Different Cytokines with Autocrine and Immunosuppressive Activities are Secreted by a Small-Cell Lung Cancer Cell Line

In recent years, knowledge about growth control of solid tumors and lymphoid malignancies by soluble growth factors has increased considerably. Experimental evidence is accumulating which indicates that autocrine growth stimulation or impaired autocrine growth inhibition may be one mechanism enabling malignant cells to escape normal growth control [1–3]. Detailed insight has also been obtained into the control mechanisms that regulate activation, proliferation, and differentiation of normal lymphocytes by cytokines. In contrast, the mechanisms leading to downregulation of immune functions are largely unknown. Interestingly, one factor that has been identified as an inhibitor of immune functions is released not only by lymphocytes but also by a variety of tumor cells [4, 5]. Whether malignant outgrowth is influenced by inhibition of an intact immune system is an open question. It has been speculated that the release of immunosuppressive factors represents another important step in malignant evolution [4]. Thus, tumor cells may escape not only growth control by secretion of autocrine growth factors but also immune surveillance by secretion of factors inhibitory to the immune system.