. It enables the rapid and effective clearance of a pathogen after re-exposure, to minimize damage to the host. When antigen-experienced, memory T cells become activated, they proliferate and produce effector molecules at faster rates and in greater magnitudes than antigen-inexperienced, naive cells. Similarly, memory B cells become activated and differentiate into antibody-secreting cells more rapidly than naive B cells, and they undergo processes that increase their affinity for antigen. The ability of T cells and B cells to form memory cells after antigen exposure is the rationale behind vaccination. Understanding immune memory not only is crucial for the design of more-efficacious vaccines but also has important implications for immunotherapies in infectious disease and cancer. This 'guide to' article provides an overview of the current understanding of the phenotype, function, location, and pathways for the generation, maintenance and protective capacity of memory T cells and memory B cells.

Immunization-induced antigen archiving enhances local memory CD8+ T cell responses following an unrelated viral infection

Antigens from viruses or immunizations can persist or are archived in lymph node stromal cells such as lymphatic endothelial cells (LEC) and fibroblastic reticular cells (FRC). Here, we find that, during the time frame of antigen archiving, LEC apoptosis caused by a second, but unrelated, innate immune stimulus such as vaccina viral infection or CpG DNA administration resulted in cross-presentation of archived antigens and boosted memory CD8 + T cells specific to the archived antigen. In contrast to "bystander" activation associated with unrelated infections, the memory CD8 + T cells specific to the archived antigen from the immunization were significantly higher than memory CD8 + T cells of a different antigen specificity. Finally, the boosted memory CD8 + T cells resulted in increased protection against Listeria monocytogenes expressing the antigen from the immunization, but only for the duration that the antigen was archived. These findings outline an important mechanism by which lymph node stromal cell archived antigens, in addition to bystander activation, can augment memory CD8 + T cell responses during repeated inflammatory insults.

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.

Vaccine-induced antigen archiving enhances local memory CD8+ T cell responses following an unrelated viral infection

Viral and vaccine antigens persist or are archived in lymph node stromal cells (LNSC) such as lymphatic endothelial cells (LEC) and fibroblastic reticular cells (FRC). Here, we find that, during the time frame of antigen archiving, LEC apoptosis caused by a second, but unrelated, innate immune stimulus such as vaccina viral infection or CpG DNA administration boosted memory CD8+ T cells specific to the archived antigen. In contrast to "bystander" activation associated with unrelated infections, the memory CD8+ T cells specific to the vaccine archived antigen were significantly higher than memory CD8+ T cells of a different antigen specificity. Finally, the boosted memory CD8+ T cells resulted in increased protection against Listeria monocytogenes expressing the vaccine antigen, but only for the duration that the vaccine antigen was archived. These findings outline a novel mechanism by which LNSC archived antigens, in addition to bystander activation, can augment memory CD8+ T cell responses during repeated inflammatory insults.

Tissue-resident memory T cell maintenance during antigen persistence requires both cognate antigen and interleukin-15

Our understanding of tissue-resident memory T (TRM) cell biology has been largely developed from acute infection models in which antigen is cleared and sterilizing immunity is achieved. Less is known about TRM cells in the context of chronic antigen persistence and inflammation. We investigated factors that underlie TRM maintenance in a kidney transplantation model in which TRM cells drive rejection. In contrast to acute infection, we found that TRM cells declined markedly in the absence of cognate antigen, antigen presentation, or antigen sensing by the T cells. Depletion of graft-infiltrating dendritic cells or interruption of antigen presentation after TRM cells were established was sufficient to disrupt TRM maintenance and reduce allograft pathology. Likewise, removal of IL-15 transpresentation or of the IL-15 receptor on T cells during TRM maintenance led to a decline in TRM cells, and IL-15 receptor blockade prevented chronic rejection. Therefore, antigen and IL-15 presented by dendritic cells play nonredundant key roles in CD8 TRM cell maintenance in settings of antigen persistence and inflammation. These findings provide insights that could lead to improved treatment of chronic transplant rejection and autoimmunity.

Quantification of lymphocytic choriomeningitis virus specific T cells and LCMV viral titers

Lymphocytic choriomeningitis virus (LCMV) is a frequently used animal model to study immune responses against acute and chronic viral infections. LCMV is a non-cytopathic virus, but destruction of infected cells by cytotoxic T lymphocytes (CTLs) can lead to severe damage of tissues. Virus-specific T cell responses have to be balanced. A low virus load leads to a strong T cell response and subsequently to viral control. In contrast, a high viral titer is associated with T cell exhaustion and chronic viral infections. During an intermediate LCMV viral load CD8⁺ T cells can cause immunopathology, which can have detrimental outcomes. The LCMV infection model offers the opportunity to study virus-specific CD4⁺ and CD8⁺ T cell responses during chronic and acute infections by quantifying LCMV-specific T cells by tetramer staining and measuring cytokine production and viral titers in different organs.

Loss of CD47 alters CD8+ T cell activation in vitro and immunodynamics in mice

CD47 has established roles in the immune system for regulating macrophage phagocytosis and lymphocyte activation, with growing evidence of its cell-intrinsic regulatory roles in natural killer and CD8+ T cells. CD47 limits antigen-dependent cytotoxic activities of human and murine CD8+ T cells, but its role in T cell activation kinetics remains unclear. Using in vitro and in vivo models, we show here that CD47 differentially regulates CD8+ T cell responses to short- versus long-term activation. Although CD47 was not required for T cell development in mice and early activation in vitro, short-term stimuli elevated pathogen-reactive gene expression and enhanced proliferation and the effector phenotypes of Cd47-deficient relative to Cd47-sufficient CD8+ T cells. In contrast, persistent TCR stimulation limited the effector phenotypes of Cd47^−/− CD8+ T cells and enhanced their apoptosis signature. CD8+ T cell expansion and activation in vivo induced by acute lymphocytic choriomeningitis virus (LCMV) infection did not differ in the absence of CD47. However, the frequency and effector phenotypes of Cd47^−/− CD8+ T cells were constrained in chronic LCMV-infected as well as in mice bearing B16 melanoma tumors. Therefore, CD47 regulates CD8+ T cell activation, proliferation, and fitness in a context-dependent manner.

Modifying enzyme replacement therapy - A perspective

Several diseases are caused by the lack of functional proteins, including lysosomal storage diseases or haemophilia A and B. Patients suffering from one of these diseases are treated via enzyme replacement therapies to restore the missing protein. Although this treatment strategy prevents some disease symptoms, enzyme replacement therapies are very expensive and require very frequent infusions, which can cause infusion adverse reactions and massively impair the quality of life of the patients. This review proposes a technology to sustainably produce proteins within the patient to potentially make frequent protein-infusions redundant. This technology is based on blood circulating immune cells as producers of the needed therapeutic protein. To ensure a stable protein concentration over time the cells are equipped with a system, which induces cell proliferation when low therapeutic protein levels are detected and a system inhibiting cell proliferation when high therapeutic protein levels are detected.

The molecular basis of immuno-radiotherapy

PURPOSE

Radiotherapy (RT) and immunotherapy are powerful anti-tumor treatment modalities. Experimental research has demonstrated an important interplay between the cytotoxic effects of RT and the immune system. This systematic review provides an overview of the basics of anti-tumor immunity and focuses on the mechanisms underlying the interplay between RT and immune anti-tumor response that set the molecular basis of immuno-RT.

