Re-examination of earlier work on repetitive DNA and mosquito infectivity in rodent malaria

Plasmodium falciparum gametocyte carriage in symptomatic patients shows significant association with genetically diverse infections, anaemia, and asexual stage density

Background

Multi-genotype malaria infections are frequent in endemic area, and people commonly harbour several genetically distinct Plasmodium falciparum variants. The influence of genetic multiplicity and whether some specific genetic variants are more or less likely to invest into gametocyte production is not clearly understood. This study explored host and parasite-related risk factors for gametocyte carriage, and the extent to which some specific P. falciparum genetic variants are associated with gametocyte carriage.

Methods

Gametocytes and asexual forms were detected by light microscopy on thick smears collected between 2010 and 2012 in Nanoro, Burkina Faso. Merozoite surface protein 1 and 2 were genotyped by nested PCR on clinical samples. Associations between gametocyte carriage and factors, including multiplicity of infection, parasite density, patient age, gender, haemoglobin (Hb) level, and body temperature were assessed. The relationship between the presence of a particular msp1 and msp2 genetic variants and gametocyte carriage was also explored.

Results

Of the 724 samples positive to P. falciparum and successfully genotyped, gametocytes were found in 48 samples (6.63%). There was no effect of patient gender, age and body temperature on gametocyte carriage. However, the probability of gametocyte carriage significantly increased with increasing values of multiplicity of infection (MOI). Furthermore, there was a negative association between parasite density and gametocyte carriage. MOI decreased with parasite density in gametocyte-negative patients, but increased in gametocyte carriers. The probability of gametocyte carriage decreased with Hb level. Finally, the genetic composition of the infection influenced gametocyte carriage. In particular, the presence of RO33 increased the odds of developing gametocytes by 2 while the other allelic families K1, MAD20, FC27, and 3D7 had no significant impact on the occurrence of gametocytes in infected patients.

Conclusion

This study provides insight into potential factors influencing gametocyte production in symptomatic patients. The findings contribute to enhance understanding of risk factors associated with gametocyte carriage in humans.

Trial registration NCT01232530.

Sexual development in malarial parasites: gametocyte production, fertility and infectivity to the mosquito vector

Using cloned lines of Plasmodium berghei producing mixed asexual and sexual (clone 234L) and purely asexual (clone 233L) parasitaemias, the courses of parasitaemia, gametocytogenesis, exflagellation, ookinete production in vitro and mosquito infectivity have been followed. For clone 234L mosquito infectivity is maximal at day 3 and has ceased by day 6 post-infection. Conversely, gametocytogenesis, exflagellation and ookinete production are at minimal levels at day 3 and rise to peaks between days 10 and 15 of infection (in TO mice infected with blood at mechanical blood passage 3). Sexual potential declines progressively with sustained mechanical passage (up to P14). Gametocyte conversion is highest early in infection and declines exponentially; however, upon each mechanical passage conversion is again raised but decreases more rapidly with succeeding passages. For practical mosquito transmission in the laboratory we consider P8 to be the useful limit for mechanical transmission for this parasite clone. Asexual parasite growth (virulence) is more rapid with increased mechanical passage inducing a rapid fall in haematocrit. By P14 the course of infection closely parallels that of the purely asexual clone 233L.

Plasmodium berghei: gametocyte production, DNA content, and chromosome-size polymorphisms during asexual multiplication in vivo

In this study the DNA content and the karyotype of clones of Plasmodium berghei, which differed in the capability to produce gametocytes, were determined. The DNA content per haploid genome was established by cytofluorometric methods after staining of the haploid merozoites with DNA-specific fluorescent dyes. Field inversion gel electrophoresis was used to establish the number and size of the chromosomes. Parasites of a high gametocyte producer clone (original HP) and a low producer clone (original LP) contained 13 to 14 chromosomes in the size range of 0.5-3.8 megabase. In four independent experiments parasites of the original HP clone were maintained in mice and were mechanically transmitted for prolonged periods of time (up to 90 weeks). During the transmission period the capability to produce gametocytes decreased in all four lines. After mosquito transmission of parasites from these low producer lines, the gametocyte production returned to the level of the original HP clone. The total DNA content per haploid genome of low producer parasites was not significantly different from that of HP parasites. During prolonged periods of asexual multiplication of the HP clone in vivo, changes in the relative size of several chromosomes were detected. Mosquito transmission of the original HP clone did not result in a change of the karyotype. However, novel karyotypes were readily observed after mosquito transmission of parasites of the low producer lines. The decrease of the capability to produce gametocytes did not necessarily involve detectable changes in DNA content or in karyotype.

Variation among circumsporozoite protein genes from rodent malarias

We investigated the effect of long term passage of parasites in naive animals on the circumsporozoite protein (CSP) gene of Plasmodium yoelii. The CSP gene sequence was determined from a non-lethal cloned line of P. yoelii and compared to the CSP gene sequence from a lethal strain of P. yoelii. The two parasite lines were originally derived from the same isolate, but were separated 17 years ago followed by continued passage. The sequence of the CSP gene and its surroundings from the non-lethal line remains identical to that from the lethal isolate except for a deletion within the repeated central domain. This result contrasts with the results obtained by sequencing a number of clones from different geographical field isolates of Plasmodium falciparum where there appears to be rapid accumulation of sense mutations within putative functional domains. These observations are consistent with the suggestion that strong biological pressure in a field environment results in selection of parasite types on the basis of different CSP gene sequences.

In vivo testing of subunit vaccines against malaria sporozoites using a rodent system

To test the putative in vivo protective effects of antibodies to circumsporozoite (CS) protein repeats against malarial infection, different strains of mice were immunized against various repetitive regions of the Plasmodium yoelii CS protein in the form of synthetic peptides conjugated to keyhole limpet hemocyanin. Complete Freund's adjuvant or saponin was used as adjuvant. When vaccinated mice were challenged with 500 sporozoites almost all animals became infected. There were no significant protective effects in vaccinated versus unvaccinated mice. Furthermore, there was no correlation between the antibody titer to the CS repeats and infection. The parasites from infected animals were shown to encode a CS protein containing the same repeats as those used for immunization, indicating that the infections were not due to selection for variant parasites. These experiments demonstrate that antibodies to the CS repeats, as derived in vivo with peptides, despite being surface reactive, do not provide protection against sporozoite challenge in vivo. This conclusion is in contrast to previous conclusions based on studies showing protection by way of in vitro sporozoite neutralization procedures and passive transfer of monoclonal antibody.