Emerging biomarkers of lymphangioleiomyomatosis

Pulmonary tuberculosis related diffuse cystic lung disease with recurrent pneumothorax mimicking pulmonary lymphangioleomyomatosis in Ethiopia: A review and case report

Millions of individuals worldwide are afflicted by the fatal infectious disease tuberculosis, which accounts for thousands of avoidable deaths. The literature has provided a good description of the clinical manifestation and radiologic features of pulmonary tuberculosis. However, the parenchymal complication of pulmonary tuberculosis presenting as cystic lung disease, has not been widely documented in the literature and is one of the incredibly uncommon causes of diffuse cystic lung disease. It is very uncommon to have a patient with possible pulmonary lymphangioleomyomatosis to be superinfected with bacteriologically confirmed tuberculosis. This report describes a young female patient who was admitted to the hospital, had repeated chest tube insertions and drainage of recurrent spontaneous pneumothoraxes secondary to likely diffuse cystic lung disease related to pulmonary tuberculosis. First, it was thought that the most likely diagnosis was pulmonary lymphangioleomyomatosis. The patient ultimately diagnosed with diffuse cystic lung disease associated with pulmonary tuberculosis as the most likely cause of her clinical presentation considering the high index of suspicion and her sputum gene xpert results. For drug-susceptible tuberculosis, the patient was finally started on anti-tuberculosis medication. She had both clinical and radiological improvement after completion of her anti tuberculosis treatment. Thus, it is reasonable to conclude that tuberculosis may contribute to diffuse cystic lung disease (DCLD) in tuberculosis endemic settings such as Ethiopia, and that appropriate diagnostic efforts should be undertaken to make the diagnosis. A high index of clinical suspicion is crucial to prevent delays in the diagnosis of diffuse cystic lung disease associated with pulmonary tuberculosis.

Circulating biomarkers of kidney angiomyolipoma and cysts in tuberous sclerosis complex patients

Patients with tuberous sclerosis complex (TSC) develop multi-organ disease manifestations, with kidney angiomyolipomas (AML) and cysts being one of the most common and deadly. Early and regular AML/cyst detection and monitoring are vital to lower TSC patient morbidity and mortality. However, the current standard of care involves imaging-based methods that are not designed for rapid screening, posing challenges for early detection. To identify potential diagnostic screening biomarkers of AML/cysts, we performed global untargeted metabolomics in blood samples from 283 kidney AML/cyst-positive or -negative TSC patients using mass spectrometry. We identified 7 highly sensitive chemical features, including octanoic acid, that predict kidney AML/cysts in TSC patients. Patients with elevated octanoic acid have lower levels of very long-chain fatty acids (VLCFAs), suggesting that dysregulated peroxisome activity leads to overproduction of octanoic acid via VLCFA oxidation. These data highlight AML/cysts blood biomarkers for TSC patients and offers valuable metabolic insights into the disease.

A case report of lymphangioleiomyomatosis with retroperitoneal masses in pregnancy

Background

Lymphangioleiomyomatosis (LAM) is a rare, gradually advancing tumor of unknown origin. It is distinguished by the anomalous proliferation of pulmonary smooth muscle cells and predominantly manifests in women of childbearing age. In this study, we aim to present a noteworthy case of LAM accompanied by lymphangioleiomyoma in the retroperitoneal space during pregnancy, a scenario susceptible to misdiagnosis.

Case presentation

A 31-year-old woman, facing an unintended pregnancy, presented during the 13th week with a cystic-solid mass exhibiting abundant blood signals in the pelvic cavity, as revealed by routine obstetrical ultrasound. Concurrently, her chest CT disclosed diffuse thin-walled cavities in both lungs. Despite the absence of clinical symptoms, the patient abandoned pregnancy and underwent a complete curettage. However, 24 days post-operation, she was readmitted for further assessment, revealing an enlargement of the mass encompassing the abdominal aorta and inferior vena cava, along with compression on the middle and lower segments of the ureter. After a multi-disciplinary discussion and patient explanation, an exploratory laparotomy was performed, resulting in the complete removal of the tumor. Intraoperative pathological examination and immunohistochemical staining indicated a retroperitoneal mass devoid of malignant evidence. The comprehensive morphologic and immunophenotypic features substantiated the diagnosis of lymphangioleiomyomatosis. The postoperative course was uneventful, culminating in the patient's discharge.

