Sera from Phylogenetically Related Alligators, Crocodiles and Domestic Chickens Exhibit Comparable Anti-Cancer Activity

Background: Crocodilians rarely develop cancer despite long lifespans and continuous exposure to environmental carcinogens, suggesting robust natural anti-tumor defense mechanisms. Methods: We investigated the anti-cancer activity of sera derived from the phylogenetically related species—alligators, crocodiles, and chickens, and studied their underlying immune mechanisms. The anti-tumor activity of alligator serum was tested in murine models of melanoma and lymphoma. Results: Alligator serum (AS) and its (NH4)2SO4-precipitated fraction (ASa) showed rapid and potent cytotoxicity toward multiple murine and human cancer cell lines while sparing non-malignant human cells. Importantly, ASa attenuated melanoma and lymphoma tumor growth in mice. Electrophysiological analyses in PN71 cancer cells treated with ASa revealed rapid membrane depolarization and formation of high-conductance pores consistent with Complement-mediated membrane attack complex (MAC) activity. Proteomic analyses identified the Complement component C5 as a major protein enriched in active fractions, implicating the Complement system in cancer cell killing. Based on phylogenetic similarity of C5, crocodile and chicken sera exhibit alligator-like comparable anti-cancer activity. Mechanistic studies in chicken serum showed that the anti-cancer activity depends on Ca2+ and Mg2+ ions, terminal Complement components (C5–C8), and IgM antibodies that initiate Complement activation. Immunodepletion of IgM from CSa significantly reduced cytotoxicity, whereas purified chicken IgM activated human Complement to induce cancer cell death. Conclusions: These findings identify a conserved IgM–Complement immune mechanism capable of selectively targeting malignant cells. The evolutionary conservation and cross-species functionality of this pathway highlight its potential as a bio-inspired strategy for developing novel Complement-based cancer immunotherapies.