Bio 54 Schistosomes: Immunology and Host Response

1
Bio 54 Schistosomes Immunology and Host Response

• Pammi Suri, Ph.D.
• Pammi_Suri_at_Brown.edu

2
Role of immune response in disease and pathology
of schistosomes

• Cercarial Dermatitis (Swimmer's itch)
• Hepatosplenomegaly
• Esophageal varices
• increased portal blood pressure
• esophageal varix rupture is leading cause of
  death
• Reduced childhood growth and nutritional status

3
Chronic Schistosomiasis
4
Immunopathology

• Katayama fever (acute) immune complex disease?
• Neurological disease due to ectopic egg
  deposition
• End organ Pathology from granulomatous
  inflammation and fibrosis induced by parasite
  eggs trapped in host tissues
• Hepatic or Urinary bladder fibrosis and bleeding
• Periportal fibrosis (clay pipe stem) does not
  bridge adjacent triads

5
Pipestem Liver Fibrosis
Consequences

• Obstruction of blood flow
• Portal hypertension
• Enlargement of collateral vessels (Abdominal,
  Esophageal)
• Hepatosplenomegaly

Cross-section of liver from an S. mansoni
infected human
6
Role of Immune Response in Egg Induced Pathology

• Immune response is acutely manifested by large
  macrophage dominated granulomas with lymphatic
  infiltrates
• T-cell mediated delayed type hypersensitivity
  reaction (DTH)
• not seen in SCID mice
• requires tumor necrosis factor TNF
• TNF restores granuloma in SCID mice

7
The Schistosome Granuloma

• Eosinophils
• Macrophages
• T cells
• B cells
• Neutrophils
• Plasma Cells
• Mast Cells
• Fibroblasts

8
Diagnostic assays (Prevalance and Intensity of
infection)

• greatest prevalence and intensity in children and
  young adults
• greater pathology in older individuals
• Detected by Kato Katz smears, ultrasonography,
  and eggs in urine
• Diagnostic antibody based assays
• Soluble Egg Antigen (SEA)
• Gut associated proteoglycans referred to as
  Circulating Cathodic Antigens (CCA) and
  Circulating anodic antigens (CAA)

9
Diagnostic assays

• an antibody based diagnostic test developed for
  S. mansoni and S. haematobium infections
• single serum dip stick test with four
  indicators
• a positive reference, a negative reference and
  two cloned shistosome antigens one for S. mansoni
  and one forS. haematobium
• detection rate of 80 with 95 specificity noted
  in field studies in Egypt
• These antigens have positive correlation with
  worm burden as determined by egg count
• disappear completly within 3 months after
  treatment
• relationship between periportal fibrosis, bladder
  morbidity and these antigens is also noted

10
Control Strategies

• CHEMOTHERAPY cure Infection by Chemotherapy
• advantage highly effective
• drugs such as PRAZIQUANTEL or OXAMNIQUINE are
  highly effective against adult worms, often
  requiring only a single dose for complete cure
• problems cost, logistics, resistance,
  reinfection
• reinfection is common, especially in areas of
  high endemicity
• drug resistance resistant shistosme strains are
  emerging
• VECTOR CONTROL (Snail Control)
• advantageinterruption of transmission using
  molluscicides
• problems cost, logistics, environmental

11
Control Strategies

• IMPROVED SANITATION
• advantage most preferable, would also impact
  other diseases
• problems cost, logistics, developing countries
  often do not have the resources to provide
  sanitary facilities to all their citizens
• PROPHYLACTIC VACCINE for vaccinating potential
  hosts
• under development
• despite a concerted effort, no vaccine is
  currently available
• candidate vaccine (GST) entering Phase I clinical
  trials

12
General considerations

• Worms and eggs are large they cannot be
  phagocytized intact
• Worms do not replicate within the host (unlike
  bacteria, viruses, protists)
• Sterilizing immunity not needed partial immunity
  would reduce disease and transmission
• The "window of opportunity" worms are
  susceptible to immune attack in the skin and
  lungs, adult worms are refractory

13
Rationale for a Vaccine

• Existing control methods need help
• Preventing infection would be more effective than
  curing it
• Likely to be more cost effective, easier than
  sanitation and snail control
• Sterile immunity is not required - a 60
  reduction in average worm burden would have a
  major impact on disease and transmission
• Can protective immunity be induced?

