Abstract

Myeloid derived suppressor cells
(MDSCs) are a population of pathologically activated, immature myeloid cells
with potent immunosuppressive capabilities. As a key mediator of immune
suppression, in the setting of many tumours, the infiltration of MDSCs may be a
significant obstacle in achieving effective tumour immunotherapy, which is a
viable and rapidly advancing strategy for cancer treatment. It is well
established that depletion of MDSCs improves T cell immune responses, whilst
delaying tumour progression and prolonging survival, in both cancer patients
and animal models. As a result, increasing attention has been focused on
understanding the mechanisms underlying MDSC induction.

Using an in vitro approach, we have generated CD33⁺ HLA-DR^low
MDSCs from healthy donor PBMCs via cytokine induction using recombinant human
GM-CSF, in combination with either IL-6 or VEGF, over a period of 6-8 days. The
clinically relevant A375 melanoma cell line, known to induce formation of
suppressive MDSCs, was found to secrete higher levels of both IL-6 and VEGF,
via MSD and ELISA, underscoring the selection of these factors for MDSC
induction.

Co-culture of the in vitro generated MDSCs with autologous
T cells, at a ratio of 2:1 T cells: MDSC, demonstrated significant inhibition
of T cell proliferation in response to CD3/CD28 stimulation of T cells using an
ImmunoCult reagent, during a 4-day co-culture period (one-way ANOVA, p<0.01).
This suppressive effect on T cell proliferation, which was accompanied by reduced
T cell secretion of IFN-γ in response to CD3/CD28 activation, indicated acquisition
of an immunosuppressive phenotype by the in vitro generated MDSCs; characteristic
of this myeloid cell population.

Overall, suppressive CD33⁺ cells
generated from human peripheral blood PBMCs via GM-CSF and IL-6 or GM-CSF and VEGF
stimulation were consistent with human MDSCs. The next phase of this study will
be to induce MDSCs using physiologically relevant A375 conditioned media, to
more closely mimic the conditions experienced by MDSC precursors in the tumour microenvironment.
 Further study of this cell population could
enable the development of novel therapeutic reagents for cancer immunotherapy.