Immunomodulatory effects of combination of pooled human gammaglobulin and rapamycin on cell proliferation and apoptosis in the mixed lymphocyte reaction.

Background: Multidrug immunosuppressive regimens benefit transplant recipients, reducing side effects and creating synergy between medications with different mechanisms of action. We have shown that pooled human gammaglobulin (intravenous immunoglobulin [IVIG]) inhibits the mixed lymphocyte reaction (MLR) and induces apoptosis primarily in B cells. Rapamycin (RAPA), a potent macrolide immunosuppressant, inhibits B- and T-cell proliferation through G1 cell-cycle blockade and purportedly induces apoptosis. Here we examined the possible synergistic effects of IVIG and RAPA on cell proliferation and apoptosis induction in the MLR.

Methods: MLR was performed with IVIG (0.2-5 mg/mL), RAPA (0.02-40 ng/mL), alone or in combination. Cell proliferation was detected by H-thymidine incorporation, and apoptosis by Annexin V and terminal deoxynucleotide transferase-mediated dUTP nick-end labeling flow cytometry.

Results: IVIG or RAPA inhibited cell proliferation in a dose-dependent manner. RAPA (0.02-40 ng/mL) in combination with IVIG (5 mg/mL) significantly augmented the inhibition compared with RAPA alone (70% vs. 34% at 0.2 ng/mL; 90% vs. 76% at 2 ng/mL). Apoptosis was significantly higher in IVIG-treated (5 mg/mL) CD19+ cells and less so in CD3+ cells. However, RAPA (0.2-40 ng/mL) neither induced apoptosis nor altered apoptosis induced by IVIG.

Conclusions: Combined RAPA and IVIG at subtherapeutic concentrations inhibits cell proliferation in the MLR. RAPA neither induces apoptosis nor augments apoptosis induced by IVIG in the MLR. Lower-concentration RAPA (0.2-2 ng/mL) in combination with IVIG (5 mg/mL) versus therapeutic levels (2-50 ng/mL and 10-40 mg/mL, respectively) could represent an effective immunomodulatory drug combination.