Molecular Mechanism Of Mechanism

1. Molecular Mechanism Of Parasitism
Hamid-Ur-Rahman, M.Phil Ist

2. Some Basic Terminologies
 Parasite
A parasite is an organism that is entirely dependent
on another organism referred to as its host, for all or
part of its lifecycle and metabolic requirements.
 Host
An organism which harbors the parasite and provides
the nourishment and shelter to the latter
 Parasitism
A relationship in which a parasite benefits and the
host provides the benefit. The host gets nothing in
return and always suffers from some injury.

3.  Parasitism is differentiated from parasitoidism, a
relationship in which the host is always killed by the
parasite; parasitoidism occurs in some Hymenoptera
(ants, wasps, and bees), Diptera (flies), and a few
Lepidoptera (butterflies and moths): the female lays
her eggs in or on the host, upon which the larvae feed
on hatching.
 Brood parasitism
A form of parasitism called brood parasitism is
practiced by the cuckoo and the cowbird, which do
not build nests of their own but deposit their eggs in
the nests of other species and abandon them there.
Though the cowbird’s parasitism does not necessarily
harm its host’s brood, the cuckoo may remove one or
more host eggs to avoid detection, and the young
cuckoo may heave the host’s eggs and nestlings from
the nest.

4. Fig. Parasitiodism
Female wasp lay egg on Tomato horn worm
caterpillar

5. Fig.Brood Parasitism

6.  Social Parasitism
Another form of parasitism, such as that practiced
by some ants on ants of other species, is known as
social parasitism.
 Hyperparasitism
Parasites may also become parasitized; such a
relationship, known as hyperparasitism, may be
exemplified by a protozoan (the hyperparasite) living
in the digestive tract of a flea living on a dog.
 Parasitism can take the form of isolated cheating or
exploitation among more generalized mutualistic
interactions. For example, broad classes of plants and
fungi exchange carbon and nutrients in common
mutualistic mycorrhizal relationships; however, some
plant species known as myco-heterotrophs "cheat" by
taking carbon from a fungus rather than donating it.

7. Fig. Social Parasitism

8. Fig. Hyperasatism
A leaf-mining caterpillar (left) is blackened and dead.
It was killed by a wasp larva

9.  The initial interaction between parasite and host
often involves invasion of a parasite into host tissue
or host cells.
 Single cell protozoan parasites often evade the host
immune response by residing within host cells.
 They must enter the host cell with minimal trauma to
ensure that they preserve an environment suitable
for their replication and not trigger a lethal host
immune response
 This intracellular invasion paradigm is also shared by
a helminth parasite, Trichinella spiralis.
 Most other helminths reside in the extracellular
space.
 Often an invasive larval form will penetrate skin or
the mucosa of the gastrointestinal tract to gain entry
into the host. Migration can be fairly extensive.

10.  To complete this invasion process, helminth parasites
have evolved sensory organs for finding and
navigating within the host, attachment mechanisms
utilizing specialized mouth structures or glue-like
secretions, and hydrolytic enzymes to digest
macromolecular barriers in the extracellular space.
 INVASION INTO CELLS
How to Enter a Cell: Phagocytosis or Invasion????
 Intracellular parasitism implies entry of a pathogen
into the cytoplasm of a host cell. Various strategies
have been evolved by microorganisms to achieve this
goal.
 Because some host cells may have the ability to
internalize foreign material by phagocytosis,
distinction between parasite invasion or uptake is
often a challenging issue.

