Reproducibility of ventilation of thresholds in trained cyclists during ramp cycle exercise

Insight into the maintenance of naive T cells is essential to understand defective immune responses in the context of aging and other immune compromised states. In humans, naive CD4+ T cells, in contrast to CD8+ T cells, are remarkably well retained with aging. Here, we show that low-affinity TCR engagement is the main driving force behind the emergence and accumulation of naive-like CD4+ T cells with enhanced sensitivity to IL-2 in aged humans. In vitro, we show that these CD45RA(+) CD25(dim) CD4(+) T cells can develop from conventional naive CD25(-) CD4+ T cells upon CD3 cross-linking alone, in the absence of costimulation, rather than via stimulation by the homeostatic cytokines IL-2, IL-7, or IL-15. In vivo, TCR engagement likely occurs in secondary lymphoid organs as these cells were detected in lymph nodes and spleen where they showed signs of recent activation. CD45RA(+) CD25(dim) CD4+ T cells expressed a broad TCRVbeta repertoire and could readily differentiate into functional T helper cells. Strikingly, no expansion of CD45RA(+) CD25(dim) CD8+ T cells was detected with aging, thereby implying that maintenance of naive CD4+ T cells is uniquely regulated. Our data provide novel insight into the homeostasis of naive T cells and may guide the development of therapies to preserve or restore immunity in the elderly

Mechanisms of naive CD4+ T cell maintenance in the elderly and its implications for Autoimmunity

CD4+ T cells are critical players in the immune system. CD4+ T cells coordinate both innate and adaptive immune responses. When naive CD4+ T cells become activated via their antigen-specific T cell receptor in the presence of costimulation, these cells differentiate into effector and memory T cells. Maintenance of a large and diverse naive CD4+ T cell repertoire over time is thus essential for developing immunity to a multitude of novel antigens. Despite a strong decline in thymic production of naive CD4+ T cells with aging, the circulating pool of naive CD4+ T cells is well-maintained in elderly humans. The preservation of naive CD4+ T cells in aged subjects contrasts sharply with that of naive CD8+ T cells, which decline markedly with age. In the current chapter, the mechanisms facilitating the remarkable maintenance of the naive CD4+ T cell pool with age are discussed. Important mechanisms include recognition of self-peptides by T cell receptors and stimulation by homeostatic cytokines, including interleukin-7 and interleukin-2. Furthermore, we address the implications of naive CD4+ T cell maintenance for the development of autoimmune diseases in the elderly. Lastly, two models for the development of aging-associated autoimmunity are proposed, and suggestions for further investigation are provided