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The use of preemptive antiviral therapy to prevent cytomegalovirus ( CMV) disease in allogeneic hematopoietic stem cell transplantation (allo- HSCT) recipients might result in over-treatment, inducing drug-related toxicity and viral resistance. A search for predictive markers is needed to determine requirement for antiviral therapy. Clinical follow-up, in combination with the use of streptamers ( STs) and cytokine-intracellular staining, could help to identify patients at high risk for CMV reactivations. To study the immune response and reactivation control by CMV-specific CD8+ T-cell ( CMV- CTL) populations, we monitored 25 patients who have undergone allo- HSCT by using ST multimer and intracellular cytokine staining. Our study has revealed that the presence of functional CMV-specific T cells, determined by early interferon γ production or by significant T-cell expansion after first CMV reactivation, correlated with short CMV viremia duration and low number of CMV reactivations. By contrast, the absence of functional CMV- CTLs does correlate with CMV recurrence. These results support that behavior of CMV-specific subpopulations after reactivation influences reactivations and can guide preemptive therapy. [ABSTRACT FROM AUTHOR]