Antigen-presenting cells (APCs) are essential for stimulating antigen-specific immunity, including immunity against tumor cells. We hypothesized that systemic administration of granulocyte macrophage colony-stimulating factor (GM-CSF) and interleukin (IL)-4, which promote monocytes to differentiate into dendritic cells in vitro, might enhance the number and antigen-presenting activity of CD14+ cells in vivo. Patients with metastatic solid malignancies were treated with daily s.c. injections of either GM-CSF alone (2.5 microg/kg/day) or GM-CSF in combination with IL-4 (0.5-6.0 microg/kg/day) in a multicohort study. When given alone, GM-CSF increased the number of CD14+ cells but did not enhance the cells' expression of APC markers or antigen-presenting activity. In contrast, combination therapy with GM-CSF and IL-4 stimulated CD14+ cells to acquire several APC characteristics including increased expression of HLA-DR and CD11c, decreased CD14, increased endocytotic activity, and the ability to stimulate T cells in a mixed leukocyte reaction. Combination therapy also induced a dose-dependent increase in the number of CD14-/CD83+ cells with APC activity. Clinically significant and sustained tumor regression was observed in one patient. Systemic therapy with GM-CSF and IL-4 may provide a mechanism for increasing the number and function of APCs in patients with cancer.