Absent Bactericidal Activity of Mouse Serum against Invasive African Nontyphoidal Salmonella Results from Impaired Complement Function but Not a Lack of Antibody

Abstract

Nontyphoidal strains of Salmonella are a major cause of fatal bacteremia in Africa. Developing a vaccine requires an improved understanding of the relevant mechanisms of protective immunity, and the mouse model of Salmonella infection is useful for studying immunity to Salmonella in vivo. It is important to appreciate the similarities and differences between immunity to Salmonella in mice and men. Ab is important for protection against nontyphoidal Salmonella in both species, and we have previously found an important role for Ab in cell-free complement-mediated bactericidal activity against Salmonella in Africans. It is unclear whether this modality of immunity is relevant in the mouse model. C57BL/6, BALB/c, and C3H mice immunized with heat-killed Salmonella Typhimurium strains D23580 (African invasive strain) and SL1344 and live-attenuated strain SL3261 produced a Salmonella-specific Ab response. Sera from these mice deposited reduced levels of C3 on Salmonella compared with human sera and were unable to kill both wild-type and galE− rough mutant of D23580, indicating absent cell-free killing via classical and alternative complement pathways. Supplementing immune mouse sera with human complement enabled killing of Salmonella, whereas addition of human anti-Salmonella Ab to immune mouse sera had no effect. These findings indicate that mouse serum cannot effect cell-free complement-dependent killing of Salmonella, because of the reduced mouse complement ability to kill these bacteria compared with human complement. This difference in Ab-dependent immunity to Salmonella in mice and men must be considered when applying findings from the mouse model of Salmonella disease and vaccination response to man.

Serum Bactericidal Assays To Evaluate Typhoidal and Nontyphoidal Salmonella Vaccines

“…Even when these obstacles are overcome, there remains the added complication of obtaining enough complement for use in multiple studies and across laboratories. We did not attempt to use mouse complement because Siggins et al (38) showed that mouse complement is unable to kill S. Typhimurium in the presence of human anti-Salmonella antibodies.…”

Searching for Outer Membrane Proteins Typical of Serum-Sensitive and Serum-Resistant Phenotypes of Salmonella

“…Invasive NTS is endemic in sub-Saharan Africa where it is a leading cause of fatal bacteremia among African children and HIV-infected adults. Increasing levels of antibiotic resistance among African strains of NTS indicate that a vaccine is urgently needed (Siggins et al, 2011). In Malawi, MacLennan et al (2008) recently found that NTS bacteriemia particularly affects African children between 4 months and 2 years of age, the period in which immunoglobulin levels to NTS are low or absent.…”

Identification of a common immune signature in murine and human systemic Salmonellosis

“…Although the murine model of Salmonella infection has been widely studied, its utility as a preclinical model of human Salmonellosis remains unclear (28,52). We therefore examined antigen targeting using a well-characterized set of convalescent pediatric sera from 12 children admitted to hospital with Salmonella bacteremia or meningitis in Blantyre, Malawi, where NTS bacteremia is common in young children (16), and age-and sex-matched healthy controls.…”