11.  Phagocytosis is a peculiar type of endocytosis that is
usually performed by specialized cells.
 It involves binding of a particle to the cell
plasmalemma through various ligand-receptor
interactions (integrins, Fc receptors, hydrophobic
interactions), zippering of the membrane around the
particle and internalization into the resulting vacuole.
 The vacuole membrane is directly derived from the
plasmalemma of the cell and will be enriched in some
molecules (receptors) if these were involved in the
binding.
 The vacuole enters the endocytic pathway and fuses
eventually with lysosomes after acidification by a
proton ATPase located in its membrane. Various
markers of the successive compartments are known,

12. Fig. Shows
Process of
Phagocytosis

13. especially the lysosomal membrane glycoproteins
(LGP) that are markers of late endosomes and
lysosomes.
 Host cell invasion by many microorganisms does not
utilize the phagocytosis route and the vacuole that is
formed either does not enter the endocytic pathway
or is destroyed before the phagolysosome stage.
 An artificial gradation can be described starting from
Sporozoa that appear to have created an
internalization procedure entirely distinct from
phagocytosis; through Trypanosoma cruzi that may
use the phagocytic system to get into the cell but
then escape into the cytoplasm; to Leishmania
promastigotes that are internalized and live into a
mostly typical phagolysosome.

14. Creating a New Compartment in the Host Cell:
Sporozoa
 A major feature of Sporozoa is to have evolved
specific structures and organelles for host cell
invasion; no other group except Microsporidia has
achieved such a complex differentiation.
 Among Sporozoa, the Plasmodium genus has been the
most thoroughly investigated.
 Most of our knowledge on host cell invasion by
Sporozoa has been obtained from studies of
erythrocyte invasion by malaria merozoites.
 Studies on other members of the group that invade
nucleated cells have shown that the invasion process
is not significantly different from that described in
Plasmodium

15.  The very conserved organization of the invasive stage
(zoite) among Sporozoa is a key to the explanation of
invasion in this group.
 Indeed, all zoites of Sporozoa share a cellular
polarity and specialized organelles organized into an
apical complex (from Apicomplexa).
 The apical complex occupies the anterior part of the
cell and comprises a cytoskeleton (anterior rings,
subpellicular microtubules, inner membrane complex)
and vesicular electron dense organelles (rhoptries,
micronemes, dense granules). All the components of
this structure are considered to be involved in host
cell invasion.
 The invasion phenomenon itself can be divided into
three successive steps: recognition and attachment,
internalization and vacuole development, vacuole
maturation

17. o Recognition and attachment
 Surface to surface binding between zoite and host cell is
supposed to involve not only zoite surface molecules and host
cell surface, but also bridging molecules exocytosed by the
zoite (erythrocyte binding antigen in Plasmodium falciparum,
135kDa Duffy receptor in P. knowlesi or P. vivax, both of which
are contained in micronemes) or components of the
extracellular matrix (laminin in Toxoplasma).
 The red cell surface receptors used by P. falciparum which
include glycophorins, sialic acids and band 3 are still
incompletely identified. P. vivax and P. knowlesi use different
receptors among which is a 45 kDa glycoprotein that carries
the Duffy determinant.
 A reorientation step has been observed in Plasmodium after
attachment.

18.  It may result from a gradient of receptor distribution on
merozoite surface, or from the presence of apically
located higher affinity receptors.
 This reorientation brings the apex in contact with the cell
membrane, which is necessary for internalization. Such a
passive reorientation step may not be present in other
Sporozoa where gliding motility or conoid flexing could
bring about the apical contact.
 The recognition or receptor-ligand interaction between
Plasmodium and its host cell is highly specific as illustrated
by the very narrow range of invasion capabilities of these
organisms.
 The matter is much less clear for other Sporozoa, the
prototype of which is Toxoplasma.
 These can invade any type of cell in vitro except the
erythrocyte, and the efficiency of invasion may be

19. modulated by receptor-ligand interaction or a ubiquitous
membrane molecule such as cholesterol might serve as a
receptor triggering invasion for these zoites.
o Internalization, parasitophorous vacuole
formation
 Motility and moving junction.
 Once a zoite has made an apical contact with the target
cell, there is a close junction between both plasmalemma.
 Freeze fracture shows a rhomboidal array of
intramembrane particles which is likely to be a crystalline
array of lipids.
 As demonstrated in P. knowlesi, all the preliminary steps
can occur in the presence of cytochalasin B that inhibits
both the zoite motility and invasion..

