Cell surface PCNA is a marker of pancreatic and colon cancer stem cells and inhibits NK cell effector function.

Purpose: Cancer stem cells (CSC), a unique subset of tumor cells, possess a stem-cell-like phenotype and are thought to facilitate metastasis by escaping NK cell effector function. There are numerous markers used to identify CSC; of which include surface markers CD44 and CD133, and transcription factors NANOG, SOX2, and Oct-4. NK cells participate in the innate immune response against cancer without prior sensitization. NK cell function depends on a balance of signals transmitted from activating and inhibitory receptors interacting with ligands on the surface of target cells. Cancer cells may evade NK-mediated killing by expressing or secreting ligands for NK cell inhibitory receptors. NKp44 can function as an activating receptor that induces NK cell cytotoxicity or an inhibitory receptor depending on ligand interaction. Proliferating cell nuclear antigen (PCNA) associates with Human Leukocyte Antigen I (HLA I) and forms the inhibitory ligand for NKp44, resulting in the inhibition of NK function. We hypothesize surface PCNA is a marker for CSC in pancreatic and colon cancer. Methods: Pancreatic (Panc-1) and colon (HCT 116) cancer cells were labeled with antibodies against PCNA, CD44, and CD133 and flow cytometry was performed to determine surface expression. Cells were labeled and sorted for cell surface PCNA expression via fluorescence activated cell sorting; NANOG, SOX2, and Oct-4 were analyzed by qRT-PCR from sorted cells. NK receptor-ligand interactions were blocked by incubating cells with anti-PCNA or control antibodies and a chromium release killing assay was performed. Results: In both Panc-1 and HCT 116 cells, a PCNA+CD44+CD133+ population was detected. Furthermore, cell sorting and qRT-PCR confirmed cells with cell surface PCNA have increased expression of CSC transcription factors compared to PCNA- cells. Blocking the interaction of NKp44 and PCNA enhanced the specific lysis of cells by primary NK cells. Conclusions: Cell surface PCNA is associated with co-expression of CD44 and CD133 as well as increased CSC transcription factor expression. Collectively these data demonstrate that surface PCNA is a marker of pancreatic and colon CSC. Additionally, cell surface PCNA on CSC facilitate escape from NK cell killing by interacting with NKp44 and transmission of an inhibitory signal. Our research implicates that blocking NKp44-PCNA interaction may provide novel immunotherapeutic targets for pancreatic and colon cancer stem cells and prevent metastasis.