The Effect of Thyroxine and Radioactive Iodine on Resistance of Mice to Infection with Murine Pneumonitis Virus

The relationship of thyroid activity to resistance of mice to infection with the virus of murine pneumonitis was investigated by the administration of thyroxine which increased thyroid activity and of radioactive iodine$I^131$which completely or almost completely destroyed the thyroid. Subcutaneous injection of 0.1 to 0.2 mg thyroxine on alternate days significantly lowered the survival time of mice infected with murine pneumonitis virus in a cloud chamber. The administration of thyroxine was begun 8 to 14 days before infection or on the day after infection with a more dilute inoculum. Thyroxine given late in the infection, beginning with the seventh day after exposure to the virus, did not alter the survival time of the mice. In contrast to thyroxine, a single subcutaneous injection of 0.1 me radioactive iodine per mouse increased the resistance of the mice when they were infected with murine pneumonitis virus three months later. A single injection of 0.08 me radioactive iodine had no effect. Intact lungs and liver slices from thyroxine and radioactive iodine-treated mice consumed oxygen at the same rate as tissues from untreated animals. Therefore, the observed changes in resistance to murine pneumonitis appear to be due to unknown alterations in the metabolic state of the infected animals rather than to simple quantitative changes in the level of energy metabolism of the infected cells. An understanding of the mechanisms involved requires further investigation.