Evidence that cyclic AMP phosphodiesterase inhibitors suppress interleukin‐2 release from murine splenocytes by interacting with a ‘low‐affinity’ phosphodiesterase 4 conformer

We have investigated the suppressive effects of rolipram, RP 73401 (piclamilast) and other structurally diverse inhibitors of cyclic AMP‐specific phosphodiesterase 4 (PDE4) on interleukin (IL)‐2 generation from Balb/c mouse splenocytes exposed to the superantigen, Staphylococcocal enterotoxin‐A (Staph. A). The purpose was to determine whether their potencies are more closely correlated with inhibition of PDE4 from CTLL cells, against which rolipram displays weak potency (low‐affinity PDE4), or displacement of [3H]‐(±)‐rolipram from its high‐affinity binding site (HARBS) in mouse brain cytosol. RP 73401 (IC50 0.46±0.07 nM, n=4) was a very potent inhibitor of Staph. A‐induced IL‐2 release from Balb/c mouse splenocytes, being >1100 fold more potent than (±)‐rolipram (IC50 540±67 nM, n=3). A close correlation (r=0.95) was observed between suppression of IL‐2 release by PDE inhibitors and inhibition of PDE4. In contrast, little correlation (r=0.39) was observed between suppression of IL‐2 release and their affinities for the high‐affinity rolipram binding site (HARBS). RP 73401 only inhibited partially (30–40%) Staph. A‐induced incorporation of [3H]‐thymidine into splenocyte DNA. The PDE3 inhibitor, siguazodan (10 μM), had little or no effect on IL‐2 release or DNA synthesis. This concentration of siguazodan did not enhance the inhibitory action of RP 73401 on IL‐2 release but potentiated its effect on DNA synthesis, increasing potency and efficacy. Staph. A‐induced DNA synthesis was only partially inhibited by anti‐IL‐2 neutralizing antibody, whereas dexamethazone (100 nM) and cyclosporine A (100 nM) completely blocked the response. RP 73401 (IC50 6.3±1.9 nM, n=4) was 140 fold more potent than rolipram (IC50 900±300 nM, n=3) in inhibiting Staph. A‐induced [3H]‐thymidine incorporation into splenocyte DNA. The results implicate a low‐affinity form of PDE4 in the suppression of Staph. A‐induced IL‐2 release from murine splenocytes by PDE inhibitors. The data also indicate that mitogenic factors other than IL‐2, whose elaboration or responses to which are regulated by PDE3 as well as PDE4, contribute to the superantigen‐induced DNA synthesis. British Journal of Pharmacology (1997) 121, 743–750; doi:10.1038/sj.bjp.0701200