Inflammatory triggers associated with exacerbations of COPD orchestrate plasticity of group 2 innate lymphoid cells in the lungs.

Innate lymphoid cells (ILCs) are critical mediators of mucosal immunity, and group 1 ILCs (ILC1 cells) and group 3 ILCs (ILC3 cells) have been shown to be functionally plastic. Here we found that group 2 ILCs (ILC2 cells) also exhibited phenotypic plasticity in response to infectious or noxious agents, characterized by substantially lower expression of the transcription factor GATA-3 and a concomitant switch to being ILC1 cells that produced interferon-γ (IFN-γ). Interleukin 12 (IL-12) and IL-18 regulated this conversion, and during viral infection, ILC2 cells clustered within inflamed areas and acquired an ILC1-like phenotype. Mechanistically, these ILC1 cells augmented virus-induced inflammation in a manner dependent on the transcription factor T-bet. Notably, IL-12 converted human ILC2 cells into ILC1 cells, and the frequency of ILC1 cells in patients with chronic obstructive pulmonary disease (COPD) correlated with disease severity and susceptibility to exacerbations. Thus, functional plasticity of ILC2 cells exacerbates anti-viral immunity, which may have adverse consequences in respiratory diseases such as COPD.

Cigarette smoke silences innate lymphoid cell function and facilitates an exacerbated type I interleukin-33-dependent response to infection.

Cigarette smoking is a major risk factor for chronic obstructive pulmonary disease and is presumed to be central to the altered responsiveness to recurrent infection in these patients. We examined the effects of smoke priming underlying the exacerbated response to viral infection in mice. Lack of interleukin-33 (IL-33) signaling conferred complete protection during exacerbation and prevented enhanced inflammation and exaggerated weight loss. Mechanistically, smoke was required to upregulate epithelial-derived IL-33 and simultaneously alter the distribution of the IL-33 receptor ST2. Specifically, smoke decreased ST2 expression on group 2 innate lymphoid cells (ILC2s) while elevating ST2 expression on macrophages and natural killer (NK) cells, thus altering IL-33 responsiveness within the lung. Consequently, upon infection and release, increased local IL-33 significantly amplified type I proinflammatory responses via synergistic modulation of macrophage and NK cell function. Therefore, in COPD, smoke alters the lung microenvironment to facilitate an alternative IL-33-dependent exaggerated proinflammatory response to infection, exacerbating disease.

Eosinophils, basophils and type 2 immune microenvironments in COPD-affected lung tissue.

Although elevated blood or sputum eosinophils are present in many patients with COPD, uncertainties remain regarding the anatomical distribution pattern of lung-infiltrating eosinophils. Basophils have remained virtually unexplored in COPD. This study mapped tissue-infiltrating eosinophils, basophils and eosinophil-promoting immune mechanisms in COPD-affected lungs.Surgical lung tissue and biopsies from major anatomical compartments were obtained from COPD patients with severity grades Global Initiative for Chronic Obstructive Lung Disease stages I-IV; never-smokers/smokers served as controls. Automated immunohistochemistry and in situ hybridisation identified immune cells, the type 2 immunity marker GATA3 and eotaxins (CCL11, CCL24).Eosinophils and basophils were present in all anatomical compartments of COPD-affected lungs and increased significantly in very severe COPD. The eosinophilia was strikingly patchy, and focal eosinophil-rich microenvironments were spatially linked with GATA3+ cells, including type 2 helper T-cell lymphocytes and type 2 innate lymphoid cells. A similarly localised and interleukin-33/ST2-dependent eosinophilia was demonstrated in influenza-infected mice. Both mice and patients displayed spatially confined eotaxin signatures with CCL11+ fibroblasts and CCL24+ macrophages.In addition to identifying tissue basophilia as a novel feature of advanced COPD, the identification of spatially confined eosinophil-rich type 2 microenvironments represents a novel type of heterogeneity in the immunopathology of COPD that is likely to have implications for personalised treatment.

Th2-driven, allergen-induced airway inflammation is reduced after treatment with anti-Tim-3 antibody in vivo.

