Insect hemocytes and their role in immunity

Antimicrobial peptides: premises and promises.

Cell-mediated immunity in arthropods: hematopoiesis, coagulation, melanization and opsonization.

Evolutionary ecology seeks to understand the selective reasons for the design features of the immune defense, especially with respect to parasitism. The molecular processes thereby set limitations, such as the failure to recognize an antigen, response specificity, the cost of defense, and the risk of autoimmunity. Sex, resource availability, and interference by parasites also affect a response. In turn, the defense repertoire consists of different kinds of immune responses--constitutive or induced, general or specific--and involves memory and lasting protection. Because the situation often defies intuition, mathematical analysis is typically required to identify the costs and benefits of variation in design, but such studies are few. In all, insect immune defense is much more similar to that of vertebrates than previously thought. In addition, the field is now rapidly becoming revolutionized by molecular data and methods that allow unprecedented access to study evolution in action.

Evolutionary ecology of insect immune defenses

The African mosquito species Anopheles gambiae sensu lato s.l. and Anopheles funestus rank among the world's most efficient vectors of human malaria. Their unique bionomics, particularly their anthropophilic, endophagic and endophilic characters, guarantee a strong mosquito-host interaction, favorable to malaria transmission. Olfactory cues govern the various behaviors of female mosquitoes and here we review the role of semiochemicals in the life history of African malaria vectors. Recent evidence points towards the existence of human-specific kairomones affecting host-seeking A. gambiae s.l., and efforts are under way to identify the volatiles mediating this behavior. Based on examples from other Culicidae spp., it is argued that there is good reason to assume that mating, sugar feeding, and oviposition behavior in Afrotropical malaria vectors may also be mediated by semiochemicals. It is foreseen that increased knowledge of odor-mediated behaviors will be applied in the development of novel sampling techniques and possibly alternative methods of intervention to control malaria.

Odor-mediated behavior of afrotropical malaria mosquitoes

Many of the most harmful parasitic diseases are transmitted by blood-feeding insect vectors. During this stage of their life cycles, selection pressures favor parasites that can manipulate their vectors to enhance transmission. Strategies may include increasing the amount of contact between vector and host, reducing vector reproductive output and consequently altering vector resource management to increase available nutrient reserves, and increasing vector longevity. Manipulation of these life-history traits may be more beneficial at some phase of the parasite's developmental process than at others. This review examines empirical, experimental, and field-based evidence to evaluate examples of changes in vector behavior and physiology that might be construed to be manipulative. Examples are mainly drawn from malaria-infected mosquitoes, Leishmania-infected sandflies, and Trypanosoma-infected tsetse flies.

Manipulation of medically important insect vectors by their parasites.

Host fecundity reduction: a strategy for damage limitation?

Apoptosis in the malaria protozoan, Plasmodium berghei: a possible mechanism for limiting intensity of infection in the mosquito.

We have used Anopheles gambiae, a major vector of malaria in Africa, to test the hypothesis that the operation of a surveillance or immune system against microorganisms and parasites can be costly to the reproductive success of the host. Blood-fed mosquitoes were challenged with an immune elicitor, lipopolysaccharide (LPS), and their resultant antimicrobial activity, accumulation of yolk protein in the ovary and egg production was monitored. Humoral activity against the Gram-positive bacterium Micrococcus luteus was induced by LPS injection in a dose-responsive manner. LPS treatment also caused a concomitant significant reduction in the accumulation of protein in ovaries 24 h after injection and in the production of eggs during the same gonotrophic cycle. Unlike immune stimulation, reduction in reproductive fitness was not dose responsive. Oral administration of LPS also significantly reduced ovarian protein content although we could not detect the presence of anti-M. luteus activity in the gut tissue by using an inhibition zone assay. These findings indicate that immune stimulation imposed reproductive fitness costs on mosquitoes.

The costs of mounting an immune response are reflected in the reproductive fitness of the mosquito Anopheles gambiae

Publisher Summary This chapter focuses on the host–parasite interactions that provide new insights into the biochemistry, physiology, ethology, and ecology of the respective partners. This is, particularly, so in the case of parasitic infections of invertebrates for the following reasons: (i) the short generation time and large population size of many invertebrate hosts make them convenient models for study, and the need to understand the role of invertebrate vectors in the transmission cycles of parasitic disease is of particular relevance; and (ii) invertebrates are utilized by a variety of protozoan and helminth parasites as definitive or intermediate hosts in complex life cycles, some of which involve vertebrates. Using examples of parasitic protozoans, helminths, crustaceans, and insects in association with a wide range of invertebrate hosts, strategies, such as the synchronization of host–parasite life cycles, alteration of host growth rates, the impairment of host reproductive capacities, parasite establishment in hosts of different sexes, and alterations in host behavior are examined. A parasitic association succeeds only if the host survives long enough to enable the parasite to complete the particular phase of its life cycle and if the host population is able to persist in time. The mode and degree of integration of a variety of invertebrates and their parasites are also examined.

Hilary Hurd

Papers

Manipulation of medically important insect vectors by their parasites.

Host fecundity reduction: a strategy for damage limitation?

Apoptosis in the malaria protozoan, Plasmodium berghei: a possible mechanism for limiting intensity of infection in the mosquito.

The costs of mounting an immune response are reflected in the reproductive fitness of the mosquito Anopheles gambiae

Physiological and behavioural interactions between parasites and invertebrate hosts.