14
Experimental Vaccines

• attenuated cercaria
• pro works well
• con logistical, expense, ethical issues
• parasite extracts
• pro authentic antigens
• con expense, quantity
• recombinant molecules
• pro unlimited quantity
• con may not contain proper epitopes
  (glycosylation)

15
Experimental vaccines contd.

• naked DNA
• pro authentic antigens
• Induces both arms of immune system
• con feasibility of delivery, issues
• anti-pathology (e.g. IL-12 plus egg antigens)
• pro prevents/reduces disease
• con does not prevent infection/transmission

16
10 weeks (prechallenge)
8 weeks (postchallenge)
individual samples
ELISA
pooled samples
Western Blot
17
Acquired Resistance

• Criteria for Immunity
• Role for specific components of immune system
  humoral and / or cellular or both
• Concomitant immunity
• "Early worm gets the bird"
• Stimulation of immune response by adult worms
  that confers protection against subsequent
  infection without any effect on the established
  infection
• Supporting Studies
• constant worm burdens in persons living in
  endemic areas measured by (eggs/gm) of stool

18
Concomitant Immunity

• transfer of adult worms into experimental animals
  results in resistance to subsequent infection,
  but transferred worms are unaffected
• Resistance is positivel correlated with number of
  worm pairs transferred and the interval of time
  between transfer and challenge
• ablation of neutrophils or eosinophills with Abs
  suppresses the expression of concomitant immunity
• expression of concomitant immunity is dependent
  on intact thymus during primary infection
• mice deficient in macrophage function fail to
  demonstrate immunity despite normal granuloma
  formation

19
Why Concomitant Immunity?

• keeps host alive with a "manageable" infection,
  can serve as an "egg factory" for an extended
  period
• Evidence for induction of protective immunity in
  laboratory hosts and its expression in infected
  human populations
• laboratory rodents and infected humans display
  immune mediated resistance to reinfection

20
IMMUNITY IN EXPERIMENTAL MODELS

• Immunity in the rat (a non permissive model)
• humoral (Th2) via ADCC
• evidence
• serum immunity may be transferred passively
• protective vs. non protective strains
• in vivo depletion of IgE prevents development of
  immunity
• maternal transfer of immunity
• immunity is adaptive and Ag specific

21
Immunity in mice (permissive host)

• Single immunization with irradiated cercaria
• cell mediated (Th1) via IFN-g activated
  macrophages
• Evidence for protective cell-mediated responses
• macrophages from immunized mice can kill
  schistosomula in vitro
• antibodies to IFN-g but not IL-4, ablate
  vaccine-induced immunity
• IL-12 can augment vaccine-induced immunity
• immunity partially dependent on nitrous oxide
  (NO)
• passive transfer with serum fails to protect
• Multiple immunization with irradiated cercaria
• Ab mediated immunity
• Passive transfer with Ab protects
• BCG and IL-12 as adjuvants confer protection in
  this model alsoby enhancing antibody responses

22
Immune response in egg induced pathology

• not a "neat" Th1 (DTH) response, has Th2
  component
• egg produces factors that induce IL-10
• granulomas are downregulated over time (5-10
  patients fail to do so, unknown why?
• granulomas become down modulated in size and in
  collagen content as infection becomes chronic
  (12-15 wks) in mouse model,
• down modulation is preceded by switch from Th1 to
  Th2
• granulomas may protect host from hepatotoxins
  produced by the egg

