Thymus and T-lymphocyte development: what is new in the 21st century?

What radiation dose does the FISH translocation assay measure in cases of incorporated radionuclides for the Southern Urals populations?

The fluorescence in situ hybridisation (FISH) technique is now well established for retrospective dosimetry in cases of external radiation exposure that occurred many years ago. However, the question remains as to whether FISH provides valid estimates of cumulative red bone marrow radiation doses in cases of incorporation of radionuclides or combined external and internal exposures. This question has arisen in connection with the interpretation of results of dose assessments for epidemiological studies of plutonium workers at the Russian Mayak plant and of members of the public exposed to strontium radioisotopes and external radiation as a result of discharges from Mayak to the Techa River. Exposures to penetrating external radiation result in fairly uniform irradiation of body tissues, and hence similar doses to all tissues, for which FISH dosimetry can provide a reliable measure of this whole body dose. However, intakes of radionuclides into the body by inhalation or ingestion may result in retention in specific organs and tissues, so that the distribution of dose is highly heterogeneous. For radionuclides emitting short-range radiations (e.g. alpha particles), this heterogeneity can apply to dose delivery within tissues and between cells within tissues. In this paper, an attempt is made to address the question of what FISH measures in such circumstances by considering evidence regarding the origin and lifetime dynamics of lymphocyte subsets in the human body in relation to the localised delivery of dose from the internal emitters (90)Sr and (239)Pu, which are of particular interest for the Southern Urals Mayak and Techa River populations, and for which most evidence is available in these populations. It is concluded that the FISH translocation assay can be usefully applied for detecting internal and combined external gamma and internal doses from internally deposited (90)Sr, albeit with fairly large uncertainties. The same may be true of (239)Pu, as well as other radionuclides, although much work remains to be done to establish dose-response relationships.

ICRP publication 118: ICRP statement on tissue reactions and early and late effects of radiation in normal tissues and organs--threshold doses for tissue reactions in a radiation protection context

This report provides a review of early and late effects of radiation in normal tissues and organs with respect to radiation protection. It was instigated following a recommendation in Publication 103 (ICRP, 2007), and it provides updated estimates of 'practical' threshold doses for tissue injury defined at the level of 1% incidence. Estimates are given for morbidity and mortality endpoints in all organ systems following acute, fractionated, or chronic exposure. The organ systems comprise the haematopoietic, immune, reproductive, circulatory, respiratory, musculoskeletal, endocrine, and nervous systems; the digestive and urinary tracts; the skin; and the eye. Particular attention is paid to circulatory disease and cataracts because of recent evidence of higher incidences of injury than expected after lower doses; hence, threshold doses appear to be lower than previously considered. This is largely because of the increasing incidences with increasing times after exposure. In the context of protection, it is the threshold doses for very long follow-up times that are the most relevant for workers and the public; for example, the atomic bomb survivors with 40-50years of follow-up. Radiotherapy data generally apply for shorter follow-up times because of competing causes of death in cancer patients, and hence the risks of radiation-induced circulatory disease at those earlier times are lower. A variety of biological response modifiers have been used to help reduce late reactions in many tissues. These include antioxidants, radical scavengers, inhibitors of apoptosis, anti-inflammatory drugs, angiotensin-converting enzyme inhibitors, growth factors, and cytokines. In many cases, these give dose modification factors of 1.1-1.2, and in a few cases 1.5-2, indicating the potential for increasing threshold doses in known exposure cases. In contrast, there are agents that enhance radiation responses, notably other cytotoxic agents such as antimetabolites, alkylating agents, anti-angiogenic drugs, and antibiotics, as well as genetic and comorbidity factors. Most tissues show a sparing effect of dose fractionation, so that total doses for a given endpoint are higher if the dose is fractionated rather than when given as a single dose. However, for reactions manifesting very late after low total doses, particularly for cataracts and circulatory disease, it appears that the rate of dose delivery does not modify the low incidence. This implies that the injury in these cases and at these low dose levels is caused by single-hit irreparable-type events. For these two tissues, a threshold dose of 0.5Gy is proposed herein for practical purposes, irrespective of the rate of dose delivery, and future studies may elucidate this judgement further.

