Inducible Bronchus-Associated Lymphoid Tissues (iBALT) Serve as Sites of B Cell Selection and Maturation Following Influenza Infection in Mice

Myeloperoxidase-ANCA IgG induces different forms of small vessel vasculitis based on type of synergistic immune stimuli

Anti-neutrophil cytoplasmic autoantibody (ANCA) vasculitis has diverse patterns of injury including microscopic polyangiitis (MPA), granulomatosis with polyangiitis (GPA), and eosinophilic granulomatosis with polyangiitis (EGPA). Necrotizing and crescentic glomerulonephritis (NCGN) occurs in all syndromes and as renal limited vasculitis (RLV). Single dose intravenous ANCA IgG-specific for mouse myeloperoxidase (MPO) causes RLV in mice. Although multiple mouse models have elucidated ANCA-IgG induced necrotizing and crescentic glomerulonephritis (NCGN), pathogenesis of ANCA-induced granulomatosis and vasculitis outside the kidney has not been clarified. To investigate this, we used intravenous MPO-ANCA IgG in the same strain of mice to induce different patterns of lung disease mirroring patients with granulomatosis with polyangiitis (GPA), microscopic polyangiitis (MPA), and eosinophilic granulomatosis with polyangiitis (EGPA). Repeated intravenous MPO-ANCA IgG induced GPA with NCGN, lung capillaritis, arteritis and granulomatosis. Lung leukocyte phenotypes were evaluated by immunohistochemical image analysis and by flow cytometry. ANCA lung capillaritis and microabscesses began within one day and evolved into granulomas in under seven days. Influenza plus single dose MPO-ANCA IgG induced MPA with NCGN, lung capillaritis and arteritis, but no granulomatosis. Allergic airway disease caused by house dust mites or ovalbumin plus single dose intravenous MPO-ANCA IgG induced EGPA with eosinophilic bronchiolitis, NCGN, capillaritis, arteritis, and granulomatosis. Thus, our study shows that the occurrence and pattern of lung lesions are determined by the same ANCA IgG accompanied by different synergistic immune factors.

Apelin modulates inflammation and leukocyte recruitment in experimental autoimmune encephalomyelitis

Demyelination due to autoreactive T cells and inflammation in the central nervous system are principal features of multiple sclerosis (MS), a chronic and highly disabling human disease affecting brain and spinal cord. Here, we show that treatment with apelin, a secreted peptide ligand for the G protein-coupled receptor APJ/Aplnr, is protective in experimental autoimmune encephalomyelitis (EAE), an animal model of MS. Apelin reduces immune cell entry into the brain, delays the onset and reduces the severity of EAE. Apelin affects the trafficking of leukocytes through the lung by modulating the expression of cell adhesion molecules that mediate leukocyte recruitment. In addition, apelin induces the internalization and desensitization of its receptor in endothelial cells (ECs). Accordingly, protection against EAE major outcomes of apelin treatment are phenocopied by loss of APJ/Aplnr function, achieved by EC-specific gene inactivation in mice or knockdown experiments in cultured primary endothelial cells. Our findings highlight the importance of the lung-brain axis in neuroinflammation and indicate that apelin targets the transendothelial migration of immune cells into the lung during acute inflammation.

Regulation of pulmonary plasma cell responses during secondary infection with influenza virus

During secondary infection with influenza virus, plasma cells (PCs) develop within the lung, providing a local source of antibodies. However, the site and mechanisms that regulate this process are poorly defined. Here, we show that while circulating memory B cells entered the lung during rechallenge and were activated within inducible bronchus-associated lymphoid tissues (iBALTs), resident memory B (BRM) cells responded earlier, and their activation occurred in a different niche: directly near infected alveoli. This process required NK cells but was largely independent of CD4 and CD8 T cells. Innate stimuli induced by virus-like particles containing ssRNA triggered BRM cell differentiation in the absence of cognate antigen, suggesting a low threshold of activation. In contrast, expansion of PCs in iBALTs took longer to develop and was critically dependent on CD4 T cells. Our work demonstrates that spatially distinct mechanisms evolved to support pulmonary secondary PC responses, and it reveals a specialized function for BRM cells as guardians of the alveoli.

Colony morphotype governs innate and adaptive pulmonary immune responses to Mycobacterium abscessus infection in C3HeB/FeJ mice

Mycobacterium abscessus is an emerging pathogen that causes chronic pulmonary infection. Treatment is challenging owing in part to our incomplete understanding of M. abscessus virulence mechanisms that enable pathogen persistence, such as the differing pathogenicity of M. abscessus smooth (S) and rough (R) colony morphotype. While R M. abscessus is associated with chronic infection and worse patient outcomes, it is unknown how immune responses to S and R M. abscessus differ in an acute pulmonary infection setting. In this study, immunological outcomes of M. abscessus infection with S and R morphotypes were examined in an immune-competent C3HeB/FeJ murine model. R M. abscessus infection was associated with the rapid production of inflammatory chemokines and recruitment of activated, MHC-II+ Ly6C+ macrophages to lungs and mediastinal LN (mLN). While both S and R M. abscessus increased T helper 1 (Th1) phenotype T cells in the lung, this was markedly delayed in mice infected with S M. abscessus. However, histopathological involvement and bacterial clearance were similar regardless of colony morphotype. These results demonstrate the importance of M. abscessus colony morphotype in shaping the development of pulmonary immune responses to M. abscessus, which further informs our understanding of M. abscessus host-pathogen interactions.

