Small Thymus Research Articles

Human G protein-coupled receptor GPR-9-6/CC chemokine receptor 9 is selectively expressed on intestinal homing T lymphocytes, mucosal lymphocytes, and thymocytes and is required for thymus-expressed chemokine-mediated chemotaxis.

TECK (thymus-expressed chemokine), a recently described CC chemokine expressed in thymus and small intestine, was found to mediate chemotaxis of human G protein-coupled receptor GPR-9-6/L1.2 transfectants. This activity was blocked by anti-GPR-9-6 monoclonal antibody (mAb) 3C3. GPR-9-6 is expressed on a subset of memory alpha4beta7(high) intestinal trafficking CD4 and CD8 lymphocytes. In addition, all intestinal lamina propria and intraepithelial lymphocytes express GPR-9-6. In contrast, GPR-9-6 is not displayed on cutaneous lymphocyte antigen-positive (CLA(+)) memory CD4 and CD8 lymphocytes, which traffic to skin inflammatory sites, or on other systemic alpha4beta7(-)CLA(-) memory CD4/CD8 lymphocytes. The majority of thymocytes also express GPR-9-6, but natural killer cells, monocytes, eosinophils, basophils, and neutrophils are GPR-9-6 negative. Transcripts of GPR-9-6 and TECK are present in both small intestine and thymus. Importantly, the expression profile of GPR-9-6 correlates with migration to TECK of blood T lymphocytes and thymocytes. As migration of these cells is blocked by anti-GPR-9-6 mAb 3C3, we conclude that GPR-9-6 is the principal chemokine receptor for TECK. In agreement with the nomenclature rules for chemokine receptors, we propose the designation CCR-9 for GPR-9-6. The selective expression of TECK and GPR-9-6 in thymus and small intestine implies a dual role for GPR-9-6/CCR-9, both in T cell development and the mucosal immune response.

Small thymus in very low birth weight infants born to mothers with subclinical chorioamnionitis

Chorioamnionitis, a major cause of preterm birth with significant neonatal morbidity and mortality, frequently occurs in mothers who are free of symptoms. A combined clinical, radiologic, and pathologic study of 129 very low birth weight infants indicated a significant association between a markedly decreased thymic size at birth and subclinical chorioamnionitis. (J Pediatr 1999;135:384-6)

The nutritional toxicity of sweet lupin (Lupinus angustifolius) seed proteins

The effects of raw sweet lupin (Lupinus angustifolius) meal on the growth and N utilisation of rats were determined in two ad libitum and two restricted-feeding net protein utilisation (NPU) and five N balance experiments. Sweet lupin seed grown in Western Australia, obtained as meal, either unsupplemented (LMU) or fully supplemented with required amino acids (360 g kg−1) (LMFS), was tested. Rats fed lactalbumin (130 g kg−1) (LACT) were used as positive controls, while rats fed a non-protein diet (NPC) were used as negative controls. In addition, seed protein, extracted at pH 7.0 with water and insoluble after dialysis at pH 7.0 (LPADI; 124 g kg−1), was also used. The diets contained the same amounts of energy and protein and were supplemented with essential amino acids, vitamins and minerals to target requirements for rats. Inclusion of LPADI in the diet of growing rats caused urinary losses of N, almost all as urea, hypoproteinaemia and increase in body water that resulted in the lowest NPU values, N balance and growth rate as compared with other diets used. These rats developed atrophy of the spleen (low dry weight) and had a comparatively smaller thymus gland than those given raw meals. Furthermore, the LPADI fraction was shown by sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE) to contain three polypeptides with molecular weights between 30 and 36 kDa which are similar to lectins obtained from Phaseolus vulgaris, Abrus precatorius and Ricinus communis. It is possible that the toxic protein component in the sweet lupin, which has negligible in vitro haemagglutination properties and is extremely toxic in vivo, exerts toxicity by interfering with protein synthesis in the liver, while the immune responses are secondary to azotaemia (high level of urea in the blood) or cytotoxicity action on lymphocytes. The unusual depletion of fat from the body, however, was due to the failure of absorbed amino acids to assimilate as proteins, creating dietary protein restriction and leading to lipolysis. It is therefore tentatively suggested that sweet lupin seed contains a lectin-like protein that is concentrated in this fraction. Further purification and biological evaluation to establish the exact nature of this protein may be important. © 2000 Society of Chemical Industry

Essential defects of athymic nude mice affect also the nu/+ heterozygotes.

