Cytotoxic function in the differentiated HL60 cell line

Monocytic features can be induced in the myeloid cell line HL60 in order to provide a suitable in vitro model for the investigation of in vivo activity in mononuclear phagocytes. 1,25 dihydroxyvitamin D3 (Calcitriol) induced the HL60 cell line to express the monocytic differentiation antigen Leu M3 in about 30-50% of the cells along with an increase (up to 20%) in the expression of the HLA class II antigen recognized by the monoclonal antibody (MoAb) HLADR, but not HLADQ. Functional investigation showed that Calcitriol-treated cells formed rosettes with sheep erythrocytes coated with a specific IgG2a mouse MoAb and readily ingested them. In addition, these same sensitized erythrocytes were lysed in an 18 hrs antibody-dependent cellular cytotoxicity (ADCC) assay. All together these data indicate the presence of functionally active Fc gamma Receptors (FcR). Sorting experiment demonstrated that only Leu M3+ HLADR+ cells contained the effector cell population. The phenotypic profile was not per se predictive of FcR presence and function, as TPA induced HL60 cells neither formed rosettes nor phagocytosed nor exhibited ADCC activity, although they express Leu M3+ and HLADR+ (as well as HLADQ) antigens. These results suggest that Calcitriol and TPA cause the differentiation of HL60 cells along distinct pathways. On the other hand, different subpopulations with given predetermined differentiation capabilities may coexist in HL60 cell line.