Inflammatory monocytes contribute to the persistence of CXCR3hi CX3CR1lo circulating and lung-resident memory CD8+ T cells following respiratory virus infection

Alterations in CX3CL1 Levels and Its Role in Viral Pathogenesis

CX3CL1, also named fractalkine or neurotactin, is the only known member of the CX3C chemokine family that can chemoattract several immune cells. CX3CL1 exists in both membrane-anchored and soluble forms, with each mediating distinct biological activities. CX3CL1 signals are transmitted through its unique receptor, CX3CR1, primarily expressed in the microglia of the central nervous system (CNS). In the CNS, CX3CL1 acts as a regulator of microglia activation in response to brain disorders or inflammation. Recently, there has been a growing interest in the role of CX3CL1 in regulating cell adhesion, chemotaxis, and host immune response in viral infection. Here, we provide a comprehensive review of the changes and function of CX3CL1 in various viral infections, such as human immunodeficiency virus (HIV), SARS-CoV-2, influenza virus, and cytomegalovirus (CMV) infection, to highlight the emerging roles of CX3CL1 in viral infection and associated diseases.

Poxviridae Pneumonia

Poxviridae family includes several viruses that infecting humans usually causes skin lesions only, but in some cases their clinical course is complicated by viral pneumonia (with or without bacterial superinfections). Historically variola virus has been the poxviridae most frequently associated with the development of pneumonia with many large outbreaks worldwide before its eradication in 1980. It is still considered a biological threat for its potential in biological warfare and bioterrorism. Smallpox pneumonia can be severe with the onset of acute respiratory distress syndrome (ARDS) and death. Vaccinia virus, used for vaccination against smallpox exceptionally, in immunocompromised patients, can induce generalized (with also lung involvement) severe disease after vaccination. MPXV virus occasionally can cause pneumonia particularly in immunocompromised patients. The pathophysiology of poxviridae pneumonia is still an area of active research; however, in animal models these viruses can cause both direct damage to the lower airways epithelium and a hyperinflammatory syndrome, like a cytokine storm. Multiple mechanisms of immune evasion have also been described. The treatment of poxviridae pneumonia is mainly based on careful supportive care. Despite the absence of randomized clinical trials in patients with poxviridae pneumonia there are antiviral drugs, such as tecovirimat, cidofovir and brincidofovir, FDA-approved for use in smallpox and also available under an expanded access protocol for treatment of MPXV. There are 2 (replication-deficient modified vaccinia Ankara and replication-competent vaccinia virus) smallpox vaccines FDA-approved with the first one also approved for prevention of MPXV in adults that are at high risk of infection.

The role of monocyte-derived macrophages in the lung: it's all about context

Macrophages are present in every tissue of the body where they play crucial roles in maintaining tissue homeostasis and providing front line defence against pathogens. Arguably, this is most important at mucosal barrier tissues, such as the lung and gut, which are major ports of entry for pathogens. However, a common feature of inflammation, infection or injury is the loss of tissue resident macrophages and accumulation of monocytes from the circulation, which differentiate, to different extents, into macrophages. The exact fate and function of these elicited, monocyte-derived macrophages in infection, injury and inflammation remains contentious. While some studies have documented the indispensable nature of monocytes and their macrophage derivatives in combatting infection and restoration of lung homeostasis following insult, observations from clinical studies and preclinical models of lung infection/injury shows that monocytes and their progeny can become dysregulated in severe pathology, often perpetuating rather than resolving the insult. In this Mini Review, we aim to bring together these somewhat contradictory reports by discussing how the plasticity of monocytes allow them to assume distinct functions in different contexts in the lung, from health to infection, and effective tissue repair to fibrotic disease.

Autophagy-associated immune dysregulation and hyperplasia in a patient with compound heterozygous mutations in ATG9A

ABBREVIATIONS

BCL2: BCL2 apoptosis regulator; BCL10: BCL10 immune signaling adaptor; CARD11: caspase recruitment domain family member 11; CBM: CARD11-BCL10-MALT1; CR2: complement C3d receptor 2; EBNA: Epstein Barr nuclear antigen; EBV: Epstein-Barr virus; FCGR3A; Fc gamma receptor IIIa; GLILD: granulomatous-lymphocytic interstitial lung disease; HV: healthy volunteer; IKBKB/IKB kinase: inhibitor of nuclear factor kappa B kinase subunit beta; IL2RA: interleukin 2 receptor subunit alpha; MALT1: MALT1 paracaspase; MS4A1: membrane spanning 4-domain A1; MTOR: mechanistic target of rapamycin kinase; MYC: MYC proto-oncogene, bHLH: transcription factor; NCAM1: neural cell adhesion molecule 1; NFKB: nuclear factor kappa B; NIAID: National Institute of Allergy and Infectious Diseases; NK: natural killer; PTPRC: protein tyrosine phosphatase receptor type C; SELL: selectin L; PBMCs: peripheral blood mononuclear cells; TR: T cell receptor; Tregs: regulatory T cells; WT: wild-type.

