Exosomes are virus-size (50-100 nm) double membrane-bound microvescicles formed within the endocytic compartments and released into the extracellular space via the fusion of multivescicles bodies with the cell membrane. While all cells release exosomes under physiologic conditions, tumor cells are avid exosome producers. In this study, we isolated exosomes from plasma of acute myeloid leukemia (AML) patients at diagnosis, during and after chemotherapy and studied their protein and TGF-β1 content. The hypothesis was tested that these exosomal components may serve as biomarkers of response to chemotherapy in AML.
Exosome fractions were isolated from plasma by exclusion chromatography on Sephadex G50 columns followed by ultracentrifugation. Exosome protein content was determined and expressed in μg protein/mL plasma. Exosome-associated TGF-β1 was measured by ELISA following exosome sonication/acidification and expressed in pg/exosome protein/mL plasma.
Exosomes isolated from AML patients’ plasma (n=16) at diagnosis and prior to any treatment had a higher mean protein content (55.2 ± 13.8 μg protein/mL) than those isolated from sera of normal controls (n=7), (13.1 ± 2.4 μg protein/mL, p<0.009). Following a course of induction chemotherapy, a reduction of the exosome protein level (23.8 ± 4.4 μg protein/mL, p<0.05) was observed in AML patients’ plasma (n=9). The exosome protein levels in the patients who at 14 days after the initiation of induction chemotherapy had a significant reduction of their blasts (to <5%) were lower compared to those in the patients with residual leukemia (18.5 ± 5.8 vs 30.8 ± 7.5 μg protein/mL, p=0.2). The exosome protein levels during consolidation therapy with high dose cytarabine in patients who had achieved complete remission (n=10) was higher (52.4 ± 8.9 μg protein/mL) compared to levels measured immediately after induction chemotherapy (p<0.02) and approaching those seen at diagnosis. Protein levels of exosomes isolated from plasma of patients (n=5) in long-term remission (>2 years after completing consolidation therapy) normalized to the levels which were not different from those of normal controls. Levels of active TGF-β1 carried by exosomes obtained at AML diagnosis were high and then decreased (p<0.005) following induction chemotherapy. Similar to protein levels, TGF-β1 levels in plasma-derived exosomes were elevated (p<0.004) during consolidation chemotherapy. In AML patients in long-term remission, TGF-β1 levels carried by exosomes normalized, approaching levels seen in exosomes of normal controls.
Our results indicate that exosomal protein and TGF-β1 levels in AML plasma could serve as biomarkers of response to chemotherapy and might reflect the presence/absence of residual disease after therapy.
No relevant conflicts of interest to declare.
Asterisk with author names denotes non-ASH members.