BMI-1 - A novel Epigenetic Regulator of CD8⁺ T Cell Differentiation

Abstract

CD8⁺ T cells are important for the elimination of intracellular pathogens and tumours. Activation of naive CD8⁺ T cells triggers their differentiation and clonal expansion, resulting in the formation of effector cells which can eliminate the pathogens via direct killing of infected host cells. Importantly, once the infection is cleared, a long-lived pool of memory T cells remains, which can respond more rapidly to secondary infection without the need for further differentiation, often providing immunity. However, our understanding of gene regulatory mechanisms that control the process of differentiation are largely unknown. Posttranslational modification of histone proteins regulates gene transcription by directly affecting chromatin compaction, or by serving as substrates for binding of chromatin remodelling complexes that influence gene regulation by various mechanisms. Here we identify a well-known chromatin remodelling histone modifier, canonical Polycomb Repressive Complex 1 (cPRC1) as a major regulator of CD8⁺ T cell differentiation. The core complex of cPRC1 in CD8⁺ T cells contains RING1B – a ubiquitin ligase that catalyses ubiquitination of Lysine 119 on H2A, one of the Polycomb group Ring Finger (PCGF) proteins BMI-1 which regulates enzymatic activity, Chromobox proteins CBX4 and CBX7 which are responsible for targeting the complex to the chromatin and Polyhomeotic-like protein 3 (PHC3) which is believed to assist forming higher order chromatin structure. Our results identify that CBX7/4, and BMI-1 are differentially regulated between naïve and activated CD8⁺ T cells. Their expression was found to be regulated in accordance with TCR signal strength and pMHC-TCR affinity. We found that the deletion of BMI-1 from T cells resulted in an exaggerated effector response and skewing to terminal differentiation during primary infection with Influenza A Virus. This was accompanied by a failure to establish CD8 T cell memory. Upon secondary infection, the number and frequency of effector cells were reduced along with reduced polyfunctionality and increased EOMES and PD1 expression. By using ChIP-qPCR and ATAC-seq we understood that deletion of BMI-1 leads to reduced H2AK119ub and increased chromatin accessibility around the promoters of transcription factors that regulate effector differentiation. Overall, we demonstrate that BMI-1 cPRC1 restrains terminal differentiation by repressing key transcription factors that drives differentiation and enables memory formation. Hence, BMI-1 and cRPRC1 is a crucial regulator of viral induced CD8⁺ T cell differentiation.