Follow-up of protein release from Pseudoplusia includens hemocytes: a first step toward identification of factors mediating encapsulation in insects

Identification and Functional Characterization of Toxoneuron nigriceps Ovarian Proteins Involved in the Early Suppression of Host Immune Response

The endophagous parasitoid Toxoneuron nigriceps (Viereck) (Hymenoptera, Braconidae) of the larval stages of the tobacco budworm Heliothis virescens (Fabricius) (Lepidoptera, Noctuidae) injects the egg, the venom, the calyx fluid, which includes a Polydnavirus (T. nigriceps BracoVirus: TnBV) and the Ovarian Proteins (OPs) into the host body during oviposition. The host metabolism and immune system are disrupted prematurely shortly after parasitization by the combined action of the TnBV, venom, and OPs. OPs are involved in the early suppression of host immune response, before TnBV infects and expresses its genes in the host tissues. In this work, we evaluated the effect of HPLC fractions deriving from in toto OPs. Two fractions caused a reduction in hemocyte viability and were subsequently tested to detect changes in hemocyte morphology and functionality. The two fractions provoked severe oxidative stress and actin cytoskeleton disruption, which might explain the high rate of hemocyte mortality, loss of hemocyte functioning, and hence the host’s reduced hemocyte encapsulation ability. Moreover, through a transcriptome and proteomic approach we identify the proteins of the two fractions: eight proteins were identified that might be involved in the observed host hemocyte changes. Our findings will contribute to a better understanding of the secreted ovarian components and their role in parasitoid wasp strategy for evading host immune responses.

Expression and function of cathepsin B-like proteinase in larval hemocytes of Helicoverpa armigera during metamorphosis

Previous work has revealed that Helicoverpa armigera cathepsin B-like proteinase (HCB) is expressed in oocytes as well as fat bodies of pupae and adults. It plays key roles in the degradation of yolk proteins during embryogenesis and the decomposition of adult fat bodies of H. armigera. This study investigated the expression and function of HCB in larval hemocytes during larva-pupa metamorphosis. Results showed that the expression of HCB in hemocytes exhibited developmental stage specificity. No HCB was found in hemocytes from 5th-molting larvae. On the contrary, HCB was highly transcribed in the hemocytes from 6th-48-h larvae. Besides, it was abundantly translated in 6th-96-h larvae (prepupation). HCB is mainly expressed in plasmatocytes and granulocytes at both transcriptional and translational levels. The number of plasmatocytes and granulocytes markedly increased before pupation. In addition, hemocytes distributed in hematopoietic organs at early larval stage, then migrated to midgut and fat bodies that would undergo histolysis at later larval stage. These findings suggested that HCB is expressed in H. armigera larval hemocytes and involved in larva-pupa metamorphosis.

A review of insect stem cell types

Insect stem cells have been described from both embryonic and adult tissues from a diversity of insect species, although much of the focus in insect stem cell research has been on Drosophila. Insects are a vast and diverse group and it is surprising that a critical aspect of their development like stem cells has not received more attention. In this review we discuss the current state of knowledge of insect stem cell types. We examine what stem cell types have been identified from insects, and briefly discuss what is known about their regulation.

Interaction between non-specific electrostatic forces and humoral factors in haemocyte attachment and encapsulation in the edible cockle, Cerastoderma edule

In invertebrates, encapsulation is the common immune defence reaction towards foreign bodies, including multicellular parasites, which enter the haemocoel and are too large to be phagocytosed. This immune response has been most extensively studied in insects, in which it is highly complex, involving a diversity of cellular and molecular processes, but little is known of this process in bivalve molluscs. Non-specific physicochemical properties are known to influence parasite-haemocyte interactions in many invertebrates, and these may provide the common basis of encapsulation on which highly specific biochemical interactions are imposed. The present study uses synthetic beads and thread to mimic inactive metacercarial cysts of trematodes, and thus investigates factors involved in the basic, non-specific mechanisms of cell attachment and encapsulation in the edible cockle, Cerastoderma edule. Results showed that positively charged targets stimulated the most vigorous response, and further detailed experiments revealed that non-specific electrostatic forces and humoral plasma factors have a synergistic role in haemocyte attachment and the encapsulation response of C. edule.

