Cost of Immune Defence (Trade-off) in Invertebrates

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There is an elementary opinion based on life-history theory stats that the development of fitness-related traits will be controlled by the persistence of trade-off between those traits. Thats mean a useful amendment in the expression of one trait will frequently encourage a negative change in the expression of another, or in other words the useful change in one trait of individual is costly and the cost is being paid in term of the reduced affectivity of another life-trait. Such kinds of trade-off are omnipresent in nature, and their presence has conventionally been described in the milieu of resource restrictions, and energy is taken as a typical currency for that cost.

During the evolution of linage if more powerful immune defence is selected, there is a strong possibility that some other function of life or the overall fitness of the organism perhaps get abated because of the peliotropic effects of genes. As, in one of the study it was observed that the increased resistant to parasitic infections in host cell lines caused the reduced fecundity. From number of other studies it was concluded, those host lines which were strongly selected to develop resistance against parasites shown to have poor proficiency for some other life traits such as abate larval growth rate in resistant honey bees than sensitive to American foulbrood (122). Besides this, lower egg viability and longer time for larval development was noted in Indian meal moths' resistant to granulosis virus (15) and also, in Drosophila larvae resistant to common parasitoids the reduced competitiveness for the food was found. Interestingly, strong selection on some other life traits found to lead a reduction in immune defence; for instance, due to the strong sexual selection in dung flies resultant a decreased enzymatic activity of proPO -cascade (an important part of immune system) (60).

To interpret and measure the cost of immune response in terms of other fitness traits is handy. There are number of studies showing how the development of strong immune response can alter some other life traits. Fore instance a lower acceptance for dehydration and starvation (59) and abridged productiveness (42) was found in Drosophila melanogaster that successfully mounted a strong immune response against parasites. On other hand, fulfilment of some other challenging activity decline the efficiency of immune system; such as, reduced rate of encapsulation in damselflies (119) and, in male crickets, decreased PO-activity and impaired resistance to bacterial infections are correlated with powerful reproductive activities (3). Furthermore, many experiments have been carried out to understand whether these negative correlations are independent of individuals' stipulation or selection by the parasite and, whether they will still appear if the individuals strained into a specific treatment. Such as, male Drosophila and T. molitor (yellow mealworms) showed the reduced proficiency to neutralize the bacterial infection, lessen numbers of hemocytes and abate the activity of PO enzyme, when they are strained into mating activities (82, 105). In addition, decreases encapsulation was noted in response to experimental implant in bumble bee upon forced feeding (68). Conversely, numbers of experiments are held to stimulate the immune response in order to test their unfavourable cost on some other life traits. Example for such studies are, injection of LPS abate the fecundity in Anopheles gambiae (a mosquito) (4), reduced the survival and diminish the fitness of whole colonies of bumble bee (Bombus terrestris) (86, 87). In addition, the implantation of nylon filaments increased the standard metabolic rate by 8% compared to controls in pupae of while cabbage butterfly and (46), the same challenge shortened the life span of the yellow mealworms (6). In one of the study the reduction in fecundity in Drosophila melanogaster was demonstrated by induction of innate immunity via the immune deficiency pathway (IMD), Furthermore, they also found that older flies have decreased production of antimicrobial peptide (diptericine) upon infection to dead bacteria than the similarly treated young flies (ref A). The augment in metabolic rate by 0.43-1.14%/day was studies along with decline in mitochondrial DNA copy number by 56-1.06%/day for three different drosophila simulans fly lines; there can be two explanations for this increase in metabolic rate, either to continue a constant bioenergetics demands and/or, to supply energy to restore cellular damages or because of change in metabolic substrate used over time (ref B).

These scientific evidences suggest that evoking a strained immune response perhaps alter some other life traits (fecundity, metabolic rate, survival, competitiveness, development etc); whereas, initiating a higher performance in some other traits can abate the immune defence (111). However the nature of these tradeoffs is not well known but, it can be a consequence of sharing a biochemical pathway or receptors which contribute in more then one traits; such as, in Drosophila, the Toll receptors are involved in both larvae development and resistant to infections (77).

Hence, it can be concluded the cost of immune defence is certainty but, there are two factors which perhaps reduce the appearance of this cost; 1) the cost can depends on background situation i.e. only appears in deprived nutritional conditions and, 2) often selection may reduce the cost i.e. bacteria get resistant to a particular antibiotic upon many exposures to that. That means though the cost is there but its appearance is not that simple.