Conclusion

The consideration of Lymphangioleiomyomatosis (LAM) with a retroperitoneal tumor is crucial in the differential diagnosis of pelvic and abdominal masses. The preoperative diagnosis of this tumor poses a challenge, as ultrasound or CT scans may not yield definitive results. Accurate diagnosis necessitates not only a pathological examination of the retroperitoneal mass but also the correlation with the patient's chest High-Resolution Computed Tomography (HRCT) findings and corresponding clinical manifestations. Optimal management involves radical surgery, with surgeons comprehensively factoring in both fetal and maternal conditions when formulating a treatment plan.

Lymphangioleiomyomatosis with Tuberous Sclerosis Complex-A Case Study

Lymphangioleiomyomatosis (LAM) is characterized by lung cysts that cause lung deterioration, changes in the lymphatic system, and tumors in the kidneys. It mainly affects women of reproductive age and is a progressive disease. LAM can occur as an isolated disease or coexist with tuberous sclerosis (TSC). The source of LAM cells is unknown. Patients with confirmed LAM should be treated with an mTOR inhibitor, sirolimus, or everolimus. We present a case of LAM with TSC in a patient whose symptoms, including those in the lymph nodes and chyaloperitoneum, mainly concern the abdominal cavity.

Lymphangioleiomyomatosis: circulating levels of FGF23 and pulmonary diffusion

OBJECTIVE

Lymphangioleiomyomatosis (LAM) is a rare, destructive disease of the lungs with a limited number of determinants of disease activity, which are a critical need for clinical trials. FGF23 has been implicated in several chronic pulmonary diseases. We aimed to determine the association between serum FGF23 levels and pulmonary function in a cohort of patients with LAM.

METHODS

This was a descriptive single-center study in which subjects with LAM and controls with unreported lung disease were recruited. Serum FGF23 levels were measured in all subjects. Clinical data, including pulmonary function testing, were retrospectively obtained from electronic medical records of LAM subjects. Associations between FGF23 levels and clinical features of LAM were explored via nonparametric hypothesis testing.

RESULTS

The sample comprised 37 subjects with LAM and 16 controls. FGF23 levels were higher in the LAM group than in the control group. In the LAM group, FGF23 levels above the optimal cutoff point distinguished 33% of the subjects who had nondiagnostic VEGF-D levels. Lower FGF23 levels were associated with impaired DLCO (p = 0.04), particularly for those with isolated diffusion impairment with no other spirometric abnormalities (p = 0.04).

CONCLUSIONS

Our results suggest that FGF23 is associated with pulmonary diffusion abnormalities in LAM patients and elicit novel mechanisms of LAM pathogenesis. FGF23 alone or in combination with other molecules needs to be validated as a biomarker of LAM activity in future clinical research.

Bioinformatics analysis of inflammation and oncology in pulmonary lymphangioleiomyomatosis

This study investigates the molecular markers and biological pathways of pulmonary lymphangioleiomyomatosis. We analyzed 2 gene expression profiles in the gene expression omnibus Gene Expression Omnibus database for normal lung tissue and lymphangioleiomyomatosis and identified differential expressed genes in pulmonary lymphangioleiomyomatosis. Ninety-one differentially expressed genes were identified, including 36 upregulated genes and 55 downregulated genes. Hub genes and pathogenic pathways associated with disease development were subsequently identified by enrichment analysis and protein-protein interaction network. Analysis showed that differential expressed genes are mainly involved in the biological behavior of tumor cell proliferation and invasion as well as the inflammatory response. We have identified 10 hub genes in the protein-protein interaction network. Hub genes play an important role in the proliferation and inflammatory response involved in tumor cell proliferation. This study deepens the understanding of lymphangioleiomyomatosis disease and provides a biological basis for further clinical diagnosis and treatment.

