L-selectin and beta7 integrin synergistically mediate lymphocyte migration to mesenteric lymph nodes

A novel monoclonal antibody against 6-sulfo sialyl Lewis x glycans attenuates murine allergic rhinitis by suppressing Th2 immune responses

Lymphocyte homing is mediated by the interaction between L-selectin on lymphocytes and its glycoprotein ligands modified with 6-sulfo sialyl Lewis x (6-sulfo sLex) glycans on high endothelial venules (HEVs) in peripheral lymph nodes (PLNs). However, the lack of specific antibodies reactive with both human and mouse 6-sulfo sLex has limited our understanding of its function in vivo. Here, we generated a novel monoclonal antibody, termed SF1, that specifically reacts with 6-sulfo sLex expressed on HEVs in both species in a manner dependent on sulfate, fucose, and sialic acid modifications. Glycan array and biolayer interferometry analyses indicated that SF1 specifically bound to 6-sulfo sLex with a dissociation constant of 6.09 × 10-9 M. SF1 specifically bound to four glycoproteins from PLNs corresponding to the molecular sizes of L-selectin ligand glycoproteins. Consistently, SF1 inhibited L-selectin-dependent lymphocyte rolling on 6-sulfo sLex-expressing cells ex vivo and lymphocyte homing to PLNs and nasal-associated lymphoid tissues in vivo. Furthermore, SF1 significantly attenuated ovalbumin-induced allergic rhinitis in mice in association with significant suppression of Th2 immune responses. Collectively, these results suggest that SF1 can be useful for the functional analysis of 6-sulfo sLex and may potentially serve as a novel therapeutic agent against immune-related diseases.

The intestine: A highly dynamic microenvironment for IgA plasma cells

To achieve longevity, IgA plasma cells require a sophisticated anatomical microenvironment that provides cytokines, cell-cell contacts, and nutrients as well as metabolites. The intestinal epithelium harbors cells with distinct functions and represents an important defense line. Anti-microbial peptide-producing paneth cells, mucus-secreting goblet cells and antigen-transporting microfold (M) cells cooperate to build a protective barrier against pathogens. In addition, intestinal epithelial cells are instrumental in the transcytosis of IgA to the gut lumen, and support plasma cell survival by producing the cytokines APRIL and BAFF. Moreover, nutrients are sensed through specialized receptors such as the aryl hydrocarbon receptor (AhR) by both, intestinal epithelial cells and immune cells. However, the intestinal epithelium is highly dynamic with a high cellular turn-over rate and exposure to changing microbiota and nutritional factors. In this review, we discuss the spatial interplay of the intestinal epithelium with plasma cells and its potential contribution to IgA plasma cell generation, homing, and longevity. Moreover, we describe the impact of nutritional AhR ligands on intestinal epithelial cell-IgA plasma cell interaction. Finally, we introduce spatial transcriptomics as a new technology to address open questions in intestinal IgA plasma cell biology.

Immune Responses to IAV Infection and the Roles of L-Selectin and ADAM17 in Lymphocyte Homing

Influenza A virus (IAV) infection is a global public health burden causing up to 650,000 deaths per year. Yearly vaccination programmes and anti-viral drugs currently have limited benefits; therefore, research into IAV is fundamental. Leukocyte trafficking is a crucial process which orchestrates the immune response to infection to protect the host. It involves several homing molecules and receptors on both blood vessels and leukocytes. A key mediator of this process is the transmembrane glycoprotein L-selectin, which binds to vascular addressins on blood vessel endothelial cells. L-selectin classically mediates homing of naïve and central memory lymphocytes to lymph nodes via high endothelial venules (HEVs). Recent studies have found that L-selectin is essential for homing of activated CD8⁺ T cells to influenza-infected lungs and reduction in virus load. A disintegrin and metalloproteinase 17 (ADAM17) is the primary regulator of cell surface levels of L-selectin. Understanding the mechanisms that regulate these two proteins are central to comprehending recruitment of T cells to sites of IAV infection. This review summarises the immune response to IAV infection in humans and mice and discusses the roles of L-selectin and ADAM17 in T lymphocyte homing during IAV infection.

Nasal vaccination of β7 integrin-deficient mice retains elevated IgA immunity

Understanding the migration of lymphocytes to nonintestinal mucosal sites is fundamental to developing mucosal vaccination strategies. Studies have shown that nasal and oral immunization with cholera toxin (CT) stimulates, in addition to α4β7⁺ , the induction of αE (CD103)β7⁺ B cells. To determine the extent to which αE-associated β7 contributes to antigen (Ag)-specific immunoglobulin (Ig)A responses in the upper respiratory tract, nasal CT vaccination was performed in wild-type (wt) and β7^-/- mice. At 16 days postprimary immunization, upper respiratory tract IgA responses were greater in β7^-/- mice than in wt mice. IgA induction by distal β7^-/- Peyer's patches, mesenteric lymph nodes and small intestinal lamina propria was minimal, in contrast to elevated gut IgA responses in wt mice. By 42 days postprimary immunization, β7^-/- gut IgA responses were restored, and upper respiratory tract Ag-specific IgA responses were equivalent to those of wt mice. Examination of homing receptor expression and cell-sorting experiments revealed that β7^-/- mice have increased usage of β1 and αE integrins by upper respiratory tract B cells, suggesting that alternative integrins can facilitate lymphocyte migration to the upper respiratory tract, especially in the absence of β7.

L-Selectin/CD62L is a Key Driver of Non-Alcoholic Steatohepatitis in Mice and Men

CD62L (L-Selectin) dependent lymphocyte infiltration is known to induce inflammatory bowel disease (IBD), while its function in the liver, especially in non-alcoholic steatohepatitis (NASH), remains unclear. We here investigated the functional role of CD62L in NASH in humans as well as in two mouse models of steatohepatitis. Hepatic expression of a soluble form of CD62L (sCD62L) was measured in patients with steatosis and NASH. Furthermore, CD62L^−/− mice were fed with a methionine and choline deficient (MCD) diet for 4 weeks or with a high fat diet (HFD) for 24 weeks. Patients with NASH displayed increased serum levels of sCD62L. Hepatic CD62L expression was higher in patients with steatosis and increased dramatically in NASH patients. Interestingly, compared to wild type (WT) mice, MCD and HFD-treated CD62L^−/− mice were protected from diet-induced steatohepatitis. This was reflected by less fat accumulation in hepatocytes and a dampened manifestation of the metabolic syndrome with an improved insulin resistance and decreased cholesterol and triglyceride levels. Consistent with ameliorated disease, CD62L^−/− animals exhibited an enhanced hepatic infiltration of Treg cells and a strong activation of an anti-oxidative stress response. Those changes finally resulted in less fibrosis in CD62L^−/− mice. Additionally, this effect could be reproduced in a therapeutic setting by administrating an anti-CD62L blocking antibody. CD62L expression in humans and mice correlates with disease activity of steatohepatitis. CD62L knockout and anti-CD62L-treated mice are protected from diet-induced steatohepatitis suggesting that CD62L is a promising target for therapeutic interventions in NASH.

Vedolizumab is associated with changes in innate rather than adaptive immunity in patients with inflammatory bowel disease

OBJECTIVE

Vedolizumab, a monoclonal antibody directed against the integrin heterodimer α4β7, is approved for the treatment of Crohn's disease and ulcerative colitis. The efficacy of vedolizumab has been suggested to result from inhibition of intestinal T cell trafficking although human data to support this conclusion are scarce. We therefore performed a comprehensive analysis of vedolizumab-induced alterations in mucosal and systemic immunity in patients with inflammatory bowel disease (IBD), using anti-inflammatory therapy with the TNFα antibody infliximab as control.

