Multiple Routes for Late Intrathymic Precursors to Generate CD4+CD8+ Thymocytes

Thymic changes as a contributing factor in the increased susceptibility of old Albino Oxford rats to EAE development

The study was aimed to examine putative contribution of thymic involution to ageing-associated increase in susceptibility of Albino Oxford (AO) rats to the development of clinical EAE, and vice versa influence of the disease on the progression of thymic involution. To this end we examined (i) the parameters of thymocyte negative selection efficacy, the thymic generation of CD4+CD25+Foxp3+ T regulatory cells (Tregs) and thymic capacity to instruct/predetermine IL-17-producing T-cell differentiation, and thymopietic efficacy-associated accumulation of "inflammescent" cytotoxic CD28- T cells in the periphery, and (ii) the key underlying mechanisms in young and old non-immunised AO rats and their counterparts immunised for EAE (on the 16^th day post-immunisation when the disease in old rats reached the plateau) using flow cytometry analysis and/or RT-qPCR. It was found that thymic involution impairs: (i) the efficacy of negative selection (by affecting thymocyte expression of CD90, negative regulator of selection threshold and the expression of thymic stromal cell integrity factors) and (ii) Treg generation (by diminishing expression of cytokines supporting their differentiation/maturation). Additionally, the results suggest that thymic involution facilitates CD8+ T-cell differentiation into IL-17-producing cells (previously linked to the development of clinical EAE in old AO rats). Furthermore, they confirmed that ageing-related decrease in thymic T-cell output (as indicated by diminished frequency of recent thymic emigrants in peripheral blood) resulted in the accumulation of CD28- T cells in peripheral blood and, upon immunisation, in the target organ. On the other hand, the development of EAE (most likely by increasing circulatory levels of proinflammatory cytokines) contributed to the decline in thymic output of T cells, including Tregs, and thereby to the progression/maintenance of clinical EAE. Thus, in AO rats thymic involution via multi-layered mechanisms may favour the development of clinically manifested autoimmunity, which, in turn, precipitates the thymus atrophy.

Strain differences in thymic atrophy in rats immunized for EAE correlate with the clinical outcome of immunization

An accumulating body of evidence suggests that development of autoimmune pathologies leads to thymic dysfunction and changes in peripheral T-cell compartment, which, in turn, perpetuate their pathogenesis. To test this hypothesis, thymocyte differentiation/maturation in rats susceptible (Dark Agouti, DA) and relatively resistant (Albino Oxford, AO) to experimental autoimmune encephalomyelitis (EAE) induction was examined. Irrespective of strain, immunization for EAE (i) increased the circulating levels of IL-6, a cytokine causally linked with thymic atrophy, and (ii) led to thymic atrophy reflecting partly enhanced thymocyte apoptosis associated with downregulated thymic IL-7 expression. Additionally, immunization diminished the expression of Thy-1, a negative regulator of TCRαβ-mediated signaling and activation thresholds, on CD4+CD8+ TCRαβ^lo/hi thymocytes undergoing selection and thereby impaired thymocyte selection/survival. This diminished the generation of mature CD4+ and CD8+ single positive TCRαβ^hi thymocytes and, consequently, CD4+ and CD8+ recent thymic emigrants. In immunized rats, thymic differentiation of natural regulatory CD4+Foxp3+CD25+ T cells (nTregs) was particularly affected reflecting a diminished expression of IL-7, IL-2 and IL-15. The decline in the overall thymic T-cell output and nTreg generation was more pronounced in DA than AO rats. Additionally, differently from immunized AO rats, in DA ones the frequency of CD28- cells secreting cytolytic enzymes within peripheral blood CD4+ T lymphocytes increased, as a consequence of thymic atrophy-related replicative stress (mirrored in CD4+ cell memory pool expansion and p16^INK4a accumulation). The higher circulating level of TNF-α in DA compared with AO rats could also contribute to this difference. Consistently, higher frequency of cytolytic CD4+ granzyme B+ cells (associated with greater tissue damage) was found in spinal cord of immunized DA rats compared with their AO counterparts. In conclusion, the study indicated that strain differences in immunization-induced changes in thymopoiesis and peripheral CD4+CD28- T-cell generation could contribute to rat strain-specific clinical outcomes of immunization for EAE.

