Autoimmune hepatitis. Part A: pathogenesis

Autoimmune hepatitis is a consequence of a triggering antigen and genetic factors that favor the presentation of autoantigens, polymorphisms that affect immunocyte activation and durability, cytokine alterations that promote proliferation of liver-infiltrating cytotoxic T cells, and perturbations in the number and function of immune-regulatory cell populations, including T regulatory cells and natural killer T cells. The triggering epitope is probably a short sequence peptide that is common in multiple infectious or toxic agents. Homologies between this epitope and self-antigens (molecular mimicry) may stimulate humoral and cellular responses that are cross-reactive. Sensitized immunocytes extend and perpetuate the inflammation through imprecise targeting of self-antigens that resemble foreign antigens (promiscuous behavior). The occurrence and clinical phenotype of the disease may relate to genetic susceptibility factors that favor protracted exposure to indigenous etiological agents, and these genetic factors can vary in different geographical regions and ethnic groups. The clinical phenotype within a population can be modified further by genetic polymorphisms that are not disease specific and that affect immunocyte activation, differentiation, proliferation and programmed death (apoptosis). Autoimmune hepatitis is a model of autoreactivity that reflects multiple disturbances in the counter-regulatory mechanisms essential for immune homeostasis.     

Component quantitation of rat ileum

Precise and accurate light microscopic morphometric analyses of biological tissue can be achieved utilizing component quantitative techniques. Component quantitation refers to measurements of the relative volumes of components in tissue sections. Such an assessment is predicated upon the mathematically verifiable assumption that direct quantitative relationships exist between an aggregate of profiles of a component contained per unit area in multiple sections and an aggregate of profiles contained per unit volume. A linear scanning device (micrometer component quantitator) was initially employed for quantitative analyses of pancreas. This quantitative technique has subsequently been applied to normal rat ileum conventionally processed for light microscopy, and the requisite sampling parameters have been defined. An identical technique was then applied to physiologically manipulated rat ileum—a gnotobiotic group, a group with ileal self-filling blind loops, and a group with ileal Thiry-Vella loops. The results observed support the following conclusions. (1) The volume percentage of the various components of the rat ileal wall of control animals was defined utilizing the micrometer component quantitator. (2) Hypertrophy of the ileal muscularis externa within the ileal self-filling blind loops was observed, probably secondary to mechanical obstruction. (3) Atrophy of the ileal epithelium within the gnotobiotic group and within the Thiry-Vella loops was observed, possibly secondary to an altered endogenous microbial flora. (4) Recognition of quantitative variations among the histological components of the intestinal wall in association with physiological manipulations or pathologic states was (is) feasible by utilization of this component quantitative technique.A portion of this paper was presented from platform at the 97th Annual Session of the American Association of Anatomists, Seattle, Washington, April 8–12, 1984.     

Studies of the immunoglobulin-producing cells of the human intestine: the defunctioned bowel.

An indirect immunoperoxidase method was used to visualise immunoglobulin-containing cells in the large intestinal mucosa of 10 children who had defunctioning colostomies. Intestine deprived of its usual exposure to intraluminal antigens contained less immunocytes per unit area than intestinal mucosa subjected to normal stimulation by dietary and microbial antigens. These findings substantiate in man the conclusion based on observations made on animals that continued mucosal exposure to antigenic stimulation is necessary for the existence of an adequate population of intestinal immunocytes.     

Colonic immunity in patients and amoebic liver abscess.

Secretory immunoglobulin A (S-IgA), coproantibody titre (antiamoebic) and IgA, IgG, IgM immunocytes in rectal mucosa were studied in 13 patients with amoebic liver abscess (ALA) prior to and 4-6 weeks after completion of antiamoebic therapy. Ten asymptomatic Entamoeba histolytica cyst passers and 17 healthy age and sex matched volunteers served as controls. Fecal S-IgA levels and counts of IgA bearing immunocytes in mucosa were significantly higher in patients with ALA and cyst passers as compared to healthy controls and showed a significant fall after treatment. Fecal antiamoebic antibodies were high in cyst passers and in cases of ALA after treatment. Raised levels of S-IgA and IgA class immunocyte counts probably indicate a local mucosal immune response directed at containing the infection.     

