Epithelial heme oxygenase-1 enhances colonic tumorigenesis by inhibiting ferroptosis

Colorectal cancer has been linked to chronic colitis and red meat consumption, which can increase colonic iron and heme. Heme oxygenase-1 ( Hmox1 ) metabolizes heme and releases ferrous iron, but its role in colonic tumorigenesis is not well-described. Recent studies suggest that ferroptosis, the iron-dependent form of cell death, protects against colonic tumorigenesis. Ferroptosis culminates in excessive lipid peroxidation that is constrained by the antioxidative glutathione pathway. We observed increased mucosal markers of ferroptosis and glutathione metabolism in the setting of murine and human colitis, as well as murine colonic neoplasia. We obtained similar results in murine and human colonic epithelial organoids exposed to heme and the ferroptosis activator erastin, especially induction of Hmox1 . RNA sequencing of colonic organoids from mice with deletion of intestinal epithelial Hmox1 (Hmox1 ΔIEC ) revealed increased ferroptosis and activated glutathione metabolism after heme exposure. In a colitis-associated cancer model we observed significantly fewer and smaller tumors in Hmox1 ΔIEC mice compared to littermate controls. Transcriptional profiling of Hmox1 ΔIEC tumors and tumor organoids revealed increased ferroptosis and oxidative stress markers in tumor epithelial cells. In total, our findings reveal ferroptosis as an important colitis-associated cancer signature pathway, and Hmox1 as a key regulator in the tumor microenvironment.

IL-38 regulates intestinal stem cell homeostasis by inducing WNT signaling and beneficial IL-1β secretion

The IL-1 Family member IL-38 has been characterized primarily as an antiinflammatory cytokine in human and mouse models of systemic diseases. Here, we examined the role of IL-38 in the murine small intestine (SI). Immunostaining of SI revealed that IL-38 expression partially confines to intestinal stem cells. Cultures of intestinal organoids reveal IL-38 functions as a growth factor by increasing organoid size via inducing WNT3a. In contrast, organoids from IL-38-deficient mice develop more slowly. This reduction in size is likely due to the downregulation of intestinal stemness markers (i.e., Fzd5, Ephb2, and Olfm4) expression compared with wild-type organoids. The IL-38 binding to IL-1R6 and IL-1R9 is still a matter of debate. Therefore, to analyze the molecular mechanisms of IL-38 signaling, we also examined organoids from IL-1R9-deficient mice. Unexpectedly, these organoids, although significantly smaller than wild type, respond to IL-38, suggesting that IL-1R9 is not involved in IL-38 signaling in the stem cell crypt. Nevertheless, silencing of IL-1R6 disabled the organoid response to the growth property of IL-38, thus suggesting IL-1R6 as the main receptor used by IL-38 in the crypt compartment. In organoids from wild-type mice, IL-38 stimulation induced low concentrations of IL-1β which contribute to organoid growth. However, high concentrations of IL-1β have detrimental effects on the cultures that were prevented by treatment with recombinant IL-38. Overall, our data demonstrate an important regulatory function of IL-38 as a growth factor, and as an antiinflammatory molecule in the SI, maintaining homeostasis.

Pancreatic Ductal Adenocarcinoma Cells Regulate NLRP3 Activation to Generate a Tolerogenic Microenvironment

Defining feature of pancreatic ductal adenocarcinoma (PDAC) that participates in the high mortality rate and drug resistance is the immune-tolerant microenvironment which enables tumors to progress unabated by adaptive immunity. In this study, we report that PDAC cells release CSF-1 to induce nucleotide-binding domain, leucine-rich containing family, pyrin domain-containing-3 (NLRP3) activation in myeloid cells. Increased NLRP3 expression was found in the pancreas of patients with PDAC when compared with normal pancreas which correlated with the formation of the NLRP3 inflammasome. Using human primary cells and an orthotopic PDAC mouse model, we show that NLRP3 activation is responsible for the maturation and release of the inflammatory cytokine IL1β which selectively drives Th2-type inflammation via COX2/PGE2 induction. As a result of this inflammation, primary tumors were characterized by reduced cytotoxic CD8+ T-cell activation and increased tumor expansion. Genetic deletion and pharmacologic inhibition of NLRP3 enabled the development of Th1 immunity, increased intratumoral levels of IL2, CD8+ T cell–mediated tumor suppression, and ultimately limited tumor growth. In addition, we observed that NLRP3 inhibition in combination with gemcitabine significantly increased the efficacy of the chemotherapy. In conclusion, this study provides a mechanism by which tumor-mediated NLRP3 activation exploits a distinct adaptive immunity response that facilitates tumor escape and progression. Considering the ability to block NLRP3 activity with safe and small orally active molecules, this protein represents a new promising target to improve the limited therapeutic options in PDAC.

SIGNIFICANT

This study provides novel molecular insights on how PDAC cells exploit NLRP3 activation to suppress CD8 T-cell activation. From a translational perspective, we demonstrate that the combination of gemcitabine with the orally active NLRP3 inhibitor OLT1177 increases the efficacy of monotherapy.

Nucleocapsid protein binding DNA aptamers for detection of SARS-COV-2

The severe acute respiratory syndrome coronavirus (SARS-CoV-2) has infected millions of individuals and continues to be a major health concern worldwide. While reverse transcription-polymerase chain reaction remains a reliable method for detecting infections, limitations of this technology, particularly cost and the requirement of a dedicated laboratory, prevent rapid viral monitoring. Antigen tests filled this need to some extent but with limitations including sensitivity and specificity, particularly against emerging variants of concern. Here, we developed aptamers against the SARS-CoV-2 Nucleocapsid protein to complement or replace antibodies in antigen detection assays. As detection reagents in ELISA-like assays, our DNA aptamers were able to detect as low as 150 pg/mL of the protein and under 150 k copies of inactivated SARS-CoV-2 Wuhan Alpha strain in viral transport medium with little cross-reactivity to other human coronaviruses (HCoVs). Further, our aptamers were reselected against the SARS-CoV-2 Omicron variant of concern, and we found two sequences that had a more than two-fold increase in signal compared to our original aptamers when used as detection reagents against protein from the Omicron strain. These findings illustrate the use of aptamers as promising alternative detection reagents that may translate for use in current tests and our findings validate the method for the reselection of aptamers against emerging viral strains.