LAM Cells as Potential Drivers of Senescence in Lymphangioleiomyomatosis Microenvironment

Senescence is a stress-response process characterized by the irreversible inhibition of cell proliferation, associated to the acquisition of a senescence-associated secretory phenotype (SASP), that may drive pathological conditions. Lymphangioleiomyomatosis (LAM) is a rare disease in which LAM cells, featuring the hyperactivation of the mammalian Target of Rapamycin Complex 1 (mTORC1) for the absence of tuberin expression, cause the disruption of the lung parenchyma. Considering that LAM cells secrete SASP factors and that mTOR is also a driver of senescence, we deepened the contribution of senescence in LAM cell phenotype. We firstly demonstrated that human primary tuberin-deficient LAM cells (LAM/TSC cells) have senescent features depending on mTOR hyperactivation, since their high positivity to SA-β galactosidase and to phospho-histone H2A.X are reduced by inducing tuberin expression and by inhibiting mTOR with rapamycin. Then, we demonstrated the capability of LAM/TSC cells to induce senescence. Indeed, primary lung fibroblasts (PLFs) grown in LAM/TSC conditioned medium increased the positivity to SA-β galactosidase and to phospho-histone H2A.X, as well as p21^WAF1/CIP1 expression, and enhanced the mRNA expression and the secretion of the SASP component IL-8. Taken together, these data make senescence a novel field of study to understand LAM development and progression.

A Novel Secreted Protein-Related Gene Signature Predicts Overall Survival and Is Associated With Tumor Immunity in Patients With Lung Adenocarcinoma

Secreted proteins are important proteins in the human proteome, accounting for approximately one-tenth of the proteome. However, the prognostic value of secreted protein-related genes has not been comprehensively explored in lung adenocarcinoma (LUAD). In this study, we screened 379 differentially expressed secretory protein genes (DESPRGs) by analyzing the expression profile in patients with LUAD from The Cancer Genome Atlas database. Following univariate Cox regression and least absolute shrinkage and selection operator method regression analysis, 9 prognostic SPRGs were selected to develop secreted protein-related risk score (SPRrisk), including CLEC3B, C1QTNF6, TCN1, F2, FETUB, IGFBP1, ANGPTL4, IFNE, and CCL20. The prediction accuracy of the prognostic models was determined by Kaplan–Meier survival curve analysis and receiver operating characteristic curve analysis. Moreover, a nomogram with improved accuracy for predicting overall survival was established based on independent prognostic factors (SPRrisk and clinical stage). The DESPRGs were validated by quantitative real-time PCR and enzyme-linked immunosorbent assay by using our clinical samples and datasets. Our results demonstrated that SPRrisk can accurately predict the prognosis of patients with LUAD. Patients with a higher risk had lower immune, stromal, and ESTIMATE scores and higher tumor purity. A higher SPRrisk was also negatively associated with the abundance of CD8⁺ T cells and M1 macrophages. In addition, several genes of the human leukocyte antigen family and immune checkpoints were expressed in low levels in the high-SPRrisk group. Our results provided some insights into assessing individual prognosis and choosing personalized treatment modalities.

Pulmonary lymphangioleiomyomatosis (LAM): A literature overview and case report

Lymphangioleiomyomatosis is a rare multisystem disease associated with genetic mutations. The disease usually occurs in women of childbearing age and is characterized by infiltration of immature smooth muscle cells into the lungs, airways, and axial lymphatic systems of the chest and abdomen. The disease often destroys lung parenchyma and produces air cysts. Lymphangioleiomyomatosis cell infiltration of the lymphatic axis can affect hilar lymph nodes, mediastinal ganglia, and extrathoracic lymph nodes. The disease can cause lymphatic dilation in the lungs and thoracic ducts, causing chylous effusion into the pleural or abdominal cavities. Invasion of cells into the walls of pulmonary veins can lead to venous obstruction and pulmonary venous hypertension with hemoptysis. Most patients present with cough, dyspnea, pneumothorax, hemoptysis, and abnormal lung function. Definitive diagnosis is usually based on histopathology and immunohistochemistry. We present a case of LAM in a 36-year-old female patient who was confirmed by specimens obtained from pneumothorax surgery and positive immunohistochemical staining with HMB-45.

