Inhibition of hydrophobic protein-mediated Candida albicans attachment to endothelial cells during physiologic shear flow.

Adhesion interactions during hematogenous dissemination of Candida albicans likely involve a complex array of host and fungal factors. Possible C. albicans factors include changes in cell surface hydrophobicity and exposed antigens that have been shown in static adhesion assays to influence attachment events. We used a novel in vitro shear analysis system to investigate host-pathogen interactions and the role of fungal cell surface hydrophobicity in adhesion events with human endothelial cells under simulated physiologic shear. Endothelial monolayers were grown in capillary tubes and tested with and without interleukin-1 beta activation in buffered medium containing human serum. Hydrophobic and hydrophilic stationary-phase C. albicans yeast cells were infused into the system under shear flow and found to adhere with widely varying efficiencies. The average number of adherent foci was determined from multiple fields, sampled via video microscopy, between 8 and 12 min after infusion. Hydrophobic C. albicans cells demonstrated significantly more heterotypic binding events (Candida-endothelial cell) and greater homotypic binding events (Candida-Candida) than hydrophilic yeast cells. Cytokine activation of the endothelium significantly increased binding by hydrophobic C. albicans compared to unactivated host cells. Preincubation of hydrophobic yeast cells with a monoclonal antibody against hydrophobic cell wall proteins significantly blocked adhesion interactions with the endothelial monolayers. Because the antibody also blocks C. albicans binding to laminin and fibronectin, results suggest that vascular adhesion events with endothelial cells and exposed extracellular matrix may be blocked during C. albicans dissemination. Future studies will address the protective efficacy of blocking or redirecting blood-borne fungal cells to favor host defense mechanisms.

Oligonucleotide inhibitors of P-selectin-dependent neutrophil-platelet adhesion.

P-selectin, an inducible cell adhesion molecule, mediates rolling of neutrophils on activated vascular endothelium. Because rolling is an early event of the inflammatory response, therapeutic applications of selectin antagonists have been of broad interest. There are, however, no truly satisfactory therapeutic candidates among known inhibitors. Consequently, we have used Systematic Evolution of Ligands by Exponential Enrichment (SELEX) technology, a process based on oligonucleotide combinatorial chemistry and in vitro selection, to develop aptamer antagonists of P-selectin. Equilibrium dissociation constants for aptamer/P-selectin binding range from 16 to 710 pM, a 10(5)-10(6)-fold improvement compared with the minimal carbohydrate ligand, sialyl Lewis X (sLeX). Aptamer binding is divalent cation dependent and, unlike sLeX, is specific for P-selectin. The selectivity for human P-selectin relative to human E-selectin or human L-selectin is 10(4)-10(5). In vitro, aptamers bind with subnanomolar affinities to P-selectin expressed on thrombin-activated platelets, inhibit the binding of P-selectin-IgG chimera to sLeX and to neutrophils, and block the binding activated platelets to neutrophils in flow cytometry and in hydrodynamic assays. Extrapolating from their in vitro characteristics, these novel P-selectin-specific antagonists may be suitable candidates for therapeutic development.

DNA aptamers block L-selectin function in vivo. Inhibition of human lymphocyte trafficking in SCID mice.

Selectins participate in the initial events leading to leukocyte extravasation from the blood into tissues. Thus the selectins have generated much interest as targets for antiinflammatory agents. Therapeutic molecules based on the monomeric carbohydrate ligand sialyl Lewis X (SLe(X)) have low affinities and are not specific for a given selectin. Using SELEX (Systematic Evolution of Ligands by EXponential Enrichment) technology, we have generated aptamers specific for L-selectin that require divalent cations for binding and have low nanomolar affinity. In vitro, the deoxyoligonucleotides inhibit L-selectin binding to immobilized SLe(X) in static assays and inhibit L-selectin-mediated rolling of human lymphocytes and neutrophils on cytokine-activated endothelial cells in flow-based assays. These aptamers also block L-selectin-dependent lymphocyte trafficking in vivo, indicating their potential utility as therapeutics.

