A measles virus-based vaccine induces robust chikungunya virus-specific CD4⁺ T-cell responses in a phase II clinical trial

Chikungunya virus (CHIKV) is an alphavirus transmitted by mosquitos that causes a debilitating disease characterized by fever and long-lasting polyarthralgia. To date, no vaccine has been licensed, but multiple vaccine candidates are under evaluation in clinical trials. One of these vaccines is based on a measles virus vector encoding for the CHIKV structural genes C, E3, E2, 6K, and E1 (MV-CHIK), which proved safe in phase I and II clinical trials and elicited CHIKV-specific antibody responses in adult measles seropositive vaccine recipients. Here, we predicted T-cell epitopes in the CHIKV structural genes and investigated whether MV-CHIK vaccination induced CHIKV-specific CD4⁺ and/or CD8⁺ T-cell responses. Immune-dominant regions containing multiple epitopes in silico predicted to bind to HLA class II molecules were found for four of the five structural proteins, while no such regions were predicted for HLA class I. Experimentally, CHIKV-specific CD4⁺ T-cells were detected in six out of twelve participants after a single MV-CHIK vaccination and more robust responses were found 4 weeks after two vaccinations (ten out of twelve participants). T-cells were mainly directed against the three large structural proteins C, E2 and E1. Next, we sorted and expanded CHIKV-specific T cell clones (TCC) and identified human CHIKV T-cell epitopes by deconvolution. Interestingly, eight out of nine CD4⁺ TCC recognized an epitope in accordance with the in silico prediction. CHIKV-specific CD8⁺ T-cells induced by MV-CHIK vaccination were inconsistently detected. Our data show that the MV-CHIK vector vaccine induced a functional transgene-specific CD4⁺ T cell response which, together with the evidence of neutralizing antibodies as correlate of protection for CHIKV, makes MV-CHIK a promising vaccine candidate in the prevention of chikungunya.