CONCLUSIONS

An 'immunity acquired equilibrium' mimicking tumor dormancy can be achieved post-irradiation treatment, with the balance shifted toward tumor eradication or regrowth when immune cells' cytotoxic effects or cancer proliferation rate prevail, respectively. RT has both immunosuppressive and immune-enhancing properties. The latter effect is also known as radio-vaccination. Its mechanisms involve up- or down-regulation of membrane molecules, such as PD-L1, HLA-class-I, CD80/86, CD47, and Fas/CD95, that play a vital role in immune checkpoint pathways and increased cytokine expression (e.g. INFα,β,γ, IL1,2, and TNFα) by cancer or immune cells. Moreover, the interactions of radiation with the tumor microenvironment (fibroblasts, tumor-infiltrating lymphocytes, monocytes, and dendritic cells are also an important component of radio-vaccination. Thus, RT may have anti-tumor vaccine properties, whose sequels can be exploited by immunotherapy agents to treat different cancer subtypes effectively.

CD8+ T cell memory is sustained in mice by hepatic stellate cells

BACKGROUND AND AIMS

Long-lasting immunological memory is the ultimate goal of vaccination. Homeostatic maintenance of memory CD8⁺ cytotoxic T cells (MemCD8TCs) is thought to be mediated by IL-15/IL-15R heterodimer (15HD)-expressing myeloid cells. Nonmyeloid hepatic stellate cells (HSCs) also express 15HD, but their role in maintaining MemCD8TC homeostasis is unknown.

APPROACH AND RESULTS

We engineered a genetically engineered mouse in which IL-15R complementary DNA (cDNA) had been inserted in-frame with lecithin-retinol acyltransferase gene and bred onto an IL-15R-KO (15R-KO) genetic background (L15R) that expressed IL-15R in HSCs at normal levels, but not in other liver cells. Outside of the liver of L15R mice, IL-15R expression was found in a number of organs, but not in dendritic cells and macrophages. The low IL-15R expression in the bone marrow (BM) of L15R mice was eliminated by the reconstitution of lethally-irradiated L15R mice with 15R-KO BM to generate L15RC mice. Because MemCD8TC maintenance is mediated by 15HD, not empty IL-15R, 15HD content in L15R mice was determined and found for liver, lung, kidney, and heart. L15R and L15RC mice developed and maintained long-lasting, systemic antigen-specific MemCD8TCs that were efficacious against tumor growth and Listeria monocytogenes infection in an antigen-specific manner. Among the four organs with 15HD content, liver-associated MemCD8TCs were different from those found in the lung, kidney, and heart in two ways: (1) they were quantitatively the most numerous, and (2) they appeared uniquely in the form of clusters in a specialized structure, sinusoidal niches of the liver.

CONCLUSIONS

The liver, the largest organ of the body, is endowed with the capability of effectuating long-lasting functional cytotoxic T cell memory.

Emerging Viral Infections and the Potential Impact on Hypertension, Cardiovascular Disease, and Kidney Disease

Viruses are ubiquitous in the environment and continue to have a profound impact on human health and disease. The COVID-19 pandemic has highlighted this with impressive morbidity and mortality affecting the world's population. Importantly, the link between viruses and hypertension, cardiovascular disease, and kidney disease has resulted in a renewed focus and attention on this potential relationship. The virus responsible for COVID-19, SARS-CoV-2, has a direct link to one of the major enzymatic regulatory systems connected to blood pressure control and hypertension pathogenesis, the renin-angiotensin system. This is because the entry point for SARS-CoV-2 is the ACE2 (angiotensin-converting enzyme 2) protein. ACE2 is one of the main enzymes responsible for dampening the primary effector peptide Ang II (angiotensin II), metabolizing it to Ang-(1-7). A myriad of clinical questions has since emerged and are covered in this review. Several other viruses have been linked to hypertension, cardiovascular disease, and kidney health. Importantly, patients with high-risk apolipoprotein L1 (APOL1) alleles are at risk for developing the kidney lesion of collapsing glomerulopathy after viral infection. This review will highlight several emerging viruses and their potential unique tropisms for the kidney and cardiovascular system. We focus on SARS-CoV-2 as this body of literature in regards to cardiovascular disease has advanced significantly since the COVID-19 pandemic.

T Cell Memory in Infection, Cancer, and Autoimmunity

Long-term immunological memory represents a unique performance of the adaptive immunity selected during evolution to support long-term survival of species in vertebrates, through protection against dangerous "invaders", namely, infectious agents or unwanted (e.g., tumor) cells. The balance between the development of T cell memory and various mechanisms of immunoregulation (namely, T cell effector exhaustion and regulatory T cell suppression) dictates the fate in providing protection or not in different conditions, such as (acute or chronic) infection, vaccination, cancer, and autoimmunity. Here, these different environments are taken in consideration to outline the up-to-date cellular and molecular features regulating the development or damping of immunological memory and to delineate therapeutic strategies capable to improve or control it, in order to address pathological contexts, such as infection, tumor, and autoimmunity.

Elevated ATP via enhanced miRNA-30b, 30c, and 30e downregulates the expression of CD73 in CD8+ T cells of HIV-infected individuals

CD8⁺ T cells play a crucial role against chronic viral infections, however, their effector functions are influenced by the expression of co-stimulatory/inhibitory receptors. For example, CD73 works with CD39 to convert highly inflammatory ATP to adenosine. However, its expression on T cells in the context of viral infections has not been well defined. Here, we analyzed the expression of CD73 on human T cells in a cohort of 102 HIV-infected individuals including those on antiretroviral therapy (ART), ART-naïve, and long-term non-progressors who were not on ART. We found that the frequency of CD73⁺ T cells was markedly lower among T cell subsets (e.g. naïve, effector or memory) in the peripheral blood of all HIV-infected individuals. Notably, CD73 was decreased at the cell surface, intracellular and gene levels. Functionally, CD8⁺CD73⁺ T cells exhibited decreased cytokine expression (TNF-α, IFN-γ and IL-2) upon global or antigen-specific stimulation and impaired expression of cytolytic molecules at the gene and protein levels. In contrast, CD8⁺CD73⁺ T cells expressed elevated levels of homing receptors such as CCR7, α4β7 integrin, which suggests a migratory advantage for these cells as observed in vitro. We also observed significant migration of CD73⁺CD8⁺ T cells into the cerebrospinal fluids of multiple sclerosis (MS) patients at the time of disease relapse. Moreover, we found that elevated levels of ATP in the plasma of HIV-infected individuals upregulates the expression of miRNA30b-e in T cells in vitro. In turn, inhibition of miRNAs (30b, 30c and 30e) resulted in significant upregulation of CD73 mRNA in CD8⁺ T cells. Therefore, we provide a novel mechanism for the downregulation of CD73 via ATP-induced upregulation of miRNA30b, 30c and 30e in HIV infection. Finally, these observations imply that ATP-mediated downregulation of CD73 mainly occurs via its receptor, P2X1/P2RX1. Our results may in part explain why HIV-infected individuals have reduced risk of developing MS considering the role of CD73 for efficient T cell entry into the central nervous system.

