Effect of Rapamycin on Spleen Size in Longstanding Renal Transplant Recipients

Background. Based on evidence available in the literature, rapamycin, a mammalian target of rapamycin (mTOR) inhibitor, but not calcineurin inhibitors (CNIs), has been shown to decrease spleen size. Small spleen, in some instances, is associated with hyposplenism, a condition recently reported in patients with longstanding renal transplant. Accordingly, the effect of immunosuppressive drugs on spleen size was evaluated. Methods. Renal transplant recipients (35 taking mTOR and 68 CNI) were included, in whom a standardized investigation of the kidney allograft and spleen with the use of color Doppler ultrasound was performed and a peripheral smear were reviewed for the presence of Howell-Jolly bodies (HJBs). Results. We enrolled 103 patients (64 men; 66 from a deceased donor). The mean age was 47.7 years (range, 23.0-74.0 y). Mean transplant duration was 1,899 days (range, 181-6,883 d). According to the presence of HJBs, the prevalence of hyposplenism was 47.6% for the entire cohort. The differences between the mTOR and CNI groups regarding sex and the presence of HJBs were not statistically significant (P >.05). Age, creatinine, hemoglobin, leukocytes, platelets, and Doppler parameters in spleen and kidney were similar in both groups (P >.05). mTOR patients had a decreased spleen length size (90.09 +/- 13.02 mm vs 111.95 +/- 18.66 mm; P < .001), a longer transplant duration (3,576 1,594 d vs 1,036 1,369 d; P < .001) and higher serum cholesterol (227.50 +/- 38.75 mg/dL vs 182.67 +/- 37.74 mg/dL; P < .001) and triglycerides (194.23 +/- 79.88 mg/dL vs 148.70 +/- 55.54 mg/dL; P = .003) levels compared with the CNI group. A multivariate analysis showed mTOR inhibitor to be the most important predictor of spleen size. In both the mTOR and CNI groups, the comparison between the subgroups of present and absent HJBs did not show any difference. Conclusions. The findings of this study suggest that small spleens in transplant recipients may be linked to treatment with an mTOR inhibitor, although this apparently does not compromise splenic function.