# Naturally Acquired Transmission-Blocking Immunity Against Different Strains of Plasmodium vivax in a Malaria-Endemic Area in Thailand

## TL;DR

The study shows that natural immunity in humans can block Plasmodium vivax transmission to mosquitoes, with antibodies playing a key role and varying effectiveness against different strains.

## Contribution

The study reveals that naturally acquired transmission-blocking immunity against Plasmodium vivax is antibody-mediated and varies by parasite strain.

## Key findings

- Transmission-blocking immunity in 26 out of 37 plasma samples was inhibited before immunoglobulin depletion.
- Some plasma samples retained blocking activity even after significant immunoglobulin removal.
- Plasma samples showed varying transmission-blocking activity against different parasite strains.

## Abstract

Human immunity triggered by natural malaria infections impedes parasite transmission from humans to mosquitoes, leading to interest in transmission-blocking vaccines. However, immunity characteristics, especially strain specificity, remain largely unexplored. We investigated naturally acquired transmission-blocking immunity (TBI) against Plasmodium vivax, a major malaria parasite.

Using the direct membrane-feeding assay, we assessed TBI in plasma samples and examined the role of antibodies by removing immunoglobulins through protein G/L adsorption before mosquito feeding. Strain specificity was evaluated by conducting a direct membrane-feeding assay with plasma exchange.

Blood samples from 47 patients with P vivax were evaluated, with 37 plasma samples successfully infecting mosquitoes. Among these, 26 showed inhibition before immunoglobulin depletion. Despite substantial immunoglobulin removal, 4 samples still exhibited notable inhibition, while 22 had reduced blocking activity. Testing against heterologous strains revealed some plasma samples with broad TBI and others with strain-specific TBI.

Our findings indicate that naturally acquired TBI is mainly mediated by antibodies, with possible contributions from other serum factors. The transmission-blocking activity of plasma samples varied by the tested parasite strain, suggesting single polymorphic or multiple targets for naturally acquired TBI. These observations improve understanding of immunity against P vivax and hold implications for transmission-blocking vaccine development.

Natural malaria infections trigger human immunity against parasite transmission to mosquitoes. We studied transmission-blocking immunity in 47 patients with Plasmodium vivax. Antibodies primarily mediated transmission-blocking immunity, with varying effectiveness against different parasite strains, offering insights for vaccine development.