tokens: Although interleukin 3 ( IL - 3 ) and erythropoietin ( EPO ) are known to act synergistically on hematopoietic cell proliferation in vitro , injection of IGF - IL - 3 and EPO in AZT - treated mice resulted in a reduction of red cells and an increase of plasma EPO levels as compared to animals treated with IGF - IL - 3 or EPO alone .