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Critical influence of the thymus on peripheral T cell homeostasis

View Article: PubMed Central - PubMed

ABSTRACT

Introduction: A tight balance between regulatory CD4+Foxp3+ (Treg) and conventional CD4+Foxp3− (Tconv) T cell subsets in the peripheral compartment, maintained stable throughout most of lifetime, is essential for preserving self‐tolerance along with efficient immune responses. An excess of Treg cells, described for aged individuals, may critically contribute to their reported immunodeficiency. In this work, we investigated if quantitative changes in thymus emigration may alter the Treg/Tconv homeostasis regardless of the aging status of the peripheral compartment.

Methods: We used two different protocols to modify the rate of thymus emigration: thymectomy of adult young (4–6 weeks old) mice and grafting of young thymus onto aged (18 months old) hosts. Additionally, lymphoid cells from young and aged B6 mice were intravenously transferred to B6.RAG2−/− mice. Alterations in Treg and Tconv peripheral frequencies following these protocols were investigated after 30 days by flow cytometry.

Results: Thymectomized young mice presented a progressive increase in the Treg cell frequency, while the grafting of a functional thymus in aged mice restored the young‐like physiological Treg/Tconv proportion. Strikingly, T cells derived from young or aged splenocytes colonized the lymphopenic periphery of RAG−/− hosts to the same extent, giving rise to similarly elevated Treg cell levels irrespective of the age of the donor population. In the absence of thymus output, the Treg subset seems to survive longer, as confirmed by their lower proportion of Annexin‐V+ cells.

Conclusions: Our data suggest that the thymus‐emigrating population, harboring an adequate proportion of Treg/Tconv lymphocytes, may be essential to keep the Treg cell balance, independently of age‐related shifts intrinsic to the peripheral environment or to the T cell biology.

No MeSH data available.


Aged and young T lymphocytes present similar ability to colonize the periphery when transferred to lymphopenic hosts. Young B6.RAG2−/− mice were adoptively transferred with unfractionated splenocytes from young (2 months old) or aged (18 months old) B6.Ba mice and the frequencies of T lymphocyte subpopulations were determined, by flow‐cytometry, in the peripheral blood 1 and 2 months after reconstitution. (A–C) Column bar graphs compare the percentages of (A) CD4+, (B) CD4+Foxp3+, and (C) CD8+ T cells found in hosts receiving young (n = 5) or aged (n = 5) donor splenocytes. (D–E) Frequencies of (D) CD44+ among CD4+ cells and (E) CD25low and CD25hi among CD4+Foxp3+ cells at 1 month after transfer (n = 5). Data are pooled from two independent experiments and statistical significance was determined by one‐way ANOVA with Bonferroni's Multiple Comparison Test (A–C) and unpaired student's t‐test (D) (*p < 0.05, **p < 0.01, ***p < 0.001).
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iid3132-fig-0005: Aged and young T lymphocytes present similar ability to colonize the periphery when transferred to lymphopenic hosts. Young B6.RAG2−/− mice were adoptively transferred with unfractionated splenocytes from young (2 months old) or aged (18 months old) B6.Ba mice and the frequencies of T lymphocyte subpopulations were determined, by flow‐cytometry, in the peripheral blood 1 and 2 months after reconstitution. (A–C) Column bar graphs compare the percentages of (A) CD4+, (B) CD4+Foxp3+, and (C) CD8+ T cells found in hosts receiving young (n = 5) or aged (n = 5) donor splenocytes. (D–E) Frequencies of (D) CD44+ among CD4+ cells and (E) CD25low and CD25hi among CD4+Foxp3+ cells at 1 month after transfer (n = 5). Data are pooled from two independent experiments and statistical significance was determined by one‐way ANOVA with Bonferroni's Multiple Comparison Test (A–C) and unpaired student's t‐test (D) (*p < 0.05, **p < 0.01, ***p < 0.001).

Mentions: The progressive rise in Treg cell frequencies after natural or experimental reduction/interruption of thymus output might derive from an enhanced competence of Treg cells to persist in the peripheral compartment. In order to verify if the survival and colonization potential of Treg and Tconv lymphocytes varies with age, we independently transferred young and aged splenocytes, normalized to contain the same absolute number of CD4+Foxp3+ cells in the inocula, to lymphopenic RAG−/− hosts. Similar frequencies of Treg/Tconv lymphocyte subpopulations were found in the hosts receiving young or aged splenocytes at one and two months after transfer (Fig. 5A–D). Equivalent proportions of CD25high/CD25neg/low among Foxp3+ cells were also found in both groups (Fig. 5E).