Protease activity sensors enable real-time treatment response monitoring in lymphangioleiomyomatosis

BACKGROUND

Biomarkers of disease progression and treatment response are urgently needed for patients with lymphangioleiomyomatosis (LAM). Activity-based nanosensors, an emerging biosensor class, detect dysregulated proteases in vivo and release a reporter to provide a urinary readout of disease. Because proteases are dysregulated in LAM and may directly contribute to lung function decline, activity-based nanosensors may enable quantitative, real-time monitoring of LAM progression and treatment response. We aimed to assess the diagnostic utility of activity-based nanosensors in a pre-clinical model of pulmonary LAM.

METHODS

Tsc2-null cells were injected intravenously into female nude mice to establish a mouse model of pulmonary LAM. A library of 14 activity-based nanosensors, designed to detect proteases across multiple catalytic classes, was administered into the lungs of LAM mice and healthy controls, urine was collected, and mass spectrometry was performed to measure nanosensor cleavage products. Mice were then treated with rapamycin and monitored with activity-based nanosensors. Machine learning was performed to distinguish diseased from healthy and treated from untreated mice.

RESULTS

Multiple activity-based nanosensors (PP03 (cleaved by metallo, aspartic and cysteine proteases), padjusted<0.0001; PP10 (cleaved by serine, aspartic and cysteine proteases), padjusted=0.017)) were differentially cleaved in diseased and healthy lungs, enabling strong classification with a machine learning model (area under the curve (AUC) 0.95 from healthy). Within 2 days after rapamycin initiation, we observed normalisation of PP03 and PP10 cleavage, and machine learning enabled accurate classification of treatment response (AUC 0.94 from untreated).

CONCLUSIONS

Activity-based nanosensors enable noninvasive, real-time monitoring of disease burden and treatment response in a pre-clinical model of LAM.

Diagnostic performance of VEGF-D for lymphangioleiomyomatosis: a meta-analysis

Objective:

VEGF-D is a potential biomarker for lymphangioleiomyomatosis (LAM); however, its diagnostic performance has yet to be systematically studied.

Methods:

We searched PubMed, EMBASE, Scopus, Web of Science, and Cochrane Library to identify primary studies on VEGF-D in relation to the diagnosis of LAM. The quality of the studies was evaluated using the Quality Assessment of Diagnostic Accuracy Studies-2 (QUADAS-2). Summary estimates of diagnostic accuracy were pooled using a bivariate random effects model. Subgroup and sensitivity analyses were performed to explore possible heterogeneity. The Grading of Recommendations Assessment, Development, and Evaluation (GRADE) was applied to rate the quality of evidence and indicate the strength of recommendations.

Results:

Ten studies involving 945 patients were of high risk in quality, as assessed using the QUADAS-2. The pooled diagnostic parameters were indicated as follows: sensitivity = 0.82 (95% CI, 0.71-0.90); specificity = 0.98 (95% CI, 0.94-0.99); and diagnostic OR = 197 (95% CI, 66-587). The AUC of summary ROC analysis was 0.98. The subgroup and sensitivity analyses revealed that the overall performance was not substantially affected by the composition of the control group, prespecified cutoff value, the country of origin, or different cutoff values (p > 0.05 for all). A strong recommendation for serum VEGF-D determination to aid in the diagnosis of LAM was made according to the GRADE.

Conclusions:

VEGF-D seems to have great potential implications for the diagnosis of LAM in clinical practice due to its excellent specificity and suboptimal sensitivity.

Heterogeneity and Cancer-Related Features in Lymphangioleiomyomatosis Cells and Tissue

