The potential of tumor-derived exosomes for noninvasive cancer monitoring

Abstract

Tumor-derived exosomes (TEX) are emerging as a new type of cancer biomarker. TEX are membrane-bound, virus-size vesicles of endocytic origin present in all body fluids of cancer patients. Based on the expanding albeit incomplete knowledge of their biogenesis, secretion by tumor cells and cancer cell-specific molecular and genetic contents, TEX are viewed as promising, clinically-relevant surrogates of cancer progression and response to therapy. Preliminary proteomic, genetic and functional profiling of tumor cell-derived or cancer plasma-derived exosomes confirms their unique characteristics. Alterations in protein or nucleic acid profiles of exosomes in plasma of cancer patients responding to therapies appear to correlate with clinical endpoints. However, methods for TEX isolation and separation from the bulk of human plasma-derived exosomes are not yet established and their role as biomarkers remains to be confirmed. Further development and validation of TEX as noninvasive, liquid equivalents of tumor biopsies are necessary to move this effort forward.

Keywords: cancer; cancer prognosis; molecular profiling; non-invasive biomarkers; nucleic acid content; protein cargo; tumor-derived exosome; tumor-derived exosome isolation.

Figure 1

Characteristics of exosomes isolated from the supernatant of Kasumi-1 (AML) cell line using differential centrifugation, ultrafiltration, size-exclusion chromatography and ultracentrifugation were studied. In A, following ultracentrifugation, exosomes were floated on a continuous sucrose density gradient, and the collected gradient fractions were tested by Western blots for CD34 and CD81. CD34+ and CD81+ exosomes were recovered at the sucrose density of 1.10–1.14 g/mL. In B, transmission electron microscopy of negatively-stained Kasumi-1 exosomes. In C, Western blots of Kasumi-1 exosomes (Kas) and exosomes isolated from plasma of a normal donor (NC). Each lane was loaded with 10µg exosomal protein. Reproduced from Hong CS, Muller L, Boyiadzis M, Whiteside TL, Isolation and characterization of CD34+ blast-derived exosomes in acute myeloid leukemia. PLoS One, 9(8), e103310 (2014) [80].

Figure 2

The strategy for isolation of TEX from plasma of a patient with cancer. Initially, a one-step mini-SEC is used to isolate total plasma exosomes. Next, TEX are captured on mAb-coated magnetic beads. The mAb selected for immune capture of TEX is specific for a tumor-associated antigen (TAA) overexpressed on tumor cells. Captured TEX are eluted from beads using acidified buffer. Following capture and elution, TEX retain morphologic integrity and mediate biological functions [83].

Figure 3

Protein content (normalized to µg/mL plasma) of exosomes isolated from plasma specimens of patients with glioma by differential centrifugation, ultrafiltration, SEC and ultracentrifugation. In A, mean protein levels are higher in patients than NC. In B, patients with stage IV disease have higher protein levels in exosome fractions than those with stage III disease. In C, protein levels in exosomes isolated prior to and after vaccination with a DC-based peptide vaccine. The data shown in A, B and C are reproduced from Muller L, Muller-Haegele S, Mitsuhashi M, Gooding W, Okada H, Whiteside TL, Exosomes isolated from plasma of glioma patients enrolled in a vaccination trial reflect anti-tumor immune activity and might predict survival, Oncoimmunology, 4 (6), e1008347 (2015) [99] by permission of Taylor & Francis LLC.