Evasion of serum antibodies and complement by Salmonella Typhi and Paratyphi A
Nontyphoidal and enteric fever serovars of Salmonella enterica display distinctive interactions with serum antibodies and the complement system, which initiate the host immune response to invading microbes. This study examines the contributions of lipopolysaccharide O-antigen and the S. Typhi Vi polysaccharide capsule to serum resistance, complement activation and deposition, and immunoglobulin (Ig) binding in nontyphoidal S. enterica serovar Typhimurium and the enteric fever serovars S. Typhi and S. Paratyphi A. Although all three serovars are resistant to serum killing, S. Typhi and S. Paratyphi A exhibit lower levels of Ig binding, complement binding and complement activation compared to S. Typhimurium. In S. Typhimurium, WzzB-dependent long O-antigen production and FepE-dependent very long O-antigen production are required for serum resistance but do not prevent IgM binding or complement deposition. S. Typhi lacks very long O-antigen, but its production of Vi capsule inhibits IgM binding and complement deposition, while acting in concert with long O-antigen to resist serum killing. In S. Paratyphi A, long O-antigen production is deficient due to a hypofunctional WzzB protein, but this is compensated by greater quantities of very long O-antigen, which are required for serum resistance. Restoration of WzzB function by exchange with the S. Typhimurium or S. Typhi wzzB alleles can restore long O-antigen production in S. Paratyphi A but decreases very long O-antigen production, resulting in increased IgM binding. Replacement of the S. Paratyphi A O2-type polysaccharide with the S. Typhi O9 polysaccharide further increases IgM binding of S. Paratyphi A, which enhances complement activation but not complement deposition. Lastly, a gene duplication of rfbV in S. Paratyphi A is necessary for higher levels of very long O-antigen and resistance to complement deposition and antibody binding. Collectively, these observations demonstrate fundamental differences between nontyphoidal and enteric fever Salmonella serovars in their interactions with innate immune effectors. Author SummaryEnteric fever acquired by ingestion of food or water contaminated with Salmonella Typhi or Paratyphi A is a significant cause of morbidity and mortality in low- and middle-income countries. In this study, we dissect the different mechanisms by which these bacteria avoid binding by serum proteins that are required for the initiation of protective immune responses. This is in contrast to the nontyphoidal serovar Salmonella Typhimurium, a common cause of gastroenteritis, which does not avoid complement activation nor antibody binding but instead exploits host inflammation. Our observations provide new insights into the mechanisms responsible for the distinctive immunological features of human enteric fever and can inform the development of Salmonella vaccines that target the Salmonella cell envelope.