Vitamin A deficiency impairs neutrophil-mediated control of Salmonella via SLC11A1 in mice

In sub-Saharan Africa, multidrug-resistant non-typhoidal Salmonella serovars are a common cause of fatal bloodstream infection. Malnutrition is a predisposing factor, but the underlying mechanisms are unknown. Here we show that vitamin A deficiency, one of the most prevalent micronutrient deficits afflicting African children, increases susceptibility to disseminated non-typhoidal Salmonella disease in mice and impairs terminal neutrophil maturation. Immature neutrophils had reduced expression of Slc11a1, a gene that encodes a metal ion transporter generally thought to restrict pathogen growth in macrophages. Adoptive transfer of SLC11A1-proficient neutrophils, but not SLC11A1-deficient neutrophils, reduced systemic Salmonella burden in Slc11a1-/- mice or mice with vitamin A deficiency. Loss of terminal granulopoiesis regulator CCAAT/enhancer-binding protein ϵ (C/EBPϵ) also decreased neutrophil-mediated control of Salmonella, but not that mediated by peritoneal macrophages. Susceptibility to infection increased in Cebpe-/- Slc11a1+/+ mice compared with wild-type controls, in an Slc11a1-expression-dependent manner. These data suggest that SLC11A1 deficiency impairs Salmonella control in part by blunting neutrophil-mediated defence.

Elimination of Chlamydia muridarum from the female reproductive tract is IL-12p40 dependent, but independent of Th1 and Th2 cells

Chlamydia vaccine approaches aspire to induce Th1 cells for optimal protection, despite the fact that there is no direct evidence demonstrating Th1-mediated Chlamydia clearance from the female reproductive tract (FRT). We recently reported that T-bet-deficient mice can resolve primary Chlamydia infection normally, undermining the potentially protective role of Th1 cells in Chlamydia immunity. Here, we show that T-bet-deficient mice develop robust Th17 responses and that mice deficient in Th17 cells exhibit delayed bacterial clearance, demonstrating that Chlamydia-specific Th17 cells represent an underappreciated protective population. Additionally, Th2-deficient mice competently clear cervicovaginal infection. Furthermore, we show that sensing of IFN-γ by non-hematopoietic cells is essential for Chlamydia immunity, yet bacterial clearance in the FRT does not require IFN-γ secretion by CD4 T cells. Despite the fact that Th1 cells are not necessary for Chlamydia clearance, protective immunity to Chlamydia is still dependent on MHC class-II-restricted CD4 T cells and IL-12p40. Together, these data point to IL-12p40-dependent CD4 effector maturation as essential for Chlamydia immunity, and Th17 cells to a lesser extent, yet neither Th1 nor Th2 cell development is critical. Future Chlamydia vaccination efforts will be more effective if they focus on induction of this protective CD4 T cell population.

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.

Cutting Edge: Optimal Formation of Hepatic Tissue-Resident Memory CD4 T Cells Requires T-bet Regulation of CD18

CD4 tissue-resident memory T cells (TRMs) allow robust protection of barrier surfaces against pathogens. We investigated the role of T-bet in the formation of liver CD4 TRMs using mouse models. T-bet-deficient CD4 T cells did not efficiently form liver TRMs when compared with wild-type (WT). In addition, ectopic expression of T-bet enhanced the formation of liver CD4 TRMs, but only when in competition with WT CD4 T cells. Liver TRMs also expressed higher levels of CD18, which was T-bet dependent. The WT competitive advantage was blocked by Ab neutralization of CD18. Taken together, our data show that activated CD4 T cells compete for entry to liver niches via T-bet-induced expression of CD18, allowing TRM precursors to access subsequent hepatic maturation signals. These findings uncover an essential role for T-bet in liver TRM CD4 formation and suggest targeted enhancement of this pathway could increase the efficacy of vaccines that require hepatic TRMs.

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).

Host cells subdivide nutrient niches into discrete biogeographical microhabitats for gut microbes

Changes in the microbiota composition are associated with many human diseases, but factors that govern strain abundance remain poorly defined. We show that a commensal Escherichia coli strain and a pathogenic Salmonella enterica serovar Typhimurium isolate both utilize nitrate for intestinal growth, but each accesses this resource in a distinct biogeographical niche. Commensal E. coli utilizes epithelial-derived nitrate, whereas nitrate in the niche occupied by S. Typhimurium is derived from phagocytic infiltrates. Surprisingly, avirulent S. Typhimurium was shown to be unable to utilize epithelial-derived nitrate because its chemotaxis receptors McpB and McpC exclude the pathogen from the niche occupied by E. coli. In contrast, E. coli invades the niche constructed by S. Typhimurium virulence factors and confers colonization resistance by competing for nitrate. Thus, nutrient niches are not defined solely by critical resources, but they can be further subdivided biogeographically within the host into distinct microhabitats, thereby generating new niche opportunities for distinct bacterial species.

Effective clearance of Chlamydia from the murine female reproductive requires IL-12p40, but not T-bet driven Th1 responses

Protective immune responses to Chlamydia infection within the female reproductive tract (FRT) are incompletely understood. MHC class II-restricted IFN-γ+ Th1 responses are thought to be vital for bacterial clearance, but the Th1 master transcription factor T-bet had not yet been fully interrogated in primary infection. Here, we investigated T-bet and IFN-γ involvement in clearance of primary infection from the FRT. Surprisingly, IFN-γ producing CD4+ T cells from the FRT expressed low levels of T-bet, indicating that Chlamydia-specific CD4 T cells are not classical T-bet+ Th1 cells. Additionally, mice deficient in T-bet competently cleared infection with wild-type kinetics and showed significant skewing towards Th17 responses, indicating compensation pathways through alternate Th fates. On the other hand, IFN-γ-, and IFN-γR-deficient mice resolved much of FRT infection, but displayed systemic dissemination and 100% mortality. Thus, IFN-γ protects mice from fatal systemic disease, but classical T-bet+ Th1 cells are not required for primary clearance. In investigating alternative Th fates, RORγt mutant mice deficient for Th17 responses exhibited a short delay in clearance of FRT infection, demonstrating an unanticipated role for Th17 cells in Chlamydia clearance. However, mice deficient in the cytokine subunit IL-12p40 that promotes both Th1 and Th17 cells displayed a prolonged delay in FRT clearance despite CD4 T cells retaining the capacity for IFN-γ production. In summary, the type 1 cytokine IFN-γ plays an essential role in systemic control of Chlamydia, while clearance of primary infection within the FRT is largely dependent on an IL-12p40 driven mechanism that does not require T-bet or classical IFN-γ producing Th1 cells.