Kinetics of thymocyte developmental process in fetal and neonatal mice

Kinetics of thymocyte development in vivo during embryogenesis was pursued. The early development of thymocytes in the fetal and neonatal BALB/c mice was discontinuous, with four waves of cell proliferation occurring at fetal day (Fd) 14 to 17, Fd 18 to day (D) 1 after birth, D 2 to D 5 and D6 thereafter. The first three proliferation waves coincided with the generation of CD4hiCD8hi (DP), TCR+CD4hiCD8-/lo (CD4 SP), and TCR+CD4-/loCD8int/hi (CD8 SP) thymocytes, respectively. The transition from DN to DP cells was further investigated and it was found out that there were two differential pathways via immature single positive (ISP) cells in the BALB/c mice, each functioning at different fetal ages. One is via TCR-CD4-CD8+ cells, occurring between Fd 15 and Fd 17 and the other is via TCR-CD4+CD8- cells, occurring from Fd 17 until birth. In contrast, the TCR-CD4-CD8+ pathway dominated overwhelmingly in the C57BL/6 mice. These findings shed new light on the hypothesis that the differential pathway preference varies with mouse strains. With respect to the shift in the intensity of CD4 and CD8 expression on thymocytes from fetal to adult mice, the TCR+CD4hiCD8-/lo, and TCR+CD4-/loCD8int/hi subsets might be equivalent to the medullary type TCR+CD4/CD8 SP cells.

Co-ordinate expression of the pre-T-cell receptor complex and a novel immature thymocyte-specific antigen, IMT-1, during thymocyte development

Previously we described a monoclonal antibody (mAb) that reacted with a cell-surface antigen, immature thymocyte antigen-1 (IMT-1), which is expressed on thymocytes of late CD4- CD8- (double negative) to early CD4+ CD8+ (double positive) differentiation stages. In this study, we investigated the expression of IMT-1 on various cell lineages in thymus as well as in peripheral lymphoid organs. We found that IMT-1 is expressed on T-cell receptor (TCR)-betalo and TCR-deltalo thymocytes, but not on TCR-betahi, TCR-deltahi or natural killer (NK)1.1+ thymocytes, or on peripheral alpha beta or gamma delta T cells. We also investigated the kinetics of expression of IMT-1 during fetal thymocyte development and compared it with the expression of the pre-TCR complex, comprising CD3, pre-TCR-alpha (pTalpha) and TCR-beta. We found that expression of both was similar, starting at day 14.5 of gestation, peaking on day 16.5 and gradually decreasing thereafter. Furthermore, the expression of both IMT-1 and pTalpha was drastically reduced when DN thymocytes in recombination activating gene (RAG)-2-/- mice were challenged in vivo with anti-CD3 mAb. These results indicate that IMT-1 is expressed on not only immature thymocytes of alpha beta T-cell lineage but also on those of gamma delta T-cell lineage, and that the expression of IMT-1 and the pre-TCR complex is co-ordinately regulated during the alpha beta lineage thymocyte development.

Critical Involvement of Tcf-1 in Expansion of Thymocytes

T cell maturation in Tcf-1−/− mice deteriorates progressively and halts completely around 6 mo of age. During fetal development thymocyte subpopulations seem normal, although total cell numbers are lower. By 4 to 6 wk of age, obvious blockades in the differentiation of CD4−8− thymocytes are observed at two distinct stages (CD44+25+ and CD44−25−), both of which are normally characterized by extensive proliferation. This lack of thymocyte expansion and/or differentiation was also observed when Tcf-1−/− progenitor cells from the aorta-gonad-mesonephros region (embryonic day 11.5), fetal liver (embryonic day 12.5/14.5), and fetal bone marrow (embryonic day 18.5) were allowed to differentiate in normal thymic lobes (fetal thymic organ cultures) or were injected intrathymically into normal recipients. Despite these apparent defects in thymocyte differentiation and expansion, adult Tcf-1−/− mice are immunocompetent, as they generate virus neutralizing Abs at normal titers. Furthermore, their peripheral T cells have an activated phenotype (increased CD44 and decreased CD62L expression) and proliferate normally in response to Ag or mitogen, suggesting that these cells may have arisen from the early wave of development during embryogenesis and are either long lived or have subsequently been maintained by peripheral expansion. As Tcf-1 is a critical component in the Wnt/β-catenin signaling pathway, these data suggest that Wnt-like factors play a role in the expansion of double-negative thymocytes.

