Effects of severe calorie restriction on thymic factor activity and lymphocyte stimulation response in rats.

The thymic factor (TF) activity and lymphocyte stimulation response to phytohemagglutinin (PHA) was studied in calorie-restricted rats. Sprague-Dawley rats (28 days old) were fed a calorie-restricted diet (30% of the average food intake of control) for 4 weeks. The control group was allowed food ad libitum. After this period the body weight was 232.9 +/- 41.8 g for control rats and 66.1 +/- 4.5 g for the calorie-restricted group. The mean thymus weight was 0.699 +/- 0.153 g and 0.099 +/- 0.030 g for the control and experimental groups respectively. The TF activity was makedly reduced in the calorie-restricted group compared with the control. There was an overlap between lymphocyte stimulation responses of the two groups; however, the mean number of counts per minute (cpm) of stimulated cultures with PHA was lower and the mean cpm of unstimulated cultures without PHA was higher in the starved animals. Thus, the stimulation index was significantly lower in the deprived rats. It is suggested that deficiency of thymic inductive factors may be important in the pathogenesis of impaired cell-mediated immunity in nutritional deficiency states.