Protective role of tissue-resident regulatory T cells in a murine model of beryllium-induced disease

CD4+ T cells drive the immunopathogenesis of chronic beryllium disease (CBD), and their recruitment to the lung heralds the onset of granulomatous inflammation. We have shown that regulatory CD4+ T cells (Tregs) control granuloma formation in an HLA-DP2 transgenic (Tg) model of CBD. In these mice, Be oxide (BeO) exposure resulted in the accumulation of three distinct CD4+ T cell subsets in the lung with the majority of tissue-resident memory cells expressing FoxP3. The amount of Be regulated the number of total and antigen-specific CD4+ T cells and Tregs in the lungs of HLA-DP2 Tg mice. Depletion of Tregs increased the number of IFN-γ-producing CD4+ T cells and enhanced lung injury while mice treated with IL2/αIL-2 complexes had increased Tregs and reduced inflammation and Be-responsive T cells in the lung. BeO-experienced resident Tregs suppressed anti-CD3-induced proliferation of CD4+ T cells in a contact-dependent manner. CLTLA-4 and ICOS blockade as well as addition of LPS to BeO-exposed mice increased the Teff/Treg ratio and enhanced lung injury. Collectively, these data show that the protective role of tissue-resident Tregs is dependent on quantity of Be exposure and is overcome by blocking immune regulatory molecules or additional environmental insults.

Protective Role of Tissue-Resident Regulatory T Cells in a Murine Model of Beryllium-Induced Disease

CD4+ T cells drive the immunopathogenesis of chronic beryllium disease (CBD), and their recruitment to the lung heralds the onset of granulomatous inflammation. We have shown that regulatory CD4+ T cells (Tregs) control granuloma formation in an HLA-DP2 transgenic (Tg) model of CBD. In Be oxide (BeO)-exposed HLA-DP2 Tg mice, the majority of tissue-resident memory cells (CD44+CD69+CD103+) expressed FoxP3 while resident effector cells (CD44+CD69+CD103−) lacked FoxP3. Increased numbers of tissue-resident effector and regulatory CD4+ T cells accumulated in the lung following BeO exposure. Depletion of Tregs increased the number of IFN-γ-producing CD4+ T cells and enhanced lung injury while mice treated with IL2/αIL-2 complexes had increased Tregs and reduced inflammation and Be-responsive T cells in the lung. BeO-experienced resident Tregs suppressed anti-CD3-induced proliferation of CD4+ T cells in a contact-dependent manner. Addition of LPS to BeO-exposed mice increased the Teff/Treg ratio and enhanced lung injury. Collectively, these data show that the protective role of tissue-resident Tregs is dependent on time and quantity of Be exposure and is overcome by additional environmental insults. Finally, our findings suggest a potential therapeutic role of Treg expansion as an adjunctive therapeutic option in granulomatous lung disease.

This work was supported by the following NIH grants: ES025534, HL062410, HL102245, and HL152756 (to Andrew P. Fontenot). 

Innate and Adaptive Immunity in Noninfectious Granulomatous Lung Disease

Sarcoidosis and chronic beryllium disease are noninfectious lung diseases that are characterized by the presence of noncaseating granulomatous inflammation. Chronic beryllium disease is caused by occupational exposure to beryllium containing particles, whereas the etiology of sarcoidosis is not known. Genetic susceptibility for both diseases is associated with particular MHC class II alleles, and CD4⁺ T cells are implicated in their pathogenesis. The innate immune system plays a critical role in the initiation of pathogenic CD4⁺ T cell responses as well as the transition to active lung disease and disease progression. In this review, we highlight recent insights into Ag recognition in chronic beryllium disease and sarcoidosis. In addition, we discuss the current understanding of the dynamic interactions between the innate and adaptive immune systems and their impact on disease pathogenesis.

Disruption of monocyte-macrophage differentiation and trafficking by a heme analog during active inflammation

Heme metabolism is a key regulator of inflammatory responses. Cobalt protoporphyrin IX (CoPP) is a heme analog and mimic that potently activates the NRF2/heme oxygenase-1 (HO-1) pathway, especially in monocytes and macrophages. We investigated the influence of CoPP on inflammatory responses using a murine model of colitis. Surprisingly, conditional deletion of myeloid HO-1 did not impact the colonic inflammatory response or the protective influence of CoPP in the setting of dextran sodium sulfate-induced colitis. Rather, we reveal that CoPP elicits a contradictory shift in blood myeloid populations relative to the colon during active intestinal inflammation. Major population changes include markedly diminished trafficking of CCR2⁺Ly6C^hi monocytes to the inflamed colon, despite significant mobilization of this population into circulation. This resulted in significantly diminished colonic expansion of monocyte-derived macrophages and inflammatory cytokine expression. These findings were linked with significant induction of systemic CCL2 leading to a disrupted CCL2 chemoattractant gradient toward the colon and concentration-dependent suppression of circulating monocyte CCR2 expression. Administration of CoPP also induced macrophage differentiation toward a Marco^hiHmox1^hi anti-inflammatory erythrophagocytic phenotype, contributing to an overall decreased inflammatory profile. Such findings redefine protective influences of heme metabolism during inflammation, and highlight previously unreported immunosuppressive mechanisms of endogenous CCL2 induction.

CD4+ T cells in the lungs of acute sarcoidosis patients recognize an Aspergillus nidulans epitope

Löfgren’s syndrome (LS) is an acute form of sarcoidosis characterized by a genetic association with HLA-DRB1*03 (HLA-DR3) and an accumulation of CD4⁺ T cells of unknown specificity in the bronchoalveolar lavage (BAL). Here, we screened related LS-specific TCRs for antigen specificity and identified a peptide derived from NAD-dependent histone deacetylase hst4 (NDPD) of Aspergillus nidulans that stimulated these CD4⁺ T cells in an HLA-DR3–restricted manner. Using ELISPOT analysis, a greater number of IFN-γ– and IL-2–secreting T cells in the BAL of DR3⁺ LS subjects compared with DR3⁺ control subjects was observed in response to the NDPD peptide. Finally, increased IgG antibody responses to A. nidulans NDPD were detected in the serum of DR3⁺ LS subjects. Thus, our findings identify a ligand for CD4⁺ T cells derived from the lungs of LS patients and suggest a role of A. nidulans in the etiology of LS.

