Invasion of host cells by malaria parasites: a tale of two protein families (Molecular Microbiology), a review by Iyer et al.

When I saw this review article, I was excited yet disappointed. I was excited since a few groups now are working on the Reticulocyte Binding Like homologues (RBLs) but disappointed in that I have a similar review paper in draft form that never made it past the first revision. It’s my fault for not working on it after the initial feedback from my former PI. This is now the second review paper to be published since I drafted that paper in early 2005, the first being a superb one by Alan Cowman’s group in December 2005 (PLoS Pathog. 2005 Dec;1(4):e37).

Getting back to the Iyer et al. review…this sums up pretty well the discoveries of the two major families of erythrocite binding proteins found in Plasmodium spp. The authors mention that recent evidence that P. falciparum may take alternative invasion routes by switching to more favorable receptor-ligand interatction when the need arises.

I like their thought that these EBLs and RBLs have some other function outside of erythrocyte binding, perhaps during another stage of the parasite. This makes perfect sense to me since I’ve always wondered why the parasite would invest its metabolic resources into making such extravagant proteins (the RBPs are more than 300 kDas).

Here’s a quick primer on Erythrocyte Binding Ligands (EBLs) and Reticulocyte-binding protein homologues (RBLs).
Note that I’m leaving out the murine and simian homologues for simplicity, but most of the groundwork for the EBLs and RBLs were made using non-human malaria models like P. yoelii and P. knowlesi.
EBLs:
First homologue in a human malaria spp.: P.vivax Duffy Binding Protein
Plasmodium ligand – Erythrocyte receptor Combinations

P. vivax Duffy Binding Protein – Duffy Antigen Receptor for Chemokines (DARC)
P. falciparum EBA175 – Glycophorin A
P. falciparum EBA140 (also called BAEBL) – Glycophorin C
P. falciparum EBA181 (also called JESEBL) – Receptor W
P. falciparum EBA165 (also called PEBL) – unknown receptor

RBLs:

First homologue in a human malaria spp.: P. vivax RBP1

P. falciparum RH1 (also called NBP1) – Receptor Y
P. falciparum RH2a (also called NBP2a) – unknown receptor
P. falciparum RH2b (also called NBP2b) – Receptor Z
P. falciparum RH3 – unknown receptor
P. falciparum RH4 – unknown receptor
P. falciparum RH5 – unknown receptor
P. vivax RBP1 – unknown receptor
P. vivax RBP2 – unknown receptor

As you can see, the erythrocyte targets of this EBL and esp. RBL proteins haven’t been fully elucidated. Finding the receptors is a crucial step towards defining the invasion pathways and developing therapeutic strategies to block those interactions. What strikes me is how P. vivax has an absolute requirement of DARC (Duffy negative individuals are not sucsceptible to vivax infection) while experiments have shown that P. falciparum can invade via separate pathways and an absolutely critical receptor-ligand interaction has yet to be defined. This is a smart strategy for the parasite as it can target different receptors if the host has already primed its antibody response against one or two receptor-ligand pairings. This broader range of invasion pathways may explain the increased virulence and prevalence of falciparum compared to vivax (in addition to the preference of vivax for reticulocytes). This is of importance in terms of strategies for a blood stage vaccine since one must account for all the different receptor-ligand parings and include every antigen in your formulation. A multi-antigen, shotgun-type vaccine runs the risk of antigenic competition or interference as certain antigens may be immunodominant over others. We need to understand the spatial and temporal aspects of the invasion pathways a bit better to provide more focus to our candidate vaccines. One would think that the broad invasion pathway for falciparum would eventually bottle neck at some point downstream, allowing us with a smaller target on which to set our sights. The trick is figuring out an assay to tease this out….

Reference: Mol Microbiol. 2007 Jul;65(2):231-49

tmt

Cardiotoxicity of Anti-malarial Drugs (Lancet Infectious Diseases), Nick White

I came across this nice review article by the famous Nick White after doing my weekly “malaria” search in Pubmed. Here’s what I learned:

“From the 1920s onwards the quinoline structure was
modified sequentially as a succession of synthetic
antimalarial drugs were produced. The most important
of these were pamaquine (1926), mepacrine (1932),
chloroquine (1934), amodiaquine (1951), primaquine
(1952), mefloquine (1963), halofantrine (1966), and in
the past 30 years piperaquine, lumefantrine, and
pyronaridine. All of these drugs interfere with the
intraparasitic detoxification of haem, although there are
important differences in the antiparasitic activities of the
various drugs.”

