The programmed death-1 (PD-1) pathway is important in the maintenance of 
peripheral tolerance and homeostasis through suppression of T cell receptor 
signaling. As such, it is employed by many tumors as a means of immune escape. 
We have investigated the role of this pathway in human ovarian cancer (OC) to 
assess its potential role as a diagnostic and/or prognostic marker and 
therapeutic target, following recent clinical trial success of antibody therapy 
directed at this pathway. We show programmed death ligand-1 (PD-L1) expression 
on monocytes in the ascites and blood of patients with malignant OC is 
strikingly higher than those with benign/borderline disease, with no overlap in 
the values between these groups. We characterize the regulation of this molecule 
and show a role of IL-10 present in ascitic fluid. Flow cytometric analysis of T 
cells present in the ascites and blood showed a correlation of PD-1 expression 
with malignant tumors versus benign/borderline, in a similar manner to PD-L1 
expression on monocytes. Finally, we demonstrate functional links between PD-L1 
expression on monocytes and OC tumor cells with suppression of T cell responses. 
Overall, we present data based on samples obtained from women with ovarian 
cancer, suggesting the PD-1 pathway may be used as a reliable diagnostic marker 
in OC, as well as a viable target for use with PD-1/PD-L1-directed antibody 
immunotherapy.