Treg-tissue cell interactions in repair and regeneration

Regulatory T (Treg) cells are classically known for their critical immunosuppressive functions that support peripheral tolerance. More recent work has demonstrated that Treg cells produce pro-repair mediators independent of their immunosuppressive function, a process that is critical to repair and regeneration in response to numerous tissue insults. These factors act on resident parenchymal and structural cells to initiate repair in a tissue-specific context. This review examines interactions between Treg cells and tissue-resident non-immune cells-in the context of tissue repair, fibrosis, and cancer-and discusses areas for future exploration.

Heparan sulfate regulates amphiregulin signaling towards reparative lung mesenchymal cells during influenza A infection

Amphiregulin (Areg), a growth factor produced by regulatory T (Treg) cells to facilitate tissue repair/regeneration, contains a heparan sulfate (HS) binding domain. How HS, a highly sulfated glycan subtype that alters growth factor signaling, influences Areg repair/regeneration functions is unclear. Here we report that inhibition of HS in various cell lines and primary lung mesenchymal cells (LMC) qualitatively alters downstream signaling and highlights the existence of HS-dependent vs. -independent Areg transcriptional signatures. Utilizing a panel of cell lines with targeted deletions in HS synthesis-related genes, we found that the presence of the glypican family of heparan sulfate proteoglycans is critical for Areg signaling and confirmed this dependency in primary LMC by siRNA-mediated knockdown. Furthermore, in the context of influenza A (IAV) infection in vivo , we found that an Areg-responsive subset of reparative LMC upregulate glypican-4 and HS. Conditional deletion of HS primarily within this LMC subset resulted in reduced blood oxygen saturation following infection with IAV, with no changes in viral load. Finally, we found that co-culture of HS-knockout LMC with IAV-induced Treg cells results in reduced LMC responses. Collectively, this study reveals the essentiality of HS on a specific lung mesenchymal population as a mediator of Treg cell-derived Areg reparative signaling during IAV infection.

Regulation of the alveolar regenerative niche by amphiregulin-producing regulatory T cells

Following respiratory viral infection, regeneration of the epithelial barrier is required to preserve lung function and prevent secondary infections. Lung regulatory T (Treg) cells are critical for maintaining blood oxygenation following influenza virus infection through production of the EGFR ligand amphiregulin (Areg); however, how Treg cells engage with progenitors within the alveolar niche is unknown. Here, we describe local interactions between Treg cells and an Areg-responsive population of Col14a1+EGFR+ lung mesenchymal cells that mediate type II alveolar epithelial (AT2) cell-mediated regeneration following influenza virus infection. We propose a mechanism whereby Treg cells are deployed to sites of damage and provide pro-survival cues that support mesenchymal programming of the alveolar niche. In the absence of fibroblast EGFR signaling, we observe impaired AT2 proliferation and disrupted lung remodeling following viral clearance, uncovering a crucial immune/mesenchymal/epithelial network that guides alveolar regeneration.

Comparative Study of SARS-CoV-2, SARS-CoV-1, MERS-CoV, HCoV-229E and Influenza Host Gene Expression in Asthma: Importance of Sex, Disease Severity, and Epithelial Heterogeneity

Epithelial characteristics underlying the differential susceptibility of chronic asthma to SARS-CoV-2 (COVID-19) and other viral infections are currently unclear. By revisiting transcriptomic data from patients with Th2 low versus Th2 high asthma, as well as mild, moderate, and severe asthmatics, we characterized the changes in expression of human coronavirus and influenza viral entry genes relative to sex, airway location, and disease endotype. We found sexual dimorphism in the expression of SARS-CoV-2-related genes ACE2, TMPRSS2, TMPRSS4, and SLC6A19. ACE2 receptor downregulation occurred specifically in females in Th2 high asthma, while proteases broadly assisting coronavirus and influenza viral entry, TMPRSS2, and TMPRSS4, were highly upregulated in both sexes. Overall, changes in SARS-CoV-2-related gene expression were specific to the Th2 high molecular endotype of asthma and different by asthma severity and airway location. The downregulation of ACE2 (COVID-19, SARS) and ANPEP (HCoV-229E) viral receptors wascorrelated with loss of club and ciliated cells in Th2 high asthma. Meanwhile, the increase in DPP4 (MERS-CoV), ST3GAL4, and ST6GAL1 (influenza) was associated with increased goblet and basal activated cells. Overall, this study elucidates sex, airway location, disease endotype, and changes in epithelial heterogeneity as potential factors underlying asthmatic susceptibility, or lack thereof, to SARS-CoV-2.

