Human CD8 T cells are better frozen when they are not so activated (let's say 10-15 days after stimulation). Usually when you thaw them (use DNAse, that helps!) you lose around 50 % of the cells, so be aware (we always freeze vials of at least 10-50 million cells if possible). Usually, we look for cytokine production the day after thawing (so day 0 you thaw the cells, day 1 you stimulate them and day 2 you collect the supernatants to measure cytokine production).

I hope this will help you!