Detection of an inhibitor of cell division in cultures of tumour cells with immunosuppressive activity in vitro.

A number of previous reports have described the presence of factors which inhibit the response of lymphoid cells to phytohaemagglutinin. The present study describes an inhibitor of cell division synthesized and released directly from cultured tumour cells which appears to have similar immunosuppressive effects in vitro. This factor(s) was detected in a wide variety of cultured tumour cells and some cultures of normal foetal tissue. It inhibited the mitogenic response of lymphocytes to PHA and also the spontaneous cell division of a variety of cultured cells including the cells producing the factor. Pulse cytophotometry showed that cells were inhibited in the G1 phase. Production of the factor increased with time and was related to the number of cells in culture. Its synthesis was inhibited by cycloheximide. DNA and RNA synthesis was not required for its production. The factor appeared to have a high degree of biological activity in terms of the number of cultured cells required for production and in regard to the number of cells inhibited. The biological significance of this factor in vivo is unknown but in vitro it was shown to inhibit immunoglobulin and mitogen-induced responses which suggests it may play an important role in suppression of the immune response against tumour cells in the host.