LAPCs promote follicular helper T cell differentiation of Ag-primed CD4+ T cells during respiratory virus infection

Ex Pluribus Unum: The CD4 T Cell Response against Influenza A Virus

Current Influenza A virus (IAV) vaccines, which primarily aim to generate neutralizing antibodies against the major surface proteins of specific IAV strains predicted to circulate during the annual 'flu' season, are suboptimal and are characterized by relatively low annual vaccine efficacy. One approach to improve protection is for vaccines to also target the priming of virus-specific T cells that can protect against IAV even in the absence of preexisting neutralizing antibodies. CD4 T cells represent a particularly attractive target as they help to promote responses by other innate and adaptive lymphocyte populations and can also directly mediate potent effector functions. Studies in murine models of IAV infection have been instrumental in moving this goal forward. Here, we will review these findings, focusing on distinct subsets of CD4 T cell effectors that have been shown to impact outcomes. This body of work suggests that a major challenge for next-generation vaccines will be to prime a CD4 T cell population with the same spectrum of functional diversity generated by IAV infection. This goal is encapsulated well by the motto 'ex pluribus unum': that an optimal CD4 T cell response comprises many individual specialized subsets responding together.

At the crux of maternal immune activation: Viruses, microglia, microbes, and IL-17A

Inflammation during prenatal development can be detrimental to neurodevelopmental processes, increasing the risk of neuropsychiatric disorders. Prenatal exposure to maternal viral infection during pregnancy is a leading environmental risk factor for manifestation of these disorders. Preclinical animal models of maternal immune activation (MIA), established to investigate this link, have revealed common immune and microbial signaling pathways that link mother and fetus and set the tone for prenatal neurodevelopment. In particular, maternal intestinal T helper 17 cells, educated by endogenous microbes, appear to be key drivers of effector IL-17A signals capable of reaching the fetal brain and causing neuropathologies. Fetal microglial cells are particularly sensitive to maternally derived inflammatory and microbial signals, and they shift their functional phenotype in response to MIA. Resulting cortical malformations and miswired interneuron circuits cause aberrant offspring behaviors that recapitulate core symptoms of human neurodevelopmental disorders. Still, the popular use of "sterile" immunostimulants to initiate MIA has limited translation to the clinic, as these stimulants fail to capture biologically relevant innate and adaptive inflammatory sequelae induced by live pathogen infection. Thus, there is a need for more translatable MIA models, with a focus on relevant pathogens like seasonal influenza viruses.

Enhanced IL-2 in early life limits the development of TFH and protective antiviral immunity

T follicular helper cell (TFH)-dependent antibody responses are critical for long-term immunity. Antibody responses are diminished in early life, limiting long-term protective immunity and allowing prolonged or recurrent infection, which may be important for viral lung infections that are highly prevalent in infancy. In a murine model using respiratory syncytial virus (RSV), we show that TFH and the high-affinity antibody production they promote are vital for preventing disease on RSV reinfection. Following a secondary RSV infection, TFH-deficient mice had significantly exacerbated disease characterized by delayed viral clearance, increased weight loss, and immunopathology. TFH generation in early life was compromised by heightened IL-2 and STAT5 signaling in differentiating naive T cells. Neutralization of IL-2 during early-life RSV infection resulted in a TFH-dependent increase in antibody-mediated immunity and was sufficient to limit disease severity upon reinfection. These data demonstrate the importance of TFH in protection against recurrent RSV infection and highlight a mechanism by which this is suppressed in early life.

Granulocytic myeloid-derived suppressor cells inhibit T follicular helper cells during experimental Schistosoma japonicum infection

Background

CD4⁺ T helper (Th) cells play critical roles in both host humoral and cellular immunity against parasitic infection and in the immunopathology of schistosomiasis. T follicular helper (Tfh) cells are a specialized subset of Th cells involved in immunity against infectious diseases. However, the role of Tfh cells in schistosome infection is not fully understood. In this study, the dynamics and roles of Tfh cell regulation were examined. We demonstrated that granulocytic myeloid-derived suppressor cells (G-MDSC) can suppress the proliferation of Tfh cells.

