Thymic pathology in primary and secondary immunodeficiencies

Death from human cytomegalovirus infection in a girl with congenital thymic dysplasia

We report the case of a girl with congenital thymic dysplasia and refractory disseminated Human Cytomegalovirus (CMV) infection diagnosed by autopsy. Additionally, she was diagnosed with T-cell lymphopenia immunodeficiency and received antiviral therapy with ganciclovir (GCV) /valganciclovir (V-GCV) and enhanced foscarnet. The CMV viral load (VL) monitoring was elevated with retinitis, interstitial pneumonia, and hepatitis. The phenotype of T-cell lymphopenia was uncertain, which limited any alternative therapy by whole-exome sequencing (WES) and lymphocyte subset panel until autopsy. The girl died of progressive respiratory failure and septic shock at ten months of age. Severe disseminated CMV infection typically develops in infants with primary maternal infections and occurs earlier during gestation and in people with a weakened host immune system. Individuals with CMV infection with initial immunodeficiency are associated with a poor prognosis, which is similar to patients with secondary immunodeficiency. This case describes the difficult treatment and prognosis of CMV infection in patients with congenital immunodeficiency, highlighting the importance of early aggressive anti-CMV antiviral therapy in immunodeficiencies, VL monitoring, drug resistance and the role of T-cells in CMV infection.

The molecular immunology of human susceptibility to fungal diseases: lessons from single gene defects of immunity

INTRODUCTION

Fungal diseases are a threat to human health. Therapies targeting the fungus continue to lead to disappointing results. Strategies targeting the host response represent unexplored opportunities for innovative treatments. To do so rationally requires the identification and neat delineation of critical mechanistic pathways that underpin human antifungal immunity. The study of humans with single-gene defects of the immune system, i.e. inborn errors of immunity (IEIs), provides a foundation for these paradigms. Areas covered: A systematic literature search in PubMed, Scopus, and abstracts of international congresses was performed to review the history of genetic resistance/susceptibility to fungi and identify IEIs associated with fungal diseases. Immunologic mechanisms from relevant IEIs were integrated with current definitions and understandings of mycoses to establish a framework to map out critical immunobiological pathways of human antifungal immunity. Expert opinion: Specific immune responses non-redundantly govern susceptibility to their corresponding mycoses. Defining these molecular pathways will guide the development of host-directed immunotherapies that precisely target distinct fungal diseases. These findings will pave the way for novel strategies in the treatment of these devastating infections.

Infectious and non-infectious complications in primary immunodeficiency disorders: an autopsy study from North India

BACKGROUND

Primary immunodeficiency disorders (PID) include a wide spectrum of inherited disorders characterised by functional abnormalities of one or more components of the immune system. Recent updates from the genomic data have contributed significantly to its better understanding with identification of new entities. Diagnosis is always challenging due to their variable clinical presentation. With the evolution of molecular diagnosis, many of these children are being diagnosed early and offered appropriate therapy. However, in developing countries, early diagnosis is still not being made: as a result these patients succumb to their disease. Autopsy data on PID is notably lacking in the literature with histopathological evaluation of PID being limited to rare case reports.

OBJECTIVE

To analyse the clinical, immunologic (including mutational) and morphologic features at autopsy in 10 proven and suspected cases of primary immunodeficiency disorders diagnosed at our Institute over the past decade.

METHODS

Study includes a detailed clinico-pathological analysis of 10 proven and suspected cases of primary immunodeficiency disorders.

RESULTS

A varied spectrum of infectious and non-infectious complications were identified in these cases of which fungal infections were found to be more frequent compared with viral or bacterial infections. Rare and novel morphological findings, like granulomatous involvement of the heart in a patient with chronic granulomatous disease, systemic amyloidosis in a teenage girl with X-linked agammaglobulinemia, are highlighted which is distinctly lacking in the literature.

CONCLUSIONS

The present study is perhaps the first autopsy series on PID. Even in the molecular era, such analysis is still important, as correlation of pathological features with clinical symptoms provides clues for a timely diagnosis and appropriate therapeutic intervention.