Intestinal B cell hyperactivity in AIDS is controlled by highly active antiretroviral therapy

BACKGROUND: It is well documented that highly active antiretroviral therapy (HAART) restores systemic immunity to human immunodeficiency virus (HIV) but the effect of this treatment on the mucosal immune system is less clear. AIMS: Because future preventive or therapeutic vaccines against HIV may be administered by the mucosal route, we wished to evaluate the effect of HAART on the activation level and homeostasis of the intestinal B cell system. PATIENTS AND METHODS: Duodenal biopsy specimens were collected consecutively from infection prone HIV positive adults (n = 31), mostly with advanced AIDS. In situ two colour immunofluorescence staining was performed to quantify mucosal immunoglobulin (Ig) class and subclass producing immunocytes (plasmablasts and plasma cells). RESULTS: HIV positive patients had, on average, duodenal proportions of IgA (74.6%), IgM (19.5%), and IgG (3.4%) immunocytes similar to median values recorded in 11 HIV seronegative healthy controls but the total immunocyte number per mucosal section length unit (500 microm) was significantly increased in patients (median 175 v 120 cells/unit; p<0.008), mainly comprised of IgA (p<0.02) and IgG1 (median 81.8% of total IgG; p<0.02) isotypes. Patients receiving a successful HAART regimen tended to normalise their IgG1 proportion and showed significantly lower total duodenal IgA immunocyte number than those receiving no or insufficient antiretroviral treatment (p<0.005). CONCLUSION: Our study demonstrated that advanced AIDS patients hyperactivate their intestinal B cell system. HAART could significantly reverse this perturbation, suggesting restored ability of the mucosal immune system to control intestinal infections.     

Structural and functional alterations in the colonic microbiome of the rat in a model of stress induced irritable bowel syndrome

Stress is known to perturb the microbiome and exacerbate irritable bowel syndrome (IBS) associated symptoms. Characterizing structural and functional changes in the microbiome is necessary to understand how alterations affect the biomolecular environment of the gut in IBS. Repeated water avoidance (WA) stress was used to induce IBS-like symptoms in rats. The colon-mucosa associated microbiome was characterized in 13 stressed and control animals by 16S sequencing. In silico analysis of the functional domains of microbial communities was done by inferring metagenomic profiles from 16S data. Microbial communities and functional profiles were compared between conditions. WA animals exhibited higher α-diversity and moderate divergence in community structure (β-diversity) compared with controls. Specific clades and taxa were consistently and significantly modified in the WA animals. The WA microbiome was particularly enriched in Proteobacteria and depleted in several beneficial taxa. A decreased capacity in metabolic domains, including energy- and lipid-metabolism, and an increased capacity for fatty acid and sulfur metabolism was inferred for the WA microbiome. The stressed condition favored the proliferation of a greater diversity of microbes that appear to be functionally similar, resulting in a functionally poorer microbiome with implications for epithelial health. Taxa, with known beneficial effects, were found to be depleted, which supports their relevance as therapeutic agents to restore microbial health. Microbial sulfur metabolism may form a key component of visceral nerve sensitization pathways and is therefore of interest as a target metabolic domain in microbial ecological restoration.     

A Morphometric Study of the Effects of Ethanol Consumption on Lactating Mammary Glands of Rats

Morphometric procedures were used to quantitate changes induced by ethanol in tissue components of rat mammary gland. Rats were pair-fed ethanol-containing or isocaloric control liquid diets formulated for pregnant or lactating animals, or maintained on regular laboratory chow. Short term animals were pair-fed ethanol or control diets from Day 1 of pregnancy through lactation Days 2 or 10. Long term animals were pair-fed ethanol or control diets for 25 days prior to mating, and then through pregnancy to lactation Days 2 or 10. Point counting was used to determine the volume fractions (vf) of alveolar epithelium, lumen, and connective tissue in the mammary glands. In chow-fed animals the percentage of alveolar epithelium remained constant from late pregnancy through lactation, while the amount of connective tissue decreased and that of alveolar lumen increased. This indicates the sensitivity of this procedure to detect changes in tissue volume fractions during mammary proliferation. No changes from the normal controls were found in any tissue component in short term ethanol or pair-fed animals. At lactation Day 2, long term ethanol-treated animals demonstrated a significant decrease in the percentage of alveolar epithelium and a significant increase in the percentage of total connective tissue as compared to pair-fed and chow-fed control animals. However, by Day 10 of lactation, no changes were found in any of the tissue components in long-term ethanol-fed versus control animals. These results indicate that ethanol consumption can alter mammary gland structure during the early stages of lactation, even when adequate levels of dietary protein are maintained.     

