Suppression and recovery of the immune response and CFU potential of mice during and after chronic low-dose-rate gamma-irradiation.

Chronic exposure of mice to 60Co gamma-rays at rates of 1-6 R/hr and 6 R/hr leads to profound immunosuppression, as demonstrated with the Jerne haemolysis-in-agar technique. The immunosuppressive effect is dose-rate dependent. Thus, significant reduction of the number of splenic antibody plaque-forming cells (PFC) to sheep red blood cells is seen after exposure to 1005 R at 1-6 R/hr; at the higher exposure rate of 6 R/hr, PFC suppression is seen after a total dose on only 468 R. Although recovery of the PFC response is possible after accumulated doses of 1005, 1579, and 2138 R at 6 R/hr and 5256 R at 1-6 R/hr, normal immunological capacity had not returned 120 days after irradiation. In contrast the recovery of splenic colony-forming units (CFU), as an expression of the haematological potential of chronically irradiated mice, is found to be much more rapid than the repair of the immune system. Thus, 10 days after irradiation, at a time when little or no recovery in immune competence is apparent, the spleen of the chronically irradiatee animal has a CFU capacity about 50 times greater than has that of a normal, unirradiated mouse. The CFU's in the chronically irradiated spleens were also shown to be functional by virtue of their ability to protect lethally irradiated animals from haemopoietic death.