Tu147 - CTLA4 Somatic Mosaicism Reveals Deleted Cells Show Little Difference for Big Effect

Katherine Jackson, PhD – Garvan Institute of Medical Research; Timothy Peters – Garvan Institute of Medical Research; Katrina Randall, Associate Professor – Canberra Hospital; Paul Gray, Dr – Sydney Children's Hospital; Christopher Goodnow, Professor – Garvan Institute of Medical Research

Abstract Text: CTLA4 controls a pivotal immune tolerance checkpoint, and its haploinsufficiency leads to a diverse autoimmune and inflammatory syndrome with ~67% penetrance. We report a unique case of gonosomal mosaicism involving a 250KB deletion encompassing the CTLA4 locus in an individual with adolescent-onset autoimmune cytopenia’s and inflammatory bowel disease responding to CTLA4-Ig therapy. His daughter possesses the same variant in the germline state and has severe neonatal-onset disease.

Quantitative PCR of the proband indicated the deletion at 6% in whole blood, with notable enrichment in memory B and T, Treg, and TFH cells, reaching nearly 50% of cells in some T-cell subsets. This cell-autonomous preferential expansion of CTLA4-deficient T-cells and development of autoimmune disease contrasts with studies of chimeric mice, where the presence of CTLA4-wildtype T-cells completely suppressed autoimmune disease.

Single-cell RNAseq demonstrated that deletion-bearing cells were polyclonal, exhibiting a trend towards a higher immunoglobulin somatic mutation rate and an increased proportion of IgA switched cells in memory B cells. No significant transcriptional differences were observed compared to CTLA4 wild-type memory B cells. Similar polyclonal patterns were observed in Treg and TFH subsets, with deleted cells showing no transcriptional distinctions from their wild-type counterparts.

This case represents the first instance of a mosaic deletion of CTLA4 associated with immune dysregulation. Acknowledging the limitation of deriving broad conclusions from one study, the selective accumulation of CTLA4-haploinsufficient memory lymphocytes, means low-level mosaicism for haploinsufficient cells may have delayed clinical implications, either for somatic deletions, or for residual recipient chimerism post-HSCT for CTLA4 haploinsufficiency.