Most antibody-secreting cells (ASCs) in mucosal tissues produce immunoglobulin A (IgA), the most abundant immunoglobulin in the body and the main class of antibody found in secretions. IgA-ASCs differentiate in the mucosal-associated lymphoid tissues and are usually considered as a homogeneous population of cells. However, IgA-ASCs that travel to the small intestine have unique characteristics in terms of their migratory requirements. These IgA-ASCs require the homing molecules alpha4beta7 and CCR9 to interact with their ligands, mucosal addressin cell adhesion molecule-1 and CCL25, which are constitutively expressed in the small intestine. Indeed, recent work has shown that IgA-ASCs specific for the small bowel are generated under different conditions as compared with IgA-ASCs in other mucosal compartments. Moreover, the mechanisms inducing IgA class switching may also vary according to the tissue where IgA-ASCs differentiate. Here we describe the mechanisms involved in the differentiation of IgA-ASCs in mucosal compartments, in particular those involved in the generation of gut-homing IgA-ASCs.

alpha(4) integrins play a pivotal role in leukocyte migration and tissue-specific homing. The ability of integrins to bind ligand is dynamically regulated by activation-dependent conformational changes triggered in the cytoplasmic domain. An NMR solution structure defined a putative membrane-proximal salt bridge between the alpha(IIb)beta(3) integrin cytoplasmic tails, which restrains integrins in their low-affinity state. However, the physiological importance of this salt bridge in alpha(4) integrin regulation remains to be elucidated. To address this question, we disrupted the salt bridge in murine germ line by mutating the conserved cytoplasmic arginine R(GFFKR) in alpha(4) integrins. In lymphocytes from knock-in mice (alpha(4)-R/A(GFFKR)), alpha(4)beta(1) and alpha(4)beta(7) integrins exhibited constitutively up-regulated ligand binding. However, transmigration of these cells across VCAM-1 and MAdCAM-1 substrates, or across endothelial monolayers, was reduced. Perturbed detachment of the tail appeared to cause the reduced cell migration of alpha(4)-R/A(GFFKR) lymphocytes. In vivo, alpha(4)-R/A(GFFKR) cells exhibited increased firm adhesion to Peyer patch venules but reduced homing to the gut. Our results demonstrate that the membrane-proximal salt bridge plays a critical role in supporting proper alpha(4) integrin adhesive dynamics. Loss of this interaction destabilizes the nonadhesive conformation, and thereby perturbs the properly balanced cycles of adhesion and deadhesion required for efficient cell migration.

T lymphocytes (T cells) express T-cell receptor (TCR) molecules on their surface that can recognize peptides (p) derived from antigenic proteins bound to products of the major histocompatibility complex (MHC) genes. The pMHC molecules are expressed on the surface of antigen-presenting cells, such as dendritic cells (DCs). T cells first encounter antigen on DCs in lymph nodes (LN). Intravital microscopy experiments show that upon entering the LN containing antigen, CD8+ T cells first move rapidly. After a few hours, they stop and make extended contacts with DCs. The factors that determine when and how this transition occurs are not well understood. We report results from computer simulations that suggest that the duration of phase one is related to the low probability of productive interactions between T cells and DCs. This is demonstrated by our finding that the antigen dose and type determine when such a transition occurs. These results are in agreement with experimental observations. TCR-pMHC binding characteristics and the antigen dose determine the time required for a productive T-cell-DC encounter (resulting in sustained contact). We find that the ratio of this time scale and the half-life of the pMHC complex itself provide a consolidated measure of antigen quantity and type. Results obtained upon varying different measures of antigen quantity and type fall on one curve when graphed against this ratio of time scales. Thus, we provide a mechanism for how the effects of varying one set of parameters are influenced by other prevailing conditions. This understanding should help guide future experimentation.

The rules by which naïve T cells decide whether and how to respond to antigenic stimuli are incompletely understood. Using multiphoton intravital microscopy (MP-IVM) in lymph nodes (LNs), we have shown that CD8+ T cells are primed by antigen-presenting dendritic cells (DCs) in three consecutive phases. During phase one, T cells undergo brief serial contacts with many DCs for several hours after homing into the LNs. Subsequently, during phase two, T cells engage in prolonged stable interactions with DCs. Finally, in the third phase, T cells return to transient interactions with DCs as they begin to proliferate and eventually leave the LNs. We have examined the influence of antigen dose on the duration of phase one by systematically varying both the number of cognate peptide-major histocompatability (pMHC) complexes per DC and the density of cognate pMHC complex-presenting DCs per LN. The duration of phase one and the kinetics of CD8+ T cell activation were inversely correlated with both parameters. Very few pMHC complexes were needed for full T cell activation and effector differentiation. Furthermore, there was a sharp threshold of antigen dose below which T cells did not transition to phase two but continued to migrate until they exited the LN, unactivated. The stability of peptide binding to MHC was a critical determinant of this threshold antigen dose in vivo. Our results suggest an integrative mechanism that allows T cells to reach an informed decision about whether to respond, based on the overall antigen dose encountered.

