DOSE-RESPONSE RELATIONSHIPS OF CYTOTOXICITY, PFC RESPONSE AND HISTOLOGY IN THE SPLEEN IN RATS TREATED WITH ALKYLATING-AGENTS

Our previous study revealed the appropriate conditions for the plaque- forming cell (PFC) assay in rats. Using this assay in the present stud y, dose-response relationships of cytotoxicity, PFC response and histo logy in the spleen were evaluated in rats receiving alkylating agents. Rats were given a single intravenous administration of cyclophosphami de (CY) at a dose of 3, 10 or 30 mg/kg. Spleen weights and cellularity were decreased in the rats treated with 30 mg/kg. Suppressed PFC resp onse was observed in the rats receiving 10 mg/kg or more. In the rats treated with CY at 1, 3 or 10 mg/kg for 7 days, spleen weights and cel lularity and PFC response were reduced at doses of 3 mg/kg or more. Tr eatment with the other alkylating agents, however, had a different con sequence. Namely, in the rats treated with nitromin once or for 7 days , spleen weights and cellularity were decreased at a dose lower than t hat causing a reduction in the PFC response. In the rats treated with melphalan or chlorambucil, the weights and cellularity of the spleen t ended to be decreased at a dose lower than that suppressing the PFC re sponse. Histologically, in the case of CY, the marginal zone was narro w with cellular depletion in the rats receiving 3 mg/kg, whereas littl e change was seen in the periarteriolar lymphoid sheath (PALS). At a d ose of 10 mg/kg, the marginal zone was markedly atrophied and slight a trophy of the PALS was seen. On the other hand, in the rats treated wi th nitromin, a dose-related decrease in the size of the spleen was see n without changes in the tissue architecture. Melphalan caused atrophy of both the marginal zone and the PALS at a dose suppressing PFC resp onse. Regarding the red pulp, the extramedullary hematopoiesis disappe ared with melphalan and nitromin, but not with CY. These results indic ate that the decreases in weights and cellularity and histological cha nges in the spleen caused by the alkylating agents are detectable at t he dose suppressing PFC response except for CY, which has a marked imm unosuppressive action. Furthermore, the observed histological findings in the spleen were characteristic of each alkylating agent.