Hormonal control of T-cell development in health and disease

The physiology of the thymus, the primary lymphoid organ in which T cells are generated, is controlled by hormones. Data from animal models indicate that several peptide and nonpeptide hormones act pleiotropically within the thymus to modulate the proliferation, differentiation, migration and death by apoptosis of developing thymocytes. For example, growth hormone and prolactin can enhance thymocyte proliferation and migration, whereas glucocorticoids lead to the apoptosis of these developing cells. The thymus undergoes progressive age-dependent atrophy with a loss of cells being generated and exported, therefore, hormone-based therapies are being developed as an alternative strategy to rejuvenate the organ, as well as to augment thymocyte proliferation and the export of mature T cells to peripheral lymphoid organs. Some hormones (such as growth hormone and progonadoliberin-1) are also being used as therapeutic agents to treat immunodeficiency disorders associated with thymic atrophy, such as HIV infection. In this Review, we discuss the accumulating data that shows the thymus gland is under complex and multifaceted hormonal control that affects the process of T-cell development in health and disease.

Thymopoiesis in mice depends on a Foxn1-positive thymic epithelial cell lineage

The thymus is essential for T-cell development. Here, we focus on the role of the transcription factor Foxn1 in the development and function of thymic epithelial cells (TECs) of the mouse. TECs are of endodermal origin; they initially express Foxn1 and give rise to orthotopic (thoracic) and additional (cervical) thymi. Using Foxn1-directed cytoablation, we show that during embryogenesis, cervical thymi develop a few days after the thoracic lobes, and that bipotent epithelial progenitors of cortical and medullary compartments express Foxn1. We also show that following acute selective near-total ablation during embryogenesis, complete regeneration of TECs does not occur, providing an animal model for human thymic aplasia syndromes. Finally, we address the functional role of Foxn1-negative TECs that arise postnatally in the mouse. Lineage tracing shows that such Foxn1-negative TECs are descendants of Foxn1-positive progenitors; furthermore, Foxn1-directed subacute intoxication of TECs by polyglutamine-containing EGFP proteins indicates that a presumptive Foxn1-independent lineage does not contribute to thymopoietic function of the adult thymus. Our findings therefore support the notion that Foxn1 is the essential transcription factor regulating the differentiation of TECs and that its expression marks the major functional lineage of TECs in embryonic and adult thymic tissue.

Thymus microenvironment in human primary immunodeficiency diseases

PURPOSE OF REVIEW

Severe combined immunodeficiencies represent a heterogeneous group of genetic disorders affecting genes of both early and late steps in lymphocytes development, a process tightly controlled by thymic epithelial cells. Detailed analysis of thymic morphology aids to the assessment of the severity of the immune disorder and may be critical to the understanding of the role of the genetic defects in the pathophysiology of these diseases. In this review, we highlight recent advancements in the characterization of the thymic microenvironment in primary immunodeficiencies.

RECENT FINDINGS

Crosstalk between thymocytes and thymic epithelial cells is essential to preserve thymic architecture and function, and therefore to promote T-cell maturation and development of self-tolerance. Early severe defects in T-cell development result in profound abnormalities of thymic epithelial cells differentiation with loss of AIRE expression and severe reduction of thymic dendritic and T-regulatory cells. Differently, later defects in T-cell development that are permissive for normal thymocytes development allow cortico-medullary differentiation with partially preserved AIRE expression and dendritic/T-regulatory cells distribution. Hypomorphic mutations in the same genes partially permissive to T-cell development may result in a more complex phenotype with immunodysreactivity and peculiar thymic alterations.

SUMMARY

Although the molecular and genetic bases of primary immunodeficiencies directly aid to both diagnosis and management of the patients, the detailed analysis of thymic morphology critically contributes to unveil the pathophysiology of these diseases.

Somatostatin modulates T cells development in adult rat thymus

It is well known that somatostatin modulates thymic functions, such as binding to receptors. In order to elucidate the influence of somatostatin on the thymus architecture and the T cells maturation, young adult male rats were treated with somatostatin-28. The results showed that somatostatin-28 decreased thymus weight and cellularity, probably due to alterations in the thymic morphometric parameters. Our results also demonstrated that SRIH treatment reduces number of cells with undetectable alphabetaTCR and cells with low expression of alphabetaTCR, while the number of TCRalphabeta(hi) cells remains approximately the same as the values obtained from the control rats. Besides, in the least mature thymocytes (DNTCR TCRalphabeta(-)) and among the most mature the SPCD4 TCRalphabeta(hi) subset remained unaltered, while SPCD8 TCRalphabeta(hi) decreased. At last, it should be noted that SRIH treatment increases DN thymocytes subsets expressing TCRalphabeta(low/hi) (TCRalphabeta(+)). These results suggest that somatostatin-28 induces reshaping of T cells maturation and, at least partly, contributes to thymic weight loss, through the modulation of the complex neuroendocrine-immune network.

