Thymic hormonal effect on recovery from early and late CCNU induced immunosuppression

The alkylating agent 1-(2-chloroethyl)-3-cyclohexyl-1-nitrosourea (CCNU) caused a prolonged suppression of antibody response to sheep red blood cell challenge in mice. Two phases of suppression could be observed, an early (first three weeks after CCNU administration) and late phase (4-10 weeks). The late immunosuppression was more severe than the early one. Treatment by the thymic extract thymostimulin (TP-1(R)) caused a complete recovery of antibody response, with overshoot, during early phase, but only partial recovery during late phase. It is suggested that the known CCNU induced damage to stem cells, manifested at late phase, reduces the responding pool for the restorative effect of thymostimulin.