Prestimulation of monocytes by the cytokines GM-CSF or IL-2 enhances the antibody dependent cellular cytotoxicity of monoclonal antibody 17-1A

Previously, we have shown that the cytokines IFN-alpha, IFN-gamma and IL-2 significantly enhance the antibody dependent cellular cytotoxicity (ADCC) e xerted by the monoclonal antibody (mAb) 17-1A which recognizes the tumor as sociated antigen EpCAM. ADCC was assessed by a new flow cytometric cytotoxi city assay using the PKH2 labeled colorectal tumor cell line HT29 as target cells and peripheral blood mononuclear cells as effecters. Monocytes are a ssumed to be one of the major effectors for ADCC. However, isolated monocyt es have a rather low ADCC capacity while addition of CD4(+) lymphocytes opt imizes ADCC. Since such an interaction between immune cells may act through cytokines we investigated whether a seven-day-prestimulation of monocytes by the cytokines M-CSF, CM-CSF, IFN-gamma, IFN-alpha and IL-2 enhances ADCC . Thereafter, we added for three days IL-2 and IFN-alpha with or without th e mAb 17-1A for terminal activation of monocytes. Interestingly, GM-CSF whi ch was ineffective in terminal activation, significantly enhanced ADCC of m onocytes when it was used for prestimulation. Similar results were obtained with IL-2. IFN-gamma and M-CSF were also active but less than GM-CSF. Asto nishingly, IFN-gamma and IFN-alpha prestimulation of monocytes suppressed t he enhancement of ADCC exerted by CM-CSF and IL-2, respectively. Our experi ments suggest that the timing of cytokine application is critical for the i nduction of optimal ADCC. Subcutaneous pretreatment with CM-CSF or IL-2 fol lowed by the combination of IL- 2/IFN-alpha/17-1A should be evaluated in a phase I clinical trial in patients with colorectal cancer.