Distinct roles of L-selectin and integrins alpha 4 beta 7 and LFA-1 in lymphocyte homing to Peyer's patch-HEV in situ: the multistep model confirmed and refined.

Circulating lymphocytes home to the mucosal lymphoid organs, Peyer's patches (PP), through high endothelial venules (HEV). In situ analyses revealed that transfused lymph node cells (LNCs) interact with PP-HEV in a series of overlapping adhesion events: L-selectin (CD62L) > alpha 4 beta 7 initiates interaction, L-selectin and alpha 4 beta 7 both participate in rolling, and G alpha i-linked activation triggers arrest that requires both alpha 4 beta 7 and LFA-1. alpha 4 beta 7 dramatically reduces rolling velocity, and appears to be required for engagement of LFA-1. In contrast with resting LNC, preactivated LNC or alpha 4 beta 7hi lymphoma cells require only alpha 4 beta 7 for arrest in PP-HEV. The predominant PP-HEV ligand for alpha 4 beta 7 but also apparently for L-selectin is the mucosal addressin MAd-CAM-1. These results validate the concept of multimolecular adhesion/decision cascades in physiologic lymphocyte-endothelial recognition, define a novel role for alpha 4 integrins as a "bridge" between selectin and beta 2 integrin-dependent events, and reemphasize the potential for direct adhesion through preactivated alpha 4 integrins alone.

alpha 4 integrins mediate lymphocyte attachment and rolling under physiologic flow.

Of the several families of adhesion receptors involved in leukocyte-endothelial cell interactions, only the selectins have been shown to initiate leukocyte interaction under physiologic shear; indeed, beta 2 (CD18) intergrins responsible for neutrophil arrest are unable to engage without prior selectin-mediated rolling. In contrast, alpha 4 (CD49d) integrins are shown here to initiate lymphocyte contract ("tethering") in vitro under shear and in the absence of a selectin contribution. The alpha 4 integrin ligands MAdCAM-1 and VCAM-1 support loose reversible interactions including rolling, as well as rapid sticking and arrest that is favored following integrin activation. Moreover, alpha 4 beta 7 mediates L-selectin (CD62L)-independent attachment of blood-borne lymphocytes to lamina propria venules in situ. Scanning electron microscopy of alpha 4 beta 7hi lymphoid cells reveals that, like L-selectin, alpha 4 beta 7 is highly concentrated on microvillous sites of initial cellular contact, whereas the beta 2 integrin LFA-1 is excluded from villi. Thus, alpha 4 but not beta 2 integrins can initiate leukocyte adhesion under flow, a capacity that may be in part a function of topographic presentation on microvilli.

Neutrophils roll on adherent neutrophils bound to cytokine-induced endothelial cells via L-selectin on the rolling cells.

Specific arrest of neutrophils in venules is central to their rapid accumulation during local inflammatory responses. Initial neutrophil rolling on endothelium is mediated by leukocyte L-selectin and the inducible vascular adhesion proteins P- and E-selectin. This rolling is a prerequisite for endothelial-dependent neutrophil arrest. Here we describe rolling of neutrophils on the surface of previously arrested neutrophils and demonstrate that this interaction involves L-selectin exclusively on rolling cells. The adherent neutrophil support of L-selectin-dependent neutrophil rolling in vivo can promote continuous and augmented leukocyte recruitment at sites of previous neutrophil accumulation.

Cell surface P- and E-selectin support shear-dependent rolling of bovine gamma/delta T cells.

