The immune environment of the mammary gland fluctuates during post-lactational regression and correlates with tumour growth rate.

Post-lactational mammary gland regression encompasses extensive programmed cell death and removal of milk-producing epithelial cells, breakdown of extracellular matrix components and redifferentiation of stromal adipocytes. This highly regulated involution process is associated with a transient increased risk of breast cancer in women. Using a syngeneic tumour model, we show that tumour growth is significantly altered depending on the stage of involution at which tumour cells are implanted. Tumour cells injected at day 3 involution grew faster than those in nulliparous mice, whereas tumours initiated at day 6 involution grew significantly slower. These differences in tumour progression correlate with distinct changes in innate immune cells, in particular among F4/80-expressing macrophages and among TCRδ+ unconventional T cells. Breast cancer post-pregnancy risk is exacerbated in older first-time mothers and, in our model, initial tumour growth is moderately faster in aged mice compared with young mice. Our results have implications for breast cancer risk and the use of anti-inflammatory therapeutics for postpartum breast cancers.

The Garlic Compound Z-Ajoene, S-Thiolates COX2 and STAT3 and Dampens the Inflammatory Response in RAW264.7 Macrophages.

SCOPE: Garlic (Allium sativum) has been used for centuries as a prophylactic and therapeutic medicinal agent to control inflammation-associated pathologies. To investigate the underlying mechanisms, an in vitro inflammatory model is established using RAW264.7 murine macrophages exposed to low-doses of lipopolysaccharide (LPS) in the presence of garlic compounds allicin and Z-ajoene (ZA), mimicking regular garlic consumption. METHODS AND RESULTS: Both allicin and Z-ajoene dampen both transcript and protein expression of the pro-inflammatory cytokines IL1ß, IL6, and IL12ß, and upregulate the expression of the anti-inflammatory cytokine IL10. Protein arrays of selected secreted inflammatory mediators confirm that Z-ajoene has a pronounced down-regulatory effect on LPS-induced inflammatory cytokines and chemokines. Many of these proteins are known targets of the transcription factor signal transducer and activator of transcription 3 (STAT3); and indeed, Z-ajoene or its analogue dansyl-ajoene is found to decrease phosphorylation and nuclear translocation of STAT3, and to covalently modify the protein by S-thiolation at Cys108, Cys367, and Cys687. Z-Ajoene dose-dependently and non-competitively inhibit the activity of cyclooxygenase 2 (COX2), possibly attributed to S-thiolation at Cys9 and Cys299. CONCLUSION: The characterization of Z-ajoene's activity of targeting and covalently modifying STAT3 and COX2, both important regulators of inflammation, may contribute to the health benefits of regular dietary garlic consumption.

Mice Deficient in T-bet Form Inducible NO Synthase-Positive Granulomas That Fail to Constrain Salmonella.

Clearance of intracellular infections caused by Salmonella Typhimurium (STm) requires IFN-γ and the Th1-associated transcription factor T-bet. Nevertheless, whereas IFN-γ-/- mice succumb rapidly to STm infections, T-bet-/- mice do not. In this study, we assess the anatomy of immune responses and the relationship with bacterial localization in the spleens and livers of STm-infected IFN-γ-/- and T-bet-/- mice. In IFN-γ-/- mice, there is deficient granuloma formation and inducible NO synthase (iNOS) induction, increased dissemination of bacteria throughout the organs, and rapid death. The provision of a source of IFN-γ reverses this, coincident with subsequent granuloma formation and substantially extends survival when compared with mice deficient in all sources of IFN-γ. T-bet-/- mice induce significant levels of IFN-γ- after challenge. Moreover, T-bet-/- mice have augmented IL-17 and neutrophil numbers, and neutralizing IL-17 reduces the neutrophilia but does not affect numbers of bacteria detected. Surprisingly, T-bet-/- mice exhibit surprisingly wild-type-like immune cell organization postinfection, including extensive iNOS+ granuloma formation. In wild-type mice, most bacteria are within iNOS+ granulomas, but in T-bet-/- mice, most bacteria are outside these sites. Therefore, Th1 cells act to restrict bacteria within IFN-γ-dependent iNOS+ granulomas and prevent dissemination.

Dynamic architectural interplay between leucocytes and mammary epithelial cells.

