Identification of In Vivo-Induced Bacterial Protein Antigens during Human Infection with Salmonella enterica Serovar Typhi

Abstract

We applied an immunoscreening technique, in vivo-induced antigen technology (IVIAT), to identify immunogenic bacterial proteins expressed during human infection with Salmonella enterica serovar Typhi, the cause of typhoid fever. We were able to assign a functional classification to 25 of 35 proteins identified by IVIAT. Of these 25, the majority represent proteins with known or potential roles in the pathogenesis of S. enterica. These include proteins implicated in fimbrial structure and biogenesis, antimicrobial resistance, heavy metal transport, bacterial adhesion, and extracytoplasmic substrate trafficking as well as secreted hydrolases. The 10 remaining antigens represent proteins with unknown functions. Of the 35 identified antigens, four had no immunoreactivity when probed with control sera from individuals never exposed to serovar Typhi organisms; these four included PagC, TcfB, and two antigens of unknown function encoded by STY0860 and STY3683. PagC is a virulence factor known to be upregulated in vivo in S. enterica serovar Typhimurium infection of mice. TcfB is the major structural subunit of a fimbrial operon found in serovar Typhi with no homolog in serovar Typhimurium organisms. By examining differential immunoreactivities in acute-versus convalescent-phase human serum samples, we found specific anti-PagC and anti-TcfB immunoglobulin G responses in patients with serovar Typhi bacteremia. Serovar Typhi antigens identified by IVIAT warrant further evaluation for their contributions to pathogenesis, and they may have diagnostic, therapeutic, or preventive uses.In humans, infection with Salmonella enterica serovar Typhi causes a spectrum of illness that includes diarrhea, a selflimited febrile illness, and most significantly, typhoid fever, a systemic infection characterized by persistent fever, lymphadenopathy, hepatosplenomegaly, encephalopathy, and intestinal hemorrhage and perforation (16). Worldwide, approximately 20 million cases of typhoid fever resulting in 200,000 deaths occur annually (2). The increasing incidence of disease caused by multidrug-resistant serovar Typhi organisms underscores the importance of developing novel approaches to the diagnosis, treatment, and prevention of typhoid fever (22).Although many S. enterica serovars infect a broad range of animal hosts and cause gastroenteritis in humans, serovar Typhi is among the few serovars for which natural infection appears to be limited to human hosts. In humans, serovar Typhi organisms penetrate the gastrointestinal epithelial barrier and infect phagocytes within the lamina propria. However, unlike organisms from other broad-host-range serovars, serovar Typhi organisms are adapted for prolonged intracellular survival in human macrophages, allowing the bacteria to spread to reticuloendothelial organs, including the liver, spleen, and bone marrow (16).Because serovar Typhi organisms are human specific, there is no optimal animal model of serovar Typhi infection, and presumed factors that contribute to the pathogenesis and immunology of...

Typhi genes expressed during infection or involved in pathogenesis

“…Five of these operons, sef, sta, ste, stg, and tcf, and the Type IV pili are not detected in serovar Typhimurium [1] and may be important for Typhi host specificity. In typhoid fever patients, antibody to 3 fimbrial systems, TcfB, StbD, and CsgEFG, has been detected [13]. The csg operon is also up-regulated 2 hours post infection inside macrophages [7].…”

“…The magnesium transport system mgtBC located on SPI-3 is strongly induced intracellularly by Typhi during infection of macrophages, as well as pipD located on SPI-5, and pagC, pagD, and envE of SPI-11 [7]. PagC is produced in typhoid patients [13]. On SPI-10, the prpZ locus encoding for proteins with homology to eukaryotic-type Ser/Thr protein phosphatase and kinases has been found to promote survival in macrophages [35] (Table 1).…”

“…The phage-shock-protein (Psp) system responds to stresses and may be involved in antimicrobial resistance [51]. PspC, PspD and PspE were identified by IVIAT during human infection with Typhi [13] and are also up-regulated in human macrophages [7] (Table 1).…”

“…Similarly, by using a tviB mutant, the presence of the Vi capsule has been shown to inhibit adhesion as well as invasion of epithelial cells [34] (Table 1). SPI-11 pagC Detected in human serum [13] ISP1820 Up in THP-1 [7] pagD, envE ISP1820 Up in THP-1 [7] Adhesion tcfB Detected in human serum [13] stbD Detected in human serum [13] csgGFE Detected in human serum [13] Other SPIs. The role of genes that belong to other pathogenicity islands in Typhi pathogenesis has not been investigated in depth yet.…”

“…Another strategy that has recently been developed identifies bacterial antigens that are immunogenic and produced during infection, a technique called in vivo induced antigen technology (IVIAT) [12]. This technique was successfully used to identify Typhi proteins produced in typhoid patients [13].…”