Abstract
Insects are exposed to a wide range of microorganisms (bacteria, fungi, parasites and viruses) and have interconnected powerful immune reactions. Although insects lack an acquired immune system they have well-developed innate immune defences that allow a general and rapid response to infectious agents.Over the last few decades we have observed a dramatic increase in the knowledge of insect innate immunity, which relies on both humoral and cellular responses. However, innate reactions to natural insect pathogens and insect-transmitted pathogens, such as parasites, still remain poorly understood.In this review, we briefly introduce the general immune system of insects and highlight our current knowledge of these reactions focusing on the interactions of Trypanosoma rangeli with Rhodnius prolixus, an important model for innate immunity investigation.
Highlights
The main data found by these authors were: (i) insects that had previously been fed on blood containing biosynthesis inhibitors of phospholipase A2 (PLA2) and COX and non-selective LOX inhibitor showed a significant increase in the number of free epimastigote forms of T. rangeli in the hemolymph and, increased lethality; and (ii) the parasite infection in insects treated with these compounds led to less hemocyte microaggregation and attenuated the activation of PPO system in the hemolymph
Hemocelic inoculation of epimastigotes of T. rangeli into larvae of R. prolixus previously fed with blood containing the same parasite, demonstrated a reduced number of hemocyte microaggregates, enhanced the number of parasites in the hemolymph as well as increased the mortality of these insects. All these effects were counteracted by combined injection of R. prolixus with T. rangeli and arachidonic acid [30]. These results suggest that the arachidonic acid pathway can be a mediator of hemocyte microaggregation reactions in the hemolymph of insects inoculated with T. rangeli and that oral infection with this protozoan inhibits the release of arachidonic acid (Figure 3)
The recent investigations into R. prolixus immune reactions relating to T. rangeli development have established a new conceptual hypothesis: a fine modulation of insect factors can interfere with parasite development and this is important for the establishment of infection, being an attractive target for intervention (Fig. 5)
Summary
The insect innate immune reactions There are two types of innate immune reactions: (i) the humoral response that is related to antimicrobial peptides, lectins and the prophenoloxidase (PPO) cascade and (ii) the cellular response which includes phagocytosis, hemocytes aggregation and encapsulation of pathogens. The main data found by these authors were: (i) insects that had previously been fed on blood containing biosynthesis inhibitors of PLA2 (dexamethasone) and COX (indomethacin) and non-selective LOX inhibitor (nordihydroguaiaretic acid, NDGA) showed a significant increase in the number of free epimastigote forms of T. rangeli in the hemolymph and, increased lethality; and (ii) the parasite infection in insects treated with these compounds led to less hemocyte microaggregation and attenuated the activation of PPO system in the hemolymph. One interesting novelty of this parasite-vector interaction was revealed by Machado et al [79] They demonstrated that hemocelic injection of short T. rangeli epimastigotes in R. prolixus that were previously fed with blood containing WEB 2086 [a strong platelet-activating factor (2acetyl-1-hexadecyl-sn-glycero-3-phosphocholine (PAF) antagonist] resulted in reduced hemocyte microaggregation, attenuated PPO activation in the hemolymph as well as increased the parasitemia and insect mortality. All these finding illustrate the ability of T. rangeli to modulate the cellular immune responses of R. prolixus to favor its own multiplication in the hemolymph
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