CD8⁺ T cells (killer T cells) play an important role against chronic viral infections, however, their functional properties get compromised during the course of HIV infection. CD73, is one of molecules that influences T cell functions, however, its role in the context of viral infections has not been well defined. Here, we analyzed the expression of CD73 on T cells in a cohort of 102 HIV-infected individuals including those on antiretroviral therapy (ART), ART-naïve, and long-term non-progressors who were not on ART. We found that the frequency of T cells expressing this molecule was markedly lower among different T cell subsets obtained from the blood of HIV-infected individuals. Notably, CD73 was decreased at the intracellular protein and gene levels. Furthermore, we found that T cells expressing this molecule (CD73) had impaired functional properties. In contrast, we observed that T cells expressing CD73 had elevated levels of homing receptors, which suggests a migratory advantage for these cells. This was also supported by increased CD73⁺ T cells in the cerebrospinal fluids of multiple sclerosis patients when they experienced disease replace. Moreover, we found that the elevated level of ATP in the plasma of HIV-infected individuals is responsible for the upregulation of miRNA30b, 30c and 30e, resulting in reduced expression of CD73.

Defining the Molecular Hallmarks of T-Cell Memory

The pool of memory CD8 T cells is comprised of highly specialized subpopulations of cells with both shared and distinct functions. The ongoing study of T-cell memory is focused on how these different subpopulations arise, how the cells are maintained over the life of the host, and how the cells protect a host against reinfection. As a field we have used the convenience of a narrow range of surface markers to define and study these memory T-cell subsets. However, as we learn more about these cells, it is becoming clear that these broad definitions are insufficient to capture the complexity of the CD8 memory T-cell pool, and an updated definition of these cellular states are needed. Here, we discuss data that have recently arisen that highlight the difficulty in using surface markers to functionally characterize CD8 T-cell populations, and the possibility of using the epigenetic state of cells to more clearly define the functional capacity of CD8 memory T-cell subsets.

Replication-Deficient Lymphocytic Choriomeningitis Virus-Vectored Vaccine Candidate for the Induction of T Cell Immunity against Mycobacterium tuberculosis

Mycobacterium tuberculosis (Mtb) represents a major burden to global health, and refined vaccines are needed. Replication-deficient lymphocytic choriomeningitis virus (rLCMV)-based vaccine vectors against cytomegalovirus have proven safe for human use and elicited robust T cell responses in a large proportion of vaccine recipients. Here, we developed an rLCMV vaccine expressing the Mtb antigens TB10.4 and Ag85B. In mice, rLCMV elicited high frequencies of polyfunctional Mtb-specific CD8 and CD4 T cell responses. CD8 but not CD4 T cells were efficiently boosted upon vector re-vaccination. High-frequency responses were also observed in neonatally vaccinated mice, and co-administration of rLCMV with Expanded Program of Immunization (EPI) vaccines did not result in substantial reciprocal interference. Importantly, rLCMV immunization significantly reduced the lung Mtb burden upon aerosol challenge, resulting in improved lung ventilation. Protection was associated with increased CD8 T cell recruitment but reduced CD4 T cell infiltration upon Mtb challenge. When combining rLCMV with BCG vaccination in a heterologous prime-boost regimen, responses to the rLCMV-encoded Mtb antigens were further augmented, but protection was not significantly different from rLCMV or BCG vaccination alone. This work suggests that rLCMV may show utility for neonatal and/or adult vaccination efforts against pulmonary tuberculosis.

The Interplay between Bluetongue Virus Infections and Adaptive Immunity

Viral infections have long provided a platform to understand the workings of immunity. For instance, great strides towards defining basic immunology concepts, such as MHC restriction of antigen presentation or T-cell memory development and maintenance, have been achieved thanks to the study of lymphocytic choriomeningitis virus (LCMV) infections. These studies have also shaped our understanding of antiviral immunity, and in particular T-cell responses. In the present review, we discuss how bluetongue virus (BTV), an economically important arbovirus from the Reoviridae family that affects ruminants, affects adaptive immunity in the natural hosts. During the initial stages of infection, BTV triggers leucopenia in the hosts. The host then mounts an adaptive immune response that controls the disease. In this work, we discuss how BTV triggers CD8⁺ T-cell expansion and neutralizing antibody responses, yet in some individuals viremia remains detectable after these adaptive immune mechanisms are active. We present some unpublished data showing that BTV infection also affects other T cell populations such as CD4⁺ T-cells or γδ T-cells, as well as B-cell numbers in the periphery. This review also discusses how BTV evades these adaptive immune mechanisms so that it can be transmitted back to the arthropod host. Understanding the interaction of BTV with immunity could ultimately define the correlates of protection with immune mechanisms that would improve our knowledge of ruminant immunology.

Epidemiology of Pregnancy Complications Through the Lens of Immunological Memory

In the fifteen minutes it takes to read this short commentary, more than 400 babies will have been born too early, another 300 expecting mothers will develop preeclampsia, and 75 unborn third trimester fetuses will have died in utero (stillbirth). Given the lack of meaningful progress in understanding the physiological changes that occur to allow a healthy, full term pregnancy, it is perhaps not surprising that effective therapies against these great obstetrical syndromes that include prematurity, preeclampsia, and stillbirth remain elusive. Meanwhile, pregnancy complications remain the leading cause of infant and childhood mortality under age five. Does it have to be this way? What more can we collectively, as a biomedical community, or individually, as clinicians who care for women and newborn babies at high risk for pregnancy complications, do to protect individuals in these extremely vulnerable developmental windows? The problem of pregnancy complications and neonatal mortality is extraordinarily complex, with multiple unique, but complementary perspectives from scientific, epidemiological and public health viewpoints. Herein, we discuss the epidemiology of pregnancy complications, focusing on how the outcome of prior pregnancy impacts the risk of complication in the next pregnancy — and how the fundamental immunological principle of memory may promote this adaptive response.

Prolongation of allograft survival by artemisinin treatment is associated with blockade of OX40-OX40L

OBJECTIVES

It has been demonstrated that artemisinin (ART) possesses multiple immune modulatory effects. However, its role as immunosuppressant in allogeneic transplantation is undetermined. Here, we investigated the effect of ART on co-stimulatory signaling in OX40⁺ T cells and evaluated ART as a potential immunosuppressant in transplantation.

MATERIALS AND METHODS

Allogeneic skin transplantation was performed in C57BL/6 to BALB/c mice. Recipient mice were administrated with vehicle, ART or cyclosporine A daily from day 0 to day 19 post transplantation. Proportions of splenic CD4⁺OX40⁺ and CD4⁺CD44^hiCD62L^hi cells, and serum IgG was measured by using flow cytometry. An in vitro lymphocyte stimulation with Con A or LPS under various concentrations of ART was performed, expression of CD4⁺OX40⁺ and CD4⁺CD44^hiCD62L^hi cells was evaluated, and interleukin(IL)-6 production was measured by ELISA.

RESULTS

In in vivo allogeneic skin transplant model, ART significantly prolongs allogeneic skin survival. Furthermore, our in vitro studies demonstrate that the immune suppression of ART on T cells is associated with a reduction in OX40⁺ T cells and inhibition of IL-6 secretion.

CONCLUSION

Our data indicate that the OX40-OX40L pathway and IL-6 are possibly involved in ART-induced immunosuppression, and ART is a potential novel immunosuppressant.

