Extrathymic T-lymphocyte development

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.

Routine thymectomy in congenital cardiac surgery changes adaptive immunity without clinical relevance

The actual importance of the thymus in both children and adults is largely unclear. In congenital cardiac surgery, a partial or total thymectomy is frequently performed to improve access to the heart and great vessels. We performed a literature search to evaluate the effect on the adaptive immune system of the removal of thymus tissue in patients with congenital heart disease. A PubMed search according to Dunning's standard provided 149 articles, of which 13 addressed our search question. Each study has been tabulated with author, cases, controls, follow-up, methods, results and limitations. A first group of articles repeatedly showed the effect on the T-cell compartment, including the impact on subgroups of this compartment. More recent studies, usually with a longer follow-up, confirm that the earlier changes in T-cell population appear to be permanent. Only one author found a normalization of T-cell population five years after thymectomy. In contrast to these clear changes in T-cell population, there is currently no clear clinical relevance. A literature search on thymectomy in congenital cardiac surgery revealed clear changes in T-cell-related immunity; however, there is a lack of clinical relevance. Further investigation of the adaptive immune system is required to explain this discrepancy.

Molecular evidence for a thymus-independent partial T cell development in a FOXN1-/- athymic human fetus

The thymus is the primary organ able to support T cell ontogeny, abrogated in FOXN1(-/-) human athymia. Although evidence indicates that in animal models T lymphocytes may differentiate at extrathymic sites, whether this process is really thymus-independent has still to be clarified. In an athymic FOXN1(-/-) fetus, in which we previously described a total blockage of CD4(+) and partial blockage of CD8(+) cell development, we investigated whether intestine could play a role as extrathymic site of T-lymphopoiesis in humans. We document the presence of few extrathymically developed T lymphocytes and the presence in the intestine of CD3(+) and CD8(+), but not of CD4(+) cells, a few of them exhibiting a CD45RA(+) naïve phenotype. The expression of CD3εεpTα, RAG1 and RAG2 transcripts in the intestine and TCR gene rearrangement was also documented, thus indicating that in humans the partial T cell ontogeny occurring at extrathymic sites is a thymus- and FOXN1-independent process.

Thymectomy in early childhood: a model for premature T cell immunosenescence?

The thymus is the main source of recent thymic emigrants (RTE) and naïve T cells. The aging of the immune system (immunosenescence) is characterized by loss of thymic function, decreased numbers of RTE, peripheral proliferation of mature T cells, and oligoclonal expansions of specific T cell subpopulations. As shown in several studies, thymectomized patients demonstrate signs of premature immunosenescence reminiscent of aged people, such as decreased proportions of naïve T cells and RTE, a compensatory increase of mature T cell subpopulations with increased proliferation rates, restriction of the T cell receptor repertoire, and a delayed response to new antigens and vaccinations. This review demonstrates that, despite some limitations, childhood thymectomy may serve as an useful model for premature immunosenescence, mimicking changes expected after physiological thymus involution in the elderly. Thus, it may prove an insightful tool for obtaining better understanding of human naïve T cell development, thymic function, and maintenance of the naïve T cell pool.

Vbeta usage and T regulatory cells in children following partial or total thymectomy after open heart surgery in infancy

During open heart surgery in infants the thymus was usually removed, partly or completely. Our previous studies on 16 such children indicated reduced T-cell output later in life with signs of extrathymic maturation of the T cells, but no reduction in T regulatory cells (CD4+CD25+). The diversity of the T-cell repertoire in these children was examined to test if the extrathymic microenvironment could alter Vbeta usage. The expression of Foxp3 and CD127 in CD4+CD25(high) T cells was measured in order to determine whether the T regulatory cells had the phenotype of natural T regulatory cells. There was a wide distribution of Vbeta usage in both study and control groups. Significant variability was found in Vbeta usage for CD4+ and CD8+ T cells when the distribution of the percentage of T cells expressing each Vbeta family was analysed between individuals within each group (P < 0.001; Kruskal-Wallis). Significant difference was also found in average usage of Vbeta2, Vbeta5.1 and Vbeta14 chains within CD4+ T cells and Vbeta2, Vbeta8 and Vbeta21.3 chains within CD8+ cells between the groups (P < 0.05; Student's t-test). There was no difference between the two groups with regard to the proportion of CD4+CD25(high) T cells and no difference in the average expression of Foxp3 or CD127 within the CD4+CD25(high) population. Our data provide evidence that cardiothoracic surgery in infants and total or partial thymectomy alters Vbeta usage, suggesting more limited selection in such children than in the control group. The frequency of natural T regulatory cells seems to be unimpaired.

