Preservation of wild isolates of human malaria parasites in wet ice and adaptation efficacy to in vitro culture

Cultivation of Asexual Intraerythrocytic Stages of Plasmodium falciparum

Successfully developed in 1976, the continuous in vitro culture of Plasmodium falciparum has many applications in the field of malaria research. It has become an important experimental model that directly uses a human pathogen responsible for a high prevalence of morbidity and mortality in many parts of the world and is a major source of biological material for immunological, biochemical, molecular, and pharmacological studies. Until present, the basic techniques described by Trager and Jensen and Haynes et al. remain unchanged in many malaria research laboratories. Nonetheless, different factors, including culture media, buffers, serum substitutes and supplements, sources of erythrocytes, and conditions of incubation (especially oxygen concentration), have been modified by different investigators to adapt the original technique in their laboratories or enhance the in vitro growth of the parasites. The possible effects and benefits of these modifications for the continuous cultivation of asexual intraerythrocytic stages of P. falciparum, as well as future challenges in developing a serum-free cultivation system and axenic cultures, are discussed.

Culture and transfection: Two major bottlenecks in understanding Plasmodium vivax biology

The long term in vitro culture of Plasmodium falciparum was successfully established by Trager and Jensen in 1976; however it largely remains unachieved for P. vivax. The major obstacle associated with Plasmodium vivax in vitro culture is its predilection for invading younger reticulocytes and the complex remodelling of invaded reticulocytes. There are many factors under exploration for this predilection and host–parasite interactions between merozoites and invaded reticulocytes. These include various factors related to parasite, host and environment such as compromised reticulocyte osmotic stability after invasion, abundance of iron in the reticulocytes which makes them favourable for P. vivax growth and propagation and role of a hypoxic environment in P. vivax in vitro growth. P. vivax blood stage transfection represents another major hurdle towards understanding this parasite’s complex biology. Efforts in making this parasite amenable for molecular investigation by genetic modification are limited. Newer approaches in sustaining a longer in vitro culture and thereby help advancing transfection technologies in P. vivax are urgently needed that can be explored to understand the unique biology of this parasite.

Long-lasting infectivity of Plasmodium vivax present in malarial patient blood to Anopheles aquasalis

Experimental studies for understanding the relationship between Plasmodium vivax and its vector hosts are difficult, because of to the lack of a long-term, in vitro continuous culture system unavailability of infected blood samples, seasonality of the disease, and the concentration of most cases in remote areas. This study evaluates the duration of the infectivity of P. vivax to Anopheles aquasalis after collecting blood from malaria-infected patients. Blood was collected from patients and stored at 4 °C and 37 °C. Every day, for 4 days, the blood was fed to An. aquasalis adult females, and a Giemsa-stained thick blood smear was mounted to account for sexual (gametocytes) and asexual (trophozoites and schizonts) stages and calculate parasitemia. Oocysts in the midgut of the mosquitoes were counted on the seventh day after feeding. Kruskal-Wallis test was used to compare the mean number of oocysts (MO) and the parasite density (PD) in each storage condition and post-infection time-points. The Mann-Whitney test was used to compare the number of oocysts for each day between temperatures. The results show that P. vivax stored at 4 °C and at 37 °C has its infectivity to An. aquasalis preserved for 2 days and 3 days, respectively. Infection rate (IR), PD and MO were higher on the day of blood collection and decreased gradually over time. The parasite density (number of parasites/μL) diminished faster at 4 °C than at 37 °C. In this study, a preservation protocol is shown for long-lasting infectivity of P. vivax in a blood sample taken from malaria-infected patients. These results show that infectivity of P. vivax stored at 4 °C and at 37 °C to An. aquasalis persist until 3 days after blood collection, but parasite density, infection rate, and mean of oocysts decreased 24h after blood collection. Since the malaria cases are increasingly far from the urban areas these results indicate that is possible, losing some infectivity, to realize experimental infections several dozen hours after the blood collection. However, it is necessary to improve the procedures for preserving P. vivax gametocytes for mosquito infection in the laboratory.

Plasmodium vivax in vitro continuous culture: the spoke in the wheel

Understanding the life cycle of Plasmodium vivax is fundamental for developing strategies aimed at controlling and eliminating this parasitic species. Although advances in omic sciences and high-throughput techniques in recent years have enabled the identification and characterization of proteins which might be participating in P. vivax invasion of target cells, exclusive parasite tropism for invading reticulocytes has become the main obstacle in maintaining a continuous culture for this species. Such advance that would help in defining each parasite protein’s function in the complex process of P. vivax invasion, in addition to evaluating new therapeutic agents, is still a dream. Advances related to maintenance, culture medium supplements and the use of different sources of reticulocytes and parasites (strains and isolates) have been made regarding the development of an in vitro culture for P. vivax; however, only some cultures having few replication cycles have been obtained to date, meaning that this parasite’s maintenance goes beyond the technical components involved. Although it is still not yet clear which molecular mechanisms P. vivax prefers for invading young CD71⁺ reticulocytes [early maturation stages (I–II–III)], changes related to membrane proteins remodelling of such cells could form part of the explanation. The most relevant aspects regarding P. vivax in vitro culture and host cell characteristics have been analysed in this review to explain possible reasons why the species’ continuous in vitro culture is so difficult to standardize. Some alternatives for P. vivax in vitro culture have also been described.

malERA: An updated research agenda for basic science and enabling technologies in malaria elimination and eradication

Basic science holds enormous power for revealing the biological mechanisms of disease and, in turn, paving the way toward new, effective interventions. Recognizing this power, the 2011 Research Agenda for Malaria Eradication included key priorities in fundamental research that, if attained, could help accelerate progress toward disease elimination and eradication. The Malaria Eradication Research Agenda (malERA) Consultative Panel on Basic Science and Enabling Technologies reviewed the progress, continuing challenges, and major opportunities for future research. The recommendations come from a literature of published and unpublished materials and the deliberations of the malERA Refresh Consultative Panel. These areas span multiple aspects of the Plasmodium life cycle in both the human host and the Anopheles vector and include critical, unanswered questions about parasite transmission, human infection in the liver, asexual-stage biology, and malaria persistence. We believe an integrated approach encompassing human immunology, parasitology, and entomology, and harnessing new and emerging biomedical technologies offers the best path toward addressing these questions and, ultimately, lowering the worldwide burden of malaria.