Innate Immunity in the Cottonmouth Watersnake (Agkistrodon piscivorus)

Despite their ecological importance and unique evolutionary history, reptiles remain underrepresented in immunological research. The innate immunity of the cottonmouth (Agkistrodon piscivorus), a semi-aquatic pit viper native to the southeastern United States, was characterized to provide insight into the molecular and cellular mechanisms underlying its first line of defense against pathogens. Plasma collected from wild A. piscivorus exhibited strong antibacterial activities against both Gram-negative and Gram-positive bacteria. In addition, plasma from A. piscivorus showed potent hemolytic activities in unsensitized sheep red blood cell (SRBC) hemolysis assays. This activity was concentration-, time-, and temperature-dependent. In addition, the hemolytic activity was inhibited by mild heat treatment (56 °C, 30 min) of plasma and proteases and also by EDTA, suggesting that the hemolytic activity was due to the presence of serum complement proteins. SDS-PAGE analysis of plasma proteins isolated from a mannan-agarose affinity column revealed the presence of a protein with a mass of 36 kDa, raising the strong possibility that the lectin pathway of complement activation is active. The EC₅₀ for hemolysis of SRBCs by plasma from A. piscivorus was approximately 10-100× lower than that of any other reptilian species described. This is the first study to characterize innate immunity in A. piscivorus.