Peptidoglycan Suppresses Melanoma Growth in a Murine Model with Local Immune Activation

Cutibacterium acnes (C. acnes), a Gram-positive cutaneous commensal, has been reported to have antitumor activity in melanoma, but the responsible bacterial components and innate immune pathways remain unclear. Using 8-week-old female C57BL/6NCrSlc mice bearing syngeneic B16-F10 melanoma, we examined heat-killed Cutibacterium acnes (C. acnes) and bacterial cell-wall components in an early local co-administration/booster regimen. C. acnes was fractionated into soluble and insoluble components, and peptidoglycan (PG) and lipoteichoic acid (LTA) were evaluated for cytokine induction in splenocytes and CD11b⁺ myeloid cells in vitro and for tumor control in vivo. Heat-killed C. acnes suppressed melanoma outgrowth, while the insoluble fraction retained TNF-α-inducing and antitumor activity. Both PG and LTA induced TNF-α production in vitro, whereas only PG significantly suppressed melanoma growth in vivo under the tested conditions. PG treatment was associated with extensive tumor necrosis and increased inflammatory cell infiltration. Pharmacologic inhibition of TLR2 or RIPK2 did not fully reverse PG-mediated tumor suppression. Thus, purified Gram-positive PG demonstrated local antitumor immunomodulatory activity and reproduced several features observed with the C. acnes insoluble fraction in this murine melanoma model.