Supported by grants from NIH (RO1 A1103422, T32 AI060555)

Sterile liver inflammation and T cell-intrinsic T-bet expression drive the formation of Tissue Resident Memory CD4 T cells that protect against systemic Salmonella infection

Infection with Salmonella enterica serovar Typhimurium (ST) can cause invasive Non-Typhoidal Salmonellosis (iNTS), a systemic disease which kills about 150,000 people annually. In a mouse model of systemic salmonellosis, hepatic tissue resident memory (TRM) CD4 T cells provided protection against subsequent infection. However, the signals required to generate these protective CD4 T cells remain poorly defined. To interrogate the process of hepatic CD4 TRM formation we developed a simple Salmonella-specific transfer system. ST-specific (SM1) TCR transgenic CD4 T cells were activated in vitro and adoptively transferred into C57BL/6 mice undergoing acetaminophen overdose. Sterile liver inflammation enhanced the formation of hepatic TRM SM1 T cells which expressed increased levels of T-bet compared to circulating memory cells. Subsequent mixed bone marrow chimera experiments substantiated the requirement for T-bet in hepatic TRM formation as T-bet-deficient CD4 T cells were deficient in resident memory. Bulk RNAseq analysis of hepatic CD4 TRMs uncovered a variety of cytokine and chemokine receptors that correlated with hepatic tissue residency. Using a combination of gene-deficiency and in vivo neutralization approaches, efforts are ongoing to uncover key components of sterile hepatic inflammation that drive CD4 TRM development. Understanding this process is critical to understanding the process of CD4 TRM development and will be foundational for the development of an effective vaccine against iNTS.

Supported by NIAID R01AI139410, NIH T32 Animal Models of Infectious Diseases

Cohousing with Dirty Mice Increases the Frequency of Memory T Cells and Has Variable Effects on Intracellular Bacterial Infection

The presence of memory lymphocytes in nonlymphoid tissues reflects prior immunological experience and can provide nonspecific defense against infection. In this study, we used a mouse cohousing approach to examine the effect of prior immunological experience on Salmonella and Chlamydia infection. As expected, cohousing of "dirty mice" with specific pathogen-free laboratory mice increased the frequency of effector memory T cells in laboratory mice and enhanced protection against systemic Listeria infection. In contrast, the course of systemic infection with Salmonella and mucosal infection with Chlamydia was largely unaffected by cohousing, despite enhanced frequencies of memory T cells. Thus, cohousing of laboratory mice reliably increases the proportion of memory T cells in circulation, but can it have variable effects on pathogen clearance.

Effective clearance of Chlamydia from the female reproductive tract in the absence of T-bet+ Th1 cells

Protective immune responses to Chlamydia infection within the female reproductive tract (FRT) are incompletely understood. MHC class II-restricted CD4 Th1 responses are thought to be vital for bacterial clearance, due to their capacity to secrete IFN-γ, but Th1 responses as defined by the master transcription factor T-bet had not yet been fully interrogated in primary infection. Here, we investigated T-bet and IFN-γ involvement in primary infection clearance. We found that IFN-γ producing CD4+ T cells from the FRT exhibited surprisingly low levels of T-bet throughout primary infection, indicating low involvement of classical T-bet+ Th1 cells. Mice deficient for T-bet either globally or specifically in CD4+ cells both competently cleared infection along wild-type kinetics. T-bet-deficient mice also showed significant skewing towards Th17 responses, indicating potential compensation pathways through alternate Th fates. On the other hand, IFN-γ-, and IFN-γR-deficient mice resolved much of FRT infection, but experienced systemic dissemination and 100% mortality with an average median survival of approximately 4 weeks. Together, this indicates that while IFN-γ is important to protect mice from death, classical T-bet+ Th1 cells are ultimately not required for primary clearance. Studies are underway to examine which alternative CD4 T cell mechanisms contribute to bacterial clearance within the FRT. Supported by grants from NIH (RO1 A1103422, T32 AI060555)

SARS-CoV-2 induces robust germinal center CD4 T follicular helper cell responses in rhesus macaques

CD4 T follicular helper (Tfh) cells are important for the generation of durable and specific humoral protection against viral infections. The degree to which SARS-CoV-2 infection generates Tfh cells and stimulates the germinal center (GC) response is an important question as we investigate vaccine induced immunity against COVID-19. Here, we report that SARS-CoV-2 infection in rhesus macaques, either infused with convalescent plasma, normal plasma, or receiving no infusion, resulted in transient accumulation of pro-inflammatory monocytes and proliferating Tfh cells with a Th1 profile in peripheral blood. CD4 helper cell responses skewed predominantly toward a Th1 response in blood, lung, and lymph nodes. SARS-CoV-2 Infection induced GC Tfh cells specific for the SARS-CoV-2 spike and nucleocapsid proteins, and a corresponding early appearance of antiviral serum IgG antibodies. Collectively, the data show induction of GC responses in a rhesus model of mild COVID-19.

SARS-CoV-2 infection induces germinal center responses with robust stimulation of CD4 T follicular helper cells in rhesus macaques

CD4 T follicular helper (T fh ) cells are important for the generation of long-lasting and specific humoral protection against viral infections. The degree to which SARS-CoV-2 infection generates T fh cells and stimulates the germinal center response is an important question as we investigate vaccine options for the current pandemic. Here we report that, following infection with SARS-CoV-2, adult rhesus macaques exhibited transient accumulation of activated, proliferating T fh cells in their peripheral blood on a transitory basis. The CD4 helper cell responses were skewed predominantly toward a T h 1 response in blood, lung, and lymph nodes, reflective of the interferon-rich cytokine environment following infection. We also observed the generation of germinal center T fh cells specific for the SARS-CoV-2 spike (S) and nucleocapsid (N) proteins, and a corresponding early appearance of antiviral serum IgG antibodies but delayed or absent IgA antibodies. Our data suggest that a vaccine promoting Th1-type Tfh responses that target the S protein may lead to protective immunity.

Development of canine PD-1/PD-L1 specific monoclonal antibodies and amplification of canine T cell function

Interruption of the programmed death 1 (PD-1) / programmed death ligand 1 (PD-L1) pathway is an established and effective therapeutic strategy in human oncology and holds promise for veterinary oncology. We report the generation and characterization of monoclonal antibodies specific for canine PD-1 and PD-L1. Antibodies were initially assessed for their capacity to block the binding of recombinant canine PD-1 to recombinant canine PD-L1 and then ranked based on efficiency of binding as judged by flow cytometry. Selected antibodies were capable of detecting PD-1 and PD-L1 on canine tissues by flow cytometry and Western blot. Anti-PD-L1 worked for immunocytochemistry and anti-PD-1 worked for immunohistochemistry on formalin-fixed paraffin embedded canine tissues, suggesting the usage of this antibody with archived tissues. Additionally, anti-PD-L1 (JC071) revealed significantly increased PD-L1 expression on canine monocytes after stimulation with peptidoglycan or lipopolysaccharide. Together, these antibodies display specificity for the natural canine ligand using a variety of potential diagnostic applications. Importantly, multiple PD-L1-specific antibodies amplified IFN-γ production in a canine peripheral blood mononuclear cells (PBMC) concanavlin A (Con A) stimulation assay, demonstrating functional activity.

