Memory CD8 T cell inflation vs tissue-resident memory T cells: Same patrollers, same controllers?

Relevance of tissue-resident memory CD8 T cells in the onset of Parkinson's disease and examination of its possible etiologies: infectious or autoimmune?

Tissue-resident memory CD8 T cells are responsible for local immune surveillance in different tissues, including the brain. They constitute the first line of defense against pathogens and cancer cells and play a role in autoimmunity. A recently published study demonstrated that CD8 T cells with markers of residency containing distinct granzymes and interferon-γ infiltrate the parenchyma of the substantia nigra and contact dopaminergic neurons in an early premotor stage of Parkinson's disease. This infiltration precedes α-synuclein aggregation and neuronal loss in the substantia nigra, suggesting a relevant role for CD8 T cells in the onset of the disease. To date, the nature of the antigen that initiates the adaptive immune response remains unknown. This review will discuss the role of tissue-resident memory CD8 T cells in brain immune homeostasis and in the onset of Parkinson's disease and other neurological diseases. We also discuss how aging and genetic factors can affect the CD8 T cell immune response and how animal models can be misleading when studying human-related immune response. Finally, we speculate about a possible infectious or autoimmune origin of Parkinson's disease.

Viral-vectored respiratory mucosal vaccine strategies

Increasing global concerns of pandemic respiratory viruses highlight the importance of developing optimal vaccination strategies that encompass vaccine platform, delivery route, and regimens. The decades-long effort to develop vaccines to combat respiratory infections such as influenza, respiratory syncytial virus, and tuberculosis has met with challenges, including the inability of systemically administered vaccines to induce respiratory mucosal (RM) immunity. In this regard, ample preclinical and available clinical studies have demonstrated the superiority of RM vaccination to induce RM immunity over parenteral route of vaccination. A great stride has been made in developing vaccines for RM delivery against respiratory pathogens, including M. tuberculosis and SARS-CoV-2. In particular, inhaled aerosol delivery of adenoviral-vectored vaccines has shown significant promise.

FGL2-targeting T cells exhibit antitumor effects on glioblastoma and recruit tumor-specific brain-resident memory T cells

Although tissue-resident memory T (TRM) cells specific for previously encountered pathogens have been characterized, the induction and recruitment of brain TRM cells following immune therapy has not been observed in the context of glioblastoma. Here, we show that T cells expressing fibrinogen-like 2 (FGL2)-specific single-chain variable fragments (T-αFGL2) can induce tumor-specific CD8+ TRM cells that prevent glioblastoma recurrence. These CD8+ TRM cells display a highly expanded T cell receptor repertoire distinct from that found in peripheral tissue. When adoptively transferred to the brains of either immunocompetent or T cell-deficient naïve mice, these CD8+ TRM cells reject glioma cells. Mechanistically, T-αFGL2 cell treatment increased the number of CD69+CD8+ brain-resident memory T cells in tumor-bearing mice via a CXCL9/10 and CXCR3 chemokine axis. These findings suggest that tumor-specific brain-resident CD8+ TRM cells may have promising implications for the prevention of brain tumor recurrence.

Tissue-resident memory T cells: The key frontier in local synovitis memory of rheumatoid arthritis

Rheumatoid arthritis (RA) is a highly disabling, systemic autoimmune disease. It presents a remarkable tendency to recur, which renders it almost impossible for patients to live without drugs. Under such circumstances, many patients have to suffer the pain of recurrent attacks as well as the side effects of long-term medication. Current therapies for RA are primarily systemic treatments without targeting the problem that RA is more likely to recur locally. Emerging studies suggest the existence of a mechanism mediating local memory during RA, which is closely related to the persistent residence of tissue-resident memory T cells (T_RM). T_RM, one of the memory T cell subsets, reside in tissues providing immediate immune protection but driving recurrent local inflammation on the other hand. The heterogeneity among synovial T_RM is unclear, with the dominated CD8⁺ T_RM observed in inflamed synovium of RA patients coming into focus. Besides local arthritis relapse, T_RM may also contribute to extra-articular organ involvement in RA due to their migration potential. Future integration of single-cell RNA sequencing (scRNA-seq) with spatial transcriptomics to explore the gene expression patterns of T_RM in both temporal dimension and spatial dimension may help us identify specific therapeutic targets. Targeting synovial T_RM to suppress local arthritis flares while using systemic therapies to prevent extra-articular organ involvement may provide a new perspective to address RA recurrence.

