Michael J. Briskin

Lymphocyte Trafficking and Regional Immunity

Publisher Summary This chapter discusses the lymphocyte trafficking and regional immunity. The chapter discusses the present understanding of tissue-selective lymphocyte trafficking, focusing in particular on the gut and its associated lymphoid tissues. The sophisticated cellular, developmental, and molecular mechanisms involved are discussed in the chapter with emphasis on the importance of these mechanisms for the understanding and manipulation of regional immune responses in the gastrointestinal tract. Lymphocytes are migratory cells, trafficking from their sites of origin in the bone marrow and thymus and homing to and recirculating through specialized lymphoid and extra lymphoid tissues in the periphery. Like all leukocytes, lymphocytes develop with characteristic trafficking properties. A central tenet of current thinking in the field is that this tissue-specific targeting of antigen-reactive populations must increase the efficiency of immune surveillance by circulating memory cells as well as enhancing local immune responses. From the perspective of regional physiology and pathology, selective trafficking may also provide a mechanism for segregating the specialized immune response modalities characteristic of intestinal versus systemic immune responses.

Intravascular immune surveillance by CXCR6+ NKT cells patrolling liver sinusoids.

We examined the in vivo behavior of liver natural killer T cells (NKT cells) by intravital fluorescence microscopic imaging of mice in which a green fluorescent protein cDNA was used to replace the gene encoding the chemokine receptor CXCR6. NKT cells, which account for most CXCR6+ cells in liver, were found to crawl within hepatic sinusoids at 10–20 μm/min and to stop upon T cell antigen receptor activation. CXCR6-deficient mice exhibited a selective and severe reduction of CD1d-reactive NKT cells in the liver and decreased susceptibility to T-cell-dependent hepatitis. CXCL16, the cell surface ligand for CXCR6, is expressed on sinusoidal endothelial cells, and CXCR6 deficiency resulted in reduced survival, but not in altered speed or pattern of patrolling of NKT cells. Thus, NKT cells patrol liver sinusoids to provide intravascular immune surveillance, and CXCR6 contributes to liver-based immune responses by regulating their abundance.

Reciprocal and dynamic control of CD8 T cell homing by dendritic cells from skin- and gut-associated lymphoid tissues

T cell activation by intestinal dendritic cells (DC) induces gut-tropism. We show that, reciprocally, DC from peripheral lymph nodes (PLN-DC) induce homing receptors promoting CD8 T cell accumulation in inflamed skin, particularly ligands for P- and E-selectin. Differential imprinting of tissue-tropism was independent of Th1/Th2 cytokines and not restricted to particular DC subsets. Fixed PLN-DC retained the capacity to induce selectin ligands on T cells, which was suppressed by addition of live intestinal DC. By contrast, fixed intestinal DC failed to promote gut-tropism and instead induced skin-homing receptors. Moreover, the induction of selectin ligands driven by antigen-pulsed PLN-DC could be suppressed “in trans” by adding live intestinal DC, but PLN-DC did not suppress gut-homing receptors induced by intestinal DC. Reactivation of tissue-committed memory cells modified their tissue-tropism according to the last activating DCs origin. Thus, CD8 T cells activated by DC acquire selectin ligands by default unless they encounter fixation-sensitive signal(s) for gut-tropism from intestinal DC. Memory T cells remain responsive to these signals, allowing for dynamic migratory reprogramming by skin- and gut-associated DC.

PYPAF7, a Novel PYRIN-containing Apaf1-like Protein That Regulates Activation of NF-κB and Caspase-1-dependent Cytokine Processing

PYRIN-containing Apaf1-like proteins (PYPAFs) are members of the nucleotide-binding site/leucine-rich repeat (NBS/LRR) family of signal transduction proteins. We report here that PYPAF7 is a novel PYPAF protein that activates inflammatory signaling pathways. The expression of PYPAF7 is highly restricted to immune cells, and its gene maps to chromosome 19q13.4, a locus that contains a cluster of genes encoding numerous PYPAF family members. Co-expression of PYPAF7 with ASC results in the recruitment of PYPAF7 to distinct cytoplasmic loci and a potent synergistic activation of NF-κB. To identify other proteins involved in PYPAF7 and ASC signaling pathways, we performed a mammalian two-hybrid screen and identified pro-caspase-1 as a binding partner of ASC. Co-expression of PYPAF7 and ASC results in the synergistic activation of caspase-1 and a corresponding increase in secretion of interleukin-1β. In addition, PYPAF1 induces caspase-1-dependent cytokine processing when co-expressed with ASC. These findings indicate that PYPAF family members participate in inflammatory signaling by regulating the activation of NF-κB and cytokine processing.