Effective clearance of Chlamydia from the female reproductive tract in the absence of T-bet+ Th1 cells

Protective immune responses to Chlamydia infection within the female reproductive tract (FRT) are incompletely understood. MHC class II-restricted CD4 Th1 responses are thought to be vital for bacterial clearance, due to their capacity to secrete IFN-γ. However, other studies suggest protective or pathogenic roles for additional CD4 T helper subsets. We used a variety of gene-deficient mice to re-examine bacterial clearance following primary genital infection of C57BL/6 mice with Chlamydia muridarum. As seen previously, IFN-γ-, and IFN-γR-deficient mice resolved much of FRT infection, but experienced systemic dissemination and 100% mortality with an average median survival of approximately 4 weeks. Mice lacking the Th1 subset defining transcription factor T-bet or the Th2 transcription factor STAT6 both effectively cleared infection from the reproductive tract, suggesting that neither Th1 nor Th2 cells are essential for local bacterial control. In contrast, RORγt mutant mice selectively deficient in Th17 responses exhibited a week delay in clearance compared to wild-type mice. Th17 subset markers were not observed to be upregulated in T-bet−/− mice and markers of Th1 subsets were minimally upregulated in RORγt mutant mice, indicating that compensation between these subsets does not explain bacterial clearance. In conclusion, timely local clearance of Chlamydia muridarum from the reproductive tract requires Th17 CD4 T cells, but will ultimately clear independent of classical T-bet+ Th1 and RORγt+ Th17 cells individually. Studies are underway to examine which alternative CD4 T cell mechanisms contribute to bacterial clearance within the FRT.

Circulating memory is sufficient for protective immunity to infection with Chlamydia muridarum

Chlamydia trachomatis is the most prevalent bacterial sexually transmitted infection worldwide and yet no vaccine currently exists to protect the public from this reproductive tract pathogen. Recent reports suggest that CD4 tissue resident memory cells (TRM) are essential for vaccine-induced protection against Chlamydia trachomatis in a mouse infection model. Additionally, CD4 TRM in the female reproductive tract (FRT) have been shown to co-localize with APCs in memory lymphocyte clusters (MLCs) to enhance secondary adaptive immunity to reproductive tract infections. Here, we examine a requirement for CD4 TRM and MLCs in protective immunity against infection with Chlamydia muridarum (Cm). Using parabiosis of naïve and Chlamydia-immune mice, we demonstrate that although CD4 TRM form in the FRT after primary Cm infection, they are not required for protection. Furthermore, histological analysis of FRT tissue from parabionts revealed that circulating memory is sufficient for protection against intravaginal challenge with Cm in the absence of CD4 TRM or MLCs in the reproductive tract. Moreover, this protective circulating memory can be induced through intravaginal or intranasal challenge with Chlamydia, indicating that a strong mucosal immunization can generate circulating protection in a vaccine setting. Ongoing work using adoptive transfers and antibody depletions of CD4 T cells and B cells will reveal which memory lymphocytes are responsible for the observed circulating protection.

Antibody, but not B-cell-dependent antigen presentation, plays an essential role in preventing Chlamydia systemic dissemination in mice

The obligate intracellular bacterium Chlamydia trachomatis causes the most prevalent bacterial sexually transmitted infection worldwide. CD4 T cells play a central role in the protective immunity against Chlamydia female reproductive tract (FRT) infection, while B cells are thought to be dispensable for resolution of primary Chlamydia infection in mouse models. We recently reported an unexpected requirement of B cells in local Chlamydia-specific CD4 T-cell priming and bacterial containment within the FRT. Here, we sought to tackle the precise effector function of B cells during Chlamydia primary infection. Using mixed bone marrow chimeras that lack B-cell-dependent Ag presentation (MHCII^B - / - ) or devoid of circulating antibodies (AID^-/- × μS^-/- ), we show that Chlamydia-specific CD4 T-cell expansion does not rely on Ag presentation by B cells. Importantly, we demonstrate that antibody, but not B-cell-dependent Ag presentation, is required for preventing systemic bacterial dissemination following Chlamydia FRT infection.

Metastatic immune infiltrates correlate with those of the primary tumour in canine osteosarcoma

Our lack of understanding of the immune microenvironment in canine osteosarcoma (cOSA) has limited the identification of potential immunotherapeutic targets. In particular, our ability to utilize readily available tissue from a dog's primary tumour to predict the type and extent of immune response in their pulmonary metastatic lesions is unknown. We, therefore, collected 21 matched pairs of primary tumours and pulmonary metastatic lesions from dogs with OSA and performed immunohistochemistry to quantify T-lymphocyte (CD3), FOXP3+ cell, B-lymphocyte (Pax-5), and CD204+ macrophage infiltration. We found that T-lymphocytes and FOXP3+ infiltrates in primary tumours positively correlated with that of metastatic lesions (ρ = 0.512, P = 0.038 and ρ = 0.698, P = 0.007, respectively), while a strong trend existed for CD204+ infiltrates (ρ = 0.404, P = 0.087). We also observed T- and B-lymphocytes, and CD204+ macrophages to be significantly higher in a dog's pulmonary metastasis compared to their primary tumour (P = 0.018, P = 0.018, P = 0.016, respectively), while FOXP3+ cells were only significantly higher in metastases when all primary tumour and metastasis lesions were compared without pairing (P = 0.036). Together, these findings suggest that the metastatic immune microenvironment may be influenced by that of the primary cOSA, and that primary tumour immune biomarkers could potentially be applied to predict immunotherapeutic responses in gross metastatic disease. We, therefore, provide a rationale for the treatment of cOSA pulmonary metastases with immunotherapeutics that enhance the anti-tumour activity of these immune cells, particularly in dogs with moderate to high immune cell infiltration in their primary tumours.

