Human MxA protein protects mice lacking a functional alpha/beta interferon system against La crosse virus and other lethal viral infections

Abstract

The human MxA protein is part of the antiviral state induced by alpha/beta interferon (IFN-alpha/beta). MxA inhibits the multiplication of several RNA viruses in cell culture. However, its antiviral potential in vivo has not yet been fully explored. We have generated MxA-transgenic mice that lack a functional IFN system by crossing MxA-transgenic mice constitutively expressing MxA with genetically targeted (knockout) mice lacking the beta subunit of the IFN-alpha/beta receptor (IFNAR-1(-/-) mice). These mice are an ideal animal model to investigate the unique antiviral activity of human MxA in vivo, because they are unable to express other IFN-induced proteins. Here, we show that MxA confers resistance to Thogoto virus, La Crosse virus, and Semliki Forest virus. No Thogoto virus progeny was detectable in MxA-transgenic mice, indicating an efficient block of virus replication at the primary site of infection. In the case of La Crosse virus, MxA restricted invasion of the central nervous system. In contrast, Semliki Forest virus multiplication in the brain was detectable in both MxA-expressing and nonexpressing IFNAR-1(-/-) mice. However, viral titers were clearly reduced in MxA-transgenic mice. Our results demonstrate that MxA does not need the help of other IFN-induced proteins for activity but is a powerful antiviral agent on its own. Moreover, the results suggest that MxA may protect humans from potential fatal infections by La Crosse virus and other viral pathogens.

FIG. 1

Detection of MxA protein by Western blot analysis. Various organs and tissue samples from an adult MxA^+/+ IFNAR-1^−/− mouse (originating from the MxA-transgenic L line) were collected, cell extracts were prepared, and samples of 20 μg of protein per lane were separated by SDS-polyacrylamide gel electrophoresis. MxA was detected with an MxA-specific monoclonal antibody (21).

FIG. 2

MxA-transgenic IFNAR-1^−/− mice are resistant to THOV infection. (A) Adult mice of four different genotypes were challenged with THOV and monitored for survival. MxA^+/+ IFNAR-1^−/− (■), IFNAR-1^−/− (□), MxA^+/+ IFNAR-1^+/+ (●), and IFNAR-1^+/+ mice (○) were infected with 300 PFU of THOV per ml by the intraperitoneal route. (B) Virus growth in livers of susceptible and resistant mice. Two animals per genotype were infected as described above and sacrificed 36 h after infection. Their livers were removed and assayed for infectivity. Each column represents the mean virus titer for two animals.

FIG. 3

Enhanced resistance of MxA-transgenic IFNAR-1^−/− mice to LACV infection. (A) Adult MxA^+/+ IFNAR-1^−/− (■) and MxA^−/− IFNAR-1^−/− (□) mice were infected with 10⁵ TCID₅₀ of LACV by the intraperitoneal route and monitored for survival. (B) Virus growth in brains of susceptible and resistant mice. Two MxA^+/+ IFNAR-1^−/− mice (animals 1 and 2) and three MxA^−/− IFNAR-1^−/− mice (animals 3, 4, and 5) were infected as described above and sacrificed 6 days after infection. Tissue samples were removed and assayed for infectivity.

FIG. 4

Histology and immunostaining of brain sections from an MxA^+/+ IFNAR-1^−/− and an MxA^−/− IFNAR-1^−/− mouse. Both types of animals were infected with 10⁵ TCID₅₀ of LACV by the intraperitoneal route. Mice were sacrificed 6 days after infection, and brain sections were prepared. Micrographs show the brain of an IFNAR-1^−/− mouse (panels A to D) and that of an MxA^+/+ IFNAR-1^−/− mouse (panels E to H) immunostained for MxA protein (panels A and E), the nucleocapsid protein of LACV (panels B and F), or GFAP (panels D and H) or stained with hematoxylin and eosin (HE) (panels C and G).

FIG. 5

Enhanced resistance of MxA-transgenic IFNAR-1^−/− mice to SFV infection. (A) Adult MxA^+/+ IFNAR-1^−/− (■) and MxA^−/− IFNAR-1^−/− (□) mice were infected with 100 PFU of SFV by the intraperitoneal route and monitored for survival. (B) Virus growth in various organs of susceptible and resistant mice. Two MxA^+/+ IFNAR-1^−/− and two MxA^−/− IFNAR-1^−/− mice were infected as described above and sacrificed 4 days after infection. Tissue samples were removed and assayed for infectivity. Each column represents the mean virus titer for two animals.

FIG. 6

Histology and immunostaining of brain sections from an MxA^+/+ IFNAR-1^−/− mouse and an MxA^−/− IFNAR-1^−/− mouse. Both were infected with 100 PFU of SFV per ml by the intraperitoneal route. Mice were sacrificed 4 days after infection, and brain sections were prepared. Micrographs show the brain of an MxA^+/+ IFNAR-1^−/− mouse (panels A to D) and that of an MxA^−/− IFNAR-1^−/− mouse (panels E to H) immunostained for MxA protein (panels A and E), the C protein of SFV (panels B and F), or GFAP (panels D and H) or stained with hematoxylin and eosin (HE) (panels C and G).