Neonatal thymectomy of DBA/2 mice or depiction of thymic cells by the administration of cortisone acetate enhanced the growth of both syngeneic tumors P 1534 and I. 1210. The capacity of the mice to delay the growth of the syngeneic P 1534 was restored by syngeneic neonatal thymic grafts or by syngeneic neonatal thymus placed in Millipore envelopes. The aqueous phase of P-1534 tumor, extracted with Genetron 113, was antigenic in the syngeneic host and showed the presence of a unique antigen not present in the normal tissue extract. Small doses (20 μg) of protein nitrogen of the tumor extract in Freund's adjuvant delayed the tumor growth, whereas large doses (600 μg) enhanced the growth. Node cells and sera from mice immunized with P-1534 extract given to syngeneic recipients showed tumor resistance and enhancement, respectively. The specificity of immunization was shown by failure of the normal DBA/2 mouse tissue, extracted in the same way, to induce resistance or enhancement. Low titers of P-1534 antibodies were found in those mice that showed delayed tumor growth. The negative cytotoxicity test in vitro indicates that those antibodies probably do not have an active part in such resistance. High titers of P-1534 antibodies were related to tumor enhancement. Those titers decreased after tumor growth, suggestive of a possible coating of the tumor by the antibody. Tumor antibodies could be detected only by the passive hemagglutination technique or by passive cutaneous anaphylaxis in guinea pigs. Antibodies could not be detected by cytotoxicity in vitro or by neutralization tests. © 1969 S. Karger AG, Basel.
