IMMU-18. PDL-1 EXPRESSION ON CIRCULATING CD68 (-) MONOCYTE-LIKE CELLS IN NF2 MENINGIOMA AS A BIOMARKER FOR TUMOR PROGRESSION

Program cell death ligand-1 (PD-L1) membranous expression on >5% tumor cells (PD-L1 positive tumors) is an unfavorable prognostic marker in many solid tumors. We previously showed that approximately 40% of neurofibromatosis type 2 (NF2) meningiomas are PD-L1 positive tumors. However, due to the invasive nature of biopsies, collection of tumor tissue is not always feasible. Thus, a non-invasive alternative is needed to evaluate the status of tumor growth and confirm PD-L1 positive tumors before the consideration of immunotherapy. It has recently been revealed that expression of PD-L1 on tumor associated macrophages is also a strong prognostic indicator. We retrieved formalin-fixed paraffin-embedded (FFPE) tissue from 10 NF2 meningioma cases to identify PD-L1 expression on macrophages and/or monocytes. We found that 3 out of 4 PD-L1 positive tumors were associated with expression of PDL-1 on CD68 (-) monocyte-like cells located in the peri- and intravascular lumens. These cells were only observed in 1 out of 6 PD-L1 negative tumors. Compared to others, tumors with PD-L1 expression on monocyte-like cells presented a higher Ki-67 proliferative index that was above 10%. Our results suggest that PD-L1 positive circulating CD68 (-) monocyte-like cells are correlated with tumor cell PD-L1 expression and progression in NF2 meningiomas.