Human melanins are heteropolymers synthesized by such diverse cells as those comprising portions of the skin, hair, inner ear, brain and retinal epithelium. These multifunctional pigments are derived from a complex series of enzymatic and nonenzymatic reactions initiated by the hydroxylation of l-phenylalanine to l-tyrosine. This reaction is mediated by the enzyme phenylalanine hydroxylase (EC 1.14.16.1), an iron-containing protein that requires the presence of the cofactor (6R)-l-erythro-5,6,7,8-tetrahydrobiopterin. A critical two-step reaction sequence follows involving the hydroxylation of tyrosine to DOPA (monophenolase activity), and the ensuing oxidation of the o-diphenol (diphenolase activity) to o-quinone (dopaquinone). Subsequent oxidative polymerizations of indolequinones yield brown to black eumelanins, whereas similar reactions involving cysteine and glutathione conjugates of dopaquinone form reddish-brown pheomelanins ( Fig. 1). Neuromelanin, which is also a brown-black pigment, apparently is restricted to the substantia nigra pars compacta and certain other regions of the mammalian brain. The pigment is derived in large part from the oxidation of dopamine (i.e. the decarboxylated derivative of DOPA) with a variety of nucleophiles, including thiols derived from glutathione [1][2][3]. Some of the numerous factors influencing pigment biogenesis in mammalian systems include substrate availability, the presence and concentrations of O 2 , metal ions, thiol The synthesis and involvement of H 2 O 2 during the early stages of melanogenesis involving the oxidations of DOPA and dopamine (diphenolase activity) were established by two sensitive and specific electrochemical detection systems. Catalase-treated reaction mixtures showed diminished rates of H 2 O 2 production during the autoxidation and tyrosinase-mediated oxidation of both diphenols. Inhibition studies with the radical scavenger resveratrol revealed the involvement in these reactions of additional reactive intermediate of oxygen (ROI), one of which appears to be superoxide anion. There was no evidence to suggest that H 2 O 2 or any other ROI was produced during the tyrosinase-mediated conversion of tyrosine to DOPA (monophenolase activity). Establishing by electrochemical methods the endogenous production H 2 O 2 in real time confirms recent reports, based in large part on the use of exogenous H 2 O 2 , that tyrosinase can manifest both catalase and peroxidase activities. The detection of ROI in tyrosinase-mediated in vitro reactions provides evidence for sequential univalent reductions of O 2 , most likely occurring at the enzyme active site copper. Collectively, these observations focus attention on the possible involvement of peroxidase-H 2 O 2 systems and related ROI-mediated reactions in promoting melanocytotoxic and melanoprotective processes.

Aim of this study was to investigate relationships between the red palm weevil (RPW) Rhynchophorus ferrugineus (Olivier) and the entomopathogenic nematode Steinernema carpocapsae (EPN); particularly, the work was focused on the immune response of the insect host in naive larvae and after infection with the EPN. Two main immunological processes have been addressed: the activity and modulation of host prophenoloxidase-phenoloxidase (proPO) system, involved in melanization of not-self and hemocytes recognition processes responsible for not-self encapsulation. Moreover, immune depressive and immune evasive strategies of the parasite have been investigated. Our results suggest that RPW possess an efficient immune system, however in the early phase of infection, S. carpocapsae induces a strong inhibition of the host proPO system. In addition, host cell-mediated mechanisms of encapsulation, are completely avoided by the parasite, the elusive strategies of S. carpocapsae seem to be related to the structure of its body-surface, since induced alterations of the parasite cuticle resulted in the loss of its mimetic properties. S. carpocapsae before the release of its symbiotic bacteria, depress and elude RPW immune defenses, with the aim to arrange a favorable environment for its bacteria responsible of the septicemic death of the insect target.

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Entomopathogenic nematodes (EPNs) are widely used as biological control agents against insect pests, the efficacy of these organisms strongly depends on the balance between the parasitic strategies and the immune response of the host. This review summarizes roles and relationships between insect hosts and two well-known EPN species, Steinernema feltiae and Steinernema carpocapsae and outlines the main mechanisms of immune recognition and defense of insects. Analyzing information and findings about these EPNs, it is clear that these two species use shared immunosuppression strategies, mainly mediated by their symbiotic bacteria, but there are differences in both the mechanism of evasion and interference of the two nematodes with the insect host immune pathways. Based on published data, S. feltiae takes advantage of the cross reaction between its body surface and some host functional proteins, to inhibit defensive processes; otherwise, secretion/excretion products from S. carpocapsae seem to be the main nematode components responsible for the host immunosuppression.

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Glycopeptide antibiotics (GPAs) are drugs of last resort for treating infections by Gram-positive bacteria. They inhibit bacterial cell wall assembly by binding to the d-Ala-d-Ala terminus of peptidoglycan precursors, leading to cell lysis. Vancomycin and teicoplanin are first generation GPAs, while dalbavancin is one of the few, recently approved, second generation GPAs. In this paper, we developed an in vivo insect model to compare, for the first time, the efficacy of these three GPAs in curing Staphylococcus aureus infection. Differently from previous reports, Bombyx mori larvae were reared at 37 °C, and the course of infection was monitored, following not only larval survival, but also bacterial load in the insect body, hemocyte activity, phenoloxidase activity, and antimicrobial peptide expression. We demonstrated that the injection of S. aureus into the hemolymph of B. mori larvae led to a marked reduction of their survival rate within 24–48 h. GPAs were not toxic to the larvae and cured S. aureus infection. Dalbavancin was more effective than first generation GPAs. Due to its great advantages (i.e., easy and safe handling, low rearing costs, low antibiotic amount needed for the tests, no restrictions imposed by ethical and regulatory issues), this silkworm infection model could be introduced in preclinical phases—prior to the use of mice—accelerating the discovery/development rate of novel GPAs.

Entomopathogenic nematodes have been proposed as biological agents for the control of Drosophila suzukii, an invasive pest of small-stone and soft-skinned fruits. Larvae of the fly are susceptible to Steinernema carpocapsae infection but the reaction of immune defenses of the host are unknown. To determine the immune response, larvae were infected with S. carpocapsae and Xenorhabdus nematophila to evaluate the effector mechanisms of both humoral and cellular processes. The symbiont bacteria presented an inhibitory effect on the phenoloxidase cascade with a low level of melanization. Besides, X. nematophila activated the synthesis of putative antimicrobial peptides on the hemolymph of infected larvae. However, those peptides presented a lower antimicrobial activity compared to hemolymph from larvae infected with non-symbiont bacteria. Xenorhabdus nematophila avoided also the phagocytosis response of hemocytes. During in vitro and in vivo assays, S. carpocapsae was not encapsulated by cells, unless the cuticle was damaged with a lipase-treatment. Hemocyte counts confirmed differentiation of lamellocytes in the early phase of infection despite the unrecognition of the nematodes. Both X. nematophila and S. carpocapsae avoided the cellular defenses of D. suzukii larvae and depressed the humoral response. These results confirmed the potential of entomopathogenic nematodes to control D. suzukii.