'Stem-like' precursors are the fount to sustain persistent CD8+ T cell responses

Virus-specific CD8⁺ T cells that differentiate in the context of resolved versus persisting infections exhibit divergent phenotypic and functional characteristics, which suggests that their differentiation trajectories are governed by distinct cellular dynamics, developmental pathways and molecular mechanisms. For acute infection, it is long known that antigen-specific T cell populations contain terminally differentiated effector T cells, known as short-lived effector T cells, and proliferation-competent and differentiation-competent memory precursor T cells. More recently, it was identified that a similar functional segregation occurs in chronic infections. A failure to generate proliferation-competent precursor cells in chronic infections and tumors results in the collapse of the T cell response. Thus, these precursor cells are major therapeutic and prophylactic targets of immune interventions. These observations suggest substantial commonality between T cell responses in acute and chronic infections but there are also critical differences. We are therefore reviewing the common features and peculiarities of precursor cells in acute infections, different types of persistent infection and cancer.

Effects of Astragalus Polysaccharides on CD8+ Tissue-Resident Memory T Cells in Mice with Herpes Simplex

Objective

This study aimed to explore whether astragalus polysaccharides (APS) could treat herpes simplex by increasing tissue-resident memory CD8+ T cells (CD8+ TRM cells) and analyze its potential mechanism using the network pharmacologic approach.

Methods

C57BL/6J male mice aged 6–8 weeks were divided into a model group with HSV-1 infection treated by saline, a control group without HSV-1 infection but treated by saline, and an APS group with HSV-1 infection treated by APS. Clinical signs were observed, and the disease score was recorded every day. The skin lesions on day 9 after infection were taken for flow cytometric analysis to evaluate CD8+ TRM cells. Network pharmacologic analysis was performed to select the potential protein targets of astragalus associated with herpes simplex. Besides, Gene Ontology (GO) analysis and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis were performed. The peripheral blood from the retroorbital venous plexus was collected to evaluate the levels of serum interferon-γ (IFN-γ) and interleukin 12 (IL-12). The comparisons of clinical signs, the disease score, CD8+ TRM cells, the serum IFN-γ, and IL-12 levels were performed among the three groups.

Results

Compared with the model group, the disease score in the APS group was significantly lower (p < 0.05). On the day 9 after HSV-1 infection, there was no significant difference in the body weight of mice among the three groups. However, compared with the control group or model group, the spleen weight in the APS group increased significantly (p < 0.05). The surface antigens of CD8+ TRM cells had no significant difference between the control group and the model group, while compared with the model group, the surface antigens of CD8 (p < 0.05), CD69 (p < 0.05), and CD103 (p < 0.05) in the APS group increased significantly. Moreover, the serum IL-12 (p < 0.05) and IFN-γ (p < 0.01) levels in the APS group increased significantly compared with the model group.

Conclusion

Our study suggested that APS could alleviate the symptoms of the mice infected with HSV-1, and CD8+ TRM cells in the skin lesions and the levels of IL-12 and IFN-γ in the serum of mice with HSV-1 infection increased after the APS treatment, of which the specific underlying mechanism requires further experiments to clarify. In addition, the antiviral effect of APS might be worthy of further development and utilization.

Antimetastatic defense by CD8+ T cells

Metastasis is an intricate process whereby tumor cells migrate from the primary tumor, survive in the circulation, seed distal organs, and proliferate to create metastatic foci. CD8⁺ T cells can detect and eliminate tumor cells. Research on CD8⁺ T cell-dependent antitumor immunity has classically focused on its role in the primary tumor. There is increasing evidence, however, that CD8⁺ T cells have unique antimetastatic functions in various steps of the metastatic cascade. Here, we review the mechanisms whereby CD8⁺ T cells control metastatic lesions. We discuss their role in each step of metastasis, metastatic dormancy, and metastatic clonal evolution as well as the consequent clinical repercussions.