Neuroglian-positive plasmatocytes of Manduca sexta and the initiation of hemocyte attachment to foreign surfaces

Observations of hemocyte aggregation on abiotic surfaces suggested that certain plasmatocytes from larvae of Manduca sexta act as foci for hemocyte aggregation. To establish how these particular plasmatocytes form initial attachments to foreign surfaces, they were cultured separately from other selected populations of hemocytes. While all circulating plasmatocytes immunolabel with anti-beta-integrin monoclonal antibody (MAb), only these larger plasmatocytes immunolabel with a MAb to the adhesion protein neuroglian. Neuroglian-negative plasmatocytes and granular cells that have been magnetically segregated from the majority of granular cells adhere to each other but fail to adhere to foreign substrata; by contrast, neuroglian-positive plasmatocytes that segregate with most granular cells adhere firmly to a substratum. Hemocytes form stable aggregates around the large, neuroglian-positive plasmatocytes. However, if neuroglian-positive plasmatocytes are separated from most granular cells, attachment of these plasmatocytes to foreign surfaces is suppressed.

Clustering of adhesion receptors following exposure of insect blood cells to foreign surfaces

Cell-mediated immune responses of insects involve interactions of two main classes of blood cells (hemocytes) known as granular cells and plasmatocytes. In response to a foreign surface, these hemocytes suddenly transform from circulating, non-adherent cells to cells that interact and adhere to each other and the foreign surface. This report presents evidence that during this adhesive transformation the extracellular matrix (ECM) proteins lacunin and a ligand for peanut agglutinin (PNA) lectin are released by granular cells and bind to surfaces of both granular cells and plasmatocytes. ECM protein co-localizes on cell surfaces with the adhesive receptors integrin and neuroglian, a member of the immunoglobulin superfamily. The ECM protein(s) secreted by granular cells are hypothesized to interact with adhesion receptors such as neuroglian and integrin by cross linking and clustering them on hemocyte surfaces. This clustering of receptors is known to enhance the adhesiveness (avidity) of interacting mammalian immune cells. The formation of ring-shaped clusters of these adhesion receptors on surfaces of insect immune cells represents an evolutionary antecedent of the mammalian immunological synapse.

Embryonic origins of the two main classes of hemocytes--granular cells and plasmatocytes--in Manduca sexta

Cell-mediated responses of the moth immune system involve the interaction of two main classes of hemocytes-granular cells and plasmatocytes. During embryogenesis, granular cells arise much earlier than plasmatocytes, and the presence of granular cells is closely coupled with the formation of basal laminae that line the hemocoel occupied by hemocytes. Although epithelial cells contribute the large extracellular matrix protein lacunin to embryonic matrices before granular cells begin contributing this protein to basal laminae, the spatial pattern of lacunin expression in early embryos parallels the later distribution of granular cells over surfaces of basal laminae. Plasmatocytes arise late in embryogenesis, after the cessation of the major morphogenetic movements and the establishment of intact basal laminae. Granular cells are intimately involved with remodeling of basal laminae, and disruptions in the structure of basal laminae can trigger an autoimmune response of granular cells and plasmatocytes. By arising after basal laminae have been molded and remodeled by granular cells, plasmatocytes presumably do not encounter the cues that trigger their aggregation and an autoimmune response.