Diminished response to tick-borne encephalitis vaccination in thymectomized children

In order to analyze the clinical impact of immunological alterations in thymectomized children after exposure to a new antigen (tick-borne encephalitis virus (TBEV) vaccine), 17 thymectomized children completed a three-dose immunization regimen. Thymectomized children showed significantly lower TBEV IgG antibody levels after the second vaccination when compared to healthy age-matched controls (n=30) (p=0.03), but a normal response after the third vaccination. Age at thymectomy correlated significantly with the TBEV IgG antibody levels (p=0.04). Thymectomized children also showed significantly lower total counts and percentages for naïve T cells correlating with time after thymectomy (p=0.02), than observed for controls. These changes in T cell subsets and the decreased ability to respond to new antigens in thymectomized children, as observed here, may precede more striking effects such as higher infection rates or autoimmune conditions as they age.

B-cell development fails in the absence of the Pbx1 proto-oncogene

Pbx1, a homeodomain transcription factor that was originally identified as the product of a proto-oncogene in acute pre-B-cell leukemia, is a global regulator of embryonic development. However, embryonic lethality in its absence has prevented an assessment of its role in B-cell development. Here, using Rag1-deficient blastocyst complementation assays, we demonstrate that Pbx1 null embryonic stem (ES) cells fail to generate common lymphoid progenitors (CLPs) resulting in a complete lack of B and NK cells, and a partial impairment of T-cell development in chimeric mice. A critical role for Pbx1 was confirmed by rescue of B-cell development from CLPs following restoration of its expression in Pbx1-deficient ES cells. In adoptive transfer experiments, B-cell development from Pbx1-deficient fetal liver cells was also severely compromised, but not erased, since transient B lymphopoiesis was detected in Rag-deficient recipients. Conditional inactivation of Pbx1 in pro-B (CD19(+)) cells and thereafter revealed that Pbx1 is not necessary for B-cell development to proceed from the pro-B-cell stage. Thus, Pbx1 critically functions at a stage between hematopoietic stem cell development and B-cell commitment and, therefore, is one of the earliest-acting transcription factors that regulate de novo B-lineage lymphopoiesis.

Evidence for extrathymic T cell maturation after thymectomy in infancy

Our previous study showed that children who had been partially or completely thymectomized during heart surgery as infants had lower proportions and numbers of total lymphocytes and reduced proportions of T cells (CD3(+)), helper T cells (CD4(+)) and naive T cells (CD3(+) CD4(+) CD45RA(+)), but normal proportion of cytotoxic T cells (CD8(+)). In this study T lymphocytes from a selected group of eight of these children and age- and gender-matched controls were characterized further using flow cytometry to determine phenotypes of T cells and T cell subsets related to T cell regulation and phenotypes suggestive of extrathymic maturation. Immune function was assessed by measuring autoantibodies and antibodies against vaccines. The study group had significantly lower numbers of all the main subsets of T lymphocytes and the composition was different. Thus, the proportions of lymphocytes with the following phenotypes: CD3(+), CD2(+), CD7(+), CD4(+), CD62L(+), CD4(+) CD62L(+) and CD4(+) CD69(-) were significantly reduced in the study group compared with the control group, but significantly higher proportions were seen of lymphocytes expressing CD8alpha(+) CD8beta(-) and TCRgammadelta(+) CD8alpha(+) CD8beta(-). The absolute number and proportion of CD4(+) CD25(+) cells were reduced but the proportions of the subgroup of naive regulatory T cells (CD4(+) CD25(+) CD62L(+)) and non-activated regulatory T cells (CD4(+) CD25(+) CD69(-)) were not reduced in the thymectomized children. We conclude that the phenotypic characteristics of T lymphocytes of children who have lost their thymus in infancy are indicative of extrathymic maturation. T regulatory cells appear to be less affected than other subsets by the general reduction in T cell numbers.

Cell-mediated adaptive immune defense of the lungs

Cell-mediated adaptive immune responses contribute to defense against all classes of pulmonary pathogens and are essential against viruses, mycobacteria, and fungi, including Pneumocystis carinii. Adaptive responses depend on sequential pairwise interactions between three cell types: T cells, natural killer (NK) cells, and dendritic cells (DC). Differential expression of specific adhesion molecules and chemokines regulates the location and timing of these interactions. Primary adaptive responses are triggered by immature myeloid DC, which carry antigen from the lungs to regional lymph nodes. Antigen presentation by these mature DC is required to activate naive CD4 T cells, which are essential to generate polarized type 1 or type 2 effector responses and for robust immunologic memory. Inflammation recruits NK cells and DC that interact in a contact- and tumor necrosis factor-alpha-dependent fashion within injured tissues to initiate immune response polarization. NK cells exposed to IL-12 favor survival of DC that prime for Th1 responses, whereas NK cells exposed to IL-4 do not exert DC selection, leading to tolerogenic or Th2 responses. Naive alphabeta T cells, NK cells, and DC also amplify secondary adaptive responses to previously encountered pathogens. However, secondary responses are accelerated because memory T cells can migrate directly to infected tissues where they can be activated without strenuous costimulatory requirements. Additionally, previous pulmonary infections or immune responses increase numbers of lung DC and populate the lungs with clones of memory B cells and T cells that are immediately available to respond to infections.