T h e nu gene has been mapped to a 9-cM region of chromosome 11; it encodes a winged helix transcription factor, expressed in the skin and thymus (Reth 1995). The resulting block in T cell development can be alleviated by IL-7 (Rich and Leder 1995). The thymic dysgenesis is influenced also by the lack ofmesenchymal invasion of the anlage in the fetal stage (Groscurth and Kistler 1975) and we assume that a mesenchymal defect is not secondary to the thymic epithelial defect (Holub 1989). This is shown by the present study on nu/+ hybrids which have, compared to +/+ homozygotes, a much smaller thymus, albeit with a normally differentiated lymphoid cortex and medulla.

Detection of Activated Caspase-3 by a Cleavage Site-Directed Antiserum during Naturally Occurring DRG Neurons Apoptosis

We prepared a cleavage site-directed antiserum against Caspase-3 (anti-p20/17), which reacts with the p20/17 fragment (p20/17) activated by cleavage but not proCaspase-3 (p32), and examined the relationship between the activation of Caspase-3 and apoptosis. We identified p20/17-positive cells where cell death occurs naturally: interdigits of the forelimbs, small intestine epithelium, thymus, trigeminal ganglia, and dorsal root ganglia of mouse embryos. Withdrawal of nerve growth factor induced the appearance of p20/17-positive cells with DNA fragmentation in the culture of dorsal root ganglia neurons, while DNA fragmentation was detected in both p20/17-positive and -negative neurons in dorsal root ganglia of mouse embryos. These results suggest that not only activation of Caspase-3 but also other molecular mechanism play a role in the naturally occurring dorsal root ganglia apoptosis. Cleavage site-directed antisera against Caspases will be useful for the analysis of the molecular mechanism of naturally occurring apoptosis during development.

T cell development and activation in Jak3-deficient mice.

Jak3, a member of the Janus family of tyrosine kinases, participates in signaling through cytokine receptors that contain the common gamma-chain, including the receptors for interleukin (IL)-2, IL-4, IL-7, IL-9, and IL-15. Jak3- and gamma c-deficient mice have pleiotropic defects that can be attributed to their inability to respond to multiple specific cytokines. A great deal of recent work has focused on the T cell defects in these mutant mice. Specifically, Jak3- and gamma c-deficient mice have small thymuses revealing a defect in T cell development, and in addition, have functionally unresponsive peripheral T cells with an activated/memory cell phenotype. The thymic defect in these mutant mice strongly resembles that seen in IL-7 and IL-7 receptor knockout mice, suggesting that the lack of IL-7 receptor signaling accounts for this defect in Jak3-/- and gamma c- mice. To characterize this defect further, we have examined the earliest stages of T cell development in adult and fetal Jak3-/- thymuses. These studies identify two discrete developmental defects at the CD4-CD8- stage of T cell maturation. Analyses of peripheral T cells in Jak3-/- and gamma c- mice have also revealed a number of abnormalities. All of the T cells in these mutant mice have an activated phenotype and a large fraction of them are proliferating in vivo. In addition, Jak3-/- and gamma c- T cells are more prone to undergo apoptosis than wild-type T cells. Together, these features account for the decreased IL-2 secretion by in vitro-stimulated Jak3-/- T cells. Overall, many of the lymphoid defects of Jak3- and gamma c-deficient mice can be accounted for by the lack of IL-7R and IL-2R signaling; however, other cytokine systems must also be involved in maintaining peripheral T cell homeostasis.

In situ Hybridization on Blood Smears for Diagnosis of Chicken Anemia Virus in Broiler Breeder Flocks

Chicken anemia virus (CAV) infection was suspected in progeny from broiler breeder hens raised in a location that was geographically different from where most pullets were raised for one Georgia production company. These breeder pullets were raised in new or relatively new houses. Progeny from these pullets experienced a 10-fold increase in average daily mortality over progeny from flocks raised in established breeder houses. Lesions in the broilers included severe necrotizing dermatitis, pale bone marrow, and small thymus glands. Infection with CAV was confirmed in the breeders by enzyme-linked immunosorbent assay, indirect immunofluorescence, and in situ hybridization on peripheral blood smears. The in situ hybridization identified three broiler breeder flocks that were actively infected with CAV and were probably the source of infection in broiler flocks.

Detection of canine parvovirus in naturally infected dogs with enteritis and myocarditis by in situ hybridization.