RNA interference silences Microplitis demolitor bracovirus genes and implicates glc1.8 in disruption of adhesion in infected host cells

The family Polydnaviridae consists of ds-DNA viruses that are symbiotically associated with certain parasitoid wasps. PDVs are transmitted vertically but also are injected by wasps into hosts where they cause several physiological alterations including immunosuppression. The PDV genes responsible for mediating immunosuppression and other host alterations remain poorly characterized in large measure because viral mutants cannot be produced to study gene function. Here we report the use of RNA interference (RNAi) to specifically silence the glc1.8 and egf1.0 genes from Microplitis demolitor bracovirus (MdBV) in High Five cells derived from the lepidopteran Trichoplusia ni. Dose-response studies indicated that MdBV infects High Five cells and blocks the ability of these cells to adhere to culture plates. This response was very similar to what occurs in two classes of hemocytes, granular cells, and plasmatocytes, after infection by MdBV. Screening of monoclonal antibody (mAb) markers that distinguish different classes of lepidopteran hemocytes indicated that High Five cells cross-react with three mAbs that recognize granular cells from T. ni. Double-stranded RNA (dsRNA) complementary to glc1.8 specifically silenced glc1.8 expression and rescued the adhesive phenotype of High Five cells. Reciprocally, dsRNA complementary to egf1.0 silenced egf1.0 expression but had no effect on adhesion. The simplicity and potency of RNAi could be extremely useful for analysis of other PDV genes.

Insect hemocytes and their role in immunity

The innate immune system of insects is divided into humoral and cellular defense responses. Humoral defenses include antimicrobial peptides, the cascades that regulate coagulation and melanization of hemolymph, and the production of reactive intermediates of oxygen and nitrogen. Cellular defenses refer to hemocyte-mediated responses like phagocytosis and encapsulation. In this review, we discuss the cellular immune responses of insects with emphasis on studies in Lepidoptera and Diptera. Insect hemocytes originate from mesodermally derived stem cells that differentiate into specific lineages identified by morphology, function, and molecular markers. In Lepidoptera, most cellular defense responses involve granular cells and plasmatocytes, whereas in Drosophila they involve primarily plasmatocytes and lamellocytes. Insect hemocytes recognize a variety of foreign targets as well as alterations to self. Both humoral and cell surface receptors are involved in these recognition events. Once a target is recognized as foreign, hemocyte-mediated defense responses are regulated by signaling factors and effector molecules that control cell adhesion and cytotoxicity. Several lines of evidence indicate that humoral and cellular defense responses are well-coordinated with one another. Cross-talk between the immune and nervous system may also play a role in regulating inflammation-like responses in insects during infection.

Influence of calcium on Manduca sexta plasmatocyte spreading and network formation

Plasmatocytes are a class of insect hemocytes important in the cellular defense response. In some species, they are phagocytic, protecting the insect from smaller pathogens. In many insects, they work in concert with other hemocytes (particularly other plasmatocytes and granular cells) to form nodules and to encapsulate foreign material. To perform these functions, plasmatocytes attach to, spread on, and surround suitable targets. Because of their importance, because we had previously observed that prolonged incubation of hemocytes in solutions containing the divalent cation chelator ethylenediaminetetraacetic acid (EDTA) inhibited plasmatocyte spreading, and because of the importance of divalent cations in many immune-related functions, we investigated the effect of calcium and magnesium on spreading of plasmatocytes from fifth instar Manduca sexta larvae. On glass slides, plasmatocytes spread more quickly and elongated in Grace's medium containing 5 mM calcium, compared to calcium-free medium. In the presence of calcium, plasmatocyte adhesion, spreading, and network formation were not visibly different in magnesium-free and magnesium-containing Grace's medium. Using immunomicroscopy with a monoclonal antibody specific for plasmatocytes, we measured the length and width of plasmatocytes incubated with several different concentrations of calcium. Plasmatocyte length positively correlated with calcium concentration to 5 mM (maximum concentration tested and approximately the hemolymph concentration). Mean plasmatocyte width was less in 0 and 5 mM calcium than in 0.05 or 0.5 mM calcium. On plastic, hemocytes survived longer than on glass (they survived beyond 24 h) and, in 5 mM calcium, formed an extensive network readily visible by phase-contrast microscopy. This network was never as extensive in the absence of calcium. Network formation in the absence of magnesium, but presence of calcium, resembled network formation in standard Grace's medium.

