Characterization of Serum Complement Activity in Three Species of Crocodilians from Southeast Mexico

Aims: To characterize the serum complement innate immune system in three species of crocodilians native to southeast Mexico.

Methodology: Plasma collected from three wild crocodilian species native to southeast Mexico were exposed to sheep red blood cells (SRBCs) to measure hemolysis, which is used as an indication of serum complement immune activity.

Results: Incubation of different volumes of plasma from Crocodylus acutus, Crocodylus moreletii, and Caiman crocodilus resulted in a volume-dependent increase in SRBC hemolysis. However, while maximum hemolysis for C. acutus and C. moreletii were both approximately five-fold higher than that of Ca. crocodilus. A kinetic study revealed that the hemolysis was rapid, with near-maximum activity recorded at 30 min for C. acutus and C. moreletii. However, Ca. crocodilus activity exhibited a significant increase (P<.5) only between one and two hours. A thermal analysis showed that the SRBC hemolysis was maximal at temperatures to which these species thermoregulate. The thermal profiles were similar for all three species, although the activity was lower for Ca. crocodilus (P<.01). The SRBC hemolysis was strongly inhibited by mild heat treatment (56°C,30 min) and also by EDTA, indicating that the hemolytic activity was probably due to the presence of crocodilian serum complement activity. The EDTA-inhibited activity was restored by the addition, of a 20mM excess of Ca2+or Mg2+, but not Fe2+orCu2+, thus exhibiting the specific need for Ca2+orMg2+.

Conclusions: The serum complement activities of C. acutus and C. moreletii are much higher (P<.01) than Ca. crocodilus, which may be related to the maximum sizes and increased aggressive natures of C. acutus and C. moreletii.