Effect of IL-7 Therapy on Naive and Memory T Cell Homeostasis in Aged Rhesus Macaques

Aging is associated with gradual deterioration of adaptive immune function, a hallmark of which is the profound loss of naive T cells (T N ) associated with decline in thymic output and export of new cells into the peripheral T cell pool. Because the lymphotropic cytokine IL-7 plays crucial roles in both development of T N in the thymus and T N homeostasis in the periphery, we sought to determine the extent to which therapeutic administration of IL-7 could reverse T N deficiency in aging rhesus macaques (RM), either by enhancement of the demonstrably reduced thymopoiesis or by peripheral T N expansion. Our results indicate that treatment of both adult (8–15 y) and old (>20 y) RM with recombinant simian IL-7 (rsIL-7) results in only transient increases in peripheral CD4 + and CD8 + T N numbers with no long-term benefit, even with repeated therapy. This transient effect was due to peripheral T N expansion and not enhanced thymic function, and appeared to be limited by induction of IL-7 nonresponsiveness. However, rsIL-7 therapy had a more promising effect on the central memory T cell (T CM ) population (both CD4 + and CD8 + ) in adult and old RM, doubling the numbers of these cells in circulation and maintaining this larger population long term. IL-7 therapy did not reduce TCR diversity of the memory T cell compartment, suggesting that rsIL-7–induced expansion was symmetrical. Thus, although rsIL-7 failed to counter age-associated T N loss, the ability of this therapy to expand clonotypically diverse CD4 + and CD8 + T CM populations might potentially improve adaptive immune responsiveness in the elderly.