Long-Term Acceptance of Fully Allogeneic Cardiac Grafts by Cotransplantation of Vascularized Thymus in Miniature Swine

BACKGROUND

We have previously reported the ability of both thymokidney and vascularized thymic lobe (VTL) allografts to induce transplantation tolerance to renal allografts across a full major histocompatibility complex (MHC) mismatch in thymectomized miniature swine. However, whether vascularized thymus is capable of inducing tolerance to less tolerogeneic organs when it is transplanted simultaneously is not yet known. The present study investigates cardiac allograft survival and the mechanism of long-term acceptance in recipient swine following cotransplantation of VTL and cardiac grafts from fully MHC-mismatched donors.

METHODS

Animals received a heart graft, a heart graft and a VTL, or a heart graft and a donor thymocyte infusion. Immunosuppressive regimens consisted of 12 or 28 days of tacrolimus.

RESULTS

All animals that received a VTL maintained their grafts significantly longer than their counterparts that received only a heart graft, and those receiving 28 days of tacrolimus maintained their heart grafts long-term. Recipients of a donor thymocyte infusion demonstrated slightly prolonged cardiac graft survival but all rejected their grafts, highlighting the importance of thymic stroma. Cytotoxic T-lymphocyte responses against third-party antigens by cells from tolerant animals showed restriction by both self and donor MHC, whereas responses of controls were restricted to self MHC only. The presence of donor dendritic cells in the VTL grafts and results of co-culture assays suggest that both central and regulatory mechanisms were involved in achieving long-term acceptance.

CONCLUSION

This is the first demonstration of the long-term acceptance of fully MHC-mismatched cardiac allografts in large animals.

Effects of patulin on thymus capillary of rats

Patulin is a mycotoxin that is produced by species of Penicillum, Aspergillus, and Byssochylamys molds that may grow on a variety of foods including fruit, grains and cheese. Patulin, at a dose of 0.1 mg kg(-1) bw day(-1) was administered orally to growing male rats aged 5-6 weeks for a period of 60 or 90 days. The dose of patulin used in the present study was based on estimated human exposure levels. At the end of these periods, the thymus glands of patulin-treated and control Wistar rats were removed and ultrastructural changes in capillary cells of the thymus of patulin-treated Wistar rats were determined by electron microscopy. The walls of thymus capillaries of the 60-day patulin-treated rat groups (P-60) exhibited degeneration observable in electron microscopic sections. For example, loss of cytoplasm and mitochondrial cristae of cells, swollen endothelial cells, increased thickness of the basement membrane, closed lumen of capillaries, accumulation of fibrous material at the periphery of the capillaries and nuclear anomalies were seen in these sections. Such degeneration and changes were also observed in sections of capillaries of the 90-day patulin-treated rat groups (P-90). The levels of degeneration of endothelial cell nucleus of P-90 were greater than those of P-60. This study demonstrated the ultrastructural degeneration of thymus capillary cells of patulin-treated rats. The results obtained from this study may provide a guide to research dealing with the toxic effects of patulin on tissue and organ ultrastructure.

Vascularized Thymic Lobe Transplantation in a Pig-to-Baboon Model: A Novel Strategy for Xenogeneic Tolerance Induction and T-cell Reconstitution

BACKGROUND

This laboratory has previously demonstrated the induction of allogeneic tolerance by vascularized thymic lobe (VTL) transplantation in miniature swine. We report here our initial attempt to induce tolerance by VTL transplantation in the clinically relevant, discordant, pig-to-baboon model of xenotransplantation.

METHODS

Six baboons received xenografts of hDAF VTLs. Four of these baboons also received omental thymic tissue implants. All recipients were treated with an immunosuppressive conditioning regimen that included thymectomy, splenectomy, extracorporeal immunoadsorption of anti-alpha Gal antibodies, and T-cell depletion. Two control baboons received sham operations, of which one also received 5x10 hDAF porcine thymocytes/kg intravenously.