Beryllium-specific CD4+ T cells induced by chemokine neoantigens perpetuate inflammation

Discovering dominant epitopes for T cells, particularly CD4+ T cells, in human immune-mediated diseases remains a significant challenge. Here, we used bronchoalveolar lavage (BAL) cells from HLA-DP2-expressing patients with chronic beryllium disease (CBD), a debilitating granulomatous lung disorder characterized by accumulations of beryllium-specific (Be-specific) CD4+ T cells in the lung. We discovered lung-resident CD4+ T cells that expressed a disease-specific public CDR3β T cell receptor motif and were specific to Be-modified self-peptides derived from C-C motif ligand 4 (CCL4) and CCL3. HLA-DP2-CCL/Be tetramer staining confirmed that these chemokine-derived peptides represented major antigenic targets in CBD. Furthermore, Be induced CCL3 and CCL4 secretion in the lungs of mice and humans. In a murine model of CBD, the addition of LPS to Be oxide exposure enhanced CCL4 and CCL3 secretion in the lung and significantly increased the number and percentage of CD4+ T cells specific for the HLA-DP2-CCL/Be epitope. Thus, we demonstrate a direct link between Be-induced innate production of chemokines and the development of a robust adaptive immune response to those same chemokines presented as Be-modified self-peptides, creating a cycle of innate and adaptive immune activation.

Human and Mouse Transcriptome Profiling Identifies Cross-Species Homology in Pulmonary and Lymph Node Mononuclear Phagocytes

The mononuclear phagocyte (MP) system consists of macrophages, monocytes, and dendritic cells (DCs). MP subtypes play distinct functional roles in steady-state and inflammatory conditions. Although murine MPs are well characterized, their pulmonary and lymph node (LN) human homologs remain poorly understood. To address this gap, we have created a gene expression compendium across 24 distinct human and murine lung and LN MPs, along with human blood and murine spleen MPs, to serve as validation datasets. In-depth RNA sequencing identifies corresponding human-mouse MP subtypes and determines marker genes shared and divergent across species. Unexpectedly, only 13%-23% of the top 1,000 marker genes (i.e., genes not shared across species-specific MP subtypes) overlap in corresponding human-mouse MP counterparts. Lastly, CD88 in both species helps distinguish monocytes/macrophages from DCs. Our cross-species expression compendium serves as a resource for future translational studies to investigate beforehand whether pursuing specific MP subtypes or genes will prove fruitful.

HIV Infection Is Associated with Loss of Anti-Inflammatory Alveolar Macrophages

HIV type 1 is associated with pulmonary dysfunction that is exacerbated by cigarette smoke. Alveolar macrophages (AM) are the most prominent immune cell in the alveolar space. These cells play an important role in clearing inhaled pathogens and regulating the inflammatory environment; however, how HIV infection impacts AM phenotype and function is not well understood, in part because of their autofluorescence and the absence of well-defined surface markers. The main aim of this study was to evaluate the impact of HIV infection on human AM and to compare the effect of smoking on their phenotype and function. Time-of-flight mass cytometry and RNA sequencing were used to characterize macrophages from human bronchoalveolar lavage of HIV-infected and -uninfected smokers and nonsmokers. We found that the frequency of CD163⁺ anti-inflammatory AM was decreased, whereas CD163^-CCR7⁺ proinflammatory AM were increased in HIV infection. HIV-mediated proinflammatory polarization was associated with increased levels of inflammatory cytokines and macrophage activation. Conversely, smoking heightened the inflammatory response evident by change in the expression of CXCR4 and TLR4. Altogether, these findings suggest that HIV infection, along with cigarette smoke, favors a proinflammatory macrophage phenotype associated with enhanced expression of inflammatory molecules. Further, this study highlights time-of-flight mass cytometry as a reliable method for immunophenotyping the highly autofluorescent cells present in the bronchoalveolar lavage of cigarette smokers.

A role for TNF-α in alveolar macrophage damage-associated molecular pattern release

Chronic beryllium disease (CBD) is a metal hypersensitivity/autoimmune disease in which damage-associated molecular patterns (DAMPs) promote a break in T cell tolerance and expansion of Be2+/self-peptide-reactive CD4+ T cells. In this study, we investigated the mechanism of cell death induced by beryllium particles in alveolar macrophages (AMs) and its impact on DAMP release. We found that phagocytosis of Be led to AM cell death independent of caspase, receptor-interacting protein kinases 1 and 3, or ROS activity. Before cell death, Be-exposed AMs secreted TNF-α that boosted intracellular stores of IL-1α followed by caspase-8-dependent fragmentation of DNA. IL-1α and nucleosomal DNA were subsequently released from AMs upon loss of plasma membrane integrity. In contrast, necrotic AMs released only unfragmented DNA and necroptotic AMs released only IL-1α. In mice exposed to Be, TNF-α promoted release of DAMPs and was required for the mobilization of immunogenic DCs, the expansion of Be-reactive CD4+ T cells, and pulmonary inflammation in a mouse model of CBD. Thus, early autocrine effects of particle-induced TNF-α on AMs led to a break in peripheral tolerance. This potentially novel mechanism may underlie the known relationship between fine particle inhalation, TNF-α, and loss of peripheral tolerance in T cell-mediated autoimmune disease and hypersensitivities.

