Chemokine receptor CXCR3 facilitates CD8(+) T cell differentiation into short-lived effector cells leading to memory degeneration

Simian Immunodeficiency Virus Targeting of CXCR3+ CD4+ T Cells in Secondary Lymphoid Organs Is Associated with Robust CXCL10 Expression in Monocyte/Macrophage Subsets

Glycosylation of Env defines pathogenic properties of simian immunodeficiency virus (SIV). We previously demonstrated that pathogenic SIVmac239 and a live-attenuated, quintuple deglycosylated Env mutant (Δ5G) virus target CD4⁺ T cells residing in different tissues during acute infection. SIVmac239 and Δ5G preferentially infected distinct CD4⁺ T cells in secondary lymphoid organs (SLOs) and within the lamina propria of the small intestine, respectively (C. Sugimoto et al., J Virol 86:9323-9336, 2012, https://doi.org/10.1128/JVI.00948-12). Here, we studied the host responses relevant to SIV targeting of CXCR3⁺ CCR5⁺ CD4⁺ T cells in SLOs. Genome-wide transcriptome analyses revealed that Th1-polarized inflammatory responses, defined by expression of CXCR3 chemokines, were distinctly induced in the SIVmac239-infected animals. Consistent with robust expression of CXCL10, CXCR3⁺ T cells were depleted from blood in the SIVmac239-infected animals. We also discovered that elevation of CXCL10 expression in blood and SLOs was secondary to the induction of CD14⁺ CD16⁺ monocytes and MAC387⁺ macrophages, respectively. Since the significantly higher levels of SIV infection in SLOs occurred with a massive accumulation of infiltrated MAC387⁺ macrophages, T cells, dendritic cells (DCs), and residential macrophages near high endothelial venules, the results highlight critical roles of innate/inflammatory responses in SIVmac239 infection. Restricted infection in SLOs by Δ5G also suggests that glycosylation of Env modulates innate/inflammatory responses elicited by cells of monocyte/macrophage/DC lineages.IMPORTANCE We previously demonstrated that a pathogenic SIVmac239 virus and a live-attenuated, deglycosylated mutant Δ5G virus infected distinct CD4⁺ T cell subsets in SLOs and the small intestine, respectively (C. Sugimoto et al., J Virol 86:9323-9336, 2012, https://doi.org/10.1128/JVI.00948-12). Accordingly, infections with SIVmac239, but not with Δ5G, deplete CXCR3⁺ CCR5⁺ CD4⁺ T (Th1) cells during the primary infection, thereby compromising the cellular immune response. Thus, we hypothesized that distinct host responses are elicited by the infections with two different viruses. We found that SIVmac239 induced distinctly higher levels of inflammatory Th1 responses than Δ5G. In particular, SIVmac239 infection elicited robust expression of CXCL10, a chemokine for CXCR3⁺ cells, in CD14⁺ CD16⁺ monocytes and MAC387⁺ macrophages recently infiltrated in SLOs. In contrast, Δ5G infection elicited only modest inflammatory responses. These results suggest that the glycosylation of Env modulates the inflammatory/Th1 responses through the monocyte/macrophage subsets and elicits marked differences in SIV infection and clinical outcomes.

Two distinct CXC chemokine receptors (CXCR3 and CXCR4) from the big-belly seahorse Hippocampus abdominalis: Molecular perspectives and immune defensive role upon pathogenic stress

CXC chemokine receptor 3 (CXCR3) and 4 (CXCR4) are members of the seven transmembrane G protein coupled receptor family, involved in pivotal physiological functions. In this study, seahorse CXCR3 and CXCR4 (designated as HaCXCR3 and HaCXCR4) cDNA sequences were identified from the transcriptome library and subsequently molecularly characterized. HaCXCR3 and HaCXCR4 encoded 363 and 373 amino acid long polypeptides, respectively. The HaCXCR3 and HaCXCR4 deduced proteins have typical structural features of chemokine receptors, including seven transmembrane domains and a G protein coupled receptors family 1 profile with characteristic DRY motifs. Amino acid sequence comparison and phylogenetic analysis of these two CXC chemokine receptors revealed a close relationship to their corresponding teleost counterparts. Quantitative real time PCR analysis revealed that HaCXCR3 and HaCXCR4 were ubiquitously expressed in all the tested tissues, with highest expression levels in blood cells. The seahorse blood cells and kidney HaCXCR3 and HaCXCR4 mRNA expressions were differently modulated when challenged with Edwardsiella tarda, Streptococcus iniae, lipopolysaccharide, and polyinosinic:polycytidylic acid, confirming their involvement in post immune responses.

Mechanisms and Dynamics of T Cell-Mediated Cytotoxicity In Vivo

Cytotoxic T lymphocytes (CTLs) are critical in the elimination of infected or malignant cells and are emerging as a major therapeutic target. How CTLs recognize and kill harmful cells has been characterized in vitro but little is known about these processes in the living organism. Here we review recent insights into CTL-mediated killing with an emphasis on in vivo CTL biology. Specifically, we focus on the possible rate-limiting steps determining the efficiency of CTL-mediated killing. We also highlight the need for cell-based datasets that permit the quantification of CTL dynamics, including CTL location, migration, and killing rates. A better understanding of these factors is required to predict protective CD8 T cell immunity in vivo and to design optimized vaccination protocols.

Persistence in Temporary Lung Niches: A Survival Strategy of Lung-Resident Memory CD8+ T Cells

Respiratory virus infections, such as those mediated by influenza virus, parainfluenza virus, respiratory syncytial virus (RSV), severe acute respiratory syndrome coronavirus (SARS-CoV), rhinovirus, and adenovirus, are responsible for substantial morbidity and mortality, especially in children and older adults. Furthermore, the potential emergence of highly pathogenic strains of influenza virus poses a significant public health threat. Thus, the development of vaccines capable of eliciting long-lasting protective immunity to those pathogens is a major public health priority. CD8⁺ Tissue-resident memory T (T_RM) cells are a newly defined population that resides permanently in the nonlymphoid tissues including the lung. These cells are capable of providing local protection immediately after infection, thereby promoting rapid host recovery. Recent studies have offered new insights into the anatomical niches that harbor lung CD8⁺ T_RM cells, and also identified the requirement and limitations of T_RM maintenance. However, it remains controversial whether lung CD8⁺ T_RM cells are continuously replenished by new cells from the circulation or permanently lodged in this site. A better understanding of how lung CD8⁺ T_RM cells are generated and maintained and the tissue-specific factors that drive local T_RM formation is required for optimal vaccine development. This review focuses on recent advance in our understanding of CD8⁺ T_RM cell establishment and maintenance in the lung, and describes how those processes are uniquely regulated in this tissue.

