T-cell dynamics after high-dose chemotherapy in adults: elucidation of the elusive CD8(+) subset reveals multiple homeostatic T-cell compartments with distinct implications for immune competence

Recovery of total T cell numbers after in vivo T-cell depletion in humans is accompanied by complex perturbation within the CD8(+) subset. We aimed to elucidate the reconstitution of CD8(+) T cells by separate analysis of putative naive CD95(-) CD28(+) , memory CD95(+) CD28(+) and CD28(-) T cell compartments after acute maximal depletion by high-dose chemotherapy (HD-ChT) in women with high-risk breast cancer. We found that recovery of putative naive CD8(+) CD95(-) CD28(+) and CD4(+) CD95(-) CD28(+) T cells, was compatible with a thymus-dependent regenerative pathway since their recovery was slow and time-dependent, their values were tightly related to each other, and their reconstitution patterns were inversely related to age. By analysing non-naive T cells, a striking diversion between putative memory T cells and CD28(-) T cells was found. These latter increased early well beyond normal values, thus playing a pivotal role in total T-cell homeostasis, and contributed to reduce the CD4 : CD8 ratio. In contrast, putative memory T cells returned to values not significantly different from those seen in patients at diagnosis, indicating that this compartment may recover after HD-ChT. At 3-5 years after treatment, naive T cells persisted at low levels, with expansion of CD28(-) T cells, suggesting that such alterations may extend further. These findings indicate that CD28(-) T cells were responsible for 'blind' T-cell homeostasis, but support the notion that memory and naive T cells are regulated separately. Given their distinct dynamics, quantitative evaluation of T-cell pools in patients undergoing chemotherapy should take into account separate analysis of naive, memory and CD28(-) T cells.