Serum Stem Cell Factor Level in Renal Transplant Recipients With Posttransplant Erythrocytosis *Ahmet A. Kiykim, *Gultekin Genctoy, *Mehmet Horoz, †Naci E. Tiftik, *Ebru Gok, ‡Bulent Altun, ‡Mustafa Arici, and §Ibrahim Haznedaroglu Divisions of *Nephrology; †Hematology, Department of Internal Medicine, School of Medicine, Mersin University, Mersin; Divisions of ‡Nephrology; and §Hematology, Department of Internal Medicine, School of Medicine, Hacettepe University, Ankara, Turkey Abstract: The etiology of posttransplant erythrocytosis (PTE) remains unclear, and the most frequently suggested causative factors are still a matter of controversy.We aimed to investigate serum-soluble stem cell factor (sSCF) along with serum erythropoietin (EPO) levels in renal transplant recipients (RTRs) with PTE.Thirteen RTRs with PTE, 42 RTRs without PTE, and 42 healthy controls were included. Serum sSCF and EPO levels were determined using an enzyme-linked immunosorbent assay kit. Expected and observed/expected EPO levels were calculated. Serum sSCF levels and observed/expected EPO were significantly higher in RTRs with PTE than both RTRs without PTE and controls. In RTRs with PTE, sSCF level was sig- nificantly correlated with hematocrit and observed/ expected EPO, respectively. Significant correlation was also observed between hematocrit level and observed/expected EPO in RTRs with PTE. Increased sSCF level and inadequate suppression of EPO production seem to have a role in the pathogenesis of PTE. Key Words: Kidney transplantation—Erythrocytosis—Growth factor. Posttransplant erythrocytosis (PTE) is a relatively common phenomenon affecting 15–20% of allograft recipients with normal renal function and can re- sult in serious thromboembolic complications (1). Although a variety of possible mechanisms, including abnormal erythropoietin (EPO) production (2–5), abnormal erythroid precursor sensitivity to EPO (6–8), abnormal marrow production of angiotensin II (AII), or increased erythroid precursor sensitivity to AII (9,10) have been suggested to have a role in the pathogenesis in PTE, its etiology remains unclear, and the most frequently suggested causative factors are still a matter of controversy. Stem cell factor (SCF) is produced by the fibro- blasts, stromal cells, keratinocytes, endothelial cells, neuroblastoma cells, and tumor cell lines (11). SCF triggers its biologic effects by binding to its receptor, c-kit (12), and acts synergistically with other hemopoietic growth factors to stimulate the growth and differentiation of a variety of progenitor cells (13). It has been reported that SCF alone or in com- bination with EPO stimulates proliferation and maturation of erythroid progenitors. Thus, normal erythropoiesis seems to be dependent on the intact function of both SCF-KIT and Epo-Epo receptor (EpoR) signaling pathways (14,15). Despite that SCF has been suggested to be one of the key elements during normal erythropoiesis, to our knowledge, there are no data related to the level of SCF in PTE. Therefore, in the present study, we aimed to investigate serum-soluble SCF (sSCF) levels along with serum EPO levels in renal trans- plant recipients (RTRs) with newly diagnosed PTE. PATIENTS AND METHODS Subjects Thirteen RTRs with newly diagnosed PTE, 42 RTRs without PTE, and 42 healthy controls were included in the present study. PTE was defined as a persistently elevated hematocrit level equal to or doi:10.1111/j.1525-1594.2009.00823.x Received September 2008; revised October 2008. Address correspondence and reprint requests to Dr. Mehmet Horoz, Mersin University, Faculty of Medicine, Department of Internal Medicine, Division of Nephrology, Mersin,Turkey. E-mail: mehmethoroz@yahoo.com Artificial Organs 33(12):1086–1090, Wiley Periodicals, Inc. © 2009, Copyright the Authors Journal compilation © 2009, International Center for Artificial Organs and Transplantation and Wiley Periodicals, Inc. 1086