Determination of T Follicular Helper Cell Fate by Dendritic Cells.

T follicular helper (Tfh) cells are a specialized subset of CD4+ T cells that collaborate with B cells to promote and regulate humoral responses. Unlike other CD4+ effector lineages, Tfh cells require interactions with both dendritic cells (DCs) and B cells to complete their differentiation. While numerous studies have assessed the potential of different DC subsets to support Tfh priming, the conclusions of these studies depend heavily on the model and method of immunization used. We propose that the location of different DC subsets within the lymph node (LN) and their access to antigen determine their potency in Tfh priming. Finally, we provide a three-step model that accounts for the ability of multiple DC subsets and related lineages to support the Tfh differentiation program

Determination of T Follicular Helper Cell Fate by Dendritic Cells

T follicular helper (Tfh) cells are a specialized subset of CD4+ T cells that collaborate with B cells to promote and regulate humoral responses. Unlike other CD4+ effector lineages, Tfh cells require interactions with both dendritic cells (DCs) and B cells to complete their differentiation. While numerous studies have assessed the potential of different DC subsets to support Tfh priming, the conclusions of these studies depend heavily on the model and method of immunization used. We propose that the location of different DC subsets within the lymph node (LN) and their access to antigen determine their potency in Tfh priming. Finally, we provide a three-step model that accounts for the ability of multiple DC subsets and related lineages to support the Tfh differentiation program

Absence of surrogate light chain results in spontaneous autoreactive germinal centres expanding VH81X-expressing B cells

Random recombination of antibody heavy- and light-chain genes results in a diverse B-cell receptor (BCR) repertoire including self-reactive BCRs. However, tolerance mechanisms that prevent the development of self-reactive B cells remain incompletely understood. The absence of the surrogate light chain, which assembles with antibody heavy chain forming a pre-BCR, leads to production of antinuclear antibodies (ANAs). Here we show that the naive follicular B-cell pool is enriched for cells expressing prototypic ANA heavy chains in these mice in a non-autoimmune background with a broad antibody repertoire. This results in the spontaneous formation of T-cell-dependent germinal centres that are enriched with B cells expressing prototypic ANA heavy chains. However, peripheral tolerance appears maintained by selection thresholds on cells entering the memory B-cell and plasma cell pools, as exemplified by the exclusion of cells expressing the intrinsically self-reactive VH81X from both pool

Presentation of Salmonella antigens by dendritic cells

This study investigates the interaction between dendritic cells (DC) and the Gram negative intracellular bacterium Salmonella typhimurium. Using in vitro and in vivo approaches, the capacity of DC to directly and indirectly present antigens from Salmonella-infected cells, cytokine production by infected DC, and activation of bacteria-specific T cells was analyzed. These studies revealed differential modulation of splenic DC subsets with regard to in situ distribution, absolute number, and cytokine production in mice orally infected with S. typhimurium. DC loaded with Salmonella in vitro were also capable of eliciting Salmonella-specific CD4+ and CD8+ T cells after transfer into naïve mice, and DC were activated in mice infected with Salmonella. In addition, splenic DC infected with S. typhimurium in vitro produced IL-12 and TNF-a, and the number of splenic DC producing TNF-a increased in Salmonella-infected mice. Furthermore, all three splenic DC subsets were capable of engulfing and processing Salmonella for MHC-I and MHC-II presentation of bacterial peptides to T cells, both in vitro and in vivo. In addition to this capacity to directly present Salmonella antigens, DC could also act as bystander cells capable of presenting antigens from neighboring macrophages induced to undergo apoptotic death following infection with Salmonella expressing the Type III secretion system. Together these data suggest that DC are key antigen presenting cells important in activating Salmonella-specific immunity

Antigen presentation capacity and cytokine production by murine splenic dendritic cell subsets upon Salmonella encounter.