T cell immunoglobulin and mucin domain-containing molecule-3 (Tim-3) is a surface molecule that is preferentially expressed on activated Th1 cells in comparison to Th2 cells. Blockade of Tim-3 has been shown to enhance Th1-driven pathology in vivo, suggesting that blockade of Tim-3 may improve the development of Th2-associated responses such as allergy. To examine the effects of Tim-3 blockade on the Th2 response in vivo, we administered anti-Tim-3 antibody during pulmonary inflammation induced by transfer of ovalbumin (OVA)-reactive Th2 cells, and subsequent aerosol challenge with OVA. In this model, anti-Tim-3 antibody treatment before each airway challenge significantly reduced airway hyperreactivity, with a concomitant decrease in eosinophils and Th2 cells in the lung. We examined Th1 and Th2 cytokine levels in the lung after allergen challenge and found that pulmonary expression of the Th2 cytokine IL-5 was significantly reduced, whereas IFN-gamma levels were significantly increased by anti-Tim-3 antibody treatment. Thus, blocking Tim-3 function has a beneficial effect during pulmonary inflammation by skewing the Th2 response toward that of a Th1 type, suggesting an important role for Tim-3 in the regulation of allergic disease.

Opposing roles of membrane and soluble forms of the receptor for advanced glycation end products in primary respiratory syncytial virus infection.

Respiratory syncytial virus (RSV), a common respiratory pathogen in infants and the older population, causes pulmonary inflammation and airway occlusion that leads to impairment of lung function. Here, we have established a role for receptor for advanced glycation end products (RAGE) in RSV infection. RAGE-deficient (ager(-/-)) mice were protected from RSV-induced weight loss and inflammation. This protection correlated with an early increase in type I interferons, later decreases in proinflammatory cytokines, and a reduction in viral load. To assess the contribution of soluble RAGE (sRAGE) to RSV-induced disease, wild-type and ager(-/-) mice were given doses of sRAGE following RSV infection. Of interest, sRAGE treatment prevented RSV-induced weight loss and neutrophilic inflammation to a degree similar to that observed in ager(-/-) mice. Our work further elucidates the roles of RAGE in the pathogenesis of respiratory infections and highlights the opposing roles of membrane and sRAGE in modulating the host response to RSV infection.

Nonredundant role of CCRL2 in lung dendritic cell trafficking.

Chemokine CC motif receptor-like 2 (CCRL2) is a heptahelic transmembrane receptor that shows the highest degree of homology with CCR1, an inflammatory chemokine receptor. CCRL2 mRNA was rapidly (30 minutes) and transiently (2-4 hours) regulated during dendritic cell (DC) maturation. Protein expression paralleled RNA regulation. In vivo, CCRL2 was expressed by activated DC and macrophages, but not by eosinophils and T cells. CCRL2(-/-) mice showed normal recruitment of circulating DC into the lung, but a defective trafficking of antigen-loaded lung DC to mediastinal lymph nodes. This defect was associated to a reduction in lymph node cellularity and reduced priming of T helper cell 2 response. CCRL2(-/-) mice were protected in a model of ovalbumin-induced airway inflammation, with reduced leukocyte recruitment in the BAL (eosinophils and mononuclear cells) and reduced production of the T helper cell 2 cytokines, interleukin-4 and -5, and chemokines CCL11 and CCL17. The central role of CCRL2 deficiency in DC was supported by the fact that adoptive transfer of CCRL2(-/-) antigen-loaded DC in wild-type animals recapitulated the phenotype observed in knockout mice. These data show a nonredundant role of CCRL2 in lung DC trafficking and propose a role for this receptor in the control of excessive airway inflammatory responses.

IL-9 governs allergen-induced mast cell numbers in the lung and chronic remodeling of the airways.

RATIONALE: IL-9 is a pleiotropic cytokine that has multiple effects on structural as well as numerous hematopoietic cells, which are central to the pathogenesis of asthma. OBJECTIVES: The contribution of IL-9 to asthma pathogenesis has thus far been unclear, due to conflicting reports in the literature. These earlier studies focused on the role of IL-9 in acute inflammatory models; here we have investigated the effects of IL-9 blockade during chronic allergic inflammation. METHODS: Mice were exposed to either prolonged ovalbumin or house dust mite allergen challenge to induce chronic inflammation and airway remodeling. MEASUREMENTS AND MAIN RESULTS: We found that IL-9 governs allergen-induced mast cell (MC) numbers in the lung and has pronounced effects on chronic allergic inflammation. Anti-IL-9 antibody-treated mice were protected from airway remodeling with a concomitant reduction in mature MC numbers and activation, in addition to decreased expression of the profibrotic mediators transforming growth factor-ß1, vascular endothelial growth factor, and fibroblast growth factor-2 in the lung. Airway remodeling was associated with impaired lung function in the peripheral airways and this was reversed by IL-9 neutralization. In human asthmatic lung tissue, we identified MCs as the main IL-9 receptor expressing population and found them to be sources of vascular endothelial growth factor and fibroblast growth factor-2. CONCLUSIONS: Our data suggest an important role for an IL-9-MC axis in the pathology associated with chronic asthma and demonstrate that an impact on this axis could lead to a reduction in chronic inflammation and improved lung function in patients with asthma.