A host innate immune circuit licenses downregulation of Salmonella flagellin expression inside macrophages

While the effect of bacterial molecules on host immune system is well studied, how host factors affect expression of bacterial molecules is not understood in great detail. Inflammasomes are instrumental in detection and clearance of bacterial pathogens, but little is known about whether there is an active cross-talk between the host sensing mechanism and the expression of stimulatory ligands by the pathogen. We used Salmonella-macrophage infection model to assess the impact of inflammasome activation on the expression of flagellin as a candidate Salmonella protein. We discovered that early during infection, host lysophospholipids released upon rapid NLRC4-mediated pyroptosis of macrophages increase flagellin expression by extracellular bacteria which leads to even greater pyroptosis when these bacteria infect healthy macrophages, thus facilitating NLRC4-mediated detection and clearance of Salmonella. A TLR-dependent host response, however, later inhibits expression of NLRC4 and the lysophospholipid biosynthetic enzyme iPLA2, causing a decline in intracellular lysophospholipids pool that permits downregulation of flagellin by intracellular Salmonella. Our findings reveal a novel mode of host-pathogen cross-talk wherein expression of a bacterium-derived inflammasome ligand is temporally controlled by the very process of inflammasome activation, and identify a host mechanism that controls switching of Salmonella to a flagellin-low phenotype inside macrophages with important implications for bacterial pathogenesis and immunity. Future investigations will utilize dual RNA-Seq studies to understand how Salmonella gene expression networks rewire to adapt to different host intracellular microenvironments.

Adaptive Immunity in Pulmonary Sarcoidosis and Chronic Beryllium Disease

Pulmonary sarcoidosis and chronic beryllium disease (CBD) are inflammatory granulomatous lung diseases defined by the presence of non-caseating granulomas in the lung. CBD results from beryllium exposure in the workplace, while the cause of sarcoidosis remains unknown. CBD and sarcoidosis are both immune-mediated diseases that involve Th1-polarized inflammation in the lung. Beryllium exposure induces trafficking of dendritic cells to the lung in a mechanism dependent on MyD88 and IL-1α. B cells are also recruited to the lung in a MyD88 dependent manner after beryllium exposure in order to protect the lung from beryllium-induced injury. Similar to most immune-mediated diseases, disease susceptibility in CBD and sarcoidosis is driven by the expression of certain MHCII molecules, primarily HLA-DPB1 in CBD and several HLA-DRB1 alleles in sarcoidosis. One of the defining features of both CBD and sarcoidosis is an infiltration of activated CD4+ T cells in the lung. CD4+ T cells in the bronchoalveolar lavage (BAL) of CBD and sarcoidosis patients are highly Th1 polarized, and there is a significant increase in inflammatory Th1 cytokines present in the BAL fluid. In sarcoidosis, there is also a significant population of Th17 cells in the lungs that is not present in CBD. Due to persistent antigen exposure and chronic inflammation in the lung, these activated CD4+ T cells often display either an exhausted or anergic phenotype. Evidence suggests that these T cells are responding to common antigens in the lung. In CBD there is an expansion of beryllium-responsive TRBV5.1+ TCRs expressed on pathogenic CD4+ T cells derived from the BAL of CBD patients that react with endogenous human peptides derived from the plexin A protein. In an acute form of sarcoidosis, there are expansions of specific TRAV12-1/TRBV2 T cell receptors expressed on BAL CD4+ T cells, indicating that these T cells are trafficking to and expanding in the lung in response to common antigens. The specificity of these pathogenic CD4+T cells in sarcoidosis are currently unknown.

Immune Surveillance by Natural IgM Is Required for Early Neoantigen Recognition and Initiation of Adaptive Immunity

Early recognition of neoantigen-expressing cells is complex, involving multiple immune cell types. In this study, in vivo, we examined how antigen-presenting cell subtypes coordinate and induce an immunological response against neoantigen-expressing cells, particularly in the absence of a pathogen-associated molecular pattern, which is normally required to license antigen-presenting cells to present foreign or self-antigens as immunogens. Using two reductionist models of neoantigen-expressing cells and two cancer models, we demonstrated that natural IgM is essential for the recognition and initiation of adaptive immunity against neoantigen-expressing cells. Natural IgM antibodies form a cellular immune complex with the neoantigen-expressing cells. This immune complex licenses surveying monocytes to present neoantigens as immunogens to CD4⁺ T cells. CD4⁺ T helper cells, in turn, use CD40L to license cross-presenting CD40⁺ Batf3⁺ dendritic cells to elicit a cytotoxic T cell response against neoantigen-expressing cells. Any break along this immunological chain reaction results in the escape of neoantigen-expressing cells. This study demonstrates the surprising, essential role of natural IgM as the initiator of a sequential signaling cascade involving multiple immune cell subtypes. This sequence is required to coordinate an adaptive immune response against neoantigen-expressing cells.

Protective role of B cells in sterile particulate-induced lung injury

Susceptibility to chronic beryllium (Be) disease is linked to HLA-DP molecules possessing a glutamic acid at the 69th position of the β-chain (βGlu69), with the most prevalent βGlu69-containing molecule being HLA-DP2. We have previously shown that HLA-DP2 transgenic (Tg) mice exposed to Be oxide (BeO) develop mononuclear infiltrates in a peribronchovascular distribution and a beryllium-specific, HLA-DP2-restricted CD4+ T cell response. In addition to T cells, B cells constituted a major portion of infiltrated leukocytes in the lung of BeO-exposed HLA-DP2 Tg mice and sequester BeO particles within ectopic lymphoid aggregates and granulomas. B cell depletion was associated with a loss of lymphoid aggregates and granulomas as well as a significant increase in lung injury in BeO-exposed mice. The protective role of B cells was innate in origin, and BeO-induced B cell recruitment to the lung was dependent on MyD88 signaling. Similar to BeO-exposed HLA-DP2 mice, B cells also accumulate in the lungs of CBD subjects, located at the periphery and surrounding the granuloma. Overall, our data suggest a novel modulatory role for B cells in the protection of the lung against sterile particulate exposure, with B cell recruitment to the inflamed lung occurring in an antigen-independent and MyD88-dependent manner.