Heterologous boosting of nonrelated toxoid immunity during acute Puumala hantavirus infection

Persistence of immune memory in humans is a crucial yet poorly understood aspect of immunology. Here we have studied the effect of Puumala hantavirus infection on unrelated, pre-existing immune memory by studying T cell- and antibody responses against toxoid vaccine antigens of diphtheria, tetanus and pertussis in a cohort of 45 patients. We found that tetanus- and pertussis -specific IgG concentrations elevate during acute Puumala virus infection. Increase in vaccine IgG was associated with proliferation of heterologous T cells. Interestingly, increases in tetanus-specific IgG persisted a year after the infection while pertussis-specific IgG declined rapidly; a difference in IgG kinetics resembling the difference seen after vaccination against tetanus and pertussis. These results suggest that persistence of immune memory is facilitated by heterologous boosting of old memory during memory formation against newly encountered antigens. They also show that different toxoid antigens may be treated differently. Our study gives new insight into how immune memory formation may alter pre-existing immune memory, and also shows that heterologous immunity may have an impact on vaccination outcomes.

Area under Immunosurveillance: Dedicated Roles of Memory CD8 T-Cell Subsets

Immunological memory, defined as the ability to respond in an enhanced manner upon secondary encounter with the same pathogen, can provide substantial protection against infectious disease. The improved protection is mediated in part by different populations of memory CD8 T cells that are retained after primary infection. Memory cells persist in the absence of pathogen-derived antigens and enable secondary CD8 T-cell responses with accelerated kinetics and of larger magnitude after reencounter with the same pathogen. At least three subsets of memory T cells have been defined that are referred to as central memory CD8 T cells (Tcm), effector memory CD8 T cells (Tem), and tissue-resident memory CD8 T cells (Trm). Tcm and Tem are circulating memory T cells that mediate bodywide immune surveillance in search of invading pathogens. In contrast, Trm permanently reside in peripheral barrier tissues, where they form a stationary defensive line of sentinels that alert the immune system upon pathogen reencounter. The characterization of these different subsets has been instrumental in our understanding of the strategies that memory T cells employ to counter invading pathogens. It is clear that memory T cells not only have a numerical advantage over naive T cells resulting in improved protection in secondary responses, but also acquire distinct sets of competencies that assist in pathogen clearance. Nevertheless, inherent challenges are associated with the allocation of memory T cells to a limited number of subsets. The classification of memory T cells into Tcm, Tem, and Trm may not take into account the full extent of the heterogeneity that is observed in the memory population. Therefore, in this review, we will revisit the current classification of memory subsets, elaborate on functional and migratory properties attributed to Tcm, Tem, and Trm, and discuss how potential heterogeneity within these populations arises and persists.

T Cell Receptor Diversity and Lineage Relationship between Virus-Specific CD8 T Cell Subsets during Chronic Lymphocytic Choriomeningitis Virus Infection

Recent studies on chronic viral infections have defined a novel programmed cell death 1-positive (PD-1⁺) T cell factor 1-positive (TCF1⁺) stem-like CD8 T cell subset that gives rise to the terminally differentiated exhausted CD8 T cells. In this study, we performed T cell receptor beta (TCRβ) sequencing of virus-specific CD8 T cells during chronic lymphocytic choriomeningitis virus (LCMV) infection to examine the TCR diversity and lineage relationship of these two functionally distinct subsets. We found that >95% of the TCR repertoire of the exhausted CD8 T cell subset was shared with the stem-like CD8 T cells. The TCR repertoires of both CD8 T cell subsets were composed mostly of a few dominant clonotypes, but there was slightly more breadth and diversity in the stem-like CD8 T cells than their exhausted counterpart (∼40 versus ∼15 GP33⁺ clonotypes; ∼20 versus ∼7 GP276⁺ clonotypes). Interestingly, the breadth of the TCR repertoire was broader during the early stages (day 8) of the chronic infection than the later stages (days 45 to 60), showing that there was a narrowing of the TCR repertoire during chronic infection (∼2-fold GP33⁺ and GP276⁺ stem-like subset; ∼10-fold GP33⁺ and ∼5-fold GP276⁺ exhausted subset). In contrast, during acute LCMV infection, the TCR repertoire was much broader in both GP33-specific effector (∼160 clonotypes) and memory CD8 T cells (∼160 clonotypes). Overall, our data demonstrate that the virus-specific CD8 T cell TCR repertoire is broad and remains stable after acute LCMV infection, but it contracts and is narrower during chronic infection. Our study also shows that the repertoire of the exhausted CD8 T cell subset is almost completely derived from the stem-like CD8 T cell subset during established chronic LCMV infection.IMPORTANCE CD8 TCR repertoires responding to chronic viral infections (HIV, hepatitis C virus [HCV], Epstein-Barr virus [EBV], and cytomegalovirus [CMV]) have limited breadth and diversity. How these repertoires change and are maintained throughout the chronic infection are unknown. We thus characterized the LCMV-specific CD8 TCR repertoires of stem-like and terminally exhausted subsets generated during chronic LCMV infections. During chronic LCMV infections, the repertoires started as diverse but became more clonal at the late time point. Further, the exhausted subset was composed of dominant clonotypes that were shared with the stem-like subset. Together, we demonstrate that the TCR repertoire contracts over time and is almost exclusively derived from the stem-like subset late during the persistent viral infection. Our data suggest that dominant clonotypes in the exhausted subset are derived from a diverse pool of stem-like clonotypes, which may be contributing to the clonality observed during chronic viral infections.

T Cell Immunity to Bacterial Pathogens: Mechanisms of Immune Control and Bacterial Evasion

The human body frequently encounters harmful bacterial pathogens and employs immune defense mechanisms designed to counteract such pathogenic assault. In the adaptive immune system, major histocompatibility complex (MHC)-restricted αβ T cells, along with unconventional αβ or γδ T cells, respond to bacterial antigens to orchestrate persisting protective immune responses and generate immunological memory. Research in the past ten years accelerated our knowledge of how T cells recognize bacterial antigens and how many bacterial species have evolved mechanisms to evade host antimicrobial immune responses. Such escape mechanisms act to corrupt the crosstalk between innate and adaptive immunity, potentially tipping the balance of host immune responses toward pathological rather than protective. This review examines the latest developments in our knowledge of how T cell immunity responds to bacterial pathogens and evaluates some of the mechanisms that pathogenic bacteria use to evade such T cell immunosurveillance, to promote virulence and survival in the host.

Waning immunity and re-emergence of measles and mumps in the vaccine era

Vaccine-preventable diseases (VPD) including measles and mumps have been re-emerging in countries with sustained high vaccine coverage. For mumps, waning immunity has been recognized as a major contributor to recent outbreaks. Although unvaccinated individuals account for most cases in recent measles outbreaks, the role of immune waning remains unclear. Accumulating serological and epidemiological evidence suggests that natural immunity induced by infection may be more durable compared to vaccine-induced immunity. As the proportion of population immunity via vaccination gradually increases and boosting through natural exposures becomes rare, risk of outbreaks may increase. Mechanistic insights into the coupled immuno-epidemiological dynamics of waning and boosting will be important to understand optimal vaccination strategies to combat VPD re-emergence and achieve eradication.

