Cellular defense mechanisms in C. capitata: recognition and entrapment of E. coli by hemocytes

Cyclic AMP affects the haemocyte responses of larval Galleria mellonella to selected antigens

Signal transduction of the innate immediate responses of insect haemocytes to foreign matter is rarely considered. Herein using a combination of adenylate cyclase inhibitors and activators and phosphodiesterase inhibitors we determined that cyclic adenosine monophosphate (cAMP) at high levels normally impairs non-self response. Haemocyte contact with glass and bacteria lowered cAMP in vitro. Inactive phosphodiesterases, including type 4, impaired haemocyte reactions in vitro. Using the drugs in vivo to modulate adenylate cyclase and phosphodiesterases altered the total and types of haemocytes. Adenylate cyclase inhibitors and etazolate (a type 4 phosphodiesterase inhibitor) alone produced changes in the haemograms similar to those caused by Bacillus subtilis. Sequential injections of an enzyme modulator followed by B. subtilis impaired bacterial removal due (1) in the case of enzyme inhibitors, to the removal of haemocytes prior to bacterial challenge and (2) in the case of forskolin and IBMX to the shut-down of the haemocytes. Activating adenylate cyclase or inhibiting phosphodiesterase impaired bacterial removal when co-injecting the compounds and bacteria.

Protein kinase A affectsGalleria mellonella(Insecta: Lepidoptera) larval haemocyte non‐self responses

We used the protein kinase A (PKA) specific activator Sp-8-Br-cAMPS and type I inhibitor Rp-8-Br-cAMPS alone and in combination to define the role of PKA in the non-self responses of larval Galleria mellonella haemocytes in vitro and in vivo. Active PKA depressed haemocyte responses whereas PKA inhibition enhanced activities, including bacterial phagocytosis, the number of haemocytes with adherent bacteria, bacterial-induced haemocytic protein release and haemocyte adhesion to slides in vitro, as well as in vivo bacterial removal from the haemolymph. Non-attached haemocytes had more PKA activity than attached haemocytes; therefore, active PKA limited haemocyte response to foreign materials. We found that (i) PKA inhibitor alone induced non-self responses, including haemocyte protein discharge and lowered haemocyte counts in vivo, and induced nodulation; (ii) the enzyme activator produced effects opposite to those of the inhibitor; and (iii) together, the modulators offset each others' effects and influenced haemocyte lysate PKA activity. These findings establish PKA as a mediator of haemocytic non-self responses.

Morphogenesis and cytopathic effects of the Diachasmimorpha longicaudata entomopoxvirus in host haemocytes

The Diachasmimorpha longicaudata entomopoxvirus (DlEPV), the first reported symbiotic entomopoxvirus, occurs in the venom apparatus of D. longicaudata female wasps and is introduced into Anastrepha suspensa larvae during parasitism. The DlEPV 250-300 kb double stranded DNA genome encodes putative proteins having 30 to >60% amino acid identity with poxvirus homologs such as DNA helicase, DNA-dependent RNA polymerase, and the poxvirus-specific rifampicin resistance protein. Although the molecular characterization of DlEPV is progressing, little is known about its morphogenesis in and effects on host haemocytes. This paper describes (1) haemocytes of third instar A. suspensa, (2) DlEPV infection and morphogenesis, and (3) DlEPV-induced changes in haemocytes. A. suspensa third instars have 3-4 haemocyte morphotypes. Dot blots of DNA from infected haemocytes hybridized with a digoxigenin-labeled DlEPV genomic probe as early as 4 h post-parasitism (hpp) and the intensity of the signal increased with time through 40 hpp. Immunofluorescence microscopy localized DlEPV proteins in cytoplasmic (but not nuclear) sites of infected haemocytes, within 24-36 hpp. Electron microscopy confirmed the presence of viral envelopes, immature spheroids with centric nucleoids, budding virus, and extracellular enveloped virus in three haemocyte types, 24-84 hpp and later. Infected haemocytes exhibited blebbing, DNA concatenation, and inability to encapsulate sephadex beads in vitro. These data indicate that DlEPV disrupts the normal function of host haemocytes, thereby insuring the successful development of D. longicaudata offspring and as such should be regarded as a symbiont of the wasp.

