Introduction: Immunosuppressive myeloid cells are elevated in the tumors and peripheral circulation of glioblastoma (GBM) patients. Programmed death-ligand 1 (PD-L1), expressed on myeloid derived suppressor cells (MDSCs) and other monocyte populations, contributes to immune suppression through induction of T cell apoptosis and anergy. GBM secreted factors can increase PD-L1 expression in naïve monocytes. However, it remains unknown to what extent GBM derived factors differentially induce PD-L1 on MDSCs and non-MDSCs.

Methods: Preoperative peripheral blood mononuclear cells from newly diagnosed GBM patients (n = 34) were collected and analyzed by flow cytometry for PD-L1+ MDSCs (CD11b+CD33+HLA-DRlo/-PD-L1+) and non-MDSC monocytes (CD11b+CD33+HLA-DRhiPD-L1+). To investigate the role of GBM secreted factors in MDSC and non-MDSC PD-L1 induction, naïve monocytes were isolated from healthy donor blood and stimulated with GBM-conditioned media (n = 6) for 24 hours before analysis.

Results: The median MDSC frequency in GBM patient peripheral blood was 14.6% of total myeloid cells. Median PD-L1 expression on MDSCs was 46.9%, which was lower than the median PD-L1 expression on non-MDSC monocytes of 64.2% (p < 0.05). Whereas MDSC and monocyte PD-L1 expression were positively correlated (p < 0.0001, R2 = 0.739), MDSC abundance and MDSC PD-L1 expression were not correlated (p = 0.77, R2 = 0.003). MDSCs isolated from healthy donors exhibited a 2.8-fold (95% CI: 2.5-3.1) increased baseline PD-L1 expression compared to monocytes. Upon stimulation with GBM-conditioned media, however, monocytes demonstrated a 2.2-fold (95% CI: 1.9-2.4) increased PD-L1 induction (relative to baseline), compared to a minimal 1.1-fold (95% CI: 1.0-1.2) increase observed in MDSCs (p < 0.001).

Conclusions: Although MDSCs exhibit higher baseline PD-L1 expression, non-MDSC monocytes demonstrate greater inducible PD-L1 potential. Moreover, as non-MDSCs comprise a greater proportion of myeloid cells and express elevated PD-L1 in GBM patients, the blockade of PD-L1 induction in this population may serve as a mechanism to improve GBM immunotherapy.

Patient Care: Identification of non-MDSC monocytes as the predominant inducible PD-L1 population in GBM patients provides a rationale for investigating the blockade of PD-L1 induction in this population as a means to improve immunotherapy for GBM.

Learning Objectives: By the conclusion of the session, participants should be able to (1) identify GBM secreted factors as drivers of PD-L1 expression on myeloid cells and (2) identify non-MDSCs and MDSCs as having different PD-L1 induction potentials.

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