An improved method for the diagnosis of canine parvovirus using in situ hybridization in standard formalin-fixed, paraffin-embedded tissue sections was developed. A digoxigenin-labeled probe complementary to DNA sequences that code for the entire sequence of the capsid protein VP-1 and the middle part of the sequence of the capsid protein VP-2 was designed. Specific histologic localization of canine parvovirus-infected cells was demonstrated in small intestine, tonsil, lymph node, thymus, spleen, heart, liver, and kidney from dogs diagnosed at necropsy with canine parvovirus infection. The in situ hybridization accurately pinpointed the specific sites of viral infection. The detection of canine parvovirus in liver, kidney, and heart tissues together in the same pups could represent an enhanced virulence of this strain of canine parvovirus and suggests a broadened tissue tropism not seen before in Korean strains of canine parvovirus.

Genetic mapping of genes regulating the thymus size in back-cross rats between the laboratory BUF/Mna strain and the MITE strain derived from the wild rat, Rattus norvegicus.

The thymoma prone BUF/Mna (B) rat is a useful model for studying the genes responsible for thymus enlargement during the stage of young growth. Among the strains of rats, B rats have the largest thymuses at all stages of life. A locus, Ten-1, which contributes to thymus enlargement in back-cross (BC) rats between the B and WKY/NCrj (W) strains, was mapped on chromosome 1. To determine the precise location of the locus, ¿B x(B x MITE)F1¿ BC rats were generated by crossing the B strain with the inbred MITE (M) strain, which was established from captured, Japanese wild rats, and were examined by linkage study using polymerase chain reaction with 67 microsatellite markers. Linkages with thymus enlargement were found in genotypes of seven markers, BSIS, LSN, MYL2, IGF2, PBPC2, D1Mgh11, and D1MIt6, by chi2-test and Student's t-test, which confirmed the presence of the genetic locus associated with thymus enlargement, Ten-1, in this region. Paradoxically, a suppressive locus, Tsu-1, to thymus enlargement was also found on chromosome 3, showing linkages of phenotype of the small thymus with genotypes of SCN2A, CAT, D3MIt16, and D3MIt13. By analyses of MAPMAKER/EXP and MAPMAKER/QTL, Ten-1 was mapped at 4.6 cM proximal from IGF2 locus on chromosome 1 and Tsu-1 at 4.0 cM proximal from CAT locus on chromosome 3, respectively.

The Human MCP-2 Gene (SCYA8): Cloning, Sequence Analysis, Tissue Expression, and Assignment to the CC Chemokine Gene Contig on Chromosome 17q11.2

Monocyte chemotactic proteins (MCPs) form a subfamily of chemokines that recruit leukocytes to sites of inflammation and that may contribute to tumor-associated leukocyte infiltration and to the antiviral state against HIV infection. With the use of degenerate primers that were based on CC chemokine consensus sequences, the known MIP-1α/LD78α, MCP-1, and MCP-3 genes and the previously unidentified eotaxin and MCP-2 genes were isolated from a YAC contig from human chromosome 17q11.2. The amplified genomic MCP-2 fragment was used to isolate an MCP-2 cosmid from which the gene sequence was determined. The MCP-2 gene shares with the MCP-1 and MCP-3 genes a conserved intron–exon structure and a coding nucleotide sequence homology of 77%. By Northern blot analysis the 1.0-kb MCP-2 mRNA was predominantly detectable in the small intestine, peripheral blood, heart, placenta, lung, skeletal muscle, ovary, colon, spinal cord, pancreas, and thymus. Transcripts of 1.5 and 2.4 kb were found in the testis, the small intestine, and the colon. The isolation of the MCP-2 gene from the chemokine contig localized it on YAC clones of chromosome 17q11.2, which also contain the eotaxin, MCP-1, MCP-3, and NCC-1/MCP-4 genes. The combination of using degenerate primer PCR and YACs illustrates that novel genes can efficiently be isolated from gene cluster contigs with less redundancy and effort than the isolation of novel ESTs.

The single positive T cells found in CD3-zeta/eta-/- mice overtly react with self-major histocompatibility complex molecules upon restoration of normal surface density of T cell receptor-CD3 complex.