DESIGN

Immunophenotyping, immunohistochemistry, T cell receptor profiling and RNA sequencing were performed using blood and colonic biopsies from patients with IBD before and during treatment with vedolizumab (n=18) or, as control, the anti-TNFα antibody infliximab (n=20). Leucocyte trafficking in vivo was assessed using single photon emission computed tomography and endomicroscopy.

RESULTS

Vedolizumab was not associated with alterations in the abundance or phenotype of lamina propria T cells and did not affect the mucosal T cell repertoire or leucocyte trafficking in vivo. Surprisingly, however, α4β7 antibody treatment was associated with substantial effects on innate immunity including changes in macrophage populations and pronounced alterations in the expression of molecules involved in microbial sensing, chemoattraction and regulation of the innate effector response. These effects were specific to vedolizumab, not observed in response to the TNFα antibody infliximab, and associated with inhibition of intestinal inflammation.

CONCLUSION

Our findings suggest that modulation of innate immunity contributes to the therapeutic efficacy of vedolizumab in IBD.

TRIAL REGISTRATION NUMBER

NCT02694588.

β7-Integrin and MAdCAM-1 play opposing roles during the development of non-alcoholic steatohepatitis

BACKGROUND & AIMS

Non-alcoholic steatohepatitis (NASH) is a leading cause of chronic liver disease in Western countries. It is unclear how infiltrating leukocytes affect NASH-development. Our study aims to investigate the role of the homing/receptor, pair mucosal addressin cell adhesion molecule-1 (MAdCAM-1)/β₇-Integrin, on immune cell recruitment and disease progression in a steatohepatitis model.

METHODS

Constitutive β₇-Integrin deficient (β₇^-/-) and MAdCAM-1 deficient (MAdCAM-1^-/-) mice were fed a high fat diet (HFD) for 26weeks or methionine-choline-deficient-diet (MCD) for 4weeks.

RESULTS

β₇^-/- mice displayed earlier and more progressive steatohepatitis during HFD- and MCD-treatment, while MAdCAM-1^-/- mice showed less histomorphological changes. The anti-oxidative stress response was significantly weaker in β₇^-/- mice as reflected by a significant downregulation of the transcription factors nuclear-factor(erythroid-derived 2)-like 2 (Nrf2) and heme-oxigenase-1 (HO-1). Additionally, stronger dihydroethidium-staining revealed an increased oxidative stress response in β₇^-/- animals. In contrast, MAdCAM-1^-/- mice showed an upregulation of the anti-oxidative stress response. β₇^-/- animals exhibited stronger hepatic infiltration of inflammatory cells, especially neutrophils, reflecting earlier steatohepatitis initiation. Expression of regulatory T cell (T_Reg) markers as well as numbers of anti-inflammatory macrophages was significantly enhanced in MAdCAM-1^-/- mice. Those changes finally resulted in earlier and stronger collagen accumulation in β₇^-/- mice, whereas MAdCAM-1^-/- mice were protected from fibrosis initiation.

CONCLUSIONS

Adhesion molecule mediated effector cell migration contributes to the outcome of steatohepatitis in the HFD- and the MCD model. While MAdCAM-1 promotes steatohepatitis, β₇-Integrin unexpectedly exerts protective effects. β₇^-/- mice show earlier steatohepatitis initiation and significantly stronger fibrosis progression. Accordingly, the interaction of β₇-Integrins and their receptor MAdCAM-1 provide novel targets for therapeutic interventions in steatohepatitis.

LAY SUMMARY

The mucosal addressin cell adhesion molecule 1 (MAdCAM-1) is expressed in livers upon diet-induced non-alcoholic steatohepatitis (NASH). Loss of MAdCAM-1 has beneficial effects regarding the development of NASH - manifested by reduced hepatic oxidative stress and decreased inflammation. In contrast, β₇-Integrin-deficiency results in increased steatohepatitis.

The genetic background of inflammatory bowel disease: from correlation to causality

Recent studies have greatly improved our insight into the genetic background of inflammatory bowel disease (IBD). New high-throughput technologies and large-scale international collaborations have contributed to the identification of 200 independent genetic risk loci for IBD. However, in most of these loci, it is unclear which gene conveys the risk for IBD. More importantly, it is unclear which variant within or near the gene is causal to the disease. Using targeted GWAS, imputation, resequencing of risk loci, and in silico fine-mapping of densely typed loci, several causal variants have been identified in IBD risk genes, and various pathological pathways have been uncovered. Current research in the field of IBD focuses on the effect of these causal variants on gene expression and protein function. However, more elements than only the genome must be taken into account to disentangle the multifactorial pathology of IBD. The genetic risk loci identified to date only explain a small part of genetic variance in disease risk. Currently, large multi-omics studies are incorporating factors ranging from the gut microbiome to the environment. In this review, we present the progress that has been made in IBD genetic research and stress the importance of studying causality to increase our understanding of the pathogenesis of IBD. We highlight important causal genetic variants in the candidate genes NOD2, ATG16L1, IRGM, IL23R, CARD9, RNF186, and PRDM1. We describe their downstream effects on protein function and their direct effects on the gut immune system. Furthermore, we discuss the future role of genetics in unravelling disease mechanisms in IBD. Copyright © 2016 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

Mechanisms of T cell organotropism

Protective immunity relies upon T cell differentiation and subsequent migration to target tissues. Similarly, immune homeostasis requires the localization of regulatory T cells (Tregs) to the sites where immunity takes place. While naïve T lymphocytes recirculate predominantly in secondary lymphoid tissue, primed T cells and activated Tregs must traffic to the antigen rich non-lymphoid tissue to exert effector and regulatory responses, respectively. Following priming in draining lymph nodes, T cells acquire the 'homing receptors' to facilitate their access to specific tissues and organs. An additional level of topographic specificity is provided by T cells receptor recognition of antigen displayed by the endothelium. Furthermore, co-stimulatory signals (such as those induced by CD28) have been shown not only to regulate T cell activation and differentiation, but also to orchestrate the anatomy of the ensuing T cell response. We here review the molecular mechanisms supporting trafficking of both effector and regulatory T cells to specific antigen-rich tissues.

Integrins as Therapeutic Targets for Respiratory Diseases

Integrins are a large family of transmembrane heterodimeric proteins that constitute the main receptors for extracellular matrix components. Integrins were initially thought to be primarily involved in the maintenance of cell adhesion and tissue integrity. However, it is now appreciated that integrins play important roles in many other biological processes such as cell survival, proliferation, differentiation, migration, cell shape and polarity. Lung cells express numerous combinations and permutations of integrin heterodimers. The complexity and diversity of different integrin heterodimers being implicated in different lung diseases present a major challenge for drug development. Here we provide a comprehensive overview of the current knowledge of integrins from studies in cell culture to integrin knockout mouse models and provide an update of results from clinical trials for which integrins are therapeutic targets with a focus on respiratory diseases (asthma, emphysema, pneumonia, lung cancer, pulmonary fibrosis and sarcoidosis).

