FGL2_172-220 peptides improve the antitumor effect of HCMV-IE1mut vaccine against glioblastoma by modulating immunosuppressive cells in the tumor microenvironment

Glioblastoma multiforme (GBM) is a highly aggressive primary brain tumor characterized by poor prognosis and lack of effective treatments. In recent years, peptide vaccines that use sequences based on tumor-specific or tumor-associated antigens to activate immune responses against tumor cells have emerged as a new therapeutic strategy. In this study, we developed a novel therapeutic polypeptide vaccine targeting the tumor-associated antigen Fibrinogen-Like Protein 2 (FGL2), whose dominant epitope peptide was tandemly linked to the C-terminus of HCMV-IE1mut via a linker. We used this vaccine to compare the therapeutic efficacy of HCMV-IE1mut alone versus HCMV-IE1mut-FGL2_172-220 and investigate the potential mechanism of action of HCMV-IE1mut-FGL2_172-220 in glioma treatment. An in situ GBM model (GL261-IE1-luc cells) was used to determine the efficacy of the vaccine. Treatment with HCMV-IE1mut-FGL2_172-220 exerted antitumor effects and extended the survival of the GL261 animal model. We observed reduced proportions of microglia, regulatory T cells (Treg), and myeloid-derived suppressor cells (MDSC) in the tumor microenvironment (TME) by immunofluorescence. Flow cytometry showed that compared to HCMV-IE1mut alone, treatment with HCMV-IE1mut-FGL2_172-220 increased the proportion of CD8+ T cells and tissue-resident memory T cells (TRM). ELISA analysis showed that it improved the secretion of tumor-specific IFN-γ and TNF-α by these cells and downregulated the expression of IL-6 and IL-10. Our study demonstrates that the long-peptide FGL2_172-220 improves the antitumor efficacy of HCMV-IE1mut, possibly by reshaping immune cells in the glioma microenvironment. These findings lay the groundwork for the development of therapeutic antigenic peptide vaccines to improve antitumor effects for cancer.