BeO-exposure induces B cell-mediated Noninfectious Granuloma Formation in the Lung

Susceptibility to chronic beryllium disease (CBD)is linked to HLA-DP molecules possessing a glutamic acid at the 69thposition of the β-chain (βGlu69), with the most prevalent βGlu69-containing molecule being HLA-DP2. We have previously shown that exposure of HLA-DP2 transgenic mice to beryllium oxide (BeO) results in the development of mononuclear infiltrates in a peribronchovascular distribution and a beryllium-specific, HLA-DP2-restricted CD4+T cell response. In addition to T cells, B cells constituted a major portion of infiltrated leukocytes in the lung of BeO-exposed HLA-DP2 Tg mice and were localized within ectopic lymphoid aggregates and granulomas. B cell depletion had no effect on the development of Be-specific CD4+T cells. However, B cell depletion was associated with a loss of lymphoid aggregates and granulomas as well as a significant increase in lung injury in BeO-exposed mice compared to the isotype-treated group. Furthermore, B cell recruitment to the lung was independent of antigen and dependent on MyD88 signaling through the secretion of CXCL13. Adoptively-transferred WT B cells into BeO-exposed μMT mice significantly reduced lung injury. B cells also surrounded granulomas in transbronchial biopsies from CBD patients, confirming a role for B cells in CBD. Overall, our data suggest a novel modulatory role for B cells in the formation of granulomas and lymphoid aggregates, resulting in the protection of the lung against sterile particulate exposure.

A host cell-intrinsic innate regulatory circuit limits inflammasome activity and promotes immune escape of Salmonella inside macrophages

Inflammasomes have been implicated in the detection and clearance of a variety of bacterial pathogens, but little is known about whether there is active cross-talk between the host sensing mechanism and the expression of stimulatory ligands by the pathogen. Here we show that inflammasome activation regulates expression of the NLRC4 and TLR5 ligand, flagellin, by Salmonella. Host lysophospholipids released upon NLRC4-mediated pyroptosis increase flagellin expression by extracellular bacteria that enhances pyroptosis upon internalization, establishing a positive feedback loop that potentiates Salmonella detection and clearance. A TLR-dependent host negative feedback response later inhibits inflammasome activation and lysophospholipid biosynthesis within cells, prompting the pathogen to switch to a flagellin-low phenotype and establish an intracellular survival niche inside macrophages. Ablation of this host regulatory circuit prevents downregulation of flagellin by intracellular Salmonella and promotes bacterial clearance in vivo. Our data identify modulation of expression of a bacterium-derived inflammasome ligand by the very process of inflammasome activation as a novel mode of host-pathogen cross-talk and reveal a host mechanism that is adapted by Salmonella for flagellin downregulation and immune escape within macrophages.

The NLRC4 inflammasome regulates extra- and intra-macrophage expression of its own pathogen-derived ligand flagellin to impact Salmonella survival

Inflammasomes have been implicated in the detection and clearance of a variety of bacterial pathogens, but whether the process of inflammasome activation per se regulates expression of microbial effectors is unknown. Here we show that inflammasome activation regulates expression of the NLRC4 ligand, flagellin, on Salmonella. Lysophospholipids released upon macrophage pyroptosis increase expression of flagellin on Salmonella and its ability to induce further NLRC4 inflammasome-dependent pyroptosis, establishing a positive feedback loop that potentiates Salmonella detection and clearance. As infection progresses, this positive feedback is checked by inhibition of NLRC4 expression and lysophospholipid biosynthesis, thus switching Salmonella to a flagellin-negative phenotype inside macrophages. While inhibition of inflammasome activation could be a host strategy aimed at preventing overt cell death and enabling generation of a productive adaptive immune response post infection, our study demonstrates that Salmonella hijacks this host response to facilitate its intracellular survival. Our data reveal a host-dependent mechanism by which Salmonella downregulates flagellin within macrophages and identify the modulation of expression of a pathogen-derived inflammasome ligand by the very process of inflammasome activation as a novel mode of host-pathogen cross talk during infection with a bacterial pathogen.

Isolation and Identification of Interstitial Macrophages from the Lungs Using Different Digestion Enzymes and Staining Strategies

Interstitial macrophages (IMs) are present in multiple organs. Although there is limited knowledge of the unique functional role IM subtypes play, macrophages, in general, are known for their contribution in homeostatic tissue maintenance and inflammation such as clearing pathogens and debris and secreting inflammatory mediators and growth factors. IM subtypes have been identified in the heart, skin, and gut, and more recently we identified three distinct IMs in the lung. IMs express on their surface high levels of MerTK, CD64, and CD11b, with differences in CD11c, CD206, and MHC II expression, and referred to the three pulmonary IM subtypes as IM1 (CD11c^loCD206⁺MHCII^lo), IM2 (CD11c^loCD206⁺MHCII^hi), and IM3 (CD11c^hiCD206^loMHCII^hi). In this chapter, we highlight how to extract IMs from the lung using three different digestion enzymes: elastase, collagenase D, and Liberase TM. Of these three commonly used enzymes, Liberase TM was the most effective at IM extraction, particularly IM3. Furthermore, alternative staining strategies to identify IMs were examined, which included CD64, MerTK, F4/80, and Tim4. Thus, future studies highlighting the functional role of IM subtypes will help further our understanding of how tissue homeostasis is maintained and inflammatory conditions are induced and resolved.

Three Unique Interstitial Macrophages in the Murine Lung at Steady State

The current paradigm in macrophage biology is that some tissues mainly contain macrophages from embryonic origin, such as microglia in the brain, whereas other tissues contain postnatal-derived macrophages, such as the gut. However, in the lung and in other organs, such as the skin, there are both embryonic and postnatal-derived macrophages. In this study, we demonstrate in the steady-state lung that the mononuclear phagocyte system is comprised of three newly identified interstitial macrophages (IMs), alveolar macrophages, dendritic cells, and few extravascular monocytes. We focused on similarities and differences between the three IM subtypes, specifically, their phenotype, location, transcriptional signature, phagocytic capacity, turnover, and lack of survival dependency on fractalkine receptor, CX₃CR1. Pulmonary IMs were located in the bronchial interstitium but not the alveolar interstitium. At the transcriptional level, all three IMs displayed a macrophage signature and phenotype. All IMs expressed MER proto-oncogene, tyrosine kinase, CD64, CD11b, and CX₃CR1, and were further distinguished by differences in cell surface protein expression of CD206, Lyve-1, CD11c, CCR2, and MHC class II, along with the absence of Ly6C, Ly6G, and Siglec F. Most intriguingly, in addition to the lung, similar phenotypic populations of IMs were observed in other nonlymphoid organs, perhaps highlighting conserved functions throughout the body. These findings promote future research to track four distinct pulmonary macrophages and decipher the division of labor that exists between them.