Parasitism of Lacanobia oleracea (lepidoptera) by the ectoparasitic wasp, Eulophus pennicornis, disrupts the cytoskeleton of host haemocytes and suppresses encapsulation in vivo

Parasitism of Lacanobia oleracea larvae by the ectoparasitic wasp Eulophus pennicornis suppressed host haemocyte-mediated encapsulation of Sephadex DEAE A-25 beads in vivo. Beads dissected out of parasitized larvae had fewer haemocytes associated with them. Moreover, those haemocytes that were associated with the beads tended to retain a rounded configuration and rarely flattened. Similar results were obtained using in vitro encapsulation assays. SDS PAGE indicated that for parasitized and PBS injected larvae, there were some differences in the plasma proteins that bound to Sephadex DEAE A-25 beads, suggesting that parasitism-mediated changes to host plasma proteins might contribute to the differences in the encapsulation response occurring in these larvae. However, in vitro encapsulation assays using beads that had been pre-incubated in plasma from parasitized and unparasitized larvae, demonstrated that major differences in the extent of encapsulation did not occur. These results, plus in vitro haemocyte attachment and spreading assays, suggest that parasitism-mediated suppression of encapsulation is primarily due to reductions in the ability of host haemocytes to attach to (i.e., recognize) and flatten over non-self surfaces and other haemocytes. This proposal is corroborated by staining of actin in the haemocyte cytoskeleton by FITC-labelled phalloidin, which indicated that parasitism disrupts the formation of stress fibers and focal adhesions in plasmatocytes. By contrast, experimental injection of adult female wasp venom into unparasitized L. oleracea larvae had no significant effect on in vivo encapsulation responses or the haemocyte cytoskeleton. Arch. Insect Biochem. Physiol. 49:108-124, 2002. Published 2002 Wiley-Liss, Inc.

Surface characteristics of foreign targets that elicit an encapsulation response by the moth Pseudoplusia includens

Hemocytes from the moth Pseudoplusia includens encapsulate a variety of biotic and abiotic targets. Prior studies indicated that granular cells are usually the first hemocyte type to attach to foreign targets. Thereafter, large numbers of plasmatocytes attach to the target and form a capsule. To identify surface features that induce an encapsulation response, chromatography beads that differed in matrix composition, charge, and functional groups were tested using in vitro and in vivo bioassays. We first conducted in vitro assays using hemocytes with no plasma components present. These experiments indicated that bead types having sulfonic, diethylaminoethyl, and quaternary amine functional groups were encapsulated significantly more often than beads with other functional groups. Charge also significantly affected encapsulation with positively charged beads being encapsulated more often than negatively charged or neutral beads. In vitro assays using purified populations of hemocytes confirmed that these targets were recognized as foreign by granular cells, and that plasmatocytes only formed capsules after granular cells attached to the target. Bead types that were encapsulated under these in vitro conditions were always rapidly encapsulated when injected into P. includens larvae. However, some bead types, like CM-Sephadex, not encapsulated in vitro were encapsulated in vivo if left in the insect hemocoel for a longer period of time (ca. 24 h). Purified plasmatocytes encapsulated these beads in vitro if they were preincubated in plasma. Basic characterization studies suggest these humoral recognition molecules are proteins or small peptides. Comparative studies with other species of noctuid moths also indicated that encapsulation of some bead types differed significantly among species. Collectively, these results reveal that P. includens recognizes some targets as foreign by pattern recognition receptors on granular cells, whereas others are recognized by pattern recognition molecules in plasma. The binding affinities of these recognition molecules also appear to differ among closely related species of Lepidoptera.

