A simplified method for cryopreservation ofPlasmodium falciparumfrom continuousin vitrocultures

Development of a Plasmodium vivax biobank for functional ex vivo assays

BACKGROUND

Plasmodium vivax is the second most prevalent cause of malaria yet remains challenging to study due to the lack of a continuous in vitro culture system, highlighting the need to establish a biobank of clinical isolates with multiple freezes per sample for use in functional assays. Different methods for cryopreserving parasite isolates were compared and subsequently the most promising one was validated. Enrichment of early- and late-stage parasites and parasite maturation were quantified to facilitate assay planning.

METHODS

In order to compare cryopreservation protocols, nine clinical P. vivax isolates were frozen with four glycerolyte-based mixtures. Parasite recovery post thaw, post KCl-Percoll enrichment and in short-term in vitro culture was measured via slide microscopy. Enrichment of late-stage parasites by magnetic activated cell sorting (MACS) was measured. Short and long-term storage of parasites at either - 80 °C or liquid nitrogen were also compared.

RESULTS

Of the four cryopreservation mixtures, one mixture (glycerolyte:serum:RBC at a 2.5:1.5:1 ratio) resulted in improved parasite recovery and statistically significant (P < 0.05) enhancement in parasite survival in short-term in vitro culture. A parasite biobank was subsequently generated using this protocol resulting in a collection of 106 clinical isolates, each with 8 vials. The quality of the biobank was validated by measuring several factors from 47 thaws: the average reduction in parasitaemia post-thaw (25.3%); the average fold enrichment post KCl-Percoll (6.65-fold); and the average percent recovery of parasites (22.0%, measured from 30 isolates). During short-term in vitro culture, robust maturation of ring stage parasites to later stages (> 20% trophozoites, schizonts and gametocytes) was observed in 60.0% of isolates by 48 h. Enrichment of mature parasite stages via MACS showed good reproducibility, with an average of 30.0% post-MACS parasitaemia and an average of 5.30 × 105 parasites/vial. Finally, the effect of storage temperature was tested, and no large impacts from short-term (7 days) or long-term (7-10 years) storage at - 80 °C on parasite recovery, enrichment or viability was observed.

CONCLUSIONS

Here, an optimized freezing method for P. vivax clinical isolates is demonstrated as a template for the generation and validation of a parasite biobank for use in functional assays.

Development of a Plasmodium vivax biobank for functional ex vivo assays

Background

Plasmodium vivax is the second most prevalent cause of malaria yet remains challenging to study due to the lack of a continuous in vitro culture system, highlighting the need to establish a biobank of clinical isolates with multiple freezes per sample for use in functional assays. Different methods for cryopreserving parasite isolates were compared and subsequently the most promising one was validated. Enrichment of early- and late-stage parasites and parasite maturation were quantified to facilitate assay planning.

Methods

In order to compare cryopreservation protocols, nine clinical P. vivax isolates were frozen with four glycerolyte-based mixtures. Parasite recovery post thaw, post KCl-Percoll enrichment and in short-term in vitro culture was measured via slide microscopy. Enrichment of late-stage parasites by magnetic activated cell sorting (MACS) was measured. Short and long-term storage of parasites at either -80°C or liquid nitrogen were also compared.

Results

Of the four cryopreservation mixtures, one mixture (glycerolyte:serum:RBC at a 2.5:1.5:1 ratio) resulted in improved parasite recovery and statistically significant (P<0.05) enhancement in parasite survival in short-term in vitro culture. A parasite biobank was subsequently generated using this protocol resulting in a collection with 106 clinical isolates, each with 8 vials. The quality of the biobank was validated by measuring several factors from 47 thaws: the average reduction in parasitemia post-thaw (25.3%); the average fold enrichment post KCl-Percoll (6.65-fold); and the average percent recovery of parasites (22.0%, measured from 30 isolates). During short-term in vitro culture, robust maturation of ring stage parasites to later stages (>20% trophozoites, schizonts and gametocytes) was observed in 60.0% of isolates by 48 hours. Enrichment of mature parasite stages via MACS showed good reproducibility, with an average 30.0% post-MACS parasitemia and an average 5.30 × 10 5 parasites/vial. Finally, the effect of storage temperature was tested, and no large impacts from short-term (7 day) or long term (7 - 10 year) storage at -80°C on parasite recovery, enrichment or viability was observed.

Conclusions

Here, an optimized freezing method for P. vivax clinical isolates is demonstrated as a template for the generation and validation of a parasite biobank for use in functional assays.

