Background: Following an antigen-driven proliferative response, some daughter T cells differentiate into effector-type cells while others differentiate into long-lived, “true memory” cells. Functional subpopulations of short-lived (effector) and long-lived (true memory) T cells have been proposed to exist within the phenotypically-defined memory/effector (m/e) T-cell population, but never directly demonstrated in humans.

Methods: 2 complementary in vivo stable isotope-labeling techniques were used to identify kinetic subpopulations within T-lymphocyte pools: ²H-glucose incorporation/die-away curves (48-hour infusions of ²H-glucose with repeated blood sampling over 5-7 weeks) and long-term ²H₂O administration (8-10 weeks orally to outpatients, with repeated blood sampling). Total, m/e- and naïve-phenotype T cells were sort purified by FACS. DNA was isolated, hydrolyzed to nucleosides and derivatized, and isotope enrichments were measured by mass spectrometry.

Results: Clearly biphasic die-away kinetics were observed for total and m/e-, but not naïve-, phenotype T cells after pulse labeling in vivo. A higher proportion of newly divided total and m/e-phenotype CD4+ T-cells died away within 5-7 weeks in untreated HIV-1-infected subjects (72% and 78%, respectively) than in healthy controls (45% and 59%). Effective long-term antiretroviral therapy (ARV) restored these values to normal. Biphasic label incorporation kinetics were also observed for total and m/e--, but not naïve, --phenotype T-cells during long-term continuous labeling of replicating DNA with ²H₂O. After 10 weeks of labeling, 50-60% of m/e-phenotype CD4+ and CD8+ T-cells had divided in untreated HIV-1 infection compared to only 10-15% in seronegative controls, with intermediate values after long-term ARV therapy. The great majority of proliferation during long-term ²H₂O labeling was in the first-phase (short-lived, rapid turnover pool), relative to late incorporation (long-lived, slower turnover pool), particularly in untreated HIV-1 infection.

Conclusions: We conclude that kinetic populations exist within T-cell pools in humans, including kinetic heterogeneity within the m/e-surface phenotype defined CD4+ and CD8+ populations (i.e. some are short-lived with rapid turnover, others long-lived with slow turnover). The primary kinetic abnormality in HIV-1 infection is reduced capacity to produce long-lived memory T-cells and to keep such cells quiescent over time.