Kinases, intracellular calcium, and apolipophorin-III influence the adhesion of larval hemocytes of the lepidopterous insect, Galleria mellonella

Based on the results from the use of selective inhibitors and activators, active protein kinase A, protein tyrosine kinase, and protein kinase C (PKC) isoforms decreased the adhesion of larval Galleria mellonella hemocytes to glass slides. The protein kinase A inhibitor at all concentrations increased granular cell adhesion only whereas protein tyrosine kinase elevated both granular and plasmatocyte attachment at the lowest concentration. Active, Ca(2+)- and lipid-dependent PKC isoforms limited plasmatocyte and granular cell adhesion whereas PKC that was inhibited by selected compounds (with differed modes of PKC inhibition) enhanced hemocyte attachment. The granular cells were more sensitive to the PKC inhibitors than were plasmatocytes. Phospholipase C and its diacylglyceride product were necessary to reduce hemocyte adhesion and maintain PKC activity. Extracellular Ca(2+), possibly transported through L-channels, was required for plasmatocyte attachment. In contrast, lowering the levels of cytosolic Ca(2+) was associated with decreased PKC activity and was required for hemocyte adhesion.

The ectoparasitic wasp Nasonia vitripennis (Walker) (Hymenoptera: Pteromalidae) differentially affects cells mediating the immune response of its flesh fly host, Sarcophaga bullata Parker (Diptera: Sarcophagidae)

In this study, we examined cellular immune responses in the flesh fly, Sarcophaga bullata, when parasitized by the ectoparasitoid Nasonia vitripennis. In unparasitized, young pharate adults and third instar, wandering larvae of S. bullata, four main hemocyte types were identified by light microscopy: plasmatocytes, granular cells, oenocytoids, and pro-hemocytes. Parasitism of young pharate adults had a differential effect on host hemocytes; oenocytoids and pro-hemocytes appeared to be unaltered by parasitism, whereas adhesion and spreading behavior were completely inhibited in plasmatocytes and granular cells by 60 min after oviposition. The suppression of spreading behavior in granular cells lasted the duration of parasitism. Plasmatocytes were found to decline significantly during the first hour after parasitism and this drop was attributed to cell death. Melanization and clotting of host hemolymph did not occur in parasitized flies, or the onset of both events was retarded by several hours in comparison to unparasitized pharate adults. Hemocytes from envenomated flies were altered in nearly identical fashion to that observed for natural parasitism; the total number of circulating hemocytes declined sharply by 60 min post-envenomation, the number of plasmatocytes declined but not granular cells, and the ability of plasmatocytes and granular cells to spread when cultured in vitro was abolished within 1 h. As with parasitized hosts, the decrease in plasmatocytes was due to cell death, and inhibition of spreading lasted until the host died. Isolated crude venom also blocked adhesion and spreading of these hemocyte types in vitro. Thus, it appears that maternally derived venom disrupts host immune responses almost immediately following oviposition and the inhibition is permanent. The possibility that this ectoparasite disables host defenses to afford protection to feeding larvae and adult females is discussed.

Phagocytosis of Escherichia coli by insect hemocytes requires both activation of the Ras/mitogen-activated protein kinase signal transduction pathway for attachment and beta3 integrin for internalization