Bystander activation of Bordetella pertussis-induced nasal tissue-resident memory CD4 T cells confers heterologous immunity to Klebsiella pneumoniae

Tissue-resident memory CD4 T (T_RM ) cells induced by infection with Bordetella pertussis persist in respiratory tissues and confer long-term protective immunity against reinfection. However, it is not clear how they are maintained in respiratory tissues. Here, we demonstrate that B. pertussis-specific CD4 T_RM cells produce IL-17A in response to in vitro stimulation with LPS or heat-killed Klebsiella pneumoniae (HKKP) in the presence of dendritic cells. Furthermore, IL-17A-secreting CD4 T_RM cells expand in the lung and nasal tissue of B. pertussis convalescent mice following in vivo administration of LPS or HKKP. Bystander activation of CD4 T_RM cells was suppressed by anti-IL-12p40 but not by anti-MHCII antibodies. Furthermore, purified respiratory tissue-resident, but not circulating, CD4 T cells from convalescent mice produced IL-17A following direct stimulation with IL-23 and IL-1β or IL-18. Intranasal immunization of mice with a whole-cell pertussis vaccine induced respiratory CD4 T_RM cells that were reactivated following stimulation with K. pneumoniae. Furthermore, the nasal pertussis vaccine conferred protective immunity against B. pertussis but also attenuated infection with K. pneumoniae. Our findings demonstrate that CD4 T_RM cells induced by respiratory infection or vaccination can undergo bystander activation and confer heterologous immunity to an unrelated respiratory pathogen.

The Memory T Cell “Communication Web” in Context with Gastrointestinal Disorders—How Memory T Cells Affect Their Surroundings and How They Are Influenced by It

Gut-related diseases like ulcerative colitis, Crohn’s disease, or colorectal cancer affect millions of people worldwide. It is an ongoing process finding causes leading to the development and manifestation of those disorders. This is highly relevant since understanding molecular processes and signalling pathways offers new opportunities in finding novel ways to interfere with and apply new pharmaceuticals. Memory T cells (mT cells) and their pro-inflammatory properties have been proven to play an important role in gastrointestinal diseases and are therefore increasingly spotlighted. This review focuses on mT cells and their subsets in the context of disease pathogenesis and maintenance. It illustrates the network of regulatory proteins and metabolites connecting mT cells with other cell types and tissue compartments. Furthermore, the crosstalk with various microbes will be a subject of discussion. Characterizing mT cell interactions will help to further elucidate the sophisticated molecular and cellular networking system in the intestine and may present new ideas for future research approaches to control gut-related diseases.

Establishment of a Risk Score Model for Early Prediction of Severe H1N1 Influenza

H1N1 is the most common subtype of influenza virus circulating worldwide and can cause severe disease in some populations. Early prediction and intervention for patients who develop severe influenza will greatly reduce their mortality. In this study, we conducted a comprehensive analysis of 180 PBMC samples from three published datasets from the GEO DataSets. Differentially expressed gene (DEG) analysis and weighted correlation network analysis (WGCNA) were performed to provide candidate DEGs for model building. Functional enrichment and CIBERSORT analyses were also performed to evaluate the differences in composition and function of PBMCs between patients with severe and mild disease. Finally, a risk score model was built using lasso regression analysis, with six genes (CX3CR1, KLRD1, MMP8, PRTN3, RETN and SCD) involved. The model performed moderately in the early identification of patients that develop severe H1N1 disease.