The role of chemokines in hypertension and consequent target organ damage

Immune cells infiltrate the kidney, vasculature, and central nervous system during hypertension, consequently amplifying tissue damage and/or blood pressure elevation. Mononuclear cell motility depends partly on chemokines, which are small cytokines that guide cells through an increasing concentration gradient via ligation of their receptors. Tissue expression of several chemokines is elevated in clinical and experimental hypertension. Likewise, immune cells have enhanced chemokine receptor expression during hypertension, driving immune cell infiltration and inappropriate inflammation in cardiovascular control centers. T lymphocytes and monocytes/macrophages are pivotal mediators of hypertensive inflammation, and these cells migrate in response to several chemokines. As powerful drivers of diapedesis, the chemokines CCL2 and CCL5 have long been implicated in hypertension, but experimental data highlight divergent, context-specific effects of these chemokines on blood pressure and tissue injury. Several other chemokines, particularly those of the CXC family, contribute to blood pressure elevation and target organ damage. Given the significant interplay and chemotactic redundancy among chemokines during disease, future work must not only describe the actions of individual chemokines in hypertension, but also characterize how manipulating a single chemokine modulates the expression and/or function of other chemokines and their cognate receptors. This information will facilitate the design of precise chemotactic immunotherapies to limit cardiovascular and renal morbidity in hypertensive patients.

Lymph node - an organ for T-cell activation and pathogen defense

The immune system is a multicentered organ that is characterized by intimate interactions between its cellular components to efficiently ward off invading pathogens. A key constituent of this organ system is the distinct migratory activity of its cellular elements. The lymph node represents a pivotal meeting point of immune cells where adaptive immunity is induced and regulated. Additionally, besides barrier tissues, the lymph node is a critical organ where invading pathogens need to be eliminated in order to prevent systemic distribution of virulent microbes. Here, we explain how the lymph node is structurally and functionally organized to fulfill these two critical functions - pathogen defense and orchestration of adaptive immunity. We will discuss spatio-temporal aspects of cellular immune responses focusing on CD8 T cells and review how and where these cells are activated in the context of viral infections, as well as how viral antigen expression kinetics and different antigen presentation pathways are involved. Finally, we will describe how such responses are regulated and 'helped', and discuss how this relates to intranodal positioning and cellular migration of the various cellular components that are involved in these processes.

pMHC affinity controls duration of CD8+ T cell-DC interactions and imprints timing of effector differentiation versus expansion

Ozga and colleagues use intravital two-photon microscopy and quantitative whole-organ imaging to reveal the dynamics of early affinity-driven CD8⁺ T cell activation.

During adaptive immune responses, CD8⁺ T cells with low TCR affinities are released early into the circulation before high-affinity clones become dominant at later time points. How functional avidity maturation is orchestrated in lymphoid tissue and how low-affinity cells contribute to host protection remains unclear. In this study, we used intravital imaging of reactive lymph nodes (LNs) to show that T cells rapidly attached to dendritic cells irrespective of TCR affinity, whereas one day later, the duration of these stable interactions ceased progressively with lowering peptide major histocompatibility complex (pMHC) affinity. This correlated inversely BATF (basic leucine zipper transcription factor, ATF-like) and IRF4 (interferon-regulated factor 4) induction and timing of effector differentiation, as low affinity–primed T cells acquired cytotoxic activity earlier than high affinity–primed ones. After activation, low-affinity effector CD8⁺ T cells accumulated at efferent lymphatic vessels for egress, whereas high affinity–stimulated CD8⁺ T cells moved to interfollicular regions in a CXCR3-dependent manner for sustained pMHC stimulation and prolonged expansion. The early release of low-affinity effector T cells led to rapid target cell elimination outside reactive LNs. Our data provide a model for affinity-dependent spatiotemporal orchestration of CD8⁺ T cell activation inside LNs leading to functional avidity maturation and uncover a role for low-affinity effector T cells during early microbial containment.

The Effects of Acute Neutrophil Depletion on Resolution of Acute Influenza Infection, Establishment of Tissue Resident Memory (TRM), and Heterosubtypic Immunity

After disease resolution, a small subset of influenza specific CD8⁺ T cells can remain in the airways of the lung as a tissue resident memory population (T_RM). These cells are critical for protection from subsequent infections with heterosubtypic influenza viruses. Although it is well established that expression of the collagen IV binding integrin alpha 1 is necessary for the retention and maintenance of T_RM cells, other requirements allowing them to localize to the airways and persist are less well understood. We recently demonstrated that inhibition of neutrophils or neutrophil derived chemokine CXCL12 during acute influenza virus infection reduces the effector T cell response and affects the ability of these cells to localize to the airways. We therefore sought to determine whether the defects that occur in the absence of neutrophils would persist throughout resolution of the disease and impact the development of the T_RM population. Interestingly, the early alterations in the CD8⁺ T cell response recover by two weeks post-infection, and mice form a protective population of T_RM cells. Overall, these observations show that acute neutrophil depletion results in a delay in the effector CD8⁺ T cell response, but does not adversely impact the development of T_RM.

Temporary CXCR3 and CCR5 antagonism following vaccination enhances memory CD8 T cell immune responses

Although current vaccination strategies have been successful at preventing a variety of human diseases, attempts at vaccinating against some pathogens such as AIDS and tuberculosis (TB) have been more problematic, largely in that abnormally high numbers of antigen specific CD8+ T cells are required for protection. This study assessed the effect of temporarily dampening the chemokine receptor CXCR3 and CCR5 after vaccination on host immune responses by the administration of TAK-779, a small molecule CXCR3 and CCR5 antagonists commonly used to inhibit HIV infection. Our results showed that the use of TAK-779 enhanced memory CD8+ T cell immune responses both qualitatively and quantitatively. Treatment with TAK-779 following vaccination of an influenza virus antigen resulted in enhanced memory generation with more CD8+CD127+ memory precursor and fewer terminally differentiated effector CD8+CD69+ T cells. These memory T cells were able to become IFN-γ-secreting effector cells when re-encountered the same antigen, which can further enhance the efficacy of vaccination. The mice vaccinated in the presence of TAK-779 were better protected upon influenza virus challenge than the control. These results showed that vaccination while temporarily inhibiting chemokine receptor CXCR3 and CCR5 by TAK-779 could be a promising strategy to generate large number of protective memory CD8+ T cells.

T cell migration, search strategies and mechanisms

T cell migration is essential for T cell responses; it allows for the detection of cognate antigen at the surface of antigen-presenting cells and for interactions with other cells involved in the immune response. Although appearing random, growing evidence suggests that T cell motility patterns are strategic and governed by mechanisms that are optimized for both the activation stage of the cell and for environment-specific cues. In this Opinion article, we discuss how the combined effects of T cell-intrinsic and -extrinsic forces influence T cell motility patterns in the context of highly complex tissues that are filled with other cells involved in parallel motility. In particular, we examine how insights from 'search theory' can be used to describe T cell movement across an 'exploitation-exploration trade-off' in the context of activation versus effector function and lymph nodes versus peripheral tissues.