A modeling platform for the lymphatic system

We present a physiologically-based pharmacokinetic modeling platform capable of simulating the biodistribution of different therapeutic agents, including cells, their interactions within the immune system, redistribution across lymphoid compartments, and infiltration into tumor tissues. This transport-based platform comprises a distinctive implementation of a tumor compartment with spatial heterogeneity which enables the modeling of tumors of different size, necrotic state, and agent infiltration capacity. We provide three validating and three exploratory examples that illustrate the capabilities of the proposed approach. The results show that the model can recapitulate immune cell balance across different compartments, respond to antigen stimulation, simulate immune vaccine effects, and immune cell infiltration to tumors. Based on the results, the model can be used to study problems pertinent to current immunotherapies and has the potential to assist medical techniques that rely on the transport of biological species.

T cell co-stimulation and co-inhibition in cardiovascular disease: a double-edged sword

The role of inflammation in cardiovascular disease (CVD) is now widely accepted. Immune cells, including T cells, are influenced by inflammatory signals and contribute to the onset and progression of CVD. T cell activation is modulated by T cell co-stimulation and co-inhibition pathways. Immune checkpoint inhibitors (ICIs) targeting T cell inhibition pathways have revolutionized cancer treatment and improved survival in patients with cancer. However, ICIs might induce cardiovascular toxicity via T cell re-invigoration. With the rising use of ICIs for cancer treatment, a timely overview of the role of T cell co-stimulation and inhibition molecules in CVD is desirable. In this Review, the importance of these molecules in the pathogenesis of CVD is highlighted in preclinical studies on models of CVD such as vein graft disease, myocarditis, graft arterial disease, post-ischaemic neovascularization and atherosclerosis. This Review also discusses the therapeutic potential of targeting T cell co-stimulation and inhibition pathways to treat CVD, as well as the possible cardiovascular benefits and adverse events after treatment. Finally, the Review emphasizes that patients with cancer who are treated with ICIs should be monitored for CVD given the reported association between the use of ICIs and the risk of cardiovascular toxicity.

Epigenetic signature of PD-1+ TCF1+ CD8 T cells that act as resource cells during chronic viral infection and respond to PD-1 blockade

We have recently defined a novel population of PD-1 (programmed cell death 1)+ TCF1 (T cell factor 1)+ virus-specific CD8 T cells that function as resource cells during chronic LCMV infection and provide the proliferative burst seen after PD-1 blockade. Such CD8 T cells have been found in other chronic infections and also in cancer in mice and humans. These CD8 T cells exhibit stem-like properties undergoing self-renewal and also differentiating into the terminally exhausted CD8 T cells. Here we compared the epigenetic signature of stem-like CD8 T cells with exhausted CD8 T cells. ATAC-seq analysis showed that stem-like CD8 T cells had a unique signature implicating activity of HMG (TCF) and RHD (NF-κB) transcription factor family members in contrast to higher accessibility to ETS and RUNX motifs in exhausted CD8 T cells. In addition, regulatory regions of the transcription factors Tcf7 and Id3 were more accessible in stem-like cells whereas Prdm1 and Id2 were more accessible in exhausted CD8 T cells. We also compared the epigenetic signatures of the 2 CD8 T cell subsets from chronically infected mice with effector and memory CD8 T cells generated after an acute LCMV infection. Both CD8 T cell subsets generated during chronic infection were strikingly different from CD8 T cell subsets from acute infection. Interestingly, the stem-like CD8 T cell subset from chronic infection, despite sharing key functional properties with memory CD8 T cells, had a very distinct epigenetic program. These results show that the chronic stem-like CD8 T cell program represents a specific adaptation of the T cell response to persistent antigenic stimulation.

CD8+ T Cells Protect During Vein Graft Disease Development

Aims: Vein grafts are frequently used conduits for arterial reconstruction in patients with cardiovascular disease. Unfortunately, vein graft disease (VGD) causes diminished patency rates. Innate immune system components are known to contribute to VGD. However, the role of T cells has yet to be established. The purpose of this study was to investigate the role of T cells and T cell activation pathways via the T cell receptor (TCR), co-stimulation and bystander effect in VGD.

Methods and results: Here, we show upon vein graft surgery in mice depleted of CD4+ T cells or CD8+ T cells, that CD8+ T cells are locally activated and have a major protective role for vein graft patency. In presence of CD8+ T cells vein grafts appear patent while CD8+ T cell depletion results in occluded vein grafts with increases apoptosis. Importantly, the protective effect of CD8+ T cells in VGD development was TCR and co-stimulation independent. This was demonstrated in vein grafts of OT-I mice, CD70^−/−, CD80/86^−/−, and CD70/80/86^−/− mice compared to C57BL/6 mice. Interestingly, cytokines including IL-15, IL-18, IL-33, and TNF are up-regulated in vein grafts. These cytokines are co-operatively capable to activate CD8+ T cells in a bystander-mediated fashion, in contrast to CD4+ T cells.

Conclusions: T cells are modulators of VGD with a specific protective role of CD8+ T cells, which are locally activated in vein grafts. CD8+ T cells may protect against occlusive lesions by providing survival signals, and concert their protection independent of TCR and co-stimulation signaling.

What if protective immunity is antigen-driven and not due to so-called "memory" B and T cells?

Vaccines or early childhood exposure to infection mediate immunity, that is, improved resistance against disease and death caused by a second infection with the same agent. This has been explained by and equaled to immunological memory, that is, an "altered immune system behavior" that is maintained in a presumably antigen-independent fashion. This review summarizes epidemiological and experimental data, that largely falsify this idea and that show that periodic re-exposure to antigen either, artificially as vaccines or naturally as low-level persisting antigens or infections, or immune complexes on follicular dendritic cells or endemic re-exposure is necessary for protection. Both, the huge success of vaccines in controlling childhood infections, the reduction in clinical disease and the chance of endemically re-exposure, have gradually reduced periodical re-exposure to infections and thereby endangered protective herd immunity. In parallel, vaccine deniers have created susceptibility islands even in an otherwise well vaccinated population, thereby creating a very new situation when compared to the later parts of the 20th century. If protective Immunity is-as emphasized here-antigen driven, then increasingly frequent revaccinations will be necessary (even more so with too much attenuated vaccines) to maintain both herd immunity and individual resistance to acute infections. Of course, this rule also applies to tumor vaccines.

Multifactorial heterogeneity of virus-specific T cells and association with the progression of human chronic hepatitis B infection

Associations between chronic antigen stimulation, T cell dysfunction, and the expression of various inhibitory receptors are well characterized in several mouse and human systems. During chronic hepatitis B virus (HBV) infection (CHB), T cell responses are blunted with low frequencies of virus-specific T cells observed, making these parameters difficult to study. Here, using mass cytometry and a highly multiplexed combinatorial peptide–major histocompatibility complex (pMHC) tetramer strategy that allows for the detection of rare antigen-specific T cells, we simultaneously probed 484 unique HLA-A*1101–restricted epitopes spanning the entire HBV genome on T cells from patients at various stages of CHB. Numerous HBV-specific T cell populations were detected, validated, and profiled. T cells specific for two epitopes (HBVpol387and HBVcore169) displayed differing and complex heterogeneities that were associated with the disease progression, and the expression of inhibitory receptors on these cells was not linearly related with their extent of T cell dysfunction. For HBVcore169-specific CD8+T cells, we found cellular markers associated with long-term memory, polyfunctionality, and the presence of several previously unidentified public TCR clones that correlated with viral control. Using high-dimensional trajectory analysis of these cellular phenotypes, a pseudo-time metric was constructed that fit with the status of viral infection in corresponding patients. This was validated in a longitudinal cohort of patients undergoing antiviral therapy. Our study uncovers complex relationships of inhibitory receptors between the profiles of antigen-specific T cells and the status of CHB with implications for new strategies of therapeutic intervention.

