Interstitial matrix and transendothelial fluxes in normal lung

Correlative 3D Imaging and Microfluidic Modelling of Human Pulmonary Lymphatics using Immunohistochemistry and High-resolution μCT

Lung lymphatics maintain fluid homoeostasis by providing a drainage system that returns fluid, cells and metabolites to the circulatory system. The 3D structure of the human pulmonary lymphatic network is essential to lung function, but it is poorly characterised. Image-based 3D mathematical modelling of pulmonary lymphatic microfluidics has been limited by the lack of accurate and representative image geometries. This is due to the microstructural similarity of the lymphatics to the blood vessel network, the lack of lymphatic-specific biomarkers, the technical limitations associated with image resolution in 3D, and sectioning artefacts present in 2D techniques. We present a method that combines lymphatic specific (D240 antibody) immunohistochemistry (IHC), optimised high-resolution X-ray microfocus computed tomography (μCT) and finite-element mathematical modelling to assess the function of human peripheral lung tissue. The initial results identify lymphatic heterogeneity within and between lung tissue. Lymphatic vessel volume fraction and fractal dimension significantly decreases away from the lung pleural surface (p < 0.001, n = 25 and p < 0.01, n = 20, respectively). Microfluidic modelling successfully shows that in lung tissue the fluid derived from the blood vessels drains through the interstitium into the lymphatic vessel network and this drainage is different in the subpleural space compared to the intralobular space. When comparing lung tissue from health and disease, human pulmonary lymphatics were significantly different across five morphometric measures used in this study (p ≤ 0.0001). This proof of principle study establishes a new engineering technology and workflow for further studies of pulmonary lymphatics and demonstrates for the first time the combination of correlative μCT and IHC to enable 3D mathematical modelling of human lung microfluidics at micrometre resolution.

Pulmonary lymphatic vessel morphology: a review

Our understanding of lymphatic vessels has been advanced by the recent identification of relatively specific lymphatic endothelium markers, including Prox-1, VEGFR3, podoplanin and LYVE-1. The use of lymphatic markers has led to the observation that, contrary to previous assumptions, human lymphatic vessels extend deep inside the pulmonary lobule, either in association with bronchioles, intralobular arterioles or small pulmonary veins. Pulmonary lymphatic vessels may thus be classified into pleural, interlobular (in interlobular septa) and intralobular. Intralobular lymphatic vessels may be further subdivided in: bronchovascular (associated with a bronchovascular bundle), perivascular (associated with a blood vessel), peribronchiolar (associated with a bronchiole), and interalveolar (in interalveolar septa). Most of the intralobular lymphatic vessels are in close contact with a blood vessel, either alone or within a bronchovascular bundle. A minority is associated with a bronchiole, and small lymphatics are occasionally present even in interalveolar septa, seemingly independent of blood vessels or bronchioles. The lymphatics of the interlobular septa often contain valves, are usually associated with the pulmonary veins, and connect with the pleural lymphatics. The large lymphatics associated with bronchovascular bundles have similar characteristics to pleural and interlobular lymphatics and may be considered conducting vessels. The numerous small perivascular lymphatics and the few peribronchiolar ones that are found inside the lobule are probably the absorbing compartment of the lung responsible for maintaining the alveolar interstitium relatively dry in order to provide a minimal thickness of the air-blood barrier and thus optimize gas diffusion. These lymphatic populations could be differentially involved in the pathogenesis of diseases preferentially involving distinct lung compartments.