Absence of Nkx2-3 homeodomain transcription factor reprograms the endothelial addressin preference for lymphocyte homing in Peyer's patches

Although the homing of lymphocytes to GALT has been extensively studied, little is known about how high endothelial venules (HEVs) within Peyer's patches (PPs) are patterned to display dominantly mucosal addressin cell adhesion molecule 1 (MAdCAM-1). In this study, we report that Nkx2-3-deficient mice show gradual loss of MAdCAM-1 in PPs postnatally and increased levels of mRNA for peripheral lymph node addressin (PNAd) backbone proteins as well as enhanced expression of MECA79 sulfated glycoepitope at the luminal aspect of HEVs, thus replacing MAdCAM-1 with PNAd. Induction of PNAd in mutant PPs requires lymphotoxin β receptor activity, and its upregulation needs the presence of mature T and B cells. Furthermore, treatment with MECA-79 anti-PNAd mAb in vivo effectively blocks lymphocyte homing to mutant PPs. Despite the replacement of MAdCAM-1 by PNAd in HEV endothelia, lymphocytes could efficiently home to PPs in mutant mice. We conclude that although Nkx2-3 activity controls the addressin balance of HEVs in GALT, the general HEV functionality is preserved independently from Nkx2-3, indicating a substantial plasticity in the specification of GALT HEV endothelium.

Leukocyte β7 integrin targeted by Krüppel-like factors

Constitutive expression of Krüppel-like factor 3 (KLF3, BKLF) increases marginal zone (MZ) B cell numbers, a phenotype shared with mice lacking KLF2. Ablation of KLF3, known to interact with serum response factor (SRF), or SRF itself, results in fewer MZ B cells. It is unknown how these functional equivalences result. In this study, it is shown that KLF3 acts as transcriptional repressor for the leukocyte-specific integrin β7 (Itgb7, Ly69) by binding to the β7 promoter, as revealed by chromatin immunoprecipitation. KLF2 overexpression antagonizes this repression and also binds the β7 promoter, indicating that these factors may compete for target sequence(s). Whereas β7 is identified as direct KLF target, its repression by KLF3 is not connected to the MZ B cell increase because β7-deficient mice have a normal complement of these and the KLF3-driven increase still occurs when β7 is deleted. Despite this, KLF3 overexpression abolishes lymphocyte homing to Peyer's patches, much like β7 deficiency does. Furthermore, KLF3 expression alone overcomes the MZ B cell deficiency when SRF is absent. SRF is also dispensable for the KLF3-mediated repression of β7. Thus, despite the shared phenotype of KLF3 and SRF-deficient mice, cooperation of these factors appears neither relevant for the formation of MZ B cells nor for the regulation of β7. Finally, a potent negative regulatory feedback loop limiting KLF3 expression is shown in this study, mediated by KLF3 directly repressing its own gene promoter. In summary, KLFs use regulatory circuits to steer lymphocyte maturation and homing and directly control leukocyte integrin expression.

T-bet and Eomes instruct the development of two distinct natural killer cell lineages in the liver and in the bone marrow

Trail(+)DX5(-)Eomes(-) natural killer (NK) cells arise in the mouse fetal liver and persist in the adult liver. Their relationships with Trail(-)DX5(+) NK cells remain controversial. We generated a novel Eomes-GFP reporter murine model to address this question. We found that Eomes(-) NK cells are not precursors of classical Eomes(+) NK cells but rather constitute a distinct lineage of innate lymphoid cells. Eomes(-) NK cells are strictly dependent on both T-bet and IL-15, similarly to NKT cells. We observed that, in the liver, expression of T-bet in progenitors represses Eomes expression and the development of Eomes(+) NK cells. Reciprocally, the bone marrow (BM) microenvironment restricts T-bet expression in developing NK cells. Ectopic expression of T-bet forces the development of Eomes(-) NK cells, demonstrating that repression of T-bet is essential for the development of Eomes(+) NK cells. Gene profile analyses show that Eomes(-) NK cells share part of their transcriptional program with NKT cells, including genes involved in liver homing and NK cell receptors. Moreover, Eomes(-) NK cells produce a broad range of cytokines, including IL-2 and TNF in vitro and in vivo, during immune responses against vaccinia virus. Thus, mutually exclusive expression of T-bet and Eomes drives the development of different NK cell lineages with complementary functions.

Roles of T cell-associated L-selectin and β7 integrins during induction and regulation of chronic colitis

BACKGROUND

L-selectin (CD62L) and β(7) integrins are important for trafficking of naive T cells under steady-state conditions. The objectives of this study were to dissect the requirements for T cell-associated CD62L and β(7) integrins during initiation, progression, and regulation of chronic colitis.

METHODS

Using the T-cell transfer model, we compared colitogenic potential between T cells lacking one or both of these molecules with wild-type T cells. To assess trafficking of cells to the secondary lymphoid tissue and the gut, we performed co-homing experiments.

RESULTS

Adoptive transfer of wild-type, CD62L(-/-) or β(7)(-/-) single-deficient T cells induced moderate to severe disease with slightly different kinetics. However, transfer of CD62L(-/-) β(7)(-/-) double-deficient (DKO) T cells produced significantly attenuated gut inflammation, which correlated with fewer T cells and reduced levels of proinflammatory cytokines in the colon lamina propria. Our subsequent experiments established that lack of colitogenic potential of these cells was due to inability of DKO T cells to home to the secondary lymphoid tissue. Furthermore, homing of in vitro-generated effector DKO T cells to the inflamed intestine was significantly impaired. Lastly, DKO regulatory T cells were ineffective at suppressing colitis induced by wild-type T cells.

CONCLUSIONS

We established that T cells can use either CD62L(-/-) or β(7)(-/-) integrins to induce chronic colitis, but lack of both abrogates their colitogenic potential. Effector T cells critically rely on β(7) integrin during their recruitment to the inflamed intestinal mucosa. Finally, regulation of intestinal inflammation by regulatory T cells requires one or both of these adhesion molecules.

Peripheral tissue homing receptor control of naïve, effector, and memory CD8 T cell localization in lymphoid and non-lymphoid tissues

T cell activation induces homing receptors that bind ligands on peripheral tissue vasculature, programing movement to sites of infection and injury. There are three major types of CD8 effector T cells based on homing receptor expression, which arise in distinct lymphoid organs. Recent publications indicate that naïve, effector, and memory T cell migration is more complex than once thought; while many effectors enter peripheral tissues, some re-enter lymph nodes (LN), and contain central memory precursors. LN re-entry can depend on CD62L or peripheral tissue homing receptors. Memory T cells in LN tend to express the same homing receptors as their forebears, but often are CD62Lneg. Homing receptors also control CD8 T cell tumor entry. Tumor vasculature has low levels of many peripheral tissue homing receptor ligands, but portions of it resemble high endothelial venules (HEV), enabling naïve T cell entry, activation, and subsequent effector activity. This vasculature is associated with positive prognoses in humans, suggesting it may sustain ongoing anti-tumor responses. These findings reveal new roles for homing receptors expressed by naïve, effector, and memory CD8 T cells in controlling entry into lymphoid and non-lymphoid tissues.

Association between variants of PRDM1 and NDP52 and Crohn's disease, based on exome sequencing and functional studies

BACKGROUND & AIMS

Genome-wide association studies (GWAS) have identified 140 Crohn's disease (CD) susceptibility loci. For most loci, the variants that cause disease are not known and the genes affected by these variants have not been identified. We aimed to identify variants that cause CD through detailed sequencing, genetic association, expression, and functional studies.