CD3-zeta/eta-deficient mice have small thymuses containing cells that show a profound reduction in the surface levels of T cell receptors and terminate their differentiation at the CD4+CD8+ stage. Rather unexpectedly, CD3- or very low single positive T cells accumulate over time in the spleen and lymph nodes of CD3-zeta/eta-deficient mice after a process dependent on MHC expression. Fusion of these peripheral T cells with a CD3-zeta-positive derivative of the BW5147 TCR-alpha-/beta- thymoma resulted in hybridomas that do express an heterogeneous set of T cell receptor alpha/beta dimers at their surface and at density comparable to those found in hybridomas derived from wild-type peripheral T cells. We have investigated the specificities of these T cell receptors using spleen cells from congenic and mutant mouse strains, and showed that the majority of them readily recognized self-MHC class I or class II molecules. These results demonstrate that by increasing the density and/or output of the T cell receptors expressed in peripheral T cells, one can confer them with the capacity to respond to normal density of self-MHC molecules.

Peripheral expression of Jak3 is required to maintain T lymphocyte function.

The Jak family tyrosine kinase, Jak3, is involved in signaling through cytokine receptors that utilize the common gamma chain (gammac), such as those for IL-2, IL-4, IL-7, IL-9, and IL-15. Recent studies of Jak3-deficient mice and humans have demonstrated that Jak3 plays a critical role in B and T lymphocyte maturation and function. The T lymphocyte defects in Jak3-deficient mice include a small thymus, a decrease in peripheral CD8+ cells, an increase in the surface expression of activation markers, and a severe reduction in proliferative and cytokine secretion responses to mitogenic stimuli. To determine whether the peripheral T lymphocyte defects result from aberrant maturation in the thymus or from the absence of Jak3 protein in peripheral T cells, we generated reconstituted mice that express normal levels of Jak3 protein in the thymus but lose Jak3 expression in peripheral T cells. Jak3 expression in the thymus restores normal T cell development, including CD8+, gammadelta, and natural killer cells. However, the loss of Jak3 protein in peripheral T cells leads to the Jak3-/- phenotype, demonstrating that Jak3 is constitutively required to maintain T cell function.

Developmental Toxicity Study in Rats and Rabbits Administered an Emulsion Containing Medium Chain Triglycerides as an Alternative Caloric Source

Triglyceride-containing lipid emulsions have been designed as caloric sources that can be administered intravenously to patients that cannot meet their nutritional needs by conventional parenteral therapies. In this study, we evaluate the developmental toxicity of a 20% lipid emulsion that contains a 3:1 ratio of medium chain triglyceride (MCT) to one long chain containing lipid emulsion (LCT). This emulsion was administered by intravenous infusion to rats and rabbits at dosages of 1 and 4.28 g lipid/kg body weight (g lipid/kg) at dose volumes of 5 and 21.4 mL/kg, respectively, once daily during organogenesis to assess the potential developmental toxicity of the test article. The control group received 0.9% saline at a dose volume of 21.4 mL/kg. Animals were observed for clinical signs of toxicity and adverse effects on body weights and feed consumption. On Day 20 (rats) or Day 29 (rabbits), females were necropsied and examined for maternal and embryo/fetal toxicity. Fetuses were removed, weighed, and examined for external, soft tissue, and skeletal abnormalities. Dosages of 4.28 g lipid/kg resulted in lower feed consumption for rats and rabbits, an expected finding based on the high-caloric nature of the test article. Potentially test article-related gross necropsy findings, including enlarged lymph nodes and spleen, small thymus, and enlarged renal pelvis, for rats given 4.28 g lipid/kg were present at a low incidence. There were no adverse effects on fetal parameters for rats even in the presence of some maternal toxicity. However, embryo and fetal toxicity (i.e., resorptions) and skeletal abnormalities were present for rabbits given 4.28 g lipid/kg. Under the conditions of this study, the no-observable-effect level for developmental toxicity was greater than or equal to 4.28 g lipid/kg for rats and greater than or equal to 1 g lipid/kg but less than 4.28 g lipid/kg for rabbits.