Stimulatory effects of opioid neuropeptides on locomotory activity and conformational changes in invertebrate and human immunocytes: evidence for a subtype of delta receptor.

The presence of opioid neuropeptides was shown to stimulate conformational changes and locomotory activity in immunocytes of two representatives of invertebrates as well as in human leukocytes. Cells were examined by use of phase-contrast and Nomarski optics coupled with a Zeiss Axiophot microscope, and of the Zeiss Videoplan/Vidas Image Analysis system. Immunocompetent blood cells, activated by exogenous opioids or stressful stimuli presumed to engage endogenous opioids, showed flattening, elongation, and formation of pseudopodia. In the mollusc Mytilus edulis, ameboid movements resulted in the formation of cell clusters, an activity not observed in untreated controls, or in immunocytes simultaneously exposed to opioid and naloxone. Tests with nine immunoreactive substances revealed immunocyte stimulation by delta, mu-, kappa-, and epsilon(?)-selective ligands. One of these, [D-Ala2,D-Met5]enkephalinamide (DAMA), active at a concentration of 10 pM, proved to be considerably more effective than the rest. The high pharmacological potency of DAMA, observed in both human and invertebrate immunocytes, sets this opioid apart from the closely related [D-Ala2,D-Leu5]enkephalin, a discrepancy not occurring in the mammalian nervous system. This suggests a specific function for [Met]enkephalin in immunoregulation, mediated perhaps by a special subtype of delta receptor.     

Surfactant homeostasis is maintained in vivo during keratinocyte growth factor-induced rat lung type II cell hyperplasia

Keratinocyte growth factor (KGF) induces transient proliferation of alveolar type II cells (AEII) associated with surfactant alterations. To test the hypothesis that homeostasis of intracellular phospholipid stores is maintained under KGF-induced hyperplasia, we (1) collected tissue from adult rat lungs, fixed for light and electron microscopy 3 days after intratracheal instillation of 5 mg recombinant human (rHu) KGF/kg body weight or phosphate-buffered saline (PBS), and from untreated control animals (five animals/group) for design-based stereology of AEII and lamellar body (LB) ultrastructure; and (2) we analyzed uptake and distribution of instilled radiolabeled phospholipids. After rHuKGF, AEII-coverage of alveolar walls (PBS:8.3 +/- 3.0%; rHuKGF:30.6 +/- 4.8%) and number of AEII/ml lung volume (PBS:28.5 +/- 6.5 x 10(6); rHuKGF:48.2 +/- 5.8 x 10(6)) were increased (p < 0.008). Number (PBS:97 +/- 25; rHuKGF:54 +/- 7) and volume (PBS:45.3 +/- 13.8 microm(3); rHuKGF:21.0 +/- 4.7 microm(3)) of LBs per cell were decreased (p < 0.008), but not total amount/ml lung volume (PBS:128 +/- 46. 4 x 10(7) microm(3); rHuKGF:103 +/- 34. 7 x 10(7) microm(3)). This was paralleled by a shift to larger LBs. After rHuKGF, radiolabeled phospholipids accumulated in whole lung tissue relative to lavage fluid (p < 0.01). However, less radiolabel was incorporated per cell (p < 0.01). We conclude that under rHuKGF-induced AEII proliferation intracellular surfactant was decreased per single cell, whereas a constant amount was maintained per unit lung volume. We suggest that surfactant homeostasis is regulated at the level of phospholipid transport processes, for example, secretion and reuptake.     

Immunoglobulins in jejunal mucosa and serum from patients with adult coeliac disease.

Immunoglobulin (Ig)-containing cells were quantitated immunohistochemically in biopsy specimens from the proximal jejunal mucosa. The numbers of IgA, IgM, and IgG immunocytes in a defined "mucosal tissue unit" were, on the average, raised 2.4, 4.6, and 6.5 times, respectively, when 13 adult patients with untreated coeliac disease (CD) were compared with 15 patients who had a histologically normal mucosa. The IgA-:IgM-:IgG-cell ratios averaged 66:28:6 in untreated CD and 79:18:2.6 in the controls. Similar quantitative data in 10 patients with treated CD were intermediate. IgD- and IgE-containing cells were rare in all patient groups. Most patients in a heterogeneous malabsorption group showed a jejunal Ig-containing cell population similar to that seen in CD, indicating that the local immunocyte pattern may not be specific for the latter disease. The only significant alteration in serum Ig levels related to CD was a raised concentration of IgA compared with normal. This was consistent with the increased amounts of extracellular IgA revealed in the mucosa. However, there was no indication of a defect in the transport of dimeric IgA and IgM through SC-producing cells, which in the CD mucosa were present in both crypt and surface epithelium.     