Microthymoma

We report 2 cases of microscopic-sized thymoma, which probably represents the earliest phase of thymoma development. The 2 patients presented with pure red cell aplasia and myasthenia gravis, respectively. The thymectomy specimens did not reveal tumor on gross examination, but histologically each contained small thymomas measuring 5 mm and 7 mm in largest dimension, respectively. One of the tumors was unencapsulated and involved a single lobule only, and the other was encapsulated and comprised two lobules. The tumors consisted of ovoid epithelial cells with pale nuclei and distinct nucleoli, scattered in a background of small lymphocytes. Foci of medullary differentiation and perivascular space were identified in the 2 cases, respectively. The lymphocytes were confirmed to be immature T cells on immunohistochemical studies (CD3+, TdT+). Except for the microscopic size, the morphology of the two tumors conforms to conventional type B1/B2 and type B2 thymoma, respectively. We propose calling such incidental small tumor "microthymoma" to distinguish it from the so-called microscopic thymoma, which is composed of small thymic epithelial nests and probably more appropriately termed "nodular hyperplasia" of the thymic epithelium.

Abnormal development of thymic dendritic and epithelial cells in human X-linked severe combined immunodeficiency

The X-linked form of severe combined immunodeficiency (X-SCID) is caused by mutations in the common cytokine receptor gamma chain and results in lack of T and NK cells and defective B cells. Without immune reconstitution, X-SCID patients typically die from infection during infancy. This report describes thymic epithelial (TE), lymphocyte, and dendritic cell (DC) differentiation in the thymic microenvironment of seven X-SCID patients who died before or after treatment for their immunodeficiency. X-SCID thymus consisted predominately of TE cells without grossly evident corticomedullary distinction. CD3+ and CD1a+ developing T cells and CD83+ thymic DC were reduced >50-fold when compared to age- and gender-matched control thymus (P < 0.001). TE expression of epithelial differentiation markers CK14, involucrin, and high molecular weight cytokeratins also differed in X-SCID versus normal thymus. These histopathologic findings indicate that in addition to T cells, thymic DC development and differentiation of TE cells are also abnormal in X-SCID.

Lymphodepletion in the ApcMin/+ mouse model of intestinal tumorigenesis

Germ line mutations in the Adenomatous polyposis coli tumor suppressor gene cause a hereditary form of intestinal tumorigenesis in both mice and man. Here we show that in ApcMin/+ mice, which carry a heterozygous germ line mutation at codon 850 of Apc, there is progressive loss of immature and mature thymocytes from approximately 80 days of age with complete regression of the thymus by 120 days. In addition, ApcMin/+ mice show parallel depletion of splenic natural killer (NK) cells, immature B cells, and B progenitor cells in bone marrow due to complete loss of interleukin 7 (IL-7)-dependent B-cell progenitors. Using bone marrow transplantation experiments into wild-type recipients, we have shown that the capacity of transplanted ApcMin/+ bone marrow cells for T- and B-cell development appears normal. In contrast, although the ApcMin/+ bone marrow microenvironment supported short-term reconstitution with wild-type bone marrow, ApcMin/+ animals that received transplants subsequently underwent lymphodepletion. Fibroblast colony-forming unit (CFU-F) colony assays revealed a significant reduction in colony-forming mesenchymal progenitor cells in the bone marrow of ApcMin/+ mice compared with wild-type animals prior to the onset of lymphodepletion. This suggests that an altered bone marrow microenvironment may account for the selective lymphocyte depletion observed in this model of familial adenomatous polyposis. (Blood. 2004;103:1050-1058)

Somatostatin-14 alters the thymus size and relation among the thymocyte subpopulations in peripubertal rats

