Eugene C. Butcher

Lymphocyte homing and homeostasis

The integration and control of systemic immune responses depends on the regulated trafficking of lymphocytes. This lymphocyte “homing” process disperses the immunologic repertoire, directs lymphocyte subsets to the specialized microenvironments that control their differentiation and regulate their survival, and targets immune effector cells to sites of antigenic or microbial invasion. Recent advances reveal that the exquisite specificity of lymphocyte homing is determined by combinatorial “decision processes” involving multistep sequential engagement of adhesion and signaling receptors. These homing-related interactions are seamlessly integrated into the overall interaction of the lymphocyte with its environment and participate directly in the control of lymphocyte function, life-span, and population dynamics. In this article a review of the molecular basis of lymphocyte homing is presented, and mechanisms by which homing physiology regulates the homeostasis of immunologic resources are proposed.

A cell-surface molecule involved in organ-specific homing of lymphocytes

Lymphocytes migrate from the bloodstream by recognizing and binding to specialized endothelial cells lining the high endothelial venules (HEV) in lymph nodes and Peyers patches. We describe here a monoclonal antibody, MEL-14, specific for a lymphocyte surface molecule that appears to mediate recognition of lymph node HEV, and to be required for lymphocyte homing into lymph nodes in vivo.

α4β7 integrin mediates lymphocyte binding to the mucosal vascular addressin MAdCAM-1

The mucosal vascular addressin, MAdCAM-1, is an immunoglobulin superfamily adhesion molecule for lymphocytes that is expressed by mucosal venules and helps direct lymphocyte traffic into Peyers patches (PP) and the intestinal lamina propria. We demonstrate that the lymphocyte integrin alpha 4 beta 7, also implicated in homing to PP, is a receptor for MAdCAM-1. Certain antibodies to alpha 4 and beta 7 integrin chains but not to the beta 2 integrin LFA-1 inhibit lymphocyte binding to purified MAdCAM-1 and to MAdCAM-1 transfectants. Lymph node lymphocytes, alpha 4 beta 7+ TK1 lymphoma cells, and a beta 7-transfected variant of an alpha 4+ B cell line, 38C13, bind constitutively to MAdCAM-1. Binding is enhanced by Mn(++)-induced integrin activation. The related integrin alpha 4 beta 1 supports efficient binding to VCAM-1 but not to MAdCAM-1, even after integrin activation, indicating that MAdCAM-1 is a preferential ligand for alpha 4 beta 7. Alpha 4 beta 7 can also bind VCAM-1, but this requires greater integrin activation than binding to MAdCAM-1. The findings imply a selective role for the interaction of alpha 4 beta 7 and MAdCAM-1 lymphocyte in homing to mucosal sites.

The chemokine receptor CCR4 in vascular recognition by cutaneous but not intestinal memory T cells

Lymphocytes that are responsible for regional (tissue-specific) immunity home from the blood to the intestines, inflamed skin or other sites through a multistep process involving recognition of vascular endothelial cells and extravasation. Chemoattractant cytokine molecules known as chemokines regulate this lymphocyte traffic, in part by triggering arrest (stopping) of lymphocytes rolling on endothelium. Here we show that many systemic memory T cells in blood carry the chemokine receptor CCR4 (ref. 6) and therefore respond to its ligands, the chemokines TARC and MDC. These cells include essentially all skin-homing cells expressing the cutaneous lymphocyte antigen and a subset of other systemic memory lymphocytes; however, intestinal (α4β7+) memory and naive T cells respond poorly. Immunohistochemistry reveals anti-TARC reactivity of venules and infiltration of many CCR4+ lymphocytes in chronically inflamed skin, but not in the gastrointestinal lamina propria. Moreover, TARC induces integrin-dependent adhesion of skin (but not intestinal) memory T cells to the cell-adhesion molecule ICAM-1, and causes their rapid arrest under physiological flow. Our results suggest that CCR4 and TARC are important in the recognition of skin vasculature by circulating T cells and in directing lymphocytes that are involved in systemic as opposed to intestinal immunity to their target tissues.

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.

