The immune properties of Manduca sexta transferrin

Transferrins are secreted proteins that bind iron. The well-studied transferrins are mammalian serum transferrin, which is involved in iron transport, and mammalian lactoferrin, which functions as an immune protein. Lactoferrin and lactoferrin-derived peptides have bactericidal activity, and the iron-free form of lactoferrin has bacteriostatic activity due to its ability to sequester iron. Insect transferrin is similar in sequence to both serum transferrin and lactoferrin, and its functions are not well-characterized; however, many studies of insect transferrin indicate that it has some type of immune function. The goal of this study was to determine the specific immune functions of transferrin from Manduca sexta (tobacco hornworm). We verified that transferrin expression is upregulated in response to infection in M. sexta larvae and determined that the concentration of transferrin in hemolymph increases from 2 μM to 10 μM following an immune challenge. It is also present in molting fluid and prepupal midgut fluid, two extracellular fluids with immune capabilities. No immune-induced proteolytic cleavage of transferrin in hemolymph was observed; therefore, M. sexta transferrin does not appear to be a source of antimicrobial peptides. Unlike iron-saturated lactoferrin, iron-saturated transferrin had no detectable antibacterial activity. In contrast, 1 μM iron-free transferrin inhibited bacterial growth, and this inhibition was blocked by supplementing the culture medium with 1 μM iron. Our results suggest that M. sexta transferrin does not have bactericidal activity, but that it does have a bacteriostatic function that depends on its iron sequestering ability. This study supports the hypothesis that insect transferrin participates in an iron withholding strategy to protect insects from infectious bacteria.