The adult mammary gland undergoes dynamic changes during puberty and the postnatal developmental cycle. The mammary epithelium is composed of a bilayer of outer basal, or myoepithelial, cells and inner luminal cells, the latter lineage giving rise to the milk-producing alveolar cells during pregnancy. These luminal alveolar cells undergo Stat3-mediated programmed cell death following the cessation of lactation. It is established that immune cells in the microenvironment of the gland have a role to play both in the ductal outgrowth during puberty and in the removal of dead cells and remodelling of the stroma during the process of postlactational regression. However, most studies have focussed on the role of the stromal immune cell compartment or have quantified immune cell populations in tissue extracts. Our recent development of protocols for deep imaging of the mammary gland in three dimensions (3D) has enabled the architectural relationship between immune cells and the epithelium to be examined in detail, and we have discovered a surprisingly dynamic relationship between the basal epithelium and leucocytes. Furthermore, we have observed morphological changes in the myoepithelial cells, as involution progresses, which were not revealed by previous work in 2D tissue sections and whole tissue. This dynamic architecture suggests a role for myoepithelial cells in the orderly progression of involution. We conclude that deep imaging of mammary gland and other tissues is essential for analysing complex interactions between cellular compartments.

Salmonella-induced thrombi in mice develop asynchronously in the spleen and liver and are not effective bacterial traps.

Thrombosis is a frequent, life-threatening complication of systemic infection associated with multiple organ damage. We have previously described a novel mechanism of inflammation-driven thrombosis induced by Salmonella Typhimurium infection of mice. Thrombosis in the liver develops 7 days after infection, persisting after the infection resolves, and is monocytic cell dependent. Unexpectedly, thrombosis was not prominent in the spleen at this time, despite carrying a similar bacterial burden as the liver. In this study, we show that thrombosis does occur in the spleen but with strikingly accelerated kinetics compared with the liver, being evident by 24 hours and resolving rapidly thereafter. The distinct kinetics of thrombosis and bacterial burden provides a test of the hypothesis that thrombi form in healthy vessels to trap or remove bacteria from the circulation, often termed immunothrombosis. Remarkably, despite bacteria being detected throughout infected spleens and livers in the early days of infection, immunohistological analysis of tissue sections show that thrombi contain very low numbers of bacteria. In contrast, bacteria are present throughout platelet aggregates induced by Salmonella in vitro. Therefore, we show that thrombosis develops with organ-specific kinetics and challenge the universality of immunothrombosis as a mechanism to capture bacteria in vivo.

Tumour cell invasiveness and response to chemotherapeutics in adipocyte invested 3D engineered anisotropic collagen scaffolds.

Breast cancers are highly heterogeneous and their metastatic potential and response to therapeutic drugs is difficult to predict. A tool that could accurately gauge tumour invasiveness and drug response would provide a valuable addition to the oncologist's arsenal. We have developed a 3-dimensional (3D) culture model that recapitulates the stromal environment of breast cancers by generating anisotropic (directional) collagen scaffolds seeded with adipocytes and culturing tumour fragments therein. Analysis of tumour cell invasion in the presence of various therapeutic drugs, by immunofluorescence microscopy coupled with an optical clearing technique, demonstrated the utility of this approach in determining both the rate and capacity of tumour cells to migrate through the stroma while shedding light also on the mode of migration. Furthermore, the response of different murine mammary tumour types to chemotherapeutic drugs could be readily quantified.

Neutral lineage tracing of proliferative embryonic and adult mammary stem/progenitor cells.

Mammary gland development occurs over multiple phases, beginning in the mammalian embryo and continuing throughout reproductive life. The remarkable morphogenetic capacity of the mammary gland at each stage of development is attributed to the activities of distinct populations of mammary stem cells (MaSCs) and progenitor cells. However, the relationship between embryonic and adult MaSCs, and their fate during different waves of mammary gland morphogenesis, remains unclear. By employing a neutral, low-density genetic labelling strategy, we characterised the contribution of proliferative stem/progenitor cells to embryonic, pubertal and reproductive mammary gland development. Our findings further support a model of lineage restriction of MaSCs in the postnatal mammary gland, and highlight extensive redundancy and heterogeneity within the adult stem/progenitor cell pool. Furthermore, our data suggest extensive multiplicity in their foetal precursors that give rise to the primordial mammary epithelium before birth. In addition, using a single-cell labelling approach, we revealed the extraordinary capacity of a single embryonic MaSC to contribute to postnatal ductal development. Together, these findings provide tantalising new insights into the disparate and stage-specific contribution of distinct stem/progenitor cells to mammary gland development.