Natural IgM initiates the immunological cascade against minor antigen-mismatched cells, illustrating the dependency and unique role of APC subtypes

In organ transplantation a major obstacle is the immunological response against mismatches of major and minor histocompatibility antigen (Ags). Even when MHC Ags are matched between donor and recipient, minor Ags can elicit graft rejection. To date, it is unclear how endogenous antigen-presenting cells (APCs) recognize, coordinate and induce immunological responses against minor Ag-mismatched cells, particularly in the absence of pathogen-associated molecular patterns (PAMPs). We hypothesized that each APC subtype is dependent on each other to complete an immunological response against mismatches of minor Ags. Using a well-established minor Agmismatch model, we demonstrated that the elimination of any one APC subtype (Batf3+ DCs, Irf4+ DCs, Ly6C+ monocytes or B cells) resulted in a diminished or abolished immune response against minor Ag-mismatches. Specifically, we demonstrated that this immunological response began with the recognition of minor Ag-mismatched cells by natural IgM, which resulted in an immune complex formation. This cellular immune complex was then acquired by MHC II presenting APCs, of which one was MHC II expressing Ly6C+ monocytes. Subsequently, Ly6C+ monocyte induced Ag-specific CD4+ T cells licensed Batf3+ DCs via CD40 to present mismatches of minor antigen to Ag-specific CD8+T cells, which were required for the cytotoxic elimination of minor-Ag mismatched cells. Overall, our in vivo findings suggest a sequential, coordinated immunological event is required for the rejection of minor Ag-mismatched cells.

Three unique interstitial macrophages in the murine lung at steady state

Rationale

The current paradigm in macrophage biology is that some tissues mainly contain macrophages from embryonic origin such as microglia in the brain, while other tissues contain postnatal-derived macrophages, such as the gut. However, in the lung and in other organs such as the skin, there are both embryonic and postnatal-derived macrophages.

Objectives

In this study, we demonstrate in the steady-state lung that the mononuclear phagocyte system is comprised of three newly identified interstitial macrophages (IMs), alveolar macrophages (AMs), dendritic cells and few extravascular monocytes.

Methods

We focused on similarities and differences between the three IM subtypes, specifically, their phenotype, location, transcriptional signature, phagocytic capacity, turnover and lack of survival dependency on CX3CR1.

Measurements and Main Results

Pulmonary IMs were located in the bronchial interstitium but not the alveolar interstitium. At the transcriptional level, all three IMs displayed a macrophage signature. All IMs expressed MerTK+CD64+ CD11b+ CX CR1+ and were furthermore distinguished by differences in cell surface protein expression of CD206, Lyve-1, CD11c, CCR2 and MHCII, along with the absence of Ly6C, Ly6G, and Siglec F. Ex vivo analysis revealed that all three IMs were highly phagocytic compared to AMs. Finally, similar phenotypic populations of IMs were present in other non-lymphoid organs, suggesting that these IMs may not be unique to the lung.

Conclusions

These findings promote future research to track four distinct pulmonary macrophages and decipher the division of labor that exists between them.

TLR7 enhances cross-presentation in Ly6C(+) monocytes by targeting endosomal properties

Effector functions of monocytes beyond terminal differentiation has recently come to light. In the present study we have made the novel discovery that Ly6C(+) monocytes can acquire cell-associated Ag from apoptotic cells, termed as efferocytosis. This property was enhanced by the presence of pathogenic stimuli (TLR ligands) directly acting on monocytes. Furthermore, we have shown that monocytes can cross-present Ag acquired from those apoptotic cells, with differential ability of TLR4 and TLR7 ligands in enhancing that process. This has led us to investigate the role of various pathogenic stimuli beyond these two ligands. Our data has confirmed our previous findings and further established the fact that among known TLR ligands, TLR7 selectively enhanced the presentation of peptides on MHC-I molecules. To elucidate the mechanism, we performed RNA sequencing on monocytes treated with a TLR7 ligand showing the involvement of endosomal proteins in antigen presentation, specifically subunits of the NADPH oxidase. Our data clearly demonstrated the role of Ncf-1 in TLR7 mediated enhanced activation of antigen specific CD8+ T cells. Overall, this study defines a unique role for Ly6C(+) monocytes in promoting immunity against single-stranded virus by regulating endosomal pH, a characterized process required for Batf3-dependent DC cross-presentation.

Ly6C+ monocyte efferocytosis and cross-presentation of cell-associated antigen

Recently it was shown that circulating Ly6C+ monocytes traffic from tissue to the draining lymph nodes (LNs) with minimal alteration in their overall phenotype. Furthermore, in the steady state, Ly6C+ monocytes are as abundant as classical dendritic cells (DCs) within the draining LNs, and even more abundant during inflammation. However, little is known about the functional roles of constitutively trafficking Ly6C+ monocytes. In this study we investigated whether Ly6C+ monocytes can efferocytose (acquire dying cells) and cross-present cell-associated antigen, a functional property specially attributed to Batf3+ DCs. We demonstrated that Ly6C+ monocytes intrinsically efferocytose and cross-present cell-associated antigen to CD8+ T cells. In addition, efferocytosis was enhanced upon direct activation of the Ly6C+ monocytes through its corresponding TLRs, TLR4 and TLR7. However, only ligation of TLR7, and not TLR4, enhanced cross-presentation by Ly6C+ monocytes. Overall, this study outlines two functional roles, among others, that Ly6C+ monocytes have during an adaptive immune response.

