Immune mechanisms in leukemia: suppression of cellular immunity by drugs and x-irradiation

The relative suppressive effects of x-irradiation (XR), cyclophosphamide (CY), prednisolone (PRD), and methotrexate (MTX) on the primary and secondary cellular immune response of C58/wm mice to syngeneic line Ib transplantable leukemia (Ib cells) were quantified. An LD10 dose of each agent was used for immunosuppression. XR, CY, and PRD were markedly suppressive for the primary immune response if given 24 hr before mice were immunized to Ib cells but less immunosuppressive if given 24 hr later. MTX was only slightly immunosuppressive XR, CY, and PRD also suppressed the secondary immune response if given before but not after antigen. The immunosuppressive effect of these agents was evaluated by defining their median immunosuppressive dose or the median time in days required for mice to recover from graded doses of each immunosuppressive agent. For example, the median recovery time from an LD10 of XR, CY, and PRD was 29.3, 19.7, and 3.7 days, respectively. Immunologic competence remaining after XR or drug treatment was quantified in terms of the LD50 dose of Ib cells required to kill recipient mice. For XR, CY, PRD, and MTX it was 10(6.16), 10(2.15), 10(6.90) and greater than 10(7.0) viable Ib cells, respectively. The overall results provided evidence that the primary and secondary cellular immune responses to a weak syngeneic tumor antigen were resistant to immunosuppression once they were initiated. There was a good correlation between the relative immunosuppressive effect of the test agents and the amount that they reduced the number of immune spleen cells. The agents also impaired the immunocompetence of individual spleen cells. Mechanisms by which XR or drugs might exert their immunosuppressive effects were discussed.