Differential regulation of pericyte function by the CXC receptor 3

Pericytes are mural cell that have been found to play important roles in promoting blood vessel development and regulating blood flow. The signals that attract pericytes to maturing vessels during the resolution phase of wound healing are unknown. In this study, we examine the role of the chemokine receptor CXC receptor 3 (CXCR3) ligands, as they are produced by maturing endothelial cells. Pericytes isolated from muscle and retina were found to by and large only express the B-isoform of CXCR3 (CXCR3B), with expression being independent of the mitotic state of the cells. Pericyte stimulation with the CXCR3 ligands Mig (CXCL9), IP-9/I-TAC (CXCL11), or IP-10 (CXCL10) resulted in the activation of ERK but not AKT. Treatment with Mig or IP-9, but not IP-10, enhanced p38(MAPK) phosphorylation. Interestingly, while cyclic adenosine monophosphate is generated downstream of CXCR3B in other cells, protein kinase A activation was not observed in these pericytes when treated with these three CXCR3 ligands. The increase in ERK activity resulted in a slight increase in cell transmigration, with the inhibition of ERK leading to a decrease in CXCR3B mediated migration and inhibition of p38(MAPK) reducing transmigration through small pores. These ligands did not affect proliferation. These data are the first to characterize CXCR3B as the predominant isoform expressed on pericytes, and was found on these diverse cells isolated from both muscle and eye. We also show that CXCR3B signaling stimulates transmigration of barrier pores in pericytes as opposed to its inhibitory affects on endothelial cells and fibroblasts. These findings characterize a novel role for the CXCR3B in regulating cellular function. Taken together these data show a role for CXCR3B in regulating pericyte function.

PI3Kδ Regulates the Magnitude of CD8+ T Cell Responses after Challenge with Listeria monocytogenes

PI3Ks regulate diverse immune cell functions by transmitting intracellular signals from Ag, costimulatory receptors, and cytokine receptors to control cell division, differentiation, survival, and migration. In this study, we report the effect of inhibiting the p110δ subunit of PI3Kδ on CD8(+) T cell responses to infection with the intracellular bacteria Listeria monocytogenes. A strong dependency on PI3Kδ for IFN-γ production by CD8(+) T cells in vitro was not recapitulated after Listeria infection in vivo. Inactivation of PI3Kδ resulted in enhanced bacterial elimination by the innate immune system. However, the magnitudes of the primary and secondary CD8 +: T cell responses were reduced. Moreover, PI3Kδ activity was required for CD8(+) T cells to provide help to other responding CD8(+) cells. These findings identify PI3Kδ as a key regulator of CD8(+) T cell responses that integrates extrinsic cues, including those from other responding cells, to determine the collective behavior of CD8(+) T cell populations responding to infection.

Memory programming in CD8(+) T-cell differentiation is intrinsic and is not determined by CD4 help

CD8⁺ T cells activated without CD4⁺ T-cell help are impaired in memory expansion. To understand the underlying cellular mechanism, here we track the dynamics of helper-deficient CD8⁺ T-cell response to a minor histocompatibility antigen by phenotypic and in vivo imaging analyses. Helper-deficient CD8⁺ T cells show reduced burst expansion, rapid peripheral egress, delayed antigen clearance and continuous activation, and are eventually exhausted. Contrary to the general consensus that CD4 help encodes memory programmes in CD8⁺ T cells and helper-deficient CD8⁺ T cells are abortive, these cells can differentiate into effectors and memory precursors. Importantly, accelerating antigen clearance or simply increasing the burst effector size enables generation of memory cells by CD8⁺ T cells, regardless of CD4 help. These results suggest that the memory programme is CD8⁺ T-cell-intrinsic, and provide insight into the role of CD4 help in CD8⁺ T-cell responses.

Persistent antigen stimulation can cause exhaustion and unresponsiveness of CD8 cells, impairing the immune response. Here the authors show that increasing the number of CD8 cells, decreasing the antigen load or providing CD4 help can overcome the exhaustion and establish a memory response.

An extrafollicular pathway for the generation of effector CD8(+) T cells driven by the proinflammatory cytokine, IL-12

The proinflammatory cytokine IL-12 drives the generation of terminally differentiated KLRG1⁺ effector CD8⁺ T cells. Using a Toxoplasma vaccination model, we delineate the sequence of events that naïve CD8⁺ T cells undergo to become terminal effectors and the differentiation steps controlled by IL-12. We demonstrate that direct IL-12 signaling on CD8⁺ T cells is essential for the induction of KLRG1 and IFN-γ, but the subsequent downregulation of CXCR3 is controlled by IL-12 indirectly through the actions of IFN-γ and IFN-γ-inducible chemokines. Differentiation of nascent effectors occurs in an extrafollicular splenic compartment and is driven by late IL-12 production by DCs distinct from the classical CD8α⁺ DC. Unexpectedly, we also found extensive proliferation of both KLRG1⁻ and KLRG1⁺ CD8⁺ T cells in the marginal zone and red pulp, which ceases prior to the final KLRG1^Hi CXCR3^Lo stage. Our findings highlight the notion of an extrafollicular pathway for effector T cell generation.

DOI:http://dx.doi.org/10.7554/eLife.09017.001

The immune system helps to protect us from cancer, infection by microbes and other diseases. There are several different types of immune cells that each have particular roles. For example, cytotoxic T cells can kill other cells in the body that are damaged or infected. These cells are found in various locations around the body—including a region of the spleen known as the white pulp—where they wait in an inactive state until they detect signals from a damaged or infected cell. These T cells divide and mature to produce populations of active T cells known as effector cytotoxic lymphoid cells (or CTLs for short), a process which is thought to occur within the white pulp.

A small protein called cytokine IL-12 is involved in the production of CTLs. The cytokine is released from other immune cells and causes the activated T cells to divide and mature. It has long been believed that IL-12 produced in the white pulp early on in the process is sufficient to drive this process, but more recent work suggests that sustained production of IL-12 in other areas of the spleen that are accessible to the bloodstream may be needed.

Here, Shah et al. studied the generation of cytotoxic T cells in mice that had been exposed to a vaccine against a disease called Toxoplasmosis. Their experiments show that IL-12 drives both the early and late stages of CTL production. In the early stages, the T cells respond to IL-12 that is secreted by a group of ‘lymphoid dendritic’ cells in the white pulp. However, in the later stages, the T cells move away from the white pulp to other parts of the spleen known as the marginal zone and red pulp, where a distinct group of ‘myeloid dendritic’ cells also produce IL-12 and direct the final maturation of the CTLs.

Shah et al.'s findings also show that the process in which cytotoxic T cells divide and later mature to produce CTLs involves a series of tightly controlled events that mostly occur outside of the white pulp. These observations provide a new perspective on how to develop vaccines and other treatments that more efficiently generate the CTLs needed to protect against infections and cancer.