RESULTS

Transplanted VTL grafts supported early thymopoiesis of recipient-type immature thymocytes, and facilitated engraftment of nonvascularized thymic omental implants. Recipients of the VTL grafts demonstrated donor-specific unresponsiveness in MLR assays, development of peripheral CD45RAhigh/CD4 double positive (DP) cells, and positive cytokeratin staining of thymic stroma in the grafts for 2 months following xenotransplantation. The control baboons did not show these markers of thymic reconstitution. The eventual return of Gal natural antibodies led to the destruction of graft epithelial cells and the rejection of all VTL grafts by 3 months posttransplantation.

CONCLUSIONS

VTL transplantation from hDAF swine to baboons induced early thymopoiesis in the recipients and donor-specific cellular unresponsiveness in vitro. When coupled with additional strategies aimed at silencing humoral rejection, VTL transplantation may significantly prolong xenograft survival and result in long-term tolerance.

Perivascular infiltrate of memory lymphocytes and mature dendritic cells in MG thymomas

In thymomas associated with myasthenia gravis (MG), the authors found that perivascular infiltrates of memory lymphocytes and mature dendritic cells (DCs) were more frequent in patients with early improvement after thymectomy than in patients without response to thymectomy. Although these findings may be limited to particular types of thymoma, thymectomy may interrupt the recruitment of mature DCs in thymus and export of activated T cells to extra-thymic tissues, thereby improving the disease.

[Imaging aid for thoracoscopic thymectomy; thymic vein visualization]

The spread and progress of thoracoscopic surgery has been remarkable. There are many important organs and blood vessels in the mediastinal space, thus, as inadvertent injury invites serious consequences, it is necessary to understand the anatomical position of existing structures. When enucleating the thymoma under a thoracoscope, one of the blood vessels to be careful about is the thymic vein. Therefore, we examined whether the thymic vein could be confirmed using a multidetector-row computed tomography (MD CT). We believed we could understand the anatomical position of the thymic vein by analyzing the high resolution slice data provided in MD CT using a work station. The diameter of the thymic is from a little less than 1 mm to 3 mm, and there were individual variations in position and the number of the vein. The thymic vein flowed into the left brachiocephalic vein in many cases. In some cases, the thymic vein flowed into the right internal thoracic vein or superior vena cava. It appears that visualization of the thymic vein could provide useful data when performing thoracoscopic thymectomy.

Gene targeting of VEGF-A in thymus epithelium disrupts thymus blood vessel architecture

The thymus harbors an organ-typical dense network of branching and anastomosing blood vessels. To address the molecular basis for morphogenesis of this thymus-specific vascular pattern, we have inactivated a key vascular growth factor, VEGF-A, in thymus epithelial cells (TECs). Both Vegf-A alleles were deleted in TECs by a complementation strategy termed nude mouse [mutated in the transcription factor Foxn1 (forkhead box N1)] blastocyst complementation. Injection of Foxn1(+/+) ES cells into Foxn1(nu/nu) blastocysts reconstituted a functional thymus. By dissecting thymus stromal cell subsets, we have defined, in addition to medullary TECs (mTECs) and cortical TECs (cTECs), another prominent stromal cell subset designated cortical mesenchymal cells (cMes). In chimeric thymi, mTECs and cTECs but not cMes were exclusively ES cell-derived. According to this distinct origin, the Vegf-A gene was deleted in mTECs and cTECs, whereas cMes still expressed Vegf-A. This genetic mosaic was associated with hypovascularization and disruption of the organ-typical network of vascular arcades. Thus, vascular growth factor production by TECs is required for normal thymus vascular architecture. These experiments provide insights into Foxn1-dependent and Foxn1-independent stromal cell development and demonstrate the value of this chimeric approach to analyzing gene function in thymus epithelium.

Overexpression of 15-lipoxygenase in the vascular endothelium is associated with increased thymic apoptosis in LDL receptor-deficient mice

BACKGROUND

15-Lipoxygenase (15-LO) is a nonheme iron-containing enzyme that catalyzes the peroxidation of fatty acids. Herein, we studied the effect of 15-LO overexpression in the vascular endothelium on thymocyte apoptosis by evaluating thymuses from low-density lipoprotein receptor-deficient (LDL-RD) mice and LDL-RD/15-LO mice. Thymuses were evaluated by immunohistochemistry and by TUNEL whereas in vitro studies were carried out by employing freshly isolated thymocytes from the respective mice and evaluation of apoptosis by propidium iodide and annexin V cytometry.

