Characterization of Naturally Acquired Immunity to a Panel of Antigens Expressed in Mature P. falciparum Gametocytes

Abstract

IntroductionNaturally acquired immune responses against antigens expressed on the surface of mature gametocytes develop in individuals living in malaria-endemic areas. Evidence suggests that such anti-gametocyte immunity can block the development of the parasite in the mosquito, thus playing a role in interrupting transmission. A better comprehension of naturally acquired immunity to these gametocyte antigens can aid the development of transmission-blocking vaccines and improve our understanding of the human infectious reservoir.MethodsAntigens expressed on the surface of mature gametocytes that had not previously been widely studied for evidence of naturally acquired immunity were identified for protein expression alongside Pfs230-C using either the mammalian HEK293E or the wheat germ cell-free expression systems. Where there was sequence variation in the candidate antigens (3D7 vs a clinical isolate PfKE04), both variants were expressed. ELISA was used to assess antibody responses against these antigens, as well as against crude stage V gametocyte extract (GE) and AMA1 using archived plasma samples from individuals recruited to participate in malaria cohort studies. We analyzed antibody levels (estimated from optical density units using a standardized ELISA) and seroprevalence (defined as antibody levels greater than three standard deviations above the mean levels of a pool of malaria naïve sera). We described the dynamics of antibody responses to these antigens by identifying factors predictive of antibody levels using linear regression models.ResultsOf the 25 antigens selected, seven antigens were produced successfully as recombinant proteins, with one variant antigen, giving a total of eight proteins for evaluation. Antibodies to the candidate antigens were detectable in the study population (N = 216), with seroprevalence ranging from 37.0% (95% CI: 30.6%, 43.9%) for PSOP1 to 77.8% (95% CI: 71.6%, 83.1%) for G377 (3D7 variant). Responses to AMA1 and GE were more prevalent than those to the gametocyte proteins at 87.9% (95% CI: 82.8%, 91.9%) and 88.3% (95% CI: 83.1%, 92.4%), respectively. Additionally, both antibody levels and breadth of antibody responses were associated with age and concurrent parasitaemia.ConclusionAge and concurrent parasitaemia remain important determinants of naturally acquired immunity to gametocyte antigens. Furthermore, we identify novel candidates for transmission-blocking activity evaluation.

Heterologous Expression and Evaluation of Novel Plasmodium falciparum Transmission Blocking Vaccine Candidates

“…Although the specific function of Pf3D7_1314500 needs be further investigated, this predicted function in protein transport makes it unlikely that this antigen is a promising TBV candidate. However, a recent study showed that increased antibody responses to Pf3D7_1314500 in two cohorts of malaria-exposed individuals were significantly associated with gametocytemia, demonstrating a potential role as biomarker of gametocyte carriage ( 49 ).…”

The acquisition of humoral immune responses targeting Plasmodium falciparum sexual stages in controlled human malaria infections

“…More recently, Muthui et al. set out to analyse naturally acquired antibody responses to seven antigens selected based on potential gametocyte surface expression, including Pfs230, PfMDV1 and five previously uncharacterized gametocyte antigens ( 49 ). They demonstrated antibody responses to all seven antigens, and suggested that PF3D7_0303900, PF3D7_1314500, and PF3D7_0208800 may have potential as markers of high gametocytaemia.…”

Plasmodium 6-Cysteine Proteins: Functional Diversity, Transmission-Blocking Antibodies and Structural Scaffolds

“…For this reason, cell free systems have been used to express multiple antigens for high-throughput screening of patient sera against an array of antigens. Such studies have shown that antibodies against Pf38 have an intermediate association with protection against symptomatic malaria ( Richards et al., 2013 ) and antibody levels against Pfs230C may be associated with age ( Muthui et al., 2021 ). Unfortunately, not all antigens appear to be amenable to expression via this system, with production of recombinant Pfs48/45, Pfs47 and PfPSOP12 proving unsuccessful ( Muthui et al., 2021 ).…”

“…Such studies have shown that antibodies against Pf38 have an intermediate association with protection against symptomatic malaria ( Richards et al., 2013 ) and antibody levels against Pfs230C may be associated with age ( Muthui et al., 2021 ). Unfortunately, not all antigens appear to be amenable to expression via this system, with production of recombinant Pfs48/45, Pfs47 and PfPSOP12 proving unsuccessful ( Muthui et al., 2021 ). This system has also been used to produce multiple vaccine candidates for functional comparison, supporting the further development of Pfs230C as a vaccine candidate ( Miura et al., 2013 ).…”