Respiratory viral infection promotes the awakening and outgrowth of dormant metastatic breast cancer cells in lungs

Breast cancer is the second most common cancer globally. Most deaths from breast cancer are due to metastatic disease which often follows long periods of clinical dormancy1. Understanding the mechanisms that disrupt the quiescence of dormant disseminated cancer cells (DCC) is crucial for addressing metastatic progression. Infection with respiratory viruses (e.g. influenza or SARS-CoV-2) is common and triggers an inflammatory response locally and systemically2,3. Here we show that influenza virus infection leads to loss of the pro-dormancy mesenchymal phenotype in breast DCC in the lung, causing DCC proliferation within days of infection, and a greater than 100-fold expansion of carcinoma cells into metastatic lesions within two weeks. Such DCC phenotypic change and expansion is interleukin-6 (IL-6)-dependent. We further show that CD4 T cells are required for the maintenance of pulmonary metastatic burden post-influenza virus infection, in part through attenuation of CD8 cell responses in the lungs. Single-cell RNA-seq analyses reveal DCC-dependent impairment of T-cell activation in the lungs of infected mice. SARS-CoV-2 infected mice also showed increased breast DCC expansion in lungs post-infection. Expanding our findings to human observational data, we observed that cancer survivors contracting a SARS-CoV-2 infection have substantially increased risks of lung metastatic progression and cancer-related death compared to cancer survivors who did not. These discoveries underscore the significant impact of respiratory viral infections on the resurgence of metastatic cancer, offering novel insights into the interconnection between infectious diseases and cancer metastasis.

B cell memory: from generation to reactivation: a multipronged defense wall against pathogens

Development of B cell memory is a conundrum that scientists are still exploring. Studies have been conducted in vitro and using advanced animal models to elucidate the mechanism underlying the generation of memory B cells (MBCs), the precise roles of MBCs against pathogens, and their protective functions against repeated infections throughout life. Lifelong immunity against invading diseases is mainly the result of overcoming a single infection. This protection is largely mediated by the two main components of B cell memory-MBCs and long-lived plasma cells (PCs). The chemical and cellular mechanisms that encourage fat selection for MBCs or long-lived PCs are an area of active research. Despite the fact that nearly all available vaccinations rely on the capacity to elicit B-cell memory, we have yet to develop successful vaccines that can induce broad-scale protective MBCs against some of the deadliest diseases, including malaria and AIDS. A deeper understanding of the specific cellular and molecular pathways that govern the generation, function, and reactivation of MBCs is critical for overcoming the challenges associated with vaccine development. Here, we reviewed literature on the development of MBCs and their reactivation, interaction with other cell types, strategies against invading pathogens, and function throughout life and discussed the recent advances regarding the key signals and transcription factors which regulate B cell memory and their relevance to the quest for vaccine development.

Mucosal Immunity against SARS-CoV-2 in the Respiratory Tract

The respiratory tract, the first-line defense, is constantly exposed to inhaled allergens, pollutants, and pathogens such as respiratory viruses. Emerging evidence has demonstrated that the coordination of innate and adaptive immune responses in the respiratory tract plays a crucial role in the protection against invading respiratory pathogens. Therefore, a better understanding of mucosal immunity in the airways is critical for the development of novel therapeutics and next-generation vaccines against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and other respiratory viruses. Since the coronavirus disease 2019 pandemic, our knowledge of mucosal immune responses in the airways has expanded. In this review, we describe the latest knowledge regarding the key components of the mucosal immune system in the respiratory tract. In addition, we summarize the host immune responses in the upper and lower airways following SARS-CoV-2 infection and vaccination, and discuss the impact of allergic airway inflammation on mucosal immune responses against SARS-CoV-2.

Memory B cells

Recent advances in studies of immune memory in mice and humans have reinforced the concept that memory B cells play a critical role in protection against repeated infections, particularly from variant viruses. Hence, insights into the development of high-quality memory B cells that can generate broadly neutralizing antibodies that bind such variants are key for successful vaccine development. Here, we review the cellular and molecular mechanisms by which memory B cells are generated and how these processes shape the antibody diversity and breadth of memory B cells. Then, we discuss the mechanisms of memory B cell reactivation in the context of established immune memory; the contribution of antibody feedback to this process has now begun to be reappreciated.

The effect of Toll-like receptor agonists on the immunogenicity of MVA-SARS-2-S vaccine after intranasal administration in mice

Background and aims

Modified Vaccinia virus Ankara (MVA) represents a promising vaccine vector for respiratory administration to induce protective lung immunity including tertiary lymphoid structure, the bronchus-associated lymphoid tissue (BALT). However, MVA expressing the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) Spike protein (MVA-SARS-2-S) required prime-boost administration to induce high titers of anti-Spike antibodies in serum and bronchoalveolar lavage (BAL). As the addition of adjuvants enables efficient tailoring of the immune responses even to live vaccines, we tested whether Toll-like receptor (TLR)-agonists affect immune responses induced by a single dose of intranasally applied MVA-SARS-2-S.