DOI:http://dx.doi.org/10.7554/eLife.09017.002

The Role of Invariant Natural Killer T Cells in Dendritic Cell Licensing, Cross-Priming, and Memory CD8(+) T Cell Generation

New vaccination strategies focus on achieving CD8⁺ T cell (CTL) immunity rather than on induction of protective antibody responses. While the requirement of CD4⁺ T (Th) cell help in dendritic cell (DC) activation and licensing, and in CTL memory induction has been described in several disease models, CTL responses may occur in a Th cell help-independent manner. Invariant natural killer T cells (iNKT cells) can substitute for Th cell help and license DC as well. iNKT cells produce a broad spectrum of Th1 and Th2 cytokines, thereby inducing a similar set of costimulatory molecules and cytokines in DC. This form of licensing differs from Th cell help by inducing other chemokines, while Th cell-licensed DCs produce CCR5 ligands, iNKT cell-licensed DCs produce CCL17, which attracts CCR4⁺ CD8⁺ T cells for subsequent activation. It has recently been shown that iNKT cells do not only enhance immune responses against bacterial pathogens or parasites but also play a role in viral infections. The inclusion of iNKT cell ligands in influenza virus vaccines enhanced memory CTL generation and protective immunity in a mouse model. This review will focus on the role of iNKT cells in the cross-talk with cross-priming DC and memory CD8⁺ T cell formation.

Non-redundant requirement for CXCR3 signalling during tumoricidal T-cell trafficking across tumour vascular checkpoints

T-cell trafficking at vascular sites has emerged as a key step in antitumour immunity. Chemokines are credited with guiding the multistep recruitment of CD8(+) T cells across tumour vessels. However, the multiplicity of chemokines within tumours has obscured the contributions of individual chemokine receptor/chemokine pairs to this process. Moreover, recent studies have challenged whether T cells require chemokine receptor signalling at effector sites. Here we investigate the hierarchy of chemokine receptor requirements during T-cell trafficking to murine and human melanoma. These studies reveal a non-redundant role for Gαi-coupled CXCR3 in stabilizing intravascular adhesion and extravasation of adoptively transferred CD8(+) effectors that is indispensable for therapeutic efficacy. In contrast, functional CCR2 and CCR5 on CD8(+) effectors fail to support trafficking despite the presence of intratumoral cognate chemokines. Taken together, these studies identify CXCR3-mediated trafficking at the tumour vascular interface as a critical checkpoint to effective T-cell-based cancer immunotherapy.

Moving to the suburbs: T-cell positioning within lymph nodes during activation and memory

Lymph nodes are highly organized secondary lymphoid structures crucial for the initiation of immune responses. Naive T cells are strategically located within lymph nodes to optimize their encounter with antigen-loaded dendritic cells. Recent advances in 3D lymph node imaging and tissue reconstruction along with methods for the detection of chemokine expression and gradients have highlighted how T cells position themselves during activation and memory responses. This article covers new insights into the guidance mechanisms that co-ordinate T-cell responses within draining lymph nodes. Furthering our understanding of how these pathways are regulated and promoted will lead to the exploitation of T-cell positioning to further strategize vaccine design.

CXCR3 chemokine receptor enables local CD8(+) T cell migration for the destruction of virus-infected cells

CD8(+) T cells play a critical role in limiting peripheral virus replication, yet how they locate virus-infected cells within tissues is unknown. Here, we have examined the environmental signals that CD8(+) T cells use to localize and eliminate virus-infected skin cells. Epicutaneous vaccinia virus (VV) infection, mimicking human smallpox vaccination, greatly increased expression of the CXCR3 chemokine receptor ligands CXCL9 and CXCL10 in VV-infected skin. Despite normal T cell numbers in the skin, Cxcr3(-/-) mice exhibited dramatically impaired CD8(+)-T-cell-dependent virus clearance. Intravital microscopy revealed that Cxcr3(-/-) T cells were markedly deficient in locating, engaging, and killing virus-infected cells. Further, transfer of wild-type CD8(+) T cells restored viral clearance in Cxcr3(-/-) animals. These findings demonstrate a function for CXCR3 in enhancing the ability of tissue-localized CD8(+) T cells to locate virus-infected cells and thereby exert anti-viral effector functions.

Proinflammatory microenvironments within the intestine regulate the differentiation of tissue-resident CD8⁺ T cells responding to infection

We report that oral infection with Yersinia pseudotuberculosis (Yptb) results in development of two distinct populations of pathogen-specific CD8 tissue-resident memory T (T_RM) cells in the lamina propria (LP). CD103^– T cells did not require transforming-growth factor-β (TGF-β) signaling, but were true resident memory cells. Unlike CD103⁺ CD8 T cells, which were TGF-β-dependent and scattered in the tissue, CD103^– T cells clustered with CD4 T cells and CX3CR1⁺ macrophages and/or dendritic cells around areas of bacterial infection. CXCR3-dependent recruitment to inflamed areas was critical for development of the CD103^– population and pathogen clearance. These studies have identified the preferential development of CD103^– LP T_RM cells in inflammatory microenvironments within the LP and suggest that this subset plays a critical role in controlling infection.

T cell exhaustion during persistent viral infections

Although robust and highly effective anti-viral T cells contribute to the clearance of many acute infections, viral persistence is associated with the development of functionally inferior, exhausted, T cell responses. Exhaustion develops in a step-wise and progressive manner, ranges in severity, and can culminate in the deletion of the anti-viral T cells. This disarming of the response is consequential as it compromises viral control and potentially serves to dampen immune-mediated damage. Exhausted T cells are unable to elaborate typical anti-viral effector functions. They are characterized by the sustained upregulation of inhibitory receptors and display a gene expression profile that distinguishes them from prototypic effector and memory T cell populations. In this review we discuss the properties of exhausted T cells; the virological and immunological conditions that favor their development; the cellular and molecular signals that sustain the exhausted state; and strategies for preventing and reversing exhaustion to favor viral control.

Checkpoint blockade immunotherapy relies on T-bet but not Eomes to induce effector function in tumor-infiltrating CD8+ T cells

Coinhibitory receptor blockade is a promising strategy to boost T-cell immunity against a variety of human cancers. However, many patients still do not benefit from this treatment, and responders often experience immune-related toxicities. These issues highlight the need for advanced mechanistic understanding to improve patient outcomes and uncover clinically relevant biomarkers of treatment efficacy. However, the T-cell-intrinsic signaling pathways engaged during checkpoint blockade treatment are not well defined, particularly for combination approaches. Using a murine model to study how effector CD8(+) T-cell responses to tumors may be enhanced in a tolerizing environment, we identified a critical role for the T-box transcription factor T-bet. Combination blockade of CTLA-4, PD-1, and LAG-3 induced T-bet expression in responding tumor/self-reactive CD8(+) T cells. Eradication of established leukemia using this immunotherapy regimen depended on T-bet induction, which was required for IFNγ production and cytotoxicity by tumor-infiltrating T cells, and for efficient trafficking to disseminated tumor sites. These data provide new insight into the success of checkpoint blockade for cancer immunotherapy, revealing T-bet as a key transcriptional regulator of tumor-reactive CD8(+) T-cell effector differentiation under otherwise tolerizing conditions.