Cytoskeletal dynamics and lung fluid balance

This article examines the role of the endothelial cytoskeleton in the lung's ability to restrict fluid and protein to vascular space at normal vascular pressures and thereby to protect lung alveoli from lethal flooding. The barrier properties of microvascular endothelium are dependent on endothelial cell contact with other vessel-wall lining cells and with the underlying extracellular matrix (ECM). Focal adhesion complexes are essential for attachment of endothelium to ECM. In quiescent endothelial cells, the thick cortical actin rim helps determine cell shape and stabilize endothelial adherens junctions and focal adhesions through protein bridges to actin cytoskeleton. Permeability-increasing agonists signal activation of "small GTPases" of the Rho family to reorganize the actin cytoskeleton, leading to endothelial cell shape change, disassembly of cortical actin rim, and redistribution of actin into cytoplasmic stress fibers. In association with calcium- and Src-regulated myosin light chain kinase (MLCK), stress fibers become actinomyosin-mediated contractile units. Permeability-increasing agonists stimulate calcium entry and induce tyrosine phosphorylation of VE-cadherin (vascular endothelial cadherin) and β-catenins to weaken or pull apart endothelial adherens junctions. Some permeability agonists cause latent activation of the small GTPases, Cdc42 and Rac1, which facilitate endothelial barrier recovery and eliminate interendothelial gaps. Under the influence of Cdc42 and Rac1, filopodia and lamellipodia are generated by rearrangements of actin cytoskeleton. These motile evaginations extend endothelial cell borders across interendothelial gaps, and may initiate reannealing of endothelial junctions. Endogenous barrier protective substances, such as sphingosine-1-phosphate, play an important role in maintaining a restrictive endothelial barrier and counteracting the effects of permeability-increasing agonists.

A coupled discrete/continuum model for describing cancer-therapeutic transport in the lung

We propose a computational simulation framework for describing cancer-therapeutic transport in the lung. A discrete vascular graph model (VGM) is coupled to a double-continuum model (DCM) to determine the amount of administered therapeutic agent that will reach the cancer cells. An alveolar cell carcinoma is considered. The processes in the bigger blood vessels (arteries, arterioles, venules and veins) are described by the VGM. The processes in the alveolar capillaries and the surrounding tissue are represented by a continuum approach for porous media. The system of equations of the coupled discrete/continuum model contains terms that account for degradation processes of the therapeutic agent, the reduction of the number of drug molecules by the lymphatic system and the interaction of the drug with the tissue cells. The functionality of the coupled discrete/continuum model is demonstrated in example simulations using simplified pulmonary vascular networks, which are designed to show-off the capabilities of the model rather than being physiologically accurate.

Morphometric analysis of intralobular, interlobular and pleural lymphatics in normal human lung

In spite of their presumed relevance in maintaining interalveolar septal fluid homeostasis, the knowledge of the anatomy of human lung lymphatics is still incomplete. The recent discovery of reliable markers specific for lymphatic endothelium has led to the observation that, contrary to previous assumptions, human lymphatic vessels extend deep inside the pulmonary lobule in association with bronchioles, intralobular arterioles or small pulmonary veins. The aim of this study was to provide a morphometric characterization of lymphatic vessels in the periphery of the human lung. Human lung sections were immunolabelled with the lymphatic marker D2-40, followed by blood vessel staining with von Willebrand Factor. Lymphatic vessels were classified into: intralobular (including those associated with bronchovascular bundles, perivascular, peribronchiolar and interalveolar), pleural (in the connective tissue of the visceral pleura), and interlobular (in interlobular septa). The percentage area occupied by the lymphatic lumen was much greater in the interlobular septa and in the subpleural space than in the lobule. Most of the intralobular lymphatic vessels were in close contact with a blood vessel, either alone or within a bronchovascular bundle, whereas 7% were associated with a bronchiole and < 1% were not connected to blood vessels or bronchioles (interalveolar). Intralobular lymphatic size progressively decreased from bronchovascular through to peribronchiolar, perivascular and interalveolar lymphatics. Lymphatics associated with bronchovascular bundles had similar morphometric characteristics to pleural and interlobular lymphatics. Shape factors were similar across lymphatic populations, except that peribronchiolar lymphatics had a marginally increased roundness and circularity, suggesting a more regular shape due to increased filling, and interlobular lymphatics had greater elongation, due to a greater proportion of conducting lymphatics cut longitudinally. Unsupervised cluster analysis confirmed a marked heterogeneity of lymphatic vessels both within and between groups, with a cluster of smaller vessels specifically represented in perivascular and interalveolar lymphatics within the alveolar interstitium. Our data indicate that intralobular lymphatics are a heterogeneous population, including vessels surrounding the bronchovascular bundle analogous to the conducting vessels present in the pleural and interlobular septa, many small perivascular lymphatics responsible for maintaining fluid balance in the alveolar interstitium, and a minority of intermediate lymphatics draining the peripheral airways. These lymphatic populations could be differentially involved in the pathogenesis of diseases preferentially involving distinct lung compartments.

