Inhibition of binding of interferon-γ to its receptor by salicylates used in inflammatory bowel disease

5-Aminosalicylic acid (5ASA), 4ASA, their Nacetylated metabolites N-acetyl-5ASA and Nacetyl- 4ASA, olsalazine, and colchicine impair interferon-γ (IFN γ) induced HLA-DR expression on a colonic cell line, HT-29. The mechanism of this effect is now reported. HT-29 cells were cultured with 50 U/ml IFN γ with or without drug, and northern blot analysis was performed using a probe for the β chain of the DR molecule. IFN γ led to a noticeable increase in HLA-DR mRNA which was attenuated by the drugs. Analysis of the specific binding of increasing concentrations of 125I-IFN γ by non-linear regression showed a Kd of 1.35×10-10 M and 2.3×105 binding sites per HT-29 cell. Binding of 125I-IFN γ was reduced by incubation with increasing concentrations of unlabelled IFN γ but not with IFN a. Incubation with therapeutic concentrations of drugs led to the following reductions in binding: 10 mM 5ASA, 20% (p<0001); 10 mM N-acetyl-SASA, 24% (p<0.01); 10 mM 4ASA, 21% (p<0.005); 10 mM N-acetyl-4ASA, 29% (p<0.001); and 1 mM olsalazine, 29% (p<0.001). Colchicine (10-7 M) and 10-5 M prednisolone had no effect. Incubation with higher concentrations of the drugs revealed a dose-response effect on binding with complete inhibition by 100 mM 4ASA and 10 mM olsalazine, and lesser degrees of inhibition by 100 mM SASA, N-acetyl-5ASA, and N-acetyl- 4ASA. At concentrations found in the rectal lumen, the salicylates used in inflammatory bowel disease impair the binding ofIFN γ to its receptor on colonic epithelial cells.