Total RNA was extracted from the sorted cells with RNAeasy kit (QIAGEN) according to the manufacturer's protocol. Amplicons for sequencing on the Illumina MiSEQ platform were synthesized using commercial reagents and a protocol provided by the manufacturer (New England Biolabs, Ipswich MA). Briefly, extracted RNA (100 ng) was reverse-transcribed into cDNA using a biotinylated oligo dT primer. An adaptor sequence was added to the 3' end of all cDNA, which contains a universal priming site and a 17-nucleotide unique molecular identifier (UMI). Products were purified using streptavidin-coated magnetic beads followed by a primary PCR reaction using a pool of primers targeting the IGHA, IGHD, IGHE, IGHG, IGHM, IGKC, and IGLC regions, as well as a primer against the universal adapter sequence. The Ig-specific primers contained tail sequences for the later Illumina adapter addition. PCR products were then purified using AMPure XP beads. A secondary PCR was then performed to add the Illumina P5 adapter to the C region end and a sample-indexed P7 adapter to the universal adapter end. The number of secondary PCR cycles was tailored to each sample to avoid entering the plateau phase, as judged by a prior quantitative PCR analysis. Final products were purified, then quantified with a TapeStation (Agilent Genomics).