METHODS

We sequenced whole exomes of 42 unrelated subjects with CD and 5 healthy subjects (controls) and then filtered single nucleotide variants by incorporating association results from meta-analyses of CD GWAS and in silico mutation effect prediction algorithms. We then genotyped 9348 subjects with CD, 2868 subjects with ulcerative colitis, and 14,567 control subjects and associated variants analyzed in functional studies using materials from subjects and controls and in vitro model systems.

RESULTS

We identified rare missense mutations in PR domain-containing 1 (PRDM1) and associated these with CD. These mutations increased proliferation of T cells and secretion of cytokines on activation and increased expression of the adhesion molecule L-selectin. A common CD risk allele, identified in GWAS, correlated with reduced expression of PRDM1 in ileal biopsy specimens and peripheral blood mononuclear cells (combined P = 1.6 × 10(-8)). We identified an association between CD and a common missense variant, Val248Ala, in nuclear domain 10 protein 52 (NDP52) (P = 4.83 × 10(-9)). We found that this variant impairs the regulatory functions of NDP52 to inhibit nuclear factor κB activation of genes that regulate inflammation and affect the stability of proteins in Toll-like receptor pathways.

CONCLUSIONS

We have extended the results of GWAS and provide evidence that variants in PRDM1 and NDP52 determine susceptibility to CD. PRDM1 maps adjacent to a CD interval identified in GWAS and encodes a transcription factor expressed by T and B cells. NDP52 is an adaptor protein that functions in selective autophagy of intracellular bacteria and signaling molecules, supporting the role of autophagy in the pathogenesis of CD.

Virus-associated activation of innate immunity induces rapid disruption of Peyer's patches in mice

Early in the course of infection, detection of pathogen-associated molecular patterns by innate immune receptors can shape the subsequent adaptive immune response. Here we investigate the influence of virus-associated innate immune activation on lymphocyte distribution in secondary lymphoid organs. We show for the first time that virus infection of mice induces rapid disruption of the Peyer's patches but not of other secondary lymphoid organs. The observed effect was not dependent on an active infectious process, but due to innate immune activation and could be mimicked by virus-associated molecular patterns such as the synthetic double-stranded RNA poly(I:C). Profound histomorphologic changes in Peyer's patches were associated with depletion of organ cellularity, most prominent among the B-cell subset. We demonstrate that the disruption is entirely dependent on type I interferon (IFN). At the cellular level, we show that virus-associated immune activation by IFN-α blocks B-cell trafficking to the Peyer's patches by downregulating expression of the homing molecule α4β7-integrin. In summary, our data identify a mechanism that results in type I IFN-dependent rapid but reversible disruption of intestinal lymphoid organs during systemic viral immune activation. We propose that such rerouted lymphocyte trafficking may impact the development of B-cell immunity to systemic viral pathogens.

Signaling through L-selectin mediates enhanced chemotaxis of lymphocyte subsets to secondary lymphoid tissue chemokine

L-selectin functions as an important adhesion molecule that mediates tethering and rolling of lymphocytes by binding to high endothelial venule (HEV)-expressed ligands during recirculation. Subsequent lymphocyte arrest and transmigration require activation through binding of HEV-decorated homeostatic chemokines such as secondary lymphoid tissue chemokine (SLC; CCL21) to its counterreceptor, CCR7. Importantly, L-selectin also functions as a signaling molecule. In this study, signaling induced by ligation of L-selectin using mAb or endothelial cell-expressed ligand significantly enhanced the chemotaxis of murine T cells and B cells to SLC but not to other homeostatic chemokines. Consistent with the expression levels of L-selectin in different lymphocyte subsets, L-selectin-mediated enhancement of chemotaxis to SLC was observed for all naive lymphocytes and effector/memory CD8(+) T cells, whereas only a subpopulation of effector/memory CD4(+) T cells responded. During in vivo mesenteric lymph node migration assays, the absence of L-selectin on lymphocytes significantly attenuated both their ability to migrate out of the HEV and their chemotaxis away from the vessel wall. Notably, ligation of L-selectin and/or CCR7 did not result in increased CCR7 expression levels, internalization, or re-expression. Pharmacologic inhibitor studies showed that L-selectin-mediated enhanced chemotaxis to SLC required intact intracellular kinase function. Furthermore, treatment of lymphocytes with the spleen tyrosine kinase family inhibitor piceatannol reduced their ability to migrate across the HEV in peripheral lymph nodes. Therefore, these results suggest that "cross-talk" in the signaling pathways initiated by L-selectin and CCR7 provides a novel mechanism for functional synergy between these two molecules during lymphocyte migration.

Distribution of primed T cells and antigen-loaded antigen presenting cells following intranasal immunization in mice

Priming of T cells is a key event in vaccination, since it bears a decisive influence on the type and magnitude of the immune response. T-cell priming after mucosal immunization via the nasal route was studied by investigating the distribution of antigen-loaded antigen presenting cells (APCs) and primed antigen-specific T cells. Nasal immunization studies were conducted using the model protein antigen ovalbumin (OVA) plus CpG oligodeoxynucleotide adjuvant. Trafficking of antigen-specific primed T cells was analyzed in vivo after adoptive transfer of OVA-specific transgenic T cells in the presence or absence of fingolimod, a drug that causes lymphocytes sequestration within lymph nodes. Antigen-loaded APCs were observed in mediastinal lymph nodes, draining the respiratory tract, but not in distal lymph nodes. Antigen-specific proliferating T cells were first observed within draining lymph nodes, and later in distal iliac and mesenteric lymph nodes and in the spleen. The presence at distal sites was due to migration of locally primed T cells as shown by fingolimod treatment that caused a drastic reduction of proliferated T cells in non-draining lymph nodes and an accumulation of extensively divided T cells within draining lymph nodes. Homing of nasally primed T cells in distal iliac lymph nodes was CD62L-dependent, while entry into mesenteric lymph nodes depended on both CD62L and α4β7, as shown by in vivo antibody-mediated inhibition of T-cell trafficking. These data, elucidating the trafficking of antigen-specific primed T cells to non-draining peripheral and mucosa-associated lymph nodes following nasal immunization, provide relevant insights for the design of vaccination strategies based on mucosal priming.

B cell homeostasis and plasma cell homing controlled by Krüppel-like factor 2

Krüppel-like factor 2 (KLF2) controls T lymphocyte egress from lymphoid organs by regulating sphingosin-1 phosphate receptor 1 (S1Pr1). Here we show that this is not the case for B cells. Instead, KLF2 controls homeostasis of B cells in peripheral lymphatic organs and homing of plasma cells to the bone marrow, presumably by controlling the expression of β(7)-integrin. In mice with a B cell-specific deletion of KLF2, S1Pr1 expression on B cells was only slightly affected. Accordingly, all splenic B cell subsets including B1 cells were present, but their numbers were increased with a clear bias for marginal zone (MZ) B cells. In contrast, fewer peyers patches harboring fewer B cells were found, and fewer B1 cells in the peritoneal cavity as well as recirculating B cells in the bone marrow were detected. Upon thymus-dependent immunization, IgG titers were diminished, and antigen-specific plasma cells were absent in the bone marrow, although numbers of antigen-specific splenic plasmablasts were normal. KLF2 plays also a role in determining the identity of follicular B cells, as KLF2-deficient follicular B cells showed calcium responses similar to those of MZ B cells and failed to down-regulate MZ B cell signature genes, such as CD21 and CXCR7.

