PD-1IR2 promotes tumor evasion via deregulating CD8+ T cell function.

Background: The programmed cell death 1 (PD-1) is an immune checkpoint that mediates immune evasion of tumors. Alternative splicing (AS) such as intron retention (IR) plays a crucial role in the immune-related gene processing and its function. However, it is not clear whether PDCD1 encoding PD-1 exists as an IR splicing isoform and what underlying function of such isoform plays in tumor evasion.

Methods: An AS isoform of human PDCD1, characterized by the second IR and named PD-1^IR2, was identified by reverse transcription-PCR (RT-PCR) and Sanger sequencing. The expression profile of PD1^IR2 was assessed by quantitative RT-PCR and flow cytometry, while its function was evaluated through immune cell proliferation, cytokine interleukin 2 secretion, and tumor cell killing assays. PDCD1^{IR2 CKI} mice which specifically conditional knock-in PDCD1^IR2 in T cells and humanized peripheral blood mononuclear cells (PBMC)-NOG (NOD.Cg-PrkdcscidIL2rgtm1Sug/JicCrl) mice were utilized to further confirm the physiological function of PD-1^IR2 in vivo.

Results: PD-1^IR2 is expressed in a variety of human leukemia cell lines and tumor-infiltrating lymphocytes. PD-1^IR2 expression is induced on T cell activation and regulated by the RNA-binding protein hnRNPLL. PD-1^IR2 negatively regulates the immune function of CD8⁺ T cells, indicated by inhibiting T cell proliferation, cytokine production, and tumor cell killing in vitro. PD-1^IR2+ CD8⁺ T cells show impaired antitumor function, which consequently promote tumor evasion in a conditional knock-in mouse model and a PBMC-engrafted humanized NOG mouse model. PD-1^IR2 mice exhibit resistance to anti-PD-L1 therapy compared with wild-type mice.

Conclusions: PD-1^IR2 is a potential immune checkpoint that may mediate potential resistance to immune checkpoint therapy.