Serial changes in PD1/PDL1 expression in metastatic urothelial carcinoma (mUC) patients (pts) treated with immune checkpoint blockade (CPB).

109Background: Since CPB may alter immune marker expression in key immunomodulatory populations such as myeloid-derived suppressor cells (MDSC) and CD8+ cytotoxic T lymphocytes (CTL), we evaluated PD1/PDL1 expression in longitudinal samples from mUC pts treated with CPB. Methods: Serial peripheral blood samples were collected from mUC pts who received CPB. PD1/PDL1 and VISTA expression was measured in MDSC (CD33+HLADR−) and CTL (CD8+CD4−) from live peripheral blood mononuclear cells using flow cytometry. MDSC subsets were further defined as (G)ranulocytic (CD15+CD14−), (M)onocytic (CD15−CD14+), and (I)mmature (CD15−CD14−). PD1/PDL1 and VISTA expression was presented as % of each MDSC subset or CTL. Wilcoxon signed-rank tests and mixed-model regression analyses were performed to assess changes in immune marker expression after CPB. Results: Of 30 CPB-treated pts with ≥ 2 blood samples for analysis, 21 received anti-PDL1 (20 atezolizumab/1 avelumab; [A]) and 9 received anti-PD1 (pembrolizumab [P]). Median a...