Salmonella typhimurium is an intracellular bacterium that replicates in the spleen and mesenteric lymph nodes (MLN) of orally infected mice. However, little is known about the Ag presentation and cytokine production capacity of dendritic cells (DC), particularly CD8alpha(+), CD8alpha(-)CD4(-), and CD8alpha(-)CD4(+) DC, from these organs in response to SALMONELLA: Infection of purified splenic DC with S. typhimiurium expressing green fluorescent protein (GFP) and OVA revealed that all three splenic DC subsets internalize bacteria, and splenic as well as MLN DC process Salmonella for peptide presentation. Furthermore, presentation of Salmonella Ags on MHC-I and MHC-II was evident in both CD8alpha(+) and CD8alpha(-) splenic DC subsets. Direct ex vivo analysis of splenic DC from mice infected with GFP-expressing Salmonella showed that all three subsets harbored bacteria, and splenic DC purified from mice given Salmonella-expressing OVA presented OVA-derived peptides on MHC-I and MHC-II. Cytokine production analyzed by intracellular staining of splenic DC infected with GFP-expressing Salmonella revealed that TNF-alpha was produced by a large percentage of CD8alpha(-) DC, while only a minor proportion of CD8alpha(+) DC produced this cytokine following bacterial exposure. In contrast, the greatest number of IL-12p40-producing DC were among CD8alpha(+) DC. Experiments inhibiting bacterial uptake by cytochalasin D as well as use of a Transwell system revealed that bacterial contact, but not internalization, was required for cytokine production. Thus, DC in sites of Salmonella replication and T cell activation, spleen and MLN, respond to bacterial encounter by Ag presentation and produce cytokines in a subset-specific fashion

The innate immune response differs in primary and secondary salmonella infection.

This study examines innate immunity to oral Salmonella during primary infection and after secondary challenge of immune mice. Splenic NK and NKT cells plummeted early after primary infection, while neutrophils and macrophages (M{phi}) increased 10- and 3-fold, respectively. In contrast, immune animals had only a modest reduction in NK cells, no loss of NKT cells, and a slight increase in phagocytes following secondary challenge. During primary infection, the dominant sources of IFN-{gamma} were, unexpectedly, neutrophils and M{phi}, the former having intracellular stores of IFN-{gamma} that were released during infection. IFN-{gamma}-producing phagocytes greatly outnumbered IFN-{gamma}-producing NK cells, NKT cells, and T cells during the primary response. TNF-{alpha} production was also dominated by neutrophils and M{phi}, which vastly outnumbered NKT cells producing this cytokine. Neither T cells nor NK cells produced TNF-{alpha} early during primary infection. The TNF-{alpha} response was reduced in a secondary response, but remained dominated by neutrophils and M{phi}. Moreover, no significant IFN-{gamma} production by M{phi} was associated with the secondary response. Indeed, only NK1.1+ cells and T cells produced IFN-{gamma} in these mice. These studies provide a coherent view of innate immunity to oral Salmonella infection, reveal novel sources of IFN-{gamma}, and demonstrate that immune status influences the nature of the innate response

Intestinal mononuclear phagocytes in health and disease

The intestine is the tissue of the body with the highest constitutive exposure to foreign antigen and is also a common entry portal for many local and systemic pathogens. Therefore, the local immune system has the unenviable task of balancing efficient responses to dangerous pathogens with tolerance toward beneficial microbiota and food antigens. As in most tissues, the decision between tolerance and immunity is critically governed by the activity of local myeloid cells. However, the unique challenges posed by the intestinal environment have necessitated the development of several specialized mononuclear phagocyte populations with distinct phenotypic and functional characteristics that have vital roles in maintaining barrier function and immune homeostasis in the intestine. Intestinal mononuclear phagocyte populations, comprising dendritic cells and macrophages, are crucial for raising appropriate active immune responses against ingested pathogens. Recent technical advances, including microsurgical approaches allowing collection of cells migrating in intestinal lymph, intravital microscopy, and novel gene-targeting approaches, have led to clearer distinctions between mononuclear phagocyte populations in intestinal tissue. In this review, we present an overview of the various subpopulations of intestinal mononuclear phagocytes and discuss their phenotypic and functional characteristics. We also outline their roles in host protection from infection and their regulatory functions in maintaining immune tolerance toward beneficial intestinal antigens