Methods

We intranasally immunized C57BL/6 mice with MVA-SARS-2-S vaccine in the presence of either TLR3 agonist polyinosinic polycytidylic acid [poly(I:C)], TLR4 agonist bacterial lipopolysaccharide (LPS) from Escherichia coli, or TLR9 agonist CpG oligodeoxynucleotide (CpG ODN) 1826. At different time-points after immunization, we analyzed induced immune responses using flow cytometry, immunofluorescent microscopy, and ELISA.

Results

TLR agonists had profound effects on MVA-SARS-2-S-induced immune responses. At day 1 post intranasal application, the TLR4 agonist significantly affected MVA-induced activation of dendritic cells (DCs) within the draining bronchial lymph nodes, increasing the ratio of CD11b+CD86+ to CD103+CD86+ DCs. Nevertheless, the number of Spike-specific CD8+ T cells within the lungs at day 12 after vaccination was increased in mice that received MVA-SARS-2-S co-administered with TLR3 but not TLR4 agonists. TLR9 agonist did neither significantly affect MVA-induced DC activation nor the induction of Spike-specific CD8+ T cells but reduced both number and size of bronchus-associated lymphoid tissue. Surprisingly, the addition of all TLR agonists failed to boost the levels of Spike-specific antibodies in serum and bronchoalveolar lavage.

Conclusions

Our study indicates a potential role of TLR-agonists as a tool to modulate immune responses to live vector vaccines. Particularly TLR3 agonists hold a promise to potentiate MVA-induced cellular immune responses. On the other hand, additional research is necessary to identify optimal combinations of agonists that could enhance MVA-induced humoral responses.

Generation of antigen-specific memory CD4 T cells by heterologous immunization enhances the magnitude of the germinal center response upon influenza infection

Current influenza vaccine strategies have yet to overcome significant obstacles, including rapid antigenic drift of seasonal influenza viruses, in generating efficacious long-term humoral immunity. Due to the necessity of germinal center formation in generating long-lived high affinity antibodies, the germinal center has increasingly become a target for the development of novel or improvement of less-efficacious vaccines. However, there remains a major gap in current influenza research to effectively target T follicular helper cells during vaccination to alter the germinal center reaction. In this study, we used a heterologous infection or immunization priming strategy to seed an antigen-specific memory CD4+ T cell pool prior to influenza infection in mice to evaluate the effect of recalled memory T follicular helper cells in increased help to influenza-specific primary B cells and enhanced generation of neutralizing antibodies. We found that heterologous priming with intranasal infection with acute lymphocytic choriomeningitis virus (LCMV) or intramuscular immunization with adjuvanted recombinant LCMV glycoprotein induced increased antigen-specific effector CD4+ T and B cellular responses following infection with a recombinant influenza strain that expresses LCMV glycoprotein. Heterologously primed mice had increased expansion of secondary Th1 and Tfh cell subsets, including increased CD4+ TRM cells in the lung. However, the early enhancement of the germinal center cellular response following influenza infection did not impact influenza-specific antibody generation or B cell repertoires compared to primary influenza infection. Overall, our study suggests that while heterologous infection/immunization priming of CD4+ T cells is able to enhance the early germinal center reaction, further studies to understand how to target the germinal center and CD4+ T cells specifically to increase long-lived antiviral humoral immunity are needed.

Bridging the gaps to overcome major hurdles in the development of next-generation tuberculosis vaccines

Although tuberculosis (TB) remains one of the leading causes of death from an infectious disease worldwide, the development of vaccines more effective than bacille Calmette-Guérin (BCG), the only licensed TB vaccine, has progressed slowly even in the context of the tremendous global impact of TB. Most vaccine candidates have been developed to strongly induce interferon-γ (IFN-γ)-producing T-helper type 1 (Th1) cell responses; however, accumulating evidence has suggested that other immune factors are required for optimal protection against Mycobacterium tuberculosis (Mtb) infection. In this review, we briefly describe the five hurdles that must be overcome to develop more effective TB vaccines, including those with various purposes and tested in recent promising clinical trials. In addition, we discuss the current knowledge gaps between preclinical experiments and clinical studies regarding peripheral versus tissue-specific immune responses, different underlying conditions of individuals, and newly emerging immune correlates of protection. Moreover, we propose how recently discovered TB risk or susceptibility factors can be better utilized as novel biomarkers for the evaluation of vaccine-induced protection to suggest more practical ways to develop advanced TB vaccines. Vaccines are the most effective tools for reducing mortality and morbidity from infectious diseases, and more advanced technologies and a greater understanding of host-pathogen interactions will provide feasibility and rationale for novel vaccine design and development.

CD4+ T cell memory

Specialized subpopulations of CD4+ T cells survey major histocompatibility complex class II-peptide complexes to control phagosomal infections, help B cells, regulate tissue homeostasis and repair or perform immune regulation. Memory CD4+ T cells are positioned throughout the body and not only protect the tissues from reinfection and cancer, but also participate in allergy, autoimmunity, graft rejection and chronic inflammation. Here we provide updates on our understanding of the longevity, functional heterogeneity, differentiation, plasticity, migration and human immunodeficiency virus reservoirs as well as key technological advances that are facilitating the characterization of memory CD4+ T cell biology.