Circulating Memory is sufficient for protective immunity to secondary infection with Chlamydia

Chlamydia trachomatis is the most prevalent bacterial STI worldwide and yet no vaccine currently exists to protect the public from this reproductive tract pathogen. Recent reports suggest that Tissue Resident Memory (TRM) T cells are essential for vaccine-induced protection against Chlamydia trachomatis in a mouse infection model. TRM T cells have also been implicated in protective immunity to other reproductive tract pathogens, including Herpes Simplex Virus. Here, we examine a requirement for circulating versus resident memory in protection against secondary infection with Chlamydia muridarum (Cm). When FTY720 was used to block memory lymphocyte circulation, protection against secondary infection was unaltered, demonstrating that local resident mechanisms within the female reproductive tract are sufficient for secondary protection. However, parabiosis surgery of memory and naïve mice demonstrated that naïve mice receiving circulating memory were also fully protected from Cm challenge, indicating that TRM are dispensable for protection. Ongoing experiments are examining the protective capacity of both TRM T cells and circulating memory and whether memory lymphocyte cluster (MLC) formation within the reproductive tract is an essential component of naturally acquired or transferred protective memory. Understanding the differential contribution of resident versus circulating memory to protective responses is vital for the development of new vaccines against Chlamydia infection.

Multi-color flow cytometry for evaluating age-related changes in memory lymphocyte subsets in dogs

While dogs are increasingly being utilized as large-animal models of disease, important features of age-related immunosenescence in the dog have yet to be evaluated due to the lack of defined naïve vs. memory T lymphocyte phenotypes. We therefore performed multi-color flow cytometry on peripheral blood mononuclear cells from young and aged beagles, and determined the differential cytokine production by proposed memory subsets. CD4+ and CD8+ T lymphocytes in aged dogs displayed increased cytokine production, and decreased proliferative capacity. Antibodies targeting CD45RA and CD62L, but less so CD28 or CD44, defined canine cells that consistently exhibited properties of naïve-, central memory-, effector memory-, and terminal effector-like CD4+ and CD8+ T lymphocyte subsets. Older dogs demonstrated decreased frequencies of naïve-like CD4+ and CD8+ T lymphocytes, and an increased frequency of terminal effector-like CD8+ T lymphocytes. Overall findings revealed that aged dogs displayed features of immunosenescence similar to those reported in other species.

Association of macrophage and lymphocyte infiltration with outcome in canine osteosarcoma

Immunotherapeutic strategies have shown promise for the treatment of canine osteosarcoma (cOSA). Very little is known about the immune microenvironment within cOSA, however, limiting our ability to identify potential immune targets and biomarkers of therapeutic response. We therefore prospectively assessed the disease-free interval (DFI) and overall survival time (ST) of 30 dogs with cOSA treated with amputation and six doses of adjuvant carboplatin. We then quantified lymphocytic (CD3+, FOXP3+) and macrophage (CD204+) infiltrates within the primary tumours of this cohort using immunohistochemistry, and evaluated their association with outcome. Overall, the median DFI and ST were 392 and 455 days, respectively. The median number of CD3+ and FOXP3+ infiltrates were 45.8 cells/mm² (4.6-607.6 cells/mm² ) and 8.5 mm² (0-163.1 cells/mm² ), respectively. The median area of CD204+ macrophages was 4.7% (1.3%-23.3%), and dogs with tumours containing greater than 4.7% CD204+ macrophages experienced a significantly longer DFI (P = 0.016). Interestingly, a significantly lower percentage of CD204+ macrophages was detected in cOSA arising from the proximal humerus compared to other appendicular bone locations (P = 0.016). Lymphocytic infiltrates did not appear to correlate with outcome in cOSA. Overall, our findings suggest that macrophages may play a role in inhibiting cOSA progression, as has been suggested in human osteosarcoma.

Diversity in the T cell response to Chlamydia-sum are better than one

Chlamydia trachomatis is responsible for an increasing number of sexually transmitted infections in the United States and is a common cause of serious pathology in the female reproductive tract (FRT). Given the impact and incidence of these infections, the production of an effective Chlamydia vaccine is a public health priority. Mouse models of Chlamydia infection have been utilized to develop a detailed and mechanistic understanding of protective immunity in the FRT. These studies reveal that MHC class-II restricted Chlamydia-specific CD4 T cells are critical for primary bacterial clearance and provide effective protection against secondary infection in the FRT. Despite the clear importance of IFN- γ produced by CD4 Th1 cells, there are also suggestions of wider functional heterogeneity in the CD4 T cell response to Chlamydia infection. Understanding the role of this diversity in the CD4 T helper cell response in the FRT should allow a more nuanced view of CD4 T cell biology in the context of Chlamydia infection and may be critical for vaccine development. Here, we summarize our current understanding of CD4 T helper subsets in the clearance of Chlamydia and discuss some areas where knowledge needs to be further extended by additional experimentation.

PD-1/PD-L1 Monoclonal Antibody Development for Canine Cancer Therapy

PD-1 is an inhibitory receptor that binds to PD-L1 on the surface of tumor and antigen presenting cells in the tumor microenvironment, causing suppression of T-cell activity and tumor progression. Human clinical trials with anti PD-1 monoclonal antibodies, nivolumab and pembrolizumab, showed an objective response rate (ORR) of 30% to 40% with melanoma and 87% with relapsed or refractory Hodgkin’s lymphoma. Here, we have explored the development of canine PD-1/PD-L1 reagents for potential veterinary applications. An S2 insect cell expression system was utilized to produce recombinant canine PD-1Ig and PD-L1Ig and antigen-specific B cells were enriched using PD-1 and PD-L1 tetramers. Enriched B cells were fused and resulting hybridomas were screened for specific reactivity to PD-1 and PD-L1. Then, antibodies capable of blocking PD-1Ig and PD-L1Ig were selected. A total of 5 monoclonal antibodies were generated from the two rounds of immunizations and are being studied for in vitro activity in T cell assays. These canine reagents have multiple diagnostic and therapeutic applications in the context of canine oncology.

Circulating memory is sufficient for protection against Chlamydia muridarum in the murine female reproductive tract

Chlamydia trachomatis is the most prevalent bacterial STI worldwide and yet no vaccine currently exists to protect the public from this reproductive tract pathogen. Recent studies show that Tissue Resident Memory T cells (Trm) are essential for vaccine-induced protection against Chlamydia trachomatis in mice. Moreover, Trm have been implicated in protective immunity to other reproductive tract pathogens including Herpes Simplex Virus. Here, we examine the requirement of Trm T cells in protection against the murine pathogen Chlamydia muridarum (Cm), commonly used to model human Chlamydia infection. Using parabiosis to test whether circulating memory from mice immune to Cm infection can protect naïve mice from reproductive tract challenge with Cm, we found that naïve mice receiving circulating immunity were fully protected from Cm challenge. Ongoing studies with MHC class-II tetramers will examine the phenotype of protective circulating Chlamydia-specific CD4 memory T cells and whether migration to the reproductive tract is a requirement for protection against secondary challenge.