Tissue resident memory T cells in the respiratory tract

Owing to their capacity to rapidly spread across the population, airborne pathogens represent a significant risk to global health. Indeed, several of the past major global pandemics have been instigated by respiratory pathogens. A greater understanding of the immune cells tasked with protecting the airways from infection will allow for the development of strategies that curb the spread and impact of these airborne diseases. A specific subset of memory T-cell resident in both the upper and lower respiratory tract, termed tissue-resident memory (Trm), have been shown to play an instrumental role in local immune responses against a wide breadth of both viral and bacterial infections. In this review, we discuss factors that influence respiratory tract Trm development, longevity, and immune surveillance and explore vaccination regimes that harness these cells, such approaches represent exciting new strategies that may be utilized to tackle the next global pandemic.

CD8+ T Cell Exhaustion in Cancer

A paradigm shift in the understanding of the exhausted CD8⁺ T cell (T_ex) lineage is underway. Originally thought to be a uniform population that progressively loses effector function in response to persistent antigen, single-cell analysis has now revealed that CD8⁺ T_ex is composed of multiple interconnected subpopulations. The heterogeneity within the CD8⁺ T_ex lineage is comprised of immune checkpoint blockade (ICB) permissive and refractory subsets termed stem-like and terminally differentiated cells, respectively. These populations occupy distinct peripheral and intratumoral niches and are characterized by transcriptional processes that govern transitions between cell states. This review presents key findings in the field to construct an updated view of the spatial, transcriptional, and functional heterogeneity of anti-tumoral CD8⁺ T_ex. These emerging insights broadly call for (re-)focusing cancer immunotherapies to center on the driver mechanism(s) underlying the CD8⁺ T_ex developmental continuum aimed at stabilizing functional subsets.

CCR2 Regulates Vaccine-Induced Mucosal T-Cell Memory to Influenza A Virus

Elicitation of lung tissue-resident memory CD8 T cells (T_RMs) is a goal of T cell-based vaccines against respiratory viral pathogens, such as influenza A virus (IAV). C-C chemokine receptor type 2 (CCR2)-dependent monocyte trafficking plays an essential role in the establishment of CD8 T_RMs in lungs of IAV-infected mice. Here, we used a combination adjuvant-based subunit vaccine strategy that evokes multifaceted (T_C1/T_C17/T_H1/T_H17) IAV nucleoprotein-specific lung T_RMs to determine whether CCR2 and monocyte infiltration are essential for vaccine-induced T_RM development and protective immunity to IAV in lungs. Following intranasal vaccination, neutrophils, monocytes, conventional dendritic cells (DCs), and monocyte-derived dendritic cells internalized and processed vaccine antigen in lungs. We found that basic leucine zipper ATF-like transcription factor 3 (BATF3)-dependent DCs were essential for eliciting T cell responses, but CCR2 deficiency enhanced the differentiation of CD127^hi, KLRG-1^lo, OX40^+ve CD62L^+ve, and mucosally imprinted CD69^+ve CD103^+ve effector and memory CD8 T cells in lungs and airways of vaccinated mice. Mechanistically, increased development of lung T_RMs induced by CCR2 deficiency was linked to dampened expression of T-bet but not altered TCF-1 levels or T cell receptor signaling in CD8 T cells. T1/T17 functional programming, parenchymal localization of CD8/CD4 effector and memory T cells, recall T cell responses, and protective immunity to a lethal IAV infection were unaffected in CCR2-deficient mice. Taken together, we identified a negative regulatory role for CCR2 and monocyte trafficking in mucosal imprinting and differentiation of vaccine-induced T_RMs. Mechanistic insights from this study may aid the development of T-cell-based vaccines against respiratory viral pathogens, including IAV and severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). IMPORTANCE While antibody-based immunity to influenza A virus (IAV) is type and subtype specific, lung- and airway-resident memory T cells that recognize conserved epitopes in the internal viral proteins are known to provide heterosubtypic immunity. Hence, broadly protective IAV vaccines need to elicit robust T cell memory in the respiratory tract. We have developed a combination adjuvant-based IAV nucleoprotein vaccine that elicits strong CD4 and CD8 T cell memory in lungs and protects against H1N1 and H5N1 strains of IAV. In this study, we examined the mechanisms that control vaccine-induced protective memory T cells in the respiratory tract. We found that trafficking of monocytes into lungs might limit the development of antiviral lung-resident memory T cells following intranasal vaccination. These findings suggest that strategies that limit monocyte infiltration can potentiate vaccine-induced frontline T-cell immunity to respiratory viruses, such as IAV and SARS-CoV-2.

