Influenza virus pathophysiology and brain invasion in mice with functional and dysfunctional Mx1 genes

Abstract

Mice with a dysfunctional myxovirus resistance-1 (dMx1) gene transport intranasally-instilled PR8 influenza virus to the olfactory bulb (OB) within 4 h post-infection. To determine if the presence of a functional Mx1 (fMx1) gene would influence this brain viral localization and/or disease, we infected mature C57BL/6 dMx1 and fMx1 mice under the same conditions and observed sickness behaviors, viral nucleoprotein (NP) RNA expression and innate immune mediator (IIM) mRNA expression in selected tissues at 15 and 96 h post-infection. Virus invaded the OB and lungs comparably in both sub-strains at 15 and 96 h as determined by nested PCR. In contrast, virus was present in blood and somatosensory cortex of dMx1, but not fMx1 mice at 96 h. At 15 h, sickness behaviors were comparable in both sub-strains. By 96 h dMx1, but not fMx1, were moribund. In both 15 and 96 h lungs, viral NP was significantly elevated in the dMx1 mice compared to the fMx1 mice, as determined by quantitative PCR. OB expression of most IIM mRNAs was similar at both time periods in both sub-strains. In contrast, lung IIM mRNAs were elevated in fMx1 at 15 h, but by 96 h were consistently reduced compared to dMx1 mice. In conclusion, functional Mx1 did not alter OB invasion by virus but attenuated illness compared to dMx1 mice. Inflammation was similar in OBs and lungs of both strains at 15 h but by 96 h it was suppressed in lungs, but not in OBs, of fMx1 mice.

Fig. 1

Changes in body temperature (A) and locomotor activity (B) over 96 h following PR8 infection. Locomotor activity data were normalized by averaging the number of locomotor activity counts across 2-h time periods for each mouse and then dividing by 1/12th of each mouse’s pre-infection 24-h average. * p < 0.05 comparing dMx1 to fMx1 using repeated measures two-way ANOVA.

Fig. 2

OB viral NP− and NP+ RNA detection by nPCR at 96 h in dMx1 (A) and fMx1 (B) mice. After live virus challenge (lower gels) the bands near the 500 base pair (bp) standard are viral NP− and NP+. No viral NP bands were detected in the OB if mice were challenged with boiled virus (upper gels).

Fig. 3

Quantitative mRNA expression at 15 h PI of innate immune mediators in PR8-infected OB (A) and lung (B) comparing dMx1 (black bars) to fMx1 (gray bars) values. Asterisks designate significant differences from boiled virus control values, plus signs designate significant differences between mouse sub-strains. Boiled virus values have been normalized to 1.0 and therefore are not included in this Figure. Note that while the Mx1 mRNA is expressed in both mouse strains, the protein product of the mutated Mx1 gene in dMx1 mice has no enzymatic activity. See Results text for analysis. * p < .05, ** p < 0.01, + p < 0.05. ++ p < 0.01. (Note scale differences between OB and lung.)

Fig. 4

Quantitative mRNA expression at 96 h PI of innate immune mediators (IIMs) in PR8-infected OB (A) and lung (B) comparing dMx1 (black bars) to fMx1 (gray bars) values. Asterisks designate significant differences from boiled virus control values, plus signs designate significant differences between mouse sub-strains. Boiled virus data have been normalized to 1.0 and therefore are not included in this Figure. Note that while the Mx1 mRNA is expressed in both mouse strains, the protein product of the mutated Mx1 gene in dMx1 mice has no enzymatic activity. See Results text for analysis. * p < .05, ** p < 0.01, + p < 0.05, ++ p < 0.05. (Note scale differences between OB and lung.)