Plasmodium falciparum: Enhanced Gametocyte Formationin Vitroin Reticulocyte-Rich Blood

doi:10.1006/expr.1998.4347

Experimental Parasitology

Volume 91, Issue 2, February 1999, Pages 115-118

https://doi.org/10.1006/expr.1998.4347 Get rights and content

Abstract

Trager, W., Gill, G. S., Lawrence, C., and Nagel, R. L. 1999.Plasmodium falciparum: Enhanced gametocyte formationin vitroin-reticulocyte-rich blood.Experimental Parasitology91,115–118. Concentrates of late stage parasites of the gametocyte-forming clone HB-3 were mixed with blood rich in reticulocytes from anemic patients, or with normal control blood, and kept under culture conditions for 4 days. Significantly more gametocytes were always formed in the reticulocyte-rich blood than in the control. This was true whether the anemic blood supported a larger asexual parasitemia than the control, or a lower one, or the same and without regard to the cause of the anemia. Gametocytes as a percentage of asexual forms were up to 10 times higher in reticulocyte-rich blood than in normal blood.

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Malaria parasites have coevolved with humans over thousands of years, mirroring their migration out of Africa. They persist to this day, despite continuous elimination efforts worldwide. These parasites can adapt to changing environments during infection of human and mosquito, and when expanding the geographical range by switching vector species. Recent studies in the human malaria parasite, Plasmodium falciparum, identified determinants governing the plasticity of sexual conversion rates, sex ratio, and vector competence. Here we summarize the latest literature revealing environmental, epigenetic, and genetic determinants of malaria transmission.
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Since then a wide variety of factors have been described as impacting the commitment rate. Here, we will focus on the role of metabolite signaling but hemoglobinopathies [26,27], drug treatment [28,29], hormones levels [30–32], and cholera toxin [33] have also been described to influence commitment rates. It was noted early on that the rate of P. falciparum sexual commitment in culture increases with the concentration of infected erythrocytes [25,34] or the use of conditioned or spent media [35–37].
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Among factors that may have affected gametocyte productivity and oocyst counts, we note possible influences of delayed growth kinetics, particularly during early development of ring stages, which may have contributed to the suboptimal infectivity observed in two experiments (Fig. 2A); other possible factors include RBCs sourced from different rhesus macaque and variations among the collected sera (Supplementary Table S1). Indeed, studies with P. falciparum have shown that RBC age can influence gametocyte development (Trager and Gill, 1992; Trager et al., 1999), and sera samples are known to differ in their ability to support gametocyte growth (Delves et al., 2016). Fitness and vector competence can shift with seemingly minor changes in the laboratory practices of rearing mosquitoes (Mwangangi et al., 2007; Gilles et al., 2011).
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We first considered the possibility that different host cell types and tissue compartments contain different amounts of LysoPC. Consistent with a series of reports demonstrating increased gametocyte formation at higher reticulocyte densities (Peatey et al., 2013; Trager et al., 1999), we hypothesized that the metabolic needs of reticulocytes may restrict LysoPC availability to the parasite and hence trigger sexual differentiation. We tested this hypothesis by culturing P. falciparum in reticulocyte-enriched blood; however, we did not observe any difference in sexual commitment rates (Figure 6F).
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An example of an environment to which parasites may respond to by switching to sexual stages is the bone marrow, where significant enrichment of immature P. falciparum gametocytes has been noted (Aguilar et al., 2014; Joice et al., 2014). When parasites reach the bone marrow, gametocytogenesis may be induced by high concentrations of soluble factors such as those found in erythrocyte progenitors (Trager et al., 1999; Peatey et al., 2013), or even by an insufficiency of specific metabolites required for asexual growth. There is considerable evidence to support the bone marrow as a site of specific adaption to which parasites respond by inducing gametocytogenesis, but the possibility that gametocytes also develop elsewhere and simply lodge in the bone marrow cannot currently be excluded (Rogers et al., 2000).
