based on my think:

memberance PD-1/PD-L1 increased expression caused apoptosis to lymphocyte

but soluble

i think its only soluble PD-L1 could be resposible for increased apoptosis

while serum PD-1 cants increased apoptosis, because its inhibotory effect on this pathways?

PD-1 protein expressed by peripheral or tumour-infiltrated CD8+ T cells was a promising biomarker for diagnosis and prognosis in PDAC and might help guide future immunotherapies.