Comprehensive analysis of immune checkpoint molecules profiles phenotype and function of exhausted T cells in enzootic bovine leukosis.

Bovine leukemia virus (BLV) causes enzootic bovine leukosis (EBL), a B-cell lymphoma in cattle. Previous studies have demonstrated that T cells of BLV-infected cattle show increased expression of immune checkpoint molecules, including programmed death-1 (PD-1), lymphocyte-activation gene-3 (LAG-3), cytotoxic T-lymphocyte-associated antigen-4 (CTLA-4), and T-cell immunoglobulin domain and mucin domain-3 (TIM-3), leading to T-cell exhaustion. However, the key immune checkpoint molecules driving T-cell exhaustion in BLV-induced tumorigenesis remained unclear. In this study, we identified the key immune checkpoint molecules by performing comprehensive flow cytometric analyses of T cells from EBL cattle, and elucidated the phenotype and function of exhausted T cells using a transcriptomic analysis by RNA sequencing and cell culture assays. The comprehensive expression analysis revealed that the proportion of CD4+ and CD8+ T cells co-expressing PD-1 and TIM-3 was significantly increased in the peripheral blood and tumor tissues of EBL cattle compared to healthy cattle. Transcriptomic analysis of PD-1+TIM-3+ T cells revealed the upregulation of genes related to terminal exhaustion and the downregulation of genes related to T-cell differentiation and response in this subset. Additionally, PD-1+TIM-3+ T cells exhibited higher expression of CTLA-4, LAG-3, and Eomes, and lower expression of T-bet, suggesting a terminally exhausted phenotype. Cell culture assays revealed a significant impairment in IFN-γ production in PD-1+TIM-3+ T cells upon stimulation, reflecting severe dysfunction. These findings indicate that PD-1+TIM-3+ T cells play a central role in T-cell exhaustion during BLV-induced tumorigenesis. This study provides valuable insights for future therapeutic strategies against BLV infection.