Glycophorin C (CD236R) mediates vivax malaria parasite rosetting to normocytes

Rosetting phenomenon has been linked to malaria pathogenesis. Although rosetting occurs in all causes of human malaria, most data on this subject has been derived from Plasmodium falciparum. Here, we investigate the function and factors affecting rosette formation in Plasmodium vivax. To achieve this, we used a range of novel ex vivo protocols to study fresh and cryopreserved P vivax (n = 135) and P falciparum (n = 77) isolates from Thailand. Rosetting is more common in vivax than falciparum malaria, both in terms of incidence in patient samples and percentage of infected erythrocytes forming rosettes. Rosetting to P vivax asexual and sexual stages was evident 20 hours postreticulocyte invasion, reaching a plateau after 30 hours. Host ABO blood group, reticulocyte count, and parasitemia were not correlated with P vivax rosetting. Importantly, mature erythrocytes (normocytes), rather than reticulocytes, preferentially form rosetting complexes, indicating that this process is unlikely to directly facilitate merozoite invasion. Although antibodies against host erythrocyte receptors CD235a and CD35 had no effect, Ag-binding fragment against the BRIC 4 region of CD236R significantly inhibited rosette formation. Rosetting assays using CD236R knockdown normocytes derived from hematopoietic stem cells further supports the role of glycophorin C as a receptor in P vivax rosette formation.

Peripheral blood stem cell transplant-related Plasmodium falciparum infection in a patient with sickle cell disease

BACKGROUND

Although transmission of Plasmodium falciparum (Pf) infection during red blood cell (RBC) transfusion from an infected donor has been well documented, malaria parasites are not known to infect hematopoietic stem cells. We report a case of Pf infection in a patient 11 days after peripheral blood stem cell transplant for sickle cell disease.

STUDY DESIGN AND METHODS

Malaria parasites were detected in thick blood smears by Giemsa staining. Pf HRP2 antigen was measured by enzyme-linked immunosorbent assay on whole blood and plasma. Pf DNA was detected in whole blood and stem cell retention samples by real-time polymerase chain reaction using Pf species-specific primers and probes. Genotyping of eight Pf microsatellites was performed on genomic DNA extracted from whole blood.

RESULTS

Pf was not detected by molecular, serologic, or parasitologic means in samples from the recipient until Day 11 posttransplant, coincident with the onset of symptoms. In contrast, Pf antigen was retrospectively detected in stored plasma collected 3 months before transplant from the asymptomatic donor. Pf DNA was detected in whole blood from both the donor and the recipient after transplant, and genotyping confirmed shared markers between donor and recipient Pf strains. Lookback analysis of RBC donors was negative for Pf infection.

CONCLUSIONS

These findings are consistent with transmission by the stem cell product and have profound implications with respect to the screening of potential stem cell donors and recipients from malaria-endemic regions.

Cryopreserved Plasmodium vivax and cord blood reticulocytes can be used for invasion and short term culture

The establishment of a Plasmodium vivaxin vitro culture system is critical for the development of new vaccine, drugs and diagnostic tests. Although short-term cultures have been successfully set up, their reproducibility in laboratories without direct access to P. vivax-infected patients has been limited by the need for fresh parasite isolates. We explored the possibility of using parasite isolates and reticulocytes, both cryopreserved, to perform invasion and initiate short-term culture. Invasion results obtained with both cryopreserved isolates and reticulocytes were similar to those obtained with fresh samples. This method should be easily replicated in laboratories outside endemic areas and will substantially contribute to the development of a continuous P. vivax culture. In addition, this model could be used for testing vaccine candidates as well as for studying invasion-specific molecular mechanisms.

Low temperature preservation of Plasmodium spp

Interest in the long-term low temperature preservation of malaria parasites was initiated by Coggeshall's (1939) demonstration that the intra-erythrocytic stages could survive storage at −76°C for 70 days. Numerous studies have subsequently been published in which a variety of techniques have been used. In addition, several reviews have included sections on the cryopreservation of malaria, the most recent being by James (1980), Leef, Hollingdale, & Beaudoin (1981), Nguyen-Dinh, Chemangey-Masaba, Spencer, Campbell, Chin & Collins (1981) and a WHO memorandum (1981). Most of these reports and reviews have dealt with the problems of cryopreserving malaria parasites from a parasitological viewpoint. In an attempt to provide a cryobiologically orientated review of value both to cryobiologists and practicing parasitologists the published technical reports have been summarized in chronological order in Tables 1 and 2, and the important cryobiological parameters are discussed.