Heterogeneity in thymic emigrants: implications for thymectomy and immunosenescence

Model of age-dependent dynamics and biokinetics of T-cells as natural biodosimeters

Circulating T-lymphocytes are used as "natural biodosimeters" for estimating radiation doses, since the frequency of chromosomal aberrations induced in them is proportional to the accumulated dose. Moreover, stable chromosomal aberrations (translocations) are detected years and decades after exposure. Internal incorporation of radionuclides often leads to non-uniform exposure, which resulted in difficulties in the application of retrospective biodosimetry using T-lymphocytes. Some properties of T-lymphocytes complicate retrospective biodosimetry in this case: (1) the thymic production of T-cells depends significantly on age, the maximum is observed in early childhood; (2) the "lymphocyte-dosimeter" accumulates changes (translocations) while circulating through the body. The objective of this paper is to describe the technical characteristics of the model of age dynamics and T-cell biokinetics and approaches to assessing the dose to circulating lymphocytes under various exposure scenarios. The model allows to quantify the fractions of T-lymphocytes that were formed before and after exposure. The model takes into account the time fractions that circulating lymphocytes spend in various lymphoid organs. Age-related thymic involution was also considered. The model predicts that after internal exposure to 90Sr, the doses to T-lymphocytes can differ significantly from the doses to the bone marrow and other tissues. For uniform external γ-exposure, and for internal exposure due to non-bone -seeking radionuclides (for example, 144Ce), predicted doses to T-lymphocytes are very close to bone marrow doses. The model allows to quantify the correction factors for FISH-based doses to obtain doses to organs and tissues.

The Impact of Immune System Aging on Infectious Diseases

Immune system aging is becoming a field of increasing public health interest because of prolonged life expectancy, which is not paralleled by an increase in health expectancy. As age progresses, innate and adaptive immune systems undergo changes, which are defined, respectively, as inflammaging and immune senescence. A wealth of available data demonstrates that these two conditions are closely linked, leading to a greater vulnerability of elderly subjects to viral, bacterial, and opportunistic infections as well as lower post-vaccination protection. To face this novel scenario, an in-depth assessment of the immune players involved in this changing epidemiology is demanded regarding the individual and concerted involvement of immune cells and mediators within endogenous and exogenous factors and co-morbidities. This review provides an overall updated description of the changes affecting the aging immune system, which may be of help in understanding the underlying mechanisms associated with the main age-associated infectious diseases.

Quantifying changes in the T cell receptor repertoire during thymic development

One of the feats of adaptive immunity is its ability to recognize foreign pathogens while sparing the self. During maturation in the thymus, T cells are selected through the binding properties of their antigen-specific T-cell receptor (TCR), through the elimination of both weakly (positive selection) and strongly (negative selection) self-reactive receptors. However, the impact of thymic selection on the TCR repertoire is poorly understood. Here, we use transgenic Nur77-mice expressing a T-cell activation reporter to study the repertoires of thymic T cells at various stages of their development, including cells that do not pass selection. We combine high-throughput repertoire sequencing with statistical inference techniques to characterize the selection of the TCR in these distinct subsets. We find small but significant differences in the TCR repertoire parameters between the maturation stages, which recapitulate known differentiation pathways leading to the CD4⁺ and CD8⁺ subtypes. These differences can be simulated by simple models of selection acting linearly on the sequence features. We find no evidence of specific sequences or sequence motifs or features that are suppressed by negative selection. These results favour a collective or statistical model for T-cell self non-self discrimination, where negative selection biases the repertoire away from self recognition, rather than ensuring lack of self-reactivity at the single-cell level.

The natural history of naive T cells from birth to maturity

Generating and maintaining a diverse repertoire of naive T cells is essential for protection against pathogens, and developing a mechanistic and quantitative description of the processes involved lies at the heart of our understanding of vertebrate immunity. Here, we review the biology of naive T cells from birth to maturity and outline how the integration of mathematical models and experiments has helped us to develop a full picture of their life histories.