This paper is an excellent resource–one usually doesn’t think too much about cardiac side effects when treating for malaria out in the field. That’s partly because malaria primarily affects the young and non-immune, a population that in general has good cardiac function.

To highlight the meds that we most often use:

Quinine is the levorotary diastereoisomer of quinidine but are quite different. Both cause hypotension by blocking alpha recepts, but qunidine markedly prolongs the QT interval and is four times more active than quinine.

Chloroquine is generally safe from a cardia standpoint, but overdose can cause severe hypotension through both venous and arterial vasodilation and a consistent ventricular conduction delay (widened QRS).

Amodiaquine hasn’t been studied much in terms of cardiac effects, but one study suggests that it may cause a clinically insignificant prolongation of PR, QRS, and QTc intervals.

Halofantrine isn’t used anymore since it can severely prolong QTc and cause ventricular tachyarrythmias (TdP).

Mefloquine (Lariam), primaquine, atovaquone-progaunil (Malarone), sulfadoxine-pyremethamine (Fansidar) and artesunate have little cardiac side effects.

Oh, and importantly, recovery from malaria by itself causes QT prolongation, and this effect often has been attributed, perhaps incorrectly, to the anti-malarial agent.

Reference: Lancet Infectious Diseases, Volume 7, Issue 8, August 2007, Pages 549-558

tmt

Kinyanjui et al. (from the Malaria Journal) IgG antibody responses to Plasmodium falciparum merozoite antigens in Kenyan children have a short half-life.

The human antibody response to malaria has been investigated by many research groups over the last few decades. Most have been cross-section studies looking at sero-prevalence to a particular malaria antigen in a certain endemic popultation. For obvious reasons, it has difficult to conduct longitudinal studies of antibody responses to malaria antigens over time. It’s pretty much established that the malaria immune response is short lived and requires frequent natural boosting to maintain clinical immunity. Kinyanjui et al. re-visits this concept with a convincing paper that shows how ephemeral the malaria antibody really is. Basically, they found that in their study of Kenyan children, the half-life of IgG1 antibodies was about 10 days and that of IgG3 was about 6 days. (Typical IgG1 half-life is usually 21 days. The significance of studying IgG1 and IgG3 subclasses is that these are the cytophilic antibodies and are believed to help in clear parasites by binding to both the parasite and macrophages.) The authors note that this rapid decline in antibody levels after infection necessitates closely spaced sampling in the first few weeks after immunization with a malaria vaccine. The results suggest assessing antibody levels within 14 days of vaccination—otherwise one may label a vaccine a non-immunogenic from a humoral perspective rather prematurely. In reality, I think this is only an academic matter. A good vaccine should be able to elicit a detectable humoral (and cellular) response long past 30 days.

It would have been nice had the authors measured the kinetics of a non-malaria antigen—ideal ones would be antigens of infectious agents to which the children have been vaccinated (for example, Hepatitis B surface Ab and Haemophilus influenza or Diptheria). Then, one would be able to discern if the short half lives of the cytophilic antibodies is a pan-phenomenon or specific to malaria antibodies. I’m not sure which vaccines are administered routinely in this particular group of Kenyan children, but it seems to me that this was an important omission, perhaps due to the difficulty of obtaining the proper viral or bacterial recombinant proteins.
I think the most intriguing question that arises from this paper is whether or not the short half-life is due to rapid clearing of antibodies or due to the extinction of short-lived plasma cells. The results indicate that memory B cells and long-lived plasma cells are rather lacking in the human immune response. I do not know much about the role of plasma cells in malaria immunity but plan to look into it. A brief Pubmed search of “plasma cells and malaria” didn’t show more than 20 papers. Seems like an open field….

Reference: Malaria Journal 2007, 6:82 (28 June 2007)

tmt

Malaria in Literature

A friend of mine began writing me passages about malaria from books that she read. What better way to demonstrate the scope and poignancy of the illness than to list passages of malaria found in literature.
Here are a couple of examples…