The insect immune system serves as a key defense against attack by parasitoids. Incompatible hosts often eliminate parasitoids by encapsulation, a process in which hemocytes form a multilayered envelope around the invading organism. Capsule formation involves cooperation between one or more classes of hemocytes and is likely mediated by cytokines and adhesion molecules. Reciprocally, parasitoids have evolved a variety of strategies for overcoming host immune responses. Some parasitoids passively avoid elimination by developing in locations inaccessible to host hemocytes or by possessing surface features that fail to elicit an immune response. Other species actively disrupt the host immune system by injecting specific factors into the host at oviposition. In particular, polydnaviruses associated with several taxa of parasitoids disrupt capsule formation by killing hemocytes or altering their ability to adhere to foreign surfaces. These symbionts have likely played a critical role in evolution of host range and in defining parasitoid-host compatibility.

Insect blood cells (hemocytes) play an essential role in defense against parasites and other pathogenic organisms that infect insects. A key class of hemocytes involved in insect cellular immunity is plasmatocytes. Here we describe the isolation and identification of a peptide from the moth Pseudoplusia includens that mediates the spreading of plasmatocytes to foreign surfaces. This peptide, designated plasmatocyte-spreading peptide (PSP1), contains 23 amino acid residues in the following sequence: H-ENFNGGCLAGYMRTADGRCK-PTF-OH. In vitro assays using the synthetic peptide at concentrations >2 nM induced plasmatocytes from P. includens to spread on the surface of culture dishes. Injection of this peptide into P. includens larvae caused a transient depletion of plasmatocytes from circulation. Labeling studies indicated that this peptide induced 75% of plasmatocytes that were double-labeled by the monoclonal antibodies 49G3A3 and 43E9A8 to spread, whereas plasma induced significantly more plasmatocytes to spread. This suggests that only a certain subpopulation of plasmatocytes responds to the peptide and that other peptidyl factors mediate plasmatocyte adhesion responses.

Microplitis demolitor polydnavirus (MdPDV

Hemocytes collected from larvae of Pseudoplusia includens (Lepidoptera:Noctuidae) were separated by centrifugation on Percoll cushions. The procedure resulted in 95% purity of plasmatocytes and greater than 99% purity of granular and spherule cells. Medium supplemented with chicken serum enhanced cell viability and promoted spreading of plasmatocytes. Cell-free plasma and medium preconditioned by plasmatocytes or granular cells stabilized cells in vitro and also accelerated spreading of plasmatocytes relative to medium supplemented with chicken serum. Oenocytoids were the only morphotype that exhibited endogenous phenoloxidase activity, while granular cells and plasmatocytes were the only cells that endocytosed fluorescent beads in vitro. Granular cells and plasmatocytes ingested fluorescently labelled beads, both in mixed populations of hemocytes and after separation. Plasmatocytes were the only morphotype that encapsulated large foreign targets in vitro following separation. Separated granular cells attached and spread on the surface of foreign targets but never formed a multilayered capsule.

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