U-87 MG glioblastoma multiforme cell line expresses cell surface PCNA, a prospective target for Natural Killer Cell-mediated immunotherapy.

Purpose: Glioblastoma multiforme (GBM) is the most common form of primary brain cancer in adults and carries a dreadful five-year survival rate of less than 7%. Current commonplace treatment options include surgery, chemotherapy, and radiation. Recently, there has been a move to pursue immunotherapy avenues to improve patient outcomes. These therapies often depend upon the identification of molecular antigens that are particular to cancer cells. Some antigens, such as EGFR, are overexpressed on a significant percentage of GBM tumors and are useful as targets for immunotherapies. However, to address GBM tumors that do not overexpress well-known antigens, our lab set out to identify novel antigens on GBM as future candidates for Natural Killer (NK) cell-mediated immunotherapy. Previously, our lab has demonstrated that cell surface-bound Proliferating Cell Nuclear Antigen (PCNA) can serve as a target of NK cell-mediated killing of several cancers. Cell surface PCNA is not expressed on healthy, non-malignant cells – making it an attractive immunotherapy target. We have also previously shown cell surface PCNA to be expressed on other GBM cell lines (LN-229 and LN-18). We set out here to investigate the potential expression of cell surface PCNA on U-87 MG cells. Methods: Based on the prior studies, we examined the expression of PCNA on the U-87 MG GBM cell line via flow cytometry using PE-labeled antibodies specific for PCNA. For comparison, we did the same experiment in the same setting with LN-18 cells, one of the previously mentioned cell lines that expressed PCNA. Our hypothesis: U-87 MG cells would show increased detection of fluorescence signal of anti-PCNA antibodies when compared to the fluorescence detection of negative control groups (no staining group and PE-isotype control group). Results: PCNA was identified to be expressed on U-87 MG cells via the detection of increased fluorescence signal versus negative controls, though to a lesser degree than that of LN-18 cells. Conclusions: Based on our results, we concluded that cell surface PCNA is expressed on the U-87 MG cell line and is a candidate for studying NK cell-mediated immunotherapy in in vitro contexts using U-87 MG cells. Currently, we are evaluating blocking inhibitory signals to NK cells mediated through the PCNA-NKp44 interaction to target GBM for NK cytotoxicity in U-87 MG cells.