Age-related propensity to peripheral expansion of Vgamma3+ gammadelta+ T lymphocytes after irradiation and bone marrow transplantation

Evolution of the thymus size in response to physiological and random events throughout life

During embryogenesis and in the early stages of life, the thymus is a crucial organ for the generation of the T cell repertoire. T cells are generated from hematopoietic stem cells already differentiated to precursor T cells in the bone marrow. These cells enter the thymus guided by chemotactic factors secreted by this organ. The complex maturation process takes place that ensures self-tolerance and homeostasis. Thymocytes that show autoreactivity do not leave the thymus, but rather die by apoptosis. The final percentage of mature T cells that survive to migrate from the thymus to the periphery is very low: at most 5%, under optimal conditions. The highest migration occurs in childhood and adulthood, at least in mice and humans; however, it declines throughout life and is minimal in the elderly. Under normal circumstances, the thymus commences involution soon after birth, and this involution correlates with the capacity to export mature T cells to the periphery. Hormones, cytokines, and neurotransmitters all play a role in this age-associated process, but the reasons for and mechanisms of this involution remain unknown. Apart from physiological conditions that change throughout life and govern age-related thymus evolution, random states and events provoked by intrinsic or extrinsic factors can induce either thymus involution, as in reversible transient thymic hypoplasias, or thymic hyperplasias. The age-associated involution, unlike transient involutions, follows a regular pattern for all individuals, though there are clear differences between the sexes. Nevertheless, even the age-associated involution seems to be reversible, raising the possibility of therapeutic strategies aimed at enhancing thymus function in the elderly.

Haemopoietic reconstitution after sublethal irradiation: comparison of the effects of different haemopoietic cytokines on murine lymphocytes and bone marrow cells

Sublethally irradiated mice were injected with recombinant cytokines to stimulate haemopoietic reconstitution. Interleukin (IL)-11 and IL-6 were able to significantly accelerate the recovery of thymus, spleen and bone marrow cells when used in combination with IL-3, but not alone. Stem cell factor (SCF) also displayed detectable effects when used with IL-3. Conversely, the IL-6 superagonist K-7/D-6 was able, even when used alone, to induce recovery of thymus, spleen and bone marrow cells up to the level of unirradiated controls. Together, these results indicate that it is possible to attain complete recovery of lymphoid organs and tissues as early as 7 d after irradiation by use of haemopoietic cytokines.