Sodium azide

Millipore filtered deionized water (“water”) was used as sample carrier and to prepare 1x PBS from 10x PBS (Rockland, Gilbertsville, PA) and CyPBS/0.1% BSA (Sigma) (“CyPBS/BSA”) buffer that was used as staining and washing media for PBMC. For some experiments, CyPBS/BSA was supplemented with 0.05% v/v sodium azide (Teknova, Hollister, CA) and 2 mM EDTA (Hoefer Inc., Holliston, MA). Buffers were filtered over 0.22 μm membranes (PALL, Ann Arbor, MI, or EMD Millipore, Billerica, MA). Unlabeled, carrier protein-free antibodies (Table SI) were purchased from Biolegend (San Diego, CA), BD Biosciences (San Jose, CA), Santa Cruz Biotechnology (Dallas, TX), R&D Systems (Minneapolis, MN) and Miltenyi Biotech (San Diego, CA). In-house conjugations were carried out using MAXPAR^® kits (Fluidigm, Sunnyvale, CA) according to the manufacturer’s instructions. This includes CD45-In113 and CD45-In115 barcoding agents. Highly isotopically-enriched metal salts not available through Fluidigm were purchased from Trace Sciences (Richmond Hill, ON, Canada). Antibodies were diluted to working concentrations in CyPBS/BSA and filtered over 0.1 μm spin filters (Amicon, Millipore, Billerica, MA).