Cytokine synergies in erythropoiesis: Identifying and explaining interactions through cell division tracking and factorial design

The directed differentiation of hematopoietic cells demands fine-grained control over cell decisions. Experiments were performed to distinguish the distinct and joint effects of two cytokines---erythropoietin (EPO) and stem cell factor (SCF)---on proliferation and survival in red blood cell progenitors. An in vitro model was used to quantify these effects on erythroid progenitors. Using factorial design approach, EPO and SF were shown to have merely additive effects on cell number in two cell lines: one human erythroleukemia (TF-1), and one mouse multipotential cell line (FRCP-mix). However, a synergistic expansion of erythroid cells within differentiating FDCP-mix cultures was observed. Cell division tracking, coupled with a mathematical model of cell proliferation, quantified this response as separate sequential effects: SCF promoted early proliferation of all progenitor cells, and EPO was necessary for survival of committed erythroid progenitors. This quantitative approach suggests how separation of proliferation from death rates can identify meaningful synergism.