Insect hemocytes in response to lipopolysaccharide (LPS) of Gram-negative bacteria facilitate binding and internalization of either cell-associated or cell-free LPS (Charalambidis, N. D., Foukas L. C., and Marmaras V. J. (1996) Eur. J. Biochem. 236, 200-206). An early event in LPS signaling in hemocytes involves protein tyrosine phosphorylation (Charalambidis N. D., Zervas C. G., Lambropoulou M., Katsoris P. G., and Marmaras V. J.(1995) Eur. J. Cell Biol. 67, 32-41). Here we report further data of LPS-mediated signal transduction responsible for Escherichia coli phagocytosis. We demonstrate that both adhesion of hemocytes to substrata and LPS stimulation can cause activation of p44(MAPK) in Ceratitis capitata hemocytes but with distinct kinetics indicating different functions. In addition, we showed that Drk, a homolog protein to the mammalian GRB2, is implicated in the transmission of LPS signaling, indicating that the Ras/mitogen-activated protein kinase pathway is involved. Either the cell-free or the cell-associated LPS appears to attach to the hemocyte surface by the same mechanism that is based on the cross-linking of LPS to membrane-associated p47 via the intermediacy of tyrosine derivatives generated by the action of phenol oxidase. By contrast, the cell-free LPS internalization into the hemocytes differs from the cell-associated LPS internalization. For E. coli internalization integrin receptors as well as cytoskeletal rearrangements are required, as judged by inhibition of E. coli internalization in the presence of the RGD peptide, beta3-integrin antibodies, and cytochalasin D.

Characterization of a Monoclonal Antibody Against a 180 kDa Hemocyte Polypeptide Involved in Cellular Defence Reactions of the Stick Insect Bacillus rossius

Defence properties of hemoctyes were investigated using the anti-hemocyte monoclonal antibody BrH1 obtained by immunizing mice with 2% paraformaldehyde-fixed hemoctyes of the stick insect Bacillus rossius. In Western blot analysis, the antibody recognized a 180 kDa antigen in hemocyte cell lysates, whereas fat body lysates and cell-free hemolymph were negative. In immunofluorescence analysis of cultured or freshly collected hemoctyes, BrH1 stained intracellular antigen(s) in detergent-treated cells. Transverse cryosections of adult stick insects probed by immunofluorescence with BrH1 showed in situ the scattered distribution of hemoctyes inside the haemocoel. The antigen(s) recognized by BrH1 appears to be involved in cell defence hemocyte-mediated mechanisms, as evidenced by the fact that cryosections of insects challenged in vivo with yeast cells, bacteria, or polystyrene latex particles and probed with BrH1 showed an accumulation of antigen surrounding the injected stimuli. Copyright 1997 Elsevier Science Ltd. All rights reserved

Biological mediators of insect immunity

Infection in insects stimulates a complex defensive response. Recognition of pathogens may be accomplished by plasma or hemocyte b1p4eins that bind specifically to bacterial or fungal polysaccharides. Several morphologically distinct hemocyte cell types cooperate in the immune response. Hemocytes attach to invading organisms and then isolate them by phagocytosis, by trapping them in hemocyte aggregates called nodules, or by forming an organized multicellular capsule around large parasites. These responses are often accompanied by proteolytic activation of the phenoloxidase zymogen that is present in the hemolymph. A component of insect immune responses to bacteria is the synthesis by fat body and hemocytes of a variety of antibacterial proteins and peptides, which are secreted into the hemolymph. These molecules attack bacteria by several mechanisms. Inducible antifungal proteins have also been recently discovered in insect hemolymph. The promoters for several antibacterial protein genes in insects are regulated by transcription factors similar to those involved in mammalian acute phase responses.

Hemocyte surface phenoloxidase (PO) and immune response to lipopolysaccharide (LPS) in Ceratitis capitata

Bacterial lipopolysaccharide (LPS) attachment at the hemocyte surface is based on the crosslinking of surface associated p47 to LPS, via the intermediacy of tyrosine derivatives generated by the action of phenoloxidase (PO). This attachment is an initial step for LPS internalization from hemocytes (Charalambidis et al., 1996). The results presented clearly show the critical role of hemocyte associated PO activity in the above processes. Biochemical and immunofluorescent analysis demonstrated unambiguously the presence of prophenoloxidase (proPO) on the hemocyte surface. The cell-surface expression of proPO appeared to be LPS-independent, whereas its activation was LPS-dependent. The activation of cell surface proPO involves a limited proteolysis, since upon activation with chymotrypsin proPO is converted to a set of smaller molecular weight proteins with PO activity. The activation appears to be due to enzyme activators, serine proteases, released upon LPS-stimulation. This hypothesis was supported from the activation of membrane proPO by the culture medium of hemocytes which have been triggered with LPS. In addition, proPO, activation was abolished by inhibitors of secretion and PMSF. The release of proPO activators upon LPS-stimulation is mediated via protein tyrosine phosphorylation, as genistein inhibited proPO activation, a situation similar to that reported by us for the release of the effector protein p47 (Charalambidis et al., 1995). The LPS-stimulated activation of cell-surface proPO is a prerequisite for LPS (either cell associated or cell free) internalization, as judged by the resistance of LPS binding to dissociation by proteinase K.

