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Changes in blood vessels and lymphatics in health and disease are easier to understand and interpret when studied microscopically in three dimensions. The mouse trachea is a simple, yet powerful, and versatile model system in which to achieve this. We describe practical immunohistochemical methods for fluorescence and confocal microscopy of wholemounts of the mouse trachea to achieve this purpose in which the entire vasculature can be visualized from the organ level to the cellular and subcellular level. Blood vessels and lymphatics have highly stereotyped vascular architectures that repeat in arcades between the tracheal cartilages. Arterioles, capillaries, and venules can be easily identified for the blood vessels, while the lymphatics consist of initial lymphatics and collecting lymphatics. Even small abnormalities in either blood vessels or lymphatics can be noticed and evaluated in three dimensions. We and others have used the mouse trachea for examining in situ angiogenesis and lymphangiogenesis, vascular development and regression, vessel patency, differences in transgenic mice, and pathological changes, such as increased vascular permeability induced by inflammatory mediators., (© 2022. The Author(s), under exclusive license to Springer Science+Business Media, LLC, part of Springer Nature.)

Mast cells (MC) and myeloid dendritic cells (DC) act proximally in detecting and processing antigens and immune insults. We sought to understand their comparative dynamic behavior with respect to the airway epithelium in the steady state and in response to an allergic stimulus in mouse trachea. We devised methods to label MC in living trachea and to demonstrate that MC and DC occupy distinct layers of the tracheal mucosa, with DC being closer to the lumen. DC numbers doubled after allergen challenge, but MC numbers remained stable. MC and DC migrated minimally in either steady state or allergen-challenge conditions, and their interactions with one another appeared to be stochastic and relatively infrequent. While DC, unlike MC, exhibited probing behaviors involving dendrites, these projections did not cross the epithelium into the airway lumen. MC typically were located too far from the epithelial surface to contact the tracheal lumen. However, MC had protrusions toward and into blood vessels, likely to load with IgE. Thus, DC and MC occupy distinct niches and engage in sessile surveillance in the mouse trachea. Little or no access of these cell types to the airway lumen suggests that trans-epithelial transport of proteins in the steady state would be required for them to access luminal antigens.

These studies used bi-transgenic Clara cell secretory protein (CCSP)/IL-1β mice that conditionally overexpress IL-1β in Clara cells to determine whether IL-1β can promote angiogenesis and lymphangiogenesis in airways. Doxycycline treatment induced rapid, abundant, and reversible IL-1β production, influx of neutrophils and macrophages, and conspicuous and persistent lymphangiogenesis, but surprisingly no angiogenesis. Gene profiling showed many up-regulated genes, including chemokines (Cxcl1, Ccl7), cytokines (tumor necrosis factor α, IL-1β, and lymphotoxin-β), and leukocyte genes (S100A9, Aif1/Iba1). Newly formed lymphatics persisted after IL-1β overexpression was stopped. Further studies examined how IL1R1 receptor activation by IL-1β induced lymphangiogenesis. Inactivation of vascular endothelial growth factor (VEGF)-C and VEGF-D by adeno-associated viral vector-mediated soluble VEGFR-3 (VEGF-C/D Trap) completely blocked lymphangiogenesis, showing its dependence on VEGFR-3 ligands. Consistent with this mechanism, VEGF-C immunoreactivity was present in some Aif1/Iba1-immunoreactive macrophages. Because neutrophils contribute to IL-1β-induced lung remodeling in newborn mice, we examined their potential role in lymphangiogenesis. Triple-transgenic CCSP/IL-1β/CXCR2(-/-) mice had the usual IL-1β-mediated lymphangiogenesis but no neutrophil recruitment, suggesting that neutrophils are not essential. IL1R1 immunoreactivity was found on some epithelial basal cells and neuroendocrine cells, suggesting that these cells are targets of IL-1β, but was not detected on lymphatics, blood vessels, or leukocytes. We conclude that lymphangiogenesis triggered by IL-1β overexpression in mouse airways is driven by VEGF-C/D from macrophages, but not neutrophils, recruited by chemokines from epithelial cells that express IL1R1., (Copyright © 2013 American Society for Investigative Pathology. Published by Elsevier Inc. All rights reserved.)

