EXTRACELLULAR PROLIFERATING CELL NUCLEAR ANTIGEN IS A NOVEL MARKER FOR CANCER STEM CELLS AND FACILITATES EVASION OF NK CELL EFFECTOR FUNCTION

Purpose: Natural Killer (NK) cells are a specialized population of lymphocytes of the innate immune system which provide vital first line defense against infections and cancer. NK cell function is strictly regulated by inhibitory and activating receptors binding corresponding ligands on the surface of target cells. NKp44, originally discovered as an activating NK cell receptor, was recently found to elicit inhibitory effects on NK cell effector function through recognition of cell surface Proliferation Cell Nuclear Antigen (PCNA), which is typically only found in the cell nucleus for DNA replication and repair. Other reports have pointed to potential associations between NKp44 and Human Leukocyte Antigen (HLA) I molecules. Methods: We have identified novel interactions between HLA I and PCNA on the surface of human tumor cells by confocal microscopy and immunoprecipitation. We have also confirmed the inhibitory nature of NKp44 recognition of PCNA in association with HLA I through a standard Chromium release assay. Results: We show PCNA on the cell surface in novel association with HLA I is a natural process which does not require provocation and occurs with endogenous PCNA. The association of HLA I and PCNA forms the inhibitory ligand for NKp44, resulting in inhibition of NK effector function. Furthermore, extracellular PCNA was only found on tumor cells exhibiting a cancer stem cell phenotype. Conclusions: Thus extracellular PCNA may serve as a novel marker for cancer stem cells and provide a mechanism by which these cells evade NK cell effector functions. As a rare population of cancer cells potentially responsible for relapse after treatment, metastasis, and overall tumor growth, cancer stem cells are difficult to clinically detect in circulation and are often impervious to chemotherapy and radiation treatments. Further studies will determine the efficacy of using extracellular PCNA as a cancer stem cell marker for better detection and therapeutic targeting of these rare, yet dangerous cancer cells.