Increases in T Cell Telomere Length in HIV Infection after Antiretroviral Combination Therapy for HIV-1 Infection Implicate Distinct Population Dynamics in CD4 + and CD8 + T Cells Sumesh Kaushal,* Alan L. Landay,† Michael M. Lederman,‡ Elizabeth Connick,§ John Spritzler, ¶ Daniel R. Kuritzkes,§ Harold Kessler,† Bruce L. Levine,* ,1 Daniel C. St. Louis,* and Carl H. June* ,1 *Henry M. Jackson Foundation for the Advancement of Military Medicine, U.S. Military HIV Research Program, Bethesda, Maryland 20889; †Departments of Immunology/Microbiology and Medicine, Rush-Presbyterian–St. Luke’s Medical Center, Chicago, Illinois 60637; ‡Division of Infectious Diseases, Case Western Reserve University School of Medicine, Cleveland, Ohio 44106; §Division of Infectious Diseases, University of Colorado Health Sciences Center, Denver, Colorado 80262; and ¶ Statistical and Data Analysis Center, Harvard School of Public Health, Boston, Massachusetts 02115 Changes in mean telomeric terminal restriction fragment (TRF) length were examined as a marker for cellular replicative history in HIV-1-infected individ- uals after institution of anti-retroviral therapy (ART). Increases in mean T cell TRF lengths were observed in most patients following therapy; however, the contri- bution of individual T cell subsets was complex. An elongation of CD8  T cell TRF was nearly uniformly observed while changes in mean TRF length in CD4  T cells were heterogeneous as, despite potent suppres- sion of viral replication, CD4 cell telomeres recovered in some patients, yet continued to decline in others. Increases in CD8 cell TRF correlated with decreased memory cells, suggesting a negative selection in the periphery for CD8 cells with extensive replicative his- tory. In contrast, increases in CD4  T cell TRF length correlated with increases in naive cell subsets, sug- gesting that the CD4  T cell TRF increase may reflect a thymic contribution in some patients. These are the first increases in somatic cell telomere length in a population of cells observed in vivo, and the findings are compatible with therapy-induced reconstitution of the lymphoid compartment with cells having a more extensive replicative potential. These findings further distinguish lymphocytes from other somatic cell pop- ulations where only decreases in TRF over time have been noted. Thus, institution of ART in persons with moderately advanced HIV-1 disease reveals distinct population dynamics of CD4 and CD8 T cell subsets and also shows that the lymphocyte replicative history is dynamic. © 1999 Academic Press Key Words: HIV-1; telomere; telomerase; antiretrovi- ral therapy; immunodeficiency. INTRODUCTION Telomeres are specific structures found at the ends of eukaryotic chromosomes. In somatic cells, telomere length is shortened with each cell division due to the inability of DNA polymerase to replicate the extreme 5' end of the lagging strand of DNA (1). Substantial evi- dence supports the hypothesis that telomeres function in part as a mitotic clock for the cell. In vivo, lympho- cytes have age-related reduction in telomere length (2), losing an average of 31 base pairs/year. In vitro, the rate of telomere loss from lymphocytes from normal individuals is approximately 50 to 120 bp/cell doubling, comparable to that seen in other somatic cells (2, 3). Thus, measurement of the telomeric terminal restric- tion fragment (TRF) length by Southern blot analysis is thought to serve as an index of replicative history in vivo and in vitro. It has been suggested that replicative senescence (4) might contribute to the immunodeficiency of HIV in- fection (5). A related hypothesis is that increased clonal expansion and potential clonal exhaustion of T cells occur in response to HIV infection (6 – 8). Consistent with these models of immunosenescence, the mean TRF length was shown to decrease in CD8 + T cells in most adult patients with HIV infection (9 –11). How- ever, telomere measurements of CD4 cells from HIV- infected individuals do not shorten excessively, sug- gesting that proliferative exhaustion is not a major component of CD4 cell decline in patients with HIV infection (9, 11–13). Recent measurements of T cell turnover during SIV infection in macaques using Brdu labeling document moderately elevated turnover of both CD4 and CD8 lymphocyte subsets (14, 15). Treatment of HIV-1-infected individuals with potent antiretroviral drug combinations results in a dramatic decline in viral load and a partial restoration of the circulating T cell compartment. Soon after initiation of 1 Present address: Department of Molecular and Cellular Engi- neering, University of Pennsylvania. Clinical Immunology Vol. 92, No. 1, July, pp. 14 –24, 1999 Article ID clim.1999.4726, available online at http://www.idealibrary.com on 1521-6616/99 $30.00 Copyright © 1999 by Academic Press All rights of reproduction in any form reserved. 14