Direct contact between Plasmodium falciparum and human B-cells in a novel co-culture increases parasite growth and affects B-cell growth

Sreenivasulu B Reddy,Frank Heuts,Francesca Chiodi,Noemi Nagy,Caroline Rönnberg,Allan Lugaajju,Mats Wahlgren,Laszlo Szekely,Kristina E M Persson

doi:10.1186/s12936-021-03831-x

Abstract

BackgroundPlasmodium falciparum parasites cause malaria and co-exist in humans together with B-cells for long periods of time. Immunity is only achieved after repeated exposure. There has been a lack of methods to mimic the in vivo co-occurrence, where cells and parasites can be grown together for many days, and it has been difficult with long time in vitro studies.Methods and resultsA new method for growing P. falciparum in 5% CO2 with a specially formulated culture medium is described. This knowledge was used to establish the co-culture of live P. falciparum together with human B-cells in vitro for 10 days. The presence of B-cells clearly enhanced parasite growth, but less so when Transwell inserts were used (not allowing passage of cells or merozoites), showing that direct contact is advantageous. B-cells also proliferated more in presence of parasites. Symbiotic parasitic growth was verified using CESS cell-line and it showed similar results, indicating that B-cells are indeed the cells responsible for the effect. In malaria endemic areas, people often have increased levels of atypical memory B-cells in the blood, and in this assay it was demonstrated that when parasites were present there was an increase in the proportion of CD19 + CD20 + CD27 − FCRL4 + B-cells, and a contraction of classical memory B-cells. This effect was most clearly seen when direct contact between B-cells and parasites was allowed.ConclusionsThese results demonstrate that P. falciparum and B-cells undoubtedly can affect each other when allowed to multiply together, which is valuable information for future vaccine studies.

Highlights

Plasmodium falciparum parasites cause malaria and co-exist in humans together with B-cells for long periods of time
Preparation of B‐cells As a first step in performing co-cultures of human B-cells and P. falciparum, CD19 + B-cells were isolated from Peripheral blood mononuclear cells (PBMC) and the B-cells were used directly after isolation without prior activation
Co‐culture of Plasmodium falciparum and B‐cells Different starting concentrations of parasites were tested when added to the B-cells, and when the concentration of parasites was below 0.1%, the parasites grew well for several days, when it was 0.5%, almost no parasites were observed after 96 h, and when the starting concentration was above 1%, no live parasites were observed after two parasite cycles

Summary

Introduction

Plasmodium falciparum parasites cause malaria and co-exist in humans together with B-cells for long periods of time. There has been a lack of methods to mimic the in vivo co-occurrence, where cells and parasites can be grown together for many days, and it has been difficult with long time in vitro studies. HIV infection, which like malaria is a chronic condition, results in changes in the B-cell compartment [18, 19] with an increased number of “exhausted” B-cells [20, 21]. These CD27− atypical memory B-cells, which express inhibitory Fc-receptor-like-4 (FCRL4)/FCRL5 have been proposed to have a reduced capacity to differentiate into antibody-secreting cells [17]

Methods

Results

Discussion

Conclusion