Expression Cloning of the STRL33/BONZO/TYMSTR Ligand Reveals Elements of CC, CXC, and CX3C Chemokines

STRL33/BONZO/TYMSTR is an orphan chemokine and HIV/SIV coreceptor receptor that is expressed on activated T lymphocytes. We describe an expression cloning strategy whereby we isolated a novel chemokine, which we name CXCL16. CXCL16 is an α (CXC) chemokine but also has characteristics of CC chemokines and a structure similar to fractalkine (neurotactin) in having a transmembrane region and a chemokine domain suspended by a mucin-like stalk. A recombinant version of CXCL16 fails to mediate chemotaxis to all known chemokine receptor transfectants tested but does mediate robust chemotaxis, high affinity binding, and calcium mobilization to Bonzo receptor transfectants, indicating that this is a unique receptor ligand interaction. In vitro polarized T cell subsets including Th1, Th2, and Tr1 cells express functional Bonzo, suggesting expression of this receptor in chronic inflammation, which we further verified by demonstration of CXCL16-mediated migration of tonsil-derived CD4+ T lymphocytes. CXCL16 is expressed on the surface of APCs including subsets of CD19+ B cells and CD14+ monocyte/macrophages, and functional CXCL16 is also shed from macrophages. The combination of unique structural features of both Bonzo and CXCL16 suggest that this interaction may represent a new class of ligands for this receptor family. Additionally, this chemokine might play a unique dual role of attracting activated lymphocyte subsets during inflammation as well as facilitating immune responses via cell-cell contact.

PYPAF1, a PYRIN-containing Apaf1-like Protein That Assembles with ASC and Regulates Activation of NF-κB

The PYRIN domain is a recently identified protein-protein interaction domain that is found at the N terminus of several proteins thought to function in apoptotic and inflammatory signaling pathways. We report here that PYPAF1 (PYRIN-containing Apaf1-like protein 1) is a novel PYRIN-containing signaling protein that belongs to the nucleotide-binding site/leucine-rich repeat (NBS/LRR) family of signaling proteins. The expression of PYPAF1 is highly restricted to immune cells, and its gene maps to chromosome 1q44, a locus that is associated with the rare inflammatory diseases Muckle-Wells syndrome and familial cold urticaria. To identify downstream signaling partners of PYPAF1, we performed a mammalian two-hybrid screen and identified ASC as a PYRIN-containing protein that interacts selectively with the PYRIN domain of PYPAF1. When expressed in cells, ASC recruits PYPAF1 to distinct cytoplasmic loci and induces the activation of NF-κB. Furthermore, coexpression of PYPAF1 with ASC results in a potent synergistic activation of NF-κB. These findings suggest that PYPAF1 and ASC function as upstream activators of NF-κB signaling.

Functional screening of five PYPAF family members identifies PYPAF5 as a novel regulator of NF-κB and caspase-1

PYRIN‐containing Apaf‐1‐like proteins (PYPAFs) are a recently identified family of proteins thought to function in apoptotic and inflammatory signaling pathways. PYPAF1 and PYPAF7 proteins have been found to assemble with the PYRIN–CARD protein ASC and coordinate the activation of NF‐κB and pro‐caspase‐1. To determine if other PYPAF family members function in pro‐inflammatory signaling pathways, we screened five other PYPAF proteins (PYPAF2, PYPAF3, PYPAF4, PYPAF5 and PYPAF6) for their ability to activate NF‐κB and pro‐caspase‐1. Co‐expression of PYPAF5 with ASC results in a synergistic activation of NF‐κB and the recruitment of PYPAF5 to punctate structures in the cytoplasm. The expression of PYPAF5 is highly restricted to granulocytes and T‐cells, indicating a role for this protein in inflammatory signaling. In contrast, PYPAF2, PYPAF3, PYPAF4 and PYPAF6 failed to colocalize with ASC and activate NF‐κB. PYPAF5 also synergistically activated caspase‐1‐dependent cytokine processing when co‐expressed with ASC. These findings suggest that PYPAF5 functions in immune cells to coordinate the transduction of pro‐inflammatory signals to the activation of NF‐κB and pro‐caspase‐1.