Toward managing chronic rejection after lung transplant: the fate and effects of inhaled cyclosporine in a complex environment

The fate and effects of inhaled cyclosporine A (CsA) are considered after deposition on the lung surface. Special emphasis is given to a post-lung transplant environment and to the potential effects of the drug on the various cell types it is expected to encounter. The known stability, metabolism, pharmacokinetics and pharmacodynamics of the drug have been reviewed and discussed in the context of the lung microenvironment. Arguments support the contention that the immuno-inhibitory and anti-inflammatory effects of CsA are not restricted to T-cells. It is likely that pharmacologically effective concentrations of CsA can be sustained in the lungs but due to the complexity of uptake and action, the elucidation of effective posology must ultimately rely on clinical evidence.

Hemorrhagic shock and resuscitation-mediated tissue water distribution is normalized by adjunctive peritoneal resuscitation

BACKGROUND

Adjunctive direct peritoneal resuscitation (DPR) from hemorrhagic shock (HS) improves intestinal blood flow and abrogates postresuscitation edema. HS causes water shifts as a result of sodium redistribution and changes in transcapillary Starling forces. Conventional resuscitation (CR) with crystalloid aggravates water sequestration. We examined the compartment pattern of organ tissue water after HS and CR, and modulation of tissue edema by adjunctive DPR.

STUDY DESIGN

Rats were hemorrhaged (40% mean arterial pressure for 60 minutes) and assigned to four groups (n = 7): sham, no HS; HS no resuscitation; HS+CR (shed blood plus 2 volumes Ringer's lactate); and HS+CR+DPR (20 mL clinical intraperitoneal (IP) dialysis fluid). Isotopic markers determined equilibrium distribution volumes [V(D)] in gut, liver, lung, and muscle by quantitative autoradiography (2-hour postresuscitation). Total tissue water (TTW) was determined by wet-dry weights. Extracellular water was measured from (14)C-mannitol V(D), and intravascular volume (IVV) from (131)I-labeled IgG V(D). Cellular and interstitial water volumes were calculated.

RESULTS

HS alone decreased IVV in all tissues and TTW in gut, lung, and muscle, but not liver, compared with shams. IVV remained decreased with all resuscitations despite restoration of central hemodynamics. CR caused interstitial edema in gut, liver, and muscle, and cellular edema in lung. DPR reduced (liver, muscle) or prevented (gut, lung) these volume shifts.

CONCLUSIONS

HS decreases IVV. HS-induced water shifts are organ-specific and prominent in gut, lung, and muscle. CR restores central hemodynamics, does not restore IVV, and alters organ-specific TTW distribution. Adjunctive DPR with IP dialysis fluid normalizes TTW and water compartment distribution and prevents edema. Combined effect of DPR and intravascular fluid replacement appears to prevent global tissue edema and improve outcomes from HS.

Na+-glucose cotransporter is also expressed in mesothelium of species with thick visceral pleura

Molecular evidence for Na+-glucose cotransporter (SGLT1) in rabbit pleural mesothelium has been recently provided, confirming earlier functional findings on solute-coupled liquid absorption from rabbit pleural space. In this research we checked whether SGLT1 is also expressed in pleural mesothelium of species with thick visceral pleura, which receives blood from systemic circulation, but drains it into pulmonary veins. To this end immunoblot assays were performed on total protein extract of scraped visceral and parietal mesothelium of lambs and adult sheep, and of a human mesothelial cell line. All of them showed SGLT1 specific bands. Moreover, confocal immunofluorescence images of lamb pleural mesothelium showed that SGLT1 is located in apical membrane. Therefore, a solute-coupled liquid absorption should also occur from pleural space of species with thick visceral pleura. Because of this protein-free liquid entering interstitium between visceral mesothelium and capillaries, inherent Starling forces should be different than hitherto considered, and visceral pleura capillaries could absorb liquid even in these species.