The integrative roles of chemokines at the maternal-fetal interface in early pregnancy

Embryos express paternal antigens that are foreign to the mother, but the mother provides a special immune milieu at the fetal-maternal interface to permit rather than reject the embryo growth in the uterus until parturition by establishing precise crosstalk between the mother and the fetus. There are unanswered questions in the maintenance of pregnancy, including the poorly understood phenomenon of maternal tolerance to the allogeneic conceptus, and the remarkable biological roles of placental trophoblasts that invade the uterine wall. Chemokines are multifunctional molecules initially described as having a role in leukocyte trafficking and later found to participate in developmental processes such as differentiation and directed migration. It is increasingly evident that the gestational uterine microenvironment is characterized, at least in part, by the differential expression and secretion of chemokines that induce selective trafficking of leukocyte subsets to the maternal-fetal interface and regulate multiple events that are closely associated with normal pregnancy. Here, we review the expression and function of chemokines and their receptors at the maternal-fetal interface, with a special focus on chemokine as a key component in trophoblast invasiveness and placental angiogenesis, recruitment and instruction of immune cells so as to form a fetus-supporting milieu during pregnancy. The chemokine network is also involved in pregnancy complications.

Spatiotemporal basis of innate and adaptive immunity in secondary lymphoid tissue

Secondary lymphoid tissues are the sites of both innate and adaptive host defense. Aside from the relatively static nonhematopoietic stromal elements and some macrophages and dendritic cells, most of the cells in these tissues are in constant movement, but the organs maintain a defined microanatomy with preferred locations for the bulk of T cells, B cells, and other lymphocytes and subsets of myeloid cells. Here we describe both the cell dynamics and spatial organization of lymph nodes and review how both physical features and molecular cues guide cell movement to optimize host defense. We emphasize the role of locality in improving the efficiency of a system requiring rare cells to find each other and interact productively through membrane-bound or short-range secreted mediators and highlight how changes in steady-state cell positioning during an infectious challenge contribute to rapid generation of productive responses.

The function of the chemokine receptor CXCR6 in the T cell response of mice against Listeria monocytogenes

The chemokine receptor CXCR6 is expressed on different T cell subsets and up-regulated following T cell activation. CXCR6 has been implicated in the localization of cells to the liver due to the constitutive expression of its ligand CXCL16 on liver sinusoidal endothelial cells. Here, we analyzed the role of CXCR6 in CD8+ T cell responses to infection of mice with Listeria monocytogenes. CD8+ T cells responding to listerial antigens acquired high expression levels of CXCR6. However, deficiency of mice in CXCR6 did not impair control of the L. monocytogenes infection. CXCR6-deficient mice were able to generate listeria-specific CD4+ and CD8+ T cell responses and showed accumulation of T cells in the infected liver. In transfer assays, we detected reduced accumulation of listeria-specific CXCR6-deficient CD8+ T cells in the liver at early time points post infection. Though, CXCR6 was dispensable at later time points of the CD8+ T cell response. When transferred CD8+ T cells were followed for extended time periods, we observed a decline in CXCR6-deficient CD8+ T cells. The manifestation of this cell loss depended on the tissue analyzed. In conclusion, our results demonstrate that CXCR6 is not required for the formation of a T cell response to L. monocytogenes and for the accumulation of T cells in the infected liver but CXCR6 appears to influence long-term survival and tissue distribution of activated cells.

Fatty acid-binding protein E-FABP restricts tumor growth by promoting IFN-β responses in tumor-associated macrophages

Fatty acid-binding proteins (FABP) are known central regulators of both metabolic and inflammatory pathways, but their role in tumor development remains largely unexplored. Here, we report that host expression of epidermal FABP (E-FABP) protects against mammary tumor growth. We find that E-FABP is highly expressed in macrophages, particularly in a specific subset, promoting their antitumor activity. In the tumor stroma, E-FABP-expressing tumor-associated macrophages (TAM) produce high levels of IFN-β through upregulation of lipid droplet formation in response to tumors. E-FABP-mediated IFN-β signaling can further enhance recruitment of tumoricidal effector cells, in particular natural killer cells, to the tumor stroma for antitumor activity. These findings identify E-FABP as a new protective factor to strengthen IFN-β responses against tumor growth.

The transcription factor BATF operates as an essential differentiation checkpoint in early effector CD8+ T cells

The transcription factor BATF is required for the differentiation of interleukin 17 (IL-17)-producing helper T cells (TH17 cells) and follicular helper T cells (TFH cells). Here we identified a fundamental role for BATF in regulating the differentiation of effector of CD8(+) T cells. BATF-deficient CD8(+) T cells showed profound defects in effector population expansion and underwent proliferative and metabolic catastrophe early after encountering antigen. BATF, together with the transcription factors IRF4 and Jun proteins, bound to and promoted early expression of genes encoding lineage-specific transcription-factors (T-bet and Blimp-1) and cytokine receptors while paradoxically repressing genes encoding effector molecules (IFN-γ and granzyme B). Thus, BATF amplifies T cell antigen receptor (TCR)-dependent expression of transcription factors and augments the propagation of inflammatory signals but restrains the expression of genes encoding effector molecules. This checkpoint prevents irreversible commitment to an effector fate until a critical threshold of downstream transcriptional activity has been achieved.

Sprouty-2 regulates HIV-specific T cell polyfunctionality

The ability of individual T cells to perform multiple effector functions is crucial for protective immunity against viruses and cancer. This polyfunctionality is frequently lost during chronic infections; however, the molecular mechanisms driving T cell polyfunctionality are poorly understood. We found that human T cells stimulated by a high concentration of antigen lacked polyfunctionality and expressed a transcription profile similar to that of exhausted T cells. One specific pathway implicated by the transcription profile in control of T cell polyfunctionality was the MAPK/ERK pathway. This pathway was altered in response to different antigen concentrations, and polyfunctionality correlated with upregulation of phosphorylated ERK. T cells that were stimulated with a high concentration of antigen upregulated sprouty-2 (SPRY2), a negative regulator of the MAPK/ERK pathway. The clinical relevance of SPRY2 was confirmed by examining SPRY2 expression in HIV-specific T cells, where high levels of SPRY2 were seen in HIV-specific T cells and inhibition of SPRY2 expression enhanced the HIV-specific polyfunctional response independently of the PD-1 pathway. Our findings indicate that increased SPRY2 expression during chronic viral infection reduces T cell polyfunctionality and identify SPRY2 as a potential target for immunotherapy.

Chemokines and chemokine receptors: positioning cells for host defense and immunity

Chemokines are chemotactic cytokines that control the migratory patterns and positioning of all immune cells. Although chemokines were initially appreciated as important mediators of acute inflammation, we now know that this complex system of approximately 50 endogenous chemokine ligands and 20 G protein-coupled seven-transmembrane signaling receptors is also critical for the generation of primary and secondary adaptive cellular and humoral immune responses. Recent studies demonstrate important roles for the chemokine system in the priming of naive T cells, in cell fate decisions such as effector and memory cell differentiation, and in regulatory T cell function. In this review, we focus on recent advances in understanding how the chemokine system orchestrates immune cell migration and positioning at the organismic level in homeostasis, in acute inflammation, and during the generation and regulation of adoptive primary and secondary immune responses in the lymphoid system and peripheral nonlymphoid tissue.