Hepatic Endothelial CCL25 Mediates the Recruitment of CCR9+ Gut-homing Lymphocytes to the Liver in Primary Sclerosing Cholangitis

Primary sclerosing cholangitis (PSC), a chronic inflammatory liver disease characterized by progressive bile duct destruction, develops as an extra-intestinal complication of inflammatory bowel disease (IBD) (Chapman, R.W. 1991. Gut. 32:1433–1435). However, the liver and bowel inflammation are rarely concomitant, and PSC can develop in patients whose colons have been removed previously. We hypothesized that PSC is mediated by long-lived memory T cells originally activated in the gut, but able to mediate extra-intestinal inflammation in the absence of active IBD (Grant, A.J., P.F. Lalor, M. Salmi, S. Jalkanen, and D.H. Adams. 2002. Lancet. 359:150–157). In support of this, we show that liver-infiltrating lymphocytes in PSC include mucosal T cells recruited to the liver by aberrant expression of the gut-specific chemokine CCL25 that activates α4β7 binding to mucosal addressin cell adhesion molecule 1 on the hepatic endothelium. This is the first demonstration in humans that T cells activated in the gut can be recruited to an extra-intestinal site of disease and provides a paradigm to explain the pathogenesis of extra-intestinal complications of IBD.

Essential Role for the C5a Receptor in Regulating the Effector Phase of Synovial Infiltration and Joint Destruction in Experimental Arthritis

A characteristic feature of rheumatoid arthritis is the abundance of inflammatory cells in the diseased joint. Two major components of this infiltrate are neutrophils in the synovial fluid and macrophages in the synovial tissue. These cells produce cytokines including tumor necrosis factor α and other proinflammatory mediators that likely drive the disease through its effector phases. To investigate what mechanisms underlie the recruitment of these cells into the synovial fluid and tissue, we performed expression analyses of chemoattractant receptors in a related family that includes the anaphylatoxin receptors and the formyl-MetLeuPhe receptor. We then examined the effect of targeted disruption of two abundantly expressed chemoattractant receptors, the receptors for C3a and C5a, on arthritogenesis in a mouse model of disease. We report that genetic ablation of C5a receptor expression completely protects mice from arthritis.

Hepatic Expression of Secondary Lymphoid Chemokine (CCL21) Promotes the Development of Portal-Associated Lymphoid Tissue in Chronic Inflammatory Liver Disease

The chronic inflammatory liver disease primary sclerosing cholangitis (PSC) is associated with portal inflammation and the development of neolymphoid tissue in the liver. More than 70% of patients with PSC have a history of inflammatory bowel disease and we have previously reported that mucosal addressin cell adhesion molecule-1 is induced on dendritic cells and portal vascular endothelium in PSC. We now show that the lymph node-associated chemokine, CCL21 or secondary lymphoid chemokine, is also strongly up-regulated on CD34(+) vascular endothelium in portal associated lymphoid tissue in PSC. In contrast, CCL21 is absent from LYVE-1(+) lymphatic vessel endothelium. Intrahepatic lymphocytes in PSC include a population of CCR7(+) T cells only half of which express CD45RA and which respond to CCL21 in migration assays. The expression of CCL21 in association with mucosal addressin cell adhesion molecule-1 in portal tracts in PSC may promote the recruitment and retention of CCR7(+) mucosal lymphocytes leading to the establishment of chronic portal inflammation and the expanded portal-associated lymphoid tissue. This study provides further evidence for the existence of portal-associated lymphoid tissue and is the first evidence that ectopic CCL21 is associated with lymphoid neogenesis in human inflammatory disease.