Methods

The levels of Tfh cells and two other Th cells (Th1, Th2) were quantitated at different Schistosoma japonicum infection times (0,3, 5, 8, 13 weeks) using flow cytometry. The proliferation of Tfh cells stimulated by soluble egg antigen (SEA) and soluble worm antigen (SWA) in vivo and in vitro were analyzed. Tfh cells were co-cultured with MDSC to detect the proliferation of Tfh cells labelled by 5(6)-carboxyfluorescein diacetate N-succinimidyl ester. We dynamically monitored the expression of programmed cell death protein 1 (PD-1) on the surface of Tfh cells and programmed cell death ligand 1 (PD-L1) on the surface of MDSC at different infection times (0, 3, 5, 8 weeks). Naïve CD4⁺ T cells (in Tfh cell differentiation) were co-cultured with G-MDSC or monocytic MDSC in the presence, or in the absence, of PD-L1 blocking antibody.

Results

The proportion of Tfh cells among CD4⁺ T cells increased gradually with time of S. japonicum infection, reaching a peak at 8 weeks, after which it decreased gradually. Both SEA and SWA caused an increase in Tfh cells in vitro and in vivo. It was found that MDSC can suppress the proliferation of Tfh cells. The expression of PD-1 on Tfh cells and PD-L1 from MDSC cells increased with prolongation of the infection cycle. G-MDSC might regulate Tfh cells through the PD-1/PD-L1 pathway.

Conclusions

The reported study not only reveals the dynamics of Tfh cell regulation during S. japonicum infection, but also provides evidence that G-MDSC may regulate Tfh cells by PD-1/PD-L1. This study provides strong evidence for the important role of Tfh cells in the immune response to S. japonicum infection.

Graphical abstract

Immunity to Influenza Infection in Humans

This review discusses the human immune responses to influenza infection with some insights from studies using animal models, such as experimental infection of mice. Recent technological advances in the study of human immune responses have greatly added to our knowledge of the infection and immune responses, and therefore much of the focus is on recent studies that have moved the field forward. We consider the complexity of the adaptive response generated by many sequential encounters through infection and vaccination.

Distinct and complementary roles of CD4 T cells in protective immunity to influenza virus

CD4 T cells play a multiplicity of roles in protective immunity to influenza. Included in these functions are help for high affinity antibody production, enhancement of CD8 T cell expansion, function and memory, acceleration of the early innate response to infection and direct cytotoxicity. The influenza-specific CD4 T cell repertoire in humans established through exposures to infection and vaccination has been found to be highly variable in abundance, specificity and functionality. Deficits in particular subsets of CD4 T cells recruited into the response result in diminished antibody responses and protection from infection. Therefore, improved strategies for vaccination should include better methods to identify deficiencies in the circulating CD4 T cell repertoire, and vaccine constructs that increase the representation of CD4 T cells of the correct specificity and functionality.

Double-negative T resident memory cells of the lung react to influenza virus infection via CD11c(hi) dendritic cells

Immunity to Influenza A virus (IAV) is controlled by conventional TCRαβ(+) CD4(+) and CD8(+) T lymphocytes, which mediate protection or cause immunopathology. Here, we addressed the kinetics, differentiation, and antigen specificity of CD4(-)CD8(-) double-negative (DN) T cells. DNT cells expressed intermediate levels of TCR/CD3 and could be further divided in γδ T cells, CD1d-reactive type I NKT cells, NK1.1(+) NKT-like cells, and NK1.1(-) DNT cells. NK1.1(-) DNT cells had a separate antigen-specific repertoire in the steady-state lung, and expanded rapidly in response to IAV infection, irrespectively of the severity of infection. Up to 10% of DNT cells reacted to viral nucleoprotein. Reinfection experiments with heterosubtypic IAV revealed that viral replication was a major trigger for recruitment. Unlike conventional T cells, the NK1.1(-) DNT cells were in a preactivated state, expressing memory markers CD44, CD11a, CD103, and the cytotoxic effector molecule FasL. DNT cells resided in the lung parenchyma, protected from intravascular labeling with CD45 antibody. The recruitment and maintenance of CCR2(+) CCR5(+) CXCR3(+) NK1.1(-) DNT cells depended on CD11c(hi) dendritic cells (DCs). Functionally, DNT cells controlled the lung DC subset balance, suggesting they might act as immunoregulatory cells. In conclusion, we identify activation of resident memory NK1.1(-) DNT cells as an integral component of the mucosal immune response to IAV infection.