The frequency of interleukin-1β-producing monocytes is significantly associated with varicella-zoster responses of nursing home residents

Previous studies have demonstrated that the status of the T cell compartment and inflammation-related factors are associated with the immunogenicity of the varicella-zoster virus (VZV) vaccine in older adults; however, little is known about the roles of other immune cell subsets known to influence the generation and maintenance of immunological memory. Responses to a live-attenuated VZV vaccine were studied in relation to peripheral blood mononuclear cell (PBMC) composition and function in a sample of 30 nursing home residents (aged 80-99 years). Interferon-gamma enzyme-linked immunospot (ELISPOT) was used to measure VZV responses at baseline and 6 weeks following vaccination, and associations were sought with the frequencies of monocytes and T, B and natural killer (NK) cells and the production and secretion of cytokines following their ex-vivo stimulation with different agents. While only the frequency of interleukin (IL)-6⁺ CD14⁺ monocytes was inversely associated with post-vaccination VZV response, amounts of IL-1β, IL-10, IL-17A and tumour necrosis factor (TNF) secreted by PBMCs and the frequency of IL-1β⁺ CD14⁺ monocytes was positively correlated with pre-vaccination VZV response. Furthermore, both bivariate correlation and causal mediation analyses supported the notion that IL-1β⁺ CD14⁺ monocytes were significant mediators of the associations between IL-1β and TNF secretion by PBMCs and pre-vaccination VZV responses. Our findings implicate a strong cytokine response mediated by inflammatory IL-1β⁺ monocytes in coordinating responses of long-lived VZV-reactive memory T cells, but with an opposing effect of IL-6⁺ CD14⁺ monocytes. Whether monocyte status promotes or inhibits the induction and/or maintenance of these memory T cells later in life has yet to be determined.

Engineering Vaccines for Tissue-Resident Memory T Cells

In recent years, tissue-resident memory T cells (TRM) have attracted significant attention in the field of vaccine development. Distinct from central and effector memory T cells, TRM cells take up residence in home tissues such as the lung or urogenital tract and are ideally positioned to respond quickly to pathogen encounter. TRM have been found to play a role in the immune response against many globally important infectious diseases for which new or improved vaccines are needed, including influenza and tuberculosis. It is also increasingly clear that TRM play a pivotal role in cancer immunity. Thus, vaccines that can generate this memory T cell population are highly desirable. The field of immunoengineering-that is, the application of engineering principles to study the immune system and design new and improved therapies that harness or modulate immune responses-is ideally poised to provide solutions to this need for next-generation TRM vaccines. This review covers recent developments in vaccine technologies for generating TRM and protecting against infection and cancer, including viral vectors, virus-like particles, and synthetic and natural biomaterials. In addition, it offers critical insights on the future of engineering vaccines for tissue-resident memory T cells.

CCR2 Regulates Vaccine-Induced Mucosal T-Cell Memory to Influenza A Virus

Elicitation of lung tissue-resident memory CD8 T cells (T _RM s) is a goal of T-cell based vaccines against respiratory viral pathogens such as influenza A virus (IAV). Chemokine receptor 2 (CCR2)-dependent monocyte trafficking plays an essential role in the establishment of CD8 T _RM s in lungs of IAV-infected mice. Here, we used a combination adjuvant-based subunit vaccine strategy that evokes multifaceted (T _C 1/T _C 17/T _H 1/T _H 17) IAV nucleoprotein-specific lung T _RM s, to determine whether CCR2 and monocyte infiltration are essential for vaccine-induced T _RM development and protective immunity to IAV in lungs. Following intranasal vaccination, neutrophils, monocytes, conventional dendrtitic cells (DCs) and monocyte-derived DCs internalized and processed vaccine antigen in lungs. We also found that Basic Leucine Zipper ATF-Like Transcription Factor 3 (BATF-3)-dependent DCs were essential for eliciting T cell responses, but CCR2 deficiency enhanced the differentiation of CD127 ^HI /KLRG-1 ^LO , OX40 ^+ve CD62L ^+ve and mucosally imprinted CD69 ^+ve CD103 ^+ve effector and memory CD8 T cells in lungs and airways of vaccinated mice. Mechanistically, increased development of lung T _RM s, induced by CCR2 deficiency was linked to dampened expression of T-bet, but not altered TCF-1 levels or T cell receptor signaling in CD8 T cells. T1/T17 functional programming, parenchymal localization of CD8/CD4 effector and memory T cells, recall T cell responses and protective immunity to a lethal IAV infection were unaffected in CCR2-deficient mice. Taken together, we identified a negative regulatory role for CCR2 and monocyte trafficking in mucosal imprinting and differentiation of vaccine-induced T _RM s. Mechanistic insights from this study may aid the development of T-cell-based vaccines against respiratory viral pathogens including IAV and SARS-CoV-2.