Blood vessel leakiness is an early, transient event in acute inflammation but can also persist as vessels undergo remodeling in sustained inflammation. Angiopoietin/Tie2 signaling can reduce the leakiness through changes in endothelial cells. The role of pericytes in this action has been unknown. We used the selective PDGF-B-blocking oligonucleotide aptamer AX102 to determine whether disruption of pericyte-endothelial crosstalk alters vascular leakiness or remodeling in the airways of mice under four different conditions: i) baseline, ii) acute inflammation induced by bradykinin, iii) sustained inflammation after 7-day infection by the respiratory pathogen Mycoplasma pulmonis, or iv) leakage after bradykinin challenge in the presence of vascular stabilization by the angiopoietin-1 (Ang1) mimic COMP-Ang1 for 7 days. AX102 reduced pericyte coverage but did not alter the leakage of microspheres from tracheal blood vessels at baseline or after bradykinin; however, AX102 exaggerated leakage at 7 days after M. pulmonis infection and increased vascular remodeling and disease severity at 14 days. AX102 also abolished the antileakage effect of COMP-Ang1 at 7 days. Together, these findings show that pericyte contributions to endothelial stability have greater dependence on PDGF-B during the development of sustained inflammation, when pericyte dynamics accompany vascular remodeling, than under baseline conditions or in acute inflammation. The findings also show that the antileakage action of Ang1 requires PDGF-dependent actions of pericytes in maintaining endothelial stability., (Copyright © 2011 American Society for Investigative Pathology. Published by Elsevier Inc. All rights reserved.)

Angiogenesis and microvascular remodeling are features of chronic airway inflammation caused by Mycoplasma pulmonis infection in rats. As airway blood vessels undergo remodeling, they become unusually sensitive to substance P-induced plasma leakage. Here we determined whether the remodeled vessels are leaky under baseline conditions, whether their heightened sensitivity is specific to substance P, and whether the leakage is reversible. Four weeks after infection, the amount of baseline leakage of Evans blue in the tracheal mucosa was two to five times the normal level. Gaps < 1 microm in diameter were located between endothelial cells in some remodeled vessels. Substance P, but not platelet-activating factor or 5-hydroxytryptamine, produced an exaggerated leakage response. Inhalation of the beta2-adrenergic receptor agonist salmeterol reduced the leakage by <60%. We conclude that the blood vessel remodeling after M. pulmonis infection is associated with microvascular leakiness due, in part, to the formation of endothelial gaps. This leakage is accompanied by an abnormal sensitivity to substance P but not to platelet-activating factor or 5-hydroxytryptamine and can be reduced by beta2-agonists.

1. This study sought to determine whether neurogenic inflammation occurs in the airways by examining the effects of capsaicin or substance P on microvascular plasma leakage in the trachea and lungs of male pathogen-free C57BL/6 mice. 2. Single bolus intravenous injections of capsaicin (0.5 and 1 micromol kg(-1), i.v.) or substance P (1, 10 and 37 nmol kg(-10, i.v.) failed to induce significant leakage in the trachea, assessed as extravasation of Evans blue dye, but did induce leakage in the urinary bladder and skin. 3. Pretreatment with captopril (2.5 mg kg(-1), i.v.), a selective inhibitor of angiotensin converting enzyme (ACE), either alone or in combination with phosphoramidon (2.5 mg kg(-1), i.v.), a selective inhibitor of neutral endopeptidase (NEP), increased baseline leakage of Evans blue in the absence of any exogenous inflammatory mediator. The increase was reversed by the bradykinin B2 receptor antagonist Hoe 140 (0.1 mg kg(-1), i.v.). 4. After pretreatment with phosphoramidon and captopril, capsaicin increased the Evans blue leakage above the baseline in the trachea, but not in the lung. This increase was reversed by the tachykinin (NK1) receptor antagonist SR 140333 (0.7 mg kg(-1), i.v.), but not by the NK2 receptor antagonist SR 48968 (1 mg kg(-1), i.v.). 5. Experiments using Monastral blue pigment as a tracer localized the leakage to postcapillary venules in the trachea and intrapulmonary bronchi, although the labelled vessels were less numerous in mice than in comparably treated rats. Blood vessels of the pulmonary circulation were not labelled. 6. We conclude that neurogenic inflammation can occur in airways of pathogen-free mice, but only after the inhibition of enzymes that normally degrade inflammatory peptides. Neurogenic inflammation does not involve the pulmonary microvasculature.