Beta7 integrin deficiency suppresses B cell homing and attenuates chronic ileitis in SAMP1/YitFc mice

Lymphocyte recruitment to intestinal tissues depends on β(7) integrins. In this study, we studied disease severity and lymphocyte recruitment into the small intestine in SAMP1/YitFc mice, which develop chronic ileitis with similarity to human Crohn's disease. To assess the role of β(7) integrins in chronic ileitis, we generated SAMP1/YitFc lacking β(7) integrins (SAMP1/YitFc Itgb7(-/-)) using a congenic strain developed via marker-assisted selection. We analyzed ileal inflammation in SAMP1/YitFc and SAMP1/YitFc Itgb7(-/-) mice by histopathology and the distribution of T and B lymphocytes in the mesenteric lymph nodes (MLNs) by flow cytometry. Short-term (18 h) adoptive transfer experiments were used to study the in vivo homing capacity of T and B lymphocytes. In both young (<20 wk) and old (20-50 wk) SAMP1/YitFc Itgb7(-/-) mice, ileitis was reduced by 30-50% compared with SAMP1/YitFc mice. SAMP1/YitFc Itgb7(-/-) mice showed a dramatic 67% reduction in the size of their MLNs, which was caused by a 85% reduction in lymphocyte numbers and reduced short-term B cell homing. Flow cytometric analysis revealed a highly significant decrease in the percentage of B cells in MLNs of SAMP1/YitFc Itgb7(-/-) mice. Cotransfer of SAMP1/YitFc MLN B cells but not SAMP1/YitFc Itgb7(-/-) MLN B cells along with CD4(+) T cells resulted in exacerbated ileitis severity in SCID mice. Our findings suggest that β(7) integrins play an essential role in spontaneous chronic ileitis in vivo by promoting homing of disease-exacerbating B cells to MLNs and other intestinal tissues.

Mucosal addressin cell-adhesion molecule-1 controls plasma-cell migration and function in the small intestine of mice

BACKGROUND & AIMS

Immunoglobulin (Ig) A secretion into the intestinal lumen is an important immune mechanism of the gastrointestinal (GI) tract. B cells proliferate and differentiate into IgA-secreting plasma cells (PC) within lymphoid organs then migrate directly into the intestinal lamina propria. We aimed to elucidate the in vivo role of the mucosal addressin cell-adhesion molecule-1 (MAdCAM-1), which is constitutively expressed in the GI-associated lymphoid tissue, in B-cell migration.

METHODS

We generated MAdCAM-1-deficient mice (MAdCAM(Delta)) and evaluated the B-cell compartment of the GI-associated lymphoid tissue. We also assessed PC migration to the small intestine and the intestinal immune response after oral immunization.

RESULTS

In MAdCAM(Delta) mice, the size of Peyer's patches was drastically reduced, compared with that of wild-type mice; this difference was detectable by 3 days after birth, indicating that MAdCAM-1 is dispensable for embryonic Peyer's patch development but mediates recruitment of lymphocytes into this lymphoid organ at later stages. Moreover, antigen-specific, IgA-positive PC were severely compromised in their migration to the small intestine; accordingly, there was a reduced number of IgA-secreting PC in the lamina propria of the small intestine. The MAdCAM(Delta) mice were unable to mount a normal intestinal IgA response after oral immunization with cholera toxin.

CONCLUSION

These data provide in vivo evidence that MAdCAM-1 is required for the localization and function of IgA-secreting PC in the intestine.

Identification of two major types of age-associated CD8 clonal expansions with highly divergent properties

CD8 memory T cells are tightly regulated in young, healthy individuals but are often perturbed in aged animals by the appearance of large CD8 T cell clones. These clones are associated with impaired immunity in the aged. The molecular basis of this phenomenon remains unclear. Here, it is shown that the issue is confused by the fact that the clones are heterogeneous. Some clones bear high, and others, low levels of integrin alpha(4) (itgalpha4). These subtypes differ by multiple criteria. They appear in mice of different ages, concentrate in different tissues, and have different stabilities in vivo and responses to stimulation in vitro. itgalpha4(high), but not itgalpha4(low), CD8 clonal expansions have several characteristics consistent with a chronically stimulated phenotype. These properties include lowered levels of CD8, decreased expression of some cytokine receptors, and elevated expression of various inhibitory receptors, including the programmed death-1 (PD1) receptor and the killer cell lectin-like receptor G1 (KLRG1). The characteristics of itgalpha4(high) clonal expansions suggest that they may arise from age-dependent alterations in antigen expression and tolerance. These data redefine CD8 clonal expansions into at least two distinct entities and indicate that there are multiple mechanisms that drive age-related alterations of CD8 T cell homeostasis.

Activated CD8 T cells redistribute to antigen-free lymph nodes and exhibit effector and memory characteristics

Exogenous dendritic cells display restricted trafficking when injected in vivo and stimulate CD8 T cell responses that are localized to a small number of lymphoid compartments. By examining these responses in the presence and absence of FTY720, a drug that causes sequestration of T cells in lymph nodes, we demonstrate that a significant fraction of divided CD8 T cells redistribute into Ag-free lymph nodes within 3 days of activation. Despite variation in the level of expression of CD62L, redistribution of these cells is CD62L-dependent. Redistributed CD8 T cells exhibit characteristics of differentiated effectors. However, when re-isolated from Ag-free lymph nodes 3 days after activation and transferred into naive mice, they persist for at least 3 wk and expand upon Ag challenge. Thus, CD8 T cells that redistribute to Ag-free lymph nodes 3 days after immunization contain memory precursors. We suggest that this redistribution process represents an important mechanism for establishment of lymph node resident central memory, and that redistribution to Ag-free nodes is an additional characteristic to be added to those that distinguish memory precursors from terminal effectors.

The absence of lymphoid CD8+ dendritic cell maturation in L-selectin-/- respiratory compartment attenuates antiviral immunity

Intratracheal instillation of L-selectin-deficient (L-Sel(-/-)) mice with an adenovirus 2 (Ad2) vector resulted in the lack of respiratory Ad2- or beta-galactosidase-specific CTLs with concomitant long-lived beta-galactosidase transgene expression in the lungs. The absence of Ag-specific CTLs was attributed to a deficiency in lymphoid CD11c(+)CD8(+) dendritic cells (DCs) in the lower respiratory lymph nodes (LRLNs). To enable L-Sel(-/-) CTL activity, cell-sorted L-Sel(-/-)CD8(+) T cells were cocultured with cell-sorted L-Sel(+/+)CD8(+) or CD8(-) DCs or L-Sel(-/-)CD8(-) DCs. Only the CD8(+) DCs restored CTL activity; L-Sel(-/-)CD8(-) DCs failed to support L-Sel(+/+) CTLs because these remained immature, lacking the ability to express costimulatory molecules CD40, CD80, or CD86. Although no lung CD8(+) DCs were detected, the DC environment remained suppressive in L-Sel(-/-) mice evident by the lack of CTL responses following adenoviral challenge with OVA in recipient L-Sel(-/-) adoptively transferred with OT-1 CD8(+) T cells. To assess whether the L-Sel(-/-)CD8(-) DCs could be induced into maturity, microbial stimulation studies were performed showing the failure of L-Sel(-/-) LRLN to make matured DCs. When L-Sel(-/-) mice were subjected in vivo to microbial activation before Ad2 vector dosing, CTL activity was restored stimulating the renewed presence of LRLN CD8(+) DCs in L-Sel(-/-) mice. These studies show that impairment of L-Sel(-/-) DC maturation results in insufficient mature DCs that require microbial activation to restore increases in respiratory CD8(+) DCs to support CTL responses.

