Sickle Cell Anemia and Malaria

Dr. Rajesh Karyakarte

Professor and Head,

Department of Microbiology,

Government Medical College, Akola

Introduction

• Evolutionary pressure from Plasmodium falciparum malaria on human populations has selected various erythrocyte polymorphisms that protect against severe complications and death from the disease

Introduction Cont…

• One important example is the mutation of sickle hemoglobin (HbS), a glutamate – to – valine substitution in the sixth position of the β-globin chain

• Molecular-genetic evidence suggests that this mutation has been selected independently at least five times in Africa, Arabia, and India

Both cause hemolytic anemia

• Sickle cell anemia (SCA) is a prototype congenital hemolytic anemia; and

• Malaria is a prototype acquired hemolytic anemia

• Clinical experience has shown that, not surprisingly, this combination is highly dangerous for the patient

SCA & Malaria can be a lethal combination

• Malaria makes the anemia of SCA worse, to the point of it becoming life-threatening

• In addition, malaria, like any other acute infection, can trigger a pain crisis or a sequestration crisis in a patient with SCA

But,SCA & Malaria share a complex relationship

• On one hand, as mentioned earlier, patients who are homozygous for the sickle gene (SS, SCA) are highly susceptible to the lethal effects of malaria

• On the other hand, heterozygotes for the sickle gene (AS) are relatively protected against the danger of dying of malaria

The ‘Malaria Hypothesis’

• J B S Haldane speculated that, depending on their genetic makeup, people would have a different risk of dying when they are confronted by a parasitic organism

• Furthermore, even if a gene offering protection against that parasite were otherwise harmful, its frequency would increase when a population was exposed to the parasite

• In equatorial Africa, up to 40% of people are carriers of Sickle cell gene

The ‘Malaria Hypothesis’ Cont…

• Plasmodium falciparum malaria is highly lethal

• Malaria has been around for several thousand years making it an agent of natural selection

• Deaths from malaria take place mostly in children, i.e. before reproduction, a critical criterion for effective selection

• Intra-erythrocytic forms are responsible for pathogenesis. Hence, if RBCs are abnormal, the survival of malarial parasite is affected

Balanced Polymorphism

• SCA is a disease of homozygotes (SS) – i.e. it is a recessive genetic disorder

• Heterozygotes (AS) are normal and carry the trait• A C Allison showed that:

– The S gene was frequent in areas of high malaria transmission. Thus, there were many patients of SCA

– But, AS heterozygotes seemed to have less malaria-related deaths, i.e. they were ‘malaria-resistant’

– Coexisting Homozygotes (SS) with a disadvantage of SCA and Heterozygotes (AS) with an advantage against malaria is called balanced polymorphism

Allison, A.C. (1954). Protection afforded by the sickle cell trait against subtertian malarial infection. British Medical Journal i, 290-294.

How the S Gene Affects Malarial parasite?

African studies show that:• AS heterozygotes do get malaria• AS heterozygotes with malaria tend to have lower

numbers of parasitized red cells in their blood• AS heterozygotes have a decreased incidence of

the two forms of severe life-threatening malaria – cerebral malaria and – malaria with severe anemia

• AS heterozygotes rarely die of malaria

Olumese, P.E., Adeyemo, A.A., Ademowo, et.al. (1997). The clinical manifestations of cerebral malaria among Nigerian children with the sickle cell trait. Annals of Tropical Paediatrics 17, 141-145.

How the S Gene Affects Malarial parasite? Cont…

• Data from clinical epidemiology show that AS heterozygotes have a survival advantage in an environment with malaria

• At the same time, they tell us clearly that HbS cannot prevent invasion of RBCs by malarial parasite

• It is therefore clear that it is something that takes place subsequent to invasion of RBCs that protects AS heterozygotes

Taylor, S.M., Parobek, C.M., and Fairhurst, R.M. (2012). Haemoglobinopathies and the clinical epidemiology of malaria: a systematic review and meta-analysis. Lancet Infect Dis 12, 457-468

How the S Gene Affects Malarial parasite?

• Beet first suggested that the phenomenon of sickling may be responsible for protection in AS

• Subsequently it was shown by quantitative in vitro studies that the rate of sickling of AS red cells that had been parasitized in vivo was significantly higher than that of non-parasitized red cells within the very same blood sample

1. Beet, E.A. (1946). Sickle cell disease in the Balovale District of Northern Rhodesia. East Afr Med J 23, 75-86.

2. Luzzatto, L., Nwachuku-Jarrett, E.S., and Reddy, S. (1970). Increased sickling of parasitised erythrocytes as mechanism of resistance against

malaria in the sickle-cell trait. Lancet i, 319-321.

How the S Gene Affects Malarial parasite? Cont…

• In vitro culture studies show that P. falciparum grows normally in AS red cells and even in SS red cells

• But once the parasite has triggered sickling the sickled cells would be removed by macrophages

Luzzatto, L., and Pinching, A.J. (1990). Commentary to R Nagel - Innate Resistance to Malaria: The Intraerythrocytic Cycle. Blood Cells 16, 340-347.