Dendritic cells are antigen-presenting cells that constitutively express high levels of major histocompatibility complex class II (Ia) antigen on their plasma membrane. Previous studies have shown that the number of dendritic cells in the rat airway mucosa decreases rapidly after glucocorticoid treatment. We sought to determine whether apoptosis contributes to this steroid-induced cell decrease. Dendritic cells in tracheal whole mounts were revealed by immunoperoxidase staining using the OX-6 (anti-Ia) monoclonal antibody. In untreated rats, a dense network of Ia-immunoreactive (Ia+) cells with highly branched cytoplasmic processes was observed just beneath the tracheal epithelium (1,405 +/- 140 cells/mm2 mucosa; mean +/- SEM, n = 6). In rats treated with dexamethasone (10 mg/kg, intraperitoneally), four distinct changes in dendritic cell morphology were evident 4 to 8 h after injection: (1) appearance of large Ia+ granules in cytoplasmic processes, (2) narrowing of cytoplasmic processes, (3) loss of Ia immunoreactivity from the cell surface, and (4) fragmentation of cells into small Ia+ bodies. These changes accompanied a 56% decrease in the number of Ia+ cells over 8 h. The contribution of apoptosis to this decrease in Ia+ cells was determined by terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end-labeling (TUNEL) of nucleosomal DNA fragments in histologic sections. The number of TUNEL+ bodies increased from a control value of 174 +/- 47 bodies/mm2 mucosa to 2,108 +/- 294 bodies/mm2 mucosa at 4 h and 936 +/- 343 bodies/ mm2 mucosa at 8 h (n = 4 rats per time point). The location of TUNEL+ bodies closely corresponded to that of Ia+ cells stained in adjacent histologic sections. We conclude that apoptosis contributes to the rapid decrease in airway dendritic cells after glucocorticoid treatment.

Chronic inflammation is associated with blood vessel proliferation and enlargement and changes in vessel phenotype. We sought to determine whether these changes represent different types of angiogenesis and whether they are stimulus dependent. Chronic airway inflammation, produced by infection with Mycoplasma pulmonis, was compared in strains of mice known to be resistant (C57BL/6) or susceptible (C3H). Tracheal vascularity, assessed in whole mounts after Lycopersicon esculentum lectin staining, increased in both strains at 1, 2, 4, and 8 weeks after infection, but the type of vascular remodeling was different. The number of vessels doubled in tracheas of C57BL/6 mice, with corresponding increases of capillaries and venules. In contrast, neither the number nor the length of vessels changed in C3H mice. Instead, vessel diameter and endothelial cell number doubled, and the proportion of venules doubled with a corresponding decrease of capillaries. Although the infection had no effect on baseline plasma leakage, in both strains it potentiated the leakage produced by substance P. We conclude that the same stimulus can result in blood vessel proliferation or enlargement, depending on the host response. Endothelial cells proliferate in both cases, but in one case new capillaries form whereas in the other capillaries convert to venules.

In rat airways, substance P released from sensory nerves induces plasma leakage via neurokinin-1 (NK1) receptors on endothelial cells. In pathogen-free rats, both leakage and endothelial NK1 receptors are most abundant in postcapillary venules. In Mycoplasma pulmonis-infected rats, extensive angiogenesis occurs in the tracheal mucosa. The capillary-sized (< 10 microns in diameter) angiogenic blood vessels are abnormally sensitive to substance P. The aim of this study was to determine whether increased expression of NK1 receptors contributes to this abnormal sensitivity. Fischer 344 rats were infected with M. pulmonis and were challenged with substance P (5 micrograms/kg i.v.), and then plasma leakage in the tracheal mucosa was measured by extravasation of Monastral blue (30 mg/kg i.v.). NK1 receptors on endothelial cells were localized by immunohistochemistry. Five minutes after substance P, NK1 receptor-immunoreactive endosomes were five times more abundant in endothelial cells of angiogenic capillaries in M. pulmonis-infected rats than in corresponding capillaries in pathogen-free controls (17.1 +/- 2.3 vs. 3.5 +/- 0.4 endosomes/100 micron 2 of endothelial surface). Endosomes were slightly more abundant in postcapillary venules 15-35 microns in diameter in infected rats (23.0 +/- 0.6 vs. 19.2 +/- 0.7 endosomes/100 micron 2). Similarly, after substance P, angiogenic capillaries had much more Monastral blue labeling (area density: 18.8 +/- 1.5 vs. 2.9 +/- 0.5% of vessel wall), whereas postcapillary venules had about the same amount of labeling (36.0 +/- 3.7 vs. 34.1 +/- 1.8%). We conclude that increased expression of NK1 receptors, which are internalized into endosomes after ligand binding, contributes to the abnormal sensitivity of endothelial cells of angiogenic blood vessels to substance P in the airways of M. pulmonis-infected rats.