Local immunoglobulin production is different in gastritis associated with dermatitis herpetiformis and simple gastritis

The degree of inflammation and atrophy in gastric body mucosal specimens (n = 38) from 28 patients with dermatitis herpetiformis (DH) was graded histologically. Immunoglobulin (Ig) producing cells were enumerated by paired immunofluorescence staining in a 500 microns wide section area from the muscularis mucosae to the lumen (mucosal 'tissue unit'). The number of immunocytes of the three main classes (IgA, IgM, and IgG) was significantly raised with increasing degree of gastritis. All three classes were increased in specimens showing atrophy compared with those without atrophy. IgA cells predominated as in simple gastritis, but a striking difference was a marked increase of IgM cells in specimens with the most pronounced DH-associated gastritis. Relative class distribution of immunocytes within different mucosal zones showed that the percentage of IgA cells was significantly higher in the luminal than in the basal zone, whereas the contrary was true for IgG cells. IgM cells did not show any zonal preference. No relation was seen between small bowel and gastric lesions. The disproportionate increase of gastric IgM producing cells in DH might nevertheless reflect seeding of precursor cells of the secretory immune system generated in the proximal small intestine where the local IgM response is relatively pronounced.     

Distribution of secretory component and immunoglobulins in the developing lung

The distribution of secretory component and immunoglobulins, IgA, IgG, and IgM in developing human lungs, ranging in age from 12 gestational weeks to 8 yr, was studied using the indirect peroxidase-labeled antibody method. Secretory component appeared first in the serous bronchial epithelium near the duct openings at the sixteenth gestational week, in goblet cells at the twentieth gestational week, and in bronchiolar epithelium at the twenty-second gestational week. The number of secretory component-positive cells (Y) in bronchial epithelium increased with age in weeks (X) according to the formula Y = 15.328 + 0.094 log X (p less than 0.001). Secretory component was almost absent in bronchiolar epithelium in atelectatic lungs with or without hyaline membrane. Secretory component-positive cells were never observed below the levels of respiratory bronchioles. Immunocompetent cells appeared in the bronchial walls after birth in normal lungs, but they appeared at the thirty-ninth gestational week in cases of pulmonary infection. The IgA- and IgM-containing cells were present around the bronchial glands in normal lungs, whereas IgG-containing cells were not associated with the glands. In normal lungs, IgA-containing cells were most prominent, followed by IgM-containing cells and IgG-containing cells. The apical portions of serous epithelial cells of bronchi and bronchial glands were positive for IgA and IgM. In bacterial and viral pulmonary infections, IgA-containing cells increased in number in the bronchial glands. The appearance of secretory component-positive cells long before immunoglobulin-containing cells in fetal lungs suggests that secretory component may play some role in the formation of the mucociliary blanket, instead of selective transport of IgA or IgM.     

Mucosal subclass distribution of immunoglobulin G-producing cells is different in ulcerative colitis and Crohn's disease of the colon

As a marked local immunoglobulin G (IgG) response has previously been found to be the most prominent immunopathological feature of both ulcerative colitis and Crohn's disease, the subclass distribution of colonic IgG-producing immunocytes was examined. This study included tissue specimens from 10 patients with ulcerative colitis and 8 with Crohn's colitis. Paired immunofluorescence staining was performed with subclass-specific murine monoclonal antibodies combined with a rabbit antibody reagent of IgG; the proportion of cells belonging to each subclass could thereby be determined in relation to the total number of mucosal IgG immunocytes. A significantly higher median proportion of IgG1 immunocytes was found in ulcerative colitis (81.3%) than in Crohn's colitis (66.5%). Conversely, the median proportion of IgG2 immunocytes was significantly higher in Crohn's colitis (24.9%) than in ulcerative colitis (9.4%). This disparity in the local IgG subclass response might reflect dissimilar mucosal exposure to mitogenetic or antigenic stimuli or genetically determined immunoregulatory differences in the two categories of patients.     