The role of the T-cell receptor (TCR) in alpha beta/gamma delta lineage commitment: clues from intracellular TCR staining

T cells belong to two mutually exclusive lineages expressing either alpha beta or gamma delta T-cell receptors (TCR). Although alpha beta and gamma delta cells are known to share a common precursor the role of TCR rearrangement and specificity in the lineage commitment process is controversial. Instructive lineage commitment models endow the alpha beta or gamma delta TCR with a deterministic role in lineage choice, whereas separate lineage models invoke TCR-independent lineage commitment followed by TCR-dependent selection and maturation of alpha beta and gamma delta cells. Here we review the published data pertaining to the role of the TCR in alpha beta/gamma delta lineage commitment and provide some additional information obtained from recent intracellular TCR staining studies. We conclude that a variant of the separate lineage model is best able to accommodate all of the available experimental results.

Influence of retinoic acid on the differentiation pathway of T cells in the thymus

This study investigated the ability of retinoic acid (RA) to influence T cell differentiation. All-trans-RA had marked effects on T cell differentiation in murine fetal thymic organ cultures (FTOCs). The time course of the effect of all-trans-RA in FTOC of day 14 C57BL/6 embryos revealed a twofold increase in the frequency of CD4 single-positive (SP) cells and a high level of CD3-bearing cells (CD3high cells) at a later stage of T cell development. At an earlier stage, all-trans-RA induced a twofold increase in the frequency of CD4 SP cells, but significantly suppressed the upregulation of CD3 and TCR. Reverse transcription-PCR using RA receptor (RAR) subtype-specific primers showed that RAR alpha but not beta and gamma is expressed during T cell development in the thymus and that its expression was associated with the generation of CD4/CD8 double-positive (DP) cells. In FTOC of day 16 BALB/c embryos, the level of V beta 3high cells was greatly reduced (1.4% of the CD3high cells) in response to the mouse mammary tumor virus-6-encoded superantigen, but V beta 3-bearing cells were rescued from the deletion in the presence of all-trans-RA (5.6% of the CD3high cells). Further, the inhibitory effect of all-trans-RA on thymocyte deletion was observed when the deletion was induced by a low concentration of staphylococcal enterotoxin B in FTOC. Taken together, these data suggest that RA increases the frequency of mature and self-reactive T cells in the thymus, possibly by inhibiting the process of negative selection at the DP stage of T cell differentiation.

Flow cytometric study of T cell development in NOD mice reveals a deficiency in alphabetaTCR+CDR-CD8- thymocytes

As a result of failed induction of T cell tolerance to pancreatic B cells, non-obese diabetic (NOD) mice develop spontaneous autoimmune insulin-dependent diabetes mellitus (IDDM). The thymic stroma, which plays a crucial role in thymic T cell maturation, undergoes extensive premature disorganization in NOD mice, so it is of interest to examine NOD T cell development. In this study, both major and minor developmental populations of thymocytes of NOD/Lt mice were studied and compared to those of BALB/c, C57BL/6 and CBA mice by multiparameter flow cytometry (FACS). These results are described in detail and reveal that most thymocyte subsets were normally represented, including alphaTcR-CD4-CD8- (triple negative; TN), alphabetaTcR-CD4+CD8- and alphabetaTcR-CD4-CD8+ (immature single positive; ISP), alphabetaTcR-/lowCD4+CD8+ (double positive; DP) and alphabetaTcR+CD4+CD8- and alphabetaTcR+CD4-CD8+ (mature single positive; SP) as well as gammadelta T cells. However, NOD mice exhibited a marked deficiency of thymic alphabetaTcR+CD4-CD8- (alphabeta+DN) T cells. alphabeta+DN T cells, which are included among NK1+ T cells in C57BL/6 mice, produce large amounts of IL-4 on primary stimulation. Given the potential significance of NKT cells in immunoregulation, it is possible that the scarcity of these cells in NOD mice plays a role in the pathogenesis of IDDM.

