A complete natural IgM antibody repertoire is required for antitumor immunity. (A) The scatter plot displays the number of urethane-induced tumors observed per mouse in WT, IghelMD4 Tg, CCR2−/−, and Batf3−/− mice after 6 months post–urethane treatment. (B) At 16 days after intravenous injection of B16F10, melanoma cell line in WT, IghelMD4 Tg, CCR2−/−, and Batf3−/− mice was examined for melanoma tumor development. Left: pictures illustrate metastatic melanoma on the surface of whole lungs. Right: scatter plot displays the number of surface metastatic tumors developed per mouse. Circles indicate pure KO mouse, and squares indicate WT recipient mice reconstituted with 100% BM from the indicated KO mouse. Data combine three to five independent experiments. ****P < 0.0001.