Memory B-cell diversity: From early generation to tissue residency and reactivation

Memory B cells (MBCs) have a crucial function in providing an enhanced response to repeated infections. Upon antigen encounter, MBC can either rapidly differentiate to antibody secreting cells or enter germinal centers (GC) to further diversify and affinity mature. Understanding how and when MBC are formed, where they reside and how they select their fate upon reactivation has profound implications for designing strategies to improve targeted, next-generation vaccines. Recent studies have crystallized much of our knowledge on MBC but also reported several surprising discoveries and gaps in our current understanding. Here, we review the latest advancements in the field and highlight current unknowns. In particular, we focus on timing and cues leading to MBC generation before and during the GC reaction, discuss how MBC become resident in mucosal tissues, and finally, provide an overview of factors shaping MBC fate-decision upon reactivation in mucosal and lymphoid tissues.

T-cell-B-cell collaboration in the lung

Collaboration between T and B cells in secondary lymphoid organs is a crucial component of adaptive immunity, but lymphocytes also persist in other tissues. Recent studies have examined T-cell-B-cell interactions in nonlymphoid tissues such as the lung. CD4+ T- resident helper cells (TRH) remain in the lung after influenza infection and support both resident CD8 T cells and B cells. Multiple lung-resident B-cell subsets (B-resident memory (BRM)) that exhibit spatial and phenotypic diversity have also been described. Though not generated by all types of infection, inducible bronchus-associated lymphoid tissue offers a logical place for T and B cells to interact. Perturbations to BRM and TRH cells elicit effects specific to Immunoglobulin A (IgA) production, an antibody isotype with privileged access to mucosa. Understanding the interplay of lymphocytes in mucosal tissues, which can be insulated from systemic immune responses, may improve the design of future vaccines and therapies.

Toward a general model of CD4+ T cell subset specification and memory cell formation

In the past few decades, a number of transformative discoveries have been made regarding memory CD8+ T cell biology; meanwhile, the CD4+ T cell field has lagged behind this progress. This perspective focuses on CD4+ helper T (Th) cell subset specification and memory cell formation. Here, we argue that the sheer number of Th effector and memory cell subsets and a focus on their differences have been a barrier to a general model of CD4+ memory T cell formation that applies to all immune responses. We highlight a bifurcation model that relies on an IL-2 signal-dependent switch as an explanation for the balanced production of diverse Th memory cells that participate in cell-mediated or humoral immunity in most contexts.

Human Airway Epithelium Responses to Invasive Fungal Infections: A Critical Partner in Innate Immunity

The lung epithelial lining serves as the primary barrier to inhaled environmental toxins, allergens, and invading pathogens. Pulmonary fungal infections are devastating and carry high mortality rates, particularly in those with compromised immune systems. While opportunistic fungi infect primarily immunocompromised individuals, endemic fungi cause disease in immune competent and compromised individuals. Unfortunately, in the case of inhaled fungal pathogens, the airway epithelial host response is vastly understudied. Furthering our lack of understanding, very few studies utilize primary human models displaying pseudostratified layers of various epithelial cell types at air-liquid interface. In this review, we focus on the diversity of the human airway epithelium and discuss the advantages and disadvantages of oncological cell lines, immortalized epithelial cells, and primary epithelial cell models. Additionally, the responses by human respiratory epithelial cells to invading fungal pathogens will be explored. Future investigations leveraging current human in vitro model systems will enable identification of the critical pathways that will inform the development of novel vaccines and therapeutics for pulmonary fungal infections.

Mucosal immunization with a delta-inulin adjuvanted recombinant spike vaccine elicits lung-resident immune memory and protects mice against SARS-CoV-2

Multiple SARS-CoV-2 vaccine candidates have been approved for use and have had a major impact on the COVID-19 pandemic. There remains, however, a significant need for vaccines that are safe, easily transportable and protective against infection, as well as disease. Mucosal vaccination is favored for its ability to induce immune memory at the site of infection, making it appealing for SARS-CoV-2 vaccine strategies. In this study we performed in-depth analysis of the immune responses in mice to a subunit recombinant spike protein vaccine formulated with the delta-inulin adjuvant Advax when administered intratracheally (IT), versus intramuscular delivery (IM). Both routes produced robust neutralizing antibody titers (NAb) and generated sterilizing immunity against SARS-CoV-2. IT delivery, however, produced significantly higher systemic and lung-local NAb that resisted waning up to six months post vaccination, and only IT delivery generated inducible bronchus-associated lymphoid tissue (iBALT), a site of lymphocyte antigen presentation and proliferation. This was coupled with robust and long-lasting lung tissue-resident memory CD4⁺ and CD8⁺ T cells that were not observed in IM-vaccinated mice. This study provides a detailed view of the lung-resident cellular response to IT vaccination against SARS-CoV-2 and demonstrates the importance of delivery site selection in the development of vaccine candidates.

Development and function of tissue-resident memory B cells

Barrier tissues are the primary site of infection for pathogens likely to cause future pandemics. Tissue-resident lymphocytes can rapidly detect pathogens upon infection of barrier tissues and are critical in preventing viral spread. However, most vaccines fail to induce tissue-resident lymphocytes and are instead reliant on circulating antibodies to mediate protective immunity. Circulating antibody titers wane over time following vaccination leaving individuals susceptible to breakthrough infections by variant viral strains that evade antibody neutralization. Memory B cells were recently found to establish tissue residence following infection of barrier tissues. Here, we summarize emerging evidence for the importance of tissue-resident memory B cells in the establishment of protective immunity against viral and bacterial challenge. We also discuss the role of tissue-resident memory B cells in regulating the progression of non-infectious diseases. Finally, we examine new approaches to develop vaccines capable of eliciting barrier immunity.