Splenic Long-Lived Plasma Cells Promote the Development of Follicular Helper T Cells during Autoimmune Responses

Long-lived plasma cells (LLPCs) develop under the help of follicular helper T (Tfh) cells and reside mainly in the bone marrow. However, these cells are unusually abundant in the spleen of several autoimmune models including K/BxNsf mice, yet their pathogenic impact remains unknown. To investigate a previously unappreciated role of splenic LLPCs, we sorted splenic plasma cells (PCs) from K/BxNsf and K/BxN mice, corresponding to LLPCs and conventional short-lived PCs, respectively, and compared their phenotypes and ability to prime and induce the differentiation of naive CD4(+) T cells into effector cells in vitro and in vivo. We found that K/BxNsf PCs had lower levels of the Ag presentation machinery and costimulators than K/BxN PCs, and also a lower CD4(+) T cell priming capacity. Autoantigen-pulsed K/BxNsf PCs selectively polarized cognate CD4(+) T cells toward the expression of molecules necessary for Tfh development and function. As a result, the K/BxNsf PC-primed CD4(+) T cells were more effective in stimulating B cells to produce autoantigen-specific IgGs than K/BxN PCs or even dendritic cells. Adoptive transfer of K/BxNsf PCs, but not K/BxN PCs, to K/BxN mice increased numbers of Tfh cells in draining lymph nodes. These results propose that abnormal accumulation of LLPCs in the spleen of autoimmune models drives the differentiation of autoantigen-primed CD4(+) T cells to Tfh cells. This positive feedback loop between splenic LLPCs and Tfh cells may contribute to the persistence of humoral autoimmunity.

Roads to the development of improved pertussis vaccines paved by immunology

Current acellular pertussis vaccines have various shortcomings, which may contribute to their suboptimal efficacy and waning immunity in vaccinated populations. This calls for the development of new pertussis vaccines capable of inducing long-lived protective immunity. Immunization with whole cell pertussis vaccines and natural infection with Bordetella pertussis induce distinct and more protective immune responses when compared with immunization with acellular pertussis vaccines. Therefore, the immune responses induced with whole cell vaccine or after infection can be used as a benchmark for the development of third-generation vaccines against pertussis. Here, we review the literature on the immunology of B. pertussis infection and vaccination and discuss the lessons learned that will help in the design of improved pertussis vaccines.

To develop improved pertussis vaccines capable of inducing long-lived protective immunity, lessons have to be learned from immunology of Bordetella pertussis infection and current vaccination.

The regulation of T follicular helper responses during infection

Following infection, naïve CD4 T cells can differentiate into various functionally distinct effector and memory subsets, including T follicular helper (TFH) cells that orchestrate germinal center (GC) reactions necessary for high-affinity, pathogen-specific antibody responses. The origins and function of this cell type have been extensively examined in response to subunit immunization with model antigens. More recently, we are beginning to also appreciate the extent to which microbial infections shape the generation, function and maintenance of TFH cells. Here, we review recent advances and highlight additional knowledge gaps in our understanding of how microbial infections influence priming, differentiation, localization and activity of TFH cells following acute and chronic infections.

IL-21 promotes late activator APC-mediated T follicular helper cell differentiation in experimental pulmonary virus infection

IL-21 is a type-I cytokine that has pleiotropic immuno-modulatory effects. Primarily produced by activated T cells including NKT and T_FH cells, IL-21 plays a pivotal role in promoting T_FH differentiation through poorly understood cellular and molecular mechanisms. Here, employing a mouse model of influenza A virus (IAV) infection, we demonstrate that IL-21, initially produced by NKT cells, promotes T_FH differentiation by promoting the migration of late activator antigen presenting cell (LAPC), a recently identified T_FH inducer, from the infected lungs into the draining lymph nodes (dLN). LAPC migration from IAV-infected lung into the dLN is CXCR3-CXCL9 dependent. IL-21-induced TNF-α production by conventional T cells is critical to stimulate CXCL9 expression by DCs in the dLN, which supports LAPC migration into the dLN and ultimately facilitates T_FH differentiation. Our results reveal a previously unappreciated mechanism for IL-21 modulation of T_FH responses during respiratory virus infection.

LAPCs contribute to the pathogenesis of allergen-induced allergic airway inflammation in mice

BACKGROUND

The inflammatory immune response associated with allergic airway inflammation in asthma involves T helper type 2 (Th2) immunity. Given the data that a newly described late activator antigen-presenting cell (LAPC) population promotes Th2 immunity in viral infections, we undertook studies to investigate whether LAPCs have a pathogenic role in allergic airway inflammation.

METHODS

We employed acute ovalbumin (OVA) and house dust mite (HDM) sensitization and challenge models to establish allergic airway inflammation in mice, followed by the analysis of lungs and draining lymph node (DLN) cell infiltrates, immunoglobulin E (IgE) production, and airway hyper-responsiveness (AHR). We tested whether adoptive transfer of LAPCs isolated from mice with established allergic airway inflammation augments the development of sensitization in naïve mice.

