High-dose (HD) IL-2 therapy in patients with cancer increases the general 
population of Tregs, which are positive for CD4, CD25, and the Treg-specific 
marker Foxp3. It is unknown whether specific subsets of Tregs are activated and 
expanded during HD IL-2 therapy or whether activation of any particular Treg 
subset correlates with clinical outcome. Here, we evaluated Treg population 
subsets that were induced in patients with melanoma following HD IL-2 therapy. 
We identified a Treg population that was positive for CD4, CD25, Foxp3, and the 
inducible T cell costimulator (ICOS). This Treg population increased more than 
any other lymphocyte subset during HD IL-2 therapy and had an activated Treg 
phenotype, as indicated by high levels of CD39, CD73, and TGF-β. ICOS(+) Tregs 
were the most proliferative lymphocyte population in the blood after IL-2 
therapy. Patients with melanoma with enhanced expansion of ICOS(+) Tregs in 
blood following the first cycle of HD IL-2 therapy had worse clinical outcomes 
than patients with fewer ICOS(+) Tregs. However, there was no difference in 
total Treg expansion between HD IL-2 responders and nonresponders. These data 
suggest that increased expansion of the ICOS(+) Treg population following the 
first cycle of HD IL-2 therapy may be predictive of clinical outcome.