The role of plasmatocyte spreading peptide in regulation of encapsulation

K. D. Clark 1 , B. Volkman 2 & M. R. Strand 1

Dept. of Entomology 1 and Biochemistry 2 Univ. Wisconsin, Madison, WI, USA

Circulating blood cells (hemocytes) play an essential role in defending insects against invading organisms. Hemocytes usually kill metazoan parasites by encapsulation; a process in which certain types of hemocytes spread across the surface of the foreign target to form a multilayered sheath of cells. Despite the fundamental importance of this defense response to invertebrates, the specific cellular and molecular mechanisms coordinating capsule formation are not well understood. In this competing renewal application, we propose to continue our studies on capsule formation in the lepidopteran Pseudoplusia includens. Our results indicate that encapsulation requires two types of hemocytes called granular cells and plasmatocytes. Neither of these hemocyte types is able to encapsulate targets alone. However, plasmatocytes encapsulate targets if either granular cells are present or if targets are preincubated in medium conditioned by granular cells. These results suggest that after attaching to a foreign surface, granular cells release factors into plasma that induce a change in the adhesive state of plasmatocytes. In this regard, we identified plasmatocyte spreading peptide (PSP1) which induces plasmatocytes to undergo a change in adhesive state and to release specific effector molecules. In this presentation we discuss how PSP1 activates plasmatocytes and the potential significance of this signaling pathway in defense against parasitoids.

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