Association Between the Intracellular Tacrolimus Concentration in CD3(+) T Lymphocytes and CD14(+) Monocytes and Acute Kidney Transplant Rejection

Background: Intracellular tacrolimus concentration in peripheral blood mononuclear cells (PBMCs) (TAC([PBMC])) has been proposed to better represent its active concentration than its whole blood concentration. As tacrolimus acts on T lymphocytes and other white blood cells, including monocytes, we investigated the association of tacrolimus concentration in CD3(+) T lymphocytes (TAC([CD3])) and CD14(+) monocytes (TAC([CD14])) with acute rejection after kidney transplantation. Methods: From a total of 61 samples in this case-control study, 28 samples were obtained during biopsy-proven acute rejection (rejection group), and 33 samples were obtained in the absence of rejection (control group). PBMCs were collected from both cryopreserved (retrospectively) and freshly obtained (prospectively) samples. CD3(+) T lymphocytes and CD14(+) monocytes were isolated from PBMCs, and their intracellular tacrolimus concentrations were measured. Results: The correlation between tacrolimus whole-blood and intracellular concentrations was poor. TAC([CD3]) was significantly lower than TAC([CD14]) (median 12.8 versus 81.6 pg/million cells; P < 0.001). No difference in TAC([PBMC]) (48.5 versus 44.4 pg/million cells; P = 0.82), TAC([CD3]) (13.4 versus 12.5 pg/million cells; P = 0.28), and TAC([CD14]) (90.0 versus 72.8 pg/million cells; P = 0.27) was found between the rejection and control groups. However, freshly isolated PBMCs showed significantly higher TAC([PBMC]) than PBMCs from cryopreserved samples. Subgroup analysis of intracellular tacrolimus concentrations from freshly isolated cells did not show a difference between rejectors and nonrejectors. Conclusions: Differences in TAC([CD3]) and TAC([CD14]) between patients with and without rejection could not be demonstrated. However, further optimization of the cell isolation process is required because a difference in TAC([PBMC]) between fresh and cryopreserved cells was observed. These results need to be confirmed in a study with a larger number of patients.