Scientists tested the human peptide LL‑37 as a helper (adjuvant) in a nose‑spray vaccine against MERS‑CoV in mice. Adding LL‑37 to the viral protein boosted both local (IgA) and systemic (IgG) antibodies, improved lung T‑cell responses, and helped mice survive infection better than the protein alone. While promising for vaccine design, it isn’t a ready‑to‑use supplement or protocol for individuals.

The human antimicrobial peptide LL-37 has chemotactic and modulatory activities in various immune cells, including dendritic cells. Because of its characteristics, LL-37 can be considered an adjuvant for vaccine development. In this study, we confirmed the possible adjuvant activity of LL-37 in mucosal vaccine development against Middle East respiratory syndrome-coronavirus (MERS-CoV) by means of intranasal immunization in C57BL/6 and human dipeptidyl peptidase 4 (hDPP4)-transgenic (hDPP4-Tg) mice. Intranasal immunization using the receptor-binding domain (RBD) of MERS-CoV spike protein (S-RBD) recombined with LL-37 (S-RBD-LL-37) induced an efficient mucosal IgA and systemic IgG response with virus-neutralizing activity, compared with S-RBD. Ag-specific CTL stimulation was also efficiently induced in the lungs of mice that had been intranasally immunized with S-RBD-LL-37, compared with S-RBD. Importantly, intranasal immunization of hDPP4-Tg mice with S-RBD-LL-37 led to reduced immune cell infiltration into the lungs after infection with MERS-CoV. Finally, intranasal immunization of hDPP4-Tg mice with S-RBD-LL-37 led to enhanced protective efficacy, with increased survival and reduced body weight loss after challenge infection with MERS-CoV. Collectively, these results suggest that S-RBD-LL-37 is an effective intranasal vaccine candidate molecule against MERS-CoV infection.