G protein fragment derived from Respiratory Syncytial Virus A2 as a vaccine candidate in mice administered via sublingual route

Abstract

Respiratory syncytial virus (RSV) is one of the major pathogens causing respiratory tract infection and disease in infants, young children and immunocompromised patients worldwide. In addition, almost all children have experienced RSV infection by 2 to 3 years of age. However, currently there is no vaccine available for human use. RSV G glycoprotein (RSVG) plays a role in attachment on the host cell and induces neutralizing antibody response which is involved in protective response against RSV infection in the host. However, RSVG, especially CD4+ T cell epitope region within RSV G protein, is known to induce immunopathology. Mucosal vaccination could induce humoral and cellular immune responses at systemic compartment and mucosal site. However, intranasal route is known to redirect antigen to central nerve system that leads to serious side effect like Bells palsy although it is rare. In contrast, sublingual route is seemingly safer than intranasal route despite the fact that its immunogenicity is low. The purpose of this study is to examine RSV vaccine candidate without immunopathology using RSV G protein core fragment (Gcf) administered through sublingual route in mouse. In the first study, the effect of Gcf immunization routes via sublingual or intranasal routes in mouse was compared when RSV was infected subsequently. Unexpectedly, Gcf sublingual vaccination induced vaccine-enhanced disease, mediated by Th17 response, but intranasal vaccination of Gcf did not vaccine-enhanced disease. The vaccine-enhanced disease by formalin inactivated-RSV (FI-RSV) was mediated through Th2 response as it has been already reported. In the second study, since CD4+ T cell epitope within RSV G protein was reported to be responsible for the immune response and immunopathology, it was modified for safe and effective RSV vaccine when administered by sublingual route. The amino acid (a.a.) positions 185 and 188, critical sites to activate T cells, were mutated to generate mGcf in order to vanish vaccine-enhanced disease. Then, to recover the function of CD4 helper T cell activity, mGcf was fused with CD4+ T cell epitope from RSV F (F51-66) protein, and generated Th-mGcf. As a result, Th-mGcf induced humoral immune response and provided protection against RSV without immunopathology. Taken together, vaccine-enhanced disease induced in mice sublingually immunized with Gcf following RSV challenge was mediated by Th17 response. The mutation of CD4+ T cell epitope in Gcf (mGcf) resulted in lower immunogenicity without immunopathology. However, lower immunogenicity by mutation of CD4+ T cell epitope was recovered by conjugation of another CD4+ T cell epitope from RSV F protein (Th-mGcf). Finally, this Th-mGcf immunization via sublingual route after RSV challenge induced protective immune response without vaccine enhanced disease and it might be potential candidate for RSV vaccine.