Eosinophil accumulation in postnatal lung is specific to the primary septation phase of development

Type 2 immune cells and eosinophils are transiently present in lung tissue not only in pathology but also during normal postnatal development. However, the lung developmental processes underlying postnatal airway recruitment of eosinophils after birth remain unexplored. We determined that in mice, mature eosinophils are recruited to the lung during P3–14, which corresponds to the primary septation/alveolarization phase of lung development. Developmental eosinophils peaked during P10–14 and exhibited Siglec-Fmed/highCD11c−/low phenotypes, similar to allergic asthma models. By interrogating the lung transcriptome and proteome during peak eosinophil recruitment in postnatal development, we identified markers that functionally capture mesenchymal-epithelial interface establishment (Nes, Smo, Wnt5a, Nog) and provisional extracellular matrix (ECM) deposition (Tnc, Postn, Spon2, Thbs2) as key lung morphogenetic events associating with eosinophils. We identifiedTenascin-C (TNC) as one of the key ECM markers in the lung epithelial-mesenchymal interface at the RNA and protein levels, consistently associating with eosinophils in development and disease. By RNA-seq analysis, naïve murine eosinophils cultured with TNC-enriched ECM significantly induced expression of Siglec-F, CD11c, eosinophil peroxidase, and other markers typical for activated eosinophils in development and allergic inflammatory responses. TNC knockout mice had an altered eosinophil recruitment profile in development. Collectively, our results indicate that lung morphogenetic processes associated with heightened Type 2 immunity are not merely tissue “background” but specifically guide immune cells both in development and pathology.

Eosinophil accumulation in postnatal lung is specific to the primary septation phase of development

Type 2 immune cells and eosinophils are transiently present in the lung tissue not only in pathology (allergic disease, parasite expulsion) but also during normal postnatal development. However, the lung developmental processes underlying airway recruitment of eosinophils after birth remain unexplored. We determined that in mice, mature eosinophils are transiently recruited to the lung during postnatal days 3-14, which specifically corresponds to the primary septation/alveolarization phase of lung development. Developmental eosinophils peaked during P10-14 and exhibited Siglec-Fmed/highCD11c-/low phenotypes, similar to allergic asthma models. By interrogating the lung transcriptome and proteome during peak eosinophil recruitment in postnatal development, we identified markers that functionally capture the establishment of the mesenchymal-epithelial interface (Nes, Smo, Wnt5a, Nog) and the deposition of the provisional extracellular matrix (ECM) (Tnc, Postn, Spon2, Thbs2) as a key lung morphogenetic event associating with eosinophils. Tenascin-C (TNC) was identified as one of the key ECM markers in the lung epithelial-mesenchymal interface both at the RNA and protein levels, consistently associating with eosinophils in development and disease in mice and humans. As determined by RNA-seq analysis, naïve murine eosinophils cultured with ECM enriched in TNC significantly induced expression of Siglec-F, CD11c, eosinophil peroxidase, and other markers typical for activated eosinophils in development and allergic inflammatory responses. TNC knockout mice had an altered eosinophil recruitment profile in development. Collectively, our results indicate that lung morphogenetic processes associated with heightened Type 2 immunity are not merely a tissue "background" but specifically guide immune cells both in development and pathology.

