Transmission by Insect vectors

Insect ingested (double pored tapeworm)

1) As insect bloodfeeds, parasites in the salivary glands injected into the host along with the saliva (Plasmodium, Arboviruses).

2) As insect bloodfeeds, parasites in the mouthparts recognize the host as suitable and forcibly exit the mouthparts and enter the host (filarial nematodes).

3) As insect feeds in engorges and defecates, parasites in feces enter host (Chagas disease)

4) As insect feeds it regurgitates compounds from its crop and the parasites enter the host (Plague bacterium)

5) Parasites in 3 and 4 remain in the GI tract and do not come in contact with the tissues that are important in the invertebrate immune response to these parasites.

For the others…. Why do the insects tolerate the presence of the parasites?

Why do they not kill them?

How might they kill these parasites?

Fitness Costs Associated with Parasite Infection

*** * ****

reduced nutrients available to hostreduced synthesis of vitellogenin in fat body (Hogg et al. 1997)ovary uptake of vitellogenin is impaired (Hogg et al. 1997)

resorption of developing follicles (Carwardine and Hurd 1997)reduced fecundity (Ahmed et al. 1999)reduced fertility (Hacker 1971, Hogg and Hurd 1995)bloodfeeding behavior affected (Anderson et al. 1999)

increase in hemolymph yolk proteins

Why should vectors protect themselves from parasites?

Do human parasites affect their vectors?

1) Why do insects tolerate parasites and pathogens?

2) How do insects protect themselves against parasites and pathogens? 

3) Why do insects not kill all parasites and pathogens?

The Vector‐Parasite Relationship

3

H

F

E

D

E

G

B

A

C

I

Why do the insects not kill their parasites???

To answer this we first must understand how insects CANprotect themselves, and then determine why these protective measures are not used or are not effective against the pathogens

INNATE IMMUNITY OF INSECTS

Innate immunity refers to a nonspecific defense mechanisms that a host uses immediately or within several hours after exposure to a stimulus. This is the immunity one is born with and is the initial response by the body to eliminate microbes and prevent infection.

Unlike adaptive immunity, innate immunity does not recognize every possible antigen. Instead, it is designed to recognize a few highly conserved structures present in many different microorganisms. The structures recognized are called pathogen-associated molecular patterns (PAMPs). Most defense cells have pattern-recognition receptors for these common pathogen-associated molecular patterns to allow for an immediate response against invading microorganisms.

Pathogen-associated molecular patterns can also be recognized by a series of soluble pattern-recognition receptors in the blood that function as opsoninsand initiate the complement pathways.

Examples of innate immunity include anatomical barriers, mechanical removal, bacterial antagonism, pattern-recognition receptors, antigen-nonspecific defense chemicals, the complement pathways, phagocytosis, inflammation, and fever.

pathogen-associated molecular patterns (PAMPs): conserved molecular patterns on microbes

lipopolysaccharide (LPS) from the gram-negative bacteria cell wall;

peptidoglycans found abundantly in the gram-positive cell wall and to a lesser degree in the gram-negative cell wall

lipoteichoic acids found in the gram-positive cell wall;

mannose-rich glycans (common in microbial glycoproteins and glycolipids);

Β-glucans on fungi

To recognize these microbial molecules, various body defense cells have on their surface a variety of receptors called pattern-recognition receptors capable of binding specifically to conserved portions of these molecules.

Pattern-Recognition Receptors (Including Toll-Like Receptors)

1. Pattern Recognition Receptors (PRR )Recognize pathogen associated molecular patterns (PAMP); conserved molecular patterns on microbes

Toll-Like Receptors (TLR):

First discovered in DrosophilaEleven receptors identified in mice and humans

Ligands arePAMP

(pathogen-associatedmolecularpatterns)

Receptors are

PRR(pattern-

recognitionreceptors)

Nobel prize 2011

Immune Response of InsectsPresence of Pathogens

Recognition?

Hemocytes

PhagocytosisAntimicrobial compounds

Transferrin

Hemocytes?

SerineProteases

Fat body

SerineProteases

Activation via TollIMD, y IRD

Production ofImmune peptides

AntimicrobialActivityMelanotic encapsulation

DefensinsCecropins

Proline-Rich PeptidesGlycine-Rich Peptides

Others?

Molecules of

Tyrosine

ProPO

PO

DDC

?