METHODS AND RESULTS

The apoptotic index in LDL-RD/15-LO mice was significantly higher than in the LDL-RD mice. In the thymic medulla the difference was smaller, although still significant. Freshly isolated thymus cells from LDL-RD/15-LO mice exhibited a higher rate of spontaneous cell death than controls. Incubation of thymus cells in the presence of the cell-permeable caspase-3 inhibitor DEVD-CMK resulted in a decrease in the frequency of apoptotic cells in LDL-RD/15-LO thymocytes, whereas no effect was evident in control thymocytes. The antioxidant N-acetylcysteine causes the increase in apoptosis in both groups.

CONCLUSION

LDL-RD/15-LO mice exhibit increased thymocyte apoptosis both in vivo and in vitro. These findings may suggest a role for 15-LO in the natural selection of thymocytes.

Matrix Metalloproteinase-9 in Macrophages Induces Thymic Neovascularization following Thymocyte Apoptosis

Matrix metalloproteinase-9 (MMP-9) has been implicated in the degradation of the extracellular matrix in a variety of physiological and pathological processes. We found that MMP-9 expression in thymuses of BALB/c mice that had been injected with anti-CD3 Ab to induce thymocyte apoptosis was increased both at mRNA and protein levels. Macrophages are shown to be the principal stromal cells responsible for phagocytosis of dying thymocytes, and macrophages were found to constitutively express MMP-9. The activity of plasmin, which is known as one of the activators for MMP-9, was increased in the thymuses with MMP-9 activation. Binding of Ab HUIV26, which recognizes a cryptic epitope on collagen type IV following proteolytic cleavage, was found to be reduced in MMP-9 knockout mice, suggesting that collagen type IV is a substrate of MMP-9. Although the formation of thymic neovessels was found following thymocyte apoptosis, it was diminished in anti-CD3 Ab-injected MMP-9 knockout mice. In vivo administration of Ab HUIV26 resulted in a reduction of thymic neovascularization. After clearance of apoptotic thymocytes, the number of macrophages in the thymuses was decreased, and this decrease was delayed by blocking of HUIV26 epitope. Taken together, our results suggest that MMP-9 expression in macrophages mediates degradation of collagen type IV and facilitates their migration from the thymus after clearance of apoptotic thymocytes. These studies demonstrate a potential role of macrophage MMP-9 in the remodeling of thymic extracellular matrix following thymocyte apoptosis.

[Morpho-functional changes of liver and its regional lymph nodes after exposure to magnetic field of industrial frequency]

The aim of this research was to study the morpho-functional organization of liver and its regional lymph nodes, as well as of other organs of immune system, in animals exposed for 14 days to a magnetic field of industrial frequency (MFIF) with aperiodic oscillations of amplitude of magnetic induction in the range of 5-10 _Ts. The methods of light and electron microscopy were used. Damage to DNA in hepatocyte nuclei was determined using ethidium bromide. Quantitative analysis of the cells in the organs of immune system was performed, blood leukocyte formula was calculated and the activity of oxidative enzymes in blood cells was determined. It was found that MFIF caused micro- and ultrastructural changes in animal liver and its regional lymph nodes, which included the disturbances of hemo- and lymphocirculation, destruction of cellular membranes in some cells, dilation of rough endoplasmic reticulum cisterns, appearance of degenerate mitochondrial forms, glycogen depletion, that collectively indicated the development of tissue hypoxia. The increase in the activity of nuclear endonucleases was detected. Response of organs of immune system to MFIF was manifested by shifts of blast forms/ mature lymphocytes ratio in hepatic regional lymph nodes and by the changes of energy metabolism of blood lymphocytes.