Critical influence of the thymus on peripheral T cell homeostasis
Aged and young T lymphocytes present similar ability to colonize the periphery when transferred to lymphopenic hosts. Young B6.RAG2−/− mice were adoptively transferred with unfractionated splenocytes from young (2 months old) or aged (18 months old) B6.Ba mice and the frequencies of T lymphocyte subpopulations were determined, by flow‐cytometry, in the peripheral blood 1 and 2 months after reconstitution. (A–C) Column bar graphs compare the percentages of (A) CD4+, (B) CD4+Foxp3+, and (C) CD8+ T cells found in hosts receiving young (n = 5) or aged (n = 5) donor splenocytes. (D–E) Frequencies of (D) CD44+ among CD4+ cells and (E) CD25low and CD25hi among CD4+Foxp3+ cells at 1 month after transfer (n = 5). Data are pooled from two independent experiments and statistical significance was determined by one‐way ANOVA with Bonferroni's Multiple Comparison Test (A–C) and unpaired student's t‐test (D) (*p < 0.05, **p < 0.01, ***p < 0.001).
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getmorefigures.php?uid=PMC5134722&req=5

iid3132-fig-0005: Aged and young T lymphocytes present similar ability to colonize the periphery when transferred to lymphopenic hosts. Young B6.RAG2−/− mice were adoptively transferred with unfractionated splenocytes from young (2 months old) or aged (18 months old) B6.Ba mice and the frequencies of T lymphocyte subpopulations were determined, by flow‐cytometry, in the peripheral blood 1 and 2 months after reconstitution. (A–C) Column bar graphs compare the percentages of (A) CD4+, (B) CD4+Foxp3+, and (C) CD8+ T cells found in hosts receiving young (n = 5) or aged (n = 5) donor splenocytes. (D–E) Frequencies of (D) CD44+ among CD4+ cells and (E) CD25low and CD25hi among CD4+Foxp3+ cells at 1 month after transfer (n = 5). Data are pooled from two independent experiments and statistical significance was determined by one‐way ANOVA with Bonferroni's Multiple Comparison Test (A–C) and unpaired student's t‐test (D) (*p < 0.05, **p < 0.01, ***p < 0.001).
Mentions: The progressive rise in Treg cell frequencies after natural or experimental reduction/interruption of thymus output might derive from an enhanced competence of Treg cells to persist in the peripheral compartment. In order to verify if the survival and colonization potential of Treg and Tconv lymphocytes varies with age, we independently transferred young and aged splenocytes, normalized to contain the same absolute number of CD4+Foxp3+ cells in the inocula, to lymphopenic RAG−/− hosts. Similar frequencies of Treg/Tconv lymphocyte subpopulations were found in the hosts receiving young or aged splenocytes at one and two months after transfer (Fig. 5A–D). Equivalent proportions of CD25high/CD25neg/low among Foxp3+ cells were also found in both groups (Fig. 5E).

View Article: PubMed Central - PubMed

ABSTRACT

Introduction: A tight balance between regulatory CD4+Foxp3+ (Treg) and conventional CD4+Foxp3&minus; (Tconv) T cell subsets in the peripheral compartment, maintained stable throughout most of lifetime, is essential for preserving self&#8208;tolerance along with efficient immune responses. An excess of Treg cells, described for aged individuals, may critically contribute to their reported immunodeficiency. In this work, we investigated if quantitative changes in thymus emigration may alter the Treg/Tconv homeostasis regardless of the aging status of the peripheral compartment.

Methods: We used two different protocols to modify the rate of thymus emigration: thymectomy of adult young (4&ndash;6 weeks old) mice and grafting of young thymus onto aged (18 months old) hosts. Additionally, lymphoid cells from young and aged B6 mice were intravenously transferred to B6.RAG2&minus;/&minus; mice. Alterations in Treg and Tconv peripheral frequencies following these protocols were investigated after 30 days by flow cytometry.

Results: Thymectomized young mice presented a progressive increase in the Treg cell frequency, while the grafting of a functional thymus in aged mice restored the young&#8208;like physiological Treg/Tconv proportion. Strikingly, T cells derived from young or aged splenocytes colonized the lymphopenic periphery of RAG&minus;/&minus; hosts to the same extent, giving rise to similarly elevated Treg cell levels irrespective of the age of the donor population. In the absence of thymus output, the Treg subset seems to survive longer, as confirmed by their lower proportion of Annexin&#8208;V+ cells.

Conclusions: Our data suggest that the thymus&#8208;emigrating population, harboring an adequate proportion of Treg/Tconv lymphocytes, may be essential to keep the Treg cell balance, independently of age&#8208;related shifts intrinsic to the peripheral environment or to the T cell biology.

No MeSH data available.