Cutting edge: Central memory CD8 T cells in aged mice are virtual memory cells

The number of memory phenotype CD8 T cells increases dramatically with aging in both humans and mice. However, the mechanism for this is unknown. The prevailing hypothesis is that memory T cells accumulate with aging as a result of lifelong antigenic stimulation. However, data supporting this supposition are lacking. In this study, we demonstrate that central memory CD8 T cells, which represent a large majority of memory CD8 T cells in aged mice, are not memory cells that develop in response to antigenic stimulation but are virtual memory cells that develop without antigenic stimulation. In addition to phenotypic evidence, we show that accumulation of central memory CD8 T cells is independent of CD4 T cells, CCR5, and CXCR3, all of which are known to be essential for Ag-driven development of central memory CD8 T cells. Thus, this study reveals a novel mechanism for aging-related changes in CD8 T cells.

International Union of Basic and Clinical Pharmacology. [corrected]. LXXXIX. Update on the extended family of chemokine receptors and introducing a new nomenclature for atypical chemokine receptors

Sixteen years ago, the Nomenclature Committee of the International Union of Pharmacology approved a system for naming human seven-transmembrane (7TM) G protein-coupled chemokine receptors, the large family of leukocyte chemoattractant receptors that regulates immune system development and function, in large part by mediating leukocyte trafficking. This was announced in Pharmacological Reviews in a major overview of the first decade of research in this field [Murphy PM, Baggiolini M, Charo IF, Hébert CA, Horuk R, Matsushima K, Miller LH, Oppenheim JJ, and Power CA (2000) Pharmacol Rev 52:145-176]. Since then, several new receptors have been discovered, and major advances have been made for the others in many areas, including structural biology, signal transduction mechanisms, biology, and pharmacology. New and diverse roles have been identified in infection, immunity, inflammation, development, cancer, and other areas. The first two drugs acting at chemokine receptors have been approved by the U.S. Food and Drug Administration (FDA), maraviroc targeting CCR5 in human immunodeficiency virus (HIV)/AIDS, and plerixafor targeting CXCR4 for stem cell mobilization for transplantation in cancer, and other candidates are now undergoing pivotal clinical trials for diverse disease indications. In addition, a subfamily of atypical chemokine receptors has emerged that may signal through arrestins instead of G proteins to act as chemokine scavengers, and many microbial and invertebrate G protein-coupled chemokine receptors and soluble chemokine-binding proteins have been described. Here, we review this extended family of chemokine receptors and chemokine-binding proteins at the basic, translational, and clinical levels, including an update on drug development. We also introduce a new nomenclature for atypical chemokine receptors with the stem ACKR (atypical chemokine receptor) approved by the Nomenclature Committee of the International Union of Pharmacology and the Human Genome Nomenclature Committee.

The impact of KLF2 modulation on the transcriptional program and function of CD8 T cells

Krüppel-like factor 2 (KLF2) is a transcription factor that is highly expressed in quiescent T lymphocytes and downregulated in effector T cells. We now show that antigen receptor engagement downregulates KLF2 expression in a graded response determined by the affinity of T cell antigen receptor (TCR) ligand and the integrated activation of protein kinase B and the MAP kinases ERK1/2. The present study explores the importance of KLF2 downregulation and reveals that the loss of KLF2 controls a select portion of the CD8 effector T cell transcriptional program. In particular, KLF2 loss is required for CD8 T cells to express the inflammatory chemokine receptor CXCR3 and for maximum clonal expansion of T cells. KLF2 thus negatively controls the ability of CD8 T cells to respond to the CXCR3 ligand CXCL10. Strikingly, the KLF2 threshold for restraining expression of CXCR3 is very low and quite distinct to the KLF2 threshold for restraining T cell proliferation. KLF2 is thus an analogue (tunable) not a digital (on/off) cellular switch where the magnitude of KLF2 expression differentially modifies the T cell responses.

Tissue competence imprinting and tissue residency of CD8 T cells

T cell immunity is characterized by striking tissue specialization. Tissue-specificity imprinting starts during priming by tissue-derived migratory dendritic cells in the non-random, specialized micro-anatomical area of the draining lymph node and is influenced by constitutive and induced cues from local environment. Besides tissue-specific effectors, memory cells also exhibit a tissue-specificity. Long-lived tissue-resident memory T cells likely play a considerable role in preventing pathogen invasion. Understanding of the mechanisms of tissue specialization of T cells is of major importance for the design of optimal vaccination strategies and therapeutic interventions in tissue/organ-specific inflammatory diseases. The present review summarizes our current knowledge and hypothesis about tissue-specificity imprinting and tissue residency of T cells.

Enhancement of antigen-specific CD8 T cell responses by co-delivery of Fc-fused CXCL11

Chemokines have been known to play an important role in eliciting adaptive immune responses by, selectively attracting the innate cellular components to the site of antigen presentation. In this study, we demonstrated that all three CXCR3 ligands, CXCL9, CXCL10, and CXCL11, could act as a strong, genetic adjuvant. Among them, CXCL11 increased vaccine antigen-specific CD8 T cells, including, several cytokine secretions (IFN-γ and TNF-α) to a greater degree than the other two CXCR3 ligands. Fc-fusion of CXCL11 (CXCL11-Fc) induced similar but slightly higher CD8 T cell response, which, appeared to be antigen- (ovalbumin (OVA) vs. human papillomavirus 16 (HPV16) E7) and vaccine, type- (adenovirus vs. DNA vaccine) independent. In addition, the adjuvant effect of CXCL11-Fc was, further confirmed by suppressing tumor growth and extension of survival rates in a therapeutic tumor, model, which was correlated with enhanced antigen-specific CD8 T cell responses. Interestingly, the, enhanced antigen-specific CD8 T cell responses by co-delivery of CXCL11-Fc were associated with CD8, T cell proliferation, followed by increased total and effector memory T cell frequencies. Taken together, our findings provide a novel role of CXCL11 as a strong genetic adjuvant which might be used to, increase antigen-specific CD8 T cell immunity elicited by vaccination.

Phenotypic and functional characterization of circulating polyomavirus BK VP1-specific CD8+ T cells in healthy adults

The human polyomavirus BK virus (BKV) establishes a latent and asymptomatic infection in the majority of the population. In immunocompromised individuals, the virus frequently (re)activates and may cause severe disease such as interstitial nephritis and hemorrhagic cystitis. Currently, the therapeutic options are limited to reconstitution of the antiviral immune response. T cells are particularly important for controlling this virus, and T cell therapies may provide a highly specific and effective mode of treatment. However, little is known about the phenotype and function of BKV-specific T cells in healthy individuals. Using tetrameric BKV peptide-HLA-A02 complexes, we determined the presence, phenotype, and functional characteristics of circulating BKV VP1-specific CD8(+) T cells in 5 healthy individuals. We show that these cells are present in low frequencies in the circulation and that they have a resting CD45RA(-) CD27(+) memory and predominantly CCR7(-) CD127(+) KLRG1(+) CD49d(hi) CXCR3(hi) T-bet(int) Eomesodermin(lo) phenotype. Furthermore, their direct cytotoxic capacity seems to be limited, since they do not readily express granzyme B and express only little granzyme K. We compared these cells to circulating CD8(+) T cells specific for cytomegalovirus (CMV), Epstein-Barr virus (EBV), and influenza virus (Flu) in the same donors and show that BKV-specific T cells have a phenotype that is distinct from that of CMV- and EBV-specific T cells. Lastly, we show that BKV-specific T cells are polyfunctional since they are able to rapidly express interleukin-2 (IL-2), gamma interferon (IFN-γ), tumor necrosis factor α, and also, to a much lower extent, MIP-1β and CD107a.