20.  This has been observed in all Sporozoa studied so far
and strongly suggests that invasion operates by using
the gliding motility system of the zoites.
 This is likely to involve an actin-based motor located in
the inner membrane complex of the zoite .
 The apical junction turns into a annulus through which
the zoite glides into the developing vacuole.
 Vacuole membrane
 The origin of the vacuole membrane is unclear. It is in
continuity with the plasmalemma of the host cell
through the moving junction during the invasion
process, but its structure is dramatically different.
 It is almost completely devoid of intramembranous
particles or host cell surface proteins.

21.  Exocytosis of organelles
 The zoite contribution to the vacuole is believed to occur
through the exocytosis of specialized apical organelles named
rhoptries.
 Rhoptries are pedunculated organelles, the ducts of which
extend toward the apex of the zoite and are open for
exocytosis during the invasion.
 A complex set of proteins, some of which are grouped in
families, have been identified in these organelles in both P.
falciparum and Toxoplasma.
 Some of these are associated with the developing vacuole
membrane, thus confirming the exocytic process .
 Ultrastructural (Plasmodium) or biochemical (Toxoplasma)
data on rhoptries suggest the presence of lipids that could
also be inserted in the vacuole membrane upon exocytosis.

22.  Enzyme activities such as proteases and phospholipases may
be involved in invasion. These activities could modify cell
surface proteins or lipids or the underlying cytoskeleton during
formation of the vacuolar membrane.
 Vacuole maturation
 Once the vacuole is completed a maturation step occurs, during
or shortly after invasion, that is characterized by the
exocytosis of dense granules from the parasite.
 The proteins of the dense granules are targeted to either the
vacuolar space , a vacuolar membranous network and the
vacuole membrane or the inner side of the host cell membrane
 Vacuole maturation reflects the development of metabolic
exchanges between parasite and host cell

23.  The vacuole is entirely isolated from the endosomal
traffic of the host cell and the parasite must rely on
transmembrane transport to get its nutrients.
 The contribution of the parasite to the vacuole
includes the necessary transporters or channels in
that membrane.
 In P. falciparum the existence of direct channels
between the vacuole and the extracellular space has
been claimed.
 A peculiar type of vacuole maturation occurs in
Piroplasma where the vacuole disappears after the
exocytosis of dense granules .
 The organism develops directly in the cytoplasm of
the host cell. The mechanism of vacuole lysis is not
known.

24.  This type of intracellular behavior is morphologically
shared with Trypanosoma cruzi , but whether any
molecular function is shared remains to be investigated.
o Escaping from a Phagosome into the Cytoplasm:
Trypanosoma cruzi
 This organism has an intracellular phase in the vertebrate
host only and invasion is performed by trypomastigotes
derived from the insect gut or previous round of
intracellular multiplication.
 Although these organisms are highly motile, their active
involvement in the invasion has long been controversial.
 Fibronectin augments the internalization of T. cruzi in
phagocytic and non-phagocytic cells .

25.  It acts as a molecular bridge between parasite and host
through attachment to a beta 1 integrin.
 Attachment is energy dependent on the part of the parasite
and does not need host cell metabolism since fixed cells can
be attached and invaded
 Although entry of T. cruzi in phagocytes is believed to be
mediated by phagocytosis, typical phagocytic receptors (FcR,
CR3) do not seem to be involved .
 Penetration in non-phagocytic cells seems to be an active
process that is not blocked by cytochalasin D, which inhibits
actin-based systems .
 During the first 60 min, T. cruzi resides in a vacuole that
acquires lysosomal glycoproteins in its membrane which means
that it fuses with endocytic vesicles en route to lysosomes.

26. Fig. Showing, Mechanisms of
T.cruzi Entry in to the cell

27.  The vacuole acidifies and a pore-forming protein,
antigenically similar to C9, the membranolytic
component of complement, is released in the vacuole.
 The vacuole membrane becomes discontinuous and
disappears leaving the parasite directly in the
cytoplasm where it transforms into amastigotes.
o Living in a Phagosome- Leishmania
 Infectious forms of Leishmania (metacyclic
promastigotes) possess two types of surface
molecules involved in attachment and invasion.
 These are the lipophosphoglycan (LPG) and the major
surface protease GP63.
 Both of these ligands can independently mediate
attachment of the parasite to macrophages
 Leishmania do not invade nonphagocytic cells.