End-stage renal disease, dialysis, kidney transplantation and their impact on CD4+ T-cell differentiation

Premature aging of both CD4⁺ regulatory T (Treg) and CD4⁺ responder-T (Tresp) cells in patients with end-stage renal disease (ESRD) is expected to affect the success of later kidney transplantation. Both T-cell populations are released from the thymus as inducible T-cell co-stimulator-positive (ICOS⁺ ) and ICOS^- recent thymic emigrant (RTE) Treg/Tresp cells, which differ primarily in their proliferative capacities. In this study, we analysed the effect of ESRD and subsequent renal replacement therapies on the differentiation of ICOS⁺ and ICOS^- RTE Treg/Tresp cells into ICOS⁺  CD31^- or ICOS^-  CD31^- memory Treg/Tresp cells and examined whether diverging pathways affected the suppressive activity of ICOS⁺ and ICOS^- Treg cells in co-culture with autologous Tresp cells. Compared with healthy controls, we found an increased differentiation of ICOS⁺ RTE Treg/Tresp cells and ICOS^- RTE Treg cells through CD31⁺ memory Treg/Tresp cells into CD31^- memory Treg/Tresp cells in ESRD and dialysis patients. In contrast, ICOS^- RTE Tresp cells showed an increased differentiation via ICOS^- mature naive (MN) Tresp cells into CD31^- memory Tresp cells. Thereby, the ratio of ICOS⁺ Treg/ICOS⁺ Tresp cells was not changed, whereas that of ICOS^- Treg/ICOS^- Tresp cells was significantly increased. This differentiation preserved the suppressive activity of both Treg populations in ESRD and partly in dialysis patients. After transplantation, the increased differentiation of ICOS⁺ and ICOS^- RTE Tresp cells proceeded, whereas that of ICOS⁺ RTE Treg cells ceased and that of ICOS^- RTE Treg cells switched to an increased differentiation via ICOS^- MN Treg cells. Consequently, the ratios of ICOS⁺ Treg/ICOS⁺ Tresp cells and of ICOS^- Treg/ICOS^- Tresp cells decreased significantly, reducing the suppressive activity of Treg cells markedly. Our data reveal that an increased tolerance-inducing differentiation of ICOS⁺ and ICOS^- Treg cells preserves the functional activity of Treg cells in ESRD patients, but this cannot be maintained during long-term renal replacement therapy.

The significance of neonatal thymectomy for shaping the immune system in children with congenital heart defects

The thymus plays an important role in the development of the immune cell pool; it serves as the primary location for T-lymphocyte maturation. Early cardiac surgical interventions for congenital heart defects are necessarily associated with thymectomy, i.e. the partial or complete removal of the thymus. A newborn infant already has a functioning thymus and developed cells of the immune system. However, thymectomy eliminates the primary location where T cells differentiate and mature. This study summarizes the current knowledge of the cellular disturbances and potential clinical consequences associated with performing thymectomy in children treated surgically for congenital heart defects.

Genomic deletion of GIT2 induces a premature age-related thymic dysfunction and systemic immune system disruption

