SIROLIMUS INHIBITS HUMAN IgG ANTI-ATGAM ANTIBODY FORMATION IN RENAL TRANSPLANT RECIPIENTS

Abstract

15 The new immunosuppressive drug, sirolimus (RAPA), has significant in vitro anti-B cell activity. RAPA inhibits pokeweed mitogen stimulated proliferation and IgM and IgG production, and IL-6 dependent differentiation of human B cells. In the rat, anti-allogeneic IgG and IgM antibody (Ab) responses were suppressed after graft rejection. The effect of RAPA on in vivo human Ab responses has not been assessed. We tested the hypothesis that RAPA would reduce Ab formation in humans by analyzing the anti-ATGAM response in renal transplant (Tx) recipients receiving RAPA. Sequential serum specimens were prospectively obtained from patients enrolled in 3 separate IRB approved immunosuppressive studies. The studies were of similar design: all patients were over 18 year old and had received a renal Tx. Baseline immunosuppression consisted of 1 gm methylprednisolone IV at the time of Tx, 500 mg on day 1, before the first dose of ATGAM (Pharmacia & Upjohn), followed by an oral steroid taper, oral CyA at 6 to 8 mg/kg/day; ATGAM (15 mg/kg/day). In addition to the baseline immunosuppression, the patients received, depending on the study, either azathioprine (AZA, 1-2mg/kg/d, n=14), mycophenolate mofetil (MMF, 1gm b.i.d., n=12), or RAPA (2mg/d, n=15). Sera were collected immediately pre-Tx and then weekly for at least 1 month. Sera (diluted 1:100) were tested for the presence of anti-ATGAM Ab by ELISA with ATGAM bound to the plate and peroxidase conjugated goat anti-human IgG (heavy/light chain reactive) as the secondary detecting reagent. Sera were considered positive if the mean absorbance of quadruplicate wells was ≥ 50% of a positive control sera. Sera positive at 1:100 were further diluted and retested. Patients were considered sensitized if they had a positive serum or non-sensitized if they had at least 2 negative sera between days 10 to 30 post transplant. The rate of anti-ATGAM formation was compared by Chi square with a Bonferoni correction for multiple comparisons. Demographics of the patients were similar (not shown). The overall rate of anti-ATGAM formation was significantly different among the groups (TABLE, overall Chi square p=0.01). Both RAPA and MMF significantly reduced the rate of anti-ATGAM Ab formation compared to AZA (p=0.005 and p=0.014, respectively). RAPA and MMF demonstrated similar rates (p=0.81). Both sensitized MMF patients had titers <1:500, one sensitized RAPA patient was <1:500, while the other was >1:2000, and 3 of the sensitized AZA patients were >1:500 and the other 6 <1:500. (Table)TableIn conclusion, RAPA significantly reduced the rate of de novo anti-ATGAM formation to a level at least comparable to that achieved with MMF. If generalizable to other antigens, this anti-B cell activity may contribute to a reduced rate of chronic rejection, and maybe beneficial in Ab mediated xenograft rejection.