RESULTS

We provide evidence that in both OVA and HDM mouse models of allergic inflammation, LAPCs accumulate in the lungs and draining lymph nodes (DLNs), concomitant with the onset of lung pathology, allergen-specific IgE production, eosinophilia, and Th2 cytokine production. Adoptive transfer experiments using OVA-activated LAPCs reveal exacerbation of disease pathology with an increase in lung inflammatory cells, eosinophilia, circulating IgE, Th2 cytokine production, and a worsening of AHR. OVA-activated LAPCs preferentially increased GATA-3 induction in naïve CD4(+) T cells.

CONCLUSIONS

Together, these data suggest an important role for LAPCs in polarizing the Th2 response in mouse models of allergic airway inflammation.

Follicular helper T cells promote liver pathology in mice during Schistosoma japonicum infection

Following Schistosoma japonicum (S. japonicum) infection, granulomatous responses are induced by parasite eggs trapped in host organs, particular in the liver, during the acute stage of disease. While excessive liver granulomatous responses can lead to more severe fibrosis and circulatory impairment in chronically infected host. However, the exact mechanism of hepatic granuloma formation has remained obscure. In this study, we for the first time showed that follicular helper T (Tfh) cells are recruited to the liver to upregulate hepatic granuloma formation and liver injury in S. japonicum-infected mice, and identified a novel function of macrophages in Tfh cell induction. In addition, our results showed that the generation of Tfh cells driven by macrophages is dependent on cell–cell contact and the level of inducible costimulator ligand (ICOSL) on macrophages which is regulated by CD40–CD40L signaling. Our findings uncovered a previously unappreciated role for Tfh cells in liver pathology caused by S. japonicum infection in mice.

Schistosomiasis is a chronic helminthic disease that affects approximately 200 million people. After S. japonicum infection, parasite eggs are trapped in host liver and granulomas are induced to form around eggs. Severe granuloma subsequently results in serious liver fibrosis and circulatory impairment chronically. It is important to fully elucidate the mechanism of the granuloma formation. Here, we show that Tfh cells play a novel role of promoting the hepatic granuloma formation and liver injury, and identified a novel function of macrophages in Tfh cells induction in S. japonicum-infected mouse model. In addition, we show that the generation of Tfh cells driven by macrophages is cell–cell contact dependent and regulated by CD40-CD40L signaling. Our findings revealed a novel role and mechanism of macrophages in Tfh cell generation and the liver pathogenesis in S. japonicum-infected mouse model.

The role of lung dendritic cell subsets in immunity to respiratory viruses

Viral infections are a common cause of acute respiratory disease. The clinical course of infection and symptoms depend on the viral strain, the health status of the host, and the immunological status of the host. Dendritic cells (DCs) play a crucial role in recognizing and presenting viral antigens and in inducing adaptive immune responses that clear the virus. Because the lung is continuously exposed to the air, the lung is equipped with an elaborate network of DCs to sense incoming foreign pathogens. Increasing knowledge on DC biology has informed us that DCs are not a single cell type. In the steady state lung, three DC subsets can be defined: CD11b(+) or CD103(+) conventional DCs and plasmacytoid DCs. Upon inflammation, inflammatory monocyte-derived DCs are recruited to the lung. It is only recently that tools became available to allow DC subsets to be clearly studied. This review focuses on the activation of DCs and the function of lung DCs in the context of respiratory virus infection and highlights some cautionary points for interpreting older experiments.

Multipronged CD4(+) T-cell effector and memory responses cooperate to provide potent immunity against respiratory virus

Over the last decade, the known spectrum of CD4(+) T-cell effector subsets has become much broader, and it has become clear that there are multiple dimensions by which subsets with a particular cytokine commitment can be further defined, including their stage of differentiation, their location, and, most importantly, their ability to carry out discrete functions. Here, we focus on our studies that highlight the synergy among discrete subsets, especially those defined by helper and cytotoxic function, in mediating viral protection, and on distinctions between CD4(+) T-cell effectors located in spleen, draining lymph node, and in tissue sites of infection. What emerges is a surprising multiplicity of CD4(+) T-cell functions that indicate a large arsenal of mechanisms by which CD4(+) T cells act to combat viruses.