It is certain that the same immune challenge leads to a reduced immune response in males' comparative to females. In vertebrates, there is a hormone called testosterone, responsible for a trade-off between immune defence and secondary sexual traits. However, in invertebrates this hormone is lacking but, still the trade-off between immunity and secondary sexual characteristics exist and also, the lower immune response in male insects than female is common (75, 100, 104). Ultimately, characteristics that affect immune defence and sexual selection happen to genetically correlated (60).

There are two main types of immune defensive costs: the cost to uphold defensive machinery and, the cost to mount a defensive response after attacking by pathogens; however, the former cost needs to be paid irrespective of pathogenic attack.

The earliest type of cost is an essential component for most of evolutionary models of defence against pathogens and predators but, to determine this cost experimentally has proved extremely tricky (ref c). The key reason for that is, the levels of defence can rarely be experimentally influence in the field or laboratory. A substitute approach to experimental manipulation is the use of artificial selection consecutively to increase defensive investment and then to look for their cost as correlated responses to selection. Drosophila and its natural enemies have been proved a useful model system for investigating the defensive cost as well as the mechanisms underlying this trade-off (ref d).

Artificial selection experiments have shown that the increased defensive ability of D. melanogaster against both of its natural parasitoids; Asobara tabida (RefD*) and that Leptopilina boulardi (refD*) is associated with cost and the resistant larvae are inferior competitors under conditions of resource scarcity and they fed more slowly comparative to their controls, however the resistant lines showed almost 40% more number of haemocytes compared to control lines (ref E).

Conversely, it was found that matting diminishes the activity of a major immune enzyme phenoloxidase in both sexes of mealworm beetle (Tenebrio molitor) and this down regulation was shown to be associated with the activity of juvenile hormone, it was noted that PO activity was significantly declined after 24 hours of matting in both males and females relative to unmated insects in both sexes. Additionally in the same study some more experiments were carried out to understand the mechanism underlying this down regulation and, it was concluded that mating encourages the corpora allata to release juvenile hormone, which plays a role in down regulating the PO activity. The similar investigation were made by another studies using Drosophila melanogaster flies and it was found that male flies kept with more number of female flies were significantly poor in their ability to clear the numbers of infective E. coli compared to the males with one female fly or with male flies only, in addition they also confirmed by few other experiments that this significant decrease in immunological performance of male flies is not because of reduced food availability or the density of population per vial, but, for their mating activity (ref).

There are some other factors which influence the immune response of individuals; therefore, it is worth considering them while measuring the immune cost to get the actual figures. These factors include, time, ambient temperature, age and nature of pathogen. There are different kinds of immune responses against any particular pathogen such as activation of proPO-cascade, nodulation, phagocytosis, encapsulation, melanization; which happens just few minutes after the pathogen invasion (50, 58); however, there are some other responses such as production of antimicrobial peptide (antibodies) and cytokines etc which requires more time for establishment and clearance of infection. Thus the time scale requires for the occurrence of these responses is widely different (ref E). Ambient temperature is a another significant limitation for the activation of immune response for both the larval and the adult stages, (11) it was demonstrated that the larvae kept at low temperatures show reduced encapsulation responses as adults (10). The immuo-senescence is an irreversible characteristic of immune system related to age, this has been studies for PO activity and encapsulation in crickets and bumble bees (3, 35). And the last but not least, a fast-replicating and more pathogenic parasite necessitates a rapid response, whereas slowly replicating pathogens also can be abolished taking longer time.

Trade-Off (Immunilogical Cost) in Vertebrates:

In vertebrates, the life history patterns are most importantly shaped by the plethora and the diversity of immunological dangers associated with their parasitic faunas in the environment. Usually, the evolutionary ecologist assume that mounting an immune response, as well as maintaining an efficient immune system is costly that requires trade-off decisions among nutrient-demanding traits such as growth, reproduction, thermoregulation and, fitness etc (ref H). However, the researches dutiful to measuring such costs and to understand well the mechanism underlying evolutionary trade-off among life history characters are seldom, and their results are broadly scattered all over the literature. However, together, all these studies show that substantial nutritional costs, in term of protein and energy, are linked with up-regulation of the immune system. Additionally, this trade-off perhaps is the most distinct within the acquired arm of the immune system comparative to innate arm that is considered the first-line of immunological defence against parasites (ref I). Energy and nutrients required for growth are often costly due to inherent infectivity and imbalance diet, therefore, these costs leads to the sensitivity of growth process to an alteration in immune status. Revera and his colleagues found that the mild up-regulation of the maternal immune system can suppress the foetal growth and development by direct and indirect mechanisms (ref J).