Alanine-scanning mutagenesis of plasmatocyte spreading peptide identifies critical residues for biological activity

Plasmatocyte spreading peptide (PSP) is a 23-amino acid cytokine that induces a class of insect immune cells called plasmatocytes to spread on foreign surfaces. The structure of PSP consists of a disordered N terminus (residues 1-6) and a well-defined core (residues 7-23) stabilized by a disulfide bridge between Cys(7) and Cys(19), hydrophobic interactions, and a short beta-hairpin. Structural comparisons also indicate that the core region of PSP adopts an epidermal growth factor (EGF)-like fold very similar to the C-terminal subdomain of EGF-like module 5 of thrombomodulin. To identify residues important for plasmatocyte spreading activity, we bioassayed PSP mutants in which amino acids were either replaced with alanine or deleted. Within the well-defined core of PSP, alanine replacement of Cys(7) and Cys(19) (C7.19A) eliminated all activity. Alanine replacement of Arg(13) reduced activity approximately 1000-fold in comparison to wild-type PSP, whereas replacement of the other charged residues (Asp(16), Arg(18), Lys(20)) surrounding Cys(19) diminished activity to a lesser degree. The point mutants Y11A, T14A, T22A, and F23A had activity identical or only slightly reduced to that of wild-type PSP. The mutant PSP-(7-23) lacked the entire unstructured domain of PSP and was found to have no plasmatocyte spreading activity. Surprisingly, E1A and N2A had higher activity than wild-type PSP, but F3A had almost no activity. We thus concluded that the lack of activity for PSP-(7-23) was largely due to the critical importance of Phe(3). To determine whether reductions in activity correlated with alterations in tertiary structure, we compared the C7.19A, R13A, R18A, and F3A mutants to wild-type PSP by NMR spectroscopy. As expected, the simultaneous replacement of Cys(7) and Cys(19) profoundly affected tertiary structure, but the R13A, R18A, and F3A mutants did not differ from wild-type PSP. Collectively, these results indicate that residues in both the unstructured and structured domains of PSP are required for plasmatocyte-spreading activity.

Innate immunity and its evasion and suppression by hymenopteran endoparasitoids

Recent studies suggest that insects use pattern recognition molecules to distinguish prokaryotic pathogens and fungi from "self" structures. Less understood is how the innate immune system of insects recognizes endoparasitic Hymenoptera and other eukaryotic invaders as foreign. Here we discuss candidate recognition factors and the strategies used by parasitoids to overcome host defense responses. We suggest that host-parasitoid systems are important experimental models for studying how the innate immune system of insects recognizes foreign invaders that are phylogenetically more closely related to their hosts. The strategies used by parasitoids suggest that insects may employ "hidden-self" recognition molecules for attacking foreign objects intruding the open circulatory system. BioEssays 23:344-351, 2001.

Plasmatocytes from the moth Pseudoplusia includens induce apoptosis of granular cells

The primary immune response toward internal parasites and other large foreign objects that enter the insect hemocoel is encapsulation. Prior studies indicated that granular cells and plasmatocytes are the two hemocyte types required for capsule formation by the moth Pseudoplusia includens (Lepidoptera: Noctuidae). Capsules formed by P. includens also have a defined architecture with primarily granular cells attaching directly to the target, multiple layers of plasmatocytes adhering to this inner layer of granular cells, and a monolayer of granular cells attaching to the capsule periphery. Dye-exclusion assays indicated that granular cells die shortly after attaching to the capsule periphery, leaving a basal lamina-like layer around the capsule. In examining the mechanisms underlying granular cell death, we found that culture medium preconditioned by plasmatocytes induced apoptosis of granular cells. Characteristics of plasmatocyte-induced apoptosis included condensation of chromatin, cell surface blebbing and fragmentation of nuclear DNA. Plasmatocyte-conditioned medium did not induce apoptosis of other hemocyte types, and medium conditioned by other hemocyte types did not induce apoptosis of granular cells. The adhesive state of granular cells and plasmatocytes also affected levels of apoptosis. Conditioned medium from spread plasmatocytes induced higher levels of granular cell apoptosis than medium conditioned by unspread plasmatocytes. Reciprocally, spread granular cells underwent significantly higher rates of apoptosis than unspread granular cells in medium conditioned by spread plasmatocytes. In situ analysis indicated that granular cells on the periphery of capsules also undergo apoptosis. Collectively, our results suggest that spread plasmatocytes release one or more factors that induce apoptosis of granular cells, and that this response is important in the final phases of capsule formation.

