ATAC-seq analysis of aged-associated B cells (ABC) across three TLR9 genotypes (TLR9WT, TLR9-/- and non-signaling TLR9P915H/P915H) isolated from a mixed chimeric setting in the MRL/lpr murine model of systemic lupus erythematosus.

Purpose: The goal of this study was to assess if TLR9 deficiency (TLR9-/-) or TLR9 incapacity to signal through MyD88 would differentially influence the epigenetic states of ABC, in a B cell intrinsic manner. Methods : Mixed bone marrow chimeras of one-third each TLR9+/+ CD45.1/2, TLR9-/- CD45.1/1 and TLR9P915H/P915H CD45.2/2 were generated on the MRL/lpr background. From each recipient, ABC were sorted with respect to the CD45 congenic markers at [20 weeks] post chimerism. DNA was isolated using a protocol adapted from Corces et al. Samples were sequenced to obtain 20M 2x75 bp paired-end reads using an Illumina NextSeq 550 sequencer (Illumina, Inc, California, USA). Results: The majority of differences in open chromatin regions were observed between TLR9-/- and TLR9P915H ABCs. This must reflect the MyD88-independent regulatory function of TLR9 (lost in TLR9-/- and present in TLR9P915H). Overall design: DAR profiles of TLR9 deficient (TLR9-/-), mutant TLR9 (P915H/P915H) and intact TLR9 (WT) ABC of 20-week-old (3-way-mixed) BM chimera MRL/lpr mice were generated in three replicates by sequencing, using an Illumina NextSeq 550 sequencer.