Increases in CD4+ T Lymphocytes Occur Without Increases in Thymic Size in HIV-Infected Subjects Receiving Interleukin-2 Therapy

OBJECTIVE: To examine the potential contribution of the thymus to CD4+ T-lymphocyte increases in HIV-infected patients receiving intermittent interleukin-2 (IL-2) therapy. DESIGN: Fifteen HIV-infected patients treated with antiretroviral regimens who were enrolled in a study of intermittent IL-2 therapy and were willing to undergo serial thymic computed tomography (CT) were prospectively studied. METHODS: Thymic CT was performed before and approximately 6 and 12-17 months after intermittent IL-2 therapy was started. Scans were graded in a blinded manner. Changes in lymphocyte subpopulations were determined by flow cytometry. RESULTS: Statistically significant increases in CD4+ T lymphocytes occurred with IL-2 administration, with a preferential increase in naive relative to memory CD4+ T cells. Despite this increase in naive CD4+ T cells, overall there was a modest decrease in thymic volume observed during the study period. No correlation was found between changes in thymic volume indices and total, naive, or memory CD4+ T-lymphocyte counts. CONCLUSIONS: These findings demonstrate that the profound CD4+ T-lymphocyte increases seen with intermittent IL-2 administration are not associated with increases in thymic volume and more likely are due to peripheral expansion rather than increased thymic output.