Proteomic profiling of serum identifies a molecular signature that correlates with clinical outcomes in COPD.

OBJECTIVE: Novel biomarkers related to main clinical hallmarks of Chronic obstructive pulmonary disease (COPD), a heterogeneous disorder with pulmonary and extra-pulmonary manifestations, were investigated by profiling the serum levels of 1305 proteins using Slow Off-rate Modified Aptamers (SOMA)scan technology. METHODS: Serum samples were collected from 241 COPD subjects in the multicenter French Cohort of Bronchial obstruction and Asthma to measure the expression of 1305 proteins using SOMAscan proteomic platform. Clustering of the proteomics was applied to identify disease subtypes and their functional annotation and association with key clinical parameters were examined. Cluster findings were revalidated during a follow-up visit, and compared to those obtained in a group of 47 COPD patients included in the Melbourne Longitudinal COPD Cohort. RESULTS: Unsupervised clustering identified two clusters within COPD subjects at inclusion. Cluster 1 showed elevated levels of factors contributing to tissue injury, whereas Cluster 2 had higher expression of proteins associated with enhanced immunity and host defense, cell fate, remodeling and repair and altered metabolism/mitochondrial functions. Patients in Cluster 2 had a lower incidence of exacerbations, unscheduled medical visits and prevalence of emphysema and diabetes. These protein expression patterns were conserved during a follow-up second visit, and substanciated, by a large part, in a limited series of COPD patients. Further analyses identified a signature of 15 proteins that accurately differentiated the two COPD clusters at the 2 visits. CONCLUSIONS: This study provides insights into COPD heterogeneity and suggests that overexpression of factors involved in lung immunity/host defense, cell fate/repair/ remodelling and mitochondrial/metabolic activities contribute to better clinical outcomes. Hence, high throughput proteomic assay offers a powerful tool for identifying COPD endotypes and facilitating targeted therapies.

Resolution of airway inflammation and hyperreactivity after in vivo transfer of CD4+CD25+ regulatory T cells is interleukin 10 dependent.

Deficient suppression of T cell responses to allergen by CD4+CD25+ regulatory T cells has been observed in patients with allergic disease. Our current experiments used a mouse model of airway inflammation to examine the suppressive activity of allergen-specific CD4+CD25+ T cells in vivo. Transfer of ovalbumin (OVA) peptide-specific CD4+CD25+ T cells to OVA-sensitized mice reduced airway hyperreactivity (AHR), recruitment of eosinophils, and T helper type 2 (Th2) cytokine expression in the lung after allergen challenge. This suppression was dependent on interleukin (IL) 10 because increased lung expression of IL-10 was detected after transfer of CD4+CD25+ T cells, and regulation was reversed by anti-IL-10R antibody. However, suppression of AHR, airway inflammation, and increased expression of IL-10 were still observed when CD4+CD25+ T cells from IL-10 gene-deficient mice were transferred. Intracellular cytokine staining confirmed that transfer of CD4+CD25+ T cells induced IL-10 expression in recipient CD4+ T cells, but no increase in IL-10 expression was detected in airway macrophages, dendritic cells, or B cells. These data suggest that CD4+CD25+ T cells can suppress the Th2 cell-driven response to allergen in vivo by an IL-10-dependent mechanism but that IL-10 production by the regulatory T cells themselves is not required for such suppression.

Capillary defects and exaggerated inflammatory response in the airways of EphA2-deficient mice.