Imaging the mammary gland and mammary tumours in 3D: optical tissue clearing and immunofluorescence methods.

BACKGROUND: High-resolution 3D imaging of intact tissue facilitates cellular and subcellular analyses of complex structures within their native environment. However, difficulties associated with immunolabelling and imaging fluorescent proteins deep within whole organs have restricted their applications to thin sections or processed tissue preparations, precluding comprehensive and rapid 3D visualisation. Several tissue clearing methods have been established to circumvent issues associated with depth of imaging in opaque specimens. The application of these techniques to study the elaborate architecture of the mouse mammary gland has yet to be investigated. METHODS: Multiple tissue clearing methods were applied to intact virgin and lactating mammary glands, namely 3D imaging of solvent-cleared organs, see deep brain (seeDB), clear unobstructed brain imaging cocktails (CUBIC) and passive clarity technique. Using confocal, two-photon and light sheet microscopy, their compatibility with whole-mount immunofluorescent labelling and 3D imaging of mammary tissue was examined. In addition, their suitability for the analysis of mouse mammary tumours was also assessed. RESULTS: Varying degrees of optical transparency, tissue preservation and fluorescent signal conservation were observed between the different clearing methods. SeeDB and CUBIC protocols were considered superior for volumetric fluorescence imaging and whole-mount histochemical staining, respectively. Techniques were compatible with 3D imaging on a variety of platforms, enabling visualisation of mammary ductal and lobulo-alveolar structures at vastly improved depths in cleared tissue. CONCLUSIONS: The utility of whole-organ tissue clearing protocols was assessed in the mouse mammary gland. Most methods utilised affordable and widely available reagents, and were compatible with standard confocal microscopy. These techniques enable high-resolution, 3D imaging and phenotyping of mammary cells and tumours in situ, and will significantly enhance our understanding of both normal and pathological mammary gland development.

Inflammation drives thrombosis after Salmonella infection via CLEC-2 on platelets.

Thrombosis is a common, life-threatening consequence of systemic infection; however, the underlying mechanisms that drive the formation of infection-associated thrombi are poorly understood. Here, using a mouse model of systemic Salmonella Typhimurium infection, we determined that inflammation in tissues triggers thrombosis within vessels via ligation of C-type lectin-like receptor-2 (CLEC-2) on platelets by podoplanin exposed to the vasculature following breaching of the vessel wall. During infection, mice developed thrombi that persisted for weeks within the liver. Bacteria triggered but did not maintain this process, as thrombosis peaked at times when bacteremia was absent and bacteria in tissues were reduced by more than 90% from their peak levels. Thrombus development was triggered by an innate, TLR4-dependent inflammatory cascade that was independent of classical glycoprotein VI-mediated (GPVI-mediated) platelet activation. After infection, IFN-γ release enhanced the number of podoplanin-expressing monocytes and Kupffer cells in the hepatic parenchyma and perivascular sites and absence of TLR4, IFN-γ, or depletion of monocytic-lineage cells or CLEC-2 on platelets markedly inhibited the process. Together, our data indicate that infection-driven thrombosis follows local inflammation and upregulation of podoplanin and platelet activation. The identification of this pathway offers potential therapeutic opportunities to control the devastating consequences of infection-driven thrombosis without increasing the risk of bleeding.

Anti-CCL2: building a reservoir or opening the floodgates to metastasis?

Neutralisation of macrophage chemoattractant C-C chemokine ligand 2 (CCL2) has shown reduced metastasis and enhanced survival in numerous experimental models of tumorigenesis. However, important new findings reported in Nature by Momo Bentires-Alj's laboratory demonstrate that withdrawal of anti-CCL2 treatment accelerates lung metastasis and death in mice. The study highlights the need to consider longer term consequences of therapeutic intervention of metastatic disease, especially with regard to transient interference with the tumour microenvironment.

Homeostatic regulation of T cell trafficking by a B cell-derived peptide is impaired in autoimmune and chronic inflammatory disease.

