Insect Immunity

Abstract

Two novel antimicrobial peptides, which we propose to name termicin and spinigerin, have been isolated from the fungus-growing termite Pseudacanthotermes spiniger (heterometabole insect, Isoptera). Termicin is a 36-amino acid residue antifungal peptide, with six cysteines arranged in a disulfide array similar to that of insect defensins. In contrast to most insect defensins, termicin is C-terminally amidated. Spinigerin consists of 25 amino acids and is devoid of cysteines. It is active against bacteria and fungi. Termicin and spinigerin show no obvious sequence similarities with other peptides. Termicin is constitutively present in hemocyte granules and in salivary glands. The presence of termicin and spinigerin in unchallenged termites contrasts with observations in evolutionary recent insects or insects undergoing complete metamorphosis, in which antimicrobial peptides are induced in the fat body and released into the hemolymph after septic injury.The termite species Pseudacanthotermes spiniger, like the other members of the Macrotermitinae, depends for its nutrition on a symbiotic fungus of the basidiomycete genus Termitomyces (1). This fungus grows inside the nests of the termites on piles of fecal pellets; it predigests the lignocellulosic substances and is responsible for food supply in cellulases. Macrotermitinae also live in symbiosis with anaerobic bacteria present in their posterior gut, which are in part responsible for the complete digestion of cellulose. In this environment inhabited with microorganisms, how do termites protect themselves?The first line of defense of insects against pathogens is the cuticle. Once this barrier has been breached, their defense reactions rely both on cellular and humoral mechanisms. The cellular aspects include phagocytosis and encapsulation of invading microorganisms (for a review, see Ref.2). The humoral facet involves the activation of proteolytic cascades leading to melanization and coagulation. In the evolutionary recent insect orders, the best characterized aspect of the humoral immune response is the rapid synthesis of antimicrobial peptides/ polypeptides by the fat body and certain blood cells and release of these factors into the hemolymph after bacterial challenge. Since the first report of an inducible antibacterial peptide from an insect, cecropin from the moth Hyalophora cecropia (3), more than 200 antimicrobial peptides/polypeptides have been characterized in insects. On the basis of their structural features, the peptides are classified into three classes: (i) linear peptides, devoid of cysteines and forming ␣-helices (the prototype of this family are the insect cecropins (4)), (ii) peptides with an overrepresentation in proline and/or glycine residues, and (iii) open-ended cyclic peptides containing cysteine residues (5). Cecropins, proline-rich, and glycine-rich peptides are essentially active against Gram-negative cells, but their activity spectrum sometimes includes Gram-positive bacteria as targets (6). Two recent reports have also indicated that c...

Spheniscins, Avian β-Defensins in Preserved Stomach Contents of the King Penguin, Aptenodytes patagonicus

“…The inhibition of A. fumigatus sporulation by spheniscin is similar to that described for stomoxin, an AMP from the anterior midgut of the blood-feeding insect Stomoxys calcitrans (25), and for termicin, an antifungal defensin from the termite Pseudacanthotermes spiniger (26). A. fumigatus is the most frequently isolated strain in invasive aspergillosis found in immunocompromised humans and animals.…”

Gambicin: A novel immune responsive antimicrobial peptide from the malaria vector Anopheles gambiae

“…The continuous production and secretion of active gambicin in the immune-competent cell lines, and its rapid transcriptional up-regulation on infection, indicate that gambicin is an acute phase response component of the mosquito innate immune system. This small protein exhibits a wide spectrum of antimicrobial activities, with highest potency against a Gram-negative bacterium, significant activity against a Gram-positive bacterium, and a morphogenic effect against a filamentous fungus similar to that described for the cysteine-rich termicin (22) and plant defensins (43). The observed enhancement of ookinete lethality by gambicin is modest but intriguing.…”

“…Purified gambicin (3 g) was treated with thermolysin (Boehringer Mannheim) at an enzyme͞substrate ratio of 1:3 as described (22). Digestion products were separated by microbore RP-HPLC.…”

Insect proteins as a potential source of antimicrobial peptides in livestock production. A review

“…Further, a constitutive production of AMPs has been observed, involving the storage of AMPs in particularly haemocytes and their release into the haemolymph after immune challenge (Lamberty et al, 2001).…”