sentence: For example , although memory CD8 T cells reportedly proliferate in response to lower doses of antigen than naive T cells ( Pihlgren et al ., 1996 ; Curtsinger et al ., 1998 ; London et al ., 2000 ) , little difference in peptide sensitivity was observed in the absence of exogenous IL-2 ( Curtsinger et al ., 1998 ; Zimmermann et al ., 1999 ) .