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.