During an inflammatory response, lymphocyte recruitment into tissue must be tightly controlled because dysregulated trafficking contributes to the pathogenesis of chronic disease. Here we show that during inflammation and in response to adiponectin, B cells tonically inhibit T cell trafficking by secreting a peptide (PEPITEM) proteolytically derived from 14.3.3 zeta delta (14.3.3.ζδ) protein. PEPITEM binds cadherin-15 on endothelial cells, promoting synthesis and release of sphingosine-1 phosphate, which inhibits trafficking of T cells without affecting recruitment of other leukocytes. Expression of adiponectin receptors on B cells and adiponectin-induced PEPITEM secretion wanes with age, implying immune senescence of the pathway. Additionally, these changes are evident in individuals with type 1 diabetes or rheumatoid arthritis, and circulating PEPITEM in patient serum is reduced compared to that of healthy age-matched donors. In both diseases, tonic inhibition of T cell trafficking across inflamed endothelium is lost. Control of patient T cell trafficking is re-established by treatment with exogenous PEPITEM. Moreover, in animal models of peritonitis, hepatic ischemia-reperfusion injury, Salmonella infection, uveitis and Sjögren's syndrome, PEPITEM reduced T cell recruitment into inflamed tissues.

A novel role of annexin A2 in human type I collagen gene expression.

The fibrillar collagen scaffold of the extracellular matrix provides a structural framework for cells in tissues and regulates intercellular communication; its disregulation has been associated with tumour development and progression. Previous work has shown that expression of type I collagen, the most abundant mammalian extracellular matrix protein, is decreased in chemically or virally transformed cells. This negative regulation could be mapped to a proximal COL1A2 promoter element spanning a CME (Collagen Modulating Element) site in SV40-transformed human fibroblasts (SV-WI38) that binds an unknown repressing protein. By magnetic bead pull-down, we observed a multi-protein complex bound to the CME with preference for single-stranded over conventional double-stranded DNA. MALDI-TOF mass spectrometry of the CME-binding protein complex revealed involvement of nuclear annexin A2 (AnxA2) which was increased in SV40-transformed cells. Further EMSA analysis demonstrated that AnxA2 did not directly bind to the DNA but stabilised the complex and led to an increase in protein binding to the CME in SV-WI38 but not untransformed WI38 cells. Knockdown of AnxA2 by siRNA increased type I collagen production in both WI38 and SV-WI38 cells; however, these effects were not mediated at the transcriptional level. Rather, our data indicate a novel functional role of AnxA2 in the negative post-transcriptional regulation of type I collagen synthesis in human fibroblasts. In SV40-transformed cells, AnxA2 is accumulated at the proximal COL1A2 promoter region, suggesting close association with the transcriptional machinery that possibly facilitates binding to the emerging mRNA, eventually contributing to overall repression of type I collagen protein synthesis.

Resolving Salmonella infection reveals dynamic and persisting changes in murine bone marrow progenitor cell phenotype and function.

The generation of immune cells from BM precursors is a carefully regulated process. This is essential to limit the potential for oncogenesis and autoimmunity yet protect against infection. How infection modulates this is unclear. Salmonella can colonize systemic sites including the BM and spleen. This resolving infection has multiple IFN-γ-mediated acute and chronic effects on BM progenitors, and during the first week of infection IFN-γ is produced by myeloid, NK, NKT, CD4(+) T cells, and some lineage-negative cells. After infection, the phenotype of BM progenitors rapidly but reversibly alters, with a peak ∼ 30-fold increase in Sca-1(hi) progenitors and a corresponding loss of Sca-1(lo/int) subsets. Most strikingly, the capacity of donor Sca-1(hi) cells to reconstitute an irradiated host is reduced; the longer donor mice are exposed to infection, and Sca-1(hi) c-kit(int) cells have an increased potential to generate B1a-like cells. Thus, Salmonella can have a prolonged influence on BM progenitor functionality not directly related to bacterial persistence. These results reflect changes observed in leucopoiesis during aging and suggest that BM functionality can be modulated by life-long, periodic exposure to infection. Better understanding of this process could offer novel therapeutic opportunities to modulate BM functionality and promote healthy aging.

Dynamic reciprocity: the role of annexin A2 in tissue integrity.

