Cytosolic delivery of heterologous antigens via Salmonella type 3 secretion system for prophylactic vaccine development against staphylococcus aureus infection and mesothelioma metastasis

Abstract

Live attenuated Salmonella enterica subsp. enterica serotype Typhimurium (S. Typhimurium) is widely used as a delivery system for vaccination purposes. Salmonella Type 3 Secretion Systems (T3SSs) encoded by Salmonella Pathogenesis Island-1 (SPI-1) and SPI-2 mediate translocation of effectors into the cytosol of host cells at different stages of infection. SPI-1 and SPI-2 T3SSs have been employed to deliver heterologous antigens into the cytosol of antigen presenting cells (APCs) for S. Typhimurium vaccine development. Antigenspecific immune responses could be efficiently stimulated, especially CD8+ T cell responses, which is important for protection against intracellular bacterial infection and tumor.

In this study, a series of live attenuated S. Typhimurium vaccines were developed, in which heterologous antigens were expressed in the bacterial cytosol (somatic form) or translocated via SPI-1 and SPI-2 T3SS into host cells (cytosolic delivery). BALB/c mice were orally immunized, and the immune responses stimulated by different approaches of antigen delivery were evaluated and compared. Finally, vaccine efficacy was determined in prophylactic settings.

In the first project on vaccine development against Staphylococcus aureus (S. aureus) infection, Staphylococcal antigens ess extracellular A (SaEsxA), ess extracellular A (SaEsxB), and adenosine synthase A (AdsA) were targeted. Results showed that in fusion with secretion peptide of SPI-1 effector SipA (1- 169aa), SaEsxA and SaEsxB could be translocated into the cytosol of macrophages in vitro. Oral administration of these two vaccines to BALB/c mice significantly induced antigen-specific mucosal, humoral and Th1/Th17 immune responses, which increased the survival rate for vaccinated mice when challenged with S. aureus strains.

In the second project on cancer vaccine development, HIV-1 GAG p24 antigen was used as an artificial tumor antigen for GAG-expressing mesothelioma (AB1- GAG). Through fusing with SPI-2 effector SseF (1-258aa) and SseJ (1-408aa), expression and translocation of p24 chimera proteins were detected in the cytosol of infected macrophages. After BALB/c mice were orally immunized with these two vaccines, low level of antigen-specific humoral immune responses, but high level of Th1 cellular immune responses were induced, which significantly prevented lung metastasis of GAG-expressing mesothelioma. On the contrary, somatic expression or SPI-1 T3SS-mediated secretion of p24 failed to confer significant protection.

This study provides evidence that in comparison with delivery of antigens in somatic form, cytosolic delivery of antigens via SPI-1 or SPI-2 T3SS stimulated superior antigen-specific cell-mediated immunity, which could be utilized for the development of live recombinant Salmonella vaccine against bacterial infections and tumor metastasis.