sentence: Additionally , although some in vitro studies have found that memory CD8 T cells do not require CD28-mediated co-stimulation to initiate recall expansion ( Flynn and M ü llbacher , 1996 ; Bachmann et al ., 1999 ) , B-7 expression appears to be necessary for recall expansion in vivo ( Borowski et al ., 2007 ; Boesteanu and Katsikis , 2009 ) .