The trade-off between growth and immunity has interestingly shown by researches of germ free and antigen free environments. The environment antigen city is heterogeneous in nature and comprises almost every thing from food antigens to microbial antigenicity, which all together stimulates the immune system. In one of the study, a 5 to 15% decree in growth was found when streptococci the normal gut flora inoculated into germ free chicks (ref K, L). In another study it was found that in the presence of gut bacteria the host growth most notably decreased when there is inadequate food supply in term of quality and quantity compared to optimum (ref M, N, O). This suggests that in the presence of parasite the maintenance of immunity has greater priority over body growth, particularly if there is lake of protein in diet. Additionally, the germ free chicks balance diets in term of sufficient energy, shown lower rate of metabolised energy, greater amount of protein and energy preservation, reduced need for maintenance energy, and all that leads to 5 to 30% increased growth than conventionally raised chicks. No of studies demonstrated that livestock and chickens have show enhanced growth performance, feed competitively, and decreased oxygen consumption when grown using selected antibodies in their food (ref K, P, Q, R).

It is evident that alteration of nutrient dynamics does not require very strong immune challenges in host and, sometime a mild immune stimulation such as in response to vaccination can repress the food intake and development. For example, pigs proved a 15% decrease in their food intake and 21 % decline in daily weight gain in response to vaccination against procrine respiratory and reproductive syndrome or endotoxin, in addition, similar finding was noted in lambs (ref S, T).

 Metabolic Cost for Immunity in Vertebrates:

To measure the metabolic costs for maintaining the effective immune system is not impossible but enormously difficult due to various reasons. In reality, so far, there is not a single study which has truly addressed the issues related to the cost of maintain an effective immune system for any vertebrate species. However, it is well known by now that the energy state of a host has great impact on the affectivity of immune response and the level of resistance against pathogens, as well as on the other side, immune status influence the energy level of the host body. For instance, the low energy level diet, if is taken long-standing leads to the suppression of immune system and increased risk of opportunistic infection (ref U, V)

One of the space studies also revealed the significance of energy for the host to keep effective immune system against parasites and other pathogenic intruders, in this study, a 90% decline in Cytotoxic T lymphocyte function was found in astronauts due to very low energy requirement and production manifested by zero-gravity environment (ref w). Some laboratory studies demonstrated that 30% and 25% increase, in the respiration rate of mitochondria in response to TNF-α and IL-1 stimulation and, the resting metabolic rates of healthy volunteers upon IL-6 infusion was found respectively (ref x, y). High levels of glucose and glutamine are used to fuel the up-regulation of immune cells; however, this use of extensive glucose and glucosamine leads to the speedy breakdown of the body's reserves of protein, carbohydrates, and lipids (ref Z, Za). A mild infection can simply escort to 150-200% augment in gluconeogenesis rate in the host.

Measuring the whole organism metabolic costs is really hard due to number of factors as discussed before in invertebrate section. However, dispute of this difficulty there are good number of studies trying to address this question to certain extant. In case of severe infection, humans tend to loss 15 to 30% of the body weight, along with 25 to 55% increase in resting metabolic rates compared to healthy individuals (ref Zb, Zc). To fulfil these energetic demands an individual utilised 20% protein metabolised from skeletal muscles, while rest of is managed by carbohydrate and lipids (ref Zd). Additionally, during the acute phase of infection a whole organism utilisation of glucose can elevate by 68% and, 10 to 15% increased was noted in the basal metabolic rate for 1C rise in the body temperature in humans suffering from fever, a trademark of infections (ref Ze, Zf). Due to mild stimulation of immune system by vaccination a 15 to 30% increase in host metabolic rate was noted in couple of researches (ref Zg, h, i).