T cell-associated CD18 but not CD62L, ICAM-1, or PSGL-1 is required for the induction of chronic colitis

The induction and perpetuation of chronic colitis are thought to involve a complex set of adhesive interactions between T cells and endothelial cells located on the vasculature within secondary lymphoid tissue and the intestine. The objective of this study was to assess the roles of T cell-associated CD18, CD62L (L-selectin), ICAM-1, and P-selectin glycoprotein ligand-1 (PSGL-1) in the induction of chronic colitis in mice. CD4(+)CD25(-) T cells derived from either wild-type (WT), CD18-deficient [CD18 knockout (KO)], CD62L KO, ICAM-1 KO, or PSGL-1 KO mice were adoptively transferred into recombinase activating gene-1 (RAG-1)-deficient mice (RAG KO mice) to assess the potential of these T cells to induce chronic colitis. At 8-10 wk following T cell transfer, we observed moderate to severe colitis as assessed by increases in colon weight-to-length ratios and by blinded histopathological analysis. In contrast, we found that transfer of CD18 KO T cells into RAG KO recipients resulted in the significant attenuation of colonic inflammation in these mice. Furthermore, we observed fewer infiltrating CD4(+) T cells in the colonic lamina propria in the CD18 KO-->RAG KO group compared with the WT-->RAG KO group. Finally, message levels of colonic TNF-alpha, IL-1beta, and IFN-gamma were significantly reduced in CD18 KO-->RAG KO mice compared with colitic control animals. We conclude that T cell-associated CD18, but not CD62L, ICAM-1, or PSGL-1, is required for the development of chronic colitis.

Kruppel-like factor 2 regulates thymocyte and T-cell migration

Mammalian Kruppel-like transcription factors are implicated in regulating terminal differentiation of several tissue types. Deficiency in Kruppel-like factor (KLF) 2 (also known as LKLF) leads to a massive loss of the peripheral T-cell pool, suggesting KLF2 regulates T-cell quiescence and survival. Here we show, however, that KLF2 is essential for T-cell trafficking. KLF2-deficient (Klf2-/-) thymocytes show impaired expression of several receptors required for thymocyte emigration and peripheral trafficking, including the sphingosine-1-phosphate (S1P) receptor S1P1, CD62L and beta7 integrin. Furthermore, KLF2 both binds and transactivates the promoter for S1P1--a receptor that is critical for thymocyte egress and recirculation through peripheral lymphoid organs. Our findings suggest that KLF2 serves to license mature T cells for trafficking from the thymus and recirculation through secondary lymphoid tissues.

Sugar-coated regulation of T cells

The human gut is host to hundreds of different species of commensal bacteria that live in peaceful partnership with the host immune system. These commensal bacteria are far from neutral bystanders as they are intimately involved in the development of the immune system. Reporting in this issue of Cell, Kasper and colleagues (Mazmanian et al., 2005) reveal that a bacterial polysaccharide, PSA, produced by the commensal bacterium Bacteroides fragilis directs development of the immune system of the mouse host.

L-selectin and beta7 integrin on donor CD4 T cells are required for the early migration to host mesenteric lymph nodes and acute colitis of graft-versus-host disease

The homing receptors L-selectin and alpha4beta7 integrin facilitate entry of T cells into the gut-associated organized lymphoid tissues such as the mesenteric lymph nodes and Peyer patches. We studied the impact of inactivation of genes encoding these receptors on the ability of purified donor CD4+ T cells to induce acute lethal graft-versus-host disease (GVHD) associated with severe colitis in irradiated major histocompatibility complex (MHC)-mismatched mice. Whereas lack of expression of a single receptor had no significant impact on the severity of colitis and GVHD, the lack of expression of both receptors markedly ameliorated colitis and early deaths observed with wild-type (WT) T cells. The changes in colitis and GVHD were reflected in a marked reduction in the early accumulation of donor T cells in the mesenteric lymph nodes and subsequently in the colon. The purified WT donor CD4+ T cells did not accumulate early in the Peyer patches and failed to induce acute injury to the small intestine. In conclusion, the combination of CD62L and beta7 integrin is required to induce acute colitis and facilitate entry of CD4+ donor T cells in the mesenteric nodes associated with lethal GVHD in allogeneic hosts.

L-Selectin, α4β1, and α4β7 Integrins Participate in CD4+ T Cell Recruitment to Chronically Inflamed Small Intestine

CD4+ T cells are essential for development and perpetuation of Crohn's disease, a chronic immune-mediated condition that affects primarily the small intestine. Using novel models of Crohn's disease-like ileitis (i.e., SAMP1/YitFc and CD4+ T cell transfer models), we have begun to understand the adhesive pathways that mediate lymphocyte trafficking to the chronically inflamed small bowel. Expansion of the CD4/beta7+ population and increased mucosal addressin cell adhesion molecule-1 (MAdCAM-1) expression were observed within the intestinal lamina propria with disease progression. However, Ab blockade of the beta7 integrin, the alpha4beta7 heterodimer, MAdCAM-1, or L-selectin did not attenuate inflammation. Blockade of two pathways (L-selectin and MAdCAM-1 or alpha4 integrins) was required to improve ileitis. Further analyses showed that 55 +/- 7% of the mesenteric lymph node alpha4beta7+CD4 expressed L-selectin. These L-selectin+ T cells were the main producers of TNF-alpha and the predominant ileitis-inducing subpopulation. Mechanistically, combined blockade of L-selectin and MAdCAM-1 depleted the intestinal lamina propria of CD4+ T cells that aberrantly coexpressed alpha4beta7 and alpha4beta1 integrins, markedly decreasing local production of TNF-alpha and IFN-gamma. Thus, pathogenic CD4+ T cells not only use the physiologic alpha4beta7/MAdCAM-1 pathway, but alternatively engage alpha4beta1 and L-selectin to recirculate to the chronically inflamed small intestine.

Intragraft distribution of lymphocytes expressing β7 integrins after small bowel transplantation in mice

Lymphocytes with activated beta7 integrins (alphaEbeta7 and alpha4beta7) contribute to inflammatory reactions in the small bowel. Since the selective recruitment of lymphocytes to the lymphoid compartments of the small bowel is controlled by distinct adhesion molecule interactions, a compartment-dependent use of beta7 integrins may influence the rejection response within intestinal transplants. To further delineate the nature of beta7 integrin-mediated graft infiltration, we analysed their expression on T lymphocytes in the heterotopically transplanted small bowel of BALB/c and C57BL/6 mice. Lymphocytes isolated from the epithelium, lamina propria (LP), Peyer's patches (PP), and mesenteric lymph nodes (MLN) were analysed by three-color fluorescence flow cytometry using monoclonal antibodies (mAb) to integrin the subunits, lymphocyte markers, and MHC I of the donor and recipient strains. On postoperative day 5 (POD) after allogeneic small bowel transplantation (SBT), 43% of intraepithelial lymphocytes (IEL) and 63% of LP, 93% of MLN, and 93% of PP lymphocytes were of host origin. In the MLN and PP of allografts, a major infiltrating lymphocyte population consisted of CD8+ cells with increased expression of alpha4beta7 and decreased expression of L-selectin, an adhesion molecule profile characteristic of intestinal effector cell phenotypes. An increase in alpha4beta7 levels was also found on CD8+ host lymphocytes in the LP. The integrin profile of a number of host IEL suggests an ongoing transition from the phenotype of graft infiltrating lymphocytes with high levels of alpha4beta7 and low levels of alphaepsilonbeta7 to that of resident IEL with high levels of alphaepsilonbeta7 and low levels of alpha4beta7. The importance of beta7-mediated lymphocyte trafficking to the graft is attested by the significant reduction in the host lymphocyte population in the LP, PP, and epithelium following the administration of a beta7-blocking mAb to allograft recipients. In conclusion, while the infiltration patterns of lymphocytes may vary between the lymphoid compartments of intestinal allografts, host CD8+ lymphocytes with high levels of alpha4beta7 constitute a major effector cell population that affects the entire graft.