Interactions between cells and the extracellular matrix are integral to tissue development, remodelling and pathogenesis. This is underlined by bi-directional flow of information signalling, referred to as dynamic reciprocity. Annexin A2 is a complex and multifunctional protein that belongs to a large family of Ca(2+)-dependent anionic phospholipid and membrane-binding proteins. It has been implicated in diverse cellular processes at the nuclear, cytoplasmic and extracellular compartments including Ca(2+)-dependent regulation of endocytosis and exocytosis, focal adhesion dynamics, transcription and translation, cell proliferation, oxidative stress and apoptosis. Most of these functions are mediated by the annexin A2-S100A10 heterotetramer (AIIt) via its ability to simultaneously interact with cytoskeletal, membrane and extracellular matrix components, thereby mediating regulatory effects of extracellular matrix adhesion on cell behaviour and vice versa. While Src kinase-mediated phosphorylation of filamentous actin-bound AIIt results in membrane-cytoskeletal remodelling events which control cell polarity, cell morphology and cell migration, AIIt at the cell surface can bind to a number of extracellular matrix proteins and catalyse the activation of serine and cysteine proteases which are important in facilitating tissue remodelling during tissue repair, neoangiogenesis and pathological situations. This review will focus on the role of annexin A2 in regulating tissue integrity through intercellular and cell-extracellular matrix interaction. Annexin A2 is differentially expressed in various tissue types as well as in many pathologies, particularly in several types of cancer. These together suggest that annexin A2 acts as a central player during dynamic reciprocity in tissue homeostasis.

Systemic flagellin immunization stimulates mucosal CD103+ dendritic cells and drives Foxp3+ regulatory T cell and IgA responses in the mesenteric lymph node.

Mucosal immunity is poorly activated after systemic immunization with protein Ags. Nevertheless, induction of mucosal immunity in such a manner would be an attractive and simple way to overcome the intrinsic difficulties in delivering Ag to such sites. Flagellin from Salmonella enterica serovar Typhimurium (FliC) can impact markedly on host immunity, in part via its recognition by TLR5. In this study, we show that systemic immunization with soluble FliC (sFliC) drives distinct immune responses concurrently in the mesenteric lymph nodes (MLN) and the spleen after i.p. and s.c. immunization. In the MLN, but not the spleen, sFliC drives a TLR5-dependent recruitment of CD103(+) dendritic cells (DCs), which correlates with a diminution in CD103(+) DC numbers in the lamina propria. In the MLN, CD103(+) DCs carry Ag and are the major primers of endogenous and transgenic T cell priming. A key consequence of these interactions with CD103(+) DCs in the MLN is an increase in local regulatory T cell differentiation. In parallel, systemic sFliC immunization results in a pronounced switching of FliC-specific B cells to IgA in the MLN but not elsewhere. Loss of TLR5 has more impact on MLN than splenic Ab responses, reflected in an ablation of IgA, but not IgG, serum Ab titers. Therefore, systemic sFliC immunization targets CD103(+) DCs and drives distinct mucosal T and B cell responses. This offers a potential "Trojan horse" approach to modulate mucosal immunity by systemically immunizing with sFliC.

The capsular polysaccharide Vi from Salmonella typhi is a B1b antigen.

Vaccination with purified capsular polysaccharide Vi Ag from Salmonella typhi can protect against typhoid fever, although the mechanism for its efficacy is not clearly established. In this study, we have characterized the B cell response to this vaccine in wild-type and T cell-deficient mice. We show that immunization with typhoid Vi polysaccharide vaccine rapidly induces proliferation in B1b peritoneal cells, but not in B1a cells or marginal zone B cells. This induction of B1b proliferation is concomitant with the detection of splenic Vi-specific Ab-secreting cells and protective Ab in Rag1-deficient B1b cell chimeras generated by adoptive transfer-induced specific Ab after Vi immunization. Furthermore, Ab derived from peritoneal B cells is sufficient to confer protection against Salmonella that express Vi Ag. Expression of Vi by Salmonella during infection did not inhibit the development of early Ab responses to non-Vi Ags. Despite this, the protection conferred by immunization of mice with porin proteins from Salmonella, which induce Ab-mediated protection, was reduced postinfection with Vi-expressing Salmonella, although protection was not totally abrogated. This work therefore suggests that, in mice, B1b cells contribute to the protection induced by Vi Ag, and targeting non-Vi Ags as subunit vaccines may offer an attractive strategy to augment current Vi-based vaccine strategies.

Thymic function is maintained during Salmonella-induced atrophy and recovery.

