Parasite Evasion of Host: Perkinsus marinus/Eastern Oyster as a Model

Interaction

G. Burresonwww.disease-watch.com/.../ CD/index/images/pm.jpg

Christina Panko

Introduction

Disease induction:

www.nwfsc.noaa.gov/.../ ec/ecotox/immuno2.jpg

Pathogen: protozoan parasite Perkinsus marinus

Host: eastern oyster, Crassostrea virginica

Environmental factors: temperature and salinity

History

Late 1940’s (Mackin, Owen and Collier)

Discovered and recorded in Louisianna oysters

Early 1950’s (Mackin)Characterized and termed “Dermo” disease

(Dermocysidium marinun)

1952 (Ray)Developed a culture technique to quantify disease

1980’s (Cheng) Oyster hemocyte research began

1990 (Vasta) Continuous in vitro culture

1995-present (Volety, Anderson, Ahmed, Schott, Vasta)

Parasite/hemocyte interaction research

Pathogen

80% infection of oysters in the Gulf of Mexico and along the U.S. Atlantic coast

Favorable environmental conditions are temp >20◦C and salinity >15ppt

Depletes host energy resources causing decreased growth and reproduction and increased mortality

Rapidly proliferates and spreads via hemolymph (oyster blood) through tissues causing deterioration and organ failure

www.jaxshells.org/ guide.htm

S. Bowerwww.disease-watch.com/.../ CD/index/images/pm.jpg

www.vims.edu/env/research/ shellfish/gallery.html

Host

Innate immune system, both humoral and cell-mediated

Humoral=lectins, lysozyme, and uncharacterized hemolymph proteins

Cell-mediated=hemocytes (main effector cells)

Hemocytes function in wound repair, shell repair, internal defense mechanisms, excretion, and nutrient digestion and transport

Phagocytosis is the main effector function of hemocytes

Hemocyte Phagocytosis

Fluorescent bead engulfed by hemocyte

P. marinus/hemocyte Recognition and Eventual Phagocytosis

Conjugate cell surface receptors mediate pathogen entry into the hemocyte

Hemocytes have both membrane-bound and soluble lectins with specificity for galactose residues

Coincidentally, P. marinus has cell-surface ligands composed of galactose

Lectin recognition of non-self induces binding between the conjugate receptors on hemocytes and P. marinus. Once bound, signal transduction occurs and the hemocyte receives signals to activate a cellular response (i.e. phagocytosis)

Soluble lectins in the circulating hemolymph may enhance cell surface receptor

recognition through opsonization (flag to mark particles as foreign and facilitate their recognition and phagocytic removal)

P. marinus Engulfed by a Hemocyte

S. Bower www.disease-watch.com/.../ CD/index/images/pm.jpg

Contrary to other protozoan parasites, P. marinus adapted a mechanism to

gain entry into hemocytes by specifically presenting cell surface ligands

that are rapidly recognized by the hemocyte cell surface lectins

When hemocytes phagocytose P. marinus, it facilitates rapid intracellular

proliferation

Oxygen-dependent Defense Pathway Reactive Oxygen Intermediates

P. marinus Secreted/Excreted

Scavenging Enzymes

P. marinus subverts an array of intracellular toxic ROI’s through an

“offensive response” before a host defensive challenge

Enzymes: 1. Superoxide dismutase (SOD)--catalyzes the oxidation and

reduction of superoxide to oxygen and hydrogen

peroxide

2. Ascorbate dependent peroxidase (APX)—may

potentially convert hydrogen peroxide to water

3. Protein phosphatases (PP)—may destabilize

NADPH oxidase(remove phosphate group)

Collectively the P. marinus SODs, APXs, and PPs may

provide an advantage for the parasite to evade the host

P. marinus SOD, APX and PP Subvert Reactive Oxygen Intermediates

Significance

1. Design more insightful therapeutic targets

2. Apply therapies in aquaculture to rebuild commercial output and potentially restore wild populations

3. Serve as a model of intracellular parasite evasion of host that parallels human intracellular parasites (i.e. Malaria parasite)

Summary

1. Disease induction- role of host/pathogen/environment

2. Role of the innate immune system (vertebrate and invertebrate)

3. P. marinus/hemocyte recognition and phagocytosis

4. ROI pathway (similar to human neutrophils)

5. Scavenging enzymes that subvert ROI’s