Characterization of vascular adhesion molecules that may facilitate progenitor homing in the post-natal mouse thymus

T cell progenitors derive from the bone marrow but must migrate via bloodstream to the thymus in order to differentiate. The mechanism by which the thymus recruits progenitors from the blood is unknown. It is known, however, that there are receptive and refractory periods for progenitor recruitment and that when cells are imported, they enter the thymus through post-capillary venules. Therefore, recruitment is an active process temporally and spatially regulated. In order to characterize the mechanism of recruitment, we evaluated vascular signals known to regulate leukocyte extravasation, with respect to their intrathymic location and temporal fluctuations. We find that CD34, MECA79, VCAM-1, ICAM-1 and VAP-1 are all expressed in thymic blood vessels. MECA79 and VAP-1 appear to be specific for post-capillary venules, while ICAM-1 and VCAM-1 are also found on intrathymic stromal cells. MAdCAM is also expressed in the thymus, but is not associated with vascular tissues. Only MECA79 is upregulated during recruitment peaks, suggesting a role for this molecule in the periodicity of recruitment. Together, these studies reveal potential roles for l-selectin ligands, VCAM-1, ICAM-1 and VAP-1 in progenitor recruitment to the thymus, and implicate the presence of other periodic signals, such as chemokines and cytokines, that cooperate to execute this essential function.

A case of thymoma protruding into the superior vena cava through the thymic vein

We describe an unusual case of an invasive thymoma protruding into the superior vena cava and left brachiocephalic vein through the thymic veins in a 64-year-old patient. The tumor was resected with a bypass of the right brachiocephalic vein and right atrium. Although this type of growth form is rare for an invasive thymoma, this case suggests that in surgical procedures for thymomas, meticulous examination of the thymic veins is necessary to avoid leaving residual tumor.

Responses to human CGRP, ADM, and PAMP in human thymic arteries

Responses to human CGRP, adrenomedullin (ADM), and proadrenomedullin NH2-terminal 20 peptide (PAMP) were studied in small human thymic arteries. CGRP, ADM, and PAMP produced concentration-dependent vasodilator responses in arteries preconstricted with the thromboxane mimic U-46619. Responses to ADM and PAMP were attenuated, whereas responses to CGRP were not altered by endothelial denudation. Inhibitors of nitric oxide synthase and guanylyl cyclase attenuated responses to ADM and PAMP but not to CGRP. The CGRP1 receptor antagonist CGRP(8-37) attenuated responses to CGRP and ADM but not to PAMP. Responses to CGRP were reduced by SQ-22536 and Rp-cAMPS, inhibitors of adenylyl cyclase and PKA. These data suggest that responses to CGRP and ADM are mediated by CGRP(8-37)-sensitive receptors and that the endothelial ADM receptor induces vasodilation by a nitric oxide-guanylyl cyclase mechanism, whereas a smooth muscle CGRP receptor signals by a cAMP-dependent mechanism. A different endothelial receptor recognizes PAMP and signals by a nitric oxide-dependent mechanism.

Vascularized islet-cell transplantation in miniature swine. I. Preparation of vascularized islet kidneys

BACKGROUND

Whereas clinical pancreatic transplantation has been highly successful in correcting the hyperglycemia of insulin-dependent diabetes mellitus (type 1), the results of islet transplantation have been disappointing. This discrepancy may be because of, at least in part, nonspecific loss of islets during the time required for revascularization. To test this hypothesis, we have designed composite kidney grafts containing vascularized autologous islets that can be used to compare the engraftment potential of vascularized versus nonvascularized islet tissue.

METHODS

(1) Islet-cell isolation: miniature swine underwent either partial pancreatectomy to isolate autologous islets or total pancreatectomy to isolate minor antigen-mismatched islets. Islets were purified from excised pancreatic tissue by enzymatic digestion and discontinuous density gradient purification. Isolated islets were cultured for 3 days before transplant. (2) Creation of vascularized islet kidneys (IK): autologous islets alone (n=6), minor-mismatched islets alone (n=3), and minor-mismatched islets plus simultaneous autologous thymic tissue (n=3) were transplanted beneath the renal capsule of juvenile miniature swine. Minor antigen-mismatched islets were also transplanted into both the vascularized thymic graft of a thymokidney (to produce a thymo-islet kidney [TIK]) and the contralateral native kidney (n=3) and both the host thymus and beneath the renal capsule (n=2). All recipients receiving minor-mismatched islets were treated with a 12-day intravenous (IV) course of either cyclosporine A (CsA) at 10 mg/kg per day or FK506 at 0.15 mg/kg per day. (3) Assessment of Function: to evaluate the function of the transplanted islets, three animals bearing TIK and IK underwent total pancreatectomy 3 months following islet transplantation.