It is well known that somatostatin exerts a wide range of effects in the body, and acts as an autocrine or paracrine factor in the thymus. However, it has not been investigated yet whether somatostatin alters the thymus size and relation among the thymocyte subpopulations in the peripubertal rats. For this purpose, the peripubertal AO male rats were cannulated intracerebroventriculary and treated with repeated, low doses of somatostatin-14 (experimental group) or saline (control group). Twenty-four hours after the last treatment, we removed and prepared the thymuses for determination of thymocyte subpopulations by flow cytometry. After five days, animals were sacrificed and their thymuses taken for morphometrical analysis by stereological methods. We noticed that somatostatin-14 decreased volumes of thymus cortex and medulla, total number of thymocytes, number of thymocytes in the cortex and medulla and numerical density of thymocytes in deeper cortex. As a consequence of these changes, thymus size was also diminished. The phenotypic analysis of thymocyte subpopulations showed that somatostatin-14 decreased the percentage of CD4(+)CD8(+) cells with low level of TCR alphabeta expression, positively selected CD4(+)CD8(+)TCRalphabeta (high) cells and the most mature CD4(-)CD8(+)TCRalphabeta (high) cells, while the percentage of CD4(+)CD8(-)TCRalphabeta (high) thymocytes was slightly increased. Somatostatin-14 increased the relative proportion of the least mature CD4(-)CD8(-)TCRalphabeta (-/low), CD4(+)CD8(+)TCRalphabeta (-) cells and both of TCRalphabeta (-/low) single positive subpopulations. These results show that centrally applied somatostatin-14, induces hypotrophy of the thymus in peripubertal rats by changing the volumes and cellularities of the thymic compartments. Additionally, increased number of the least mature thymocytes and a deficiency of double positive cells indicate the involvement of somatostatin in the modulation of T cells maturation.

Gastrointestinal cryptosporidiosis in a puppy

An 8-week-old female Yorkshire terrier with a history of weakness, diarrhea and intestinal isosporiasis was euthanized and a post mortem examination performed. Histologically, there was severe gastrointestinal cryptosporidiosis, severe intestinal isosporiasis and thymic lymphoid depletion. PCR revealed visible bands for the actin and 18S rRNA genes but not for the acetyl CoA synthetase gene for Cryptosporidium spp. The PCR product for the actin gene was sequenced and found to have a 97.6-99.8% similarity to that of Cryptosporidium canis. To our knowledge, this is the first report of gastric cryptosporidiosis in a canine.

Thymic function after hematopoietic stem-cell transplantation for the treatment of severe combined immunodeficiency

BACKGROUND

Immune function can be restored in infants with severe combined immunodeficiency by transplantation of unfractionated bone marrow from HLA-identical donors or T-cell-depleted marrow stem cells from haploidentical donors, with whom there is a single haplotype mismatch, without the need for chemotherapy before transplantation or prophylaxis against graft-versus-host disease. The role of the thymus in this process is unknown.

METHODS

We analyzed the phenotypes of circulating T cells and the proliferative responses of peripheral-blood mononuclear cells to phytohemagglutinin in 83 patients with severe combined immunodeficiency who received allogeneic marrow transplants without T-cell ablation from related donors over an 18-year period. We also tested for the presence of episomes of T-cell antigen receptors (extrachromosomal DNA circles formed during intrathymic T-cell development) to assess thymus-dependent T-cell reconstitution.

RESULTS

Before and early after transplantation, the numbers of circulating T cells were low, with a predominance of mature CD45RO+ T cells (primarily resulting from the transplacental transfer of maternal cells); T-cell antigen-receptor episomes were undetectable in peripheral-blood mononuclear cells. In 73 of the infants, thymus-derived T cells expressing CD45RA and T-cell antigen-receptor episomes were detected within three to six weeks after transplantation. The mean (+/-SD) value for thymus-dependent T-cell antigen-receptor episomes peaked (at 7311+/-8652 per microgram of peripheral-blood mononuclear-cell DNA) 1 to 2 years after transplantation and declined to low levels (less than 100 episomes per microgram of DNA) within 14 years, as compared with a gradual decline from birth to the age of about 80 years in normal subjects.

CONCLUSIONS

The vestigial thymus in infants with severe combined immunodeficiency is functional and can produce enough T cells after bone marrow transplantation to provide normal immune function.