T-cell generation by lymph node resident progenitor cells

In the thymus, 2 types of Lin–Sca-1+ (lineage-negative stem cell antigen-1–positive) progenitors can generate T-lineage cells: c-Kithi interleukin-7 receptor α–negative (c-KithiIL-7Rα–) and c-KitloIL-7Rα+. While c-KithiIL-7Rα– progenitors are absent, c-KitloIL-7Rα+ progenitors are abundant in the lymph nodes (LNs). c-KitloIL-7Rα+ progenitors undergo abortive T-cell commitment in the LNs and become arrested in the G1 phase of the cell cycle because they fail both to up-regulate c-myb, c-myc, and cyclin D2 and to repress junB, p16INK4a, and p21Cip1/WAF. As a result, development of LN c-KitloIL-7Rα+ progenitors is blocked at an intermediate CD44+CD25lo development stage in vivo, and LN-derived progenitors fail to generate mature T cells when cultured with OP9-DL1 stromal cells. LN stroma can provide key signals for T-cell development including IL-7, Kit ligand, and Delta-like–1 but lacks Wnt4 and Wnt7b transcripts. LN c-KitloIL-7Rα+ progenitors are able to generate mature T cells when cultured with stromal cells producing wingless-related MMTV integration site 4 (Wnt4) or upon in vivo exposure to oncostatin M whose signaling pathway intersects with Wnt. Thus, supplying Wnt signals to c-KitloIL-7Rα+ progenitors may be sufficient to transform the LN into a primary T-lymphoid organ. These data provide unique insights into the essence of a primary T-lymphoid organ and into how a cryptic extrathymic T-cell development pathway can be amplified.

Tumor necrosis factor promotes human T-cell development in nonobese diabetic/severe combined immunodeficient mice

A major problem after clinical hematopoietic stem cell transplantations is poor T-cell reconstitution. Studying the mechanisms underlying this concern is hampered, because experimental transplantation of human stem and progenitor cells into nonobese diabetic/severe combined immunodeficient (NOD/SCID) mice usually results in low T-lymphocyte reconstitution. Because tumor necrosis factor alpha (TNFalpha) has been proposed to play a role in T-lineage commitment and differentiation in vitro, we investigated its potential to augment human T-cell development in vivo. Administration of TNF to irradiated NOD/SCID mice before transplantation of human mononuclear cells from either cord blood or adult G-CSF-mobilized peripheral blood (MPBL) led 2-3 weeks after transplantation to the emergence of human immature CD4(+)CD8(+) double-positive T-cells in the bone marrow (BM), spleen, and thymus, and in this organ, the human cells also express CD1a marker. One to 2 weeks later, single-positive CD4(+) and CD8(+) cells expressing heterogenous T-cell receptor alpha beta were detected in all three organs. These cells were also capable of migrating through the blood circulation. Interestingly, human T-cell development in these mice was associated with a significant reduction in immature lymphoid human CD19(+) B cells and natural killer progenitors in the murine BM. The human T cells were mostly derived from the transplanted immature CD34(+) cells. This study demonstrates the potential of TNF to rapidly augment human T lymphopoiesis in vivo and also provides clinically relevant evidence for this process with adult MPBL progenitors.

Extrathymic Hemopoietic Progenitors Committed to T Cell Differentiation in the Adult Mouse

The role of the thymus in T cell commitment of hemopoietic precursor is yet controversial. We previously identified a major T cell progenitor activity in precursor cells isolated from bone marrow-derived spleen colonies. In this study, we characterize the properties of these pre-T cells. We demonstrate that they have unique phenotype and can be generated in a total absence of any thymic influence. Indeed, even when studied at the single-cell level, extrathymic T cell-committed precursors express T cell-specific genes. Moreover, these cells are not committed to a particular T cell differentiation pathway because they can generate both extrathymic CD8alphaalpha+ intraepithelial lymphocytes and thymus-derived conventional thymocytes. We also compared these pre-T cells with fully T cell-committed thymic progenitors. When tested in vitro or by direct intrathymic transfer, these cells have a low clonogenic activity. However, after i.v. transfer, thymus repopulation is efficient and these precursors generate very high numbers of peripheral T cells. These results suggest the existence of extra steps of pre-T cell maturation that improve thymus reconstitution capacity and that can be delivered even after full T cell commitment. Consequently, our studies identify a source of extrathymic progenitors that will be helpful in defining the role of the thymus in the earliest steps of T cell differentiation.