Immunity to malaria: more questions than answers

Jean Langhorne, Francis M Ndungu, Anne-Marit Sponaas & Kevin Marsh

Nature Immunology, July 2008

12th January 2009

Overview

1. Introduction to malaria

2. Immunity to malaria• mechanisms of immunity• immune response • Immune memory

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Malaria Vector-borne infectious disease

Caused by protozoan parasites of the genus plasmodium

Human malaria P. falciparum P. vivax P. ovale P. malariae

all persons become ill on the first exposure

Causes flu-like illness anemia, fever, chills followed by control of parasitemia in severe cases coma and death

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Center of disease control and prevention, Atlanta, USA.

common & severe

rare & mild

Malaria

Wide spread in tropical and subtropical regions 400 million cases per year 3 million death Majority 2-5 years

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© DPDx: CDC's web site for laboratory identification of parasites

Life cycle of Plasmodium falcipare

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Involves two hosts- Anopheles mosquito- Human host

three distinct cycles- Sporogonic cycle (mosquito)- Exo-erythrocytic cycle- Erytrocytic cycle

© DPDx: CDC's web site for laboratory identification of parasites

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Life cycle of Plasmodium falcipare

Exo-erythrocytic cycle infected (female anopheles)

mosquito bites through the skin

sporozoites travel from dermis to the liver/hepatocytes

undergo an amplification phase lasting between 2 and 9 days (asymptomatic stage)

parasite reenters the bloodstream by rupture of hepatocytes (Merozoites)

© DPDx: CDC's web site for laboratory identification of parasites

Life cycle of Plasmodium falcipare

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Erythrocytic cycle merozoites invade red blood cells

(RBC) and initiate the asexual cycle in RBCs

exponential expansion of parasite populations leads to febrile illness

sexual stages (male and female gametocytes) are formed during the erythrocytic cycle

this stage continues the life cycle in the mosquito after a blood meal

© DPDx: CDC's web site for laboratory identification of parasites

Life cycle of Plasmodium falcipare

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© DPDx: CDC's web site for laboratory identification of parasites

Sporogonic cycle While in the mosquito's stomach,

the microgametes penetrate the macrogametes generating zygotes

zygotes become motile and elongated (ookinetes)

invade the midgut wall of the mosquito to develop into oocysts

oocysts grow, rupture, and release

sporozoites which make their way to the mosquito's salivary glands.

2. Immunity to malaria• mechanisms of immunity• immune response • Immune memory

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Immunity to malarianaturally acquired immunity (NAI) to malaria

Immunity to malaria is seen as asymptomatic infection more resistant to new

infections limits parasitemia reduces the frequency

and severity of illness

human studies showed that immunity to malaria is relatively slow to develop

and incomplete immunity to death is

acquired more quicklyLanghorne et al., 2008

Mechanisms of Immunity

Picture of human immunity to malaria has been provided by two main sources: Deliberately induced malaria in non immune persons Natural history studies in endemic populations

Immune attack could be directed at any point in the life cycle of plasmodium

Pre-erythrocytic cycle (asymptomatic stage) Probably has limited involvement

Erythrocytic cycle

Mechanisms of Immunity Pre-erythrocytic cycle

targets are free sporocytes and infected hepatocytes

Requires mainly CD8+effector cells producing interferon-γ

Need the help of CD4+ T cells Antibodies to sporozoites are thought

to have a lesser function Kill parasites in infected hepatocytes

1. Antibodies to sporocyte neutralize sprozoites and/or block invasion of hepatocytes

2. IFN-γ and CD8+ T, NK, NKT, and γδT cells kill intrahepatic parasites

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Langhorne et al., 2008

Mechanisms of Immunity Erythrocytic cycle

targets are free merozoites or intraerythrocytic parasites (Schizont)

Humoral responses are the key to blood stage immunity (protective antibody response)

3. Antibodies to merozoites opsonize merozites for uptake and/or inhibit invasion of RBCs

4. Antibodies to • block infection of RBCs by merozoites• Antibody-dependent cellular killing

mediated by cytophilic antibodies• Block adhesion of infected RBCs to

endothelium• Neutralize parasite toxins and prevent

the induction of excessive inflammationLanghorne et al., 2008

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Mechanisms of Immunity Erythrocytic cycle

targets are free gametocytes Humoral responses are the key to

blood stage immunity (protective antibody response)