Age- and disease-related decline in immune function: an opportunity for "thymus-boosting" therapies

The thymus is the site of production of mature T lymphocytes and thus is indispensable for the development and maintenance of the T cell-mediated arm of the immune system. Thymic production of mature T cells is critically dependent on an influx of bone marrow-derived progenitor T cells that undergo replication and selection within the thymus. Thymus cellularity and thymic hormone secretion reach a peak during the first year of life and then decline gradually until the age of 50-60 years, a process known as "thymic involution." A rapid reduction of thymus cellularity occurs in young patients following injuries, chemotherapy, and other forms of stress. The mechanisms underlying the involution process appear to be dependent on factors intrinsic to the thymic tissue, such as the local production of cytokines and chemoattractants, promoting the recruitment, growth, and differentiation of bone marrow-derived T cell progenitors in the thymus, as well as extrinsic factors, such as systemic levels of endocrine hormones and mediators released by intrathymic nerves of the autonomic nervous system. Knowledge of these factors provides a rational basis for the development of an approach based on tissue engineering that could be used to provide either temporary or permanent reconstitution of thymic function.

From basic immunobiology to the upcoming WHO-classification of tumors of the thymus. The Second Conference on Biological and Clinical Aspects of Thymic Epithelial Tumors and related recent developments

The Second Conference on Biological and Clinical Aspects of Thymic Epithelial Tumors in Leiden, The Netherlands, 1998, set the stage for an interdisciplinary meeting of immunologists, pathologists and members of various clinical disciplines to exchange their recent findings in the field of thymus-related biology, pathology, and medicine. The contributions covered such diverse subjects as the role of transcription factors and cytokines in the development of the thymic microenvironment, thymic T, B and NK cell development, the pathogenesis of myasthenia gravis and other thymoma-associated autoimmunities, the pathology of thymic epithelial tumors and germ cell neoplasms, and new approaches to their diagnosis and treatment. This editorial will briefly sum up the data presented at the Conference and will comment on related novel findings that have been reported since then. Because it was also at the Leiden Conference, that the proposal of the WHO committee for the classification of thymic tumors was discussed for the first time, a description of the upcoming WHO Classification of Tumors of the Thymus is given with emphasis on the diagnostic criteria of thymic epithelial tumors, that should now be termed as type A, AB, B1-3 and type C thymomas, to make pathological and clinical studies comparable in the future.

Selective thymocyte depletion and immunoglobulin coating in the thymus of cats infected with feline immunodeficiency virus

Thymus alterations associated with feline immunodeficiency virus (FIV), an AIDS animal model, were investigated by measuring phenotypic composition of thymocytes, structure of thymic epithelial cells, and transcription of viral RNA in the thymus of FIV-infected juvenile kittens. These kittens either acquired infection by natural vertical transmission or were experimentally inoculated with the virus at defined times of fetal or neonatal life. Thymocytes from FIV-infected cats were analyzed by flow cytometry for the differential expression of CD4, CD8, Pan T, and IgG and subpopulation percentages were compared to values from uninfected littermates. Infected cats demonstrated a decrease in the percentage of CD4+/CD8+ lymphocytes and a concurrent increase in the percentage of CD4-/CD8-, CD4-/CD8+, and IgG+ lymphocytes. Absolute numbers of IgG+ cells were increased with FIV infection. On bivariate distribution scatter plots generated by two-color flow cytometry, this population of IgG+ cells overlapped extensively with cells having low to minimally detectable levels of a pan-T lymphocyte marker, suggesting that thymocytes were coated with IgG. Immunohistochemical detection of feline IgG defined a broad zone of IgG+ cells within the residual cortex but outside lymphoid follicles. However, cells stained with B5, a feline B lymphocyte marker, localized almost exclusively to the centers of lymphoid follicles that were also characterized by a lack of internal cytokeratin staining. FIV RNA transcripts detected by in situ hybridization using an FIVgag RNA probe were evenly distributed throughout the thymic parenchyma except in lymphoid follicles, which were generally devoid of FIV expression. Despite these phenotypic and structural changes, thymus weight, expressed as a percentage of body weight, was not significantly reduced. From these data, we conclude that the clinically asymptomatic stage of FIV infection is associated with two distinct B cell-related phenomena within the thymus-the formation of germinal centers and the coating of thymocytes with IgG. These changes accompany a distorted thymocyte distribution characterized by a reduced percentage of CD4+/CD8+ lymphocytes and a relative increase in CD4-/CD8+ and CD4-/CD8- lymphocytes. Together, these findings suggest that degenerative thymic changes after lentivirus infection may involve humoral immune mechanisms.

