The Role of Granulocytes in Colony Stimulation by Human White Blood Cells in Agar Cultures

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Abstract
The effect of polymorphonuclear granulocytes (PMN) on colony stimulating activity (CSA) was studied in double layer cultures of human Ficoll Isopaque separated white blood cells (mononuclear cells = MNC). Previously published data confirmed that granulocytes are able to enhance or inhibit MNC derived CSA. Further analysis of the mode of action of PMN in vitro indicates that the enhancing activity ascribed to granulocytes coincides with low CSA in MNC basal layers. In contrast, in cultures with high levels of CSA as provided by lysed red blood cell enhancement, rather low concentrations of PMN are sufficient to induce inhibition of colony growth. A very similar effect to that achieved with basal layer derived CSA could be obtained with conditioned media of PMN and MNC short term liquid cultures. Enhancement and inhibition of colony growth apparently reflect a specific reactivity of granulocytes (PMN) to a given CSA level in the cultures. These findings are discussed in terms of a speculative role of PMN in a negative feed back control mechanism regulating granulopoiesis in vivo.