Memory B-Cell Responses Against Merozoite Antigens After Acute Plasmodium falciparum Malaria, Assessed Over One Year Using a Novel Multiplexed FluoroSpot Assay.

Peter Jahnmatz,Francis Ndungu,Christopher Sundling,Muhammad Asghar,Victor Yman,Christine Stenström,Christian Smedman,Linnea Widman,Anna Färnert,Niklas Ahlborg,Klara Sondén

doi:10.3389/fimmu.2020.619398

Abstract

Memory B cells (MBCs) are believed to be important for the maintenance of immunity to malaria, and these cells need to be explored in the context of different parasite antigens and their breadth and kinetics after natural infections. However, frequencies of antigen-specific MBCs are low in peripheral blood, limiting the number of antigens that can be studied, especially when small blood volumes are available. Here, we developed a multiplexed reversed B-cell FluoroSpot assay capable of simultaneously detecting MBCs specific for the four Plasmodium falciparum blood-stage antigens, MSP-119, MSP-2, MSP-3 and AMA-1. We used the assay to study the kinetics of the MBC response after an acute episode of malaria and up to one year following treatment in travelers returning to Sweden from sub-Saharan Africa. We show that the FluoroSpot assay can detect MBCs to all four merozoite antigens in the same well, and that the breadth and kinetics varied between individuals. We further found that individuals experiencing a primary infection could mount and maintain parasite-specific MBCs to a similar extent as previously exposed adults, already after a single infection. We conclude that the multiplexed B-cell FluoroSpot is a powerful tool for assessing antigen-specific MBC responses to several antigens simultaneously, and that the kinetics of MBC responses against merozoite surface antigens differ over the course of one year. These findings contribute to the understanding of acquisition and maintenance of immune responses to malaria.

Highlights

Acquisition of clinical immunity to malaria is achieved after multiple infections in individuals living in areas with high transmission [1, 2]
By studying Memory B cells (MBCs) responses in travelers returning with malaria to a country without transmission, the immune response after a natural infection can be assessed in the absence of re-exposure
The optimal concentrations of the expressed P. falciparum antigens were established by serial dilutions in a series of FluoroSpot assays and defined as the dilution where the number of counted spots plateaued without increasing background noise

Summary

Introduction

Acquisition of clinical immunity to malaria is achieved after multiple infections in individuals living in areas with high transmission [1, 2]. Maintaining protection without repeated exposure is challenging as antibody levels against parasite antigens wane rapidly after both natural infections and vaccination [4,5,6]. By studying MBC responses in travelers returning with malaria to a country without transmission, the immune response after a natural infection can be assessed in the absence of re-exposure. We have previously shown, using a cross-sectional approach, that after a single infection, MBCs specific for P. falciparum blood stage antigens can be maintained by some individuals for many years without re-exposure [11]. In order to further investigate how antigen-specific immune responses are acquired and maintained after acute malaria, we set up a prospective longitudinal cohort study of travelers sampled several times over one year after treatment

Methods

Results

Conclusion