EJea European Journal of Cell Biology 78, 675-689 (1999, October) . © Urban & Fischer Verlag· Jena 675 hltp:llwww.urbanfischer.de/journals/ejcb Filamentous proteophosphoglycan secreted by Leishmania promastigotes forms gel-like three- dimensional networks that obstruct the digestive tract of infected sandfly vectors York-Dieter Stierhot1 l a, Paul A. Bates b , Raymond L. lacobson c , Matthew E. Rogers b , Yosef Schlein c , Emanuela Handman d , Thomas IIg1)a a Max-Planck-Institut fUr Biologie, Abteilung Membranbiochemie, Tiibingen/Germany b Liverpool School of Tropical Medicine, LiverpooVUnited Kingdom C The Hebrew University Hadassah Medical School, Department of Parasitology, JerusalemlIsrael d The Walter and Eliza Hall Institute of Medical Research, P.O. Royal Melbourne Hospital, Melbourne/Australia Received April 16, 1999 Accepted June 22, 1999 Immunoelectron microscopy - proteophosphoglycan - secreted acid phosphatase - sand fly Development of Leishmania parasites in the digestive tract of their sandfly vectors involves several morphological transfor- mations from the intracellular mammalian amastigote via a succession of free and gut wall-attached pro mastigote stages to the infective metacyclic promastigotes. At the foregut - midgut transition of Leishmania-infected sandDies a gel-like plug of unknown origin and composition is formed, which contains high numbers of parasites, that occludes the gut lumen and which may be responsible for the often observed inability of infected sandDies to draw blood. This "blocked fly" phenotype has been linked to efficient transmission of infectious meta cyclic promastigotes from the vector to the mammalian host. We show by immunofluorescence and immu- no electron microscopy on two Leishmania/sandfly vector com- binations (Leishmania mexicana/Lutzomyia longipalpis and L. major/Phlebotomus papatasi) that the gel-like mass is formed mainly by a parasite-derived mucin-like filamentous proteo- phosphoglycan (fPPG) whereas the Leishmania polymeric secreted acid phosphatase (SAP) is not a major component of this plug. fPPG forms a dense three-dimensional network of filaments which engulf the pro mastigote cell bodies in a gel- like mass. We propose that the continuous secretion of fPPG by promastigotes in the sandfly gut, that causes plug forma- tion, is an important factor for the efficient transmission to the mammalian host. 1) Dr. York-Dieter Stierhof and Dr. Thomas Ilg, Max-Planck-Institut fiir Biologie, Corrensstr. 38, D-72076 Tiibingen/Germany, FAX: + + 497071601235. Introduction Digenetic protozoan parasites of the genus Leishmania are the causative agents of a diversity of pathological conditions that are prevalent in the tropical and subtropical zones. Their life cycle includes several forms of flagellated, extracellular pro- mastigotes in the alimentary tract of their sandfly vectors and a non-flagellated intracellular amastigote form that resides within macrophages of their mammalian hosts (Alexander and Russell, 1992). The Leishmania life cycle within the sandfly vectors has been previously described (Killick-Kendrick 1979, Killick- Kendrick et al. 1974, 1977a, b, 1988, Molyneux and Killick- Kendrick 1987, Walters et al. 1987, 1989a, b, Lawyer et al. 1987, 1990, Bates 1994, and references therein). Infections begin when a sandfly bites an infected host. The ingested blood and amastigotes are enclosed within a peritrophic mem- brane which is formed from secretions of the epithelial cells of the insect's abdominal midgut. Amastigotes of suprapylarian Leishmania parasites transform to short free-swimming pro- cyclics and then to long slender nectomonads which exit for- wards from the disrupted peritrophic membrane at the end of the blood meal digestion. Many of the released nectomonads insert their flagella between the microvilli of the epithelial cells, another group attaches to the cuticular lining of the sto- modeal valve and the rest remains free in the gut lumen. Fur- ther morphological changes lead to the formation of haptomo- nads, either free-swimming or attached to the cuticle by modi- fied shortened flagella, and free-swimming, highly motile, non-dividing metacyclic promastigotes, which are mainly pre- sent in the foregut (reviewed by Bates 1994, compare Fig. 1). This latter stage is transmitted to the mammalian host during 0171-9335/99/78/10-675 $12.00/0