Importance

While antibody-based immunity to influenza A virus (IAV) is type and sub-type specific, lung and airway-resident memory T cells that recognize conserved epitopes in the internal viral proteins are known to provide heterosubtypic immunity. Hence, broadly protective IAV vaccines need to elicit robust T-cell memory in the respiratory tract. We have developed a combination adjuvant-based IAV nucleoprotein vaccine that elicits strong CD4 and CD8 T cell memory in lungs and protects against H1N1 and H5N1 strains of IAV. In this study, we examined the mechanisms that control vaccine-induced protective memory T cells in the respiratory tract. We found that trafficking of monocytes into lungs might limit the development of anti-viral lung-resident memory T cells, following intranasal vaccination. These findings suggested that strategies that limit monocyte infiltration can potentiate vaccine-induced frontline T-cell immunity to respiratory viruses such as IAV and SARS-CoV-2.

CXCL9-expressing tumor-associated macrophages: new players in the fight against cancer

Tumor-associated macrophages (TAMs) are among the main contributors to immune suppression in the tumor microenvironment, however, TAM depletion strategies have yielded little clinical benefit. Here, we discuss the concept that TAMs are also key regulators of anti-PD(L)-1-mediated CD8 T cell-dependent immunity. Emerging data suggest that expression of the chemokine CXCL9 by TAMs regulates the recruitment and positioning of CXCR3-expressing stem-like CD8 T (T_stem) cells that underlie clinical responses to anti-PD(L)-1 treatment. We evaluate clinical and mechanistic studies that establish relationships between CXCL9-expressing TAMs, T_stem and antitumor immunity. Therapies that enhance anti-PD(L)-1 response rates must consider TAM CXCL9 expression. In this perspective, we discuss opportunities to enhance the frequency and function of CXCL9 expressing TAMs and draw on comparative analyzes from infectious disease models to highlight potential functions of these cells beyond T_stem recruitment.

Divergence of Tissue-Memory T Cells: Distribution and Function-Based Classification

Tissue-resident memory T cells (Trm) comprise the majority of memory cells in nonlymphoid tissues and play a predominant role in immunity at barrier surfaces. A better understanding of Trm cell maintenance and function is essential for the development of vaccines that confer frontline protection. However, it is currently challenging to precisely distinguish Trm cells from other T cells, and this has led to confusion in the literature. Here we highlight gaps in our understanding of tissue memory and discuss recent advances in the classification of Trm cell subsets based on their distribution and functional characteristics.

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.

Monocyte-Derived Cells in Tissue-Resident Memory T Cell Formation

There is currently much interest in how different dendritic cell and macrophage populations contribute to T cell-mediated immunity. Although conventional dendritic cell subsets have received much attention for their role in T cell priming, there is emerging evidence for a role for monocyte-derived APC (MoAPC) in tissue-resident memory T cell (Trm) formation. Cells of the monocyte/macrophage lineage play a key role in providing chemokines and cytokines for the localization, differentiation, and survival of Trm and Trm precursors. In addition, inflammatory MoAPC are the key providers of TNF superfamily costimulatory signals, a signal we refer to as signal 4 for T cell activation. Recent evidence suggests that signal 4 from MoAPC occurs postpriming and substantially increases Trm formation. Key questions remain, such as the Ag dependence of signal 4 and the specific mechanisms by which MoAPC-Trm interactions affect the long-term maintenance of Trm.

Androgen receptor signaling in the lungs mitigates inflammation and improves the outcome of influenza in mice