Cytokine-Mediated Regulation of CD8 T-Cell Responses During Acute and Chronic Viral Infection

The common γ-chain cytokines, interleukin (IL)-2, IL-7, and IL-15, regulate critical aspects of antiviral CD8 T-cell responses. During acute infections, IL-2 controls expansion and differentiation of antiviral CD8 T cells, whereas IL-7 and IL-15 are key cytokines to maintain memory CD8 T cells long term in an antigen-independent manner. On the other hand, during chronic infections, in which T-cell exhaustion is established, precise roles of these cytokines in regulation of antiviral CD8 T-cell responses are not well defined. Nonetheless, administration of IL-2, IL-7, or IL-15 can increase function of exhausted CD8 T cells, and thus can be an attractive therapeutic approach. A new subset of stem-cell-like CD8 T cells, which provides a proliferative burst after programmed cell death (PD)-1 therapy, has been recently described during chronic viral infection. Further understanding of cytokine-mediated regulation of this CD8 T-cell subset will improve cytokine therapies to treat chronic infections and cancer in combination with immune checkpoint inhibitors.

Immunological memories of the bone marrow

Memory for antigens once encountered is a hallmark of the immune system of vertebrates, providing us with an immunity adapted to pathogens of our environment. Despite its fundamental relevance, the cells and genes representing immunological memory are still poorly understood. Here we discuss the concept of a circulating, proliferating, and ubiquitous population of effector lymphocytes vs concepts of resting and dormant populations of dedicated memory lymphocytes, distinct from effector lymphocytes and residing in defined tissues, particularly in barrier tissues and in the bone marrow. The lifestyle of memory plasma cells of the bone marrow may serve as a paradigm, showing that persistence of memory lymphocytes is not defined by intrinsic "half-lives", but rather conditional on distinct survival signals provided by dedicated niches. These niches are organized by individual mesenchymal stromal cells. They define the capacity of immunological memory and regulate its homeostasis.

The Respiratory Environment Diverts the Development of Antiviral Memory CD8 T Cells

Our understanding of memory CD8⁺ T cells has been largely derived from acute, systemic infection models. However, memory CD8⁺ T cells generated from mucosal infection exhibit unique properties and, following respiratory infection, are not maintained in the lung long term. To better understand how infection route modifies memory differentiation, we compared murine CD8⁺ T cell responses to a vesicular stomatitis virus (VSV) challenge generated intranasally (i.n.) or i.v. The i.n. infection resulted in greater peak expansion of VSV-specific CD8⁺ T cells. However, this numerical advantage was rapidly lost during the contraction phase of the immune response, resulting in memory CD8⁺ T cell numerical deficiencies when compared with i.v. infection. Interestingly, the antiviral CD8⁺ T cells generated in response to i.n. VSV exhibited a biased and sustained proportion of early effector cells (CD127^loKLRG1^lo) akin to the developmental program favored after i.n. influenza infection, suggesting that respiratory infection broadly favors an incomplete memory differentiation program. Correspondingly, i.n. VSV infection resulted in lower CD122 expression and eomesodermin levels by VSV-specific CD8⁺ T cells, further indicative of an inferior transition to bona fide memory. These results may be due to distinct (CD103⁺CD11b⁺) dendritic cell subsets in the i.n. versus i.v. T cell priming environments, which express molecules that regulate T cell signaling and the balance between tolerance and immunity. Therefore, we propose that distinct immunization routes modulate both the quality and quantity of antiviral effector and memory CD8⁺ T cells in response to an identical pathogen and should be considered in CD8⁺ T cell-based vaccine design.

Epigenetic Maintenance of Acquired Gene Expression Programs during Memory CD8 T Cell Homeostasis

Memory CD8 T cells have a unique ability to provide lifelong immunity against pathogens containing their cognate epitope. Because of their ability to provide lifelong protection, the generation of memory T cells is now a major focus for current vaccination or adoptive cell therapy approaches to treat chronic viral infections and cancer. It is now clear that maintenance of memory CD8 T cells occurs through a process of antigen-independent homeostatic proliferation, which is regulated in part by the gamma chain cytokines IL-7 and IL-15. Here, we will describe the role of these cytokines in the survival and self-renewal of memory CD8 T cells. Further, we will describe the role of epigenetics in the maintenance of acquired functions among memory CD8 T cells during homeostatic proliferation.

Origin and differentiation of human memory CD8 T cells after vaccination

The differentiation of human memory CD8 T cells is not well understood. Here we address this issue using the live yellow fever virus (YFV) vaccine, which induces long-term immunity in humans. We used in vivo deuterium labelling to mark CD8 T cells that proliferated in response to the virus and then assessed cellular turnover and longevity by quantifying deuterium dilution kinetics in YFV-specific CD8 T cells using mass spectrometry. This longitudinal analysis showed that the memory pool originates from CD8 T cells that divided extensively during the first two weeks after infection and is maintained by quiescent cells that divide less than once every year (doubling time of over 450 days). Although these long-lived YFV-specific memory CD8 T cells did not express effector molecules, their epigenetic landscape resembled that of effector CD8 T cells. This open chromatin profile at effector genes was maintained in memory CD8 T cells isolated even a decade after vaccination, indicating that these cells retain an epigenetic fingerprint of their effector history and remain poised to respond rapidly upon re-exposure to the pathogen.

Interconnected subsets of memory follicular helper T cells have different effector functions

Follicular helper T cells regulate high-affinity antibody production. Memory follicular helper T cells can be local in draining lymphoid organs and circulate in the blood, but the underlying mechanisms of this subdivision are unresolved. Here we show that both memory follicular helper T subsets sustain B-cell responses after reactivation. Local cells promote more plasma cell differentiation, whereas circulating cells promote more secondary germinal centers. In parallel, local memory B cells are homogeneous and programmed to become plasma cells, whereas circulating memory B cells are able to rediversify. Local memory follicular helper T cells have higher affinity T-cell receptors, which correlates with expression of peptide MHC-II at the surface of local memory B cells only. Blocking T-cell receptor-peptide MHC-II interactions induces the release of local memory follicular helper T cells in the circulating compartment. Our studies show that memory follicular helper T localization is highly intertwined with memory B cells, a finding that has important implications for vaccine design.Tfh cells can differentiate into memory cells. Here the authors describe distinct functional and phenotypic profiles of these memory Tfh cells dependent on their anatomical localization to the lymphoid organs or to the circulation.