5. Antibodies prevent sequestration and maturation of gametocytes

6. Antibody and complement mediate lysis of gametocytes and prevent further development

Langhorne et al., 2008

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Immune response to plasmodium

Many features are similar to those of sepsis, over-vigorous or disordered immune responses are central

CD4+ and CD8+ T cells are chief participants in acute pathology

Enhanced amounts of cytokine responses TNF IL-1 β, IL-6, IL-10 IFN-γ CCL3 (MIP-1α), CCL4 (MIP-1β)

Immune response to plasmodium

Specific cytokine profiles associated with different syndromes Relatively low IL-10 in severe malaria anaemia Large amounts of IL-10 in respiratory distress Low amounts of CCL5 in severe disease and

mortality

Polymorphism in host genes is associated with susceptibility IFN-γ Interferon regulatory factors TNF IL-10, IL-4

Balance in the regulation of pro- and anti-inflammatory cytokines may be critical in determining the extent of pathology

Trigger for cytokine production

Depends on interaction of host cells and parasite via pattern-recognition receptors (PRR)

Can be induced by infected RBCs or parasite products with several Toll-like receptors (erythrocytic cycle)

Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1)

Glycosylphosphatidylinositol anchors (GPI) Hemazoin Ds DNA

Trigger for cytokine production

Trigger for cytokine production

MyD88 Intracellular adaptor for several TLRs Important for the induction of some but not all of

the pathology Also required for controlling acute-stage parasitemia

Immune response via several PRR Quantity of inflammatory cytokines? Qualitiy of a person’s response to a particular

clone?

Difficult to distinguish pathological from protective responses

Immune memory in malaria

Immunity to malaria develops Relatively slow Is not sterile

Continued exposure to malarial antigens is needed for Generation of memory and effector cells Persistence or memory and effector cells Maintenance of immunity

It is still unclear what components of immunity is lost without exposure Short lived immunity Longlived immunity

Immune memory in malaria

Rapid boosting of antibody responses indicate the presence of memory B cells

Accumulation of memory B cell specific for malaria indicates the presence of memory B cells

anti-P. falciparum memory B cells present in adults for over 8 years without evident re-

exposure

a subsequent study in children in contrast presence of anti-P. falciparum serum antibody only very low frequencies of malaria-specific memory B

cells

Immune memory in malaria

Pre-Erythrocytic cycle

To be protective specific CD4+ and CD8+ T cells must be maintained as effector cells to differentiate

very rapidly into effector-killer cells

Population shift between mostly protective effector phenotype to the less protective memory phenotype

This could explain why immunological memory is not always correlated with immunity

Immune memory in malaria

Erythrocytic cycle

CD4+ T cells are induced by natural infection Frequencies of responding cells and the prevalence of

exposed people with measurable malaria-specific CD4+ T cells are often low

lower concentrations of malaria-specific antibodies in malaria-infected people positive for HIV

suggests that CD4+ T cell help is necessary for the induction and also for the maintenance of protective antibody

Requirment for memory CD4+ T cells in immunity has still to be elucidated

Immune memory in malaria

1. Inhibition of DC maturation

2. Hemozoin can inhibit macrophage monoyte function

3. IL-10 from DCs and macrophages modulated by infect RBCs inhibit CD4+ T cell activation

4. CD4+ T cells produce IL-10 and transforming growth factor-β, which inhibts the generation of central and memory-effector cells

Immune memory in malaria

5. Infected RBCs induce apoptosis and/or depletion of memory B cells

6. The niche for plasma cells of many different specificities may be limiting

7. Acceleration of the catabolism of immunoglobulin molecules

8. Antigenic variation for immune escape

9. Circulating immune complexes and low-affinity immunoglobulin molecules can trigger apoptosis of long-lived plasma cells

Conclusions - more questions than answers

Advanced understanding the host response to plasmodium

But many questions remain: What to measure as a correlate for immunity? What mechanisms regulate immune pathology in

semi-immune people? What defects contribute to the relatively

ineffective immunity in children? Why immunity to plasmodium infection can be

short-lived?

Thank you for your attention!

Complete life cycle

Trigger for cytokine production

Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1) Adhesive molecules sequestration in the post-capillary venules

Glycosylphosphatidylinositol anchors (GPI) Anchor on many plamodium membrane proteins induction of proinflammatory responses

Hemazoin disposal product formed from the digestion of

erythrocytes