Regulatory multitasking of tolerogenic dendritic cells - lessons taken from vitamin d3-treated tolerogenic dendritic cells

Tolerogenic dendritic cells (DCs) work through silencing of differentiated antigen-specific T cells, activation and expansion of naturally occurring T regulatory cells (Tregs), transfer of regulatory properties to T cells, and the differentiation of naïve T cells into Tregs. Due to an operational definition based on T cell activation assays, the identity of tolerogenic DCs has been a matter of debate and it need not represent a specialized DC subset. Human tolerogenic DCs generated in vitro using inhibitory cytokines, growth factors, natural immunomodulators, or genetic manipulation have been effective and several of these tolerogenic DCs are currently being tested for clinical use. Ex vivo generated tolerogenic DCs reduce activation of naïve T cells using various means, promote a variety of regulatory T cells and most importantly, frequently show stable inhibitory phenotypes upon repetitive maturation with inflammatory factors. Yet, tolerogenic DCs differ with respect to the phenotype or the number of regulatory mechanisms they employ to modulate the immune system. In our experience, tolerogenic DCs generated using the biologically active form of vitamin D (VD3-DCs), alone, or combined with dexamethasone are proficient in their immunoregulatory functions. These tolerogenic DCs show a stable maturation-resistant semi-mature phenotype with low expression of activating co-stimulatory molecules, no production of the IL-12 family of cytokines and high expression of inhibitory molecules and IL-10. VD3-DCs induce increased apoptosis of effector T cells and induce antigen-specific regulatory T cells, which work through linked suppression ensuring infectious tolerance. Lessons learned on VD3-DCs help understanding the contribution of different pattern-recognition receptors (PRRs) and secondary signals to the tolerogenic function and how a cross-talk between DCs and T cells translates into immune regulation.

Peripheral prepositioning and local CXCL9 chemokine-mediated guidance orchestrate rapid memory CD8+ T cell responses in the lymph node

After an infection, the immune system generates long-lived memory lymphocytes whose increased frequency and altered state of differentiation enhance host defense against reinfection. Recently, the spatial distribution of memory cells was found to contribute to their protective function. Effector memory CD8+ T cells reside in peripheral tissue sites of initial pathogen encounter, in apparent anticipation of reinfection. Here we show that within lymph nodes (LNs), memory CD8+ T cells were concentrated near peripheral entry portals of lymph-borne pathogens, promoting rapid engagement of infected sentinel macrophages. A feed-forward CXCL9-dependent circuit provided additional chemotactic cues that further increase local memory cell density. Memory CD8+ T cells also produced effector responses to local cytokine triggers, but their dynamic behavior differed from that seen after antigen recognition. These data reveal the distinct localization and dynamic behavior of naive versus memory T cells within LNs and how these differences contribute to host defense.

mTOR and metabolic pathways in T cell quiescence and functional activation

The mechanistic target of rapamycin (mTOR), an evolutionally conserved serine and threonine kinase, plays a critical role in the promotion of cell growth and proliferation via integration of cellular and environmental cues. In adaptive immunity, the mTOR pathway orchestrates multiple physiological processes including the development and homeostasis of T cells under steady state, and their subsequent activation and differentiation upon antigen recognition. Associated with such fate decisions is the dynamic reprogramming of T cell metabolic pathways, as naïve, activated and memory cells are defined by distinct bioenergetic and biosynthetic activities. Emerging evidence indicates that mTOR signaling intersects with T cell metabolism at two major levels to constitute a critical control mechanism of T cell fate decisions. First, as a central environmental sensor, mTOR links immune signaling and the availability of nutrients, especially amino acids. Second, mTOR activates specific metabolic pathways in T cells such as aerobic glycolysis (also known as the "Warburg effect") in a process dependent upon the induction of transcription factors MYC and HIF1α. Understanding how mTOR interplays with T cell metabolism to dictate T cell fates and functions will provide fundamental insights into the mechanism of immune responses and the development of novel therapeutics against immune-mediated diseases. In this review, we summarize the current advances on mTOR signaling and T cell metabolism in the control of development, homeostasis, activation and differentiation of T cells.

Role of PI3K/Akt signaling in memory CD8 T cell differentiation

The clonal expansion, differentiation into effectors and establishing an immunological memory are crucial components of the adaptive immune response. Following the initial encounter with a pathogen, clonal CD8 T cell expansion yields at least two distinct populations of effector cells, short-lived effector cells (SLECs) and memory precursor effector cells (MPECs). SLECs are the terminally differentiated cells, which play an active role in pathogen clearance and undergo apoptosis once the pathogen is eliminated. In contrast, MPECs persist and give rise to self-renewing memory cells. These memory CD8 T cells maintain a state of heightened alertness and are poised to rapidly respond and swiftly clear the pathogen upon antigen re-encounter. As one of the goals of vaccination is to induce the development of these memory CD8 T cells, understanding the cellular and molecular basis of memory cell differentiation is critical to rational vaccine design. It is clear that memory differentiation is complex and involves multiple interrelated signaling pathways. It is influenced by factors such as the strength and duration of antigen receptor signaling and concurrent exposure to cytokines. Several signaling pathways that influence T cell fate have been recently described, and many culminate in the differential expression of specific transcription factors. Unfortunately, the mechanisms underlying the coordination and confluence of these signaling pathways remain largely unknown. In this review, we will discuss the role of the phosphatidylinositol 3-kinase signaling pathway as a central signaling node, and the function of Akt as a rheostat in orchestrating the differentiation of memory CD8 T cells.

Protecting and rescuing the effectors: roles of differentiation and survival in the control of memory T cell development

Vaccines, arguably the single most important intervention in improving human health, have exploited the phenomenon of immunological memory. The elicitation of memory T cells is often an essential part of successful long-lived protective immunity. Our understanding of T cell memory has been greatly aided by the development of TCR Tg mice and MHC tetrameric staining reagents that have allowed the precise tracking of antigen-specific T cell responses. Indeed, following acute infection or immunization, naïve T cells undergo a massive expansion culminating in the generation of a robust effector T cell population. This peak effector response is relatively short-lived and, while most effector T cells die by apoptosis, some remain and develop into memory cells. Although the molecular mechanisms underlying this cell fate decision remain incompletely defined, substantial progress has been made, particularly with regards to CD8(+) T cells. For example, the effector CD8(+) T cells generated during a response are heterogeneous, consisting of cells with more or less potential to develop into full-fledged memory cells. Development of CD8(+) T cell memory is regulated by the transcriptional programs that control the differentiation and survival of effector T cells. While the type of antigenic stimulation and level of inflammation control effector CD8(+) T cell differentiation, availability of cytokines and their ability to control expression and function of Bcl-2 family members governs their survival. These distinct differentiation and survival programs may allow for finer therapeutic intervention to control both the quality and quantity of CD8(+) T cell memory. Effector to memory transition of CD4(+) T cells is less well characterized than CD8(+) T cells, emerging details will be discussed. This review will focus on the recent progress made in our understanding of the mechanisms underlying the development of T cell memory with an emphasis on factors controlling survival of effector T cells.

