INHIBITION OF CTL INDUCTION BY RAPAMYCIN - IL-2 RESCUES GRANZYME-B AND PERFORIN EXPRESSION BUT ONLY PARTIALLY RESTORES CYTOTOXIC ACTIVITY

Rapamycin (RAP) is a potent inhibitor of CTL induction, Since RAP is k nown to block IL-2 signaling through the IL-2R, we hypothesized that R AP may interfere with CTL generation by inhibiting IL-2-induced expres sion of granzyme (Gzm) B, perforin, and/or Fas ligand (FasL). MHC-unre stricted mouse CTL induced in vitro with anti-CD3 mAb in the presence of RAP (1 ng/ml) exhibited dramatically reduced cellular proliferation and cytotoxicity against P815 tumor target cells, Gzm B mRNA expressi on and enzymatic activity in RAP-treated CTL were greatly reduced comp ared with those in control CTL, Perforin mRNA expression was also redu ced by RAP, In contrast, RAP failed to inhibit FasL mRNA expression, R AP, therefore, inhibits induction of the perforin/Gzm B cytolytic path way but spares Fas/FasL-mediated cytotoxicity. To determine whether RA P exerts a total blockade of the IL-2R signaling pathway, we induced C TL in the presence of both RAP and exogenous rIL-2 (100 U/ml), Under t hese conditions, Gzm B and perforin mRNA and protein expression as wel l as cellular proliferation were restored to at least control levels, Surprisingly, inhibition of cytotoxicity was only partially alleviated when CTL were induced in the presence of RAP plus rIL-2, even though CTL conjugated normally with target cells and had an intact granule se cretory pathway, We conclude that 1) the inhibitory effect of RAP at t he level of the IL-2R is incomplete; and 2) the suppressive effect of RAP on CTL induction is only partly due to inhibition of Gzm B and per forin gene expression.