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

The development of a subunit Salmonella vaccine has been hindered by the absence of detailed information about antigenic targets of protective Salmonella-specific T and B cells. Recent studies have identified SseB as a modestly protective Ag in susceptible C57BL/6 mice, but the mechanism of protective immunity remains undefined. In this article, we report that simply combining Salmonella SseB with flagellin substantially enhances protective immunity, allowing immunized C57BL/6 mice to survive for up to 30 d following challenge with virulent bacteria. Surprisingly, the enhancing effect of flagellin did not require flagellin Ag targeting during secondary responses or recognition of flagellin by TLR5. Although coimmunization with flagellin did not affect SseB-specific Ab responses, it modestly boosted CD4 responses. In addition, protective immunity was effectively transferred in circulation to parabionts of immunized mice, demonstrating that tissue-resident memory is not required for vaccine-induced protection. Finally, protective immunity required host expression of IFN-γR but was independent of induced NO synthase expression. Taken together, these data indicate that Salmonella flagellin has unique adjuvant properties that improve SseB-mediated protective immunity provided by circulating memory.

CCR7 Deficiency Allows Accelerated Clearance of Chlamydia from the Female Reproductive Tract

Immune mechanisms responsible for pathogen clearance from the female reproductive tract (FRT) are incompletely defined; in particular, the contribution of lymphocyte trafficking to this process is unclear. CCR7-deficient mice have profoundly altered lymphocyte recirculation and display ectopic formation of lymphocyte aggregates within mucosal nonlymphoid tissues, including the FRT. In this study, we investigated how altered lymphocyte distribution in CCR7-deficient mice would affect host responses to Chlamydia muridarum within the reproductive tract. As expected, CCR7-deficient mice exhibited reduced lymphocyte trafficking to lymph nodes and a corresponding increase in T cell populations within the FRT. After intravaginal infection with Chlamydia, CCR7-deficient mice displayed markedly reduced Ag-specific CD4 T cell responses within the local draining iliac lymph nodes, yet robust Th1 and Th17 responses were prominent in the FRT. In addition, Chlamydia-specific Ab responses were dysregulated in CCR7-deficient mice, displaying an unexpected increase in the systemic IgA responses. Importantly, prominent mucosal immune responses in CCR7-deficient mice increased the efficiency of bacteria clearance from the FRT while reducing tissue-associated inflammation and pathology. Thus, increased numbers of lymphocytes within the FRT result in pathogen clearance with reduced immune-mediated pathology.

T cell expression of IL-18R and DR3 is essential for non-cognate stimulation of Th1 cells and optimal clearance of intracellular bacteria

Th1 cells can be activated by TCR-independent stimuli, but the importance of this pathway in vivo and the precise mechanisms involved require further investigation. Here, we used a simple model of non-cognate Th1 cell stimulation in Salmonella-infected mice to examine these issues. CD4 Th1 cell expression of both IL-18R and DR3 was required for optimal IFN-γ induction in response to non-cognate stimulation, while IL-15R expression was dispensable. Interestingly, effector Th1 cells generated by immunization rather than live infection had lower non-cognate activity despite comparable IL-18R and DR3 expression. Mice lacking T cell intrinsic expression of MyD88, an important adapter molecule in non-cognate T cell stimulation, exhibited higher bacterial burdens upon infection with Salmonella, Chlamydia or Brucella, suggesting that non-cognate Th1 stimulation is a critical element of efficient bacterial clearance. Thus, IL-18R and DR3 are critical players in non-cognate stimulation of Th1 cells and this response plays an important role in protection against intracellular bacteria.

Antibiotic treatment causes a reduction in antigen-specific T cell memory and increased susceptibility to secondary infection

Salmonella enterica serovar Typhi causes recurrent and relapsing infection in antibiotic-treated individuals, suggesting that rapid bacterial clearance hinders the development of adaptive immunity. We have developed an antibiotic-treatment model in mice to examine this issue. This model demonstrates poor secondary protection after antibiotic treatment and allows the use of antigen-specific reagents to examine this issue in detail. Infection with Salmonella Typhimurium (BRD509-2W1S) caused expansion of Salmonella-specific CD4 T cells that were detected in peripheral blood, lymphoid tissues, and various non-lymphoid tissues. In contrast, antibiotic-treated mice had lower CD4 clonal expansion that persisted for months after infection. This reduced clonal frequency of Salmonella-specific CD4 T cells correlated with diminished protective immunity to secondary infection. In order to examine the protective contribution of circulating and non-circulating CD4 T cells, parabiosis experiments were performed whereby mice previously infected with BRD509-2W1S were paired with naïve mice for 30 days. Previously infected mice were more capable of controlling secondary infection compared to naïve parabionts or naïve control mice, however naïve parabionts were partially protected, indicating the requirement of both tissue resident and circulating cellular populations. Current experiments are underway to restore full protection during antibiotic intervention. Greater understanding of how antibiotics hinder CD4 memory development may allow for therapeutics to boost protective immunity to secondary or relapsing Salmonella infections. (Funding: NIH 5P01AI056172-09)

U-Omp19 from Brucella abortus Is a Useful Adjuvant for Vaccine Formulations against Salmonella Infection in Mice

Most pathogens infect through mucosal surfaces, and parenteral immunization typically fails to induce effective immune responses at these sites. Development of oral-administered vaccines capable of inducing mucosal as well as systemic immunity while bypassing the issues of antigen degradation and immune tolerance could be crucial for the control of enteropathogens. This study demonstrates that U-Omp19, a bacterial protease inhibitor with immunostimulatory features, coadministered with Salmonella antigens by the oral route, enhances mucosal and systemic immune responses in mice. U-Omp19 was able to increase antigen-specific production of IFN-γ and IL-17 and mucosal (IgA) antibody response. Finally, oral vaccination with U-Omp19 plus Salmonella antigens conferred protection against virulent challenge with Salmonella Typhimurium, with a significant reduction in bacterial loads. These findings prove the efficacy of this novel adjuvant in the Salmonella infection model and support the potential of U-Omp19 as a suitable adjuvant in oral vaccine formulations against mucosal pathogens requiring T helper (Th)1-Th17 protective immune responses.