Th1 cell induction in lymph nodes according to a red-blue chemokine map

Lymph nodes are compartmentalized organs but whether this feature has a role in T cell differentiation has been unclear. In this issue, Groom et al. (2012) reveal that spatially separated expression of two CXCR3 ligands guides Th1 cell development.

Memory T cell subsets, migration patterns, and tissue residence

Tissues such as the skin and mucosae are frequently exposed to microbial pathogens. Infectious agents must be quickly and efficiently controlled by our immune system, but the low frequency of naive T cells specific for any one pathogen means dependence on primary responses initiated in draining lymph nodes, often allowing time for serious infection to develop. These responses imprint effectors with the capacity to home to infected tissues; this process, combined with inflammatory signals, ensures the effective targeting of primary immunity. Upon vaccination or previous pathogen exposure, increased pathogen-specific T cell numbers together with altered migratory patterns of memory T cells can greatly improve immune efficacy, ensuring infections are prevented or at least remain subclinical. Until recently, memory T cell populations were considered to comprise central memory T cells (TCM), which are restricted to the secondary lymphoid tissues and blood, and effector memory T cells (TEM), which broadly migrate between peripheral tissues, the blood, and the spleen. Here we review evidence for these two memory populations, highlight a relatively new player, the tissue-resident memory T cell (TRM), and emphasize the potential differences between the migratory patterns of CD4(+) and CD8(+) T cells. This new understanding raises important considerations for vaccine design and for the measurement of immune parameters critical to the control of infectious disease, autoimmunity, and cancer.

CD4 T cell depletion exacerbates acute Mycobacterium tuberculosis while reactivation of latent infection is dependent on severity of tissue depletion in cynomolgus macaques

CD4 T cells are believed to be important in protection against Mycobacterium tuberculosis, but the relative contribution to control of initial or latent infection is not known. Antibody-mediated depletion of CD4 T cells in M. tuberculosis-infected cynomolgus macaques was used to study the role of CD4 T cells during acute and latent infection. Anti-CD4 antibody severely reduced levels of CD4 T cells in blood, airways, and lymph nodes. Increased pathology and bacterial burden were observed in CD4-depleted monkeys during the first 8 weeks of infection compared to controls. CD4-depleted monkeys had greater interferon (IFN)-γ expression and altered expression of CD8 T cell activation markers. During latent infection, CD4 depletion resulted in clinical reactivation in only three of six monkeys. Reactivation was associated with lower CD4 T cells in the hilar lymph nodes. During both acute and latent infection, CD4 depletion was associated with reduced percentages of CXCR3(+) expressing CD8 T cells, reported to be involved in T cell recruitment, regulatory function, and effector and memory T cell maturation. CXCR3(+) CD8 T cells from hilar lymph nodes had more mycobacteria-specific cytokine expression and greater coexpression of multiple cytokines compared to CXCR3(-) CD8 T cells. CD4 T cells are required for protection against acute infection but reactivation from latent infection is dependent on the severity of depletion in the draining lymph nodes. CD4 depletion influences CD8 T cell function. This study has important implications for human HIV-M. tuberculosis coinfection.

CD8 T cell memory: it takes all kinds

Understanding the mechanisms that regulate the differentiation and maintenance of CD8⁺ memory T cells is fundamental to the development of effective T cell-based vaccines. Memory cell differentiation is influenced by the cytokines that accompany T cell priming, the history of previous antigen encounters, and the tissue sites into which memory cells migrate. These cues combine to influence the developing CD8⁺ memory pool, and recent work has revealed the importance of multiple transcription factors, metabolic molecules, and surface receptors in revealing the type of memory cell that is generated. Paired with increasingly meticulous subsetting and sorting of memory populations, we now know the CD8⁺ memory pool to be phenotypically and functionally heterogeneous in nature. This includes both recirculating and tissue-resident memory populations, and cells with varying degrees of inherent longevity and protective function. These data point to the importance of tailored vaccine design. Here we discuss how the diversity of the memory CD8⁺ T cell pool challenges the notion that “one size fits all” for pathogen control, and how distinct memory subsets may be suited for distinct aspects of protective immunity.

Chemokine guidance of central memory T cells is critical for antiviral recall responses in lymph nodes

A defining feature of vertebrate immunity is the acquisition of immunological memory, which confers enhanced protection against pathogens by mechanisms that are incompletely understood. Here, we compared responses by virus-specific naive T cells (T(N)) and central memory T cells (T(CM)) to viral antigen challenge in lymph nodes (LNs). In steady-state LNs, both T cell subsets localized in the deep T cell area and interacted similarly with antigen-presenting dendritic cells. However, upon entry of lymph-borne virus, only T(CM) relocalized rapidly and efficiently toward the outermost LN regions in the medullary, interfollicular, and subcapsular areas where viral infection was initially confined. This rapid peripheralization was coordinated by a cascade of cytokines and chemokines, particularly ligands for T(CM)-expressed CXCR3. Consequently, in vivo recall responses to viral infection by CXCR3-deficient T(CM) were markedly compromised, indicating that early antigen detection afforded by intranodal chemokine guidance of T(CM) is essential for efficient antiviral memory.

CXCR3 chemokine receptor-ligand interactions in the lymph node optimize CD4+ T helper 1 cell differentiation

Differentiation of naive CD4(+) T cells into T helper (Th) cells is a defining event in adaptive immunity. The cytokines and transcription factors that control Th cell differentiation are understood, but it is not known how this process is orchestrated within lymph nodes (LNs). Here we have shown that the CXCR3 chemokine receptor was required for optimal generation of interferon-γ (IFN-γ)-secreting Th1 cells in vivo. By using a CXCR3 ligand reporter mouse, we found that stromal cells predominately expressed the chemokine ligand CXCL9 whereas hematopoietic cells expressed CXCL10 in LNs. Dendritic cell (DC)-derived CXCL10 facilitated T cell-DC interactions in LNs during T cell priming while both chemokines guided intranodal positioning of CD4(+) T cells to interfollicular and medullary zones. Thus, different chemokines acting on the same receptor can function locally to facilitate DC-T cell interactions and globally to influence intranodal positioning, and both functions contribute to Th1 cell differentiation.

Transcriptional control of effector and memory CD8+ T cell differentiation

During an infection, T cells can differentiate into multiple types of effector and memory T cells, which help to mediate pathogen clearance and provide long-term protective immunity. These cells can vary in their phenotype, function and location, and in their long-term fate in terms of their ability to populate the memory T cell pool. Over the past decade, the signalling pathways and transcriptional programmes that regulate the formation of heterogeneous populations of effector and memory CD8(+) T cells have started to be characterized, and this Review discusses the major advances in these areas.