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Immunization with an autotransporter protein of Orientia tsutsugamushi provides protective immunity against scrub typhus.

Ha NY, Sharma P, Kim G, Kim Y, Min CK, Choi MS, Kim IS, Cho NH - PLoS Negl Trop Dis (2015)

Bottom Line: 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.Our findings demonstrate that ScaA functions as a bacterial adhesion factor, and anti-ScaA antibody significantly neutralizes bacterial infection of host cells.

View Article: PubMed Central - PubMed

Affiliation: Department of Microbiology and Immunology, Seoul National University College of Medicine, Seoul, Republic of Korea; Department of Biomedical Sciences, Seoul National University College of Medicine, Seoul, Republic of Korea.

ABSTRACT

Background: Scrub 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 findings: We 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/significance: Immunization 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.

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Related in: MedlinePlus

Similarity plots of a set of tsa56 and scaA sequences from the indicated strains compared to sequences from the Boryong strain.Each plotted point is the percent identity within a sliding window of 100 bp or 100 amino acids wide centered on the position plotted, with a step size between points of 10 bp or amino acids. Diagrams above the graphs show the relative sizes of TSA56 and ScaA proteins and their sequence motiffs. Yellow box: signal peptide, gray box: antigenic domain, green box: variable domain, blue box: transmembrane domain, pink box: repeated sequences, brown box: autotransporter domain.
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pntd.0003585.g005: Similarity plots of a set of tsa56 and scaA sequences from the indicated strains compared to sequences from the Boryong strain.Each plotted point is the percent identity within a sliding window of 100 bp or 100 amino acids wide centered on the position plotted, with a step size between points of 10 bp or amino acids. Diagrams above the graphs show the relative sizes of TSA56 and ScaA proteins and their sequence motiffs. Yellow box: signal peptide, gray box: antigenic domain, green box: variable domain, blue box: transmembrane domain, pink box: repeated sequences, brown box: autotransporter domain.

Mentions: In this study, we examined the neutralizing activity of antibodies against four Sca proteins (ScaA, B, C, and E) encoded in the O. tsutsugamushi genome and found that only the antibody against ScaA inhibited bacterial infection in a cell culture model, whereas antibodies against other Sca proteins of O. tsutsugamushi had marginal effects (Fig. 3). In addition, immunization with ScaA provided protective immunity against O. tsutsugamushi infection in mice as efficiently as TSA56, whereas ScaC failed to induce protection, indicating that ScaA could provide specific and protective immunity against O. tsutsugamushi, at least against the homologous strain (Fig. 3C). When combined with TSA56, ScaA immunization significantly enhanced protective immunity against infection with heterologous strains, resulting in better survival or extended half-life of infected mice (Fig. 4). To our knowledge, this is the most promising result of scrub typhus vaccination against infection of heterologous strains in a mouse model. When we compared the sequences of scaA from the four different strains, the overall level of sequence similarity of scaA nucleotides (83.5 ~ 87.5%) and amino acids (81.0 ~ 88.4%) is similar to those of tsa56 (nucleotides: 77.5 ~ 88.4%, amino acids: 78.8 ~ 90.2%) (S6 Fig). However, a similarity plot shows that tsa56 has more local variation among the four strains than scaA (Fig. 5). Sequence variation observed in scaA is mainly due to the differential presence of repeated sequences found in the 5’-region (nucleotides 203 ~ 241 in Boryong strain) and 3’-end of the passenger domain (nucleotides 3243 ~ 3314 in Boryong strain) of each strain. When we examined the neutralizing activity of anti-ScaA antibody generated by immunizing ScaA protein from Boryong strain, it showed less inhibitory effect on the cellular invasion of Kato strain than Boryong strain (S2B Fig), suggesting that the sequence variation of ScaA may also affect in vitro neutralizing activity of anti-ScaA antibody. It remains to be determined whether the variable regions and their repeated sequences affect the antigenicity or neutralizing activity of antibodies against ScaA protein. Since protective immunity against O. tsutsugamushi infection is provided by antigen-specific IFN-γ-producing T cells [60,61] as well as humoral immunity [23,62], the protective role of ScaA-specific Th1 cells also needs to be investigated. Nevertheless, the passenger domains of ScaA proteins from different strains are relatively well conserved and those conserved areas could make it a better antigen for scrub typhus vaccine for targeting multiple strains of O. tsutsugamushi.


