CD28 and neoplasm: To evaluate whether the high number of macrophages present in the tumor microenvironment of the KPC mice contribute to the production of these cytokines as well as their capacity to suppress T cells, we removed F4/80+ macrophages from tumor cell suspensions obtained from collagenase-digested tumors and then cultured the cells from the tumor in the presence of plate-bound anti-CD3 and soluble CD28.