The Immunosuppressor Sirolimus Reduces the Number and Maturation of Colony-Forming Units of Erythroid Progenitor Cells.

Introduction: Kidney transplantation is the treatment of choice in patients with end-stage renal failure. However, the immunosuppression has severe side effects, such as chronic allograft nephropathy, cardiovascular morbidity and neoplasia. An additional effect of inhibiting mTOR is microcytic anemia. We have explored the pathophysiology of this effect.

Methods: Erythroid progenitor cells were isolated from peripheral blood of healthy control persons (HC; n=8), or kidney transplant recipients with chronic sirolimus treatment with (SRL+MC; n=8) or without (SRL-MC; n=8) microcytosis. The isolated progenitor cells were then cultured in a semi-solid medium, containing 3 U/ml erythropoietin, in the absence or presence of SRL (5ng/mL) for 14 days. Burst forming unit erythroid (BFU-E) derived colonies were then counted through an inverted microscope considering that each colony consists of more than 40 cells. Cultures were performed in duplicate and colonies were counted in the entire culture dish.

Results: Hemoglobin was 13.17 (SRL+MC) and 13.23 (SRL-MC) g/dL. RBC count was 5.22 and 4.55x10⁶ /μL in SRL+MC and SRL-MC respectively. MCV was 76 in SRL+MC and 87 fL in SRL-MC. Presence of SRL in the culture medium led to a decrease in the number of colonies in healthy controls and kidney transplant patients (43.2±4.5 vs. 28.5±3.6 BFU-E derived colonies, Mean±SEM, p<0.05). Similar differences were seen if the three groups were analyzed separately (HC, SRL-MC, SRL+MC). This effect was not observed in the presence of tacrolimus, an immunosuppressor with different mechanism of action. Interestingly, culture dishes corresponding to SRL+MC patients contained a higher number of colonies when compared to SRL-MC and HC.

Conclusion: mTOR inhibition leads to a reduced number of erythroid colonies in culture. There is a higher number of circulating progenitor cells in the microcytic patients. Both these observations indicate that microcytosis may be compensated by a higher number of erythrocytes and circulating progenitor cells.