Increased susceptibility of diabetic mice to influenza virus infection: compromise of collectin-mediated host defense of the lung by glucose?

Abstract

The influence of diabetes on susceptibility to influenza virus infection was examined in a mouse model in which RIP-Kb transgenic mice and their nontransgenic littermates were used as the diabetic and nondiabetic hosts, respectively. Influenza virus A/Phil/82 (H3N2) grew to significantly higher titers in the lungs of diabetic than nondiabetic mice. The extent of viral replication in the lungs was proportional to blood glucose levels in the mice at the time of infection, and the enhanced susceptibility of diabetic mice was reversed with insulin. Growth of A/HKx31 (H3N2) virus was also enhanced in diabetic mice, whereas the highly virulent strain A/PR/8/34 (H1N1) showed no difference in virus yields in diabetic and nondiabetic mice, even with low inocula. A/Phil/82 and A/HKx31 are sensitive to neutralization in vitro by the pulmonary collectin surfactant protein D (SP-D), whereas A/PR/8/34 is essentially resistant. Glucose is a ligand for SP-D, and neutralization of A/Phil/82 virus by SP-D was abolished in the presence of glucose at levels commonly found in diabetic mice. These findings suggest that in mice, and perhaps in humans, diabetes predisposes to influenza virus infection through compromise of collectin-mediated host defense of the lung by glucose.

FIG. 1

Replication of influenza A viruses in the lungs of diabetic and nondiabetic mice. (A) Virus titers in the lungs of individual diabetic mice (•) and nondiabetic littermates (○) at various times after i.n. inoculation with 10⁵ PFU of A/Phil/82 virus. The dashed line represents the lower limit of detection of virus in lung homogenates. At all time points except 4 h postinfection, virus titers for diabetic mice were significantly higher than those for nondiabetic mice (P < 0.01, Student’s t test). (B and C) Virus titers in the lungs of diabetic (▪) and nondiabetic (□) mice 3 days after i.n. inoculation with different doses of A/HKx31 (B) or A/PR/8/34 (C) influenza virus. Data are the mean lung virus titers (± 1 standard error) for groups of 3 mice. Asterisks indicate lung virus titers of diabetic mice that were significantly different from those of the corresponding nondiabetic controls. ∗, P < 0.02; ∗∗, P < 0.01, Student’s t test.

FIG. 2

Relationship between blood glucose levels and the ability of influenza virus to replicate in the lungs of mice. (A) Blood glucose levels of male (▪ and □) and female (• and ○) mice were determined 4 h before i.n. infection with 10⁵ PFU of A/Phil/82, and virus titers in the lungs were determined at 3 days postinfection. Diabetic animals are indicated by closed symbols, and nondiabetic animals are indicated by open symbols. (B) Effect of insulin treatment on replication of influenza virus in the lungs of diabetic mice. Data are the lung virus titers for individual diabetic mice (•), diabetic mice given subcutaneous injections of insulin every 6 h (⊕), and nondiabetic controls (○) 24 h after i.n. infection with 10⁵ PFU of A/Phil/82 virus. The dashed line represents the lower limit of detection of virus in lung homogenates.

FIG. 3

Effect of glucose on neutralization of influenza virus by SP-D. A/Phil/82 virus was incubated with (•) or without (○) recombinant rat SP-D (final concentration, 2 μg/ml) in the absence or presence of increasing concentrations of glucose (5 to 25 mM) for 60 min at 37°C, and the residual infectivity of the samples was determined by fluorescent focus assay on MDCK cell monolayers.