Analysis of a novel immune checkpoint, Siglec-15, in pancreatic ductal adenocarcinoma.

Siglec-15, a novel immune checkpoint, is an emerging target for next-generation cancer immunotherapy. However, the role of Siglec-15 in pancreatic ductal adenocarcinoma (PDAC) remains poorly understood. We investigated the expression of Siglec-15 and its association with clinicopathological characteristics, programmed cell death-ligand 1 (PD-L1), immune cells, and DNA damage repair (DDR) molecules in a cohort of 291 patients with PDAC. Positive tumour cell expression of Siglec-15 and PD-L1 was observed in 18.6 and 30.3% of the samples, respectively. We also detected Siglec-15 positivity in macrophages in 3.4% of patients. Co-expression of Siglec-15 with PD-L1 was observed in 6.1% of the patients. A total of 33 PD-L1-negative samples (18.0%) were Siglec-15-positive. Siglec-15 was observed more frequently in moderate-to-well-differentiated tumours. Siglec-15 was associated with a low density of Tregs and CD45RO T cells, high BRCA1 expression, and improved survival. Both Siglec-15 and PD-L1 are independent factors of patient outcomes. The prognostic significance of Siglec-15 for survival was more discriminative in lymph node-negative, high BRCA1 expression, or low BRCA2 expression tumours than in lymph node-positive, low BRCA1 expression, or high BRCA2 expression tumours. In conclusion, we identified Siglec-15 as a promising predictor for prognosis combined with different DDR molecular statuses and complex tumour-infiltrating cells in PDAC. Targeting Siglec-15 may be a novel therapeutic option for patients who are unresponsive to anti-PD-1 therapy. Future studies are needed to validate the prognostic significance of Siglec-15 and to investigate its regulatory mechanisms in this disease.