Detection of phosphatidylserine-positive exosomes for the diagnosis of early-stage malignancies

Abstract

Background: There has been increasing interest in the detection of tumour exosomes in blood for cancer diagnostics. Most studies have focussed on miRNA and protein signatures that are surrogate markers for specific tumour types. Because tumour cells and tumour-derived exosomes display phosphatidylserine (PS) in their outer membrane leaflet, we developed a highly sensitive ELISA-based system that detects picogram amounts of exosomal phospholipid in plasma as a cancer biomarker.

Methods: This report describes the development of a highly specific and sensitive ELISA for the capture of PS-expressing tumour exosomes in the blood of tumour-bearing mice. To monitor the relationship between tumour burden and tumour exosome plasma concentrations, plasma from one transplantable breast cancer model (MDA-MB-231) and three genetic mouse models (MMTV-PyMT; breast and KIC and KPC; pancreatic) were screened for captured exosomal phospholipid.

Results: We show that quantitative assessment of PS-expressing tumour exosomes detected very early-stage malignancies before clinical evidence of disease in all four model systems. Tumour exosome levels showed significant increases by day 7 after tumour implantation in the MDA-MB-231 model while palpable tumours appeared only after day 27. For the MMTV-PyMT and KIC models, tumour exosome levels increased significantly by day 49 (P⩽0.0002) and day 21 (P⩽0.001) while tumours developed only after days 60 and 40, respectively. For the KPC model, a significant increase in blood exosome levels was detected by day 70 (P=0.023) when only preinvasive lesions are microscopically detectable.

Conclusions: These data indicate that blood PS exosome levels is a specific indicator of cancer and suggest that blood PS is a biomarker for early-stage malignancies.

Figure 1

Schematic diagram of betabody, KL15C. Domains 1 and 5 of the plasma PS-binding protein, β2GP1, were genetically fused to the C-terminus of the CH3 domains of the Fc fragment of human IgG1. A Gly4Ser linker was inserted between the CH3 domains and domains 1 and 5 of β2GP1. The recombinant betabody was expressed as a dimer.

Figure 2

Determination of blood exosomal PS levels for the detection of tumours. (A) ELISA plates coated with KL15C (PS capture) and duramycin (PE capture) were incubated with the indicated amounts of vesicles containing 50% PS or 50% PE (wt/wt) in PC. After washing, the plates were incubated with biotinylated annexin 5 in the presence of Ca²⁺ (2 mM) or biotinylated duramycin (no Ca²⁺) and the amounts of PS or PE captured was quantified with HRP–streptavidin. The amounts of phospholipid indicated on the abscissa represents the assumed amounts of aminophospholipids expressed on the vesicle surface that is available for binding (50% aminophospholipid in PC with half of that in the particles outer leaflet). Closed circles, PS/PC vesicles; open circles, PE/PC vesicles; solid line, annexin 5 detection (PS); dotted line, duramycin detection (PE). (B and C) Human EDTA plasma (100 μl of 50% diluted plasma in PBS) from two confirmed ovarian carcinoma patients (dark grey and light grey) or two sex-matched healthy individuals (diagonal lines and crossed lines) were incubated on ELISA plates coated with (B) KL15C or (C) duramycin. After exosome capture, the plates were developed with both probes as described for the synthetic vesicles in (A). The values shown in (B and C) were calculated from the standard PS and PE curves shown in (A) (PS, left; PE, right) and are the average of two confirmed cancer patients and two healthy individuals.

Figure 3

Detection of PS-expressing tumour-derived exosomes in the blood of tumour-bearing mice. (A) MDA-MB231 breast cancer model: 8-week-old SCID Balb/c mice were injected orthotopically with 4 × 10⁶ MDA-MB231 cells on day 0. Day 0 blood was taken just before inoculation with tumour cells. The mice were then bled at weekly intervals. Each data point represent 50 μl of pooled plasma from 5 mice. Squares, exposed PE levels on PS-captured exosomes; circles, average tumour volume of the 5 mice. (B) Blood exosome numbers in pooled plasma of mice bearing MDA-MB231 tumours (bars): Aliquots of plasma obtained from animals described in (A) were quantified for total exosome number using the Exocet Exosome Quantification Assay system as described by the manufacturer (Systems Biosciences Inc., Palo Alto, CA, USA). The ratio of PS to particle number (circles) was obtained by dividing the PE values in (A) by the particle count. (C) PYMT breast cancer model: Five FVB PyMT mice were monitored for tumour size and bled weekly for exosome quantification beginning at day 35. Squares, exposed PE levels on PS-captured exosomes; circles, tumour volume. The data represent the average and s.d. for all animals. (D) Student’s t-test P-values for days 35, 42 and 49. Dark grey, PYMT mice; light grey, littermate controls. Individual tumour and exosome plots are shown in Supplementary Figure S5. (E) KIC pancreatic cancer model: Fourteen KIC mice and 14 littermate controls were monitored for body weight and bled weekly for exosome quantification beginning at day 21. Squares, exposed PE levels on PS-captured exosomes; circles, exposed PE levels in littermate control animals. The data represent the average and s.d. for all animals. (F) Student’s t-test P-values for day 21. Dark grey, KIC mice; light grey, KIC control littermates. Individual exosome plots are shown in Supplementary Figure S6A. (G) KPC pancreatic cancer model: Nine KPC mice and 8 littermate controls were monitored for body weight and bled weekly for exosome quantification beginning at day 21. Squares, exposed PE levels on PS-captured exosomes; circles, exposed PE levels in littermate control animals. The data represent the average and s.d. for all animals. (H) Student’s t-test P-values for days 56–84. Dark grey, KPC mice; light grey, KPC control littermates. Individual data sets are shown in Supplementary Figure S7.