The adrenal cortex and life

The template for our understanding of the physiological role of the adrenal cortex was set by Hans Selye, who demonstrated its key involvement in the response to stress, of whatever origin, and who also introduced the terms glucocorticoid and mineralocorticoid. Despite this, from the late 1940s on there was certainly general awareness of the multiple actions of glucocorticoids, including effects on the thymus and immune system, cardiovascular system, water balance, and the CNS. For these reasons, and perhaps because in the early studies of the actions of individual steroids there was less clear-cut difference between them, there was some initial resistance to the use of these terms. Today they are universal and unchallenged. It can be argued that, with respect to the glucocorticoids, this term colours our perception of their physiological importance, and may be misleading. By taking evidence from disease states, emphasis is placed on extreme conditions that do not necessarily reveal normal physiology. In particular, evidence for the role of glucocorticoid regulation of gluconeogenesis and blood glucose in the normal subject or animal is inconclusive. Similarly, while highly plausible theories explaining glucocorticoid actions on inflammation or the immune system as part of normal physiology have been presented, direct evidence to support them is hard to find. Under extreme conditions of chronic stress, the cumulative actions of glucocorticoids on insulin resistance or immunocompromise may indeed seem to be actually damaging. Two well-documented and long recognized situations create huge variation in glucocorticoid secretion. These are the circadian rhythm, and the acute response to mild stress, such as handling, in the rat. Neither of these can be adequately explained by the need for glucocorticoid action, as we currently understand it, particularly on carbohydrate metabolism or on the immune system. Perhaps we should re-examine other targets at the physiological level. At the present time, some of these seem to be out of fashion.

Diverse T-cell differentiation potentials of human fetal thymus, fetal liver, cord blood and adult bone marrow CD34 cells on lentiviral Delta-like-1-modified mouse stromal cells

Human haematopoietic progenitor/stem cells (HPCs) differentiate into functional T cells in the thymus through a series of checkpoints. A convenient in vitro system will greatly facilitate the understanding of T-cell development and future engineering of therapeutic T cells. In this report, we established a lentiviral vector-engineered stromal cell line (LSC) expressing the key lymphopoiesis regulator Notch ligand, Delta-like 1 (DL1), as feeder cells (LSC-mDL1) supplemented with Flt3 ligand (fms-like tyrosine kinase 3, Flt3L or FL) and interleukin-7 for the development of T cells from CD34(+) HPCs. We demonstrated T-cell development from human HPCs with various origins including fetal thymus (FT), fetal liver (FL), cord blood (CB) and adult bone marrow (BM). The CD34(+) HPCs from FT, FL and adult BM expanded more than 100-fold before reaching the beta-selection and CD4/CD8 double-positive T-cell stage. The CB HPCs, on the other hand, expanded more than 1000-fold before beta-selection. Furthermore, the time required to reach beta-selection differed for the various HPCs, 7 days for FT, 14 days for FL and CB, and 35 days for adult BM. Nevertheless, all of the T cells developed in vitro were stalled at the double-positive or immature single-positive stage with the exception that some CB-derived T cells arrived at a positive selection stage. Consequently, the LSC-mDL1 culture system illustrated diverse T-cell development potentials of pre- and post-natal and adult human BM HPCs. However, further modification of this in vitro T-cell development system is necessary to attain fully functional T cells.

