IL-10 is excluded from the functional cytokine memory of human CD4+ memory T lymphocytes.

Epigenetic modifications, including DNA methylation, profoundly influence gene expression of CD4(+) Th-specific cells thereby shaping memory Th cell function. We demonstrate here a correlation between a lacking fixed potential of human memory Th cells to re-express the immunoregulatory cytokine gene IL10 and its DNA methylation status. Memory Th cells secreting IL-10 or IFN-gamma were directly isolated ex vivo from peripheral blood of healthy volunteers, and the DNA methylation status of IL10 and IFNG was assessed. Limited difference in methylation was found for the IL10 gene locus in IL-10-secreting Th cells, as compared with Th cells not secreting IL-10 isolated directly ex vivo or from in vitro-established human Th1 and Th2 clones. In contrast, in IFN-gamma(+) memory Th cells the promoter of the IFNG gene was hypomethylated, as compared with IFN-gamma-nonsecreting memory Th cells. In accordance with the lack of epigenetic memory, almost 90% of ex vivo-isolated IL-10-secreting Th cells lacked a functional memory for IL-10 re-expression after restimulation. Our data indicate that IL10 does not become epigenetically marked in human memory Th cells unlike effector cytokine genes such as IFNG. The exclusion of IL-10, but not effector cytokines, from the functional memory of human CD4(+) T lymphocytes ex vivo may reflect the need for appropriate regulation of IL-10 secretion, due to its potent immunoregulatory potential.

Post-thymic in vivo proliferation of naive CD4+ T cells constrains the TCR repertoire in healthy human adults.

In spite of thymic involution early in life, the numbers of naive CD4(+) T cells only slowly decline in ageing humans implying peripheral post-thymic naive CD4(+) T cell expansion. This proliferation may compensate for continuous activation and death of naive CD4(+) T cells but may also have negative consequences for protective immunity. Here we show that naive CD4(+) T cells that have proliferated in the periphery are characterized by a highly restricted oligoclonal TCR repertoire. Additionally these cells, which constitute the majority of naive CD4(+) T cells in the elderly, display signatures of recent TCR engagement. Our results demonstrate for the first time that peripheral post-thymic proliferation of naive CD4(+) T cells in healthy human individuals causes a significant contraction of the peripheral TCR repertoire. This age-dependent deterioration of CD4(+) T cell immunity could entail ageing-associated autoimmunity, increased susceptibility to infection or cancer and decreased efficiency of vaccination.

Two subsets of naive T helper cells with distinct T cell receptor excision circle content in human adult peripheral blood.

During ageing thymic function declines and is unable to meet the demand for peripheral T helper (Th) cell replenishment. Therefore, population maintenance of naive Th cells must be at least partly peripherally based. Such peripheral postthymic expansion of recent thymic emigrants (RTEs) during ageing consequently should lead to loss or dilution of T cell receptor excision circles (TRECs) from a subset of naive T cells. We have identified two subsets of naive Th cells in human adult peripheral blood characterized by a striking unequal content of TRECs, indicating different peripheral proliferative histories. TRECs are highly enriched in peripheral naive CD45RA(+) Th cells coexpressing CD31 compared with peripheral naive CD45RA(+) Th cells lacking CD31 expression, in which TRECs can hardly be detected. Furthermore we show that CD31(-)CD45RA(+) Th cells account for increasing percentages of the naive peripheral Th cell pool during ageing but retain phenotypic and functional features of naive Th cells. As CD31 is lost upon T cell receptor (TCR) engagement in vitro, we hypothesize that TCR triggering is a prerequisite for homeostatically driven peripheral postthymic expansion of human naive RTEs. We describe here the identification of peripherally expanded naive Th cells in human adult blood characterized by the loss of CD31 expression and a highly reduced TREC content.