Effect of Mycoplasma pheumoniae infection on cell-mediated immunity

The effect of Mycoplasma pneumoniae infection on cell-mediated immunity was examined by two methods: the first method was the tuberculin skin test and the other was the measurement of the in vitro lymphocyte stimulation response to purified protein derivative (PPD) of tuberculin. Twenty-nine patients out of a total of 47 patients with a lower respiratory tract illness caused by M. pneumoniae had a negative tuberculin skin test when first tested. Twenty-three out of 26 patients with a negative tuberculin reaction in the early phase of the illness had a positive skin test when they were retested several weeks or months after the illness. The lymphocyte stimulation response to PPD was examined in 13 patients. In eight of these cases the lymphocyte response to PPD was significantly lower during illness than after recovery. Six control subjects showed no significant difference in their lymphocyte responsiveness to PPD when examined on two different occasions. These studies show that M.pneumoniae infection causes transient depression of cell-mediated immunity.