A Protective Vaccine against Chlamydia Genital Infection Using Vault Nanoparticles without an Added Adjuvant

Chlamydia trachomatis genital infection is the most common sexually transmitted bacterial disease, causing a significant burden to females due to reproductive dysfunction. Intensive screening and antibiotic treatment are unable to completely prevent female reproductive dysfunction, thus, efforts have become focused on developing a vaccine. A major impediment is identifying a safe and effective adjuvant which induces cluster of differentiation 4 (CD4) cells with attributes capable of halting genital infection and inflammation. Previously, we described a natural nanocapsule called the vault which was engineered to contain major outer membrane protein (MOMP) and was an effective vaccine which significantly reduced early infection and favored development of a cellular immune response in a mouse model. In the current study, we used another chlamydial antigen, a polymorphic membrane protein G-1 (PmpG) peptide, to track antigen-specific cells and evaluate, in depth, the vault vaccine for its protective capacity in the absence of an added adjuvant. We found PmpG-vault immunized mice significantly reduced the genital bacterial burden and histopathologic parameters of inflammation following a C. muridarum challenge. Immunization boosted antigen-specific CD4 cells with a multiple cytokine secretion pattern and reduced the number of inflammatory cells in the genital tract making the vault vaccine platform safe and effective for chlamydial genital infection. We conclude that vaccination with a Chlamydia-vault vaccine boosts antigen-specific immunities that are effective at eradicating infection and preventing reproductive tract inflammation.

Adhesion Molecules Associated with Female Genital Tract Infection

Efforts to develop vaccines that can elicit mucosal immune responses in the female genital tract against sexually transmitted infections have been hampered by an inability to measure immune responses in these tissues. The differential expression of adhesion molecules is known to confer site-dependent homing of circulating effector T cells to mucosal tissues. Specific homing molecules have been defined that can be measured in blood as surrogate markers of local immunity (e.g. α4β7 for gut). Here we analyzed the expression pattern of adhesion molecules by circulating effector T cells following mucosal infection of the female genital tract in mice and during a symptomatic episode of vaginosis in women. While CCR2, CCR5, CXCR6 and CD11c were preferentially expressed in a mouse model of Chlamydia infection, only CCR5 and CD11c were clearly expressed by effector T cells during bacterial vaginosis in women. Other homing molecules previously suggested as required for homing to the genital mucosa such as α4β1 and α4β7 were also differentially expressed in these patients. However, CD11c expression, an integrin chain rarely analyzed in the context of T cell immunity, was the most consistently elevated in all activated effector CD8⁺ T cell subsets analyzed. This molecule was also induced after systemic infection in mice, suggesting that CD11c is not exclusive of genital tract infection. Still, its increase in response to genital tract disorders may represent a novel surrogate marker of mucosal immunity in women, and warrants further exploration for diagnostic and therapeutic purposes.

Contaminated water delivery as a simple and effective method of experimental Salmonella infection

AIM

In most infectious disease models, it is assumed that gavage needle infection is the most reliable means of pathogen delivery to the GI tract. However, this methodology can cause esophageal tearing and induces stress in experimental animals, both of which have the potential to impact early infection and the subsequent immune response.

MATERIALS & METHODS

C57BL/6 mice were orally infected with virulent Salmonella Typhimurium SL1344 either by intragastric gavage preceded by sodium bicarbonate, or by contamination of drinking water.

RESULTS

We demonstrate that water contamination delivery of Salmonella is equivalent to gavage inoculation in providing a consistent model of infection. Furthermore, exposure of mice to contaminated drinking water for as little as 4 h allowed maximal mucosal and systemic infection, suggesting an abbreviated window exists for natural intestinal entry.

CONCLUSION

Together, these data question the need for gavage delivery for infection with oral pathogens.

Transient Loss of Protection Afforded by a Live Attenuated Non-typhoidal Salmonella Vaccine in Mice Co-infected with Malaria

In immunocompetent individuals, non-typhoidal Salmonella serovars (NTS) are associated with gastroenteritis, however, there is currently an epidemic of NTS bloodstream infections in sub-Saharan Africa. Plasmodium falciparum malaria is an important risk factor for invasive NTS bloodstream in African children. Here we investigated whether a live, attenuated Salmonella vaccine could be protective in mice, in the setting of concurrent malaria. Surprisingly, mice acutely infected with the nonlethal malaria parasite Plasmodium yoelii 17XNL exhibited a profound loss of protective immunity to NTS, but vaccine-mediated protection was restored after resolution of malaria. Absence of protective immunity during acute malaria correlated with maintenance of antibodies to NTS, but a marked reduction in effector capability of Salmonella-specific CD4 and CD8 T cells. Further, increased expression of the inhibitory molecule PD1 was identified on memory CD4 T cells induced by vaccination. Blockade of IL-10 restored protection against S. Typhimurium, without restoring CD4 T cell effector function. Simultaneous blockade of CTLA-4, LAG3, and PDL1 restored IFN-γ production by vaccine-induced memory CD4 T cells but was not sufficient to restore protection. Together, these data demonstrate that malaria parasite infection induces a temporary loss of an established adaptive immune response via multiple mechanisms, and suggest that in the setting of acute malaria, protection against NTS mediated by live vaccines may be interrupted.

In children, malaria is a predisposing factor for invasive bacterial infections with non-typhoidal Salmonella (NTS) serovars, a frequent cause of morbidity and mortality in sub-Saharan Africa. Since development of vaccines against NTS has been proposed as a strategy to protect African children against disseminated NTS infection, we interrogated the effect of malaria on vaccine-induced memory responses to NTS. Our results from a mouse infection model show that infection with malaria parasites temporarily suspends protective immunity conferred by a live, attenuated vaccine and suppresses adaptive immune responses to NTS that are mediated by T cells. These results suggest that in the setting of acute malaria, live attenuated NTS vaccines may lose their effectiveness.

Protective host immune responses to Salmonella infection

Salmonella enterica serovars Typhi and Paratyphi are the causative agents of human typhoid fever. Current typhoid vaccines are ineffective and are not widely used in endemic areas. Greater understanding of host-pathogen interactions during Salmonella infection should facilitate the development of improved vaccines to combat typhoid and nontyphoidal Salmonellosis. This review will focus on our current understanding of Salmonella pathogenesis and the major host immune components that participate in immunity to Salmonella infection. In addition, recent findings regarding host immune mechanisms in response to Salmonella infection will be also discussed, providing a new perspective on the utility of improved tools to study the immune response to Salmonella infections.

Salmonella Infection Drives Promiscuous B Cell Activation Followed by Extrafollicular Affinity Maturation

The B cell response to Salmonella typhimurium (STm) occurs massively at extrafollicular sites, without notable germinal centers (GCs). Little is known in terms of its specificity. To expand the knowledge of antigen targets, we screened plasmablast (PB)-derived monoclonal antibodies (mAbs) for Salmonella specificity, using ELISA, flow cytometry, and antigen microarray. Only a small fraction (0.5%-2%) of the response appeared to be Salmonella-specific. Yet, infection of mice with limited B cell receptor (BCR) repertoires impaired the response, suggesting that BCR specificity was important. We showed, using laser microdissection, that somatic hypermutation (SHM) occurred efficiently at extrafollicular sites leading to affinity maturation that in turn led to detectable STm Ag-binding. These results suggest a revised vision of how clonal selection and affinity maturation operate in response to Salmonella. Clonal selection initially is promiscuous, activating cells with virtually undetectable affinity, yet SHM and selection occur during the extrafollicular response yielding higher affinity, detectable antibodies.