Recent research has proposed that GIT2 (G protein-coupled receptor kinase interacting protein 2) acts as an integrator of the aging process through regulation of 'neurometabolic' integrity. One of the commonly accepted hallmarks of the aging process is thymic involution. At a relatively young age, 12 months old, GIT2^-/- mice present a prematurely distorted thymic structure and dysfunction compared to age-matched 12 month-old wild-type control (C57BL/6) mice. Disruption of thymic structure in GIT2^-/- (GIT2KO) mice was associated with a significant reduction in the expression of the cortical thymic marker, Troma-I (cytokeratin 8). Double positive (CD4⁺CD8⁺) and single positive CD4⁺ T cells were also markedly reduced in 12 month-old GIT2KO mice compared to age-matched control wild-type mice. Coincident with this premature thymic disruption in GIT2KO mice was the unique generation of a novel cervical 'organ', i.e. 'parathymic lobes'. These novel organs did not exhibit classical peripheral lymph node-like characteristics but expressed high levels of T cell progenitors that were reflexively reduced in GIT2KO thymi. Using signaling pathway analysis of GIT2KO thymus and parathymic lobe transcriptomic data we found that the molecular signaling functions lost in the dysfunctional GIT2KO thymus were selectively reinstated in the novel parathymic lobe - suggestive of a compensatory effect for the premature thymic disruption. Broader inspection of high-dimensionality transcriptomic data from GIT2KO lymph nodes, spleen, thymus and parathymic lobes revealed a systemic alteration of multiple proteins (Dbp, Tef, Per1, Per2, Fbxl3, Ddit4, Sin3a) involved in the multidimensional control of cell cycle clock regulation, cell senescence, cellular metabolism and DNA damage. Altered cell clock regulation across both immune and non-immune tissues therefore may be responsible for the premature 'aging' phenotype of GIT2KO mice.

Local bone-marrow exposure: how to interpret the data on stable chromosome aberrations in circulating lymphocytes? (some comments on the use of FISH method for dose reconstruction for Techa riverside Residents)

The method of fluorescence in situ hybridization (FISH) applied to peripheral blood T lymphocytes is used for retrospective dose estimation, and the results obtained from the analysis of stable chromosomal aberrations are usually interpreted as a dose accumulated in the red bone marrow (RBM). However, after local internal exposure of the RBM, doses derived from FISH were found to be lower than those derived from direct measurements of radionuclides accumulated in the bodies of exposed persons. These results were obtained for people residing near the Techa River contaminated by ^89,90Sr (beta-emitters) in 1949-1956 (Chelyabinsk Oblast, Russia). A new analysis has been performed of the combined results of FISH studies (n = 178) undertaken during 1994-2012 for persons living on the Techa Riverside. Analysis confirms the lower slope of the translocation yield per Gy (8.0 ± 0.7 × 10^-3) for Techa residents in comparison with FISH data for donors with external exposures (11.6 ± 1.6 × 10^-3, Tawn et al., Radiat Res 184(3):296-303, 2015). It was suggested that some portion of T cells remained unexposed, because they represented the descendants of T cell progenitors, which had migrated to the thymus before the start of ^89,90Sr intakes. To clarify this problem, the dynamics of T-cell Genera (TG), combining all descendants of specific T-cell progenitor reaching the thymus, was considered. Rates of TGs produced by RBM over different age periods of human life were estimated with the use of the mathematic model of T-cell homeostasis (Bains, Mathematical modeling of T-cell homeostasis. A thesis submitted for the degree of Doctor of Philosophy of the University College London. http://discovery.ucl.ac.uk/20159/1/20159.pdf , 2010). The rate of TG loss during the lifetime was assumed to be very small in comparison with production rate. The recirculation of mature T lymphocytes in contaminated RBM was taken into account. According to our model estimates, at the time of blood sampling, the fraction of exposed T lymphocytes (whose progenitors were irradiated) ranged from 20 to 80% depending on the donors' age at the start of exposure to ^89,90Sr. Dose to T lymphocytes, estimated from FISH studies, should be about 0.6-0.9 of RBM dose for residents of the upper Techa region and about 0.4-0.8 in the middle Techa region. Our results could explain the lower value of translocation yield per Gy obtained for Techa residents. The approaches for further model improvement and validation are discussed in this paper.

