Natural murine interferon-γ (naMuIFN-γ) produced by T-lymphoma cells (L12-R4) stimulated with phorbol myristic acetate was purified by use of an anti-MuIFN-γ immunoadsorbent and was labelled with I to study its binding to murine cell receptors. All the cell lines examined bound naMuIFN-γ, although the binding affinity varied considerably. By adding increasing concentrations of unlabelled naMuIFN-γ in competition binding assays we determined dissociation constants ( ) of 8.2 × 10 and 7.4 × 10 for L1210 and TS/A cells, respectively, and of 6.5 × 10 for L-929 cells. The numbers of receptors present per cell of these lines were 3000, 1000 and 2000 respectively. Highly purified naMuIFN-γ as well as recombinant MuIFN-γ competed for binding sites with I-labelled IFN-γ on L1210 cells, although the latter displayed a greater than the former (5.8 × 10 compared to 8.2 × 10 ). Moreover, protease, but not endoglycosidase, treatment of target cells prevented the subsequent binding of I-labelled IFN-γ, suggesting that a protein moiety is involved in the binding of the ligand. These studies demonstrate that naMuIFN-γ binds in a specific manner and with high affinity to murine cell receptors.


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