Brief DeNnitive Report

Interferon, or ig ina l ly de tec ted by its capac i ty to inhib i t viral rep l ica t ion in vi tro (1), has long been suspected to p l ay a role in viral infections in vivo (2). Thus , it was no ted tha t interferon a p p e a r e d at the r ight t ime a n d in the r ight p lace to l imi t viral growth (3). Nevertheless, a s imple corre la t ion be tween the a m o u n t of in terferon a n d the severi ty of a viral disease seems insufficient to make a s t rong case for in terferon as a defense mechanism. W i t h the ava i l ab i l i ty of po ten t an t i in te r feron ant isera , it became possible to demons t r a t e more convinc ingly tha t in terferon m a y de t e rmine viral pa thogen ic i ty (4-6). An interes t ing i l lus t ra t ion for this is inf luenza virus infect ion in mice ca r ry ing the d o m i n a n t allele Mx. Adul t an imals wi th this gene resist the le thal effect o f large doses of m o u s e a d a p t e d inf luenza viruses, t i ters of virus grown in thei r organs r ema in low, and in terferon is ha rd ly de tec tab le (7, 8). Nevertheless, inject ion o f po ten t ant i interferon an t ibodies renders Mx-bear ing mice as suscept ible to le thal inf luenza virus infect ion as non-Mx-bear ing mice (8). T h a t interferon has a pro tec t ive role in vivo, however, has so far not been demons t r a t ed direct ly. Because newborn Mx-bear ing mice are v i r tua l ly as suscept ible to or thomyxovi ruses as their Mx-negat ive coun te rpa r t s (9), we were ab le to de te rmine the efficacy o f in terferon in p ro tec t ing newborn mice differing at the Mx locus.