Pre-treatment virus-specific B and T cell reactivities in 17 patients with MCPyV-positive MCC receiving pembrolizumab

Patient no.Antibodies to small T-antigenaMCPyV tetramer analysisbMCPyV intracellular cytokine reactivitycResponse assessed by RECIST 1.1d
3++CR
7++CR
8+++PR
6++PR
9++PD
16++PR
12+PR
21+CR
19++PD
4+N/APR
13+N/APR
26+N/APR
23+N/APD
15+N/APD
25+N/APR
14N/ACR
10+PR

a Baseline serum samples from all patients were used to measure MCPyV small T-antigen oncoprotein antibody titers at Laboratory Medicine (University of Washington, Seattle, WA) as described [6]. Titers above 74 STU were considered positive as negative control sera titers fall below 74 STU [7]

b All patients were low-resolution HLA class I genotyped to determine eligibility for CD8 T cell specific MCPyV peptide-HLA class I tetramer screening (Bloodworks Northwest, Seattle, WA). Pre- and post-treatment peripheral blood mononuclear cells (PBMCs) collected from patients with HLA class I types that corresponded to available MCPyV-specific tetramers (A*02:01, A*24:02, B*07:02, B*35:02, or B*37:01; n = 17 patients) were stained with appropriate tetramers and analyzed by flow cytometry. Samples with > 0.01% of CD8+ T cells co-staining with tetramers were considered positive. N/A (Not Available): nine patients, regardless of tumor viral status, had HLA class I types not amenable to tetramer staining and could thus not be evaluated for the presence of T cells recognizing MCPyV

c PBMCs pre-treatment and post-treatment blood collections (week 12 or 21) were stimulated with pools of MCPyV-specific peptides in a flow cytometry-based intracellular cytokine secretion assay (HIV Vaccine Trials Network, Seattle, WA). PBMCs that secreted interferon-gamma and/or IL-2 robustly (≥0.1% of CD8 T cells after background subtraction) were considered reactive to MCPyV

d Abbreviations for RECIST 1.1 response criteria are as follows: CR complete response, PR partial response, PD progressive disease