Both Eph receptors and ephrin ligands have been implicated in blood vessel and neuronal development. Recent studies suggested that EphA2 inhibition reduces tumor angiogenesis, but its role in blood vessel development and inflammation is unclear. We examined these issues using either airways of pathogen-free, EphA2-deficient mice at various ages or EphA2-deficient mice whose airways were inflamed by either Mycoplasma pulmonis infection or ovalbumin sensitization and challenge. EphA2-deficient mice had fewer capillaries, a greater number of endothelial sprouts, and greater capillary diameters than age-matched, wild-type control mice. Moreover, capillaries in EphA2-deficient mice had significantly less pericyte coverage, suggesting abnormal interactions between endothelial cells and pericytes. These differences were apparent in early postnatal life but decreased during progression into adulthood. In inflamed airways, significantly more angiogenesis and lymphangiogenesis, a greater number of infiltrating leukocytes, and higher expression levels of inflammatory cytokine mRNA were present in EphA2-deficient mice after M. pulmonis infection. Additionally, in allergic airway inflammation with ovalbumin sensitization and challenge, a greater number of lymphatic sprouts and infiltrating leukocytes, higher mRNA expression levels of TH2 cytokines and chemokines related to allergic airway inflammation, and enhanced airway hyper-responsiveness were present in EphA2-deficient mice. We conclude that defective pericyte coverage causes capillary defects, abundant endothelial sprouts, and thick capillary diameters in EphA2-deficient mice, indicating that these animals have exaggerated responses to airway inflammation.

Glycerol-3-phosphate acyltransferases gat1p and gat2p are microsomal phosphoproteins with differential contributions to polarized cell growth.

Glycerol-3-phosphate acyltransferase (GPAT) catalyzes the initial step in the synthesis of all glycerolipids. It is the committed and rate-limiting step and is redundant in Saccharomyces cerevisiae, mammals, and plants. GPAT controls the formation of lipid intermediates that serve not only as precursors of more-complex lipids but also as intracellular signaling molecules. Saccharomyces cerevisiae possesses two GPATs, encoded by the GAT1 and GAT2 genes. Metabolic analysis of yeast lacking either GAT1 or GAT2 indicated partitioning of the two main branches of phospholipid synthesis at the initial and rate-limiting GPAT step. We are particularly interested in identifying molecular determinants mediating lipid metabolic pathway partitioning; therefore, as a starting point, we have performed a detailed study of Gat1p and Gat2p cellular localization. We have compared Gat1p and Gat2p localization by fluorescence microscopy and subcellular fractionation using equilibrium density gradients. Our results indicate Gat1p and Gat2p overlap mostly in their localization and are in fact microsomal GPATs, localized to both perinuclear and cortical endoplasmic reticula in actively proliferating cells. A more detailed analysis suggests a differential enrichment of Gat1p and Gat2p in distinct ER fractions. Furthermore, overexpression of these enzymes in the absence of endogenous GPATs induces proliferation of distinct ER arrays, differentially affecting cortical ER morphology and polarized cell growth. In addition, our studies also uncovered a dynamic posttranslational regulation of Gat1p and Gat2p and a compensation mechanism through phosphorylation that responds to a cellular GPAT imbalance.

Resolution of allergic inflammation and airway hyperreactivity is dependent upon disruption of the T1/ST2-IL-33 pathway.

RATIONALE: Although there have been numerous studies on the development of allergen-induced inflammation, the mechanisms leading to resolution of inflammation remain poorly understood. This represents an important consideration because failure to resolve allergen driven inflammation potentially leads to irreversible airway remodeling, characteristic of chronic asthma. OBJECTIVES: We investigated the resolution of allergic inflammation and identified the factors responsible. METHODS: BALB/c and C57BL/6 mice were sensitized to ovalbumin and challenged through the airways to induce allergic inflammation. Mice were analyzed at 24 hours and 7 days after the final challenge. MEASUREMENTS AND MAIN RESULTS: Airway hyperreactivity (AHR) and increased mucus production were present 7 days after the cessation of allergen challenge in BALB/c mice. Persisting AHR correlated with the continued presence of Th2 cells but not eosinophils in the lungs. The role of Th2 cells in maintaining AHR was confirmed using blocking antibodies against T1/ST2, IL-4, and IL-13 during the resolution period. Moreover, AHR in the "Th1 type" C57BL/6 mouse strain was resolved 1 week after allergen challenge, concomitant with clearance of Th2 cells from the lung. Expression of the T1/ST2 ligand, IL-33, also correlated with maintenance of AHR. CONCLUSIONS: We have used blockade of Th2 function and strain differences to show for the first time that resolution of allergic inflammation and AHR may be dependent on the T1/ST2-IL-33 pathway and the presence of Th2 cells, suggesting they are necessary not only for the development of an allergic response but also for its maintenance.

CD4+CD25+ regulatory T cells reverse established allergic airway inflammation and prevent airway remodeling.