How the S Gene Affects Malarial parasite? Cont…

• Friedman found that parasite development was impaired in AS erythrocytes under reduced oxygen conditions

• He proposed that the sequestration of parasitized AS erythrocytes in the low-oxygen environment of post-capillary venules could result in parasite death in situ

Friedman, M.J. (1978). Erythrocytic mechanism of sickle cell resistance to malaria. Proc Natl Acad Sci USA 75, 1994-1997.

Does Immunity play a role?

• Acquired immunity is a major determinant of the clinical outcome of malarial infection

• A recent study carried out in Uganda has shown that AS heterozygous children (age 1-10) are protected from:– The establishment of blood-stage infection– The development of high densities of parasites– The progression of infection to symptomatic malaria– The authors infer that both innate and acquired

mechanisms are involved in protection from malaria

Gong, L., Maiteki-Sebuguzi, C., Rosenthal, et. al.(2012). Evidence or both innate and acquired mechanisms of protection from Plasmodium falciparum in children with sickle cell trait. Blood 19, 3808-3814.

How does Immunity play a role?

• The main advantage of AS heterozygotes in areas with heavy malaria endemicity is in increasing the probability of survival until acquired immunity steps in regardless of their hemoglobin type

In an area of heavy malaria (Abeokuta, SW Nigeria) the P. falciparum density is significantly reduced in AS versus AA children, specifically between the age of 3 and 5.

Guggenmoos-Holzmann, I., Bienzle, U., and Luzzatto, L. (1981). Plasmodium falciparum malaria and human red cells. II. Red cell genetic traits and resistance against malaria. 10, 16-22.

Hemoglobin S Interferes with Actin Remodeling in Plasmodium falciparum Infected Erythrocytes

• Hemoglobin S affected the trafficking system that transports P. falciparum erythrocyte membrane protein-1 (PfEMP-1) - to the surface of infected erythrocytes

• The parasite generates a host-derived actin cytoskeleton in RBCs

• This connects the Maurer’s clefts with the host cell membrane that transports vesicles with PfEMP-1 to the cell membrane (knobs)

Cyrklaff, M. et al. (2011). Hemoglobins S and C Interfere with Actin Remodeling in Plasmodium falciparum–Infected Erythrocytes. Science 334 (6060): 1283-1286

Hemoglobin S Interferes with Actin Remodeling in Plasmodium falciparum

Infected Erythrocytes Cont…

• The actin cytoskeleton and the Maurer’s clefts were aberrant in erythrocytes containing hemoglobin S

• Hemoglobin oxidation products, enriched in hemoglobin S erythrocytes, inhibited actin polymerization in vitro and may account for the protective role in malaria

Sickle cells infected with Plasmodium falciparum (green) collapse and prevent the parasite from interfering with the cell's actin proteins, protecting the host against malaria.

Impaired cytoadherence of Plasmodiumfalciparum-infected erythrocytes containing sickle

hemoglobin Cont…

• P. falciparum erythrocyte membrane protein-1 (PfEMP-1) is expressed as knob-like protrusions at the surface of parasitized erythrocytes

• PfEMP-1 promotes adherence of parasitized erythrocytes to microvascular endothelial cells

• This enables parasites to avoid clearance from the bloodstream by the spleen

Impaired cytoadherence of Plasmodiumfalciparum-infected erythrocytes containing sickle

hemoglobin Cont…

• PfEMP-1 typically attaches to CD36, the main host cytoadherence receptor on the surface of endothelial cells and blood monocytes

• PfEMP-1 attaches to ICAM-1 in cerebral microvessels (which generally do not express CD36)

• These interactions contribute to pathogenesis through processes of monocyte and platelet recruitment, cytokine release, and fibrin deposition, aggravating inflammation

Impaired cytoadherence of Plasmodiumfalciparum-infected erythrocytes containing sickle

hemoglobin Cont…

• PfEMP-1 also binds complement receptor 1 on non-infected erythrocytes to form rosettes, which are believed to impair microcirculatory flow and to contribute to the ischemic complications of malaria

• These PfEMP-1-dependent cytoadherence interactions that promote symptomatic and severe malaria are impaired due to sickle hemoglobin

Knob morphology and distribution on the surface of parasitized AS and SS erythrocytes. (a–e) AFM images of parasitized AA (a), AS (b and c), and SS (d and e) erythrocytes

How does malaria Affect Patients with SCA?

• If sickle hemoglobin is protective then why is it of no consequence in SS homozygotes, i.e. in patients with SCA?

• It is because of following factors:– Normally the spleen plays an important role in

filtering and removing parasitized red cells; but patients with SCA have an impaired splenic function due to:• Functional asplenia (common)• Anatomical atrophy of the spleen from multiple

infarcts (so-called auto-splenectomy) - - Less common

How Malaria Affects Patients with SCA? Cont…

• A recent population study carried out in Kenya has shown that malaria is no more common in SCA children than in controls: however, the mortality of SCA children who had malaria was about 10 times higher than in controls

McAuley, C.F., Webb, C., Makani, J., et al. (2010). High mortality from Plasmodium falciparum malaria in children living with sickle cell anemia on the coast of Kenya. Blood 116, 1663-1668.

Conclusion

• Malaria contributes substantially to the early mortality of patients with SCA, which makes it imperative that they should be protected by life-long anti-malarial prophylaxis (Based on African studies)

Thank you!