IL-7 plays a critical role for the homeostasis of allergen-specific memory CD4 T cells in the lung and airways

Memory T cells respond rapidly to repeated antigen exposure and can maintain their population for extended periods through self-renewal. These characteristics of memory T cells have mainly been studied during viral infections, whereas their existence and functions in allergic diseases have been studied incompletely. Since allergic patients can suffer repeated relapses caused by intermittent allergen exposure, we hypothesized that allergen- specific memory Th2 cells are present and the factors necessary for the maintenance of these cells are provided by the lung and airways. Using a murine model of airway inflammation, we found that allergen-specific CD4 T cells survived longer than 70 days in the lung and airways in an IL-7 dependent fashion. These T cells showing homeostatic proliferation were largely found in the mediastinal lymph node (mLN), rather than the airways; however, cells residing in the lung and airways developed recall responses successfully. We also found that CD4 T cells exhibited differential phenotypes in the mLN and in the lung. Altogether, we believe that allergen-specific memory T cells reside and function in the lung and airways, while their numbers are replenished through homeostatic turnover in the mLNs. Furthermore, we determined that IL-7 signaling is important for the homeostasis of these cells.

Role of Different Subpopulations of CD8+ T Cells during HIV Exposure and Infection

During HIV infection, specific responses exhibited by CD8⁺ T cells are crucial to establish an early, effective, and sustained viral control, preventing severe immune alterations and organ dysfunction. Several CD8⁺ T cells subsets have been identified, exhibiting differences in terms of activation, functional profile, and ability to limit HIV replication. Some of the most important CD8⁺ T cells subsets associated with viral control, production of potent antiviral molecules, and strong polyfunctional responses include Th1-like cytokine pattern and Tc17 cells. In addition, the expression of specific activation markers has been also associated with a more effective response of CD8⁺ T cells, as evidenced in HLA-DR⁺ CD38⁻ cells. CD8⁺ T cells in both, peripheral blood and gut mucosa, are particularly important in individuals with a resistant phenotype, including HIV-exposed seronegative individuals (HESNs), long-term non-progressors (LTNPs) and HIV-controllers. Although the role of CD8⁺ T cells has been extensively explored in the context of an established HIV-1 infection, the presence of HIV-specific cells with effector abilities and a defined functional profile in HESNs, remain poorly understood. Here, we reviewed studies carried out on different subpopulations of CD8⁺ T cells in relation with natural resistance to HIV infection and progression.

Unsupervised learning techniques reveal heterogeneity in memory CD8+ T cell differentiation following acute, chronic and latent viral infections

CD8⁺ T lymphocytes are critical for the control of gammaherpesvirus latency. To determine how memory CD8⁺ T cells generated during latency differ from those primed during acute or chronic viral infection, we adoptively transferred naive P14 CD8⁺ T cells into uninfected recipients, and examined surface proteins, cytokines and transcription factors following infection with the Armstrong (acute) or Clone 13 (chronic) strains of lymphocytic choriomeningitis virus (LCMV), or murine gammaherpesvirus 68 (MHV68) expressing the LCMV epitope D^bGP33-41. By performing k-means clustering and generating self organizing maps (SOM), we observed increased short-lived effector-like, CD27^lo CD62L^lo and Bcl-6^lo CD8⁺ T cells following latent infection. In addition, we found that memory CD8⁺ T cells from latent primed mice underwent less expansion following adoptive transfer and antigen rechallenge. Data from cluster models were combined and visualized by principal component analysis (PCA) demonstrating memory CD8⁺ T cells from latent infection occupy an intermediate differentiation space.

Understanding memory CD8+ T cells

Memory CD8+ T cells were originally thought to exist as two populations (effector and central memory). In recent years, a third population called resident memory T cells has been discovered and further to this these populations are being divided into different subtypes. Understanding the function and developmental pathways of memory CD8+ T cells is key to developing effective therapies against cancer and infectious diseases. Here we have reviewed what is currently known about all three subsets of memory CD8+ T populations and as to how each population was originally discovered and the developmental pathways of each subpopulation. Each memory population appears to play a distinct role in adaptive immune responses but we are still a long way from understanding how the populations are generated and what roles they play in protection against invading pathogens and if they contribute to the pathogenesis of inflammatory diseases.

Heterologous Immunity and Persistent Murine Cytomegalovirus Infection

One's history of infections can affect the immune response to unrelated pathogens and influence disease outcome through the process of heterologous immunity. This can occur after acute viral infections, such as infections with lymphocytic choriomeningitis virus (LCMV) and vaccinia virus, where the pathogens are cleared, but it becomes a more complex issue in the context of persistent infections. In this study, murine cytomegalovirus (MCMV) was used as a persistent infection model to study heterologous immunity with LCMV. If mice were previously immune to LCMV and then infected with MCMV (LCMV+MCMV), they had more severe immunopathology, enhanced viral burden in multiple organs, and suppression of MCMV-specific T cell memory inflation. MCMV infection initially reduced the numbers of LCMV-specific memory T cells, but continued MCMV persistence did not further erode memory T cells specific to LCMV. When MCMV infection was given first (MCMV+LCMV), the magnitude of the acute T cell response to LCMV declined with age though this age-dependent decline was not dependent on MCMV. However, some of these MCMV persistently infected mice with acute LCMV infection (7 of 36) developed a robust immunodominant CD8 T cell response apparently cross-reactive between a newly defined putative MCMV epitope sequence, M57_727-734, and the normally subdominant LCMV epitope L_2062-2069, indicating a profound private specificity effect in heterologous immunity between these two viruses. These results further illustrate how a history of an acute or a persistent virus infection can substantially influence the immune responses and immune pathology associated with acute or persistent infections with an unrelated virus.

IMPORTANCE

This study extends our understanding of heterologous immunity in the context of persistent viral infection. The phenomenon has been studied mostly with viruses such as LCMV that are cleared, but the situation can be more complex with a persistent virus such as MCMV. We found that the history of LCMV infection intensifies MCMV immunopathology, enhances MCMV burden in multiple organs, and suppresses MCMV-specific T cell memory inflation. In the reverse infection sequence, we show that some of the long-term MCMV-immune mice mount a robust CD8 T cell cross-reactive response between a newly defined putative MCMV epitope sequence and a normally subdominant LCMV epitope. These results further illustrate how a history of infection can substantially influence the immune responses and immune pathology associated with infections with an unrelated virus.

Cytomegalovirus Reinfections Stimulate CD8 T-Memory Inflation

Cytomegalovirus (CMV) has been shown to induce large populations of CD8 T-effector memory cells that unlike central memory persist in large quantities following infection, a phenomenon commonly termed “memory inflation”. Although murine models to date have shown very large and persistent CMV-specific T-cell expansions following infection, there is considerable variability in CMV-specific T-memory responses in humans. Historically such memory inflation in humans has been assumed a consequence of reactivation events during the life of the host. Because basic information about CMV infection/re-infection and reactivation in immune competent humans is not available, we used a murine model to test how primary infection, reinfection, and reactivation stimuli influence memory inflation. We show that low titer infections induce “partial” memory inflation of both mCMV specific CD8 T-cells and antibody. We show further that reinfection with different strains can boost partial memory inflation. Finally, we show preliminary results suggesting that a single strong reactivation stimulus does not stimulate memory inflation. Altogether, our results suggest that while high titer primary infections can induce memory inflation, reinfections during the life of a host may be more important than previously appreciated.