Dendritic cells (DCs) are critical for adaptive immunity and tolerance. Most DCs are strategically positioned as immune sentinels poised to respond to invading pathogens in tissues throughout the body. Differentiated DCs and their precursors also circulate in blood and can get rapidly recruited to sites of challenge. Within peripheral tissues, DCs collect antigenic material and then traffic to secondary lymphoid organs, where they communicate with lymphocytes to orchestrate adaptive immune responses. Hence, the migration and accurate positioning of DCs is indispensable for immune surveillance. Here, we review the molecular traffic signals that govern the migration of DCs throughout their life cycle.

Resolvin E1 (RvE1) is an omega-3 eicosapentaenoic acid (EPA)-derived lipid mediator generated during resolution of inflammation and in human vasculature via leukocyte-endothelial cell interactions. RvE1 possesses anti-inflammatory and proresolving actions. Here, we report that RvE1 in human whole blood rapidly regulates leukocyte expression of adhesion molecules. RvE1 in the 10- to 100-nM range stimulated L-selectin shedding, while reducing CD18 expression in both neutrophils and monocytes. When added to whole blood, RvE1 did not stimulate reactive oxygen species by either neutrophils or monocytes, nor did it directly stimulate cytokine/chemokine production in heparinized blood. Intravital microscopy (IVM) demonstrated that RvE1 rapidly reduced leukocyte rolling (approximately 40%) in venules of mice. In human platelet-rich plasma (PRP), RvE1 selectively blocked both ADP-stimulated and thromboxane receptor agonist U46619-stimulated platelet aggregation in a concentration-dependent manner. In contrast, Delta 6,14-trans-RvE1 isomer was inactive. RvE1 did not block collagen-stimulated aggregation, and regulation of ADP-induced platelet aggregation was not further enhanced with aspirin treatment. These results indicate RvE1 is a potent modulator of leukocytes as well as selective platelet responses in blood and PRP, respectively. Moreover, the results demonstrate novel agonist-specific antiplatelet actions of RvE1 that are potent and may underlie some of the beneficial actions of EPA in humans.

Regulated movement of stem cells is critical for organogenesis during development and for homeostasis and repair in adulthood. Here we analyze the biological significance and molecular mechanisms underlying stem cell trafficking in the generation of the germline, and the generation and regeneration of blood and muscle. Comparison across organisms and lineages reveals remarkable conservation as well as specialization in homing and migration mechanisms used by mature leukocytes, adult and fetal stem cells, and cancer stem cells. In vivo trafficking underpins the successful therapeutic application of hematopoietic stem cells for bone-marrow transplant, and further elucidation of homing and migration pathways in other systems will enable broader application of stem cells for targeted cell therapy and drug delivery.

After homing to lymph nodes, CD8+ T cells are primed by dendritic cells (DCs) in three phases. During phase one, T cells undergo brief serial contacts with DCs for several hours, whereas phase two is characterized by stable T cell-DC interactions. We show here that the duration of phase one and T cell activation kinetics correlated inversely with the number of complexes of cognate peptide and major histocompatibility complex (pMHC) per DC and with the density of antigen-presenting DCs per lymph node. Very few pMHC complexes were necessary for the induction of full-fledged T cell activation and effector differentiation. However, neither T cell activation nor transition to phase two occurred below a threshold antigen dose determined in part by pMHC stability. Thus, phase one permits T cells to make integrated 'measurements' of antigen dose that determine subsequent T cell participation in immune responses.

There is growing evidence that plasmacytoid dendritic cells (pDC) are involved in the innate recognition of various microbes. However, the precise consequences of pathogen recognition on pDC activation and function are incompletely understood. Using a novel transgenic mouse model that facilitates the isolation of highly pure pDC populations, we found that influenza virus PR/8, a TLR7 ligand, and CpG 1826 oligonucleotide, a TLR9 ligand, induced surprisingly divergent activation programs in these cells. pDC stimulated with PR/8 produced large amounts of type I IFNs, and CpG 1826-stimulated pDC expressed higher levels of costimulatory molecules and proinflammatory cytokines and induced stronger proliferation of T cells. Transcriptome analysis uncovered the differential regulation in pDC of 178 and 1577 genes by PR/8 and CpG 1826, respectively. These differences may relate to the activation of discrete signaling pathways, as evidenced by distinct ERK1/2 and p38 MAPK phosphorylation kinetics. Finally, pDC isolated ex vivo during PR/8 infection or after i.v. CpG 1826 injection resembled their in vitro counterparts, corroborating that these cells can adopt specialized phenotypes in vivo. Thus, pDC display remarkable functional flexibility, which emphasizes their versatile functions in antimicrobial immunity and inflammatory processes.

Vitamins are essential constituents of our diet that have long been known to influence the immune system. Vitamins A and D have received particular attention in recent years as these vitamins have been shown to have an unexpected and crucial effect on the immune response. We present and discuss our current understanding of the essential roles of vitamins in modulating a broad range of immune processes, such as lymphocyte activation and proliferation, T-helper-cell differentiation, tissue-specific lymphocyte homing, the production of specific antibody isotypes and regulation of the immune response. Finally, we discuss the clinical potential of vitamin A and D metabolites for modulating tissue-specific immune responses and for preventing and/or treating inflammation and autoimmunity.