Carcinogenicity of the Anticancer Topoisomerase Inhibitor, Amsacrine, in Wistar Rats

Amsacrine is an antineoplastic drug used in the treatment of acute adult leukemias. To assess its carcinogenic potential, groups of 50 male and 50 female rats were administered amsacrine by lateral tail vein injection at 0 (vehicle control), 0.25, 1, or 3 mg/kg once daily for 5 days, followed by a 23-day recovery period. This cycle of dosing and recovery was repeated a total of six times. The animals were then maintained without dosing for an 18-month observation period. During the dosing phase, signs of toxicity were limited to the 3 mg/kg animals and included alopecia, diarrhea, injection site lesions, and skin and subcutaneous nodules. Statistically significant reductions in body weight gain and food consumption also occurred at 3 mg/kg during each 5-day dosing period followed by recovery during the latter 3 weeks of each cycle. Except for skin and subcutaneous nodules, signs of toxicity in the 3 mg/kg animals ultimately disappeared during the 18-month observation phase. Survival at study termination for the vehicle control, 0.25, 1, and 3 mg/kg groups was 56, 52, 34, and 0%, respectively, in males, and 64, 48, 54, and 4%, respectively, in females. Mortality was primarily due to bone marrow suppression during the dosing phase, chronic progressive nephropathy, or development of tumors. Incidences of the following tumors were significantly increased in the 3 mg/kg groups of both sexes (Fisher exact test, two-tailed,p< 0.01): all malignancies; all tumors of the small intestine, adenocarcinoma and adenoma of the small intestine, all tumors of the skin, and squamous cell papilloma. Other tumor incidences that were significantly increased in the 3 mg/kg males were thymoma and multiple neoplastic histotypes of the skin and adnexa including basal cell tumor, fibroma, sebaceous gland adenoma, and squamous cell carcinoma. A disproportionate number of the skin tumors were located on the tail, suggesting a localized tissue concentration effect. In the 3 mg/kg females, significantly increased tumor incidences also included all tumors of the mammary gland, adenocarcinoma of the mammary gland, all tumors of the uterine horn, and endometrial stromal polyps of the uterine horn. The 1 mg/kg males had significantly increased incidences of all tumors of the small intestine and skin, adenocarcinoma of the small intestine, and fibroma of the skin. Fibroma of the skin was also significantly increased in the 0.25 mg/kg males. Incidences of all tumors and all benign tumors were significantly increased in the 1 mg/kg females. There were no significantly increased tumor incidences in the 0.25 mg/kg females. The results of this study show that amsacrine is carcinogenic in Wistar rats. Target organs for tumorigenicity include small intestine, skin, mammary gland, thymus, and uterus.

Expression of the winged helix genes fkh-4 and fkh-5 defines domains in the central nervous system

The ‘winged helix’ or ‘forkhead’ transcription factor gene family is defined by a common 100 amino acid DNA binding domain which is a variant of the helix-turn-helix motif. Here we describe the structure and expression of the mouse fkh-4 and fkh-5 genes. The two genes encode proteins of 427 and 324 amino acids, respectively, with highly similar winged helix domains. Both genes are expressed in adjacent domains in the developing diencephalon from the headfold stage onward. Linkage analysis localised fkh-5 to chromosome 9 at 34.5 centiMorgans (cM) and fkh-4 to chromosome 19 at 10.5 cM. The potential relationship of the two genes to the mouse mutations staggerer and small thymus (for fkh-5) and muscle deficient (for fkh-4) is discussed.

Enhancement of TdT-mediated dUTP-biotin nick end-labeling (TUNEL) method using mung bean nuclease, a single-stranded DNA digestion enzyme.

The TdT-mediated dUTP-biotin nick end-labeling (TUNEL) method has been employed widely to demonstrate apoptotic cells in routinely prepared paraffin sections. Because the apoptotic cells were reactive with the antibody to single-stranded DNA, we attempted to enhance the TUNEL positivity by pretreatment with single-stranded DNA digestion enzymes, S1 nuclease, and mung bean nuclease. When mung bean nuclease (5 U/50 microliter/section) was incubated at 37 degrees C for 30 min, the TUNEL reaction was most effectively enhanced. Pretreatment with S1 nuclease (0.25 U/50 microliter/section) at 37 degrees C for 45 min was less reliable. Compared with the conventional TUNEL sequence, the enhancement technique using mung bean nuclease enabled us to detect more apoptotic cells in human small intestine, colon, tonsil, thymus, endometrium, ovary, liver, kidney, and pancreas. The positivity was not affected by autolytic change. The mechanism of enhancement is discussed.