Malaria is the disease caused by the apicomplexan parasites belonging to the genus Plasmodium. Expanding our arsenal to include transmission-blocking agents in our fight against malaria is becoming increasingly important. Such an implementation requires detailed understanding of the biology of the Plasmodium life cycle stages that are transmissible. Plasmodium gametocytes are the only parasite stage that can be transmitted to the mosquito vector and are the product of sexual development in a small percentage of parasites that continually proliferate in host blood. The critical decision made by asexual erythrocytic stages to cease further proliferation and differentiate into gametocytes, as well as the first steps they take into maturity, have long remained unknown. Recent studies have contributed to a breakthrough in our understanding of this branch point in development. In this review, we will discuss the findings that have allowed us to make this major leap forward in our knowledge of sexual commitment in Plasmodium. We will further propose a model for the mechanism triggering the switch to sexual development, constructed around the proteins currently known to regulate this process. Further insight into sexual commitment and gametocyte development will help identify targets for the development of transmission-blocking malaria therapies.
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Citation Excerpt :
Selective accumulation of immature gametocytes in bone marrow during the 8 to 12 days they need for maturation may provide them with a better niche for survival than the lumen of small vessels if, for example, their clearance is reduced in the special immune environment of the bone marrow.40 However, it is still not clear if gametocytogenesis can be induced in bone marrow by the high concentration of erythrocyte progenitors41,42 or other soluble factors43 or alternatively gametocytes are produced elsewhere and sequester in the bone marrow. This study showed an elevated carriage of mature gametocytes in peripheral blood of P falciparum-infected children with severe anemia,2,44,45 representing an important infectious reservoir given the high numbers of anemic children in Mozambique (3.8 million children younger than age 10 years, 11.5% of them with severe anemia46) and in other African countries where malaria is endemic.47,48
Plasmodium falciparum immature gametocytes are not observed in peripheral blood. However, gametocyte stages in organs such as bone marrow have never been assessed by molecular techniques, which are more sensitive than optical microscopy. We quantified P falciparum sexual stages in bone marrow (n = 174) and peripheral blood (n = 70) of Mozambican anemic children by quantitative polymerase chain reaction targeting transcripts specific for early (PF14_0748; PHISTa), intermediate (PF13_0247; Pfs48/45), and mature (PF10_0303; Pfs25) gametocytes. Among children positive for the P falciparum housekeeping gene (PF08_0085; ubiquitin-conjugating enzyme gene) in bone marrow (n = 136) and peripheral blood (n = 25), prevalence of immature gametocytes was higher in bone marrow than peripheral blood (early: 95% vs 20%, P < .001; intermediate: 80% vs 16%; P < .001), as were transcript levels (P < .001 for both stages). In contrast, mature gametocytes were more prevalent (100% vs 51%, P < .001) and abundant (P < .001) in peripheral blood than in the bone marrow. Severe anemia (3.57, 95% confidence interval 1.49-8.53) and dyserythropoiesis (6.21, 95% confidence interval 2.24-17.25) were independently associated with a higher prevalence of mature gametocytes in bone marrow. Our results highlight the high prevalence and abundance of early sexual stages in bone marrow, as well as the relationship between hematological disturbances and gametocyte development in this tissue.

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^☆: The work at the Rockefeller University was supported by a grant from the National Institute of Allergy and Infectious Diseases. We thank James Stanorski for pickup and delivery of the blood samples and for preparation of the manuscript and Terezinha Kuzemka for technical assistance.

¹: To whom correspondence should be addressed: Fax: 212-327-7974.

²: Present address: 54 Burns Way, Heston, Hounslow Middlesex TW5-9BA, United Kingdom.

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Regular ArticlePlasmodium falciparum: Enhanced Gametocyte Formationin Vitroin Reticulocyte-Rich Blood☆

Abstract

Blood

Blood

Gametocyte-forming and non-gametocyte-forming clones ofPlasmodium falciparum

American Journal of Tropical Medicine and Hygiene

Evidence for environmental modulation of gametocytogenesis inPlasmodium falciparum

Bulletin of the World Health Organization

Enhanced gametocyte formation byPlasmodium chabaudi

Journal of Parasitology

Regular Article
Plasmodium falciparum: Enhanced Gametocyte Formationin Vitroin Reticulocyte-Rich Blood☆