It was shown that although intravenous administration of bacterial endotoxin caused extensive hemorrhagic necrosis of four different syngeneic murine tumors, only two of these tumors subsequently underwent complete regression: the two that were shown to be immunogeneic as classically defined. An immunologic basis for endotoxin-induced regression was further indicated by the additional findings that regression, but not hemorrhagic necrosis, of the two immunogenic tumors failed to occur in mice that were immunodepressed by whole-body gamma-irradiation, or that were made T-cell deficient by thymectomy and irradiation. That endotoxin-induced regression is T-cell mediated was suggested by the findings that tumor regression was followed by a state of long-lived immunity to a tumor cell challenge implant, and with the possession by the host of T cells that were capable of passively transferring this state of immunity to normal recipients. It is concluded that although parenteral injection of endotoxin causes hemorrhagic necrosis of most solid murine tumors, it is only those tumors that are immunogenic enough to evoke the generation of T-cell-mediated immunity which subsequently go on to completely regress.

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