Critical Role of T Cell-Dependent Serum Antibody, but Not the Gut-Associated Lymphoid Tissue, for Surviving Acute Mucosal Infection withCitrobacter rodentium, an Attaching and Effacing Pathogen

Citrobacter rodentium uses virulence factors similar to the enteropathogenic Escherichia coli to produce attaching and effacing lesions in the distal colon of mice. We used this infection model to determine components of adaptive immunity needed to survive infection. During acute infection, wild-type mice develop breaks across infected epithelial surfaces but resolve infection. Surprisingly, mice markedly deficient in mucosal lymphocyte populations from beta(7) integrin deficiency resolve infection, as do CD8alpha-/- or TCR-delta-/- mice. In contrast, CD4-/- or TCR-beta-/- mice develop polymicrobial sepsis and end-organ damage, and succumb during acute infection, despite epithelial damage similar to wild-type mice. B cell-deficient (MuMT-/-) or B and T cell-deficient (recombinase-activating gene 2-/-) mice develop severe pathology in colon and internal organs, and deteriorate rapidly during acute infection. Surviving mice develop robust Citrobacter-specific serum IgM responses during acute infection, whereas mice that succumb do not. However, CD4-/- mice receiving serum Igs from infected wild-type mice survive and clear the infection. Our data show that survival of apparently self-limited and luminal mucosal infections requires a systemic T cell-dependent Ab response against bacteria that enter through damaged mucosa. These findings have implications for understanding host defense against mucosal infections, including the pathogenesis of these diseases in immunocompromised populations.

Intravital microscopy comparing T lymphocyte trafficking to the spleen and the mesenteric lymph node

Lymphocyte rolling velocity is determined largely by interactions between leukocyte alpha(4)-integrin (CD49d) and L-selectin and mucosal addressin cell adhesion molecule-1 (MAdCAM-1) in mesenteric postcapillary venules and Peyer's patch high endothelial venules (HEVs). The role of these interactions in other tissue sites of lymphocyte emigration is not known. With the use of real-time intravital confocal microscopy, we found that rolling velocities of T lymphocytes in the murine mesenteric lymph node (MLN) HEV also depend on L-selectin and CD49d. However, in the murine spleen, rolling velocities of T lymphocytes are not influenced by the loss of L-selectin and CD49d. With the use of FITC-dextran and TIE2-GFP mice, we further defined the microvascular compartments of the spleen and showed that adherence of T cells is localized to regions in the white pulp that are not lined by endothelial cells and have shear rates similar to bone marrow sinusoids. These results establish that T cell trafficking to the spleen differs from trafficking to other secondary lymphoid organs and suggest that the mechanical properties of the blood-filtering role of the spleen are important in T cell accumulation in the organ.

Anatomical basis of tolerance and immunity to intestinal antigens

The intestinal immune system has to discriminate between harmful and beneficial antigens. Although strong protective immunity is essential to prevent invasion by pathogens, equivalent responses against dietary proteins or commensal bacteria can lead to chronic disease. These responses are normally prevented by a complex interplay of regulatory mechanisms. This article reviews the unique aspects of the local microenvironment of the intestinal immune system and discuss how these promote the development of regulatory responses that ensure the maintenance of homeostasis in the gut.

alpha4 integrins and L-selectin differently orchestrate T-cell activity during diabetes prevention following oral administration of CTB-insulin

Oral administration of insulin conjugated to the B chain of cholera toxin (CTB-insulin) in non-obese diabetic (NOD) mice results in diabetes prevention. We investigated the respective contributions of L-selectin (CD62L) and alpha4-integrin pathways during CTB-driven tolerance. Purified CD62L+CD4+ cells from CTB-insulin fed mice significantly reduced the capacity of diabetogenic T cells to transfer diabetes in syngeneic recipients. In vivo antibody blockade of fed animals during adoptive co-transfer experiments indicated that both CD62L and alpha4-integrins pathways were necessary to develop a protective response after oral tolerance induction. In contrast, when antibodies were given to recipient mice, only CD62L was critical for the protection. In vitro stimulated CD62L+CD4+ cells from the spleen of fed animals secreted lower amounts of IL-4 and IL-10 but comparable levels of TGFbeta than CD62L-cells. A reduced IFN-gamma production between the two cell subsets was specifically observed in CTB-insulin fed mice. Furthermore, antibody treatments induced changes in T-cell migration to the spleen, mesenteric and pancreatic lymph nodes. The protective effect was also associated with migration of regulatory T cells into pancreatic islets. Taken together, our results suggest that L-selectin and alpha4-integrin have distinct but complementary roles in the generation and function of regulatory CD4+ T cells following CTB-insulin administration.

Leukocyte rolling velocities and migration are optimized by cooperative L-selectin and intercellular adhesion molecule-1 functions

Selectin family members largely mediate initial tethering and rolling of leukocytes on vascular endothelium, whereas integrin and Ig family members are essential for leukocyte firm adhesion. To quantify functional synergy between L-selectin and Ig family members during leukocyte rolling, the EA.hy926 human vascular endothelial line was transfected with either fucosyltransferase VII (926-FtVII) cDNA to generate L-selectin ligands alone or together with ICAM-1 cDNA (926-FtVII/ICAM-1). The ability of transfected 926 cells to support human leukocyte interactions was assessed in vitro using parallel plate flow chamber assays. Lymphocyte rolling on 926-FtVII cells was increased by approximately 70% when ICAM-1 was expressed at physiological levels. Although initial tether formation was similar for both cell types, lymphocyte rolling was 26% slower on 926-FtVII/ICAM-1 cells. Pretreatment of lymphocytes with an anti-CD18 mAb eliminated the increase in rolling, and all rolling was blocked by anti-L-selectin mAb. In addition, rolling velocities of lymphocytes from CD18-hypomorphic mice were 48% faster on 926-FtVII/ICAM-1 cells, with a similar reduction in rolling frequency relative to wild-type lymphocytes. CD18-hypomorphic lymphocytes also showed an approximately 40% decrease in migration to peripheral and mesenteric lymph nodes during in vivo migration assays compared with wild-type lymphocytes. Likewise, wild-type lymphocyte migration to peripheral lymph nodes was reduced by approximately 50% in ICAM-1(-/-) recipient mice. Similar to human lymphocytes, human neutrophils showed enhanced rolling interactions on 926-FtVII/ICAM-1 cells, but also firmly adhered. Thus, in addition to mediating leukocyte firm adhesion, CD18 integrin/ICAM-1 interactions regulate leukocyte rolling velocities and thereby optimize L-selectin-mediated leukocyte rolling.

