Multiplex serum immune profiling reveals circulating LAG-3 is associated with improved patient survival in high grade serous ovarian cancer

•Serum LAG-3 is detectable in HGSOC and is associated with improved patient PFS and OS.•Serum LAG-3 demonstrated superior prognostic utility compared to PD-1 and a diverse array of cytokines and chemokines.•This study suggests that serum LAG-3 could be employed as a predictive marker of improved HGSOC patient outcomes.