Lymphangioleiomyomatosis (LAM) is a rare, low-grade metastasizing disease characterized by cystic lung destruction. LAM can exhibit extensive heterogeneity at the molecular, cellular, and tissue levels. However, the molecular similarities and differences among LAM cells and tissue, and their connection to cancer features are not fully understood. By integrating complementary gene and protein LAM signatures, and single-cell and bulk tissue transcriptome profiles, we show sources of disease heterogeneity, and how they correspond to cancer molecular portraits. Subsets of LAM diseased cells differ with respect to gene expression profiles related to hormones, metabolism, proliferation, and stemness. Phenotypic diseased cell differences are identified by evaluating lumican (LUM) proteoglycan and YB1 transcription factor expression in LAM lung lesions. The RUNX1 and IRF1 transcription factors are predicted to regulate LAM cell signatures, and both regulators are expressed in LAM lung lesions, with differences between spindle-like and epithelioid LAM cells. The cancer single-cell transcriptome profiles most similar to those of LAM cells include a breast cancer mesenchymal cell model and lines derived from pleural mesotheliomas. Heterogeneity is also found in LAM lung tissue, where it is mainly determined by immune system factors. Variable expression of the multifunctional innate immunity protein LCN2 is linked to disease heterogeneity. This protein is found to be more abundant in blood plasma from LAM patients than from healthy women. IMPLICATIONS: This study identifies LAM molecular and cellular features, master regulators, cancer similarities, and potential causes of disease heterogeneity.

Distinct Characteristics of VEGF-D and VEGF-C to Predict Mortality in Patients With Suspected or Known Coronary Artery Disease

Background

VEGF‐D (vascular endothelial growth factor D) and VEGF‐C are secreted glycoproteins that can induce lymphangiogenesis and angiogenesis. They exhibit structural homology but have differential receptor binding and regulatory mechanisms. We recently demonstrated that the serum VEGF‐C level is inversely and independently associated with all‐cause mortality in patients with suspected or known coronary artery disease. We investigated whether VEGF‐D had distinct relationships with mortality and cardiovascular events in those patients.

Methods and Results

We performed a multicenter, prospective cohort study of 2418 patients with suspected or known coronary artery disease undergoing elective coronary angiography. The serum level of VEGF‐D was measured. The primary outcome was all‐cause death. The secondary outcomes were cardiovascular death and major adverse cardiovascular events defined as a composite of cardiovascular death, nonfatal myocardial infarction, and nonfatal stroke. During the 3‐year follow‐up, 254 patients died from any cause, 88 died from cardiovascular disease, and 165 developed major adverse cardiovascular events. After adjustment for possible clinical confounders, cardiovascular biomarkers (N‐terminal pro‐B‐type natriuretic peptide, cardiac troponin‐I, and high‐sensitivity C‐reactive protein), and VEGF‐C, the VEGF‐D level was significantly associated with all‐cause death and cardiovascular death but not with major adverse cardiovascular events.. Moreover, the addition of VEGF‐D, either alone or in combination with VEGF‐C, to the model with possible clinical confounders and cardiovascular biomarkers significantly improved the prediction of all‐cause death but not that of cardiovascular death or major adverse cardiovascular events. Consistent results were observed within patients over 75 years old.

Conclusions

In patients with suspected or known coronary artery disease undergoing elective coronary angiography, an elevated VEGF‐D value seems to independently predict all‐cause mortality.

Alterations in Polyamine Metabolism in Patients With Lymphangioleiomyomatosis and Tuberous Sclerosis Complex 2-Deficient Cells

BACKGROUND

Lymphangioleiomyomatosis (LAM), a destructive lung disease that affects primarily women, is caused by loss-of-function mutations in TSC1 or TSC2, leading to hyperactivation of mechanistic/mammalian target of rapamycin complex 1 (mTORC1). Rapamycin (sirolimus) treatment suppresses mTORC1 but also induces autophagy, which promotes the survival of TSC2-deficient cells. Based on the hypothesis that simultaneous inhibition of mTORC1 and autophagy would limit the availability of critical nutrients and inhibit LAM cells, we conducted a phase 1 clinical trial of sirolimus and hydroxychloroquine for LAM. Here, we report the analyses of plasma metabolomic profiles from the clinical trial.

METHODS

We analyzed the plasma metabolome in samples obtained before, during, and after 6 months of treatment with sirolimus and hydroxychloroquine, using univariate statistical models and machine learning approaches. Metabolites and metabolic pathways were validated in TSC2-deficient cells derived from patients with LAM. Single-cell RNA-Seq was employed to assess metabolic enzymes in an early-passage culture from an LAM lung.