Assessing response to treatment of exacerbations of bronchiectasis in adults

The present study aimed to assess the effect of intravenous antibiotic therapy on clinical and laboratory end-points in exacerbations of noncystic fibrosis bronchiectasis and to determine whether the outcomes were influenced by the pathogenic organism isolated. A prospective cohort study was conducted from November 2006 to March 2008 of exacerbations requiring intravenous antibiotics. End-points included 24-h sputum volume, forced expiratory volume in one second (FEV(1)), forced vital capacity (FVC), incremental shuttle walk test, qualitative sputum microbiology, white cell count, erythrocyte sedimentation rate, C-reactive protein (CRP) and St George's Respiratory Questionnaire (SGRQ). Exacerbations due to Pseudomonas aeruginosa were compared with exacerbations due to other potential pathogenic organisms. In total, 32 exacerbations were studied. Following 14 days of intravenous antibiotics, all outcomes significantly improved independent of a pathogenic organism, except FEV(1) and FVC. The most responsive markers were: 24-h sputum volume (reduced in all patients and 80% had >/=50% reduction); sputum bacterial clearance (78.1%); CRP (>/=75% reduction in 62.5%) and SGRQ (>/=4 unit improvement in 89.7%). CRP, 24-h sputum volume and SGRQ improved independent of microbial clearance. In the current study, 24-h sputum volume, microbial clearance, C-reactive protein and St George's Respiratory Questionnaire were the most useful parameters to assess response to treatment of exacerbations of bronchiectasis. Outcomes were similar independent of the pathogenic organism with the exception of forced expiratory volume in one second and forced vital capacity.     

Effect of alcohol-feeding on IgA-producing immunocytes in the small intestine of rats with and without jejunoileal bypass

Alcohol-feeding to rats subjected to jejunoileal bypass operation has been shown to lead to marked liver injury (fatty liver, necrosis and inflammation). This study investigated the influence of alcohol-feeding over a period of 3 months on the number of IgA-producing immunocytes and the villus surface area in various sections of the small intestine of rats subjected to a jejunoileal bypass or a sham operation. A jejunoileal bypass in animals receiving the control diet led to a decrease in the number of IgA-producing immunocytes in the duodenum and ileum, but not in the bypassed (blind) loop of the jejunum. In animals subjected to a jejunoileal bypass, alcohol-feeding led to an increase in the number of IgA-producing immunocytes in the duodenum and the bypassed jejunal loop as compared with animals with a jejunoileal bypass receiving the control diet. Among the animals with a jejunoileal bypass fed the control diet, the villus surface area within the duodenum and ileum increased as compared with the groups of sham-operated rats. The feeding of alcohol prevented this increase in the villus surface area in animals with a jejunoileal bypass. The increase in the number of IgA-producing immunocytes induced by alcohol in the animals with a jejunoileal bypass, in the duodenum and bypassed jejunum, supports the assumption of a change in antigen uptake in these parts of the small intestine, brought about by alcohol.     

Vitamin D3 administration induces nuclear p27 accumulation, restores differentiation, and reduces tumor burden in a mouse model of metastatic follicular thyroid cancer

We have previously demonstrated in vitro that 1alpha,25-dihydroxyvitamin D3 (calcitriol) treatment increases p27 expression and decreases cell proliferation in cultured thyroid carcinoma cell lines. We hypothesized that in vivo treatment with calcitriol would have a beneficial effect on thyroid carcinoma growth and progression. Five x 10(6) WRO (human thyroid follicular carcinoma derived) cells were implanted in the neck in 4- to 5-wk-old female SCID mice in an orthotopic xenograft model. Animals (n = 15) were treated i.p. three times a week for 21 d with 0.75 microg/kg calcitriol or vehicle. Mice were killed 21 d after tumor implantation, tumor volume was measured, and excised tumor tissue was examined by light microscopy and immunohistochemistry for p27 and thyroglobulin reactivity. Average tumor volume in control mice after 21 d of vehicle treatment was 2002 +/- 207 mm3 compared with a mean tumor volume of 1241 +/- 115 mm3 in animals receiving calcitriol, reflecting a 38% reduction in tumor volume size (P < 0.003). Tumors from vehicle-treated animals demonstrated morphological features of epithelial malignancies with characteristics of insular carcinoma and multiple metastases to the lungs. Tumors excised from calcitriol-treated animals demonstrated signs of differentiation with restoration of thyroglobulin staining. This was associated with a marked accumulation of p27 immunoreactivity in the nuclear compartment. These studies demonstrate that in vivo calcitriol administration can effectively restore p27 accumulation in thyroid carcinoma cells, an effect associated with appreciably enhanced cellular differentiation, reduction in tumor burden, and prevention of metastatic growth.     