28.  Internalization may require synergy of both parasite
ligands as has been shown with artificial systems of
beads coated with both molecules separately or
associated.
 The cellular receptors involved in binding and
internalization may be multiple but the complement
component CR3 seems to be a major receptor that
binds both LPG and GP63.
 The vacuole containing the parasite is acidified and
its membrane becomes LGP-positive but MPR
(mannose-6-phosphate receptor) negative.
 This corresponds to a lysosomal compartment. Only
some extracellular ligands appear in the vacuole via
receptor-mediated endocytosis; this means that it
fuses only with some of the receptor-mediated
endocytic pathways

29. Fig.
Living in a Phagosome-
Leishmania

30.  The survival and development of Leishmania sp. within
the hostile environment represented by the lysosomal
contents is supposed to be mediated by several
protective mechanisms including the inhibition of the
macrophage respiratory burst by LPG and the activity
of parasitic enzymes (proteases, superoxide
dismutase and acid phosphatase) that may counteract
the activity of lysosomal enzymes.
o A Multicellular Organism Within a Cell:
Trichinella
 The L1 larvae of this nematode are born in the lamina
propria of the gut, travel in the blood and eventually
enter a striated skeletal muscle cell.
 The mechanism of host cell penetration is not known
but what has been extensively studied is the
transformation of the host cell by the parasite after
entry.

31.  The multinucleated muscle fiber loses the myofibrils
that are replaced by smooth membranes and
mitochondria; the nuclei enlarge and develop
prominent nucleoli; the cell glycocalix is replaced by a
thick collagen coat and around the nurse cell thus
formed, angiogenesis is triggered and a circulatory
plexus develops.
 This transformation reflects a dramatic alteration of
the host cell. This is believed to be triggered by the
worm which sends information modifying gene
expression.
 The exact nature of the message is not known but
proteins secreted in the cell by the larva are likely to
play a role in this modulation.
 A 43 kDa glycoprotein exocytosed from granules
found in specialized cells of the worm (stichocytes) is

32. Fig. Trichinella in Muscle Cell

33. targeted to the nuclei of the host cell where it can be
immunodetected and may play a role in modulation of
host genomic expression; other secreted parasite
molecules could also be involved in this
transformation.
o Injection into a Cell: Microsporidia
 These parasites are quite original with respect to
invasion since they inject themselves through the
plasmalemma of the host cell.
 The microsporidian spore is a highly organized system
of a coiled hollow polar filament that everts at
excystation to open a hole in a target cell and inject
the microsporidium.
 The parasite then develops directly in the cytoplasm
of the host cell.
 Triggering of the spore discharge seems to be

34.  INVASION BY HELMINTHS
 Helminths are multicellular parasites, often a
millimeter or more in length.
 With rare exceptions (see Trichinella), invasion of
and seclusion within host cells is not feasible.
 When speaking of host invasion by helminth parasites,
one considers tissue invasion or, more specifically,
migration through extracellular barriers.
 Different helminth parasites may have quite distinct
pathways of invasion and migration in the host, and
may employ different mechanisms to facilitate that
invasion
 The port of entry for several parasitic nematodes
and trematodes is the skin.
 However, even within this group, the exact mode of
entry may vary significantly.

35.  Invasive larvae of Stronglyloides, hookworm and
schistosomes may enter the skin directly, without the
need for an insect vector bite or accidental trauma.
 Larvae of Bruoia and Wuchereria are deposited on
skin from the salivary gland of the mosquito vector
and follow the mosquito bite path to a lymphatic
vessel.
 The L3 larvae of Onchocerca, are also deposited at
the vector bite site, but then migrate for a
considerable distance through dermal connective
tissue before reaching their final site of adult
residence.
 The intestinal wall is another barrier for helminth
parasite invasion.
 Here, the common endpoint of invasion is the lumen of
a blood vessel, with subsequent dissemination to
multiple organs.