The Differentiation and Maintenance of SARS-CoV-2-Specific Follicular Helper T Cells

Upon acute viral infection, virus-specific CD4+ T cells differentiate into either TH1 cells or follicular helper T (TFH) cells. The molecular pathways governing such bimodal cell fate commitment remain elusive. Additionally, effector virus-specific TFH cells further differentiate into corresponding memory population, which confer long-term protection against re-infection of same viruses by providing immediate help to virus-specific memory B cells. Currently, the molecular mechanisms underlying the long-term maintenance of memory TFH cells are largely unknown. In this review, we discuss current understanding of early differentiation of virus-specific effector TFH cells and long-term maintenance of virus-specific memory TFH cells in mouse models of viral infection and patients of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection.

Mucosal immune responses to infection and vaccination in the respiratory tract

The lungs are constantly exposed to inhaled debris, allergens, pollutants, commensal or pathogenic microorganisms, and respiratory viruses. As a result, innate and adaptive immune responses in the respiratory tract are tightly regulated and are in continual flux between states of enhanced pathogen clearance, immune-modulation, and tissue repair. New single-cell-sequencing techniques are expanding our knowledge of airway cellular complexity and the nuanced connections between structural and immune cell compartments. Understanding these varied interactions is critical in treatment of human pulmonary disease and infections and in next-generation vaccine design. Here, we review the innate and adaptive immune responses in the lung and airways following infection and vaccination, with particular focus on influenza virus and severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). The ongoing SARS-CoV-2 pandemic has put pulmonary research firmly into the global spotlight, challenging previously held notions of respiratory immunity and helping identify new populations at high risk for respiratory distress.

Murine Respiratory Tract Infection with Classical Klebsiella pneumoniae Induces Bronchus-Associated Lymphoid Tissue

Klebsiella pneumoniae is a Gram-negative, opportunistic pathogen that commonly causes nosocomial pneumonia, urinary tract infection, and septicemia. Our recent work utilizing a murine model of respiratory tract infection with classical K. pneumoniae demonstrated leukocyte aggregates in the lungs of mice at 28 days postinfection. Here, we sought to characterize the composition and development of these structures. Histopathological analyses of murine lungs revealed immune cell clusters surrounding the pulmonary vasculature and airways by 14 days postinfection, resembling inducible bronchus-associated lymphoid tissue (iBALT). Further investigation of these structures demonstrated central B cell aggregates with concomitant dispersed T cells. At day 28 postinfection, these lymphoid clusters expressed germinal center markers and CXCL12, qualifying these structures as iBALT with nonclassical B cell follicles. Investigations in mutant mice revealed that those lacking B and/or T cells were not able to form fully defined iBALT structures, although some rudimentary B cell clusters were identified in mice lacking T cells. The longevity of K. pneumoniae-induced BALT was assessed for up to 120 days postinfection. Lymphoid aggregates significantly decreased in size and quantity by 90 days after K. pneumoniae infection; however, aggregates persisted in mice that were restimulated with K. pneumoniae every 30 days. Finally, infections of mice with an array of classical K. pneumoniae clinical isolates demonstrated that the development of these structures is a common feature of K. pneumoniae lung infection. Together, these data confirm that murine lungs infected with K. pneumoniae develop iBALT, which may play a role in pulmonary immunity to this troublesome pathogen.

Lung-resident memory B cells established after pulmonary influenza infection display distinct transcriptional and phenotypic profiles

Recent studies have established that memory B cells, largely thought to be circulatory in the blood, can take up long-term residency in inflamed tissues, analogous to widely described tissue-resident T cells. The dynamics of recruitment and retention of memory B cells to tissues and their immunological purpose remains unclear. Here, we characterized tissue-resident memory B cells (B_RM) that are stably maintained in the lungs of mice after pulmonary influenza infection. Influenza-specific B_RM were localized within inducible bronchus-associated lymphoid tissues (iBALTs) and displayed transcriptional signatures distinct from classical memory B cells in the blood or spleen while showing partial overlap with memory B cells in lung-draining lymph nodes. We identified lung-resident markers, including elevated expression of CXCR3, CCR6, and CD69, on hemagglutinin (HA)- and nucleoprotein (NP)-specific lung B_RM. We found that CCR6 facilitates increased recruitment and/or retention of B_RM in lungs and differentiation into antibody-secreting cells upon recall. Although expression of CXCR3 and CCR6 was comparable in total and influenza-specific memory B cells isolated across tissues of human donors, CD69 expression was higher in memory B cells from lung and draining lymph nodes of human organ donors relative to splenic and PBMC-derived populations, indicating that mechanisms underpinning B_RM localization may be evolutionarily conserved. Last, we demonstrate that human memory B cells in lungs are transcriptionally distinct to populations in lung-draining lymph nodes or PBMCs. These data suggest that B_RM may constitute a discrete component of B cell immunity, positioned at the lung mucosa for rapid humoral response against respiratory viral infections.