A functional role for circulating mouse L-selectin in regulating leukocyte/endothelial cell interactions in vivo

L-selectin mediates the initial capture and subsequent rolling of leukocytes along inflamed vascular endothelium and mediates lymphocyte migration to peripheral lymphoid tissues. Leukocyte activation induces rapid endoproteolytic cleavage of L-selectin from the cell surface, generating soluble L-selectin (sL-selectin). Because human sL-selectin retains ligand-binding activity in vitro, mouse sL-selectin and its in vivo relevance were characterized. Comparable with humans, sL-selectin was present in adult C57BL/6 mouse sera at approximately 1.7 micro g/ml. Similar levels of sL-selectin were present in sera from multiple mouse strains, despite their pronounced differences in cell surface L-selectin expression levels. Adhesion molecule-deficient mice prone to spontaneous chronic inflammation and mice suffering from leukemia/lymphoma had 2.5- and 20-fold increased serum sL-selectin levels, respectively. By contrast, serum sL-selectin levels were reduced by 70% in Rag-deficient mice lacking mature lymphocytes. The majority of serum sL-selectin had a molecular mass of 65-75 kDa, consistent with its lymphocyte origin. Slow turnover may explain the relatively high levels of sL-selectin in vivo. The t(1/2) of sL-selectin, assessed by transferring sera from wild-type mice into L-selectin-deficient mice and monitoring serum sL-selectin levels by ELISA, was >20 h, and it remained detectable for longer than 1 wk. Short-term in vivo lymphocyte migration assays demonstrated that near physiologic levels ( approximately 0.9 micro g/ml) of sL-selectin decreased lymphocyte migration to peripheral lymph nodes by >30%, with dose-dependent inhibition occurring with increasing sL-selectin concentrations. These results suggest that sL-selectin influences lymphocyte migration in vivo and that the increased sL-selectin levels present in certain pathologic conditions may adversely affect leukocyte migration.

T cell homeostatic proliferation elicits effective antitumor autoimmunity

Development of tumor immunotherapies focuses on inducing autoimmune responses against tumor-associated self-antigens primarily encoded by normal, unmutated genes. We hypothesized that such responses could be elicited by T cell homeostatic proliferation in the periphery, involving expansion of T cells recognizing self-MHC/peptide ligands. Herein, we demonstrate that sublethally irradiated lymphopenic mice transfused with autologous or syngeneic T cells showed tumor growth inhibition when challenged with melanoma or colon carcinoma cells. Importantly, the antitumor response depended on homeostatic expansion of a polyclonal T cell population within lymph nodes. This response was effective even for established tumors, was characterized by CD8(+) T cell-mediated tumor-specific cytotoxicity and IFN-gamma production, and was associated with long-term memory. The results indicate that concomitant induction of the physiologic processes of homeostatic T cell proliferation and tumor antigen presentation in lymph nodes triggers a beneficial antitumor autoimmune response.

beta7 Integrin expression is not required for the localization of T cells to the intestine and colitis pathogenesis

beta7 Integrins have been shown to have an important role in the localization of T cells to the intestine. Utilizing two different experimental mouse models of inflammatory bowel disease (IBD), this study was undertaken to determine if beta7 integrin expression is critical for T cell localization to the intestine and colitis pathogenesis. Transfer of CD4+ CD45RBhigh cells into immunodeficient mice results in colitis. To examine the role of beta7 integrins, donor cells were obtained from beta7 integrin gene-deficient animals and disease induction was examined following transfer into severe combined immunodeficiency (SCID) mice. Additionally, beta7 integrin gene-deficient animals were crossed to IL-2-deficient mice and the onset of spontaneous colitis that normally occurs in IL-2-deficient animals was examined. No differences in the onset or severity of spontaneous colitis was noted in animals that were deficient in both beta7 integrin and IL-2. In contrast, the onset of colitis in recipients of T cells from beta7 integrin-deficient donors was delayed significantly. In mice receiving beta7 integrin negative cells, the initial lack of colitis appeared to correlate with fewer numbers of CD3+beta7 integrin -/- donor lymphocytes present in the host colon. The eventual development of disease, however, was associated with increased numbers of donor beta7 integrin -/- lymphocytes. These results show that beta7 integrin expression is not absolutely required for T cell localization to the intestine and colitis pathogenesis.

L-selectin is not required for T cell-mediated autoimmune diabetes

Administration of anti-L-selectin (CD62L) mAb to neonatal nonobese diabetic (NOD) mice mediates long term protection against the development of insulitis and overt diabetes. These results suggested that CD62L has a key role in the general function of beta cell-specific T cells. To further examine the role of CD62L in the development of type 1 diabetes, NOD mice lacking CD62L were established. The onset and frequency of overt diabetes were equivalent among CD62L(+/+), CD62L(+/-), and CD62L(-/-) NOD littermates. Furthermore, patterns of T cell activation, migration, and beta cell-specific reactivity were similar in NOD mice of all three genotypes. Adoptive transfer experiments with CD62L(-/-) CD4(+) T cells prepared from BDC2.5 TCR transgenic mice revealed no apparent defects in migration to pancreatic lymph nodes, proliferation in response to beta cell Ag, or induction of diabetes in NOD.scid recipients. In conclusion, CD62L expression is not essential for the development of type 1 diabetes in NOD mice.

L-selectin in health and disease

This article reviews recent advances in the knowledge of the role of L-selectin, an adhesion molecule that is expressed on the surface of circulating leucocytes, in animal and human physiology and pathophysiology. After a brief discussion on nomenclature and structure, it progresses through the evidence for expression and regulation of L-selectin, cell collection and purification, physiological function and roles. The special role of knock out mice and monoclonal antibodies in determining a role for L-selectin in inflammatory states is described before proceeding to discuss the importance of L-selectin ligands and shed L-selectin. A second section describes a role for L-selectin in pathophysiological states in animals and man, with special reference to trauma, systemic inflammatory syndromes and sepsis. The review concludes with a summary of the potential role of anti-inflammatory medication and L-selectin blockers in the management of inflammation.

Adhesion molecule cascades direct lymphocyte recirculation and leukocyte migration during inflammation

Leukocyte interactions with vascular endothelium are highly orchestrated processes that include the capture of free-flowing leukocytes from the blood with subsequent leukocyte rolling, arrest, firm adhesion, and ensuing diapedesis. These interactions occur under high shear stresses within venules and depend on multiple families of adhesion molecules. Many of the adhesion molecules involved are now identified. In addition, precise mechanisms underlying their regulation and our understanding of how different families of adhesion molecules work together is becoming clearer. Specifically, leukocyte/endothelial cell interactions such as capture, rolling, and firm adhesion can no longer be viewed as occurring in discrete steps mediated by individual families of adhesion molecules, but rather as a series of overlapping synergistic interactions among adhesion molecules resulting in an adhesion cascade. Although long thought to be mediated by distinct adhesion pathways, overlapping adhesion cascades mediate normal lymphocyte recirculation to peripheral lymphoid tissues and inflammation-induced leukocyte migration. These cascades thereby direct leukocyte migration, which is essential for the generation of effective inflammatory responses and the development of rapid immune responses.

Functional consequences of integrin gene mutations in mice

Integrins are cell-surface receptors responsible for cell attachment to extracellular matrices and to other cells. The application of mouse genetics has significantly increased our understanding of integrin function in vivo. In this review, we summarize the phenotypes of mice carrying mutant integrin genes and compare them with phenotypes of mice lacking the integrin ligands.