Increased splenic transplantable colony-forming units related to increased radioresistance after rauscher leukemia virus infection of the SJL/J mouse

Using the exogenous colony-forming unit (CFU) assay of Till and McCulloch, we studied changes in splenic CFU content as a function of time after Rauscher leukemia virus (RLV) infection of the SJL/J mouse, and we related splenic CFU content to previously reported changes in radiosensitivity after infection. Total splenic CFU content [colony-forming potential (CFP)/spleen] first increased after the 6th day of infection, correlating with the appearance of splenomegaly and of radioresistance. CFP/spleen continued to increase until day 22 after infection, reaching 16 times normal levels. This increase was positively correlated with greater radioresistance. Beyond the 22d day of infection, CFP/spleen fell somewhat but was never `9 times normal values. However, calculations based on existing information regarding spleen cell radiation survival and turnover indicated that this level of splenic CFU was sufficient to provide radioprotection at the level of X-ray exposure used. We conclude, from these studies, that RLV infection increased the number of splenic hematopoietic stem cells of the SJL/J mouse and this increase resulted in the greater radioresistance.