The germinal centre B cell response to SARS-CoV-2

The germinal centre (GC) response is critical for the generation of affinity-matured plasma cells and memory B cells capable of mediating long-term protective immunity. Understanding whether severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection or vaccination elicits a GC response has profound implications for the capacity of responding B cells to contribute to protection against infection. However, direct assessment of the GC response in humans remains a major challenge. Here we summarize emerging evidence for the importance of the GC response in the establishment of durable and broad immunity against SARS-CoV-2 and discuss new approaches to modulate the GC response to better protect against newly emerging SARS-CoV-2 variants. We also discuss new findings showing that the GC B cell response persists in the draining lymph nodes for at least 6 months in some individuals following vaccination with SARS-CoV-2 mRNA-based vaccines.

Intranasal Delivery of MVA Vector Vaccine Induces Effective Pulmonary Immunity Against SARS-CoV-2 in Rodents

Antigen-specific tissue-resident memory T cells (Trms) and neutralizing IgA antibodies provide the most effective protection of the lungs from viral infections. To induce those essential components of lung immunity against SARS-CoV-2, we tested various immunization protocols involving intranasal delivery of a novel Modified Vaccinia virus Ankara (MVA)-SARS-2-spike vaccine candidate. We show that a single intranasal MVA-SARS-CoV-2-S application in mice strongly induced pulmonary spike-specific CD8+ T cells, albeit restricted production of neutralizing antibodies. In prime-boost protocols, intranasal booster vaccine delivery proved to be crucial for a massive expansion of systemic and lung tissue-resident spike-specific CD8+ T cells and the development of Th1 - but not Th2 - CD4+ T cells. Likewise, very high titers of IgG and IgA anti-spike antibodies were present in serum and broncho-alveolar lavages that possessed high virus neutralization capacities to all current SARS-CoV-2 variants of concern. Importantly, the MVA-SARS-2-spike vaccine applied in intramuscular priming and intranasal boosting treatment regimen completely protected hamsters from developing SARS-CoV-2 lung infection and pathology. Together, these results identify intramuscular priming followed by respiratory tract boosting with MVA-SARS-2-S as a promising approach for the induction of local, respiratory as well as systemic immune responses suited to protect from SARS-CoV-2 infections.

Poor protective potential of influenza nucleoprotein antibodies despite wide prevalence

Humans are exposed to influenza virus through periodic infections. Due to these repeated exposures, human populations commonly have elevated antibody titers targeting the conserved internal influenza virus nucleoprotein (NP). Despite the presence of anti-NP antibodies, humans are acutely susceptible to drifted influenza viruses with antigenically different surface proteins and the protective potential of human NP antibodies is unclear. In this study, high levels of anti-NP antibody and NP-specific B cells were detected in both adult humans and influenza-infected mice, confirming that NP is a major target of humoral immunity. Through sorting single B cells from influenza-exposed human adults, we generated a panel of 11 anti-NP monoclonal antibodies (mAbs). The majority of anti-NP human mAbs generated were capable of engaging cellular Fc receptors and bound NP on the surface of influenza-infected cell lines in vitro, suggesting that anti-NP mAbs have the potential to mediate downstream Fc effector functions such as antibody-dependent cellular cytotoxicity and antibody-dependent phagocytosis. However, human anti-NP mAbs were not protective in vivo when passively transferred into a murine influenza challenge model. Future in vivo studies examining the synergistic effect of anti-NP mAbs infused with other influenza-specific mAbs are warranted.

Pulmonary-Resident Memory Lymphocytes: Pivotal Orchestrators of Local Immunity Against Respiratory Infections

There is increasing evidence that lung-resident memory T and B cells play a critical role in protecting against respiratory reinfection. With a unique transcriptional and phenotypic profile, resident memory lymphocytes are maintained in a quiescent state, constantly surveying the lung for microbial intruders. Upon reactivation with cognate antigen, these cells provide rapid effector function to enhance immunity and prevent infection. Immunization strategies designed to induce their formation, alongside novel techniques enabling their detection, have the potential to accelerate and transform vaccine development. Despite most data originating from murine studies, this review will discuss recent insights into the generation, maintenance and characterisation of pulmonary resident memory lymphocytes in the context of respiratory infection and vaccination using recent findings from human and non-human primate studies.

Know your enemy or find your friend?-Induction of IgA at mucosal surfaces

Most antibodies produced in the body are of the IgA class. The dominant cell population producing them are plasma cells within the lamina propria of the gastrointestinal tract, but many IgA-producing cells are also found in the airways, within mammary tissues, the urogenital tract and inside the bone marrow. Most IgA antibodies are transported into the lumen by epithelial cells as part of the mucosal secretions, but they are also present in serum and other body fluids. A large part of the commensal microbiota in the gut is covered with IgA antibodies, and it has been demonstrated that this plays a role in maintaining a healthy balance between the host and the bacteria. However, IgA antibodies also play important roles in neutralizing pathogens in the gastrointestinal tract and the upper airways. The distinction between the two roles of IgA - protective and balance-maintaining - not only has implications on function but also on how the production is regulated. Here, we discuss these issues with a special focus on gut and airways.

