Immune mechanisms in insects

Understanding immune evasion by parasites in their insect vectors requires some understanding of the insect immune system. Until fairly recently, technical difficulties in handling cells and plasma hampered laboratory investigations into insect immunology, but modern techniques combined with a sound knowledge of insect physiology are now permitting rapid advances. Rather than discussing the many controversies, this review aims to point out current areas of research into cellular and 'humoral' mechanisms that might be followed up by parasitologists.     

Immune defense and suppression in insects

Insect endoparasitoids are able to circumvent the defense reactions of their habitual hosts. In a hymenopteran wasp species, virus-like particles, found on the egg surface are responsible for the protection against the encapsulation reaction of the host caterpillar. Some of the particle proteins are structurally and probably functionally related to host protein(s). Biological properties of some of the host proteins suggest that they might be involved in the insect defense reaction.     

Density-dependence in parasite transmission dynamics

The transmission of vector-borne parasites is complex, yet to a large extent this complexity can be unravelled through the insights gained from simple mathematical models of the transmission system. The principle is simple because the key question is merely "what is the rate of increase in numbers of hosts affected?" Clearly, if this rate of increase is greater than unity then the infection can spread, while if it is less than unity it will decline. Ronald Ross in 1911 was the first to formulate this idea for malarial(1) and malaria transmission has since attracted most attention from modellers of parasitic diseases(2-4). But although it is implicitly recognized that nothing- not even parasitic transmission - can increase indefinitely, the importance of some degree of density-dependence in regulating the system tends to be neglected (see Box 1). In this article, Klaus Dietz explores some classical ideas of modelling parasitic disease transmission, emphasizing not only the importance of density dependence but also the importance of knowing exactly where such effects operate in the system.     

Immune response inhibits associative learning in insects

In vertebrates, it is well established that there are many intricate interactions between the immune system and the nervous system, and vice versa. Regarding insects, until now little has been known about the link between these two systems. Here, we present behavioural evidence indicating a link between the immune system and the nervous system in insects. We show that otherwise non-infected honeybees whose immune systems are challenged by a non-pathogenic immunogenic elicitor lipopolysaccharide (LPS) have reduced abilities to associate an odour with sugar reward in a classical conditioning paradigm. The cost of an immune response therefore not only affects survival of the host, as previously shown, but also everyday behaviour and memory formation.     

Vertical transmission of nucleopolyhedrovirus in insects.

A review of the literature on transmission studies of nucleopolyhedroviruses showed that low levels of viral infection were common among studies of pupae of insects but rarer in adults. Virus could be transmitted from parent to progeny and could be found in caterpillars reared from surface-decontaminated eggs. Persistent low levels of infection were observed in many of the studies considered. These could contribute to the persistence of virus in low-density populations, but the dominant source of virus among generations is probably through environmental contamination.     

Immune responses in vitro

The Mls locus was originally defined to have four alleles; three controlled products that were detectable in primary mixed leukocyte reactions (MLR), whereas one, b, was described as being null. Recently, other investigators postulated that the Mls locus is nonpolymorphic, being composed of the b null allele and of a singly expressed allele previously thought to be the a and d alleles. We previously reported that products controlled by Mls ^aand Mls ^dwere antigenically distinct and therefore are not controlled by the same allele, and the product of Mls ^bon cells of three different strains was easily detectable by Mls ^aand Mls ^dresponding cells. Thus the b allele is not null. In the present report evidence is presented which indicates that both Mls ^band Mls ^cencoded products were undetectable by MLR when in the presence of Mls ^aor Mls ^d. This was demonstrated by (a) the inability of Mls ^a/Mls ^cand Mls ^a/Mls ^bF₁ cells to stimulate Mls ^aresponding cells and Mls ^d/Mls ^cand Mls ^d/Mls ^bcells to stimulate Mls ^dcells; (b) the positive response of Mls ^a/Mls ^band Mls ^d/Mls ^bF₁-hybrid cells to Mls ^b-encoded products; and (c) the reactivity of Mls ^a/Mls ^cand Mls ^d/Mls ^cF₁ hybrid cells to Mls ^c-encoded determinants.     

Immune reactions of insects on bacterial pathogens and mutualists

In the past few years the knowledge of insect defense mechanisms against pathogenic microorganisms and parasites has significantly increased on both the molecular and the organismic level. These investigations have led to new concepts of immune protection also relevant for mammals with the identification of the Toll receptor family as an eminent example. This review provides a brief overview of insect strategies to on the one hand defeat bacterial pathogens while on the other hand cooperating with symbiotic bacteria beneficial for the insects.     

Gut microbiota and parasite transmission by insect vectors

In the gut of some insect vectors, parasites ingested with the bloodmeal decrease in number before coming into contact with host tissues. Many factors could be responsible for this reduction in parasite number but the potentially important role of the large communities of naturally occurring microorganisms that exist alongside the newly ingested parasites in the vector midgut has been largely overlooked. Some previous reports exist of the inhibition of parasite development by vector gut microbiota and of the killing of Trypanosoma cruzi and Plasmodium spp. by prodigiosin produced by bacteria. Based on this evidence, we believe that the microbiota present in the midgut of vector insects could have important roles as determinants of parasite survival and development in insect vector hosts and, therefore, contribute to the modulation of vector competence for many important diseases.     

Immune responses in Anopheles gambiae

Transmission of human malaria requires a successful development of Plasmodium parasites in anopheline mosquitoes. Insects have developed efficient immune responses to oppose microbial and eukaryotic invaders. The completion of the sequencing of the Anopheles genome provides a wealth of information on putative immune genes that are homologous to components of the Drosophila and mammalian immune systems. In this review, we will summarize our present knowledge of immune responses in the mosquito Anopheles gambiae and attempt a comparative analysis of insect immune systems.     

Constraints on parasite fecundity and transmission in an insect-STD system

Parasites that are sexually transmitted (causing sexually transmitted diseases, or STDs) can have important effects on host population dynamics, but we know almost nothing for such parasites about constraints on fecundity and transmission. In this study, we have examined the effect of two potentially important constraints in one of the few empirically well-studied animal-STD systems, the ladybird, Adalia bipunctata , and its sexually transmitted mite, Coccipolipus hippodamiae . Using a factorial design, we manipulated: (i) within-host competition, by varying infection intensity; and (ii) host condition, by introducing dietary stress. Infection with C . hippodamiae significantly reduced ladybird survival whether or not diet was restricted, and restricting diet led to reduced survival regardless of infection status. Increased infection intensity and reduced host condition (dietary stress) both independently constrained per capita rates of parasite egg production and the development of infective larvae. Furthermore, when host condition was compromised, significantly fewer larvae were transmitted per adult mite during copulation. The effect of infection intensity on per mite transmission was more complex: there was no significant effect when hosts were fed normally, but when the ladybirds were nutritionally stressed higher infection intensity was associated with a slight increase in the numbers that were transmitted (despite the fact that mite fecundity was reduced under these conditions). These results indicate that host condition and within-host competition may both play an important role in shaping the epidemiology of the A . bipunctata – C . hippodamiae system, by influencing the parasite's basic reproductive ratio (R₀) and the rate of epidemic spread. Our data also extend general insights into STD ecology, by highlighting the importance of constraints on disease dynamics that are likely to be widespread.