1 Group for the Study of Viral Hepatitis and AIDS, Service of Internal Medicine and Biochemistry Department, University Hospital Virgen del Rocio, Seville, Spain. 2 Radiology Department, Virgen del Rocio University Hospital, Seville, Spain. *This work was presented in the 13th International Symposium on HIV, and Emerging Infectious Diseases. Toulon, France, June 3–5, 2004. VIRAL IMMUNOLOGY Volume 18, Number 4, 2005 © Mary Ann Liebert, Inc. Pp. 740–746 Brief Report High Thymic Volume Is Associated With Viral Replication and Immunologic Impairment Only Early After HAART Interruption in Chronic HIV Infection* ALEJANDRO VALLEJO, 1 ANGELA VALLADARES, 2 BEATRIZ DE FELIPE, 1 JORGE VIVANCOS, 2 SONIA GUTIERREZ, 1 NATALIA SORIANO-SARABIA, 1 EZEQUIEL RUIZ-MATEOS, 1 EDUARDO LISSEN, 1 and MANUEL LEAL 1 ABSTRACT One of the strategies that has been investigated to reduce antiretroviral treatment toxicity in patients infected with human immunodefiency virus (HIV) is structured treatment interruption (STI). Our aim was to analyze early viral and immune dynamics after interruption of highly active antiretrovi- ral therapy (HAART) and to determine whether thymic function–related markers play a role in pre- venting CD4 count decline caused by increased viral replication. This was a prospective study of an open cohort of 47 HIV-infected patients with a median 969 CD4 count and prolonged viral suppres- sion. They were followed every 4 weeks though week 24. Thymic volume and TREC level were an- alyzed at baseline. Increased thymic volume was associated with higher plasma viral load and greater CD4 count decline early after interruption. Three virologic patterns were observed: rapid/high (RH), delayed/high (DH), and low/slow (LS) viral replication. RH correlated with higher thymic volume at baseline and with higher CD4 count decline at week 4. Patients with greater thymic volume was as- sociated with an immune and virologic impairment only early after interruption, probably because of infection of the increased number of available target cells. As the long-term consequences of these observations are unknown, the safety of treatment interruption must be further studied. 740 INTRODUCTION S INCE HIGHLY ACTIVE antiretrovial therapy (HAART) was introduced in the history of the aquired immun- odeficency syndrome (AIDS) epidemic, mortality has re- markably declined and morbidity has diminished (21). These regimens maximally have suppressed viral repli- cation and have unequivocally reduced the risk of human immunodeficiency (HIV)–related complications. Never- theless, therapy itself has produced an entirely new set of clinical complications including lipodystrophy, lactic acidosis, osteoporosis, neuropathy, and cardiovascular disease (16). Moreover, the mechanisms of these toxici- ties as well as long-term consequences are still unknown.