The involvement of neurokinin NK1 receptors in cigarette-induced adhesion of neutrophils and eosinophils to venules of the airway mucosa was investigated. Rats were pretreated with the NK1 receptor antagonist CP-99,994 (4 mg/kg IV), its vehicle, or its inactive enantiomer CP-100,263 before exposure to cigarette smoke. Adherent neutrophils and eosinophils were stained histochemically for endogenous peroxidase activity and were counted in tracheal whole mounts. Plasma leakage was quantified be stereological measurements of the extravasation of Monastral blue. Cigarette smoke induced the adhesion of 104 + 17 neutrophils and 10.4 +/- 1.7 eosinophils per square millimeter of mucosa. CP-99,994 reduced neutrophil and eosinophil adhesion by 66 and 61%, respectively, and reduced plasma extravasation by 61% (p < .05), but CP-100,263 had no significant effect. The inhibitory effects of CP-99,994 appeared to be specific because CP-99,994 had no effect on neutrophil and eosinophil adhesion, or on plasma extravasation induced by platelet activating factor, an inflammatory stimulus acting independently of NK1 receptors. These results suggest that NK1 receptors are involved in cigarette smoke-induced adhesion of neutrophils and eosinophils to the endothelium of venules in the rat tracheal mucosa.

Substance P released from sensory nerve fibers causes plasma leakage through an action on neurokinin-1 (NK1 or substance P) receptors. However, it is unknown whether the leakage results from a direct action of substance P on endothelial cells. We determined the distribution of NK1 receptors at sites of plasma leakage in the rat tracheal mucosa, using NK1 receptor-immunoreactive endosomes as markers of substance P-induced receptor internalization. We found that immunoreactive endosomes were located in the endothelial cells of venules and capillaries but not in those of arterioles. Five minutes after vagal stimulation for 1 min, the number of immunoreactive endosomes in endothelial cells was increased 5-fold in postcapillary venules (mean of 17.4 endosomes/100 micron2 compared with a baseline value of 3.4), 15-fold in collecting venules (12.1 compared with 0.8), and 4-fold in capillaries (2.5 compared with 0.7). No endosomes were found in arterioles under either condition. The number of immunoreactive endosomes in individual vessels corresponded to the amount of stimulus-induced plasma leakage. Both the receptor internalization and the plasma leakage were blocked by the selective NK1 receptor antagonist SR-140333 (100 microgram/kg iv). Although both substance P (5 microgram/kg iv) and platelet-activating factor (5 microgram/kg iv) caused plasma leakage, only substance P induced receptor internalization. We conclude that substance P, released from sensory nerve fibers, causes plasma leakage through a direct action on endothelial cells of venules, and that this action is followed by the internalization of NK1 receptors into endosomes.

6-Hydroxydopamine (6-OHDA) stimulates the release of catecholamines from sympathetic nerves. This stimulation has been proposed as the basis of the 6-OHDA-induced increase in vascular permeability in the rat knee joint. We sought to determine whether 6-OHDA increases vascular permeability in the rat trachea through a similar mechanism. We also sought to determine whether sympathetic nerves contribute to the plasma extravasation caused by stimulating sensory nerves with capsaicin. In anesthetized rats, an intratracheal infusion of 6-OHDA caused more Monastral blue extravasation than did an infusion of vehicle (area density, 23 +/- 3% vs. 9 +/- 1%). Chemical sympathectomy, which reduced the number of tyrosine hydroxylase-immunoreactive nerves by 98%, did not reduce the amount of extravasation induced by 6-OHDA. However, pretreatment with capsaicin, which reduced the number of substance P-immunoreactive nerves by 95%, reduced the Monastral blue extravasation induced by 6-OHDA by 98%. Extravasation induced by stimulating sensory nerves with capsaicin was not significantly different in tracheae with or without sympathetic nerves. We conclude that in the rat trachea infusion of 6-OHDA causes plasma extravasation by stimulating sensory nerves, not by stimulating sympathetic nerves. Furthermore, sympathetic nerves are not essential for the plasma extravasation induced by capsaicin.

The tracheal epithelium of pathogen-free rats consists mainly of serous-type secretory cells, ciliated cells, basal cells, and a few neuroendocrine cells. Mucus-containing goblet cells are rare. Calcitonin gene-related peptide (CGRP) is known to exist in the neuroendocrine cells and in sensory nerves of the tracheal mucosa and is released into the airway lumen by sensory nerve stimulation. In this study, we determined whether epithelial serous cells are another source of CGRP. Tracheas of adult male specific pathogen-free F344 rats were immunostained by an avidin-biotin technique either as whole mounts or as cryostat sections using two different polyclonal primary antibodies to rat CGRP. Some specimens were stained for CGRP-like immunofluorescence and examined with a confocal microscope. CGRP immunoreactivity was present in granules of serous cells throughout the trachea. In whole mounts, the stained cells were most abundant between the cartilaginous rings, especially in the rostral trachea, where they constituted 56% of the epithelial cells in contact with the tracheal lumen. Serous cells were easily distinguished from neuroendocrine cells and nerve fibers with CGRP immunoreactivity. In evidence that the CGRP immunoreactivity was specific, the staining of serous cells was abolished by omitting the primary antibody and by absorption with 10 micrograms/ml CGRP. Antibodies to substance P, vasoactive intestinal polypeptide, and tyrosine hydroxylase did not stain epithelial serous cells. An antibody to protein gene product 9.5 labeled neuroendocrine cells, but not serous cells. Injection of capsaicin (150 micrograms/kg intravenously), a substance known to degranulate epithelial serous cells, reduced the staining of the serous cells for CGRP.(ABSTRACT TRUNCATED AT 250 WORDS)