On Peptides and Altered Peptide Ligands: From Origin, Mode of Action and Design to Clinical Application (Immunotherapy)

T lymphocytes equipped with clonotypic T cell antigen receptors (TCR) recognize immunogenic peptides only when presented in the context of their own major histocompatibility complex (MHC) molecules. Peptide loading to MHC molecules occurs in intracellular compartments (ER for class I and MIIC for class II molecules) and relies on the interaction of the respective peptides and peptide binding pockets on MHC molecules. Those peptide residues not engaged in MHC binding point towards the TCR screening for possible peptide MHC complex binding partners. Natural or intentional modification of both MHC binding registers and TCR interacting residues of peptides – leading to the formation of altered peptide ligands (APLs) – might alter the way peptides interact with TCRs and hence influence subsequent T cell activation events, and consequently T cell effector functions. This review article summarizes how APLs were detected and first described, current concepts of how APLs modify T cellular signaling, which biological mechanisms might force the generation of APLs in vivo, and how peptides and APLs might be used for the benefit of patients suffering from allergic or autoimmune diseases.

Tracking genetically engineered lymphocytes long-term reveals the dynamics of T cell immunological memory

Long-lasting immune protection from pathogens and cancer requires the generation of memory T cells able to survive long-term. To unravel the immunological requirements for long-term persistence of human memory T cells, we characterized and traced, over several years, T lymphocytes genetically modified to express the thymidine kinase (TK) suicide gene that were infused in 10 patients after haploidentical hematopoietic stem cell transplantation (HSCT). At 2 to 14 years after infusion and in the presence of a broad and resting immune system, we could still detect effectors/effector memory (TEM/EFF), central memory (TCM), and stem memory (TSCM) TK(+) cells, circulating at low but stable levels in all patients. Longitudinal analysis of cytomegalovirus (CMV)- and Flu-specific TK(+) cells indicated that antigen recognition was dominant in driving in vivo expansion and persistence at detectable levels. The amount of infused TSCM cells positively correlated with early expansion and with the absolute counts of long-term persisting gene-marked cells. By combining T cell sorting with sequencing of integration (IS), TCRα and TCRβ clonal markers, we showed that T cells retrieved long-term were enriched in clones originally shared in different memory T cell subsets, whereas dominant long-term clonotypes appeared to preferentially originate from infused TSCM and TCM clones. Together, these results indicate that long-term persistence of gene-modified memory T cells after haploidentical HSCT is influenced by antigen exposure and by the original phenotype of infused cells. Cancer adoptive immunotherapy might thus benefit from cellular products enriched in lymphocytes with an early-differentiated phenotype.

A Single 17D Yellow Fever Vaccination Provides Lifelong Immunity; Characterization of Yellow-Fever-Specific Neutralizing Antibody and T-Cell Responses after Vaccination

Introduction

Prompted by recent amendments of Yellow Fever (YF) vaccination guidelines from boost to single vaccination strategy and the paucity of clinical data to support this adjustment, we used the profile of the YF-specific CD8⁺ T-cell subset profiles after primary vaccination and neutralizing antibodies as a proxy for potentially longer lasting immunity.

Methods and Findings

PBMCs and serum were collected in six individuals on days 0, 3, 5, 12, 28 and 180, and in 99 individuals >10 years after YF-vaccination. Phenotypic characteristics of YF- tetramer⁺ CD8⁺ T-cells were determined using class I tetramers. Antibody responses were measured using a standardized plaque reduction neutralization test (PRNT). Also, characteristics of YF-tetramer positive CD8⁺ T-cells were compared between individuals who had received a primary- and a booster vaccination. YF-tetramer⁺ CD8⁺ T-cells were detectable on day 12 (median tetramer⁺ cells as percentage of CD8⁺ T-cells 0.2%, range 0.07–3.1%). On day 180, these cells were still present (median 0.06%, range 0.02–0.78%). The phenotype of YF-tetramer positive CD8⁺ T-cells shifted from acute phase effector cells on day 12, to late differentiated or effector memory phenotype (CD45RA^-/+CD27^-) on day 28. Two subsets of YF-tetramer positive T-cells (CD45RA⁺CD27^- and CD45RA⁺CD27⁺) persisted until day 180. Within all phenotypic subsets, the T-bet: Eomes ratio tended to be high on day 28 after vaccination and shifted towards predominant Eomes expression on day 180 (median 6.0 (day 28) vs. 2.2 (day 180) p = 0.0625), suggestive of imprinting compatible with long-lived memory properties. YF-tetramer positive CD8⁺ T-cells were detectable up to 18 years post vaccination, YF-specific antibodies were detectable up to 40 years after single vaccination. Booster vaccination did not increase titers of YF-specific antibodies (mean 12.5 vs. 13.1, p = 0.583), nor induce frequencies or alter phenotypes of YF-tetramer⁺ CD8⁺ T-cells.

Conclusion

The presence of a functionally competent YF-specific memory T-cell pool 18 years and sufficient titers of neutralizing antibodies 35–40 years after first vaccination suggest that single vaccination may be sufficient to provide long-term immunity.

Activation of CD8 T Lymphocytes during Viral Infections

CD8 T lymphocytes are a major cell population of the adaptive immune system. A fundamental characteristic of the CD8 T lymphocyte pool is that it is composed of millions of clones; each with a unique T cell receptor capable of recognizing a limited number of peptides displayed at the cell surface bound to the grooves of major histocompatibility complex class I (MHC I) molecules. Naïve CD8 T lymphocytes are normally resting and circulate between the blood and secondary lymphoid organs in search of their cognate peptide–MHC complexes. During viral infections, bone marrow–derived professional antigen-presenting cells (pAPCs) in secondary lymphoid organs display viral peptides on their MHC I molecules. Specific CD8 T lymphocytes that recognize these peptide–MHC adducts become activated (primed), proliferate extensively, and develop into effectors capable of killing infected cells, identified by the presence at their surface of the pertinent viral peptide–MHC complexes. This article describes how the process of priming naïve CD8 T lymphocytes occurs.

The Pathogenesis and Immunobiology of Mousepox

Ectromelia virus is a mouse-specific orthopoxvirus that, following footpad infection or natural transmission, causes mousepox in most strains of mice, while a few strains, such as C57BL/6, are resistant to the disease but not to the infection. Mousepox is an acute, systemic, highly lethal disease of remarkable semblance to smallpox, caused by the human-specific variola virus. Starting in 1929 with its discovery by Marchal, work with ECTV has provided essential information for our current understanding on how viruses spread lympho-hematogenously, the genetic control of antiviral resistance, the role of different components of the innate and adaptive immune system in the control of primary and secondary infections with acute viruses, and how the mechanisms of immune evasion deployed by the virus affect virulence in vivo. Here, I review the literature on the pathogenesis and immunobiology of ECTV infection in vivo.

Interaction between unrelated viruses during in vivo co-infection to limit pathology and immunity

Great progress has been made in understanding immunity to viral infection. However, infection can occur in the context of co-infection by unrelated pathogens that modulate immune responses and/or disease. We have studied immunity and disease during co-infection with two unrelated viruses: Ectromelia virus (ECTV) and Lymphocytic Choriomeningitis virus (LCMV). ECTV infection can be a lethal in mice due in part to the blockade of Type I Interferons (IFN-I). We show that ECTV/LCMV co-infection results in decreased ECTV viral load and amelioration of ECTV-induced disease, likely due to IFN-I induction by LCMV, as rescue is not observed in IFN-I receptor deficient mice. However, immune responses to LCMV in ECTV co-infected mice were also lower compared to mice infected with LCMV alone and potentially biased toward effector-memory cell generation. Thus, we provide evidence for bi-directional effects of viral co-infection that modulate disease and immunity.