BACKGROUND: CD4(+)CD25(+) regulatory T cells can inhibit excessive T-cell responses in vivo. We have previously demonstrated that prophylactic administration of CD4(+)CD25(+) regulatory T cells suppresses the development of acute allergen-induced airway inflammation in vivo. OBJECTIVE: We sought to determine the effect of therapeutic transfer of CD4(+)CD25(+) regulatory T cells on established pulmonary inflammation and the subsequent development of airway remodeling. METHODS: CD4(+)CD25(+) cells were transferred after the onset of allergic inflammation, and airway challenges were continued to induce chronic inflammation and airway remodeling. RESULTS: Administration of CD4(+)CD25(+) regulatory T cells reduced established lung eosinophilia, T(H)2 infiltration, and expression of IL-5, IL-13, and TGF-beta. Moreover, subsequent mucus hypersecretion and peribronchial collagen deposition were reduced after prolonged challenge. In contrast, transfer of CD4(+)CD25(+) regulatory T cells had no effect on established airway hyperreactivity either 7 days or 4 weeks after transfer. CONCLUSIONS: In this study we demonstrate for the first time that therapeutic transfer of CD4(+)CD25(+) regulatory T cells can resolve features of chronic allergen-induced inflammation and prevent development of airway remodeling.

Targeting p16-induced senescence prevents cigarette smoke-induced emphysema by promoting IGF1/Akt1 signaling in mice.

Senescence is a mechanism associated with aging that alters tissue regeneration by depleting the stem cell pool. Chronic obstructive pulmonary disease (COPD) displays hallmarks of senescence, including a diminished stem cell population. DNA damage from cigarette smoke (CS) induces senescence via the p16 pathway. This study evaluated the contribution of p16 to CS-associated lung pathologies. p16 expression was prominent in human COPD lungs compared with normal subjects. CS induces impaired pulmonary function, emphysema, and increased alveolar epithelial cell (AECII) senescence in wild-type mice, whereas CS-exposed p16-/- mice exhibit normal pulmonary function, reduced emphysema, diminished AECII senescence, and increased pro-growth IGF1 signaling, suggesting that improved lung function in p16-/- mice was due to increased alveolar progenitor cell proliferation. In conclusion, our study suggests that targeting senescence may facilitate alveolar regeneration in COPD emphysema by promoting IGF1 proliferative signaling.

Altered expression of the CCN genes in the lungs of mice in response to cigarette smoke exposure and viral and bacterial infections.

The CCN proteins are key signaling and regulatory molecules involved in many biological functions and contribute to malignant and non-malignant lung diseases. Despite the high morbidity and mortality of the lung respiratory infectious diseases, there is very little data related to the expression of the CCNs during infection. We investigated in mice the pulmonary mRNA expression levels of five CCNs (1 to 5) in response to influenza A virus (IAV) and bacterial agents (Nontypeable Haemophilus influenzae (NTHi), lipopolysaccharide (LPS) and lipoteichoic acid (LTA)). IAV, NTHi, LPS or LTA were instilled intranasally into mice. Mice were also exposed for 4days or 8weeks to cigarette smoke alone or prior infection to IAV in order to determine if CS modifies the CCN response to a viral infection. All challenges induced a robust inflammation. The mRNA expression of CCN1, CCN2 and CCN3 was decreased after short exposure to CS whereas prolonged exposure altered the expression of CCN1, CCN3 and CCN4. Influenza A virus infection increased CCN1, 2, 4 and 5 mRNA levels but expression of CCN3 was significantly decreased. Acute CS exposure prior infection had little effect on the expression of CCN genes but prolonged exposure abolished the IAV-dependent induction. Treatment with LPS or LTA and infection with NTHi revealed that both Gram-positive and Gram-negative bacteria rapidly modulate the expression of the CCN genes. Our findings reveal that several triggers of lung inflammation influence differently the CCN genes. CCN3 deserves special attention since its mRNA expression is decreased by all the triggers studied.

Peptide immunotherapy in allergic asthma generates IL-10-dependent immunological tolerance associated with linked epitope suppression.