Monoclonal antibody MS13 identifies a plasmatocyte membrane protein and inhibits encapsulation and spreading reactions of Manduca sexta hemocytes

Lepidopterans generally can successfully defend themselves against a variety of parasites or parasitoids. One mechanism they use is to encapsulate the invader in many layers of hemocytes. For encapsulation to occur, the hemocytes must attach to the foreign material, spread, and adhere to each other. The molecules that mediate these processes are not known. One method to identify proteins potentially necessary for adhesion, spreading, and, thus, encapsulation is to use monoclonal antibodies that interfere with these functions. In this paper, we report that a monoclonal antibody against Manduca sexta plasmatocytes effectively inhibited encapsulation of synthetic beads in vitro and in vivo. Furthermore, it inhibited plasmatocyte spreading in vitro. Other anti-hemocyte antibodies did not have these effects. The plasmatocyte-specific monoclonal antibody, mAb MS13, recognized a protein of approximately 90,000 daltons as indicated by Western blot analysis of hemocyte lysate proteins. The epitope recognized by mAb MS13 was present on the exterior surface of plasmatocytes. Using indirect immunohistochemistry with hemocyte-specific antibodies, we also determined that during encapsulation plasmatocytes were the first cells bound to latex beads and later layers consisted of both plasmatocytes and granular cells. Arch.

Hematopoiesis in larval Pseudoplusia includens and Spodoptera frugiperda

Maintenance of circulating hemocytes in larval Lepidoptera has been attributed to both mitosis of hemocytes already in circulation and the release of hemocytes from hematopoietic organs. In this study, we compared hematopoiesis in the noctuids Pseudoplusia includens and Spodoptera frugiperda. For both species, hemocyte densities per microl of blood increased with instar. Differential hemocyte counts indicated that plasmatocytes were the most abundant hemocyte type during early instars but granular cells were the most abundant hemocyte type in the last instar. Hematopoietic organs were located in the meso- and metathorax of S. Frugiperda and P. Includens. These organs contained large numbers of hemocytes in S. Frugiperda, but contained few hemocytes in P. Includens. The majority of the hemocytes recovered from hematopoietic organs were identified as plasmatocytes. Using hemocyte type-specific markers and bromodeoxyuridine (BrdU) incorporation experiments, we determined that all hemocyte types with the exception of oenocytoids synthesize DNA. BrdU labeling indices for both species also fluctuated with the molting cycle. Ligation experiments suggested that hematopoietic organs are an important source of circulating plasmatocytes in S. Frugiperda but not in P. Includens. Injection of heat killed bacteria into larvae induced higher levels of BrdU labeling than injection of sterile saline, suggesting that infection and wounding induce different levels of hemocyte proliferation. Arch.

