Philip R. Streeter

Homing Receptors and Vascular Addressins: Cell Adhesion Molecules that Direct Lymphocyte Traffic

The trafficking of lymphocytes and other leukocytes plays a major role in the physiology of the itnmune system and the implementation of immune mechanisms. The recirculation of lymphocytes allows the full repertoire of antigenic specificities to be continuously represented throughout the body, and may also facilitate interactions between lymphocyte subsets and accessory cells required for antigen-specific effector functions. In addition, lymphocyte trafficking plays a role in segregating particular effector and memory subpopulations permitting the development of unique responses in, for example, mucosal versus non-mucosal sites. The selective extravasation of other classes of leukocytes, such as monocytes and neutrophils, contributes to inflammatory reactions resulting in the destruction of foreign microorganisms, clearance of tumors and the expulsion of parasites. The first step reqtiired for lymphocyte entry into peripheral lymph nodes and mucosal lymphoid organs (Peyers patches and appendix) from the blood is the recognition and adhesion to postcapillary venules lined by high endothelium (HEV). Lymphocytes as well as HEV employ specific cell surface molecules for this function and these or related molecules are also important in directing the extravasation of other leukocytes. In this manuscript we will briefly outline our view of the role of lymphocyte-HEV interactions in lymphocyte traffic, and will summarize our studies of some of the molecules involved. We will discuss the

Vascular addressins are induced on islet vessels during insulitis in nonobese diabetic mice and are involved in lymphoid cell binding to islet endothelium.

In the nonobese diabetic (NOD) mouse, lymphocytic and monocytic infiltration of the pancreatic islets leads to beta cell destruction. To investigate the mechanisms by which lymphocytes enter the NOD pancreas, pancreata were immunostained using monoclonal antibodies to a variety of adhesion molecules known to be involved in lymphocyte binding to vascular endothelium, an initial step in the migration of lymphocytes from blood into organized lymphoid and inflamed tissues. These adhesion molecules include: lymphocyte homing receptors involved in tissue-selective binding of lymphocytes to peripheral lymph node (L-selectin) or mucosal lymphoid tissue (LPAM-1, alpha 4 beta 7-integrin) high-endothelial venules (HEV); and HEV ligands peripheral vascular addressin (PNAd) and mucosal vascular addressin (MAdCAM-1). In NOD pancreata, alpha 4 beta 7 is expressed on most infiltrating cells at all stages of insulitis, whereas L-selectin expression is more pronounced on cells in the islets at later stages. During the development of insulitis, MAdCAM-1 and to a lesser extent PNAd became detectable on vascular endothelium adjacent to and within the inflamed islets. The Stamper-Woodruff in vitro assay was used to examine lymphoid cell binding to such vessels. These functional assays show that both the mucosal (MAdCAM-1/alpha 4 beta 7) and the peripheral (PNAd/L-selectin) recognition systems are involved in this binding. Our findings demonstrate that expression of peripheral and mucosal vascular addressins is induced on endothelium in inflamed islets in NOD pancreas, and that these addressins participate in binding lymphoid cells via their homing receptors. This suggests that these adhesion molecules play a role in the pathogenesis of diabetes in these mice by being involved in the migration of lymphocytes from blood into the inflamed pancreas.

Preclinical Evaluation of Human Hematolymphoid Function in the SCID‐hu Mouse

Work in the SCID-hu system has progressed significantly over the past 3-4 years. Reliable SCID-hu constructs can now be created for the evaluation of long-term multilineage human hematopoiesis, human immune function, and HIV infection of human hematolymphoid organs in vivo. Given these constructs, a variety of therapeutically important modalities can now be discovered and developed with a relevant in vivo model, prior to the time that they are moved into the clinic. With further progress in this and other preclinical models, useful therapies for important human disease states will hopefully be forthcoming at an even faster pace.

Immune response to ultraviolet-induced tumors. II. Effector cells in tumor immunity.

Skin tumors induced in mice by chronic ultraviolet irradiation are highly antigenic and can induce a state of transplantation immunity in syngeneic hosts. In the present study, we compared the in vitro cytolytic activity of splenic lymphocytes from mice immunized with either a regressor or a progressor UV-tumor. The results of this comparison supported previous work implicating a role for tumor-specific cytolytic T lymphocytes in the rejection of regressor UV-tumors. The results also revealed that immunization with the progressor UV-tumor 2237 failed to elicit detectable levels of progressor tumor-specific CTL in animals capable of rejecting the immunizing tumor. Interestingly, following in vitro resensitization of both regressor and progressor immune spleen cells, we found a previously undetected lymphocyte population with anti-UV-tumor activity. Besides lysing UV-tumors in vitro, these lymphocytes also lysed a wide variety of additional tumor targets. This effector activity along with the analysis of cell surface markers indicated that these lymphocytes belong to that category of effector cells mediating natural-cell-mediated cytotoxicity (NCMC). As we had not detected cells with this activity in splenic lymphocyte preparations prior to in vitro resensitization, we examined lymphocytes from the local tumor environment during the course of progressor 2237 tumor rejection for either NCMC activity or tumor-specific CTL activity. This in situ analysis revealed lymphocytes exhibiting significant levels of cytolytic activity against several UV-tumors, thus implicating NK cells as effector cells in the rejection of progressor UV-tumors by immune animals. The mechanisms whereby NK cells with NCMC activity could be induced in immune animals are discussed in the context of class-II-restricted immune responses by helper/inducer T lymphocytes.

Immune response to ultraviolet-induced tumors. III. Analysis of cloned lymphocyte populations exhibiting antitumor activity.

Previously, we hypothesized that natural killer lymphocytes could function as effector cells in the rejection of UV-induced tumors in tumor-immune animals. Immunization with progressor UV-tumor 2237 induced lymphocytes exhibiting natural cell-mediated cytotoxicity but failed to elicit tumor-specific cytolytic T lymphocytes. In the present investigation, T lymphocyte cloning technology provided a means of isolating homogeneous lymphocyte populations exhibiting CTL and NK activities. Clones with both CTL and NK activity were isolated from regressor-1316-immune mice, but NK-like clones only were isolated from progressor-2237-immune mice. An evaluation of the in situ anti-UV-tumor action of a representative NK lymphocyte clone revealed that these cells could in fact prevent tumor outgrowth, supporting our hypothesis that these cells could function as effector cells in UV-tumor rejection responses in tumor-immune animals.

Lymphocyte migration molecules.

Recirculating lymphocytes leave the blood by selectively binding to specialized high endothelial venules (HEV) in the lymphoid organs of the body1. This binding is mediated by specific lymphocyte surface molecules called homing receptors2. Functionally distinct homing receptors control lymphocyte migration to peripheral lymph nodes, to mucosa-associated lymphatic tissues (appendix, Peyer’s patches) or to inflamed joint tissue (synovium)3,4. Most virgin lymphocytes appear to express all three different types of receptors and can, therefore, migrate throughout the body in search of antigen. Following antigenic activation the lymphocytes go through a nonmigratory phase after which many of them, as memory and effector cells, appear to express a single homing receptor class that controls their migration to particular tissues of the body similar to the site where they first encountered their cognate antigen. Thus, organ-specific homing receptors are thought to play an essential role in determining the characteristics of local immune responses2,5.