CD4 and infection: The absolute number of CD4+CD25+FoxP3+ cells was increased at day 14 p.i. and remained elevated through day 35 p.i., reaching a plateau or declining by day 54 p.i. We hypothesize that elevation in peripheral CD4+CD25+FoxP3+ cell numbers was due to new expression of CD25 by cells that were CD4+CD25−FoxP3+ at the time of FIV infection.