36.  Schistosome eggs must leave the lumen of mesenteric
blood vessels and cross the intestinal wall in a path
exactly opposite that of invasive gastrointestinal
parasites.
o Pathways of Helminthes Invasion
 Specific Steps in the Invasion Process
1). Tactic responses
 For the parasite entering the host from the
external environment, this is the process by
which it finds the host.
 For the parasite in the lumen of the gut or in
the bloodstream, this is the process that
defines where it will go and, equally important,
when it will stop.

37. 2). Attachment
 Prior to invasion it is necessary for parasites,
particularly those coming from the external
environment, to attach to the host.
 During the actual process of invasion, cyclical
attachment and release from extracellular matrix or
other structures may be necessary for motility
3). Digestion of macromolecular barriers
 Particularly in migration through skin and connective
tissue, the interactive extracellular matrix must be
breached before a multicellular organism can invade
4). Evasion of host immunity
 Implicit in the ability to invade is the ability to evade
host responses that would block parasite migration or
be directly lethal to the organism

38.  A). Tactic responses and migration
 Some information is available for those helminth
parasites which enter the host from external
environments.
 Schistosome cercariae and larvae of Stronglyloides
and hookworm respond to specific signals that
identify the host as such and have been the most
extensively studied.
 A response to light ensures that schistosome
cercariae reside in the same part of a pond or lake as
the host.
 Schistosome cercariae which infect humans are shed
from the intermediate host snail under stimulus of
light following a period of darkness.
 Schistosoma douthitti, in contrast, which infects
nocturnal mammals such as voles and muskrats, leaves
the snail in darkness following a period of light.

39.  Cercariae of Diplostomums pathaceum, which infect
fish, are stimulated to swimming bursts by water
turbulence or touch; Trichobilharzia ocellata, which
infects ducks, is stimulated by shadows.
 To contact host skin and invade, schistosome
cercariae follow a thermal gradient and then use an
elegantly adapted penetration response triggered by
specific fatty acids present on the skin.
 Specific free fatty acids like linoleic acid will
stimulate cercariae to invade in vitro.
 Whether this is receptor-mediated is not known, but
cercariae can metabolize linoleic acid to eicosanoids ,
which are potential 'second messengers'.
 Upon stimulation cercariae lose their glyocalyx and
tails and release a protease from preacetabular gland
cells to facilitate invasion.

40.  Hookworm and Stronglyloides larvae enhance their
chance of encountering host skin by aggregating in
clusters at the highest points of blades of grass .
 Hookworm larvae also recognize mammalian hosts by
sensing CO2 and are stimulated to penetrate by host
skin proteins .
 The molecular details of this interaction have yet to
be elucidated.
 Cercariae of A. brauni do not recognize small
molecules such as amino acids, monosaccharides or
electrolytes, but do respond to hyaluronic acid and
glycoproteins for attachment.
 Penetration of the host is triggered by free fatty
acids and mucus components present on the fish skin
surface .
 Monogenean parasites of fish skin lay eggs which fall
to the sea or river bottom.

41.  Hatching of eggs of Entobdella soleae is timed by
photoperiod to occur just after dawn, when the
common sole (Solea solea) begins resting on the sandy
bottom.
 Hatching is also stimulated by mucus from the host
fish skin.
 The ciliated oncomiracidium that emerges is
phototactic and may also sense currents .
 Thermosensing undoubtedly plays a role in the
directional migration or invasion of some nematode
parasites as it does in schistosome cercariae.
 Caenorhabditis elegans larvae and adults migrate to
the temperature at which they have been growing
when they have been placed in thermal gradient.

42.  Mutants with thermotactic abnormalities have been
isolated. Many of them have lost the ability to
migrate towards a specific temperature, whereas
others show reversal of the usual behavior.
 Many, although not all, of these thermotactic mutants
are also defective in chemotaxis.
 Chemotaxis was one of the first sensory behaviors
noted in C. elegans
 Worms are attracted to cyclic nucleotides, both
anions and cations, some amino acids, and extracts of
bacteria.
 All of the chemotactic mutants have been
morphologically mapped to specific cells with
corresponding sensory abnormalities in the head of
the worm.

