Oral administration of high molecular mass poly-γ-glutamate induces NK cell-mediated antitumor immunity

We analyzed the in vivo tumor regression activity of high molecular mass poly-γ-glutamate (γ-PGA) from Bacillus subtilis sups. chungkookjang. C57BL/6 mice were orally administered 10-, 100-, or 2000-kDa γ-PGA or β-glucan (positive control), and antitumor immunity was examined. Our results revealed higher levels of NK cell-mediated cytotoxicity and IFN-γ secretion in mice treated with higher molecular mass γ-PGA (2000 kDa) vs those treated with lower molecular mass γ-PGA (10 or 100 kDa) or β-glucan. We then examined the effect of oral administration of 10- or 2000-kDa γ-PGA on protection against B16 tumor challenge in C57BL/6 mice. Mice receiving high molecular mass γ-PGA (2000 kDa) showed significantly smaller tumor sizes following challenge with the MHC class I-down-regulated tumor cell lines, B16 and TC-1 P3 (A15), but not with TC-1 cells, which have normal MHC class I expression. Lastly, we found that γ-PGA-induced antitumor effect was decreased by in vivo depletion of NK cells using mAb PK136 or anti-asialo GM1 Ab, and that was completely blocked in NK cell-deficient B6 beige mice or IFN-γ knockout mice. Taken together, we demonstrated that oral administration of high molecular mass γ-PGA (2000 kDa) generated significant NK cell-mediated antitumor activity in mice bearing MHC class I-deficient tumors.