The trachea of guinea-pigs was stained as a whole-mount preparation with the zinc iodide-osmium technique. A distinct class of nerve endings was observed associated with the tracheal muscle. The endings, issued from myelinated fibres of the vagus nerve via the recurrent laryngeal nerve, are distributed on either side of the midline and ventral to the tips of cartilages. They are interpreted as afferent nerve endings that may correspond to slow adapting stretch receptors identified by physiological studies. Each nerve contributes predominantly, but not exclusively, to the receptors of the ipsilateral side. There are 120-180 receptors along the full length of the guinea-pig trachea, their density being higher at the cranial end. The receptors are variable in size and structural complexity, and, to some extent, also in spatial orientation, but distinct subtypes are not recognizable. Receptors of similar morphology and distribution are found also in the rat trachea. The receptors can also be visualized with a cytochrome oxidase method for nerve endings, but they do not stain with immunohistochemistry for the neuropeptides substance P, calcitonin gene-related peptide, vasointestinal polypeptide and neurotensin.

Changes in blood vessels and lymphatics in health and disease are easier to understand and interpret when studied microscopically in three dimensions. The mouse trachea is a simple, yet powerful, and versatile model system in which to achieve this. We describe practical immunohistochemical methods for fluorescence and confocal microscopy of wholemounts of the mouse trachea to achieve this purpose in which the entire vasculature can be visualized from the organ level to the cellular and subcellular level. Blood vessels and lymphatics have highly stereotyped vascular architectures that repeat in arcades between the tracheal cartilages. Arterioles, capillaries, and venules can be easily identified for the blood vessels, while the lymphatics consist of initial lymphatics and collecting lymphatics. Even small abnormalities in either blood vessels or lymphatics can be noticed and evaluated in three dimensions. We and others have used the mouse trachea for examining in situ angiogenesis and lymphangiogenesis, vascular development and regression, vessel patency, differences in transgenic mice, and pathological changes, such as increased vascular permeability induced by inflammatory mediators.

RationaleLymphatic vessels in the respiratory tract normally mature into a functional network during the neonatal period, but under some pathological conditions they can grow as enlarged, dilated sacs that result in the potentially lethal condition of pulmonary lymphangiectasia.ObjectiveWe sought to determine whether overexpression of the lymphangiogenic growth factor (vascular endothelial growth factor-C [VEGF-C]) can promote lymphatic growth and maturation in the respiratory tract. Unexpectedly, perinatal overexpression of VEGF-C in the respiratory epithelium led to a condition resembling human pulmonary lymphangiectasia, a life-threatening disorder of the newborn characterized by respiratory distress and the presence of widely dilated lymphatics.Methods and resultsAdministration of doxycycline to Clara cell secretory protein-reverse tetracycline-controlled transactivator/tetracycline operator-VEGF-C double-transgenic mice during a critical period from embryonic day 15.5 to postnatal day 14 was accompanied by respiratory distress, chylothorax, pulmonary lymphangiectasia, and high mortality. Enlarged sac-like lymphatics were abundant near major airways, pulmonary vessels, and visceral pleura. Side-by-side comparison revealed morphological features similar to pulmonary lymphangiectasia in humans. The condition was milder in mice given doxycycline after age postnatal day 14 and did not develop after postnatal day 35. Mechanistic studies revealed that VEGF recptor (VEGFR)-3 alone drove lymphatic growth in adult mice, but both VEGFR-2 and VEGFR-3 were required for the development of lymphangiectasia in neonates. VEGFR-2/VEGFR-3 heterodimers were more abundant in the dilated lymphatics, consistent with the involvement of both receptors. Despite the dependence of lymphangiectasia on VEGFR-2 and VEGFR-3, the condition was not reversed by blocking both receptors together or by withdrawing VEGF-C.ConclusionsThe findings indicate that VEGF-C overexpression can induce pulmonary lymphangiectasia during a critical period in perinatal development.