RESULTS

(1) Islet-cell yields: an average of 254,960+/-51,879 (4,452+/-932 islet equivalents [IEQ]/gram of pancreas) and 374,410+/-9,548 (4,183+/-721 IEQ/gram of pancreas) viable islets were obtained by partial pancreatectomy and complete pancreatectomy, respectively. (2) Creation of IK: autologous islets engrafted indefinitely, whereas recipients of minor-mismatched islets alone rejected the islets within 2 months. However, when minor-mismatched islets were implanted into both the thymokidney and the contralateral kidney of animals bearing a thymokidney, the islets engrafted indefinitely in both sites (>3 months). Simultaneous implantation of islets into the host thymus and under the renal capsule also led to permanent engraftment of minor-mismatched islets. (3) Function of vascularized islets: three animals with both a TIK and an IK in place for 3 months underwent total pancreatectomy. All three animals maintained normoglycemia thereafter. In two of these animals, the IKs were removed 2 months after the pancreatectomy, and in both cases normoglycemia was maintained thereafter by the TIK.

CONCLUSIONS

The implantation of islets beneath the autologous renal capsule permitted the establishment of a vascular supply and thereby supported normal islet-cell growth and function. The presence of thymic tissue beneath the autologous renal capsule facilitated the engraftment of minor-mismatched islets, and such grafts achieved results similar to autologous islet transplants. Therefore, the ability to create vascularized islet grafts may provide a strategy for successful islet transplantation across allogeneic and potentially across xenogeneic barriers.

Correlation between tumor angiogenesis and invasiveness in thymic epithelial tumors

OBJECTIVE

Because it is difficult to predict the behavior of thymomas on the basis of morphology alone, other methods for determining tumor aggressiveness must be explored. This study investigated the correlation between angiogenic grade and invasiveness in thymic epithelial tumors.

METHODS

Immunohistochemical studies of 46 surgically resected thymic epithelial tumors (18 noninvasive thymomas, 20 invasive thymomas, and 8 thymic carcinomas) were conducted. To highlight the microvessels, we used a specific monoclonal antibody against factor VIII. Expression of vascular endothelial growth factor and basic fibroblast growth factor was determined by using polyclonal antibodies.

RESULTS

Mean microvessel density readings for noninvasive thymomas, invasive thymomas, and thymic carcinomas were 4.6 +/- 3.2, 12.4 +/- 7.5, and 34.4 +/- 16.7, respectively. Stages I, II, III, and IV of thymoma had microvessel density readings of 4.6 +/- 3.2, 8.5 +/- 4.3, 13.8 +/- 7.7, and 22.0 +/- 6.8, respectively. These findings suggest a significant correlation between microvessel density and tumor invasiveness. Vascular endothelial growth factor expression in noninvasive thymomas, invasive thymomas, and thymic carcinomas was present in 1 (5.6%) of 18, 11 (55.0%) of 20, and 5 (62.5%) of 8 patients, respectively, thereby indicating a distinct association between vascular endothelial growth factor expression and increased microvessel density. Basic fibroblast growth factor expression was present in only 8 patients.

CONCLUSIONS

In patients with thymic epithelial tumors, there appears to be a significant correlation between tumor angiogenesis and invasiveness. Furthermore, our data suggests that this angiogenesis in thymic epithelial tumors might be, at least in part, dependent on vascular endothelial growth factor expression.

A novel technique for en bloc, vascularized, composite thymic, and cardiac co-transplantation1

BACKGROUND

A surgical technique for co-transplantation of fully vascularized thymus and heart potentially applicable to the clinical setting has not been developed and is the subject of this article.

METHODS

Vascularized right lobe of the thymus was transplanted heterotopically with the heart as a composite graft in rats. This co-transplantation technique was developed and assessed, and viability of the grafted thymus was evaluated histologically.

