Microplitis demolitor bracovirus inhibits phagocytosis by hemocytes from Pseudoplusia includens

Abstract

The braconid wasp Microplitis demolitor carries Microplitis demolitor bracovirus (MdBV) and parasitizes the larval stage of several noctuid moths. A key function of MdBV in parasitism is suppression of the host's cellular immune response. Prior studies in the host Pseudoplusia includens indicated that MdBV blocks encapsulation by preventing two types of hemocytes, plasmatocytes and granulocytes, from adhering to foreign targets. The other main immune response mediated by insect hemocytes is phagocytosis. The goal of this study was to determine which hemocyte types were phagocytic in P. includens and to assess whether MdBV infection affects this defense response. Using the bacterium Escherichia coli and inert polystyrene beads as targets, our results indicated that the professional phagocyte in P. includens is granulocytes. The phagocytic responses of granulocytes were very similar to those of High Five cells that prior studies have suggested are a granulocyte-like cell line. MdBV infection dose-dependently disrupted phagocytosis in both cell types by inhibiting adhesion of targets to the cell surface. The MdBV glc1.8 gene encodes a cell surface glycoprotein that had previously been implicated in disruption of adhesion and encapsulation responses by immune cells. Knockdown of glc1.8 expression by RNA interference (RNAi) during the current study rescued the ability of MdBV-infected High Five cells to phagocytize targets. Collectively, these results indicate that glc1.8 is a key virulence determinant in disruption of both adhesion and phagocytosis by insect immune cells.

Characterization of hemocytes from the mosquitoes Anopheles gambiae and Aedes aegypti

“…Our functional bioassays and markers also did not indicate that mosquitoes produce more than one adhesive type of hemocyte even though lepidopterans (granulocytes and plasmatocytes) and Drosophila (plasmatocytes and lamellocytes) produce two adhesive hemocyte types specialized for phagocytosis and capsule formation, respectively (Gillespie et al, 1997;Strand, 2002, 2003;Lebestky et al, 2000;Irving et al, 2005;Strand et al, 2006). Mosquito oenocytoids are identified by the combination of morphology, MCB labeling, and phenoloxidase activity.…”

“…We originally thought this difference in spreading morphology could be functionally significant since other insects produce adhesive hemocyte types that assume different spreading morphologies in vitro which in turn correlate with different immune functions. In most Lepidoptera, for example, granulocytes spread symmetrically in vitro and are the professional phagocytes, whereas plasmatocytes spread asymmetrically, are non-phagocytic, and function as the main capsule-forming hemocyte type (summarized by Lavine and Strand, 2002;Strand et al, 2006). Analogously, the professional phagocytic hemocytes in Drosophila (called plasmatocytes) differ in size and assume a different spread morphology from lamellocytes that form capsules (Lanot et al, 2001;Irving et al, 2005).…”

Pteromalus puparum venom impairs host cellular immune responses by decreasing expression of its scavenger receptor gene

“…Insects possess effective innate immunity, typically assorted into physical (i.e., integument), cellular and humoral immune responses, although these distinctions are becoming blurred with new knowledge. Once invaders enter the insect hemocoel, they are met with an arsenal of immune responses (Strand, 2008;Strand et al, 2006). Humoral immunity involves induced biosynthesis of anti-microbial proteins (e.g., the classical cecropins) and enzymes (lysozyme and activation of prophenoloxidase).…”

PTP-H2 and PTP-H3 from Microplitis demolitor Bracovirus Localize to Focal Adhesions and Are Antiphagocytic in Insect Immune Cells

“…A previously made Glc1.8 expression construct (pIZT/Glc1.8) was also used in functional assays (5). All constructs were transiently expressed in S2 cells by cationic lipid-mediated transfection with 1 g of plasmid per ml of medium (5,50). Transfection efficiencies averaged 75 to 80% as measured by GFP expression.…”

“…Previous studies have implicated two MdBV gene families in immunosuppression. The Glc genes encode cell surface mucins that disrupt adhesion and phagocytosis of hemocytes and hemocyte-like cell lines (5,6,50). The vankyrin (IB-like) gene family encodes homologs related to insect and mammalian inhibitor Bs (IBs) that function as negative regulators of nuclear factor B transcription factors (NFBs).…”