Coordinated immune activation by distinct mononuclear phagocytes subtypes against the mismatch of minor antigens

Recently, we identified that Batf3-dependent DCs were solely responsible for the rejection of minor antigen-mismatched grafts, and demonstrated in Batf3−/− female mice, lacking Batf3-dependent DCs, the complete acceptance of minor antigen-mismatched male cells, which are normally rejected in WT female mice. Intriguingly, this rejection occurs in the absence of PAMPs, leading to considerable uncertainty of how endogenous APC subtypes become activated and licensed to induce an adaptive immune response against male cells in female mice. Investigating this gap in knowledge is the main goal of this study. Rejection of minor antigen-mismatched grafts involves all branches of the immune system: adaptive immunity (T cells), humoral immunity (B cells) and innate immunity (DCs and antigen-presenting monocytes). Although we showed that depletion of one cell type dampens or omits the inflammatory outcome of minor antigen-mismatched graft rejection, here we show how lymphoid and myeloid cell subtypes act sequentially and in concert for the induction of minor antigen-mismatched graft rejection without the presence of PAMPs. In conclusion, we believe that understanding the mechanisms of action against minor antigen-mismatched graft rejection will also apply more broadly to other diseases such as autoimmunity and cancer, in which an immune response is elicited against self-associated minor antigens.

Ly6C(+) monocyte efferocytosis and cross-presentation of cell-associated antigens

Recently it was shown that circulating Ly6C⁺ monocytes traffic from tissue to the draining lymph nodes (LNs) with minimal alteration in their overall phenotype. Furthermore, in the steady state, Ly6C⁺ monocytes are as abundant as classical dendritic cells (DCs) within the draining LNs, and even more abundant during inflammation. However, little is known about the functional roles of constitutively trafficking Ly6C⁺ monocytes. In this study we investigated whether Ly6C⁺ monocytes can efferocytose (acquire dying cells) and cross-present cell-associated antigen, a functional property particularly attributed to Batf3⁺ DCs. We demonstrated that Ly6C⁺ monocytes intrinsically efferocytose and cross-present cell-associated antigen to CD8⁺ T cells. In addition, efferocytosis was enhanced upon direct activation of the Ly6C⁺ monocytes through its corresponding TLRs, TLR4 and TLR7. However, only ligation of TLR7, and not TLR4, enhanced cross-presentation by Ly6C⁺ monocytes. Overall, this study outlines two functional roles, among others, that Ly6C⁺ monocytes have during an adaptive immune response.

Flow Cytometric Analysis of Mononuclear Phagocytes in Nondiseased Human Lung and Lung-Draining Lymph Nodes

RATIONALE

The pulmonary mononuclear phagocyte system is a critical host defense mechanism composed of macrophages, monocytes, monocyte-derived cells, and dendritic cells. However, our current characterization of these cells is limited because it is derived largely from animal studies and analysis of human mononuclear phagocytes from blood and small tissue resections around tumors.

OBJECTIVES

Phenotypic and morphologic characterization of mononuclear phagocytes that potentially access inhaled antigens in human lungs.

METHODS

We acquired and analyzed pulmonary mononuclear phagocytes from fully intact nondiseased human lungs (including the major blood vessels and draining lymph nodes) obtained en bloc from 72 individual donors. Differential labeling of hematopoietic cells via intrabronchial and intravenous administration of antibodies within the same lobe was used to identify extravascular tissue-resident mononuclear phagocytes and exclude cells within the vascular lumen. Multiparameter flow cytometry was used to identify mononuclear phagocyte populations among cells labeled by each route of antibody delivery.

MEASUREMENTS AND MAIN RESULTS

We performed a phenotypic analysis of pulmonary mononuclear phagocytes isolated from whole nondiseased human lungs and lung-draining lymph nodes. Five pulmonary mononuclear phagocytes were observed, including macrophages, monocyte-derived cells, and dendritic cells that were phenotypically distinct from cell populations found in blood.

CONCLUSIONS

Different mononuclear phagocytes, particularly dendritic cells, were labeled by intravascular and intrabronchial antibody delivery, countering the notion that tissue and blood mononuclear phagocytes are equivalent systems. Phenotypic descriptions of the mononuclear phagocytes in nondiseased lungs provide a precedent for comparative studies in diseased lungs and potential targets for therapeutics.

Cutting Edge: Roles for Batf3-Dependent APCs in the Rejection of Minor Histocompatibility Antigen-Mismatched Grafts

In transplantation, a major obstacle for graft acceptance in MHC-matched individuals is the mismatch of minor histocompatibility Ags. Minor histocompatibility Ags are peptides derived from polymorphic proteins that can be presented by APCs on MHC molecules. The APC subtype uniquely responsible for the rejection of minor Ag-mismatched grafts has not yet been identified. In this study, we examined graft rejection in three mouse models: 1) mismatch of male-specific minor Ags, 2) mismatch of minor Ags distinct from male-specific minor Ags, and 3) skin transplant. This study demonstrates that in the absence of pathogen-associated molecular patterns, Batf3-dependent dendritic cells elicit the rejection of cells and grafts expressing mismatched minor Ags. The implication of our findings in clinical transplantation may be significant, as minor Ag reactivity has been implicated in the pathogenesis of multiple allograft tissues.

Rapid CD4+ T-cell responses to bacterial flagellin require dendritic cell expression of Syk and CARD9

Toll-like receptors (TLRs) can recognize microbial patterns and utilize adaptor molecules, such as-MyD88 or (TRIF TIR-domain-containing adapter-inducing interferon-β), to initiate downstream signaling that ultimately affects the initiation of adaptive immunity. In addition to this inflammatory role, TLR5 expression on dendritic cells can favor antigen presentation of flagellin peptides and thus increase the sensitivity of flagellin-specific T-cell responses in vitro and in vivo. Here, we examined the role of alternative signaling pathways that might regulate flagellin antigen presentation in addition to MyD88. These studies suggest a requirement for spleen tyrosine kinase, a noncanonical TLR-signaling adaptor molecule, and its downstream molecule CARD9 in regulating the sensitivity of flagellin-specific CD4(+) T-cell responses in vitro and in vivo. Thus, a previously unappreciated signaling pathway plays an important role in regulating the dominance of flagellin-specific T-cell responses.

