Provided herein are isolated populations of kidney cells harvested from differentiated cells of the kidney, wherein cells have been expanded in vitro. The kidney cells may include peritubular interstitial cells of the kidney, and preferably produce erythropoietin (EPO). The kidney cells may also be selected based upon EPO production. Methods of producing an isolated population of EPO producing cells are also provided, and methods of treating a kidney disease resulting in decreased EPO production in a patient in need thereof are provided, including administering the population to the patient, whereby the cells produce EPO in vivo.