RESULTS

Surgical mortality was identical to isolated heart transplantation, and all grafted hearts functioned well. Histology of the thymic grafts at explantation revealed viable thymus with preservation of normal thymic microarchitecture.

CONCLUSION

We developed a novel technique to create a composite graft in which fully vascularized and viable thymus was harvested en bloc and co-transplanted with a donor heart allograft.

Developmentally regulated interactions of human thymocytes with different laminin isoforms

The gene family of heterotrimeric laminin molecules consists of at least 15 naturally occurring isoforms which are formed by five different alpha, three beta and three gamma subunits. The expression pattern of the individual laminin chains in the human thymus was comprehensively analysed in the present study. Whereas laminin isoforms containing the laminin alpha1 chain (e.g. LN-1) were not present in the human thymus, laminin isoforms containing the alpha2 chain (LN-2/4) or the alpha5 chain (LN-10/11) were expressed in the subcapsular epithelium and in thymic blood vessels. Expression of the laminin alpha4 chain seemed to be restricted to endothelial cells of the thymus, whereas the LN-5 isoform containing the alpha3 chain could be detected on medullary thymic epithelial cells and weakly in the subcapsular epithelium. As revealed by cell attachment assays, early CD4- CD8- thymocytes which are localized in the thymus beneath the subcapsular epithelium adhered strongly to LN-10/11, but not to LN-1, LN-2/4 or LN-5. Adhesion of these thymocytes to LN-10/11 was mediated by the integrin alpha6beta1. During further development, the cortically localized CD4+ CD8+ thymocytes have lost the capacity to adhere to laminin-10/11. Neither do these cells adhere to any other laminin isoform tested. However, the more differentiated single positive CD8+ thymocytes which were mainly found in the medulla were able to bind to LN-5 which is expressed by medullary epithelial cells. Interactions of CD8+ thymocytes with LN-5 were integrin alpha6beta4-dependent. These results show that interactions of developing human thymocytes with different laminin isoforms are spatially and developmentally regulated.

Vascularized thymic lobe transplantation in miniature swine: I. Vascularized thymic lobe allografts support thymopoiesis

BACKGROUND

Vascularized thymokidney transplants have previously been shown to induce tolerance across major histocompatibility complex barriers. The ability to perform vascularized thymic lobe transplantation could permit such tolerance to be induced with any cotransplanted solid organ or tissue. For this reason, we have developed a technique for vascularized thymic lobe transplantation in miniature swine.

METHODS

Thymic vessels (n=2) were anastomosed to the carotid artery and the external jugular vein of naïve minor-mismatched recipients treated with a 12-day course of cyclosporine A (10 mg/kg/day). Graft survival and thymopoiesis were assessed by histology, immunohistochemistry, and fluorescence-activated cell sorting. Allele-specific antibodies 74-12-4 and pig allelic antigen (PAA) were used to distinguish donor and recipient cells.

RESULTS

Allografts showed intact cortical and medullary structure posttransplantation, without evidence of rejection or ischemia. Recipient thymocytes repopulated the donor cortical thymus by POD30 and increased in the cortex and medulla by POD60.

CONCLUSIONS

Our study demonstrates the technical feasibility of vascularized thymic lobe transplantation and the support of thymopoiesis by such transplants in a large animal model. This technique may offer a novel strategy to induce transplant tolerance across allogeneic and xenogeneic barriers, and to support long-term thymopoiesis in immunodeficient hosts.

Functional demonstration of intrathymic binding sites and microvascular gates for prothymocytes in irradiated mice