Transcription factor achaete-scute homologue 2 initiates follicular T-helper-cell development

In immune responses, activated T cells migrate to B cell follicles and develop to T follicular helper (Tfh) cells, a new subset of CD4⁺ T cells specialized in providing help to B lymphocytes in the induction of germinal centers ^1,2. Although Bcl6 has been shown to be essential in Tfh cell function, it may not regulate the initial migration of T cells ³ or the induction of Tfh program as exampled by C-X-C chemokine receptor type 5 (CXCR5) upregulation ⁴. Here, we show that Achaete-Scute homologue 2 (Ascl2), a basic helix-loop-helix (bHLH) transcription factor ⁵, is selectively upregulated in its expression in Tfh cells. Ectopic expression of Ascl2 upregulates CXCR5 but not Bcl6 and downregulates C-C chemokine receptor 7 (CCR7) expression in T cells in vitro and accelerates T cell migration to the follicles and Tfh cell development in vivo. Genome-wide analysis indicates that Ascl2 directly regulates Tfh-related genes while inhibits expression of Th1 and Th17 genes. Acute deletion of Ascl2 as well as blockade of its function with the Id3 protein in CD4⁺ T cells results in impaired Tfh cell development and the germinal center response. Conversely, mutation of Id3, known to cause antibody-mediated autoimmunity, greatly enhances Tfh cell generation. Thus, Ascl2 directly initiates Tfh cell development.

Priming of T follicular helper cells by dendritic cells

T follicular helper cells (Tfh) are required to generate long-lived antibody responses, which confer long-term protection to pathogens following vaccination or infection. Despite significant advances in the field, however, little is known about the early steps that drive Tfh cell differentiation. In this review, we will discuss the mechanisms by which dendritic cells promote the initial commitment of activated CD4(+) T cells to the Tfh cell differentiation pathway.

Viral infection of the lung: host response and sequelae

Because of its essential role in gas exchange and oxygen delivery, the lung has evolved a variety of strategies to control inflammation and maintain homeostasis. Invasion of the lung by pathogens (and in some instances exposure to certain noninfectious particulates) disrupts this equilibrium and triggers a cascade of events aimed at preventing or limiting colonization (and more importantly infection) by pathogenic microorganisms. In this review we focus on viral infection of the lung and summarize recent advances in our understanding of the triggering of innate and adaptive immune responses to viral respiratory tract infection, mechanisms of viral clearance, and the well-recognized consequences of acute viral infection complicating underlying lung diseases, such as asthma.

Role of T cell immunity in recovery from influenza virus infection

Influenza virus infection has the potential to induce excess pulmonary inflammation and massive tissue damage in the infected host. Conventional CD4(+) and CD8(+) as well as nonconventional innate like T cells respond to infection and make an essential contribution to the clearance of virus infected cells and the resolution of pulmonary inflammation and injury. Emerging evidence in recent years has suggested a critical role of local interactions between lung effector T cells and antigen presenting cells in guiding the accumulation, differentiation and function of effector T cells beyond their initial activation in the draining lymph nodes during influenza infection. As such, lung effector CD4(+) and CD8(+) T cells utilize multiple effector and regulatory mechanisms to eliminate virus infected cells as well as fine tune the control of pulmonary inflammation and injury. Elucidating the mechanisms by which conventional and nonconventional T cells orchestrate their response in the lung as well as defining the downstream events required for the resolution of influenza infection will be important areas of future basic research which in turn may result in new therapeutic strategies to control the severity of influenza virus infection.

Splenic priming of virus-specific CD8 T cells following influenza virus infection

In healthy individuals, influenza virus (IAV) infection generally remains localized to the epithelial cells of the respiratory tract. Previously, IAV-specific effector CD8 T cells found systemically during the course of IAV infection were thought to have been primed in lung-draining lymph nodes with subsequent migration to other tissues. However, little is known about whether other lymphoid sites participate in the generation of virus-specific CD8 T cells during localized IAV infection. Here, we present evidence of early CD8 T cell priming in the spleen following respiratory IAV infection independent of lung-draining lymph node priming of T cells. Although we found early indications of CD8 T cell activation in the lymph nodes draining the respiratory tract, we also saw evidence of virus-specific CD8 T cell activation in the spleen. Furthermore, CD8 T cells primed in the spleen differentiated into memory cells of equivalent longevity and with similar recall capacity as CD8 T cells primed in the draining lymph nodes. These data showed that the spleen contributes to the virus-specific effector and memory CD8 T cell populations that are generated in response to respiratory infection.