Chemokines and the Tissue-Specific Migration of Lymphocytes

Tissue-selective trafficking of memory and effector T and B lymphocytes is mediated by unique combinations of adhesion molecules and chemokines. The discovery of several related epithelial-expressed chemokines (TECK/CCL25 in small intestine, CTACK/CCL27 in skin, and MEC/CCL28 in diverse mucosal sites) now highlights an important role for epithelial cells in controlling homeostatic lymphocyte trafficking, including the localization of cutaneous and intestinal memory T cells, and of IgA plasma cells. Constitutively expressed epithelial chemokines may help determine the character of local immune responses and contribute to the systemic organization of the immune system.

Chemokines in tissue-specific and microenvironment-specific lymphocyte homing

This review describes recent breakthroughs in our understanding of the roles played by chemokines in lymphocyte trafficking. These include the first demonstration that chemokines control lymphocyte/vascular recognition by shear-resistant rapid adhesion; the first example of specialized tissue-specific homing mediated by chemokines; and the implication that chemokines may control microenvironmental segregation within lymphoid organs.

Chemokines trigger immediate beta2 integrin affinity and mobility changes: differential regulation and roles in lymphocyte arrest under flow.

Chemokines trigger rapid integrin-dependent lymphocyte arrest to vascular endothelium. We show that the chemokines SLC, ELC, and SDF-1alpha rapidly induce lateral mobility and transient increase of affinity of the beta2 integrin LFA-1. Inhibition of phosphatidylinositol 3-OH kinase (PI(3)K) activity blocks mobility but not affinity changes and prevents lymphocyte adhesion to ICAM-1 immobilized at low but not high densities, suggesting that mobility enhances the frequency of encounters between high-affinity integrin and ligand but that at higher ligand density affinity changes are sufficient for arrest. Thus, chemokines trigger, through distinct signaling pathways, both a high-affinity state and lateral mobility of LFA-1 that can coordinately determine the vascular arrest of circulating lymphocytes under physiologic conditions.

Role of Rho in chemoattractant-activated leukocyte adhesion through integrins.

Heterotrimeric guanine nucleotide binding protein (G protein)-linked receptors of the chemoattractant subfamily can trigger adhesion through leukocyte integrins, and in this role they are thought to regulate immune cell-cell interactions and trafficking. In lymphoid cells transfected with formyl peptide or interleukin-8 receptors, agonist stimulation activated nucleotide exchange on the small guanosine triphosphate-binding protein RhoA in seconds. Inactivation of Rho by C3 transferase exoenzyme blocked agonist-induced lymphocyte α4β1 adhesion to vascular cell adhesion molecule-1 and neutrophil β2 integrin adhesion to fibrinogen. These findings suggest that Rho participates in signaling from chemoattractant receptors to trigger rapid adhesion in leukocytes.

Rapid Acquisition of Tissue-specific Homing Phenotypes by CD4+ T Cells Activated in Cutaneous or Mucosal Lymphoid Tissues

Effector and memory T cells can be subdivided based on their ability to traffic through peripheral tissues such as inflamed skin and intestinal lamina propria, a property controlled by expression of ‘tissue-specific’ adhesion and chemoattractant receptors. However, little is known about the development of these selectively homing T cell subsets, and it is unclear whether activation in cutaneous versus intestinal lymphoid organs directly results in effector/memory T cells that differentially express adhesion and chemoattracant receptors targeting them to the corresponding nonlymphoid site. We define two murine CD4+ effector/memory T cell subsets that preferentially localize in cutaneous or intestinal lymphoid organs by their reciprocal expression of the adhesion molecules P-selectin ligand (P-lig) and α4β7, respectively. We show that within 2 d of systemic immunization CD4+ T cells activated in cutaneous lymph nodes upregulate P-lig, and downregulate α4β7, while those responding to antigen in intestinal lymph nodes selectively express high levels of α4β7 and acquire responsiveness to the intestinal chemokine thymus-expressed chemokine (TECK). Thus, during an immune response, local microenvironments within cutaneous and intestinal secondary lymphoid organs differentially direct T cell expression of these adhesion and chemoattractant receptors, targeting the resulting effector T cells to the inflamed skin or intestinal lamina propria.