Ab-mediated signaling in tumor cells and Ab-dependent cell-mediated cytotoxicity (ADCC) are both considered as relevant effector mechanisms for Abs in tumor therapy. To address potential interactions between these two mechanisms, we generated HER-2/neu- and CD19-derived chimeric target Ags, which were expressed in experimental tumor target cells. HER-2/neu-directed Abs were documented to mediate effective ADCC with both mononuclear cells (MNCs) and polymorphonuclear granulocytes (PMNs), whereas Abs against CD19 were effective only with MNCs and not with PMNs. We generated cDNA encoding HER-2/CD19 or CD19/HER-2 (extracellular/intracellular) chimeric fusion proteins by combining cDNA encoding extracellular domains of HER-2/neu or CD19 with intracellular domains of CD19 or HER-2/neu, respectively. After transfecting wild-type HER-2/neu or chimeric HER-2/CD19 into Raji Burkitt's lymphoma cells and wild-type CD19 or chimeric CD19/HER-2 into SK-BR-3 breast cancer cells, target cell lines were selected for high membrane expression of transfected Ags. We then investigated the efficacy of tumor cell lysis by PMNs or MNCs with CD19- or HER-2/neu-directed Ab constructs. MNCs triggered effective ADCC against target cells expressing wild-type or chimeric target Ag. As expected, PMNs killed wild-type HER-2/neu-transfected, but not wild-type CD19-transfected target cells. Interestingly, however, PMNs were also effective against chimeric CD19/HER-2-transfected, but not HER-2/CD19-transfected target cells. In conclusion, these results demonstrate that intracellular domains of target Ags contribute substantially to effective Ab-mediated tumor cell killing by PMNs.