Immunization with an autotransporter protein of Orientia tsutsugamushi provides protective immunity against scrub typhus.

Ha NY, Sharma P, Kim G, Kim Y, Min CK, Choi MS, Kim IS, Cho NH - PLoS Negl Trop Dis (2015)

Similarity plots of a set of tsa56 and scaA sequences from the indicated strains compared to sequences from the Boryong strain.Each plotted point is the percent identity within a sliding window of 100 bp or 100 amino acids wide centered on the position plotted, with a step size between points of 10 bp or amino acids. Diagrams above the graphs show the relative sizes of TSA56 and ScaA proteins and their sequence motiffs. Yellow box: signal peptide, gray box: antigenic domain, green box: variable domain, blue box: transmembrane domain, pink box: repeated sequences, brown box: autotransporter domain.
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Related In: Results  -  Collection

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getmorefigures.php?uid=PMC4359152&req=5

pntd.0003585.g005: Similarity plots of a set of tsa56 and scaA sequences from the indicated strains compared to sequences from the Boryong strain.Each plotted point is the percent identity within a sliding window of 100 bp or 100 amino acids wide centered on the position plotted, with a step size between points of 10 bp or amino acids. Diagrams above the graphs show the relative sizes of TSA56 and ScaA proteins and their sequence motiffs. Yellow box: signal peptide, gray box: antigenic domain, green box: variable domain, blue box: transmembrane domain, pink box: repeated sequences, brown box: autotransporter domain.
Mentions: In this study, we examined the neutralizing activity of antibodies against four Sca proteins (ScaA, B, C, and E) encoded in the O. tsutsugamushi genome and found that only the antibody against ScaA inhibited bacterial infection in a cell culture model, whereas antibodies against other Sca proteins of O. tsutsugamushi had marginal effects (Fig. 3). In addition, immunization with ScaA provided protective immunity against O. tsutsugamushi infection in mice as efficiently as TSA56, whereas ScaC failed to induce protection, indicating that ScaA could provide specific and protective immunity against O. tsutsugamushi, at least against the homologous strain (Fig. 3C). When combined with TSA56, ScaA immunization significantly enhanced protective immunity against infection with heterologous strains, resulting in better survival or extended half-life of infected mice (Fig. 4). To our knowledge, this is the most promising result of scrub typhus vaccination against infection of heterologous strains in a mouse model. When we compared the sequences of scaA from the four different strains, the overall level of sequence similarity of scaA nucleotides (83.5 ~ 87.5%) and amino acids (81.0 ~ 88.4%) is similar to those of tsa56 (nucleotides: 77.5 ~ 88.4%, amino acids: 78.8 ~ 90.2%) (S6 Fig). However, a similarity plot shows that tsa56 has more local variation among the four strains than scaA (Fig. 5). Sequence variation observed in scaA is mainly due to the differential presence of repeated sequences found in the 5’-region (nucleotides 203 ~ 241 in Boryong strain) and 3’-end of the passenger domain (nucleotides 3243 ~ 3314 in Boryong strain) of each strain. When we examined the neutralizing activity of anti-ScaA antibody generated by immunizing ScaA protein from Boryong strain, it showed less inhibitory effect on the cellular invasion of Kato strain than Boryong strain (S2B Fig), suggesting that the sequence variation of ScaA may also affect in vitro neutralizing activity of anti-ScaA antibody. It remains to be determined whether the variable regions and their repeated sequences affect the antigenicity or neutralizing activity of antibodies against ScaA protein. Since protective immunity against O. tsutsugamushi infection is provided by antigen-specific IFN-γ-producing T cells [60,61] as well as humoral immunity [23,62], the protective role of ScaA-specific Th1 cells also needs to be investigated. Nevertheless, the passenger domains of ScaA proteins from different strains are relatively well conserved and those conserved areas could make it a better antigen for scrub typhus vaccine for targeting multiple strains of O. tsutsugamushi.

Bottom Line: 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.Our findings demonstrate that ScaA functions as a bacterial adhesion factor, and anti-ScaA antibody significantly neutralizes bacterial infection of host cells.

View Article: PubMed Central - PubMed

Affiliation: Department of Microbiology and Immunology, Seoul National University College of Medicine, Seoul, Republic of Korea; Department of Biomedical Sciences, Seoul National University College of Medicine, Seoul, Republic of Korea.

ABSTRACT

Background: Scrub 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 findings: We 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/significance: Immunization 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.

Show MeSH
Related in: MedlinePlus