Humoral mechanisms in the immune response of insects to larvae of Hymenolepis diminuta (Cestoda)

Phagocytosis of microvilli of the metacestode ofHymenolepis diminutabyTenebrio molitorhaemocytes

A limited number of metacestodes ofHymenolepis diminutafromTenebrio molitorhad host plasmatocytes and cell debris of coagulocyte origin associated with the microvillar surface. There was evidence of phagocytosis of microvilli by plasmatocytes, involving clathrin-like endocytotic vesicles. In contrast to these otherwise ‘normal’ cysticercoids, occasional batches were ‘sticky’ and, on flushing into saline, rapidly developed large bladders possibly associated with osmotic stress. In addition, numerous small surface blebs were present. These ‘surface-stressed’ cysticercoids also had host plasmatocytes and cell debris associated with the bladder bases and the blebs, and phagocytosis of microvilli was observed. In no case had precipitation of haemolymph, melanization or encapsulation (features typical of the insect immune response) occurred. The possibility that the limited host response indicates a spatially restricted impairment of the surface compatibility of the parasite, and thus insufficient to elicit the full immune response, is discussed.

Induction of cysteine proteinase in the encapsulation of Hymenolepis diminuta eggs in the American cockroach, Periplaneta americana

Haemocytes play an essential role in defending invertebrates against pathogens and parasites that enter their haemocoel. In the present study, the cockroach, Periplaneta americana was able to encapsulate Hymenolepis diminuta eggs within 24 h after injection. Proteolytic activity of egg capsules was determined by gelatin zymography. A gelatinase-type proteolytic enzyme with molecular weight about 65 kDa was present at the time of capsule formation. Enzyme activity was obviously inhibited by leupeptin but not by ethylenediamine tetraacetic acid (EDTA) or 1,10-phenanthroline or phenylmethanesulfonyl fluoride (PMSF). Hence, we tentatively characterized this enzyme as a cysteine proteinase. The specificity of the cellular immune response in vivo and the increased cysteine proteinase activity coincided with the capsule size and encapsulation process. The possible function of this cysteine proteinase activity during encapsulation of the H. diminuta eggs by P. americana is discussed.

Ontogenetic changes in helminth membrane function

During their life-cycle many parasites experience a wide range of environments including free living and those provided by a variety of intermediate and final hosts. The nutritional requirements of parasites are met by physiological processes adapted to exploit the physicochemical characteristics provided by different hosts. In helminth parasites these adaptations are frequently expressed on the tegumentary surface. As an example of adaptations within the Trematoda, the control of monosaccharide transport in Proterometra sp. is described. Environmental sodium, although not directly involved in the uptake process, nevertheless regulates the expression of transport capabilities. In the Cestoda, the uptake of monosaccharides and amino acids is described for Hymenolepis diminuta. The metacestode of this tapeworm inhabits the blood system of an arthropod, and the adult the gut of a mammal. There are quantitative and qualitative differences in the amino acids and monosaccharides in these two environments and these are reflected in the transport mechanisms exhibited by the two forms of the life-cycle. In Echinococcus granulosus the transfer of amino acids, sugars and macromolecules across the membranes of hydatid cysts and protoscoleces is described. The major difference between these two stages in the life-cycle relates to the ability of hydatid cysts to absorb macromolecules, whereas protoscoleces are impermeable to these compounds. The potential for future work is emphasized.

Naturally occurring agglutinins against trypanosomatid flagellates in the haemolymph of insects

In vitro studies of the behaviour of the trypanosomatid flagellates Trypanosoma brucei and Leishmania hertigi in the presence of cell-free haemolymph of locusts, Schistocerca gregaria and cockroaches, Periplaneta americana revealed the presence of parasite agglutinins. The range of normal values of agglutination titres was 2(-4) to 2(-13). Physico-chemical treatment of haemolymph indicated that these agglutinins are protein or glycoprotein in nature and are only partially affected by heat treatment below 65 degrees C, at which temperature incubation of haemolymph for 30 min abrogated all agglutination. Agglutination was not dependent on the presence of Ca2+ or Mg2+. Prior injection of locusts and cockroaches with T. brucei and L. hertigi significantly increased agglutinin titres between Days 4 and 6 in cockroaches (P less than 0.05) and from Days 2 to 4 when L. hertigi was inoculated into locusts. The induced differences in titres observed in locusts infected with T. brucei were not significant. Lysozyme levels were significantly increased after inoculation of T. brucei into cockroaches compared with placebo-inoculated and uninoculated controls. L. hertigi inoculation produced significant increases in lysozyme levels compared with controls between Days 1 and 7 in locusts and 3 to 6 in cockroaches. These studies indicate that, at least in easily manipulated model systems, induced responses to intrahaemocoelic inoculation to trypanosomes and Leishmania can occur. As far as we are aware this is the first report of an induced response of an insect to such important parasites. The possibility that induced responses in natural vector to this parasites occurs requires investigation.