A re-evaluation of the role of B cells in protective immunity to Chlamydia infection

Chlamydia trachomatis is the etiological agent of the most commonly reported bacterial sexual transmitted infection (STI) in North America and Europe. The control of Chlamydia infection is hindered by the asymptomatic nature of initial infection but the consequence of untreated infection seriously threatens the reproductive health of young women. Unfortunately, there is no licensed vaccine for Chlamydia vaccine, in part due to our incomplete understanding of the immune response to Chlamydia urogenital infection. It has been well established that T cell-mediated immunity plays a dominant role in protective immunity against Chlamydia and thus the importance of B cells is somewhat underappreciated. Here, we summarize recent progress on understanding the role of B cells during Chlamydia genital tract infections and discuss how B cells and humoral immunity make an effective contribution to host defense against important intracellular pathogens, including Chlamydia.

Immunity to intestinal pathogens: lessons learned from Salmonella

Salmonella are a common source of food- or water-borne infection and cause a wide range of clinical disease in human and animal hosts. Salmonella are relatively easy to culture and manipulate in a laboratory setting, and the infection of laboratory animals induces robust innate and adaptive immune responses. Thus, immunologists have frequently turned to Salmonella infection models to expand understanding of host immunity to intestinal pathogens. In this review, I summarize current knowledge of innate and adaptive immunity to Salmonella and highlight features of this response that have emerged from recent studies. These include the heterogeneity of the antigen-specific T-cell response to intestinal infection, the prominence of microbial mechanisms to impede T- and B-cell responses, and the contribution of non-cognate pathways for elicitation of T-cell effector functions. Together, these different issues challenge an overly simplistic view of host-pathogen interaction during mucosal infection, but also allow deeper insight into the real-world dynamic of protective immunity to intestinal pathogens.

Salmonella as a model for non-cognate Th1 cell stimulation

Salmonella has been a model pathogen for examining CD4 T cell activation and effector functions for many years due to the strength of the Th1 cell response observed during Salmonella infections, the relative ease of use of Salmonella, the availability of Salmonella-specific T cell reagents, and the well-characterized nature of the model system, the pathogen, and the immune response elicited. Herein, we discuss the use of Salmonella as a model pathogen to explore the complex interaction of T cells with their inflammatory environment. In particular, we address the issue of bystander activation of naïve T cells and non-cognate stimulation of activated and memory T cells. Further, we compare and contrast our current knowledge of these non-cognate responses in CD8 versus CD4 T cells. Finally, we make a case for Salmonella as a particularly appropriate model pathogen in the study of non-cognate CD4 T cell responses based on the strength of the Th1 response during infection, the requirement for CD4 T cells in bacterial clearance, and the well-characterized inflammatory response to conserved molecular patterns induced by Salmonella infection.

Rapid CD4+ T-cell responses to bacterial flagellin require dendritic cell expression of Syk and CARD9

Toll-like receptors (TLRs) can recognize microbial patterns and utilize adaptor molecules, such as-MyD88 or (TRIF TIR-domain-containing adapter-inducing interferon-β), to initiate downstream signaling that ultimately affects the initiation of adaptive immunity. In addition to this inflammatory role, TLR5 expression on dendritic cells can favor antigen presentation of flagellin peptides and thus increase the sensitivity of flagellin-specific T-cell responses in vitro and in vivo. Here, we examined the role of alternative signaling pathways that might regulate flagellin antigen presentation in addition to MyD88. These studies suggest a requirement for spleen tyrosine kinase, a noncanonical TLR-signaling adaptor molecule, and its downstream molecule CARD9 in regulating the sensitivity of flagellin-specific CD4(+) T-cell responses in vitro and in vivo. Thus, a previously unappreciated signaling pathway plays an important role in regulating the dominance of flagellin-specific T-cell responses.

Direct visualization of endogenous Salmonella-specific B cells reveals a marked delay in clonal expansion and germinal center development

CD4(+) T cells and B cells are both essential for acquired immunity to Salmonella infection. It is well established that Salmonella inhibit host CD4(+) T-cell responses, but a corresponding inhibitory effect on B cells is less well defined. Here, we utilize an Ag tetramer and pull-down enrichment strategy to directly visualize OVA-specific B cells in mice, as they respond to infection with Salmonella-OVA. Surprisingly, OVA-specific B-cell expansion and germinal center formation was not detected until bacteria were cleared from the host. Furthermore, Salmonella infection also actively inhibited both B- and T-cell responses to the same coinjected Ag but this did not require the presence of iNOS. The Salmonella Pathogenicity Island 2 (SPI2) locus has been shown to be responsible for inhibition of Salmonella-specific CD4(+) T-cell responses, and an examination of SPI2-deficient bacteria demonstrated a recovery in B-cell expansion in infected mice. Together, these data suggest that Salmonella can simultaneously inhibit host B- and T-cell responses using SPI2-dependent mechanisms.

The Role of Non-Cognate T Cell Stimulation during Intracellular Bacterial Infection

Intra-macrophage bacterial infections cause significant morbidity and mortality in both the developed and developing world. Protective host immune responses to these infections initially requires the activation and expansion of pathogen-specific CD4 Th1 cells within lymphoid tissues and subsequent relocation of these effector cells to sites of infection. After entering infected tissues, the elicitation of Th1 bactericidal activity can be triggered by cognate or non-cognate signals that are delivered by locally infected antigen-presenting cells and innate cells. However, the contribution of non-cognate stimulation to the resolution of bacterial infection remains poorly understood, especially in the context of a Th1 response. Here, we review the current data on Th1 cell activation and expansion in mouse models of Salmonella and Chlamydia infection and discuss the potential role of non-cognate Th1 cell stimulation in these disease models. Greater understanding of this pathway of T cell activation may lead to the design of therapeutics or vaccines to combat intra-macrophage pathogens.

Toll-like receptor and inflammasome signals converge to amplify the innate bactericidal capacity of T helper 1 cells

T cell effector functions can be elicited by noncognate stimuli, but the mechanism and contribution of this pathway to the resolution of intracellular macrophage infections have not been defined. Here, we show that CD4(+) T helper 1 (Th1) cells could be rapidly stimulated by microbe-associated molecular patterns during active infection with Salmonella or Chlamydia. Further, maximal stimulation of Th1 cells by lipopolysaccharide (LPS) did not require T-cell-intrinsic expression of toll-like receptor 4 (TLR4), interleukin-1 receptor (IL-1R), or interferon-γ receptor (IFN-γR) but instead required IL-18R, IL-33R, and adaptor protein MyD88. Innate stimulation of Th1 cells also required host expression of TLR4 and inflammasome components that together increased serum concentrations of IL-18. Finally, the elimination of noncognate Th1 cell stimulation hindered the resolution of primary Salmonella infection. Thus, the in vivo bactericidal capacity of Th1 cells is regulated by the response to noncognate stimuli elicited by multiple innate immune receptors.