Arrest of in vitro T cell differentiation of normal bone marrow-derived CD34+ stem cells with thymic epithelial fragments from children with AIDS

A novel approach is presented to assess the ability of thymic tissues obtained from children with end stage AIDS to attract normal bone marrow (BM)-derived CD34+ (lineage negative) stem cells (SCs) and support lymphopoiesis in vitro. Chemokinesis of BM-derived CD34+ SCs was analyzed by time-lapse videomicroscopy to ascertain whether an alteration in SC motility could contribute to abnormal thymopoiesis under conditions of HIV infection. The migration of SCs derived from an HIV+ donor into thymic tissue was not significantly altered compared to normal controls, as were normal SCs migrating toward thymic epithelial cell monolayers derived from an HIV+ patient. Thymic tissue obtained from children with AIDS contained nests of CD34+ SCs identified by immunofluorescence, indicating SC homing to the thymus is apparently supported in HIV infection. The ability of HIV-affected thymic epithelial fragments to support lymphopoiesis was determined by examining the initial thymocyte populations present, compared to thymocytes produced de novo in T cell-depleted thymic fragments, following a single pulse of lineage negative CD34+ CD38- SCs. In comparison to normal controls, thymocytes derived from the HIV-affected thymic epithelial fragment coculture had an increased percentage of triple negative thymocytes (28% of lymphocytes from HIV-affected tissue versus 1.5% in controls, p < 0.01) and a decreased percentage of double and single positive CD4+ thymocytes. However, CD3+CD8+ TCR alpha beta + expression was comparable to control cultured thymic epithelial fragments indicating that HIV-affected thymic epithelia were capable of supporting the development of the CD8+ lineage. In an effort to extend the information obtained to date from the histological examination of HIV-affected thymic tissue, select patient thymic tissues were maintained in culture to evaluate the capacity of undifferentiated thymic epithelial cell guirlandes to differentiate in vitro. A partial regeneration of certain subpopulations of the thymic epithelium defined by TE-4 monoclonal antibody (mAb) and CDR2 mAbs occurred during the in vitro culture. The epithelial and mesenchymal components of thymic tissues were distinguished by immunostaining for keratins (indicative of epithelium) and vimentin (a mesenchymal marker). Further evaluation of the modulation of HIV thymus, with respect to the testing of new therapeutic strategies on SCs, will be possible with this in vitro model.

Developmental control point in induction of thymic cortex regulated by a subpopulation of prothymocytes

T lymphocytes of the alpha/beta T-cell receptor (TCR) lineage mature in the thymus, where they undergo a series of differentiation, expansion and selection events. For normal T-cell ontogeny to occur, thymocytes must interact physically with cortical and medullary thymic stroma cells. In parallel, interactions of the thymic stromal cells with TCR-positive thymocytes are necessary for the development of the thymic medulla. Comparable requirements for the differentiation of the cortex have not been defined, however. Here we analyse mutant mouse strains to assess the function of early prothymocytes in the induction of the thymic cortex. We find that animals with a developmental block at the earliest stage of T-lineage commitment lack a functional thymic cortex. This abnormality could be corrected in fetal but not adult animals by transplantation of either fetal or adult wild-type haematopoietic stem cells. Thus a developmentally restricted interaction of fetal stromal cells with early prothymocytes is required for the induction of a cortical microenvironment. In addition, a normal thymic architecture is necessary for sustained T-cell ontogeny.

Development in the thymus: it takes two to tango

Intrathymic T-cell development is dependent upon signals provided by the thymic stromal cell microenvironment. However, loss of thymic T cells in natural and experimentally induced situations is associated with a reduction in the surrounding epithelium, suggesting an interdependence between thymocytes and their microenvironment. Here, the authors review the evidence in favour of this intrathymic symbiosis, and hypothesize that T cells may provide maturation and survival signals that are necessary for the development and maintenance of their microenvironment.