Thymic atrophy is a frequent consequence of infection with bacteria, viruses, and parasites and is considered a common virulence trait between pathogens. Multiple reasons have been proposed to explain this atrophy, including premature egress of immature thymocytes, increased apoptosis, or thymic shutdown to prevent tolerance to the pathogen from developing. The severe loss in thymic cell number can reflect an equally dramatic reduction in thymic output, potentially reducing peripheral T cell numbers. In this study, we examine the relationship between systemic Salmonella infection and thymic function. During infection, naive T cell numbers in peripheral lymphoid organs increase. Nevertheless, this occurs despite a pronounced thymic atrophy caused by viable bacteria, with a peak 50-fold reduction in thymocyte numbers. Thymic atrophy is not dependent upon homeostatic feedback from peripheral T cells or on regulation of endogenous glucocorticoids, as demonstrated by infection of genetically altered mice. Once bacterial numbers fall, thymocyte numbers recover, and this is associated with increases in the proportion and proliferation of early thymic progenitors. During atrophy, thymic T cell maturation is maintained, and single-joint TCR rearrangement excision circle analysis reveals there is only a modest fall in recent CD4(+) thymic emigrants in secondary lymphoid tissues. Thus, thymic atrophy does not necessarily result in a matching dysfunctional T cell output, and thymic homeostasis can constantly adjust to systemic infection to ensure that naive T cell output is maintained.

SadA, a trimeric autotransporter from Salmonella enterica serovar Typhimurium, can promote biofilm formation and provides limited protection against infection.

Salmonella enterica is a major cause of morbidity worldwide and mortality in children and immunocompromised individuals in sub-Saharan Africa. Outer membrane proteins of Salmonella are of significance because they are at the interface between the pathogen and the host, they can contribute to adherence, colonization, and virulence, and they are frequently targets of antibody-mediated immunity. In this study, the properties of SadA, a purported trimeric autotransporter adhesin of Salmonella enterica serovar Typhimurium, were examined. We demonstrated that SadA is exposed on the Salmonella cell surface in vitro and in vivo during infection of mice. Expression of SadA resulted in cell aggregation, biofilm formation, and increased adhesion to human intestinal Caco-2 epithelial cells. Immunization of mice with folded, full-length, purified SadA elicited an IgG response which provided limited protection against bacterial challenge. When anti-SadA IgG titers were enhanced by administering alum-precipitated protein, a modest additional protection was afforded. Therefore, despite SadA having pleiotropic functions, it is not a dominant, protective antigen for antibody-mediated protection against Salmonella.

CD31 is required on CD4+ T cells to promote T cell survival during Salmonella infection.

Hematopoietic cells constitutively express CD31/PECAM1, a signaling adhesion receptor associated with controlling responses to inflammatory stimuli. Although expressed on CD4(+) T cells, its function on these cells is unclear. To address this, we have used a model of systemic Salmonella infection that induces high levels of T cell activation and depends on CD4(+) T cells for resolution. Infection of CD31-deficient (CD31KO) mice demonstrates that these mice fail to control infection effectively. During infection, CD31KO mice have diminished numbers of total CD4(+) T cells and IFN-γ-secreting Th1 cells. This is despite a higher proportion of CD31KO CD4(+) T cells exhibiting an activated phenotype and an undiminished capacity to prime normally and polarize to Th1. Reduced numbers of T cells reflected the increased propensity of naive and activated CD31KO T cells to undergo apoptosis postinfection compared with wild-type T cells. Using adoptive transfer experiments, we show that loss of CD31 on CD4(+) T cells alone is sufficient to account for the defective CD31KO T cell accumulation. These data are consistent with CD31 helping to control T cell activation, because in its absence, T cells have a greater propensity to become activated, resulting in increased susceptibility to become apoptotic. The impact of CD31 loss on T cell homeostasis becomes most pronounced during severe, inflammatory, and immunological stresses such as those caused by systemic Salmonella infection. This identifies a novel role for CD31 in regulating CD4 T cell homeostasis.

T-zone localized monocyte-derived dendritic cells promote Th1 priming to Salmonella.

Control of intracellular Salmonella infection requires Th1 priming and IFN-γ production. Here, we show that efficient Th1 priming after Salmonella infection requires CD11c(+) CD11b(hi) F4/80(+) monocyte-derived dendritic cells (moDCs). In non-infected spleens, moDCs are absent from T-cell zones (T zones) of secondary lymphoid tissues, but by 24 h post-infection moDCs are readily discernible in these sites. The accumulation of moDCs is more dependent upon bacterial viability than bacterial virulence. Kinetic studies showed that moDCs were necessary to prime but not sustain Th1 responses, while ex vivo studies showed that antigen-experienced moDCs were sufficient to induce T-cell proliferation and IFN-γ production via a TNF-α-dependent mechanism. Importantly, moDCs and cDCs when co-cultured induced superior Th1 differentiation than either subset alone, and this activity was independent of TNF-α. Thus, optimal Th1 development to Salmonella requires the rapid accumulation of moDCs within T zones and their collaboration with cDCs.