Immunological Correlates of Response to Immune Checkpoint Inhibitors in Metastatic Urothelial Carcinoma

Background The identification of prognostic and/or predictive biomarkers for response to immune checkpoint inhibitors (ICI) could help guide treatment decisions. Objective We assessed changes in programmed cell death-1 (PD1)/PD1 ligand (PDL1) expression in key immunomodulatory cell subsets (myeloid-derived suppressor cells [MDSC]; cytotoxic T lymphocytes [CTL]) following ICI therapy and investigated whether these changes correlated with outcomes in patients with metastatic urothelial carcinoma (mUC). Patients and Methods Serial peripheral blood samples were collected from ICI-treated mUC patients. Flow cytometry was used to quantify PD1/PDL1 expression on MDSC (CD33(+)HLADR(-)) and CTL (CD8(+)CD4(-)) from peripheral blood mononuclear cells. MDSC were grouped into monocytic (M)-MDSC (CD14(+)CD15(-)), polymorphonuclear (PMN)-MDSC (CD14(-)CD15(+)), and immature (I)-MDSC (CD14(-)CD15(-)). Mixed-model regression and Wilcoxon signed-rank or rank-sum tests were performed to assess post-ICI changes in immune biomarker expression and identify correlations between PD1/PDL1 expression and objective response to ICI. Results Of 41 ICI-treated patients, 26 received anti-PDL1 (23 atezolizumab/3 avelumab) and 15 received anti-PD1 (pembrolizumab) therapy. Based on available data, 27.5% had prior intravesical Bacillus Calmette-Guerin therapy, 42% had prior neoadjuvant chemotherapy, and 70% had prior cystectomy or nephroureterectomy. Successive doses of anti-PDL1 correlated with decreased percentage of PDL1(+) (%PDL1(+)) M-MDSC, while doses of anti-PD1 correlated with decreased %PD1(+) M- and I-MDSC. Although pre-treatment %PD1(+) CTL did not predict response, a greater %PD1(+) CTL within 9 weeks after ICI initiation correlated with objective response. Conclusions Treatment with ICI correlated with distinct changes in PD1/PDL1-expressing peripheral immune cell subsets, which may predict objective response to ICI. Further studies are required to validate immune molecular expression as a prognostic and/or predictive biomarker for long-term outcomes in mUC.