Immunity to Intracellular Salmonella Depends on Surface-associated Antigens

Abstract

Invasive Salmonella infection is an important health problem that is worsening because of rising antimicrobial resistance and changing Salmonella serovar spectrum. Novel vaccines with broad serovar coverage are needed, but suitable protective antigens remain largely unknown. Here, we tested 37 broadly conserved Salmonella antigens in a mouse typhoid fever model, and identified antigen candidates that conferred partial protection against lethal disease. Antigen properties such as high in vivo abundance or immunodominance in convalescent individuals were not required for protectivity, but all promising antigen candidates were associated with the Salmonella surface. Surprisingly, this was not due to superior immunogenicity of surface antigens compared to internal antigens as had been suggested by previous studies and novel findings for CD4 T cell responses to model antigens. Confocal microscopy of infected tissues revealed that many live Salmonella resided alone in infected host macrophages with no damaged Salmonella releasing internal antigens in their vicinity. In the absence of accessible internal antigens, detection of these infected cells might require CD4 T cell recognition of Salmonella surface-associated antigens that could be processed and presented even from intact Salmonella. In conclusion, our findings might pave the way for development of an efficacious Salmonella vaccine with broad serovar coverage, and suggest a similar crucial role of surface antigens for immunity to both extracellular and intracellular pathogens.

Vaccination with a Single CD4 T Cell Peptide Epitope from a Salmonella Type III-Secreted Effector Protein Provides Protection against Lethal Infection

“…(ii) Protection was mediated solely by immunity to a single CD4 T cell epitope from a secreted effector protein injected by the T3SS. This finding agrees with some previous studies showing that protection against Salmonella can be mediated by immunity to other whole proteins of the Salmonella T3SS, particularly Salmonella secreted effector B (SseB) (40,41); however, it contrasts somewhat with a recent report showing that the predominant protective response relies on immunity to surface-associated antigens (21). (iii) Protection was antibody independent, appearing to rely entirely on CD4 T cell responses against the expressed Salmonella peptide.…”

“…While protective responses may be generated against whole protein (33), the protective efficacy of these peptide epitopes has never been demonstrated. Additionally, it has been speculated that protection against Salmonella Typhimurium relies on surface-associated proteins (21). We set out to determine if Salmonella Typhimurium MHC-II epitopes, and particularly an epitope from a type III-secreted effector protein, were sufficient to provide protection to susceptible C57BL/6J mice upon virulent Salmonella challenge.…”

“…Notably, a recent study showed that the predominant protective responses against Salmonella infection are mediated by surface-associated antigens; however, many of the most common proteins from the Salmonella type III secretion systems (T3SS), located on Salmonella pathogenicity island 1 (SPI1) and SPI2, have not been tested for protection (21). Many of these effector proteins are injected directly into the cytosol from within the Salmonella-containing vacuole (SCV) (22).…”

Bacterial Stimulation of Toll-Like Receptor 4 Drives Macrophages To Hemophagocytose

“…enterica serovar Typhimurium wild-type strain SL1344 was transformed with pRFPTag (ESH829, Fig. 1, 2, and 4) (26) or transduced with green fluorescent protein (GFP) and a kanamycin resistance cassette at sifB (sifB::GFP-Kan STm, CSD1021) (27). S. Typhimurium was grown overnight at 37°C in Luria-Bertani broth with aeration.…”

A Bivalent Typhoid Live Vector Vaccine Expressing both Chromosome- and Plasmid-Encoded Yersinia pestis Antigens Fully Protects against Murine Lethal Pulmonary Plague Infection

“…Antigen export systems have also been devised to export antigens out of the cytoplasm and either onto the cell surface or out into the surrounding milieu (9-11). Export of foreign antigens is now appreciated to improve immune responses, possibly by avoiding proteolytic degradation of antigens within the cytoplasm or periplasmic space of the vaccine organism (10,(12)(13)(14)(15)(16)(17).However, there can be additional pitfalls introduced by stabilized expression plasmids. Sustained production of large amounts of foreign antigen can impose a metabolic burden upon the vaccine that overattenuates the strain and results in reduced immunogenicity (1,(18)(19)(20)(21)(22).…”