Localization of Viral Epitope-Specific CD8 T Cells during Cytomegalovirus Latency in the Lungs and Recruitment to Lung Parenchyma by Airway Challenge Infection

Interstitial pneumonia is a life-threatening clinical manifestation of cytomegalovirus infection in recipients of hematopoietic cell transplantation (HCT). The mouse model of experimental HCT and infection with murine cytomegalovirus revealed that reconstitution of virus-specific CD8⁺ T cells is critical for resolving productive lung infection. CD8⁺ T-cell infiltrates persisted in the lungs after the establishment of latent infection. A subset defined by the phenotype KLRG1⁺CD62L⁻ expanded over time, a phenomenon known as memory inflation (MI). Here we studied the localization of these inflationary T effector-memory cells (iTEM) by comparing their frequencies in the intravascular and transmigration compartments, the IVC and TMC, respectively, with their frequency in the extravascular compartment (EVC), the alveolar epithelium. Frequencies of viral epitope-specific iTEM were comparable in the IVC and TMC but were reduced in the EVC, corresponding to an increase in KLRG1⁻CD62L⁻ conventional T effector-memory cells (cTEM) and a decrease in functional IFNγ⁺CD8⁺ T cells. As maintained expression of KLRG1 requires stimulation by antigen, we conclude that iTEM lose KLRG1 and convert to cTEM after transmigration into the EVC because pneumocytes are not latently infected and, therefore, do not express antigens. Accordingly, antigen re-expression upon airway challenge infection recruited virus-specific CD8⁺ T cells to TMC and EVC.

Immunodominant Cytomegalovirus Epitopes Suppress Subdominant Epitopes in the Generation of High-Avidity CD8 T Cells

CD8⁺ T-cell responses to pathogens are directed against infected cells that present pathogen-encoded peptides on MHC class-I molecules. Although natural responses are polyclonal, the spectrum of peptides that qualify for epitopes is remarkably small even for pathogens with high coding capacity. Among those few that are successful at all, a hierarchy exists in the magnitude of the response that they elicit in terms of numbers of CD8⁺ T cells generated. This led to a classification into immunodominant and non-immunodominant or subordinate epitopes, IDEs and non-IDEs, respectively. IDEs are favored in the design of vaccines and are chosen for CD8⁺ T-cell immunotherapy. Using murine cytomegalovirus as a model, we provide evidence to conclude that epitope hierarchy reflects competition on the level of antigen recognition. Notably, high-avidity cells specific for non-IDEs were found to expand only when IDEs were deleted. This may be a host’s back-up strategy to avoid viral immune escape through antigenic drift caused by IDE mutations. Importantly, our results are relevant for the design of vaccines based on cytomegaloviruses as vectors to generate high-avidity CD8⁺ T-cell memory specific for unrelated pathogens or tumors. We propose the deletion of vector-encoded IDEs to avoid the suppression of epitopes of the vaccine target.

Direct Evidence for Viral Antigen Presentation during Latent Cytomegalovirus Infection

Murine models of cytomegalovirus (CMV) infection have revealed an immunological phenomenon known as “memory inflation” (MI). After a peak of a primary CD8⁺ T-cell response, the pool of epitope-specific cells contracts in parallel to the resolution of productive infection and the establishment of a latent infection, referred to as “latency.” CMV latency is associated with an increase in the number of cells specific for certain viral epitopes over time. The inflationary subset was identified as effector-memory T cells (iTEM) characterized by the cell surface phenotype KLRG1⁺CD127⁻CD62L⁻. As we have shown recently, latent viral genomes are not transcriptionally silent. Rather, viral genes are sporadically desilenced in a stochastic fashion. The current hypothesis proposes MI to be driven by presented viral antigenic peptides encoded by the corresponding, stochastically expressed viral genes. Although this mechanism suggests itself, independent evidence for antigen presentation during viral latency is pending. Here we fill this gap by showing that T cell-receptor transgenic OT-I cells that are specific for peptide SIINFEKL proliferate upon adoptive cell transfer in C57BL/6 recipients latently infected with murine CMV encoding SIINFEKL (mCMV-SIINFEKL), but not in those latently infected with mCMV-SIINFEKA, in which antigenicity is lost by mutation L8A of the C-terminal amino acid residue.

