Cytostasis of tumor cell lines by human granulocytes

Purified peripheral blood granulocytes from normal adult donors were tested for cytolytic and cytostatic activity against a variety of tumor-derived, virus-transformed, and normal cell lines. Altogether, 45 donors and 16 cell lines were tested. Although granulocytes mediated antibody-dependent cell-mediated cytolysis, no spontaneous cytolysis, as measured by chromium-51 (⁵¹Cr) or ³H]thymidine (³H]TdR) release could be detected in assays performed for up to 12 hr, even at an effector:target (E:T) cell ratio of 100:1. In contrast, granulocytes exhibited substantial growth-inhibitory activity (GIA) against most target cells, as measured by uptake of ³H]TdR by the target cells. These results were confirmed by visual counting of target cells. The degree of cytostasis was dependent on the E:T ratio, with a plateau of 80–95% inhibition usually reached at a ratio of 40:1. Inhibition of growth of adherent tumor target cells was accompanied by cell detachment, with both effects apparent by 5 hr and reaching a peak after 15 hr of incubation. With nonadherent targets, the onset and the peak of cytostasis were delayed, being observed after 8 and 24 hr, respectively. Growth of target cells remained inhibited for up to 4 days of culture. A wide variety of target cells were sensitive to granulocyte-mediated cytostasis, including tumor-derived human and mouse cell lines, lymphoblastoid cell lines from normal donors, and embryo fibroblasts. Normal human fibroblasts were inhibited only at high E:T ratios (40:1). PHA-induced lymphoblasts were the only target cells tested that were completely resistant to the cytostatic effects of granulocytes and in fact, their growth was slightly stimulated. There appeared to be two somewhat different mechanisms of growth inhibition by granulocytes, which varied with the target cell. Trypsinization of granulocytes markedly reduced their reactivity against adherent target cells but had little effect on GIA against suspension target cells. Also, the activity against F-265, but not against other target cells, was almost completely abrogated in the presence of catalase, suggesting an important role of hydrogen peroxide in one mechanism of granulocytemediated cytostasis.