Circulating androgens can modulate immune cell activity, but the impact of androgens on viral pathogenesis remains unclear. Previous data demonstrate that testosterone reduces the severity of influenza A virus (IAV) infection in male mice by mitigating pulmonary inflammation rather than by affecting viral replication. To examine the immune responses mediated by testosterone to mitigate IAV-induced inflammation, adult male mice remained gonadally intact or were gonadectomized and treated with either placebo or androgen-filled (i.e., testosterone or dihydrotestosterone) capsules prior to sublethal IAV infection. Like intact males, treatment of gonadectomized males with androgens improved the outcome of IAV infection, which was not mediated by changes in the control of virus replication or pulmonary cytokine activity. Instead, androgens accelerated pulmonary leukocyte contraction to limit inflammation. To identify which immune cells were contracting in response to androgens, the composition of pulmonary cellular infiltrates was analyzed and revealed that androgens specifically accelerated the contraction of total pulmonary inflammatory monocytes during peak disease, as well as CD8+ T cells, IAV-specific CD8+ T numbers, cytokine production and degranulation by IAV-specific CD8+ T cells, and the influx of eosinophils into the lungs following clearance of IAV. Neither depletion of eosinophils nor adoptive transfer of CD8+ T cells could reverse the ability of testosterone to protect males against IAV suggesting these were secondary immunologic effects. The effects of testosterone on the contraction of immune cell numbers and activity were blocked by co-administration of the androgen receptor antagonist flutamide and mimicked by treatment with dihydrotestosterone, which was also able to reduce the severity of IAV in female mice. These data suggest that androgen receptor signaling creates a local pulmonary environment that promotes downregulation of detrimental inflammatory immune responses to protect against prolonged influenza disease.

The Suppressive Attitude of Inflammatory Monocytes in Antiviral Antibody Responses

Inflammatory monocytes play important functions in antiviral immune responses, including release of inflammatory cytokines and antigen presentation to T lymphocytes. Depending on the pathological context, these functions might translate into beneficial or detrimental effects in the resolution of the disease. Recent literature has highlighted a role for inflammatory monocytes also in direct suppression of B cell responses. In this review, we will briefly discuss research showing the relationship between inflammatory monocytes and B lymphocytes, its functional consequences on antiviral antibody responses, and possible implications in the design of future vaccination strategies.

Lack of B Lymphocytes Enhances CD8 T Cell-Mediated Resistance against Respiratory Viral Infection but Compromises Memory Cell Formation

Following a respiratory virus infection, CXCR3^hi CX3CR1^lo and CXCR3^lo CX3CR1^hi CD8 T cells localize to different compartments within the lung and play an important role in host resistance, but mechanisms governing their optimal generation are poorly defined. We serendipitously found that B cell-deficient (μMT^-/-) mice were highly resistant to lethal infection with a virulent poxvirus strain and that depletion of CD8 T cells rendered these mice susceptible to infection. B cells were not required for the expansion of virus-specific CD8 T cells, but a greater proportion of activated CD8 T cells acquired an effector-like CXCR3^lo CX3CR1^hi phenotype in the absence of B cells. After recovery from infection, CD8 T cells in μMT^-/- mice contracted normally but failed to survive and seed the memory cell pool in both the lungs and spleen. These findings reveal a previously unappreciated role for B cells in regulating the balance between CD8 T cell-mediated resistance against respiratory viral infection and memory cell development.IMPORTANCE B cells play critical role in host resistance against many respiratory viral infections. However, the role of B cells beyond antibody-producing cells is less well defined. In this study, we made a surprising observation that mice lacking B cells were more resistant to respiratory infection with vaccinia virus than wild-type mice. This enhanced resistance was mediated by CD8 T cells because when we depleted CD8 T cells in B cell-deficient mice, these mice were unable to survive the infection. Interestingly, CD8 T cells in B cell-deficient mice were skewed more toward effector phenotype and less toward memory phenotype, which resulted in severely compromised memory CD8 T cell development. Thus, our study shows a novel role of B cells as regulators of CD8 T cell-mediated host resistance and memory CD8 T cell formation during respiratory viral infection.

Pulmonary monocytes interact with effector T cells in the lung tissue to drive TRM differentiation following viral infection

Lung resident memory CD8 T cells (T_RM) are critical for protection against respiratory viruses, but the cellular interactions required for their development are poorly understood. Herein we describe the necessity of classical monocytes for the establishment of lung T_RM following influenza infection. We find that, during the initial appearance of lung T_RM, monocytes and dendritic cells are the most numerous influenza antigen-bearing APCs in the lung. Surprisingly, depletion of DCs after initial T cell priming did not impact lung T_RM development or maintenance. In contrast, a monocyte deficient pulmonary environment in CCR2^-/- mice results in significantly less lung T_RM following influenza infection, despite no defect in the antiviral effector response or in the peripheral memory pool. Imaging shows direct interaction of antigen-specific T cells with antigen-bearing monocytes in the lung, and pulmonary classical monocytes from the lungs of influenza infected mice are sufficient to drive differentiation of T cells in vitro. These data describe a novel role for pulmonary monocytes in mediating lung T_RM development through direct interaction with T cells in the lung.

