Several tissue culture lines of 6C3HED, a murine lymphoma, were more susceptible to immunologic destruction in vivo than the highly virulent 6C3HED line maintained by serial intramuscular transplantation. The attenuated tissue culture cells were rejected by normal syngeneic recipients, but thymectomized mice were unable to reject attenuated cells. In such mice, the growth rate of attenuated cells was equivalent to the growth rate of virulent cells in normal syngeneic mice. The increased susceptibility of attenuated cells to destruction by syngeneic hosts was shown to correlate with decreased production by the tumor cells of a macrophage chemotaxis inhibitor, and not with altered antigen density. In addition, when inhibitor isolated from virulent cells was administered to mice challenged with attenuated cells, the latter cells became virulent in vivo. When attenuated and virulent cells were administered simultaneously in the same host, the attenuated cells were able to develop into progressively growing tumors. The data suggest that the successful growth of neoplastic cells in normal may require tumor cells to produce factors which subvert the ability of the host to mobilize macrophages rapidly at the tumor site.

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