Fibrinogen Is a Specific Trigger for Cytolytic Eosinophil Degranulation

In inflamed human tissues, we often find intact eosinophilic granules, but not eosinophils themselves. Eosinophils, tissue-dwelling granulocytes with several homeostatic roles, have a surprising association with fibrinogen and tissue remodeling. Fibrinogen is a complex glycoprotein with regulatory roles in hemostasis, tumor development, wound healing, and atherogenesis. Despite its significance, the functional link between eosinophils and fibrinogen is not understood. We tested IL-5-primed mouse bone marrow-derived and human blood-sorted eosinophil activity against FITC-linked fibrinogen substrates. The interactions between these scaffolds and adhering eosinophils were quantified using three-dimensional laser spectral, confocal, and transmission electron microscopy. Eosinophils were labeled with major basic protein (MBP) Ab to visualize granules and assessed by flow cytometry. Both mouse and human eosinophils showed firm adhesion and degraded up to 27 ± 3.1% of the substrate area. This co-occurred with active MBP-positive granule release and the expression of integrin CD11b. Mass spectrometry analysis of fibrinogen proteolytic reactions detected the presence of eosinophil peroxidase, MBP, and fibrin α-, β-, and γ-chains. Eosinophil activity was adhesion dependent, as a blocking Ab against CD11b significantly reduced adhesion, degranulation, and fibrinogenolysis. Although adhered, eosinophils exhibited no proteolytic activity on collagen matrices. Cytolytic degranulation was defined by loss of membrane integrity, cell death, and presence of cell-free granules. From transmission electron microscopy images, we observed only fibrinogen-exposed eosinophils undergoing this process. To our knowledge, this is the first report to show that fibrinogen is a specific trigger for cytolytic eosinophil degranulation with implications in human disease.

Tetraspanin CD151 Is a Negative Regulator of FcεRI-Mediated Mast Cell Activation

Mast cells are critical in the pathogenesis of allergic disease due to the release of preformed and newly synthesized mediators, yet the mechanisms controlling mast cell activation are not well understood. Members of the tetraspanin family are recently emerging as modulators of FcεRI-mediated mast cell activation; however, mechanistic understanding of their function is currently lacking. The tetraspanin CD151 is a poorly understood member of this family and is specifically induced on mouse and human mast cells upon FcεRI aggregation but its functional effects are unknown. In this study, we show that CD151 deficiency significantly exacerbates the IgE-mediated late phase inflammation in a murine model of passive cutaneous anaphylaxis. Ex vivo, FcεRI stimulation of bone marrow-derived mast cells from CD151(-/-) mice resulted in significantly enhanced expression of proinflammatory cytokines IL-4, IL-13, and TNF-α compared with wild-type controls. However, FcεRI-induced mast cell degranulation was unaffected. At the molecular signaling level, CD151 selectively regulated IgE-induced activation of ERK1/2 and PI3K, associated with cytokine production, but had no effect on the phospholipase Cγ1 signaling, associated with degranulation. Collectively, our data indicate that CD151 exerts negative regulation over IgE-induced late phase responses and cytokine production in mast cells.

Is Rapoport's rule a recent phenomenon? A deep time perspective on potential causal mechanisms

Macroecology strives to identify ecological patterns on broad spatial and temporal scales. One such pattern, Rapoport's rule, describes the tendency of species' latitudinal ranges to increase with increasing latitude. Several mechanisms have been proposed to explain this rule. Some invoke climate, either through glaciation driving differential extinction of northern species or through increased seasonal variability at higher latitudes causing higher thermal tolerances and subsequently larger ranges. Alternatively, continental tapering or higher interspecific competition at lower latitudes may be responsible. Assessing the incidence of Rapoport's rule through deep time can help to distinguish between competing explanations. Using fossil occurrence data from the Palaeobiology Database, we test these hypotheses by evaluating mammalian compliance with the rule throughout the Caenozoic of North America. Adherence to Rapoport's rule primarily coincides with periods of intense cooling and increased seasonality, suggesting that extinctions caused by changing climate may have played an important role in erecting the latitudinal gradients in range sizes seen today.