Predictors of Change in Left-Ventricular Structure and Function in a Trial of Extended Hours Hemodialysis

BackgroundMyocardial pathology is common in patients undergoing hemodialysis. To explore the effects of differing aspects of dialysis treatment on its evolution, we examined the impact of change in markers of volume status, hemodynamics and solute clearance on left ventricular (LV) parameters in a randomized trial of extended hours dialysis.Methods and ResultsA Clinical Trial of IntensiVE (ACTIVE) Dialysis randomized 200 patients undergoing hemodialysis to extended dialysis hours (≥ 24 hours/week) or standard hours (12–18 hours/week) for 12 months. In a prespecified substudy, 95 participants underwent cardiac magnetic resonance imaging (CMR) at baseline and at the study's end. Generalized linear regression was used to model the relationship between changes in LV parameters and markers of volume status (normalized ultrafiltration rate and total weekly interdialytic weight gain), hemodynamic changes (systolic and diastolic blood pressure) and solute control (urea clearance, dialysis hours and phosphate). Randomization to extended hours dialysis was not associated with change in any CMR parameter. Reduction in ultrafiltration rate was associated with reduction in LV mass index (P = 0.049) and improved ejection fraction (P = 0.024); reduction in systolic blood pressure was also associated with improvement in ejection fraction (P = 0.045); reduction in interdialytic weight gain was associated with reduced stroke volume (P = 0.038). There were no associations between change in urea clearance, phosphate or total hours per week and CMR parameters.ConclusionsReduction in ultrafiltration rate and blood pressure are associated with improved myocardial parameters in hemodialysis recipients independently of solute clearance or dialysis time. These findings underscore the importance of fluid status and related parameters as potential treatment targets in this population.     

Predictors of outcome of percutaneous catheter drainage in patients with acute pancreatitis having acute fluid collection and development of a predictive model

BackgroundPercutaneous catheter drainage (PCD) is effective initial strategy in the step-up approach of management of acute pancreatitis (AP). The objective of this study was to identify factors associated with outcomes after PCD and develop a predictive model.Method and materialsIn a prospective observational study between July 2016 and Nov 2017, 101 consecutive AP patients were treated using a “step-up approach” in which PCD was used as the first step. We evaluated the association between success of PCD (survival without necrosectomy) and baseline parameters viz. etiology, demography, severity scores, C-reactive protein (CRP), and intra-abdominal pressure (IAP), morphologic characteristics on computed tomography (CT) [percentage of necrosis, CT severity index (CTSI), characteristics of collection prior to PCD (volume, site and solid component of the collection), PCD parameters (initial size, maximum size, number and duration of drainage) and factors after PCD insertion (fall in IAP, reduction in volume of collection).ResultsAmong 101 patients, 51 required PCD. The success rate of PCD was 66.66% (34/51). Four patients required additional surgical necrosectomy after PCD. Overall mortality was 29.4% (15/51). Multivariate analysis showed percentage of volume reduction of fluid collection (p = 0.016) and organ failure (OF) resolution (p = 0.023) after one week of PCD to be independent predictors of success of PCD. A predictive model based on these two factors resulted in area under curve (AUROC) of 0.915. Nomogram was developed with these two factors to predict the probability of success of PCD.ConclusionOrgan failure resolution and reduction in volume of collection after one week of PCD are significant predictors of successful PCD outcomes in patients with fluid collection following AP     

High-dose synthetic progestogens inhibit foam and smooth muscle cell proliferation and atherosclerotic plaque formation in aortas of rabbits fed a hypercholesterolemic diet

Female rabbits on a hypercholesterolemic atherogenic diet were treated with high doses of the synthetic progestogens norethisterone and medroxyprogesterone acetate in order to clarify the effect and possibly some of the mechanism of action of these hormones on diet-induced atherogenesis. We employed morphometric studies to determine the surface area of the rabbit aorta occupied by and the maximum thickness of lipid plaques. Autoradiography with tritiated thymidine was performed to demonstrate the effect of the progestogens on cell proliferation, which plays a key role in atherogenesis. Medroxyprogesterone acetate-treated and, above all, norethisterone-treated animals exhibit a more marked reduction of atherosclerosis than control rabbits fed the same diet. Our results suggest that both progestogens we used inhibit the development of atherosclerosis mainly by blocking the proliferation of smooth muscle cells in the tunica media and the cell population of the plaque.