Quantitative intrathymic (i.t.) and i.v. adoptive transfer assays for prothymocytes show strict log dose saturation kinetics, consistent with a finite number of i.t. binding sites (microenvironmental niches). This inference is supported here by demonstration of competitive antagonism obeying one-on-one receptor occupancy kinetics during the establishment of thymic chimerism in irradiated adult mice. The results of primary and secondary transfer experiments suggested that hematogenous precursors (i) enter specific i.t. niches between 4 and 24 h after injection, (ii) compete reversibly with subsequently introduced precursors, (iii) establish insurmountable competition within 5-7 days, (iv) mature through the initial stages of thymocytopoiesis preceding proliferative expansion, and (v) vacate the niches between 7 and 14 days after entry. The results also suggested that, as in non-irradiated mice, prothymocyte importation in irradiated mice is a gated phenomenon. Gate closure was indicated by the inability of i.v.-, but not i.t.-, injected bone marrow (BM) cells to induce thymic chimerism when administered 7--14 days after a primary injection and gate opening by the ability of i.v.-injected BM cells to induce thymic chimerism in competition with circulating host prothymocytes. Gate closing was log dose-responsive and could be induced in individual thymic lobes by unilateral i.t. injection, whereas gate opening, which occurs bilaterally, was not initiated until most of the niches for prothymocytes had been vacated. We therefore posit the existence of a series of associated microvascular gates and microenvironmental niches that act in concert to regulate prothymocyte importation and early thymocyte differentiation.

Thymus size and head circumference at birth and the development of allergic diseases

BACKGROUND

The positive association between a large head circumference at birth and total serum IgE levels has been suggested to be due to negative associations between head circumference at birth and thymus development and between thymus development and total serum IgE levels.

OBJECTIVES

To examine the associations between head circumference and thymus size at birth and the development of allergic disease.

METHODS

The size of the thymus was assessed by sonography during the first week of life in 149 healthy term infants. Information on birth characteristics and mode of delivery was collected at delivery. The presence of allergic disease was assessed 5 years later by mailed questionnaires, which were returned by 85% of the eligible families.

RESULTS

At birth, head circumference was positively associated with thymus size (P < 0.001). In all, 27 (23%) of the children had developed at least one allergic disease. Multivariate analysis revealed that both parental allergy (Prevalence Ratio and 95% CI) = 3.18 (1.49-6.78)) and caesarean delivery (2.62 (1.48-4.64)) were independently correlated with allergic disease, whereas thymus size was not.

CONCLUSIONS

Our study does not support that a large head circumference is associated with a small thymus size, nor that a small thymus size is associated with allergic disease. Whether thymus size at birth is related to total serum IgE levels still remains to be elucidated.

B cells in epithelial and perivascular compartments of human adult thymus

The thymus is the site of T-cell differentiation. However, the relatively recent observation that B cells are also present in the human thymus has prompted studies to determine the origin and function of these B cells. Our studies show that phenotypically distinguishable B cell populations are located within both the thymic medulla and the thymic perivascular space and that cellular trafficking occurs between these compartments, including B cells trafficking from the periphery. The numbers of thymic B cells increase with age, correlating with increases in lymphocyte-rich regions of thymic perivascular space that are prominent between ages 10 and 50 years. B cells within both thymic epithelial and perivascular compartments contain mutated immunoglobulin VH sequences characteristic of post-germinal center B cells, suggesting that the B cells that most often give rise to thymic B-cell lymphomas may originate from either the thymic medulla or perivascular space.

The thymus of the hairless rhino-j (hr/rh-j) mice

The hairless (hr) gene is expressed in a large number of tissues, primarily the skin, and a mutation in the hr gene is responsible for the typical cutaneous phenotype of hairless mice. Mutant hr mouse strains show immune defects involving especially T cells and macrophages, as well as an age-related immunodeficiency and an accelerated atrophy of the thymus. These data suggest that the hr mutation causes a defect of this organ, although hr transcripts have not been detected in fetal or adult mice thymus. The present study analyses the thymus of young (3 mo) and adult (9 mo) homozygous hr-rh-j mice (a strain of hairless mice) by means of structural techniques and immunohistochemistry to selectively identify thymic epithelial cells, dendritic cells, and macrophages. There were structural alterations in the thymus of both young and adult rh-rh-j mice, which were more severe in older animals. These alterations consisted of relative cortical atrophy, enlargement of blood vessels, proliferation of perivascular connective tissue, and the appearance of cysts. hr-rh-j mice also showed a decrease in the number of epithelial and dendritic cells, and macrophages. Taken together, present results strongly suggest degeneration and accelerated age-dependent regression of the thymus in hr-rh-j mice, which could explain at least in part the immune defects reported in hairless mouse strains.