Abstract
Tumors were induced in adult and newborn rabbits by inoculation of fibroma virus. Whereas tumors completely regressed in adult rabbits by 3 weeks after virus inoculation, newborn rabbits supported tumor growth for 3 to 4 weeks. In the latter case, some animals died at this time, others survived with a gradual regression of the tumors over an additional period of 5 to 6 weeks. Virus neutralization studies demonstrated antibodies to fibroma virus in the serum from both adult and newborn tumor-bearing rabbits. Newborn rabbits with progressively growing tumors failed to elicit a delayed cutaneous hypersensitivity reaction to fibroma antigens, whereas adult rabbits showed strong reactions as early as 7 days after tumor induction. Similarly, macrophage migration inhibition tests revealed that the lymphocytes from newborn rabbits with progressively growing tumors were only weakly reactive to fibroma antigens, as compared to lymphocytes from adult tumorbearing rabbits. In contrast, newborn rabbits that survived and regressed the tumors demonstrated strong cell-mediated immunity both by skin testing and migration inhibition. Virus growth studies in cell culture demonstrated that fibroma was unable to replicate in peritoneal macrophages from either newborn or adult rabbits. No differences were observed in growth of the virus in macrophages from tumor-bearing rabbits. The significance of these observations is discussed in respect of the possible role of cell-mediated immunity in fibroma tumor regression.