Tissue-Resident Macrophages Limit Pulmonary CD8 Resident Memory T Cell Establishment

Tissue resident memory CD8 T cells (T_RM) serve as potent local sentinels and contribute significantly to protective immunity against intracellular mucosal pathogens. While the molecular and transcriptional underpinnings of T_RM differentiation are emerging, how T_RM establishment is regulated by other leukocytes in vivo is largely unclear. Here, we observed that expression of PPAR-γ in the myeloid compartment was a negative regulator of CD8 T_RM establishment following influenza virus infection. Interestingly, myeloid deficiency of PPAR-γ resulted in selective impairment of the tissue-resident alveolar macrophage (AM) compartment during primary influenza infection, suggesting that AM are likely negative regulators of CD8 T_RM differentiation. Indeed, influenza-specific CD8 T_RM cell numbers were increased following early, but not late ablation of AM using the CD169-DTR model. Importantly, these findings were specific to the parenchyma of infected tissue as circulating memory T cell frequencies in lung and T_CM and T_EM in spleen were largely unaltered following macrophage ablation. Further, the magnitude of the effector response could not explain these observations. These data indicate local regulation of pulmonary T_RM differentiation is alveolar macrophage dependent. These, findings could aid in vaccine design aimed at increasing T_RM density to enhance protective immunity, or deflating their numbers in conditions where they cause overt or veiled chronic pathologies.

IRF4-dependent dendritic cells regulate CD8+ T-cell differentiation and memory responses in influenza infection

Acute respiratory disease caused by influenza viruses is imperfectly mitigated by annual vaccination to select strains. Development of vaccines that elicit lung-resident memory CD8⁺ T cells (T_RM) would offer more universal protection to seasonal and emerging pandemic viruses. Understanding how lung-resident dendritic cells (DCs) regulate T_RM differentiation would be an important step in this process. Here, we used CD11c-cre-Irf4^f/f (KO) mice, which lack lung-resident IRF4-dependent CD11b⁺CD24^hi DCs and show IRF4 deficiency in other lung cDC subsets, to determine if IRF4-expressing DCs regulate CD8⁺ memory precursor cells and T_RM during influenza A virus (IAV) infection. KO mice showed defective CD8⁺ T-cell memory, stemming from a deficit of T regulatory cells and memory precursor cells with decreased Foxo1 expression. Transfer of wild-type CD11b⁺CD24^hi DCs into KO mice restored CD8⁺ memory precursor cell numbers to wild-type levels. KO mice recovered from a primary infection harbored reduced numbers of CD8⁺ T_RM and showed deficient expansion of IFNγ⁺CD8⁺ T cells and increased lung pathology upon challenge with heterosubtypic IAV. Thus, vaccination strategies that harness the function of IRF4-dependent DCs could promote the differentiation of CD8⁺ T_RM during IAV infection.

Establishment and Maintenance of Conventional and Circulation-Driven Lung-Resident Memory CD8+ T Cells Following Respiratory Virus Infections

Antigen-specific CD8⁺ tissue-resident memory T cells (T_RM cells) persist in the lung following resolution of a respiratory virus infection and provide first-line defense against reinfection. In contrast to other memory T cell populations, such as central memory T cells that circulate between lymph and blood, and effector memory T cells (T_EM cells) that circulate between blood and peripheral tissues, T_RM cells are best defined by their permanent residency in the tissues and their independence from circulatory T cell populations. Consistent with this, we recently demonstrated that CD8⁺ T_RM cells primarily reside within specific niches in the lung (Repair-Associated Memory Depots; RAMD) that normally exclude CD8⁺ T_EM cells. However, it has also been reported that circulating CD8⁺ T_EM cells continuously convert into CD8⁺ T_RM cells in the lung interstitium, helping to sustain T_RM numbers. The relative contributions of these two mechanisms of CD8⁺ T_RM cells maintenance in the lung has been the source of vigorous debate. Here we propose a model in which the majority of CD8⁺ T_RM cells are maintained within RAMD (conventional T_RM) whereas a small fraction of T_RM are derived from circulating CD8⁺ T_EM cells and maintained in the interstitium. The numbers of both types of T_RM cells wane over time due to declines in both RAMD availability and the overall number of T_EM in the circulation. This model is consistent with most published reports and has important implications for the development of vaccines designed to elicit protective T cell memory in the lung.