Influenza- and MCMV-induced memory CD8 T cells control respiratory vaccinia virus infection despite residence in distinct anatomical niches

Induction of memory CD8 T cells residing in peripheral tissues is of interest for T cell-based vaccines as these cells are located at mucosal and barrier sites and can immediately exert effector functions, thus providing protection in case of local pathogen encounter. Different memory CD8 T cell subsets patrol peripheral tissues, but it is unclear which subset is superior in providing protection upon secondary infections. We used influenza virus to induce predominantly tissue resident memory T cells or cytomegalovirus to elicit a large pool of effector-like memory cells in the lungs and determined their early protective capacity and mechanism of reactivation. Both memory CD8 T cell pools have unique characteristics with respect to their phenotype, localization, and maintenance. However, these distinct features do not translate into different capacities to control a respiratory vaccinia virus challenge in an antigen-specific manner, although differential activation mechanisms are utilized. While influenza-induced memory CD8 T cells respond to antigen by local proliferation, MCMV-induced memory CD8 T cells relocate from the vasculature into the tissue in an antigen-independent and partially chemokine-driven manner. Together these results bear relevance for the development of vaccines aimed at eliciting a protective memory CD8 T cell pool at mucosal sites.

Innate and Adaptive Immunity in Giant Cell Arteritis

Autoimmune diseases can afflict every organ system, including blood vessels that are critically important for host survival. The most frequent autoimmune vasculitis is giant cell arteritis (GCA), which causes aggressive wall inflammation in medium and large arteries and results in vaso-occlusive wall remodeling. GCA shares with other autoimmune diseases that it occurs in genetically predisposed individuals, that females are at higher risk, and that environmental triggers are suspected to beget the loss of immunological tolerance. GCA has features that distinguish it from other autoimmune diseases and predict the need for tailored diagnostic and therapeutic approaches. At the core of GCA pathology are CD4⁺ T cells that gain access to the protected tissue niche of the vessel wall, differentiate into cytokine producers, attain tissue residency, and enforce macrophages differentiation into tissue-destructive effector cells. Several signaling pathways have been implicated in initiating and sustaining pathogenic CD4⁺ T cell function, including the NOTCH1-Jagged1 pathway, the CD28 co-stimulatory pathway, the PD-1/PD-L1 co-inhibitory pathway, and the JAK/STAT signaling pathway. Inadequacy of mechanisms that normally dampen immune responses, such as defective expression of the PD-L1 ligand and malfunction of immunosuppressive CD8⁺ T regulatory cells are a common theme in GCA immunopathology. Recent studies are providing a string of novel mechanisms that will permit more precise pathogenic modeling and therapeutic targeting in GCA and will fundamentally inform how abnormal immune responses in blood vessels lead to disease.

The many faces of islet antigen-specific CD8 T cells: clues to clinical outcome in type 1 diabetes

Immune monitoring enables a better understanding of disease processes and response to therapy, but has been challenging in the setting of chronic autoimmunity because of unknown etiology, variable and protracted kinetics of the disease process, heterogeneity across patients and the complexity of immune interactions. To begin to parse this complexity, we focus here on type 1 diabetes (T1D) and CD8 T cells as a cell type that has features that are associated with different stages of disease, rates of progression and response to therapy. Specifically, we discuss the current understanding of the role of autoreactive CD8 T cells in disease outcome, which implicates particular CD8 functional subsets, rather than unique antigens or total number of autoreactive T cells. Next, we discuss how autoreactive CD8 T‐cell features can be reflected in measures of global CD8 T cells, and then pull these concepts together by highlighting immune therapies recently shown to modulate both CD8 T cells and disease progression. We end by discussing outstanding questions about the role of specific subsets of autoreactive CD8 T cells in disease progression and how they may be optimally modulated to treat and prevent T1D.

Surface NKG2C Identifies Differentiated αβT-Cell Clones Expanded in Peripheral Blood

T cells that express CD56 in peripheral blood of healthy humans represent a heterogeneous and poorly studied subset. In this work, we analyzed this subset for NKG2C expression. In both CD56+ and CD56- subsets most of the NKG2C+ T cells had a phenotype of highly differentiated CD8+ TEMRA cells. The CD56+NKG2C+ T cells also expressed a number of NK cell receptors, such as NKG2D, CD16, KIR2DL2/DL3, and maturation marker CD57 more often than the CD56-NKG2C+CD3+ cells. TCR β-chain repertoire of the CD3+CD56+NKG2C+ cell fraction was limited by the prevalence of one or several clonotypes which can be found within the most abundant clonotypes in total or CD8+ T cell fraction TCRβ repertoire. Thus, NKG2C expression in highly differentiated CD56+ T cells was associated with the most expanded αβ T cell clones. NKG2C+ T cells produced almost no IFN-γ in response to stimulation with HCMV pp65-derived peptides. This may be partially due to the high content of CD45RA+CD57+ cells in the fraction. CD3+NKG2C+ cells showed signs of activation, and the frequency of this T-cell subset in HCMV-positive individuals was positively correlated with the frequency of NKG2C+ NK cells that may imply a coordinated in a certain extent development of the NKG2C+ T and NK cell subsets under HCMV infection.