CD103-CD11b+ dendritic cells regulate the sensitivity of CD4 T-cell responses to bacterial flagellin

Toll-like receptor 5 (TLR5) has been widely studied in an inflammatory context, but the effect of TLR5 on the adaptive response to bacterial flagellin has received considerably less attention. Here, we demonstrate that TLR5 expression by dendritic cells (DCs) allows a 1,000-fold enhancement of T-cell sensitivity to flagellin, and this enhancement did not require the expression of NLRC4 or Myd88. The effect of TLR5 on CD4 T-cell sensitivity was independent of the adjuvant effect of flagellin and TLR5 ligation did not alter the sensitivity of ovalbumin (OVA)-specific T cells to OVA. In the spleen, the exquisite T-cell sensitivity to flagellin was regulated by CD4-CD8α- DCs and was blocked by a monoclonal antibody to TLR5. In the mesenteric lymph nodes, flagellin-specific T-cell activation was regulated by a population of CD103-CD11b+ DCs. Thus, TLR5 expression by mucosal and systemic DC subsets controls the sensitivity of the adaptive immune response to flagellated pathogens.

Microbial-induced Th17: superhero or supervillain?

Th17 cells are an effector lineage of CD4 T cells that can contribute to protection against microbial pathogens and to the development of harmful autoimmune and inflammatory conditions. An increasing number of studies suggests that Th17 cells play an important protective role in mobilizing host immunity to extracellular and intracellular microbial pathogens, such as Candida and Salmonella. Furthermore, the generation of Th17 cells is heavily influenced by the normal microbial flora, highlighting the complex interplay among harmless microbes, pathogens, and host immunity in the regulation of pathogen-specific Th17 responses. In this article, we review the current understanding of microbe-induced Th17 cells in the context of infectious and inflammatory disease.

Cutting edge: B cells are essential for protective immunity against Salmonella independent of antibody secretion

Typhoid fever and nontyphoidal bacteremia caused by Salmonella remain critical human health problems. B cells are required for protective immunity to Salmonella, but the mechanism of protection remains unclear. In this study, we immunized wild-type, B cell-deficient, Ab-deficient, and class-switched Ab-deficient mice with attenuated Salmonella and examined protection against secondary infection. As expected, wild-type mice were protected and B cell-deficient mice succumbed to secondary infection. Interestingly, mice with B cells but lacking secreted Ab or class-switched Ab had little deficiency in resistance to Salmonella infection. The susceptibility of B cell-deficient mice correlated with marked reductions in CD4 T cell IFN-γ production after secondary infection. Taken together, these data suggest that the primary role of B cells in acquired immunity to Salmonella is via the development of protective T cell immunity.

MHC class-I-restricted CD8 T cells play a protective role during primary Salmonella infection

Protective immunity against Salmonella infection is known to require CD4 Th1 cells and B cells, but the role of MHC class-I-restricted CD8 T cells is less clear. Previous studies have suggested that CD8 T cells participate in secondary, but not primary, bacterial clearance. However, these studies have used experimental models that are difficult to interpret and do not clearly isolate the role of MHC class-I-restricted CD8 T cells from other cell populations. Here, we examined the role of class-I-restricted T cells in protection against Salmonella infection using mice lacking all classical MHC class-Ia molecules, perforin, or granzyme B. Immunized K(b)D(b)-, perforin-, granzyme B-, or perforin/granzyme B-deficient mice were able to resolve secondary infection with virulent Salmonella, demonstrating that class-I-restricted CTLs are not required for acquired immunity. However, during primary infection with attenuated bacteria, bacterial clearance was delayed in each of these mouse strains when compared to wild-type mice. Taken together, these data demonstrate that CD8 T cells are not required for acquired immunity to Salmonella, but can play a protective role in resolving primary infection with attenuated bacteria.

Increased susceptibility to Salmonella infection in signal regulatory protein α-deficient mice

Recent studies have shed light on the connection between elevated erythropoetin production/spleen erythropoiesis and increased susceptibility to Salmonella infection. In this article, we provide another mouse model, the SIRPα-deficient (Sirpα⁻/⁻) mouse, that manifests increased erythropoiesis as well as heightened susceptibility to Salmonella infection. Sirpα⁻/⁻ mice succumbed to systemic infection with attenuated Salmonella, possessing significantly higher bacterial loads in both the spleen and the liver. Moreover, Salmonella-specific Ab production and Ag-specific CD4 T cells were reduced in Sirpα⁻/⁻ mice compared with wild-type controls. To further characterize the potential mechanism underlying SIRPα-dependent Ag-specific CD4 T cell priming, we demonstrate that lack of SIRPα expression on dendritic cells results in less efficient Ag processing and presentation in vitro. Collectively, these findings demonstrate an indispensable role of SIRPα for protective immunity to Salmonella infection.

TLR5-deficient mice lack basal inflammatory and metabolic defects but exhibit impaired CD4 T cell responses to a flagellated pathogen

TLR5-deficient mice have been reported to develop spontaneous intestinal inflammation and metabolic abnormalities. However, we report that TLR5-deficient mice from two different animal colonies display no evidence of basal inflammatory disease, metabolic abnormalities, or enhanced resistance to Salmonella infection. In contrast, the absence of TLR5 hindered the initial activation and clonal expansion of intestinal flagellin-specific CD4 T cells following oral Salmonella infection. Together, these data demonstrate that a basal inflammatory phenotype is not a consistent feature of TLR5-deficient mice and document a novel role for TLR5 in the rapid targeting of flagellin by intestinal pathogen-specific CD4 T cells.

Generation of Salmonella-specific Th1 cells requires sustained antigen stimulation

The administration of live attenuated Salmonella strains has proven to be an effective way to generate protective immunity against Salmonella infection in humans and mice. Studies in the mouse model have shown that protection requires Salmonella-specific Th1 cells, however the timing and stimulatory requirements for generating optimal Th1 responses have not been carefully examined. We used antibiotic interruption of vaccination with live attenuated Salmonella to examine the requirements for Salmonella-specific Th1 development and protective immunity. Optimal development of protective immunity to Salmonella infection required at least one week of exposure to the live attenuated Salmonella strain. In contrast, optimal development of Salmonella-specific Th1 cells required two weeks of in vivo colonization. Thus, sustained in vivo stimulation with a live vaccine strain is essential for the development of robust Salmonella-specific Th1 cells.