Covalent association of lipopolysaccharide at the hemocyte surface of insects is an initial step for its internalization--Protein-tyrosine phosphorylation requirement

It is well known that lipopolysaccharide (LPS) of Gram-negative bacteria triggers antibacterial responses to mammalian macrophages [Weinstein, S., Gold, M. R. & DeFranco, A. (1991) Proc. Natl Acad. Sci. USA 88, 4148-4152] and insect hemocytes [Charalambidis, N.D., Zervas, C.G., Lambropoulou, M., Katsoris, P.G. & Marmaras, V.J. (1995) Eur J. Cell Biol. 67, 32-41], via protein-tyrosine phosphorylation. In this study we show that insect hemocytes in response to LPS facilitate internalization of LPS (either cell-associated or cell-free). According to our data, the recognition and covalent association of LPS (either cell-associated or cell-free) to the hemocyte surface are essential initial steps for LPS internalization. LPS (Escherichia coli) recognizes membrane effector 47-kDa protein (p47) and then crosslinks to membrane-associated p47 (mp47) via the intermediacy of tyrosine derivatives generated by the action of phenol oxidase, as is the case for cuticular protein-chitin crosslinks during sclerotization [Shaefer, J., Kramer, K.J., Garbow, J.R., Jacob, G.S., Stejskal, E.O., Hopkins, T.L. & Speirs, R.D. (1987) Science 235, 1200-1204]. The covalent association of LPS to the hemocyte surface appears to be a prerequisite for LPS internalization as judged by the resistance of LPS binding to dissociation by proteinase K. In addition, our results show that the effector molecules participating in LPS covalent association at the cell surface and LPS internalization are not involved in LPS-induced activation of hemocytes. However, the fact that genistein, as well as the inhibitors of LPS-dependent secretion, block LPS covalent association at the cell surface and LPS internalization provides a preliminary characterization of an LPS signal-transduction-dependent process which is apparently involved.

Immune response in insects: the role of phenoloxidase in defense reactions in relation to melanization and sclerotization

It is well known that activated prophenoloxidase (proPO) plays an important role in cuticular melanization and sclerotization. In addition, studies dealing with immune response of insects suggest that phenoloxidase (PO) is also critical in the defense reactions of insects against invaders. proPO is activated by elicitors derived from microbial cell wall components such as peptidoglycan, beta-1,3-glucan, and lipopolysaccharide (LPS). According to our recent studies we proposed a model clarifying the role of PO in both cellular and humoral immune responses. LPS triggers Ceratitis capitata hemocytes via induced protein tyrosine phosphorylation to release biologically active molecules, including p47 and proPO-activators. Furthermore, hemocytes in response to LPS facilitate clearance of LPS from the hemocoel of medfly. The effector molecules involved in the LPS clearance are hemocyte surface-associated p47 (mp47), soluble p47 (sp47), activated proPO, and tyrosine. A similar LPS clearance system in the integument of medfly in vitro was also demonstrated. According to our data, the proposed mechanism for LPS clearance from hemocoel and from integument is the crosslinking of LPS to p47 or certain integumental proteins via the intermediacy of reactive tyrosine derivatives generated by PO activity, as is the case for cuticular protein-chitin crosslinks during sclerotization. We also demonstrated that metabolites of the eumelanin biosynthesis and not melanin itself or N-acetyldopamine (NADA), the key precursor of sclerotizing agent, were necessary for the immune responses by hemocytes and integument.