Iron withholding as an innate immune mechanism in the American alligator (Alligator mississippiensis)

Abstract

Pathogenic microbes require Fe and Zn for growth and proliferation. Upon infection, microbes produce proteins, called sidephores, designed to strip serum divalent metals away from host proteins. Higher vertebrates respond to infection by increasing the expression of proteins that sequester serum iron away from bacteria. As a result, host plasma Fe levels decrease during the initial phases of infection. This study was conducted to determine if the American alligator, an ancient reptile, exhibits the same innate immune mechanism to protect against in vivo microbial proliferation. Intraperitoneal injection of juvenile captive alligators with bacterial lipopolysaccharide (LPS) resulted in a time-dependent decrease in plasma Fe, as determined by inductively coupled plasma emission spectroscopy. Plasma Fe levels decreased by 5.9, 10.6, and 18.6% relative to untreated control levels at 3, 6, and 12 hr post-injection, respectively, and remained decreased by 12.0% at 48 hr. Alligators injected with pyrogen-free saline did not exhibit statistically significant changes in plasma Fe concentrations at any time point observed. In contrast, serum Zn and Cu remained unchanged relative to untreated controls. To insure that the decreases in plasma Fe were not due to the repeated blood collections during the course of the kinetic study, another experiment was conducted in which plasma metals were measured at 24 hr post-injection. Once again, plasma Fe was reduced by 30.2%, whereas Zn and Cu did not exhibit appreciable changes. These results show that alligators exhibit low plasma Fe levels during an inflammatory response induced by bacterial lipopolysaccharide.

Serum Complement Activity in Two Species of Divergent Central African Crocodiles

“…Some crocodilian species are known to exhibit potent and broad-acting serum complement activities and have been shown to demonstrate other effective innate immune components including serum phospholipase A 2 activity [12,22,23], the production of superoxide [24], and iron withholding [14]. However, at least some components of acquired immunity, including the presence of well developed B-and T-lymphocytes, seem to be less developed than those of mammalians [25].…”

Constitutive Innate Immunity and Systemic Responses to Infection of the American Alligator (Alligator mississippiensis)

“…American alligators ( Alligator mississippiensis ) have potent and complex innate immune systems that include phospholipase A 2 [ 12 ], dipeptidyl peptidase [ 13 ], and serum complement activities [ 14 ]. Furthermore, alligators utilize iron withholding [ 15 ] and behavioral febrile responses [ 16 ] to combat infection. However, these animals have less complex immune systems than endothermic vertebrates [ 17 ].…”