Human NK cell-mediated ADCC activity was also evaluated in vitro using purified human NK cells as effector cells. As shown in Figure 4c , human NK cells were able to kill Mac-1 target cells in the presence of h1567 in a dose dependent fashion. Control 11A minibody showed only very low levels of killing. As both mouse neutrophils and human NK cells express FccRIIIA receptors (CD16A) on their surface that can bind h1567, these in vitro and in vivo data strongly support that h1567 mediated killing occurs, at least in part, through FccRIIIA engagement and activation of immune cell effector functions.