CALCINEURIN ACTIVITY IS ONLY PARTIALLY INHIBITED IN LEUKOCYTES OF CYCLOSPORINE-TREATED PATIENTS

Measurement of the degree of immunosuppression induced clinically by d rugs such as cyclosporine is an important but elusive goal. In lymphoc ytes in vitro, cyclosporine (CsA) blocks the phosphatase activity of t he enzyme calcineurin, preventing cytokine induction. We sought to mea sure the degree of calcineurin blockade in patients on CsA. Calcineuri n activity was measured in peripheral blood mononuclear cells (PBL) fr om stable CsA-treated renal transplant patients, compared with control s. Cytokine expression was assessed by challenging ex vivo PBL with ca lcium ionophore A23187 (5 mu M) for 60 min and measuring interferon-ga mma (IFN-gamma) and interleukin 2 (IL-2) mRNA induction. In vitro, CsA inhibited both calcineurin activity and cytokine induction with an IC 50 of 10-20 mu g/L. In CsA-treated patients with therapeutic CsA level s (mean trough CsA blood level = 180+/-55 mu g/L), calcineurin activit y was detectable but reduced by 50% compared with controls (P less tha n or equal to 0.001) and correlated with CsA trough levels (r= -0.390, P less than or equal to 0.01). The induction of cytokine mRNA in such patients was not blocked, but was sensitive to CsA in vitro, suggesti ng that CsA is much less available in vivo in body fluids than it is f or isolated cells in vitro. In lymphocytes of patients on CsA, calcine urin activity is reduced but 50% of the activity persists, permitting strong signals to trigger cytokine expression. Partial calcineurin inh ibition may explain why the immune responsiveness of patients on CsA i s reduced but still sufficient for host defense.