Direct visualization of endogenous Salmonella-specific B cells reveals a marked delay in clonal expansion and germinal center development

CD4(+) T cells and B cells are both essential for acquired immunity to Salmonella infection. It is well established that Salmonella inhibit host CD4(+) T-cell responses, but a corresponding inhibitory effect on B cells is less well defined. Here, we utilize an Ag tetramer and pull-down enrichment strategy to directly visualize OVA-specific B cells in mice, as they respond to infection with Salmonella-OVA. Surprisingly, OVA-specific B-cell expansion and germinal center formation was not detected until bacteria were cleared from the host. Furthermore, Salmonella infection also actively inhibited both B- and T-cell responses to the same coinjected Ag but this did not require the presence of iNOS. The Salmonella Pathogenicity Island 2 (SPI2) locus has been shown to be responsible for inhibition of Salmonella-specific CD4(+) T-cell responses, and an examination of SPI2-deficient bacteria demonstrated a recovery in B-cell expansion in infected mice. Together, these data suggest that Salmonella can simultaneously inhibit host B- and T-cell responses using SPI2-dependent mechanisms.

Toll-like receptor and inflammasome signals converge to amplify the innate bactericidal capacity of T helper 1 cells

T cell effector functions can be elicited by noncognate stimuli, but the mechanism and contribution of this pathway to the resolution of intracellular macrophage infections have not been defined. Here, we show that CD4(+) T helper 1 (Th1) cells could be rapidly stimulated by microbe-associated molecular patterns during active infection with Salmonella or Chlamydia. Further, maximal stimulation of Th1 cells by lipopolysaccharide (LPS) did not require T-cell-intrinsic expression of toll-like receptor 4 (TLR4), interleukin-1 receptor (IL-1R), or interferon-γ receptor (IFN-γR) but instead required IL-18R, IL-33R, and adaptor protein MyD88. Innate stimulation of Th1 cells also required host expression of TLR4 and inflammasome components that together increased serum concentrations of IL-18. Finally, the elimination of noncognate Th1 cell stimulation hindered the resolution of primary Salmonella infection. Thus, the in vivo bactericidal capacity of Th1 cells is regulated by the response to noncognate stimuli elicited by multiple innate immune receptors.

CD103-CD11b+ dendritic cells regulate the sensitivity of CD4 T-cell responses to bacterial flagellin

Toll-like receptor 5 (TLR5) has been widely studied in an inflammatory context, but the effect of TLR5 on the adaptive response to bacterial flagellin has received considerably less attention. Here, we demonstrate that TLR5 expression by dendritic cells (DCs) allows a 1,000-fold enhancement of T-cell sensitivity to flagellin, and this enhancement did not require the expression of NLRC4 or Myd88. The effect of TLR5 on CD4 T-cell sensitivity was independent of the adjuvant effect of flagellin and TLR5 ligation did not alter the sensitivity of ovalbumin (OVA)-specific T cells to OVA. In the spleen, the exquisite T-cell sensitivity to flagellin was regulated by CD4-CD8α- DCs and was blocked by a monoclonal antibody to TLR5. In the mesenteric lymph nodes, flagellin-specific T-cell activation was regulated by a population of CD103-CD11b+ DCs. Thus, TLR5 expression by mucosal and systemic DC subsets controls the sensitivity of the adaptive immune response to flagellated pathogens.

Increased susceptibility to Salmonella infection in signal regulatory protein α-deficient mice

Recent studies have shed light on the connection between elevated erythropoetin production/spleen erythropoiesis and increased susceptibility to Salmonella infection. In this article, we provide another mouse model, the SIRPα-deficient (Sirpα⁻/⁻) mouse, that manifests increased erythropoiesis as well as heightened susceptibility to Salmonella infection. Sirpα⁻/⁻ mice succumbed to systemic infection with attenuated Salmonella, possessing significantly higher bacterial loads in both the spleen and the liver. Moreover, Salmonella-specific Ab production and Ag-specific CD4 T cells were reduced in Sirpα⁻/⁻ mice compared with wild-type controls. To further characterize the potential mechanism underlying SIRPα-dependent Ag-specific CD4 T cell priming, we demonstrate that lack of SIRPα expression on dendritic cells results in less efficient Ag processing and presentation in vitro. Collectively, these findings demonstrate an indispensable role of SIRPα for protective immunity to Salmonella infection.

TLR5-deficient mice lack basal inflammatory and metabolic defects but exhibit impaired CD4 T cell responses to a flagellated pathogen

TLR5-deficient mice have been reported to develop spontaneous intestinal inflammation and metabolic abnormalities. However, we report that TLR5-deficient mice from two different animal colonies display no evidence of basal inflammatory disease, metabolic abnormalities, or enhanced resistance to Salmonella infection. In contrast, the absence of TLR5 hindered the initial activation and clonal expansion of intestinal flagellin-specific CD4 T cells following oral Salmonella infection. Together, these data demonstrate that a basal inflammatory phenotype is not a consistent feature of TLR5-deficient mice and document a novel role for TLR5 in the rapid targeting of flagellin by intestinal pathogen-specific CD4 T cells.

TLR5 functions as an endocytic receptor to enhance flagellin-specific adaptive immunity

Innate immune activation via TLR induces dendritic cell maturation and secretion of inflammatory mediators, generating favorable conditions for naïve T-cell activation. Here, we demonstrate a previously unknown function for TLR5, namely that it enhances MHC class-II presentation of flagellin epitopes to CD4(+) T cells and is required for induction of robust flagellin-specific adaptive immune responses. Flagellin-specific CD4(+) T cells expanded poorly in TLR5-deficient mice immunized with flagellin, a deficiency that persisted even when additional TLR agonists were provided. Flagellin-specific IgG responses were similarly depressed in the absence of TLR5. In marked contrast, TLR5-deficient mice developed robust flagellin-specific T-cell responses when immunized with processed flagellin peptide. Surprisingly, the adaptor molecule Myd88 was not required for robust CD4(+) T-cell responses to flagellin, indicating that TLR5 enhances flagellin-specific CD4(+) T-cell responses in the absence of conventional TLR signaling. A requirement for TLR5 in generating flagellin-specific CD4(+) T-cell activation was also observed when using an in vitro dendritic cell culture system. Together, these data uncover an Myd88-independent function for dendritic cell TLR5 in enhancing the presentation of peptides to flagellin-specific CD4(+) T cells.