Background: Graft survival in transplant recipients depends on pharmacokinetics and on individual susceptibility
towards immunosuppressive drugs. Nevertheless, pharmacodynamic changes in T-cell functionality in response to
drugs and in relation to pharmacokinetics are poorly characterized. We therefore investigated the immunosuppressive
effect of calcineurin inhibitors and steroids on general T-cell functionality after polyclonal stimulation of whole blood
samples.
Methods: General T-cell functionality in the absence or presence of immunosuppressive drugs was determined in vitro
directly from whole blood based on cytokine induction after stimulation with the polyclonal stimulus Staphylococcus
aureus enterotoxin B. In addition, diurnal changes in leukocyte and lymphocyte subsets, and on T-cell function after
intake of immunosuppressive drugs were analyzed in 19 patients during one day and compared to respective kinetics
in six immunocompetent controls. Statistical analysis was performed using non-parametric and parametric tests.
Results: Susceptibility towards calcineurin inhibitors showed interindividual differences. When combined with steroids,
tacrolimus led to more pronounced increase in the inhibitory activity as compared to cyclosporine A. While circadian
alterations in leukocyte subpopulations and T-cell function in controls were related to endogenous cortisol levels, T-cell
functionality in transplant recipients decreased after intake of the morning medication, which was more pronounced in
patients with higher drug-dosages. Interestingly, calcineurin inhibitors differentially affected circadian rhythm of T-cell
function, as patients on cyclosporine A showed a biphasic decrease in T-cell reactivity after drug-intake in the morning
and evening, whereas T-cell reactivity in patients on tacrolimus remained rather stable.
Conclusions: The whole blood assay allows assessment of the inhibitory activity of immunosuppressive drugs in
clinically relevant concentrations. Circadian alterations in T-cell function are determined by dose and type of
immunosuppressive drugs and show distinct differences between cyclosporine A and tacrolimus. In future these
findings may have practical implications to estimate the net immunosuppressive effect of a given drug-regimen
that daily acts in an individual patient, and may contribute to individualize immunosuppressio
