The serodominant secreted effector protein of Salmonella, SseB, is a strong CD4 antigen containing an immunodominant epitope presented by diverse HLA class II alleles

Abstract

Detailed characterization of the protective T-cell response in salmonellosis is a pressing unmet need in light of the global burden of human Salmonella infections and the likely contribution of CD4 T cells to immunity against this intracellular infection. In previous studies screening patient sera against antigen arrays, SseB was noteworthy as a serodominant target of adaptive immunity, inducing significantly raised antibody responses in HIV-seronegative compared with seropositive patients. SseB is a secreted protein, part of the Espa superfamily, localized to the bacterial surface and forming part of the translocon of the type III secretion system (T3SS) encoded by Salmonella pathogenicity island 2. We demonstrate here that SseB is also a target of CD4 T-cell immunity, generating a substantial response after experimental infection in human volunteers, with around 0·1% of the peripheral repertoire responding to it. HLA-DR/peptide binding studies indicate that this protein encompasses a number of peptides with ability to bind to several different HLA-DR alleles. Of these, peptide 11 (p11) was shown in priming of both HLA-DR1 and HLA-DR4 transgenic mice to contain an immunodominant CD4 epitope. Analysis of responses in human donors showed immunity focused on p11 and another epitope in peptide 2. The high frequency of SseB-reactive CD4 T cells and the broad applicability to diverse HLA genotypes coupled with previous observations of serodominance and protective vaccination in mouse challenge experiments, make SseB a plausible candidate for next-generation Salmonella vaccines.

Clonal analysis of Salmonella-specific effector T cells reveals serovar-specific and cross-reactive T cell responses

“…Cells were then washed with CyFACS buffer (PBS + 4% FBS + 0.05% sodium azide) and stained with streptavidin-alpha Galcer with 10 μM free biotin for 30 minutes at room temperature. This was followed by30 minutes incubation in primary antibody cocktail on ice. Cells were then washed twice in CyFACS and stained with metal-tagged surface antibodies.…”

Dual Immunization with SseB/Flagellin Provides Enhanced Protection against Salmonella Infection Mediated by Circulating Memory Cells

“…Thus, modification of specific IgG2c did not correlate with the observed enhanced protection. Although epitope mapping has identified an immunodominant epitope of SseB in humans (37), peptide library screening of SseB-specific T cells in our laboratory has so far failed to uncover the immunodominant epitope in mice (Lee at al, unpublished). Therefore, to examine the effect of flagellin on CD4 T cell responses to SseB, we fused the 2W1S epitope to the carboxy-terminus of SseB and used 2W1S MHC class-II tetramers to examine SseB-2W-specific CD4 T cells.…”

Protective Host Immune Responses to Salmonella Infection

“…In particular, it is not yet clear if the mouse model of infection with serovar Typhimurium effectively models typhoid or systemic nontyphoidal Salmonellosis. Recent studies have found overlap between antigens targeted in mouse infection, children suffering from nontyphoidal Salmonellosis and experimental infection in volunteers [85,86], providing some hope that mouse studies represent a viable preclinical model. However, the development of a murine model that is permissive for S. Typhi infection would still be an important future goal.…”