Effect of prednisolone on the leukocyte counts of ponies and on the reactivity of lymphocytes in vitro and in vivo.

Treatment of ponies with a single dose of prednisolone markedly reduced the number of blood lymphocytes. A decrease of the number of eosinophils was also observed. In contrast, the number of neutrophils significantly increased. These profound changes were temporary and returned to the pretreatment level within 48 h. The number of monocytes did not show any of the significant changes post-prednisolone treatment. The reactivity of the blood lymphocytes of these ponies, in vitro, to stimulation with phytohemagglutinin (PHA) or streptokinase-strepto-dornase (SK-SD) was measured by incorporation of (3-H)thymidine by deoxyribonucleic acid of lymphocytes. The ponies' blood lymphocytes responded very well to PHA stimulation. The incorporation of (3-H)thymidine into deoxyribonucleic acid of the PHA-stimulated lymphocytes was 14 times greater than incorporation of the nonstimulated lymphocytes. SK-SD-stimulated lymphocytes incorporated only three times more (3-H)thymidine than nonstimulated lymphocytes. There was no significant difference in stimulation of lymphocytes with PHA or SK-SD before or after the prednisolone treatment. However, a significant decrease in the dermal response to the homologous antigen after this treatment in sensitive ponies was observed.