CD39+PD-1+CD8+ T cells mediate metastatic dormancy in breast cancer

Some breast tumors metastasize aggressively whereas others remain dormant for years. The mechanism governing metastatic dormancy remains largely unknown. Through high-parametric single-cell mapping in mice, we identify a discrete population of CD39+PD-1+CD8+ T cells in primary tumors and in dormant metastasis, which is hardly found in aggressively metastasizing tumors. Using blocking antibodies, we find that dormancy depends on TNFα and IFNγ. Immunotherapy reduces the number of dormant cancer cells in the lungs. Adoptive transfer of purified CD39+PD-1+CD8+ T cells prevents metastatic outgrowth. In human breast cancer, the frequency of CD39+PD-1+CD8+ but not total CD8+ T cells correlates with delayed metastatic relapse after resection (disease-free survival), thus underlining the biological relevance of CD39+PD-1+CD8+ T cells for controlling experimental and human breast cancer. Thus, we suggest that a primary breast tumor could prime a systemic, CD39+PD-1+CD8+ T cell response that favors metastatic dormancy in the lungs.

Thymus involution sets the clock of the aging T-cell landscape: Implications for declined immunity and tissue repair

Aging is generally characterized as a gradual increase in tissue damage, which is associated with senescence and chronic systemic inflammation and is evident in a variety of age-related diseases. The extent to which such tissue damage is a result of a gradual decline in immune regulation, which consequently compromises the capacity of the body to repair damages, has not been fully explored. Whereas CD4 T lymphocytes play a critical role in the orchestration of immunity, thymus involution initiates gradual changes in the CD4 T-cell landscape, which may significantly compromise tissue repair. In this review, we describe the lifespan accumulation of specific dysregulated CD4 T-cell subsets and their coevolution with systemic inflammation in the process of declined immunity and tissue repair capacity with age. Then, we discuss the process of thymus involution-which appears to be most pronounced around puberty-as a possible driver of the aging T-cell landscape. Finally, we identify individualized T cell-based early diagnostic biomarkers and therapeutic strategies for age-related diseases.

Impact of multiple hits with cognate antigen on memory CD8+ T-cell fate

Antigen-driven activation of CD8+ T cells results in the development of a robust anti-pathogen response and ultimately leads to the establishment of long-lived memory T cells. During the primary response, CD8+ T cells interact multiple times with cognate antigen on distinct types of antigen-presenting cells. The timing, location and context of these antigen encounters significantly impact the differentiation programs initiated in the cells. Moderate re-activation in the periphery promotes the establishment of the tissue-resident memory T cells that serve as sentinels at the portal of pathogen entry. Under some circumstances, moderate re-activation of T cells in the periphery can result in the excessive expansion and accumulation of circulatory memory T cells, a process called memory inflation. In contrast, excessive re-activation stimuli generally impede conventional T-cell differentiation programs and can result in T-cell exhaustion. However, these conditions can also elicit a small population of exhausted T cells with a memory-like signature and self-renewal capability that are capable of responding to immunotherapy, and restoration of functional activity. Although it is clear that antigen re-encounter during the primary immune response has a significant impact on memory T-cell development, we still do not understand the molecular details that drive these fate decisions. Here, we review our understanding of how antigen encounters and re-activation events impact the array of memory CD8+ T-cell subsets subsequently generated. Identification of the molecular programs that drive memory T-cell generation will advance the development of new vaccine strategies that elicit high-quality CD8+ T-cell memory.