43.  Onchocerca cervipedis, (which infects deer)The adult
Onchocerca live in the hindquarters of the deer and
the microfilariae migrate across the body of the
deer, up the neck, and into the ears.
 The blackfly vectors recognize the upright ears of
the deer and congregate there.
 Movement in aqueous environments of the ciliated
miracidia and fork-tailed cercariae of schistosomes,
and oncomiracidia of monogenean fish parasites, is
very rapid.
o Attachment
 Many nematode parasites, such as hookworm adults
and Anisakis adults, have evolved mouth structures
which allow attachment to intestinal mucosa .
 Monogenean parasites of fish skin, which must attach
to a rapidly moving host and resist water shear, also
have highly evolved attachment structures

44.  Schistosome cercariae emit a sticky mucus substance
from the posterior acetabular glands, which allow
them to attach to each other as well as to skin
 Cercariae may be induced to cluster together on skin
in response to L-arginine emitted from their
secretory glands during the initial stages of skin
exploration .
o Digestion of Macromolecular Barriers
 Although it is plausible that enzymes such as
hyaluronidases, glycosidases or lipases may digest
certain barriers that face invading helminths,
evidence to date suggests that these functions are
largely performed by proteolytic enzymes.
 The best evidence is from studies of Strongyloides
L3 larvae and schistosome cercariae.

45.  In both of these cases, secreted proteases with the
capacity to degrade connective tissue or basement
membrane molecules have been identified.
 Specific inhibitors of these proteases will inhibit
invasion of skin by the larvae.
 Related proteases have been identified in many other
parasites, including hookworm , Anisakis , Ascaris ,
Toxocara and Onchocerca.
 The enzymes discovered in excretory/secretory
products of invasive stages of helminth parasites also
might be important in anticoagulation, digestion,
immune evasion, or morphologic transformation.
 The most direct test of the function of these
enzymes is to show that blocking the action of the
protease by irreversible inhibitors, or active-site-
directed antibodies, inhibits invasion.

46. Entamoeba histolytica---a Protozoan Parasite that
Invades Extracellularly
 In contrast to the other protozoa , Entamoeba
histolytica is not at any stage in its life cycle an
intracellular parasite.
 Rather, it is more like the helminth parasites in that
it invades tissue extracellular matrix, producing
destructive lesions in the wall of the colon, and has
the capacity to metastasize to other organs .
 A second distinctive aspect of infection by E.
histolytica is that invasion of the host is by no means
necessary for parasite replication or transmission.
 Only 10% or less of all cases of amebiasis result in
invasive infections.
 First, trophozoites of E. histolytica have specific
surface lectins and adhesion-mediating glycoproteins
which allow……

47. attachment to intestinal mucus, target cells or host
extracellular matrix.
 Attachment appears to be necessary for target cell
lysis, which is mediated at least in part by a cytolytic
ion channel-forming protein secreted by the
trophozoites.
 Pathogenic, invasive trophozoites of E. histolytica also
release proteinases.
 The major enzyme is a cysteine proteinase with
cathepsin B-like substrate specificity and structural
homology to members of the papain superfamily.
 Release of this enzyme correlates with pathogenicity
of clinical isolates and induces an immune reponse in
infected individuals.
 Direct inhibition of this enzyme by specific,
irreversible cysteine protease inhibitors blocks the
cytopathic effect of trophozoites

49.  It also produces a specific cleavage in complement
factor C3, producing biologically active C3 cleavage
products.
 Other potential virulence factors associated with
invasion by E. histolytica include a metallocollagenase
and phospholipases
 As a group, the virulence factors identified in
extracts or secretions of pathogenic E. histolytica
can explain in large part the cytolysis and tissue
destruction that characterize E. histolytica invasion.
 Host neutrophils also contribute to the tissue
destruction seen in amebic liver abscesses .