Seroprevalence of antibodies to Plasmodium falciparum transmission-blocking target proteins Pfs230D1M and Pfs48/45 in Tanzanian populations of diverse malaria transmission intensity

Abstract

Transmission-blocking vaccines are among the novel tools under development for malaria control and elimination. Understanding the human immune response to the sexual stages of Plasmodium falciparum is essential for progressing transmission-blocking vaccine development. A serosurvey was conducted in Tanzania, from May to August 2022 among 290 participants, consisting of 114 children (5-12 years), 44 adolescents (13-17 years), and 132 adults (18-45 years). The participants were tested for malaria parasites using quantitative polymerase chain reaction, and standardized enzyme-linked immunosorbent assays were performed to detect the presence of IgG antibodies against transmission-blocking target antigens-Pfs230D1M, Pfs48/45, and Pfs25. A set of 10 plasma samples that were reactive to Pfs230DIM and/or Pfs48/45 were tested individually for transmission-reducing activity via standard membrane feeding assays. Of the participants tested, 56% (157/281) had detectable Pfs230D1M antibodies, and 49% (141/290) were positive for Pfs48/45 IgG. Approximately 30% were seropositive for both. However, Pfs25 IgG was not detected in any of the 117 participants tested. The seroprevalence for Pfs230D1M and Pfs48/45 IgG increased significantly with participants' age, with adults more likely to have antibodies than children: Pfs230D1M (adjusted odds ratio: 3.16, 95% confidence interval: 1.81-5.53, p-value ≤ 0.0001) and Pfs45/48 (OR: 3.06, 95% CI: 1.79-5.25, p ≤ 0.0001). There was no significant difference in antibody titers for Pfs230D1M and Pfs48/45 antibodies across age groups. A significant transmission-reducing activity was observed in 2/10 participants, who were highly reactive to Pfs230D1M and Pfs48/45. Naturally acquired antibody responses to both full-length Pfs48/45 and Pfs230D1M proteins are prevalent and appeared to be stable, suggesting that semi-immune populations may be ideal to evaluate boosting transmission-blocking vaccine candidates.

Keywords: Pfs230D1M; Pfs25; Plasmodium falciparum; Tanzania; and Pfs48/45; gametocytes; malaria transmission; transmission-blocking vaccines.

Figure 1

Correlation between antibody levels and parasite density. (A) Correlation between anti-Pfs230DIM antibody levels and anti-Pfs48/45 levels in the figure. (B) Correlation between anti-Pfs230D1M antibody levels and malaria parasite density. (C) Correlation between anti-Pfs48/45 levels and parasite density. The solid line shows the predicted mean from the linear regression, and the shaded area is the confidence interval.

Figure 2

Geometric mean anti-Pfs230D1M and anti-Pfs48/45 IgG titer in seropositive adults and children over a 10-month-period: (A) anti-Pfs230D1M total IgG responses; n = 31 (8 children and 23 adults); (B) anti-Pfs48/45 total IgG responses; n = 26 (9 children and 17 adults), and antibody responses are shown on the left y-axes. The error bars indicate the lowest and highest antibody titer. Average precipitation at each time point is indicated by blue bars (right y-axes). The sampling time points were between June 2022 and March 2023.

Figure 3

Antibody levels, antibody avidity, and oocyst intensity in participants assessed for antibody functionality. (A) Pfs230D1M and Pfs48/45 individual and median Pfs230D1M and Pfs48/45 antibody responses. (B) Individual and median avidity indices of total Pfs230D1M and Pfs48/45 IgG. Plasma samples were tested individually, and increasing concentrations of NaSCN (0, 1, 2, 3,4,5, 6, 7, and 8 M) were used prior to the development of the ELISA reaction. Results for both ID N006 and N008 are in red. IC50 = concentration of NaSCN needed to reduce antibodies by 50%. (C, D) Oocyst counts following SMFA with purified IgGs from Pfs230D1M and/or Pfs48/45-positive plasmas. Purified IgG (10 mg/mL) was mixed with P. falciparum NF54 cultured gametocytes and fed to mosquitoes (n = 20 per test assay), which were dissected 7 days post-feeding to determine oocyst intensity. Purified IgG from a pool of naive sera was used as negative control. Monoclonal Pfs25 antibodies (4B7) was used as the positive control. The asterisk means statistically significant difference (p-value < 0.05) between the negative control and test samples. All 10 samples were tested in SMFA1 (C), and only two samples were tested in SMFA2 (D).