The ability of BALB/c nude and C57BL/6 mice to eliminate tumor cells from the blood stream was severely impaired after a single inoculation of 0.2 ml of anti-asialo BMI (asGMI) serum, diluted 1:40 to 1:320. The number of i.v.-inoculated YAC-I cells surviving in the lungs of BALB/c nude mice pretreated with anti-asGMI serum was 28 times higher than in the control nude mice. In this respect, nude mice treated with anti-asGMI behaved similarly to beige mice. The increase in the initial survival of tumor cells in the mice that was induced by pre-treatment with anti-asGMI resulted in a substantial increase in the number of artificial lung metastases that developed. In C57BL/6 +/+ mice treated with anti-asGMI and in C57BL/6 beige mice, i.v. inoculation of B16 melanoma cells induced 10 times more metastatic foci in the lungs than in the control C57BL/6 +/+ mice. In contrast, in nude mice which possess higher levels of NK reactivity, metastatic growth was suppressed 7-fold in comparison with intact C57BL/6 +/+ mice. In beige mice and in C57BL/6 +/+ mice treated with anti-asGMI, multiple metastatic foci developed in the liver, whereas in control C57BL/6 +/+ and nude mice, no extrapulmonary metastases were found. These data indicate that B16 melanoma cells are able to grow in the liver, but their growth is ordinarily prevented by NK cells. The antimetastatic defense of C57BL/6 mice treated by anti-asGMI could be restored by transplantation of 40 X 10(6) normal spleen cells. This antimetastatic effect of transplanted spleen cells was mediated by asGMI-bearing cells, since after in vitro pre-treatment of normal spleen cells with anti-asGMI and complement, they lost their ability to inhibit the development of artificial metastases in the lungs of C57BL/6 mice. Suppression of NK reactivity by multiple injections of anti-asGMI (every 4 to 5 days), in C57BL/6 mice inoculated intrafootpad (i.f.p.) with B16 melanoma or 3LL tumor cells, did not influence the growth of local tumors, but dramatically accelerated the development of spontaneous pulmonary metastases. These data demonstrate that NK cells may play an important role in resistance to the dissemination of tumor cells, and therefore contribute to the control of metastasis formation in mice.

Many neoplasms seem to be heterogeneous in nature, producing metastases by pre-existing variant cells with inherent biochemical and biological properties. The survival and proliferation of metastatic cells depend on various biological properties, such as enzymes which degrade basement membranes, resistance to various host defence systems, association with host cellular components, adhesiveness and expression of certain membrane glycoproteins. Recent studies have indicated that metastatic cells may differ from the local tumour cells in the expression of immune recognizable membrane-associated antigens. Such antigenic differences may result from an immunoselection of cells with distinct antigenic properties due to a specific immune response evoked against the local tumour. In view of the role of the major histocompatibility complex (MHC) system in controlling and restricting the function of immune effector cells against modified self-components, one could assume that the modulation of the expression of MHC-encoded antigens on the membrane of tumour cells influenced the interclonal relationship within a local heterogeneous tumour cell population and the subsequent generation of metastasis. The modulation of the expression of H-2 antigens on several murine tumours is well documented; however, practically no attempts were made to relate H-2 modulation with invasiveness. We now describe principal differences in the expression of H-2 parental haplotypes between a local F1 methylcholanthrene-induced tumour and its descendant pulmonary metastases. These results suggest that both the expression and the immunogenicity of MHC products strongly influence the immune relationship between the tumour and the host's immune system, thus determining the generation and dissemination of metastases.

The antimetastatic effects of heparin (40 units) and prostacyclin (PGI2, 100 microgram)1 were investigated in normal mice and in mice with depressed or activated natural killer (NK) cell activity. Both anticoagulants inhibited the formation of lung metastases after inoculation of the FI or F10 sublines of B16 melanoma. Inhibition of NK activity by treatment of mice with anti-asialo GM1 serum abrogated the antimetastatic effects of PGI2 or heparin. Conversely, augmentation of NK-cell activity by poly I:C plus treatment with anticoagulants produced synergistic antimetastatic effects. A similar pattern of results was obtained with heparin treatment of mice challenged with the Madison lung carcinoma (M109), but PGI2 alone or in combination with theophylline had little or no detectable antimetastatic effect on M109 or on the parental B16 melanoma. Studies of the mechanism of the interaction between heparin nd NK cells revealed that the anticoagulant treatment did not affect splenic NK activity in vitro. However, heparin treatment caused a significant increase in the clearance of radiolabelled tumor cells from the lungs of normal mice. Combined treatment of mice with poly I:C and heparin synergistically accelerated the elimination of radiolabelled tumor cells. In contrast, heparin did not affect the clearance of tumor cells from the lungs of mice with depressed NK activity. Thus the antimetastatic effects of heparin and PGI2 are dependent on levels of NK activity in the host. Platelet aggregation and fibrin coating of the surface of tumor cells may be among the mechanisms by which hematogenously spread tumor cells are protected from destruction by NK cells. Anticoagulant drugs may exert antimetastatic effects by making tumor cells more vulnerable to the cytotoxic effects of NK cells, rather than by blocking adherence of tumor cells to vascular endothelium.

Differs significantly from group 1 (p<0.05).

Experiments were carried out relevant to the mechanisms involved in tumor "concomitant immunity". Mice bearing 3LL, 816, EL4 or the methylcholanthrene-induced T-I0 tumors were capable of completely