Peripherally induced brain tissue-resident memory CD8+ T cells mediate protection against CNS infection

The central nervous system (CNS) is classically viewed as immune-privileged; however, recent advances highlight interactions between the peripheral immune system and CNS in controlling infections and tissue homeostasis. Tissue-resident memory (TRM) CD8+ T cells in the CNS are generated after brain infections, but it is unknown whether CNS infection is required to generate brain TRM cells. We show that peripheral infections generate antigen-specific CD8+ memory T cells in the brain that adopt a unique TRM signature. Upon depletion of circulating and perivascular memory T cells, this brain signature was enriched and the surveilling properties of brain TRM cells was revealed by intravital imaging. Notably, peripherally induced brain TRM cells showed evidence of rapid activation and enhanced cytokine production and mediated protection after brain infections. These data reveal that peripheral immunizations can generate brain TRM cells and will guide potential use of T cells as therapeutic strategies against CNS infections and neurological diseases.

Revisiting CD8 T-cell 'Memory Inflation': New Insights with Implications for Cytomegaloviruses as Vaccine Vectors

Murine models of cytomegalovirus (CMV) infection have revealed an exceptional kinetics of the immune response. After resolution of productive infection, transient contraction of the viral epitope-specific CD8 T-cell pool was found to be followed by a pool expansion specific for certain viral epitopes during non-productive ‘latent’ infection. This phenomenon, known as ‘memory inflation’ (MI), was found to be based on inflationary KLRG1⁺CD62L⁻ effector-memory T cells (iTEM) that depend on repetitive restimulation. MI gained substantial interest for employing CMV as vaccine vector by replacing MI-driving CMV epitopes with foreign epitopes for generating high numbers of protective memory cells specific for unrelated pathogens. The concept of an MI-driving CMV vector is questioned by human studies disputing MI in humans. A bias towards MI in experimental models may have resulted from systemic infection. We have here studied local murine CMV infection as a route that is more closely matching routine human vaccine application. Notably, KLRG1⁻CD62L⁺ central memory T cells (TCM) and conventional KLRG1⁻CD62L⁻ effector memory T cells (cTEM) were found to expand, associated with ‘avidity maturation’, whereas the pool size of iTEM steadily declined over time. The establishment of high avidity CD8 T-cell central memory encourages one to pursue the concept of CMV vector-based vaccines.

Immune-Inflammation in Atherosclerosis: A New Twist in an Old Tale

Background and Objective:

Atherosclerosis, a chronic and progressive inflammatory disease, is triggered by the activation of endothelial cells followed by infiltration of innate and adaptive immune cells including monocytes and T cells in arterial walls. Major populations of T cells found in human atherosclerotic lesions are antigen-specific activated CD4+ effectors and/or memory T cells from Th1, Th17, Th2 and Treg subsets. In this review, we will discuss the significance of T cell orchestrated immune inflammation in the development and progression of atherosclerosis.

Discussion:

Pathogen/oxidative stress/lipid induced primary endothelial wound cannot develop to a full-blown atherosclerotic lesion in the absence of chronically induced inflammation. While the primary inflammatory response might be viewed as a lone innate response, the persistence of such a profound response over time must be (and is) associated with diverse local and systemic T cell responses. The interplay between T cells and innate cells contributes to a phenomenon called immuneinflammation and has an impact on the progression and outcome of the lesion. In recent years immuneinflammation, an old term, has had a comeback in connecting the puzzle pieces of chronic inflammatory diseases.

Conclusion:

Taking one-step back and looking from afar at the players of immune-inflammation may help us provide a broader perspective of these complicated interactions. This may lead to the identification of new drug targets and the development of new therapies as well as preventative measures.

Striking a Balance-Cellular and Molecular Drivers of Memory T Cell Development and Responses to Chronic Stimulation

Effective adaptive immune responses are characterized by stages of development and maturation of T and B cell populations that respond to disturbances in the host homeostasis in cases of both infections and cancer. For the T cell compartment, this begins with recognition of specific peptides by naïve, antigen-inexperienced T cells that results in their activation, proliferation, and differentiation, which generates an effector population that clears the antigen. Loss of stimulation eventually returns the host to a homeostatic state, with a heterogeneous memory T cell population that persists in the absence of antigen and is primed for rapid responses to a repeat antigen exposure. However, in chronic infections and cancers, continued antigen persistence impedes a successful adaptive immune response and the formation of a stereotypical memory population of T cells is compromised. With repeated antigen stimulation, responding T cells proceed down an altered path of differentiation that allows for antigen persistence, but much less is known regarding the heterogeneity of these cells and the extent to which they can become “memory-like,” with a capacity for self-renewal and recall responses that are characteristic of bona fide memory cells. This review focuses on the differentiation of CD4⁺ and CD8⁺ T cells in the context of chronic antigen stimulation, highlighting the central observations in both human and mouse studies regarding the differentiation of memory or “memory-like” T cells. The importance of both the cellular and molecular drivers of memory T cell development are emphasized to better understand the consequences of persisting antigen on T cell fates. Integrating what is known and is common across model systems and patients can instruct future studies aimed at further understanding T cell differentiation and development, with the goal of developing novel methods to direct T cells toward the generation of effective memory populations.