Effect of Growth Hormone Administration on Thymus Size, Peripheral T Lymphocyte and T Lymphocyte Subsets in Growth Hormone Deficiency

Experimental evidence suggests that growth hormone (GH) has an important role in the development of T lymphocytes within the thymus. In this study, we investigated the effect of 3-month therapy with GH on the peripheral T lymphocytes, T lymphocyte subsets and thymus size with magnetic resonance imaging measurement of GH-deficient children. We found that GH-deficient chlidren had smaller thymi when compared with healthy age-matched children, but their peripheral T lymphocytes and T lymphocte subsets were within normal range. There were no change in the total peripheral T lymphocytes and T lymphocte subtsets, but thymus size significantly increased wtih GH administration. These findings suggest that GH acts as a thymopoietic agent wtithout any effect on the number of peripheral T lymhocytes.

A block in both early T lymphocyte and natural killer cell development in transgenic mice with high-copy numbers of the human CD3E gene.

A severe immunodeficiency involving a complete loss of T lymphocytes and natural killer cells was observed in independent lines of transgenic mice containing > 30 copies of the human CD3E gene (pL12). T-cell- natural killer (NK)- mice could also be generated by using a gene fragment pL12 delta 1 (without exons 4A and 5) coding for the CD3-epsilon transmembrane region and its 55-amino acid nonenzymatic cytoplasmic tail. The abnormally small thymus gland in the homozygous transgenic animals, which was approximately 1% the size of a wild-type thymus, contained only a few (2-4%) prethymocytes with a Thy-1+Pgp-1+IL-2R alpha- CD3-4-8- phenotype. In mice with lower copy numbers of the transgene thymocyte development was blocked at the Thy-1+Pgp-1-IL-2R alpha+CD3-4-8- stage, and normal NK activity was detected. Mice generated with high-copy numbers of a transgene pL12 delta 2 (pL12 delta 1 minus exons 6), coding for a truncated protein from which the CD3-epsilon extracellular domain, its transmembrane region, and most of its cytoplasmic region were absent, contained normal numbers of T lymphocytes and NK cells. These transgene effects suggested that recruitment of signal-transduction molecules by the cytoplasmic tail of this protein played an important role in the abrogation of both lineages. Taken together these observations support the notion that T lymphocytes and NK cells stemmed from a common precursor.

Purification and characterization of diamine oxidase (histaminase) from rat small intestine.

Diamine oxidase (DAO) was purified to homogeneity from rat small intestine, and its biochemical and immunochemical properties were studied. DAO was suggested to be a dimer of a 92 kDa subunit, and its isoelectric point was found to be 6.0. Histamine, putrescine, N tau-methylhistamine, and cadaverine were good substrates, with Km values ranging from 9.4 to 16.0 microM. Spermine and spermidine were not substrates. Both an immunoprecipitation study and Ouchterlony's double diffusion test involving antiserum against the purified DAO showed that the immunological properties of the DAOs from rat small intestine, thymus, and placenta were identical. Among small intestinal DAOs from different species, this antibody reacted to the guinea pig enzyme as strongly as to the rat enzyme, but the reaction was much weaker to the mouse enzyme than to the rat enzyme. The DAOs from rabbit and dog small intestine, pig kidney, and human placenta showed no reactivity toward this antibody.

Reciprocal changes in plasma corticosterone and testosterone in stressed male rats maintained in a visible burrow system: evidence for a mediating role of testicular 11 beta-hydroxysteroid dehydrogenase

The purpose of these studies was to investigate the possible role of rat Leydig cell 11 beta-hydroxysteroid dehydrogenase (11HSD) in mediating the inhibitory effects of corticosterone on testosterone production. In a unique communal environment, the visible burrow system, male Long-Evans rats spontaneously segregated into unstressed dominant and stressed subordinate social relationships. Subordinate animals had elevated plasma corticosterone and diminished circulating testosterone levels relative to the dominant animals. The categories of animals were distinguished by behavioral criteria: weight change, wounds received, offensive and defensive behavior, and freedom of movement. As a result of their persistently elevated corticosterone levels, subordinate animals had smaller thymi and larger adrenals and spleens than dominants. We have postulated that Leydig cells are protected against the inhibitory effects of glucocorticoids on testosterone secretion by the inactivating effects of 11HSD. High corticosterone and low 11HSD are predicted to suppress testosterone production, and normal or diminished corticosterone levels combined with normal or elevated 11HSD should permit undiminished testosterone production. Consistent with these predictions, the testes of subordinate animals contained significantly lower 11HSD activity than those of dominant animals. The 11HSD of livers of subordinate and dominant animals were statistically indistinguishable. The results of this study support the postulated role of 11HSD as a protector of Leydig cell function.