Treatment of patients with allergic asthma using low doses of peptides containing T cell epitopes from Fel d 1, the major cat allergen, reduces allergic sensitization and improves surrogate markers of disease. Here, we demonstrate a key immunological mechanism, linked epitope suppression, associated with this therapeutic effect. Treatment with selected epitopes from a single allergen resulted in suppression of responses to other ("linked") epitopes within the same molecule. This phenomenon was induced after peptide immunotherapy in human asthmatic subjects and in a novel HLA-DR1 transgenic mouse model of asthma. Tracking of allergen-specific T cells using DR1 tetramers determined that suppression was associated with the induction of interleukin (IL)-10(+) T cells that were more abundant than T cells specific for the single-treatment peptide and was reversed by anti-IL-10 receptor administration. Resolution of airway pathophysiology in this model was associated with reduced recruitment, proliferation, and effector function of allergen-specific Th2 cells. Our results provide, for the first time, in vivo evidence of linked epitope suppression and IL-10 induction in both human allergic disease and a mouse model designed to closely mimic peptide therapy in humans.

Role of breast regression protein 39 (BRP-39)/chitinase 3-like-1 in Th2 and IL-13-induced tissue responses and apoptosis.

Mouse breast regression protein 39 (BRP-39; Chi3l1) and its human homologue YKL-40 are chitinase-like proteins that lack chitinase activity. Although YKL-40 is expressed in exaggerated quantities and correlates with disease activity in asthma and many other disorders, the biological properties of BRP-39/YKL-40 have only been rudimentarily defined. We describe the generation and characterization of BRP-39(-/-) mice, YKL-40 transgenic mice, and mice that lack BRP-39 and produce YKL-40 only in their pulmonary epithelium. Studies of these mice demonstrated that BRP-39(-/-) animals have markedly diminished antigen-induced Th2 responses and that epithelial YKL-40 rescues the Th2 responses in these animals. The ability of interleukin13 to induce tissue inflammation and fibrosis was also markedly diminished in the absence of BRP-39. Mechanistic investigations demonstrated that BRP-39 and YKL-40 play an essential role in antigen sensitization and immunoglobulin E induction, stimulate dendritic cell accumulation and activation, and induce alternative macrophage activation. These proteins also inhibit inflammatory cell apoptosis/cell death while inhibiting Fas expression, activating protein kinase B/AKT, and inducing Faim 3. These studies establish novel regulatory roles for BRP-39/YKL-40 in the initiation and effector phases of Th2 inflammation and remodeling and suggest that these proteins are therapeutic targets in Th2- and macrophage-mediated disorders.

Strain-specific requirement for eosinophils in the recruitment of T cells to the lung during the development of allergic asthma.

Eosinophils have been implicated as playing a major role in allergic airway responses. However, the importance of these cells to the development of this disease has remained ambiguous despite many studies, partly because of lack of appropriate model systems. In this study, using transgenic murine models, we more clearly delineate a role for eosinophils in asthma. We report that, in contrast to results obtained on a BALB/c background, eosinophil-deficient C57BL/6 Delta dblGATA mice (eosinophil-null mice via the Delta DblGATA1 mutation) have reduced airway hyperresponsiveness, and cytokine production of interleukin (IL)-4, -5, and -13 in ovalbumin-induced allergic airway inflammation. This was caused by reduced T cell recruitment into the lung, as these mouse lungs had reduced expression of CCL7/MCP-3, CC11/eotaxin-1, and CCL24/eotaxin-2. Transferring eosinophils into these eosinophil-deficient mice and, more importantly, delivery of CCL11/eotaxin-1 into the lung during the development of this disease rescued lung T cell infiltration and airway inflammation when delivered together with allergen. These studies indicate that on the C57BL/6 background, eosinophils are integral to the development of airway allergic responses by modulating chemokine and/or cytokine production in the lung, leading to T cell recruitment.

Y-chromosomal insights into the genetic impact of the caste system in India.

The caste system has persisted in Indian Hindu society for around 3,500 years. Like the Y chromosome, caste is defined at birth, and males cannot change their caste. In order to investigate the genetic consequences of this system, we have analysed male-lineage variation in a sample of 227 Indian men of known caste, 141 from the Jaunpur district of Uttar Pradesh and 86 from the rest of India. We typed 131 Y-chromosomal binary markers and 16 microsatellites. We find striking evidence for male substructure: in particular, Brahmins and Kshatriyas (but not other castes) from Jaunpur each show low diversity and the predominance of a single distinct cluster of haplotypes. These findings confirm the genetic isolation and drift within the Jaunpur upper castes, which are likely to result from founder effects and social factors. In the other castes, there may be either larger effective population sizes, or less strict isolation, or both.