Modulation of host resistance to metastasis in the lungs of aged retired breeder mice.

: Pretreatment with a single injection of poly I.C inhibited lung colonization by tumor line 4T07 cells in mice regardless of age. However, preimmunization inhibited lung colonization by intravenously injected 4T07 cells in young mice (10-16 weeks of age) but not in retired breeders (median age greater than 72 weeks). Analysis of kinetics of clearance and expansion of the clonogenic tumor stem cell population revealed that the initial rates of clearance were similar for control young mice and retired breeders, but the subsequent rate of expansion in control retired breeders was half that in young mice. As in young mice, pretreatment of retired breeders with poly I.C increased the initial rate of clearance of intravenously injected 4T07 cells but did not inhibit growth measured between days 1 and 8. Preimmunization, which inhibits growth of neoplastic cells in young mice, did not inhibit expansion of the clonogenic population in the lungs of retired breeders.