Figure 1.
Isolated merozoites maintain surface coat integrity.
A) E64-treated schizonts were filtered to release free merozoites and haemozoin crystals, and the filtrate was passed over magnetic columns. Merozoite purification was confirmed by Giemsa-stained smears of i) filtrate, ii) retained haemozoin and iii) purified merozoites. B) Merozites were stained with EtBr and enumerated by flow cytometry. C) Washed Merozoites retained surface proteins MSP-3, MSP-6 and AMA-1 by western blot (S: purified Schizonts; M: filtered merozoites; SM: EtBr stained merozoites). D) Merozoite surface proteins are maintained during merozoite isolation and wash steps as shown by surface localisation of MSP-3 and MSP119 by immunofluorescence microscopy. Antigens were stained with Alexa-594 and the nucleus with DAPI (panels in order Alexa549; DAPI; brightfield; Alexa549/DAPI; merge).
Figure 2.
Phagocytosis by THP-1 cells is antibody and Fc Receptor dependent.
A) EtBr stained merozoites were incubated with i) non-immune plasma or ii) immune PNG plasma, and were added to THP-1 cells. THP-1 cells were gated by forward and side scatter, and EtBr fluorescence was determined by flow cytometry. A non-immune control sample was used to set the EtBr positive gate. B) THP-1 cells were stained with anti-CD14 antibody and treated with trypan blue (TB) buffer which quenched all surface FITC fluorescence (black:THP-1 cells only, white: CD14-FITC stained THP-1, grey: CD14-FITC stained THP-1 with TB buffer). C) FITC stained merozoites were opsonised with i) non-immune or ii) immune PNG plasma and added to THP-1 cells. FITC fluorescence was measured by flow cytometry before and after quenching, and phagocytosed merozoites were resistant to quenching (black:THP-1 cells only, white: fluorescence with FITC stained merozoites, grey: fluorescence with FITC stained merozoites after TB buffer). D) The % phagocytosis measured for EtBr stained merozoites was equivalent to FITC stained merozoites after quenching. Opsonised EtBr stained and FITC stained merozoites were added to THP-1 cells at 3∶1 and 10∶1 merozoite:THP-1 ratios. E) Phagocytosis is active and Fc Receptor dependent. THP-1 cells were treated with cytochalasin D (CytoD) or blocked with non-specific IgG prior to addition to immune plasma opsonised merozoites in the phagocytosis assay. Each point represents the mean ± standard error. *, p<0.05; **, p<0.01; ***, p<0.005.
Figure 3.
Efficient merozoite phagocytosis at low antibody concentrations.
A) Increasing numbers of EtBr labeled merozoites were incubated with a pool of immune plasma from PNG children and then added to THP-1 cells. The % phagocytosis was determined by flow cytometry. The ratio of 4∶1 was chosen for subsequent assays, and is indicated by the arrow above. B) Titration of a pool of immune plasma from PNG children for opsonising activity using the 4∶1 merozoite:THP-1 cell ratio. % phagocytosis was determined by flow cytometry. The chosen dilution, 1/30,000, is indicated by an arrow above.
Figure 4.
Optimised Phagocytosis Assay is sensitive and reproducible.
A) Percentage phagocytosis for 22 plasma samples from PNG children, compared to a pooled PNG adult positive control. B) Interassay comparisons as determined by Spearman Correlation, or (C) Bland-Altman test.
Figure 5.
Merozoite opsonisation proceeds rapidly at low antibody concentrations.
Merozoites were added to wells containing THP-1 cells simultaneously (white bars), or after 40 mins of preincubation (grey bars), with varying dilutions of an immune plasma pool. % phagocytosis was determined by flow cytometry.