Alternative Splicing of Apoptosis Genes Promotes Human T Cell Survival

Abstract

Alternative splicing occurs in the vast majority of human genes, giving rise to distinct mRNA and protein isoforms. We, and others, have previously identified hundreds of genes that change their isoform expression upon T cell activation via alternative splicing; however, how these changes link activation input with functional output remains largely unknown. Here we investigate how costimulation of T cells through the CD28 receptor impacts alternative splicing in T cells activated through the T cell receptor (CD3) and find that while CD28 signaling alone has minimal impact on splicing, it enhances the extent of change for up to 20% of TCR-induced alternative splicing events. Interestingly, a set of CD28-enhanced splicing events occur within genes encoding key components of the apoptotic signaling pathway; namely caspase-9, Bax and Bim. Using both CRISPR-edited cells and antisense oligos to force expression of specific isoforms, we show for all three of these genes that the isoform induced by CD3/CD28 costimulation promotes resistance to apoptosis, and that changes in all three genes together function combinatorially to further promote cell viability. Finally, we show that the JNK signaling pathway, induced downstream of CD3/CD28 costimulation, is required for each of these splicing events, further highlighting their co-regulation. Together these findings demonstrate that alternative splicing is a key mechanism by which costimulation of CD28 promotes viability of activated T cells.

Molecular and temporal control of restimulation-induced cell death (RICD) in T lymphocytes

“…Previous work has discovered a role for both coinhibitory and costimulatory molecules in T cell apoptosis, albeit with confusing results. Costimulatory molecules CD28, OX40, and CD27 have classically been associated with apoptotic resistance in the event of T cell (re)stimulation (Chen and Flies, 2013;Blake et al, 2022). However, subsequent data from our lab and others suggest CD28 engagement does not have an appreciable effect on RICD, and might serve to boost RICD sensitivity slightly in expanding CD4 + T cells.…”