Immunity in Drosophila:

Before going on, to discuss the specific details of Drosophila immunity here is the briefly summarise what is known of immunity and its basic types. Immunity is a biological state of a living body that provides defensive mechanisms against infections, diseases and to other unwanted exposures to the body such as allergens and, the these defensive immune responses works on the basic principal of differentiating between self and non self. Immunity can broadly be classified into innate immunity (non specific); that confers an immediate protection against infecting organisms in a general way without resulting in long-lasting immunity in the host, unlike acquired immune responses [29] and the adaptive (antigen-specific) immunity. The latter one is associated with long term memory to respond to a particular antigen, and can be subdivided into natural acquired and artificial acquired

Figure 1: Diagrammatic presentation for the types of immunity.

immunity. Either of them can be active or passive depending on the route to generate the immune responses; such as if the immunity elicits in response to pathogens would be active immunity, whereas if it is administrating by another immune host or source (vaccination or parental transfer) would be called as passive immunisation. Adaptive immunity can also be categorised into acquired humoral immunity and acquired cell mediated immunity depending on the involvement of the cell types.

Immune Recognition in Drosophila:

The recognition step is the hallmark of immune system; the immune cells are capable to differentiate between self and non self, so recognise only foreign intruders and efficiently induces the immune response against them. In invertebrates recognition of foreign intruders is carried out by microbial recognition receptors (MRR) such as peptidoglycan binding proteins (PGBP) and gram-negative binding proteins (GNBP) (ref zr). The Toll and IMD pathway which are functionally resembled to the mammalian T lymphocyte receptors (TLR) and tumor necrosis factor receptor (TNFR) signaling pathways respectively are involved in pathogen-recognition process that leads to the initiation of local and systemic innate immune responses in Drosophila (ref zs, zt). The Toll pathway predominantly triggered by PGBP in the presence of gram-positive and fungal infections; however, the IMD pathway turns on by GNBP in response to gram-negative bacteria (ref zw). Once a pathogen like bacteria or fungus inters in fruit fly body the innate immune response gets activated by MRR recognising the repetitive microbial surface determinants which are conserved in microbes but not found in host genome such as lipopolyseccaride (LPA), peptidoglycane and manan etc. Upon recognition the host receptors activate the signal cascade for the transcription of regulator and effectors molecules against that invader (ref zj).

General Features of Drosophila Innate Immune Responses:

As far as insects are concerned they mainly rely on innate arm of the immunity for their defence against parasites as they lake the adaptive immunity. Drosophila has multiple innate defensive mechanisms, many of which are shared with higher organisms such as Toll signaling pathway of Drosophila is also involved in mammalian innate immunity; this makes Drosophila a highly competent model to study innate immunity in animals (ref). The innate defensive mechanisms in Drosophila include the use of physical barrier (epithelia) along with local (production of antimicrobial peptides) and systemic (specialized haemocytes) immune responses.

The epithelia of Drosophila provide the front line of defence against pathogens by producing antimicrobial peptides (ref zk) However, the microbes which have prevented by that front line of defence and have successfully entered into hemococle are then dealt by both the cellular and humoral responses (ref zl, zm). Cellular responses are based on encapsulation and phagocytosis of parasite, special phagocytic cells such as plasmactocytes (look like macrophages) and lamellocytes (special flattened cells) are responsible for cellular responses in Drosophila (ref zn). Whereas, humoral immune responses are involved in the production of antimicrobial peptides (so far seven antimicrobial peptides have been found in Drosophila which are Diptericin, Drosocin, Defensin, Attacin, Cecropin, Drosomycin and Metchnikowin) into the heamolymph and the activation of numerous proteolytic cascades of phenoloxidase (proPO cascade) such as coagulation cascade and melanisation cascade that produces melanin around the invader or at the wound site (ref zm, zo). Additionally, melanization cascade also involves in the production of quinones and toxic reactive oxygen species (ROS) (ref zp). Drosophila mutants that are unable to produce antimicrobial peptides found to be more susceptible to microbial infection which indicates the vital importance of these antimicrobial peptides and innate immune defence of Drosophila (ref zq). The antimicrobial peptides are synthesized by the fat body in Drosophila that is an equivalent to mammalian liver.

Protolytic Phenoloxidase Cascade (ProPO Cascade)

It is well known that humoral defence in invertebrates depends primarily on the activation of ProPO cascade in addition to the production of antimicrobial peptides. PO is produced by the special type of haemocytes as an inactive form which then converted into active PO by the action of proPO-activating enzyme; this enzyme induces upon microbial recognition. This activated PO involves in phenol oxidation to convert it into quinones which have dual action that can directly target the pathogenic invader and can also polymerize into melanin which melanise the microbes and wounded lesions.

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