In a recent phase I clinical trial, a vaccine consisting of glypican-3 (GPC3)-derived CTL epitopes was found to be safe and induced measurable immune and clinical responses in patients with hepatocellular carcinoma (HCC). The aim of this study was to identify GPC3-derived long peptides (GPC3-LPs) carrying promiscuous HLA class II-restricted T helper (Th) cell epitopes. Using a computer algorithm, we predicted GPC3-LPs that can bind to promiscuous HLA class II molecules. Their antigenicity for induction of specific CD4+ T cells in healthy donors or patients with HCC, before and after vaccination with GPC3-SPs, was proven by IFNγ enzyme-linked immunospot assays. Natural processing of these epitopes was confirmed by the immune response of helper T cells to dendritic cells (DCs) loaded with GPC3 proteins. Cross-presentation capacity was assessed in vitro using human DCs and LPs encapsulated in liposomes and in vivo in HLA-A2 transgenic mice (Tgm). All five LPs could induce Th1 cells and were presented by several frequently occurring HLA class II moleculesin vitro. Four of them were likely to be naturally processed. One of the LPs encapsulated in liposomes was well cross-presented in vitro; it cross-primed CTLs in HLA-A2 Tgm. LP-specific and HLA class II-restricted CD4+ T-cell responses were observed in 14 of 20 HCC patients vaccinated with GPC3-SPs. Repeated vaccinations enhanced GPC3-LP-specific responses in 8 of 13 patients with HCC. Moreover, the presence of the specific Th cell was correlated with prolonged overall survival (OS). GPC3-LPs can be useful for cancer immunotherapy.
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