DCE

communication

Molecules of

communicacion

E. coli 3 hr: Phagocytosis

Immune Response of InsectsPresence of Pathogens

Recognition?

Hemocytes

PhagocytosisAntimicrobial compounds

Transferrin

Hemocytes?

SerineProteases

Fat body

SerineProteases

Activation via TollIMD, y IRD

Production ofImmune peptides

AntimicrobialActivityMelanotic encapsulation

DefensinsCecropins

Proline-Rich PeptidesGlycine-Rich Peptides

Others?

Molecules of

Tyrosine

ProPO

PO

DDC

?

DCE

communication

Molecules of

communicacion

Melanotic encapsulation

Immune Response of InsectsPresence of Pathogens

Recognition?

Hemocytes

PhagocytosisAntimicrobial compounds

Transferrin

Hemocytes?

SerineProteases

Fat body

SerineProteases

Activation via TollIMD, y IRD

Production ofImmune peptides

AntimicrobialActivityMelanotic encapsulation

DefensinsCecropins

Proline-Rich PeptidesGlycine-Rich Peptides

Others?

Molecules of

Tyrosine

ProPO

PO

DDC

?

DCE

communication

Molecules of

communicacion

Mosquito Antimicrobial Peptides

defensin

cecropin

gambicin

lysozyme

Bomanin

Jacob

Advantages of Insect Innate Immune System

* Antimicrobial peptide generation is very fast

* Peptides are potent with wide spectrum of activity

* Small peptides diffuse quickly

* Insect immune peptides do not need special cells for production

Where do peptides act? How can parasites survive?

The majority of insect immune peptides are expressed in hemocytes and fat body tissues and secreted into the hemolymph

Parasites can:

1) Evade immune response

2) Inactivate immune response

3) Avoid contact with immune response

defensindefensin

DENv comprise 4 antigenically distinct serotypes: DENv‐1, ‐2, ‐3, ‐4

• 2.5 billion people at risk• 50-100 million new infections/year • ~500,000 cases of DHF, DSS• No vaccine, no drugs

Transmitted by mosquitoes• Aedes aegypti, Aedes albopictus, 

Aedes polynesiensis

19

What happens with Intracellular Parasites?

Intracellular viruses are not freely exposed to classical components of the vector immune response

Apoptosis: Programmed Cell Death

• Cellular response to damage, age, and stress– Intracellular infection

• Cells respond to viral infection by initiating apoptotic cell death

• Powerful immune response – severely limit virus 

production– reduce or eliminate the 

spread of progeny virus20

We believe:

Dengue enters cells- Mosquito activates apoptosis

virus over expresses IAP1

Apoptosis inhibited until virus has replicated

Cells allowed to burst- releasing virions

How can we prove/disprove/study this?

• Cali, Colombia:

• >120,000 dengue cases

• 115 deaths

• Naturally DENv resistant field population of Aedes aegypti

22

Costs associated with Parasitism:

Reduced nutrients available to host

Reduced synthesis of vitellogenin in the fat body

Uptake of vitellogenin by the ovary is impaired

Increased concentration of egg yolk proteins

Resorption of developing follicles

Reduced fecundity & fertility

Bloodfeeding behaviour may be altered

Fitness Costs Associated with Immune Response

Melanization

****

reduced fertility increased time to ovipositionreduced longevitycompetition for resources needed for egg

* development and melanin synthesis

Immune peptides and Phagocytosis

***

No apparent reduction in fertilityNo significant reduction in longevityCompetition for resources?

Tyrosine

Phenylalanine

Defense Egg Development

PAH

Competition

TRADE OFFS

Undergoing immune response Controls(Ferdig et al. 1993)

Why study insect immune responses?

•Understand general insect immunity: the innate responses that insects use to protect themselves from pathogens

•By understanding how this system works we may exploit it to enhance the success of management strategies

•Invertebrates have no antigen:antibody system, and lack a memory function. But their innate immune response is extremely similar to that of the vertebrate acute phase response. We can look at the origin and progenitor responses that have arisen through different evolutionary periods and which form the only immune response in invertebrates.

• Identify several novel immune peptides that have potent antimicrobial activity. These have a broad spectrum of activity, and can be produced and released with no known toxicity to eukaryotic organisms.

Rhodnius prolixus Anopheles gambiae

T. Cruzi P. falciparum

Brugia malayiBrugia pahangi

develops killed

Armigeres subalbatus