CD4+ T cell immunity to Salmonella is transient in the circulation

The role of tissue resident memory CD4 T cells in Salmonella infection: Implications for future vaccines

Salmonella infections cause a wide range of intestinal and systemic disease that affects global human health. While some vaccines are available, they do not mitigate the impact of Salmonella on endemic areas. Research using Salmonella mouse models has revealed the important role of CD4 T cells and antibody in the development of protective immunity against Salmonella infection. Recent work points to a critical role for hepatic tissue-resident memory lymphocytes in naturally acquired immunity to systemic infection. Thus, understanding the genesis and function of this Salmonella-specific population is an important objective and is the primary focus of this review. Greater understanding of how these memory lymphocytes contribute to bacterial elimination could suggest new approaches to vaccination against an important human pathogen.

Vaccine-induced inflammation and inflammatory monocytes promote CD4+ T cell-dependent immunity against murine salmonellosis

Prior infection can generate protective immunity against subsequent infection, although the efficacy of such immunity can vary considerably. Live-attenuated vaccines (LAVs) are one of the most effective methods for mimicking this natural process, and analysis of their efficacy has proven instrumental in the identification of protective immune mechanisms. Here, we address the question of what makes a LAV efficacious by characterising immune responses to a LAV, termed TAS2010, which is highly protective (80-90%) against lethal murine salmonellosis, in comparison with a moderately protective (40-50%) LAV, BRD509. Mice vaccinated with TAS2010 developed immunity systemically and were protected against gut-associated virulent infection in a CD4+ T cell-dependent manner. TAS2010-vaccinated mice showed increased activation of Th1 responses compared with their BRD509-vaccinated counterparts, leading to increased Th1 memory populations in both lymphoid and non-lymphoid organs. The optimal development of Th1-driven immunity was closely correlated with the activation of CD11b+Ly6GnegLy6Chi inflammatory monocytes (IMs), the activation of which can be modulated proportionally by bacterial load in vivo. Upon vaccination with the LAV, IMs expressed T cell chemoattractant CXCL9 that attracted CD4+ T cells to the foci of infection, where IMs also served as a potent source of antigen presentation and Th1-promoting cytokine IL-12. The expression of MHC-II in IMs was rapidly upregulated following vaccination and then maintained at an elevated level in immune mice, suggesting IMs may have a role in sustained antigen stimulation. Our findings present a longitudinal analysis of CD4+ T cell development post-vaccination with an intracellular bacterial LAV, and highlight the benefit of inflammation in the development of Th1 immunity. Future studies focusing on the induction of IMs may reveal key strategies for improving vaccine-induced T cell immunity.

In silico design and immunoinformatics analysis of a chimeric vaccine construct based on Salmonella pathogenesis factors

Currently, there are two vaccines based on killed and/or weakened Salmonella bacteria, but no recombinant vaccine is available for preventing or treating the disease. We used an in silico approach to design a multi-epitope vaccine against Salmonella using OmpA, OmpS, SopB, SseB, SthA and FilC antigens. We predicted helper T lymphocyte, cytotoxic T lymphocyte, and IFN-γ epitopes. The FilC sequence was used as a bovine TLR5 agonist, and the linkers KK, AAY, GPGPG and EAAAK were used to connect epitopes. The final sequence consisted of 747 amino acid residues, and the expressed soluble protein (∼79.6 kDa) was predicted to be both non-allergenic and antigenic. The tertiary structure of modeled protein was refined and validated, and the interactions of vaccine 3D structure were evaluated using molecular docking, and molecular dynamics simulation (RMSD, RMSF and Gyration). This structurally stable protein could interact with human TLR5. The C-ImmSim server predicted that this proposed vaccine likely induces an immune response by stimulating T and B cells, making it a potential candidate for further evaluation for the prevention and treatment of Salmonella infection.

Optimal generation of hepatic tissue-resident memory CD4 T cells requires IL-1 and IL-2

Hepatic CD4 tissue-resident memory T cells (TRM) are required for robust protection against Salmonella infection; however, the generation of this T cell population is poorly understood. To interrogate the contribution of inflammation, we developed a simple Salmonella-specific T cell transfer system that allowed direct visualization of hepatic TRM formation. Salmonella-specific (SM1) T cell receptor (TCR) transgenic CD4 T cells were activated in vitro and adoptively transferred into C57BL/6 mice while hepatic inflammation was induced by acetaminophen overdose or L. monocytogenes infection. In both model systems, hepatic CD4 TRM formation was accentuated by local tissue responses. Liver inflammation also enhanced the suboptimal protection provided by a subunit Salmonella vaccine which typically induces circulating memory CD4 T cells. To further elucidate the mechanism of CD4 TRM formation in response to liver inflammation, various cytokines were examined by RNAseq, bone marrow chimeras, and in vivo neutralization. Surprisingly, IL-2 and IL-1 were found to enhance CD4 TRM formation. Thus, local inflammatory mediators enhance CD4 TRM populations and can boost the protective immunity provided by a suboptimal vaccine. This knowledge will be foundational for the development of a more effective vaccine against invasive nontyphoidal salmonellosis (iNTS).

Temporally Evolving and Context-Dependent Functions of Cytokines That Regulate Murine Anti-Plasmodium Humoral Immunity

Protective immunity against blood-stage Plasmodium infection and the disease malaria depends on antibodies secreted from high-affinity B cells selected during the germinal center (GC) response. The induction and stability of the GC response require the activation and direct cell–cell communication between parasite-specific CD4 helper T cells and B cells. However, cytokines secreted by helper T cells, B cells, and multiple other innate and adaptive immune cells also contribute to regulating the magnitude and protective functions of GC-dependent humoral immune responses. Here, we briefly review emerging data supporting the finding that specific cytokines can exhibit temporally distinct and context-dependent influences on the induction and maintenance of antimalarial humoral immunity.