Challenges and Concluding Thoughts
Since exosomes are released by multiple cell types, an obvious challenge
in the use of exosomal cargo or liquid biopsy techniques in general as
biomarkers of DLBCL is the enrichment of tumor specific exosomes in
order to avoid dilution of information in tumor exosomes by exosomes
from non-neoplastic tissue. Enrichment for tumor derived exosomes before
subsequent analysis of exosomal cargo would address the issue of
background noise from non-neoplastic exosomes. Castillo et al identified
specific surface proteins on prostate cancer cell lines derived exosomes
and used antibodies to these proteins for immunocapture in plasma of
prostate cancer patients to enrich for tumor derived exosomes for
downstream analysis of the tumor derived exosomal cargo (107). Mizutani
et al also demonstrated isolation of prostate cancer specific exosomes
using immunocapture with prostate specific antibodies. (108) Similar
approaches can be employed as pre-analytical step in studying exosomes
in fluids from DLBCL patients.
Another challenge in routine use of exosomes in cancer management is the
difference in detected cargo observed using different methods of
isolation. Jeppesen et al used high resolution gradient fractionation to
separate small extracellular vesicles from non-vesicular proteins and
direct immunoaffinity capture to isolate exosomes from other vesicles
(109). Analyses of these exosomes showed absence in the cargo of some
nucleic acids and proteins hitherto found to be present in exosomes
using other methods. Contamination of exosomal isolates by other
vesicles and non-vesicular molecules probably accounts for such
variation. Guidelines to confirm purity of isolation would need to be
developed to prevent inter-laboratory variation of exosomal content
analyses.
Exosomes thus have great potential for not only improving diagnosis,
prognostication, and monitoring of DLBCL, but provide a means of
understanding the biology of DLBCL with the ultimate goal of improving
care for patients.