The antitumor effect of lentinan in syngeneic and autochthonous tumor-host systems and its suppressive effect on 3-methylcholanthrene (MC)-induced carcinogenesis were confirmed using DBA/2 and SWM/MS hosts. The regressive activity of lentinan against the solid form of Sarcoma 180 was the most effective in DBA/2, SWM/Ms, or A/J mice and less effective in C3H/He or C57BL/6 mice. The growth of a syngeneic MC-induced DBA/2.MC.CS-1 fibrosarcoma (native and trypsinized) was markedly inhibited, and the regression of tumors was detected by the i.p. injection of minute amounts of lentinan into DBA/2 mice, which were the most suitable host in lentinan treatment. When DBA/2 mice were used, lentinan was also effective for even autochthonous primary tumors induced within 15 weeks after MC inoculation, but less effective for tumors induced during the 16 to 36 weeks after MC treatment. Lentinan showed a prominent suppressive effect in MC-induced carcinogenesis using DBA/2 and SWM/Ms mice but no effect when BALB/c, C57BL/6, or C3H/He mice were used. The timing of lentinan administration in the latter result was examined using SWM/Ms mice, and lentinan, when it was given daily for 10 days after the third week of MC inoculation, was strikingly effective (33%), but not so effective (63%) when lentinan was given after the sixth week of MC treatment, compared with tumor-occurrence rate in the control group (88%).

The reason why DBA/2, SWM/Ms, or A/J mice were suitable hosts for lentinan treatment is not clear, but the natural killer capability or phagocytic macrophage function in these strains seems to have no relation to lentinan action, because A/J mice are deficient in natural killer function, and in these strains of mice the phagocytic function of macrophages is weak. It may be quite possible that these strains of mice are most sensitive to delayed-type hypersensitivity and/or cytotoxic T-cell response in which T-cells and lentinan play important roles. The tumor-host systems presented here provide a good model in which lentinan retains an inhibitory capacity in syngeneic and autochthonous hosts, and such a model offers the possibility for further study of the host defense mechanism against cancer.

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