GITRL on inflammatory antigen presenting cells in the lung parenchyma provides signal 4 for T-cell accumulation and tissue-resident memory T-cell formation

T-cell responses in the lung are critical for protection against respiratory pathogens. TNFR superfamily members play important roles in providing survival signals to T cells during respiratory infections. However, whether these signals take place mainly during priming in the secondary lymphoid organs and/or in the peripheral tissues remains unknown. Here we show that under conditions of competition, GITR provides a T-cell intrinsic advantage to both CD4 and CD8 effector T cells in the lung tissue, as well as for the formation of CD4 and CD8 tissue-resident memory T cells during respiratory influenza infection in mice. In contrast, under non-competitive conditions, GITR has a preferential effect on CD8 over CD4 T cells. The nucleoprotein-specific CD8 T-cell response partially compensated for GITR deficiency by expansion of higher affinity T cells; whereas, the polymerase-specific response was less flexible and more GITR dependent. Following influenza infection, GITR is expressed on lung T cells and GITRL is preferentially expressed on lung monocyte-derived inflammatory antigen presenting cells. Accordingly, we show that GITR+/+ T cells in the lung parenchyma express more phosphorylated-ribosomal protein S6 than their GITR-/- counterparts. Thus, GITR signaling within the lung tissue critically regulates effector and tissue-resident memory T-cell accumulation.

Batf3-Dependent Dendritic Cells Promote Optimal CD8 T Cell Responses against Respiratory Poxvirus Infection

Respiratory infection with vaccinia virus (VacV) elicits robust CD8⁺ T cell responses that play an important role in host resistance. In the lung, VacV encounters multiple tissue-resident antigen-presenting cell (APC) populations, but which cell plays a dominant role in priming of virus-specific CD8⁺ effector T cell responses remains poorly defined. We used Batf3^-/- mice to investigate the impact of CD103⁺ and CD8α⁺ dendritic cell (DC) deficiency on anti-VacV CD8⁺ T cell responses. We found that Batf3^-/- mice were more susceptible to VacV infection, exhibiting profound weight loss, which correlated with impaired accumulation of gamma interferon (IFN-γ)-producing CD8⁺ T cells in the lungs. This was largely due to defective priming since early in the response, antigen-specific CD8⁺ T cells in the draining lymph nodes of Batf3^-/- mice expressed significantly reduced levels of Ki67, CD25, and T-bet. These results underscore a specific role for Batf3-dependent DCs in regulating priming and expansion of effector CD8⁺ T cells necessary for host resistance against acute respiratory VacV infection.IMPORTANCE During respiratory infection with vaccinia virus (VacV), a member of Poxviridae family, CD8⁺ T cells play important role in resolving the primary infection. Effector CD8⁺ T cells clear the virus by accumulating in the infected lungs in large numbers and secreting molecules such as IFN-γ that kill virally infected cells. However, precise cell types that regulate the generation of effector CD8⁺ T cells in the lungs are not well defined. Dendritic cells (DCs) are a heterogeneous population of immune cells that are recognized as key initiators and regulators of T-cell-mediated immunity. In this study, we reveal that a specific subset of DCs that are dependent on the transcription factor Batf3 for their development regulate the magnitude of CD8⁺ T cell effector responses in the lungs, thereby providing protection during pulmonary VacV infection.

Genealogy, Dendritic Cell Priming, and Differentiation of Tissue-Resident Memory CD8+ T Cells

Tissue-resident memory CD8⁺ T (Trm) cells define a distinct non-recirculating subset. Trm cells constitute a first line of defense against local infections in barrier tissues, but they are also found in non-barrier tissues and play a role in antitumor immunity. Their differentiation in tissues and their phenotypical, transcriptional, and functional characteristics are the object of active research. Herein, we will discuss the potential existence of committed CD8⁺ Trm precursors and the genealogy of memory CD8⁺ T cell subsets. In addition to the priming of naive T cells, there is some plasticity of antigen-experienced effector and memory T cell subsets to generate Trm precursors. Local inflammation, antigen presentation, and cytokines drive Trm differentiation. It is of prime interest how specific dendritic cell subsets modulate priming and differentiation of Trm cells, as well as their reactivation within tissues. Research on how we can manipulate generation of memory T cells subsets is key for improved vaccination strategies.