Preliminary characterization and purification of in vitro encapsulation promoting factor: a peptide that mediates insect haemocyte adhesion.

The granular cells and plasmatocytes (PLs) of Heliothis virescens form multicellular aggregations in vitro. This allows capsules to form around suitable targets. Prior trypsinization of the haemocytes abolishes their ability to encapsulate, and this function can be restored by adding plasma to the trypsinized cells. Trypsinized PLs were also unable to spread on a planar glass surface unless normal plasma was present. Plasma was subjected to a variety of treatments to determine the nature of the encapsulation promoting factor (EPF) using the in vitro encapsulation system as a bioassay. The data suggest EPF is a peptide; it is trypsin sensitive and moderately heat stable. Similar results were obtained when using spreading by trypsinized PLs as the bioassay. Dialysis using a 3,500 MW cut-off membrane also abolished encapsulation promoting activity. Protein-free extracts of plasma (crude EPF) has strong activity in both bioassays but does not agglutinate human erythrocytes. A single peak with strong activity in both bioassays was resolved after subjecting crude EPF to reversed-phase HPLC. This active material was purified after additional HPLC with a different solvent system.