Establishment and Maintenance of the Human Naïve CD4+ T-Cell Compartment

The naïve CD4⁺ T-cell compartment is considered essential to guarantee immune competence throughout life. Its replenishment with naïve cells with broad diverse receptor repertoire, albeit with reduced self-reactivity, is ensured by the thymus. Nevertheless, cumulative data support a major requirement of post-thymic proliferation both for the establishment of the human peripheral naïve compartment during the accelerated somatic growth of childhood, as well as for its lifelong maintenance. Additionally, a dynamic equilibrium is operating at the cell level to fine-tune the T-cell receptor threshold to activation and survival cues, in order to counteract the continuous naïve cell loss by death or conversion into memory/effector cells. The main players in these processes are low-affinity self-peptide/MHC and cytokines, particularly IL-7. Moreover, although naïve CD4⁺ T-cells are usually seen as a homogeneous population regarding stage of maturation and cell differentiation, increasing evidence points to a variety of phenotypic and functional subsets with distinct homeostatic requirements. The paradigm of cells committed to a distinct lineage in the thymus are the naïve regulatory T-cells, but other functional subpopulations have been identified based on their time span after thymic egress, phenotypic markers, such as CD31, or cytokine production, namely IL-8. Understanding the regulation of these processes is of utmost importance to promote immune reconstitution in several clinical settings, namely transplantation, persistent infections, and aging. In this mini review, we provide an overview of the mechanisms underlying human naïve CD4⁺ T-cell homeostasis, combining clinical data, experimental studies, and modeling approaches.

Temporal fate mapping reveals age-linked heterogeneity in naive T lymphocytes in mice

Understanding how our T-cell compartments are maintained requires knowledge of their population dynamics, which are typically quantified over days to weeks using the administration of labels incorporated into the DNA of dividing cells. These studies present snapshots of homeostatic dynamics and have suggested that lymphocyte populations are heterogeneous with respect to rates of division and/or death, although resolving the details of such heterogeneity is problematic. Here we present a method of studying the population dynamics of T cells in mice over timescales of months to years that reveals heterogeneity in rates of division and death with respect to the age of the host at the time of thymic export. We use the transplant conditioning drug busulfan to ablate hematopoetic stem cells in young mice but leave the peripheral lymphocyte compartments intact. Following their reconstitution with congenically labeled (donor) bone marrow, we followed the dilution of peripheral host T cells by donor-derived lymphocytes for a year after treatment. Describing these kinetics with mathematical models, we estimate rates of thymic production, division and death of naive CD4 and CD8 T cells. Population-averaged estimates of mean lifetimes are consistent with earlier studies, but we find the strongest support for a model in which both naive T-cell pools contain kinetically distinct subpopulations of older host-derived cells with self-renewing capacity that are resistant to displacement by naive donor lymphocytes. We speculate that these incumbent cells are conditioned or selected for increased fitness through homeostatic expansion into the lymphopenic neonatal environment.

Mapping the Diversity of Follicular Helper T Cells in Human Blood and Tonsils Using High-Dimensional Mass Cytometry Analysis

Single-cell analysis technologies such as mass cytometry allow for measurements of cellular heterogeneity with unprecedented dimensionality. Here, we applied dimensionality reduction and automated clustering methods on human T helper (T(H)) cells derived from peripheral blood and tonsils, which showed differential cell composition and extensive T(H) cell heterogeneity. Notably, this analysis revealed numerous subtypes of follicular helper T (T(FH)) cells that followed a continuum spanning both blood and tonsils. Furthermore, we identified tonsillar CXCR5(lo)PD-1(lo)CCR7(lo) T(FH) cells expressing interferon-γ (IFN-γ), interleukin-17 (IL-17), or Foxp3, indicating that T(FH) cells exhibit diverse functional capacities within extrafollicular stages. Regression analysis demonstrated that CXCR5(lo)PD-1(-) and CXCR5(lo)PD-1(lo) cells accumulate during childhood in secondary lymphoid organs, supporting previous findings that these subsets represent memory T(FH) cells. This study provides an in-depth comparison of human blood and tonsillar T(FH) cells and outlines a general approach for subset discovery and hypothesizing of cellular progressions.