Single-cell BCR and transcriptome analysis after influenza infection reveals spatiotemporal dynamics of antigen-specific B cells

B cell responses are critical for antiviral immunity. However, a comprehensive picture of antigen-specific B cell differentiation, clonal proliferation, and dynamics in different organs after infection is lacking. Here, by combining single-cell RNA and B cell receptor (BCR) sequencing of antigen-specific cells in lymph nodes, spleen, and lungs after influenza infection in mice, we identify several germinal center (GC) B cell subpopulations and organ-specific differences that persist over the course of the response. We discover transcriptional differences between memory cells in lungs and lymphoid organs and organ-restricted clonal expansion. Remarkably, we find significant clonal overlap between GC-derived memory and plasma cells. By combining BCR-mutational analyses with monoclonal antibody (mAb) expression and affinity measurements, we find that memory B cells are highly diverse and can be selected from both low- and high-affinity precursors. By linking antigen recognition with transcriptional programming, clonal proliferation, and differentiation, these finding provide important advances in our understanding of antiviral immunity.

Lung-resident memory B cells protect against bacterial pneumonia

Lung-resident memory B cells (BRM cells) are elicited after influenza infections of mice, but connections to other pathogens and hosts - as well as their functional significance - have yet to be determined. We postulate that BRM cells are core components of lung immunity. To test this, we examined whether lung BRM cells are elicited by the respiratory pathogen pneumococcus, are present in humans, and are important in pneumonia defense. Lungs of mice that had recovered from pneumococcal infections did not contain organized tertiary lymphoid organs, but did have plasma cells and noncirculating memory B cells. The latter expressed distinctive surface markers (including CD69, PD-L2, CD80, and CD73) and were poised to secrete antibodies upon stimulation. Human lungs also contained B cells with a resident memory phenotype. In mice recovered from pneumococcal pneumonia, depletion of PD-L2+ B cells, including lung BRM cells, diminished bacterial clearance and the level of pneumococcus-reactive antibodies in the lung. These data define lung BRM cells as a common feature of pathogen-experienced lungs and provide direct evidence of a role for these cells in pulmonary antibacterial immunity.

Effective control of Staphylococcus aureus lung infection despite tertiary lymphoid structure disorganisation

BACKGROUND

Tertiary lymphoid structures (TLS) are triggered by persistent bronchopulmonary infection with Staphylococcus aureus, but their roles remain elusive. The present study sought to examine the effects of B- and/or T-cell depletion on S. aureus infection and TLS development (lymphoid neogenesis) in mice.

METHODS

C57Bl/6 mice were pre-treated with 1) an anti-CD20 monoclonal antibody (mAb) (B-cell depletion) or 2) an anti-CD4 and/or an anti-CD8 mAb (T-cell depletion) or 3) a combination of anti-CD20, anti-CD4 and anti-CD8 mAbs (combined B- and T-cell depletion) or 4) isotype control mAbs. After lymphocyte depletion, mice were infected by intratracheal instillation of agarose beads containing S. aureus (10⁶ CFU per mouse). 14 days later, bacterial load and lung inflammatory cell infiltration were assessed by cultures and immunohistochemistry, respectively.

RESULTS

14 days after S. aureus-bead instillation, lung bacterial load was comparable between control and lymphocyte-depleted mice. While TLS were observed in the lungs of infected mice pre-treated with control mAbs, these structures were disorganised or abolished in the lungs of lymphocyte-depleted mice. The absence of CD20⁺ B-lymphocytes had no effect on CD3⁺ T-lymphocyte infiltration, whereas CD4⁺/CD8⁺ T-cell depletion markedly reduced CD20⁺ B-cell infiltration. Depletion of CD4⁺ or CD8⁺ T-cells separately had limited effect on B-cell infiltration, but led to the absence of germinal centres.

CONCLUSION

TLS disorganisation is not associated with loss of infection control in mice persistently infected with S. aureus.

T resident helper cells promote humoral responses in the lung

Influenza is a deadly and costly infectious disease, even during flu seasons when an effective vaccine has been developed. To improve vaccines against respiratory viruses, a better understanding of the immune response at the site of infection is crucial. After influenza infection, clonally expanded T cells take up permanent residence in the lung, poised to rapidly respond to subsequent infection. Here, we characterized the dynamics and transcriptional regulation of lung-resident CD4⁺ T cells during influenza infection and identified a long-lived, Bcl6-dependent population that we have termed T resident helper (T_RH) cells. T_RH cells arise in the lung independently of lymph node T follicular helper cells but are dependent on B cells, with which they tightly colocalize in inducible bronchus-associated lymphoid tissue (iBALT). Deletion of Bcl6 in CD4⁺ T cells before heterotypic challenge infection resulted in redistribution of CD4⁺ T cells outside of iBALT areas and impaired local antibody production. These results highlight iBALT as a homeostatic niche for T_RH cells and advocate for vaccination strategies that induce T_RH cells in the lung.

Tissue-resident CD4+ T helper cells assist the development of protective respiratory B and CD8+ T cell memory responses

Much remains unknown about the roles of CD4⁺ T helper cells in shaping localized memory B cell and CD8⁺ T cell immunity in the mucosal tissues. Here, we report that lung T helper cells provide local assistance for the optimal development of tissue-resident memory B and CD8⁺ T cells after the resolution of primary influenza virus infection. We have identified a population of T cells in the lung that exhibit characteristics of both follicular T helper and T_RM cells, and we have termed these cells as resident helper T (T_RH) cells. Optimal T_RH cell formation was dependent on transcription factors involved in T follicular helper and resident memory T cell development including BCL6 and Bhlhe40. We show that T_RH cells deliver local help to CD8⁺ T cells through IL-21-dependent mechanisms. Our data have uncovered the presence of a tissue-resident helper T cell population in the lung that plays a critical role in promoting the development of protective B cell and CD8⁺ T cell responses.