RESULTS

Metabolomic profiling revealed changes in polyamine metabolism during treatment, with 5'-methylthioadenosine and arginine among the most highly upregulated metabolites. Similar findings were observed in TSC2-deficient cells derived from patients with LAM. Single-cell transcriptomic profiling of primary LAM cultured cells revealed that mTORC1 inhibition upregulated key enzymes in the polyamine metabolism pathway, including adenosylmethionine decarboxylase 1.

CONCLUSIONS

Our data demonstrate that polyamine metabolic pathways are targeted by the combination of rapamycin and hydroxychloroquine, leading to upregulation of 5'-methylthioadenosine and arginine in the plasma of patients with LAM and in TSC2-deficient cells derived from a patient with LAM upon treatment with this drug combination.

TRIAL REGISTRY

ClinicalTrials.gov; No.: NCT01687179; URL: www.clinicaltrials.gov. Partners Human Research Committee, protocol No. 2012P000669.

Risk of breast cancer in patients with lymphangioleiomyomatosis

Lymphangioleiomyomatosis (LAM) is a rare metastasizing pulmonary disease that shares some clinical, cellular, and molecular similarities with metastatic breast cancer to lung. LAM cells have been identified circulating in various body fluids of patients and, intriguingly, diverse evidence indicates that these cells may originate from a different organ to the lung. Following on from these observations, we hypothesized the existence of a common risk basis between LAM and breast cancer, and suggested increased risk of breast cancer among LAM patients. Here, by studying two additional LAM cohorts with more detailed epidemiological, life-style, and disease-related data, we show consistent results; a potential excess of estrogen-receptor-positive young breast cancer cases in LAM. This observation further suggests the need of prospective studies to precisely assess the association between both diseases.

Circulating Biomarkers From the Phase 1 Trial of Sirolimus and Autophagy Inhibition for Patients With Lymphangioleiomyomatosis

BACKGROUND

We have previously conducted the Sirolimus and Autophagy Inhibition in LAM (SAIL) trial, a phase 1 dose-escalation study of the combination of sirolimus and hydroxychloroquine in patients with lymphangioleiomyomatosis (LAM). The goal of the present study was to analyze sera from the SAIL trial to identify novel biomarkers that could shed light into disease pathogenesis and response to therapy.

METHODS

We used the DiscoveryMAP platform from Rules Based Medicine to simultaneously measure 279 analytes in sera collected at each visit from subjects enrolled in the SAIL trial. We used longitudinal regression and pathway analysis to examine analyte rate of change and corresponding effect on lung function and to identify networks and potential nodes of interest.

RESULTS

A total of 222 analytes were included in the analysis. We identified 32 analytes that changed over the treatment period of the study. Pathway analysis revealed enrichment in cytokine-receptor interaction and mechanistic/mammalian target of rapamycin-related pathways, in addition to seemingly unrelated processes such as rheumatoid arthritis. Search Tool for the Retrieval of Interacting Genes/Proteins analysis identified two hubs centered around acetyl-CoA carboxylase alpha and beta and coagulation factor II. In addition, we identified vascular endothelial growth factor receptor-3 and CCL21 as molecules significantly associated with changes in FEV₁ during the study period.

CONCLUSIONS

We performed a large-scale analyte study in sera of women with LAM and identified potential markers that could be linked to disease pathogenesis, lung injury, and therapeutic response. These data will enable future investigation into the specific roles of these molecules in LAM.

TRIAL REGISTRY

ClinicalTrials.gov; No. NCT01687179; URL: www.clinicaltrials.gov).

Pulmonary manifestations in tuberous sclerosis complex

Tuberous sclerosis complex has manifestations in many organ systems, including brain, heart, kidney, skin, and lung. The primary manifestations in the lung are lymphangioleiomyomatosis (LAM) and multifocal micronodular pneumocyte hyperplasia (MMPH). LAM affects almost exclusively women, and causes cystic lung destruction, pneumothorax, and chylous pleural effusions. LAM can lead to dyspnea, oxygen dependence, and respiratory failure, with more rapid disease progression during the premenopausal years. In contrast, MMPH affects men and women equally, causing small nodular pulmonary deposits of type II pneumocytes that rarely progress to symptomatic disease. Here, we review the clinical features and pathogenesis of LAM and MMPH.

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.