The immunopathology of thymic GVHD

The clinical success of allogeneic hematopoietic stem cell transplantation (HSCT) depends on the appropriate reconstitution of the host's immune system. While recovery of T-cell immunity may occur in transplant recipients via both thymus-dependent and thymus-independent pathways, the regeneration of a population of phenotypically naive T cells with a broad receptor repertoire relies entirely on the de novo generation of T-cells in the thymus. Preclinical models and clinical studies of allogeneic HSCT have identified the thymus as a target of graft-versus-host disease (GVHD), thus limiting T-cell regeneration. The present review focuses on recent insight into how GVHD affects thymic structure and function and how this knowledge may aid in the design of new strategies to improve T-cell reconstitution following allogeneic HSCT.

Histochemical and molecular overview of the thymus as site for T-cells development

The thymus represents the primary site for T cell lymphopoiesis, providing a coordinated set for critical factors to induce and support lineage commitment, differentiation and survival of thymus-seeding cells. One irrefutable fact is that the presence of non-lymphoid cells through the thymic parenchyma serves to provide coordinated migration and differentiation of T lymphocytes. Moreover, the link between foetal development and normal anatomy has been stressed in this review. Regarding thymic embryology, its epithelium is derived from the embryonic endodermal layer, with possible contributions from the ectoderm. A series of differentiating steps is essential, each of which must be completed in order to provide the optimum environment for thymic development and function. The second part of this article is focused on thymic T-cell development and differentiation, which is a stepwise process, mediated by a variety of stromal cells in different regions of the organ. It depends strongly on the thymic microenvironment, a cellular network formed by epithelial cells, macrophages, dendritic cells and fibroblasts, that provide the combination of cellular interactions, cytokines and chemokines to induce thymocyte precursors for the generation of functional T cells. The mediators of this process are not well defined but it has been demonstrated that some interactions are under neuroendocrine control. Moreover, some studies pointed out that reciprocal signals from developing T cells also are essential for establishment and maintenance of the thymic microenvironment. Finally, we have also highlighted the heterogeneity of the lymphoid, non-lymphoid components and the multi-phasic steps of thymic differentiation. In conclusion, this review contributes to an understanding of the complex mechanisms in which the foetal and postnatal thymus is involved. This could be a prerequisite for developing new therapies specifically aimed to overcome immunological defects, linked or not-linked to aging.

Gammadelta T cells promote anterior chamber-associated immune deviation and immune privilege through their production of IL-10

Anterior chamber-associated immune deviation (ACAID) is a form of peripheral tolerance that is induced by introducing Ags into the anterior chamber (AC) of the eye, and is maintained by Ag-specific regulatory T cells (Tregs). ACAID regulates harmful immune responses that can lead to irreparable injury to innocent bystander cells that are incapable of regeneration. This form of immune privilege in the eye is mediated through Tregs and is a product of complex cellular interactions. These involve F4/80+ ocular APCs, B cells, NKT cells, CD4+CD25+ Tregs, and CD8+ Tregs. gammadelta T cells are crucial for the generation of ACAID and for corneal allograft survival. However, the functions of gammadelta T cells in ACAID are unknown. Several hypotheses were proposed for determining the functions of gammadelta T cells in ACAID. The results indicate that gammadelta T cells do not cause direct suppression of delayed-type hypersensitivity nor do they act as tolerogenic APCs. In contrast, gammadelta T cells were shown to secrete IL-10 and facilitate the generation of ACAID Tregs. Moreover, the contribution of gammadelta T cells ACAID generation could be replaced by adding exogenous recombinant mouse IL-10 to ACAID spleen cell cultures lacking gammadelta T cells.

Thymic epithelial progenitor cells and thymus regeneration: an update

The thymus provides the essential microenvironment for T-cell development and maturation. Thymic epithelial cells (TECs), which are composed of thymic cortical epithelial cells (cTECs) and thymic medullary epithelial cells (mTECs), have been well documented to be critical for these tightly regulated processes. It has long been controversial whether the common progenitor cells of TECs could give rise to both cTECs and mTECs. Great progress has been made to characterize the common TEC progenitor cells in recent years. We herein discuss the sole origin paradigm with regard to TEC differentiation as well as these progenitor cells in thymus regeneration.