Senescence in Pulmonary Fibrosis: Between Aging and Exposure

To date, chronic pulmonary pathologies represent the third leading cause of death in the elderly population. Evidence-based projections suggest that >65 (years old) individuals will account for approximately a quarter of the world population before the turn of the century. Genomic instability, telomere attrition, epigenetic alterations, loss of proteostasis, deregulated nutrient sensing, mitochondrial dysfunction, cellular senescence, stem cell exhaustion, and altered intercellular communication, are described as the nine “hallmarks” that govern cellular fitness. Any deviation from the normal pattern initiates a complex cascade of events culminating to a disease state. This blueprint, originally employed to describe aberrant changes in cancer cells, can be also used to describe aging and fibrosis. Pulmonary fibrosis (PF) is the result of a progressive decline in injury resolution processes stemming from endogenous (physiological decline or somatic mutations) or exogenous stress. Environmental, dietary or occupational exposure accelerates the pathogenesis of a senescent phenotype based on (1) window of exposure; (2) dose, duration, recurrence; and (3) cells type being targeted. As the lung ages, the threshold to generate an irreversibly senescent phenotype is lowered. However, we do not have sufficient knowledge to make accurate predictions. In this review, we provide an assessment of the literature that interrogates lung epithelial, mesenchymal, and immune senescence at the intersection of aging, environmental exposure and pulmonary fibrosis.

Effect of Vitamin A Deficiency in Dysregulating Immune Responses to Influenza Virus and Increasing Mortality Rates After Bacterial Coinfections

BACKGROUND

Secondary bacterial coinfections are ranked as a leading cause of hospitalization and morbid conditions associated with influenza. Because vitamin A deficiency (VAD) and insufficiency are frequent in both developed and developing countries, we asked how VAD influences coinfection severity.

METHODS

VAD and control mice were infected with influenza virus for evaluation of inflammatory cytokines, cellular immune responses, and viral clearance. Influenza-infected mice were coinfected with Streptococcus pneumoniae to study weight loss and survival.

RESULTS

Naive VAD mouse lungs exhibited dysregulated immune function. Neutrophils were enhanced in frequency and there was a significant reduction in RANTES (regulated on activation of normal T cells expressed and secreted), a chemokine instrumental in T-cell homing and recruitment. After influenza virus infection, VAD mice experienced failures in CD4+ T-cell recruitment and B-cell organization into lymphoid structures in the lung. VAD mice exhibited higher viral titers than controls and slow viral clearance. There were elevated levels of inflammatory cytokines and innate cell subsets in the lungs. However, arginase, a marker of alternatively activated M2 macrophages, was rare. When influenza-infected VAD animals were exposed to bacteria, they experienced a 100% mortality rate.

CONCLUSION

Data showed that VAD dysregulated the immune response. Consequently, secondary bacterial infections were 100% lethal in influenza-infected VAD mice.

Stromal cell control of conventional and ectopic germinal centre reactions

The germinal centre (GC) is a specialized cellular structure that forms in response to antigenic stimulation. It generates long-term humoral immunity through the production of memory B cells and long-lived antibody-secreting plasma cells. Conventional GCs form within secondary lymphoid organs, where networks of specialised stromal cells that form during embryogenesis act as the stage upon which the various GC immune cell players are brought together, nurtured and co-ordinated to generate a productive response. In non-lymphoid organs, ectopic GCs can form in response to persistent antigenic and inflammatory stimuli. Unlike secondary lymphoid tissues, non-lymphoid organs do not have a developmentally programmed stromal cell network capable of supporting the germinal centre reaction; therefore, the local tissue stroma must be remodelled by inflammatory stimuli in order to host a GC reaction. These ectopic GCs produce memory B cells and plasma cells that form a critical component of the humoral immune response.

A murine model demonstrates capsule-independent adaptive immune protection in survivors of Klebsiella pneumoniae respiratory tract infection

Klebsiella pneumoniae represents a growing clinical threat, given its rapid development of antibiotic resistance, necessitating new therapeutic strategies. Existing live-infection models feature high mortality rates, limiting their utility in the study of natural adaptive immune response to this pathogen. We developed a preclinical model of pneumonia with low overall mortality, in which previously exposed mice are protected from subsequent respiratory tract challenge with K. pneumoniae Histologic analyses of infected murine lungs demonstrate lymphocytic aggregates surrounding vasculature and larger airways. Initial exposure in RAG1 knockout mice (lacking functional B and T cells) failed to confer protection against subsequent K. pneumoniae challenge. While administration of isolated K. pneumoniae capsule was sufficient to provide protection, we also found that initial inoculation with K. pneumoniae mutants lacking capsule (Δcps), O-antigen (ΔwecA) or both conferred protection from subsequent wild-type infection and elicited K. pneumoniae-specific antibody responses, indicating that non-capsular antigens may also elicit protective immunity. Experiments in this model will inform future development of multivalent vaccines to prevent invasive K. pneumoniae infections.