Plasmatocyte spreading peptide induces spreading of plasmatocytes but represses spreading of granulocytes

In most Lepidoptera, plasmatocytes and granulocytes are the two hemocyte classes capable of adhering to foreign targets. Previously, we identified plasmatocyte spreading peptide (PSP1) from the moth Pseudoplusia includens and reported that it induced plasmatocytes to rapidly spread on foreign surfaces. Here we examine whether the response of plasmatocytes to PSP1 was influenced by cell density or culture conditions, and whether PSP1 affected the adhesive state of granulocytes. Plasmatocyte spreading rates were clearly affected by cell density in the absence of PSP1 but spreading was density independent in the presence of PSP1. PSP1 also induced plasmatocytes in agarose-coated culture wells to form homotypic aggregations rather than spread on the surface of culture wells. In contrast, granulocytes rapidly spread in a density independent manner in the absence of PSP1, but were dose-dependently inhibited from spreading by the addition of peptide. An anti-PSP1 polyclonal antibody neutralized the spreading activity of synthetic PSP1. This antibody also neutralized the plasmatocyte spreading activity of granulocyte-conditioned medium, and significantly delayed plasmatocyte spreading when cells were cultured at a high density in unconditioned medium. These results suggested that the spreading activity derived from granulocytes is due in part to PSP1. Pretreatment of plasmatocytes with trypsin had no effect on PSP1-induced aggregation but PSP1-induced aggregations were readily dissociated by trypsin. This suggested that PSP1 is not an adhesion factor but induces adhesion by stimulating a change in the cell surface of plasmatocytes. Synthetic PSP1 also induced aggregation of plasmatocytes from other Lepidoptera indicating that regulation of hemocyte activity by PSP1-related peptides may be widespread. Arch.

Monoclonal antibodies bind distinct classes of hemocytes in the moth Pseudoplusia includens

Insect hemocytes have historically been identified on the basis of morphology, ultrastructure and hypothesized function. Among insects in the order Lepidoptera, five hemocyte classes are usually recognized: granular cells, plasmatocytes, spherule cells, oenocytoids and prohemocytes. We have generated a panel of monoclonal antibodies (mAbs) against hemocytes of the moth Pseudoplusia includens. In this study, hemocyte identification using 16 different mAbs was compared to identification methods using morphological characters. Three main categories of mAb binding activity were identified: (1) mAbs that specifically labeled only one morphological class of hemocytes, (2) mAbs that labeled granular cells and spherule cells, and (3) mAbs that labeled plasmatocytes and oenocytoids. With one exception, none of the antibodies bound to other tissues in P. includens. However, certain mAbs that specifically labeled granular cells and/or spherule cells in separated hemocyte populations also labeled plasmatocytes co-cultured with granular cells or cultured in granular cell conditioned medium. Overall, our results suggest that granular cells are antigenically related to spherule cells, and that plasmatocytes are antigenically related to oenocytoids. The use of mAbs as hemocyte markers are discussed.

Plasmatocyte spreading peptide does not induce Microplitis demolitor polydnavirus-infected plasmatocytes to spread on foreign surfaces

Capsule formation by the moth Pseudopulsia includens requires that plasmatocytes change from being nonadhesive cells in circulation to strongly adhesive cells capable of attaching to the foreign target and one another. This change in adhesive state is induced by Plasmatocyte Spreading Peptide (PSP1); a 23 amino acid peptide isolated from P. includens plasma. Plasmatocytes from hosts parasitized by Microplitis demolitor remain in a nonadhesive state after infection by Microplitis demolitor polydnavirus (MdPDV). This alteration in plasmatocyte function prevents P. includens from encapsulating the developing parasitoid. In the current study, we examined whether MdPDV infection eliminates PSP1-responsive plasmatocytes from circulation or disrupts the ability of PSP1 to induce adhesion and spreading of plasmatocytes to foreign surfaces. In vivo experiments revealed that infection of P. includens by MdPDV induced an increase in the total number of hemocytes in circulation but reduced the proportion of hemocytes in circulation that were plasmatocytes. However, plasmatocytes normally capable of responding to PSP1 were not eliminated from circulation. Both in vivo and in vitro experiments indicated that plasmatocytes inoculated with MdPDV lost the capacity to respond to PSP1 4-6 h post-infection. Infection of P. includens with MdPDV reduced expression levels of prepro-PSP1 mRNA in hemocytes but did not appear to alter expression levels in fat body.