Antiviral activity of a thymic factor in experimental viral infections. I. Thymic hormonal effect on survival, interferon production and NK cell activity in Mengo virus-infected mice

A partially purified thymic factor, thymostimulin (TS), significantly increased the survival rate of adult, immune-intact mice infected with the neurotropic Mengo virus. TS treatment was begun after virus inoculation by daily i.p. injections. In untreated C57BL/6 mice, LD₅₀ was reached with 1 x 10⁴ PFU, but 10-fold more virus (i.e., 1 x 10⁵ PFU) was needed to reach LD₅₀ in TS-treated animals. TS effect on survival, though, could be observed with several virus doses (1 x 10³ to 1 x 10⁶ PFU) (p < 0.001). A significant effect on survival was also observed with outbred ICR mice (p < 0.005). Serum interferon (IFN) levels in the mengo virus-infected mice were relatively low (average peak 300 U/ml), but were significantly increased (two- to ninefold) in the TS-treated mice. Peak serum levels were reached earlier in TS than in control animals (24 hr and 72 hr, respectively). Both acid-labile and acid-stable type I IFN production were augmented by TS in the Mengo virus-infected mice. Natural killer activity was also enhanced by TS, in particular on the second day after virus inoculation. In addition, MP-virus was used as a second, unrelated virus challenge. This virus caused a nonlethal infection, with relatively high levels of serum IFN (Average peak 10,000 U/ml). TS increase IFN levels (two- to eight-fold) also in this challenge system. In conclusion, TS causes a nonspecific enhancement of endogenous production of IFN and has a significant effect on the survival of lethally infected mice. The data indicate a potential application of thymic factors for the treatment of viral infections.