Early primed KLRG1- CMV-specific T cells determine the size of the inflationary T cell pool

Memory T cell inflation is a process in which a subset of cytomegalovirus (CMV) specific CD8 T cells continuously expands mainly during latent infection and establishes a large and stable population of effector memory cells in peripheral tissues. Here we set out to identify in vivo parameters that promote and limit CD8 T cell inflation in the context of MCMV infection. We found that the inflationary T cell pool comprised mainly high avidity CD8 T cells, outcompeting lower avidity CD8 T cells. Furthermore, the size of the inflationary T cell pool was not restricted by the availability of specific tissue niches, but it was directly related to the number of virus-specific CD8 T cells that were activated during priming. In particular, the amount of early-primed KLRG1^- cells and the number of inflationary cells with a central memory phenotype were a critical determinant for the overall magnitude of the inflationary T cell pool. Inflationary memory CD8 T cells provided protection from a Vaccinia virus challenge and this protection directly correlated with the size of the inflationary memory T cell pool in peripheral tissues. These results highlight the remarkable protective potential of inflationary CD8 T cells that can be harnessed for CMV-based T cell vaccine approaches.

Cytomegalovirus induces a lifelong infection in the majority of the world's population, due to the ability of the virus to establish latency. Upon CMV infection, large numbers of effector memory T cells are induced in peripheral tissues, a process that is termed memory inflation. As inflationary T cells are highly functional, CMV-based vaccines have gained substantial interest for vaccination purposes. Here we examine factors that promote and limit memory T cell inflation. We found that there were no constraints on the availability of specific niches for inflationary T cells in tissues and that high avidity T cells predominately contribute to the inflationary T cell population in the beginning of infection. Moreover, the number of early primed KLRG1^- CMV-specific T cells in the acute phase of infection set the limit for memory T cell inflation. Furthermore, we show that inflationary T cells provided protection from a pathogenic challenge in peripheral tissues such as the ovaries. Thus, inflationary T cells comprise a population of T cells that can protect peripheral tissues from pathogenic infections and their efficacy can be regulated by balancing the number of KLRG1^- CMV-specific cells during priming.

Fuel and brake of memory T cell inflation

Memory T cell inflation is a process in which a large number of effector memory T cells accumulates in peripheral tissues. This phenomenon is observed upon certain low level persistent virus infections, but it is most commonly described upon infection with the β-herpesvirus Cytomegalovirus. Due to the induction of this large pool of functional effector CD8 T cells in peripheral tissues, the interest in using CMV-based vaccine vectors for vaccination purposes is rising. However, the exact mechanisms of memory T cell inflation are not yet fully understood. It is clear that repetitive exposure to antigen is a key determinant for memory inflation, and therefore the viral inoculum dose and the subsequent number of viral reactivation events strongly impact on the magnitude of the inflationary T cell pool. In addition, the number of CMV-specific CD8 T cells that is able to sense these reactivation events affects the size of the inflationary T cell pool. In the following, we will discuss factors that either promote or limit T cell inflation from both the virus and host perspective. These factors mostly operate by influencing the amount of available antigen or by affecting the T cell pool that is able to respond to the antigen. Furthermore, we will discuss the recent use of CMV-based vaccines in pre-clinical experimental settings, where these vectors have shown promising results by inducing prolonged effector memory T cell responses to foreign-introduced epitopes and thereby provided protection from subsequent virus or tumour challenges.

Resident-Memory T Cells in Tissue-Restricted Immune Responses: For Better or Worse?

Tissue-resident-memory CD8+ T cells (T_RM) have been described as a non-circulating memory T cell subset that persists at sites of previous infection. While T_RM in all non-lymphoid organs probably share a core signature differentiation pathway, certain aspects of their maintenance and effector functions may vary. It is well-established that T_RM provide long-lived protective immunity through immediate effector function and accelerated recruitment of circulating immune cells. Besides immune defense against pathogens, other immunological roles of T_RM are less well-studied. Likewise, evidence of a putative detrimental role of T_RM for inflammatory diseases is only beginning to emerge. In this review, we discuss the protective and harmful role of T_RM in organ-specific immunity and immunopathology as well as prospective implications for immunomodulatory therapy.

