Immunization with an Autotransporter Protein of Orientia tsutsugamushi Provides Protective Immunity against Scrub Typhus

Abstract

BackgroundScrub typhus is an acute febrile disease caused by Orientia tsutsugamushi infection. Recently, the rapid increase of scrub typhus incidence in several countries within the endemic region has become a serious public health issue. Despite the wide range of preventative approaches that have been attempted in the past 70 years, all have failed to develop an effective prophylactic vaccine. Currently, the selection of the proper antigens is one of the critical barriers to generating cross-protective immunity against antigenically-variable strains of O. tsutsugamushi.Methodology/Principal FindingsWe examined the potential role of ScaA protein, an autotransporter protein of O. tsutsugamushi, in bacterial pathogenesis and evaluated the protective attributes of ScaA immunization in lethal O. tsutsugamushi infection in mice. Our findings demonstrate that ScaA functions as a bacterial adhesion factor, and anti-ScaA antibody significantly neutralizes bacterial infection of host cells. In addition, immunization with ScaA not only provides protective immunity against lethal challenges with the homologous strain, but also confers significant protection against heterologous strains when combined with TSA56, a major outer membrane protein of O. tsutsugamushi.Conclusions/SignificanceImmunization of ScaA proteins provides protective immunity in mice when challenged with the homologous strain and significantly enhanced protective immunity against infection with heterologous strains. To our knowledge, this is the most promising result of scrub typhus vaccination trials against infection of heterologous strains in mouse models thus far.

Generation of protective immunity against Orientia tsutsugamushi infection by immunization with a zinc oxide nanoparticle combined with ScaA antigen

“…The binding of each peptide was saturated at ~16 nmol for 1× ZBP and ~32 nmol for 3× ZBP with 50 μg of ZNPs, resulting in 0.32–0.64 nmol of peptide binding per 1 μg of nanoparticle. Next, we generated ScaA antigen [28] fused with 3× ZBP for scrub typhus vaccine study. As expected, addition of 3× ZBP to the recombinant protein enhanced the binding of ScaA to ZNPs by ~2.5 fold over that of ScaA without 3× ZBP (Fig.…”

“…5). Considering that both humoral and cell-mediated immune responses are required for efficient protection against bacterial challenge [28], we can conclude that the level of antibody and T cell responses against ScaA induced by immunization with ZNP/ZBP-ScaA were strong enough to protect the mice. Therefore, enhanced affinity of an antigen to ZNP can contribute to generating stronger adaptive immune responses when delivered together with ZNP.…”

“…Plates were spun at 500× g for 5 min to synchronize bacterial contact with the host cell monolayers and then incubated at 37 °C for the indicated times. Intracellular bacteria were stained by differential immunofluorescence as previously described [28]. DC2.4 cells taken up ZNP/ZBP or ZNP/ZBP-ScaA complexes were stained with LysoTracker Red DND-99 (Molecular probe) to examine the co-localization of the complexes with lysosomes.…”

“…While a major outer membrane protein, TSA56, has been studied as a conventional target for scrub typhus vaccine since it is an immunodominant antigen, many issues remain that need to be resolved for the development of an effective vaccine, especially for cross-protective immunity against diverse genotypes [22, 27]. Previously, our group reported the potential role of the ScaA protein, an autotransporter protein of O. tsutsugamushi , in bacterial pathogenesis and evaluated the immunogenicity of ScaA for protective immunity against lethal O. tsutsugamushi infection in mice, suggesting that ScaA should be considered as a novel target for scrub typhus vaccine [28, 29]. ScaA functions as a bacterial adhesion factor, and anti-ScaA antibody significantly neutralizes bacterial infection of host cells.…”

“…ScaA functions as a bacterial adhesion factor, and anti-ScaA antibody significantly neutralizes bacterial infection of host cells. In addition, immunization with ScaA not only provides protective immunity against lethal challenges with the homologous strain, but also confers significant protection against heterologous strains when combined with TSA56 [28]. …”