T cell receptor alpha gene rearrangement and transcription in adult thymic gamma delta cells

T cells belong to two separate lineages based on surface expression of alpha beta or gamma delta T cell receptors (TCR). Since during thymus development TCR beta, gamma, and delta genes rearrange before alpha genes, and gamma delta cells appear earlier than alpha beta cells, it has been assumed that gamma delta cells are devoid of TCR alpha rearrangements. We show here that this is not the case, since mature adult, but not fetal, thymic gamma delta cells undergo VJ alpha rearrangements more frequently than immature alpha beta lineage thymic precursors. Sequence analysis shows VJ alpha rearrangements in gamma delta cells to be mostly (70%) nonproductive. Furthermore, VJ alpha rearrangements in gamma delta cells are transcribed normally and, as shown by analysis of TCR beta-/- mice, occur independently of productive VDJ beta rearrangements. These data are interpreted in the context of a model in which precursors of alpha beta and gamma delta cells differ in their ability to express a functional pre-TCR complex.

Early T lymphocyte progenitors

The earliest steps along the pathway leading to T cells in mice and humans are reviewed. These are the steps between the multipotent hemopoietic stem cell (HSC) and the fully committed precursors undergoing T cell receptor (TCR) gene rearrangement. At this level significant differences between adult and fetal lymphopoiesis have been demonstrated. The extent of lymphoid commitment of precursors within bone marrow is still unresolved, although HSCs clearly undergo developmental changes before migration to the thymus. Both multipotent and T-restricted precursors have now been isolated from fetal blood, suggesting both may seed the thymus. Within the thymus, several minute but discrete populations of T precursors precede the stage of TCR gene rearrangement. They include precursors that are not exclusively T-lineage committed, although they are distinct from HSCs. These precursors have a potential to form NK cells, B cells, dendritic cells, and sometimes other myeloid cells. Some factors that control early lymphoid development are discussed, including IL-7 and the Ikaros transcription factors. These will eventually help to clarify the process of T-lineage commitment.

T cell receptor delta gene rearrangement and T early alpha (TEA) expression in immature alpha beta lineage thymocytes: implications for alpha beta/gamma delta lineage commitment

Mature T cells comprise two mutually exclusive lineages expressing heterodimeric alpha beta or gamma delta antigen receptors. During development, beta, gamma, and delta genes rearrange before alpha, and mature gamma delta cells arise in the thymus prior to alpha beta cells. The mechanism underlying commitment of immature T cells to the alpha beta or gamma delta lineage is controversial. Since the delta locus is located within the alpha locus, rearrangement of alpha genes leads to deletion of delta. We have examined the rearrangement status of the delta locus immediately prior to alpha rearrangement. We find that many thymic precursors of alpha beta cells undergo VDJ delta rearrangements. Furthermore, the same cells frequently coexpress sterile T early alpha (TEA) transcripts originating 3' of C delta and 5' of the most upstream J alpha, thus implying that individual alpha beta lineage cells undergo sequential VDJ delta and VJ alpha rearrangements. Finally, VDJ delta rearrangements in immature alpha beta cells appear to be random, supporting models in which alpha beta lineage commitment is determined independently of the rearrangement status at the TCR delta locus.

Control points in early T-cell development

Intrathymic T-cell differentiation involves the generation, expansion and selection of distinct T-lymphocyte subsets. While positive and negative selection have been a focal point of T-cell development, these events represent the final stages in a complicated sequence of differentiation steps. Here, Dale Godfrey and Albert Zlotnik summarize recent advances in our understanding of early T-cell development and describe five 'control points' that identify key events in this sequence.

Surface antigens of human thymocyte populations defined by CD3, CD4 and CD8 expression: CD1a is expressed by mature thymocytes but not peripheral T cells

Three colour flow cytometric analysis has been used to analyze the expression of a series of surface antigens on human thymic and peripheral T-cell populations. CD4, CD8 and CD3 were used to divide the populations into the conventional major categories, and the distribution of CD1a, CD2, CD7, CD34, CD44, class I MHC and class II MHC was then determined. Some characteristics of 'single positive' (CD4+8- and CD4-8+) T-lineage cells were unexpected. Amongst thymocytes, some 'immature single positives' were delineated as larger sized cells lacking cell surface CD3 and expressing low levels of class I MHC; however, in contrast with murine thymocytes, these were all CD4+8-, rather than being predominantly CD4-8+. Amongst peripheral T cells, a small proportion of CD7- cells were detected, within both the CD4+8-3+ and the CD4-8+3+ categories. Finally, in contrast to previous conclusions, CD1a was expressed at high levels on mature (CD4+8-3+ and CD4-8+3+) human thymocytes, although in agreement with previous reports it was absent from peripheral T cells. CD1a is therefore a useful marker of post-selection, post-thymic T-cell maturation.