Lack of IMCs prevents tumor formation. (A) S100A9 protein in skin of C57BL/6 WT and S100A9KO mice treated with TPA or acetone vehicle. Each lane represents an individual mouse. Splenocytes from a tumor–bearing mouse were used as a positive control (PC). RP, recombinant protein. (B) The number of Gr-1⁺ cells in skin of C57BL/6 mice evaluated by IHC. Mice were treated with TPA or acetone for 4 wk. Each group included four mice. **, P < 0.01. (C) The number of papillomas in WT and S100A9KO C57BL/6 mice induced by DMBA and TPA as indicated. Each group included seven mice. P = 0.003. (D) Number of papillomas in lethally irradiated C57BL/6 mice that received BM from S100A9KO or WT (control) mice. Carcinogen treatment initiated 3 wk after the BM transfer. Each group included four mice. P < 0.001. (E) Number of papillomas in lethally irradiated C57BL/6 WT or S100A9KO mice that received BM from WT mice. Carcinogen treatment initiated 4 wk after the BM transfer. Each group included four mice (P > 0.1). (B–E) Mean and SD are shown.