23
Alternate model for immunity in mice/ resistance
is non-immunological

• parabiotic partners of infected mice
• No immunity develops in single sex infections
• egg deposition is an absolute requirement
• ablation of eosinophills and IgE with anti IL-4
  and anti IL-5 Abs has no effect on resistance
• magnitude of resistance positively correlated
  with tissue egg burden, of worm pairs and of
  egg granulomas in the lung
• mechanistic interpretation
• granulomas from chronic infection result in
  compression of portal triads with subsequent
  portal hypertension- enlargement of portal-caval
  anastomoses
• upon secondary infection, larval parasites are
  unable to lodge in the enlarged portal
  mesenteries parasites are swept with blood flow
  and carried to the lungs, too large to negotiate
  the pulmonary capillary bed and die.

24
Correlates of Immunity/children more susceptible

• In children acquired resistance still undeveloped
  
• blocking antibodies of the IgM and IgG2 isotypes
  may explain slow development of immunity in
  children
• decline in prevalence and intensity with age
• acquired immunity or reduced exposure?
• Hypotheses to explain age-related resistance
• immunity develops progressively during childhood
  reaching maximum around age of puberty
• recently debated issue, hormonal changes at
  puberty have been recently suggested to play a
  role in reduced susceptibility to infection see
  ref. Fulford et al, Parasitology Today, vol 14
  Feb 1998)

25
Correlates of Immunity (humoral Immunity)

• Antibody dependent cell mediated cytotoxicity
  (ADCC)
• Infected individuals have high levels of IgE and
  eosinophilia
• In rats, protective antibodies are of IgE, IgG2a
  isotypes
• blocking antibodies (IgM, IgG2) isolated from
  sera of patients directly block
  eosinophiil-dependent killing of schistosomula
• IgG4 blocks effector functions of IgE
  significant correlation between Ig4 and
  susceptibility to reinfection in humans
• In animal models, IgG2c was shown to inhibit the
  capacity of IgG2a Ab to induce eosinophiil-depende
  nt killing of schistosomula
• A putative resistance gene is associated with
  chromosome region containing Th2 associated genes
  
• Clinical expression of immunity results from a
  balance between effector and blocking antibodies
• Immunity to reinfection after chemotherapy is
  more related to the IgE-IgG4 balance than to the
  absolute levels of these two isotypes

26
Correlates of Immunity (Cell-mediated responses)

• In humans, primates and rats, protective immunity
  to schistosomiasis is associated with Th2
  responses interleukin 4 (IL-4, IL-5 production,
  IgE, IgG4 and eosinophils
• IL-5 production associated with lymphocyte
  proliferation to specific antigens and resistance
  to reinfection of a human population
• Th-2 cytokine association with protective
  immunity is further documented by Th-cell clones
  from subjects resistant to S.mansoni are Th0/2.
• In contrast murine schistosomiasis is associated
  with Th1 responses IL-2, interferon-g (IFN-g)
  and IL-12
• In contrast to murine model, no evidence of
  significant positive association of IFN-g
  production with acquired resistance in human
  studies, rather a negative association has been
  reported in some instances

27
Conclusions

• Differences observed in different hosts (rats and
  humans vesus mice) and even in same host
  suggests
• Complex interaction between host and parasite is
  not restricted to the context of Th1and Th2 cells
  and the effects of their products
• successful immune response is the result of
  combination and appropriate balance of cytokines
  and effector cells

28
Why does the vaccine work in rats, but not mice?
29
Possible mechanisms of immune avoidance by worms

• adsorption of host macromolecules (e.g. ABO
  antigens)
• loss of parasite antigens
• tegument refractory to damage
• release of immunoregulatory mediators
• proteolytic enzymes
• lymphocyte/mast cell inhibitory factors

30
Effective Control Program

• rational vaccine strategy has been defined,
  protective role of IgA and mucosal vaccination
  being evaluated
• Ideally, a program of schistomiasis control would
  incorporate the best of both approaches
  (chemotherapy and vaccination), integrated with
  health education and the use of molluscicides