Major TCR Repertoire Perturbation by Immunodominant HLA-B*44:03-Restricted CMV-Specific T Cells

Lack of disease during chronic human cytomegalovirus (CMV) infection depends on the maintenance of a high-frequency CMV-specific T cell response. The composition of the T cell receptor (TCR) repertoire underlying this response remains poorly characterised, especially within African populations in which CMV is endemic from infancy. Here we focus on the immunodominant CD8+ T cell response to the immediate-early 2 (IE-2)-derived epitope NEGVKAAW (NW8) restricted by HLA-B^*44:03, a highly prevalent response in African populations, which in some subjects represents >10% of the circulating CD8+ T cells. Using pMHC multimer staining and sorting of NW8-specific T cells, the TCR repertoire raised against NW8 was characterised here using high-throughput sequencing in 20 HLA-B^*44:03 subjects. We found that the CD8+ T cell repertoire raised in response to NW8 was highly skewed and featured preferential use of a restricted set of V and J gene segments. Furthermore, as often seen in immunity against ancient viruses like CMV and Epstein-Barr virus (EBV), the response was strongly dominated by identical TCR sequences shared by multiple individuals, or “public” TCRs. Finally, we describe a pair “superdominant” TCR clonotypes, which were germline or nearly germline-encoded and produced at remarkably high frequencies in certain individuals, with a single CMV-specific clonotype representing up to 17% of all CD8+ T cells. Given the magnitude of the NW8 response, we propose that this major skewing of CMV-specific immunity leads to massive perturbations in the overall TCR repertoire in HLA-B^*44:03 individuals.

Cytomegalovirus infection and progressive differentiation of effector-memory T cells

Primary cytomegalovirus (CMV) infection leads to strong innate and adaptive immune responses against the virus, which prevents serious disease. However, CMV infection can cause serious morbidity and mortality in individuals who are immunocompromised. The adaptive immune response to CMV is characterized by large populations of effector-memory (EM) T cells that are maintained lifelong, a process termed memory inflation. Recent findings indicate that infection with CMV leads to continuous differentiation of CMV-specific EM-like T cells and that high-dose infection accelerates this progression. Whether measures that counteract CMV infection, such as anti-viral drugs, targeting of latently infected cells, adoptive transfer of CMV-specific T cells, and vaccination strategies, are able to impact the progressive differentiation of CMV-specific EM-like cells is discussed.

Brain-Resident T Cells Following Viral Infection

Activated CD8⁺ lymphocytes infiltrate the brain in response to many viral infections; where some remain stationed long term as memory T cells. Brain-resident memory T cells (bT_RM) are positioned to impart immediate defense against recurrent or reactivated infection. The cytokine and chemokine milieu present within a tissue is critical for T_RM generation and retention; and reciprocal interactions exist between brain-resident glia and bT_RM. High concentrations of TGF-β are found within brain and this cytokine has been shown to induce CD103 (integrin αeβ7) expression. The majority of T cells persisting within brain express CD103, which aids in retention through interaction with E-cadherin. Likewise, cytokines produced by T cells also modulate microglia. The anti-inflammatory cytokine IL-4 has been shown to preferentially polarize microglial cells toward an M2 phenotype, with a corresponding increase in E-cadherin expression. These findings demonstrate that the brain microenvironment, both during and following inflammation, prominently contributes to the role of CD103 in T cell persistence. Further evidence shows that microglia, and astrocytes, upregulate programmed death (PD) ligand 1 during neuroinflammation, likely to limit neuropathology, and the PD-1: PD-L1 pathway also aids in bT_RM generation and retention. Upon reactivation of quiescent neurotropic viruses, bT_RM may respond to small amounts of de novo-produced viral antigen by rapidly releasing IFN-γ, resulting in interferon-stimulated gene expression in surrounding glia, thereby amplifying activation of a small number of adaptive immune cells into an organ-wide innate antiviral response. While advantageous from an antiviral perspective; over time, recall response-driven, organ-wide innate immune activation likely has cumulative neurotoxic and neurocognitive consequences.