The vascular selectins P- and E-selectin are inducible adhesion proteins expressed by endothelial cells that have been shown to support shear-dependent rolling of myeloid cells. This interaction is thought to be a prerequisite event for subsequent steps, such as tight adhesion/aggregation and transendothelial cell migration, involved in the accumulation of leukocytes into tissues. Certain lymphocyte subsets have also been shown to bind the vascular selectins, but the importance of this interaction in mediating shear-dependent rolling, as described for myeloid cells, has not been demonstrated. We expand on our earlier observation that bovine gamma/delta T cells bind E-selectin by showing that this interaction leads to a reproducible rolling event in assays done under shear forces that approximate those that occur in vivo. E-selectin, expressed by L cell transfectants or cytokine-stimulated human and bovine endothelial cells, equally supports the shear-dependent rolling interaction. The lymphocyte adhesion proteins L-selectin, CD44, and CD2 do not contribute to this event. Neuraminidase treatment of the gamma/delta T cells or addition of EDTA to the assay completely blocks the rolling interaction. We further show for the first time that P-selectin expressed by thrombin-activated platelets or a soluble P-selectin/human Ig chimera specifically binds gamma/delta T cells. The P-selectin interaction is similar to the rolling event mediated by E-selectin--it requires divalent cations and sialic acid on the lymphocyte, it lacks involvement of L-selectin and CD44, and rolling occurs under physiologic shear conditions. These results provide the documentation that the vascular selectins can support shear-dependent rolling of a lymphocyte subset and that P-selectin mediates the adhesion of gamma/delta T cells.

In vivo and in vitro functional examination of a conserved epitope of L- and E-selectin crucial for leukocyte-endothelial cell interactions.

Selectins constitute a three-member gene family of carbohydrate-binding adhesion proteins found on the surface of leukocytes and endothelial cells that is central to inflammation-associated leukocyte recruitment and lymphocyte recirculation. E- and P-selectin are inducible and expressed on the surface of endothelial cells under inflammatory conditions, whereas L-selectin is constitutively expressed on most circulating leukocytes. Previously, we have characterized a unique mAb (EL-246) that recognizes a common epitope on both E- and L-selectin, which is presented or determined by their short consensus repeat domains. This report defines the functional properties of EL-246 and its cognate epitope. In a novel in vitro physiologic shear system, we show that neutrophil rolling on activated HUVECs and on E-selectin cDNA transfectants is blocked 45 to 120 s after infusion of EL-246. The examination of the binding of neutrophils to E-selectin cDNA transfectants reveals that their adhesion is blocked by EL-246 treatment of either cell type. A unique Ab transfer mechanism is demonstrated in which El-246 is delivered unidirectionally from L- to E-selectin to surpass the adhesion blocked by mAbs that recognize either L- or E-selectin alone. By using flow cytometry and in vivo homing techniques, we show that pretreating bovine lymphocytes with EL-246 blocks their ability to home to mouse peripheral lymph nodes by > 65%. Cumulatively, these results suggest that EL-246 is a uniquely potent pharmacologic inhibitor of leukocyte-endothelial cell interactions that are mediated by either E- or L-selectin.

L-selectin-mediated lymphocyte rolling on MAdCAM-1.

The L-selectin, a cell surface C-type lectin, directs lymphocyte traffic to lymph nodes, and contributes to lymphocyte homing to Peyer's patches and to leukocyte interactions with inflamed venules. Here we report that the mucosal vascular addressin MAdCAM-1, a mucosal endothelial adhesion molecule with immunoglobulin- and mucin-like domains, is a facultative ligand for L-selectin. MAdCAM-1 isolated from mesenteric lymph nodes, but not from cultured endothelioma cells, bears N-glycanase-resistant sialic acid-containing carbohydrate which supports adhesion of L-selectin-transfected lymphoid cells under shear. Interacting lymphoid cells display a 'rolling' behaviour similar to the selectin-dependent rolling of neutrophils observed in inflamed venules. MAdCAM-1 is also a ligand for the lymphocyte integrin homing receptor for Peyer's patches, alpha 4 beta 7 (ref. 7), and may be uniquely adapted to support both selectin-mediated lymphocyte rolling and integrin-mediated adhesion and arrest in vivo.

Rapid G protein-regulated activation event involved in lymphocyte binding to high endothelial venules.

The homing of blood borne lymphocytes into lymph nodes and Peyer's patches is mediated in part by recognition and binding to specialized high endothelial venules (HEV). Here we demonstrate that a rapid pertussis toxin-sensitive lymphocyte activation event can participate in lymphocyte recognition of HEV. In situ video microscopic analyses of lymphocyte interactions with HEV in exteriorized mouse Peyer's patches reveal that pertussis toxin has no effect on an initial "rolling" displayed by many lymphocytes, but inhibits an activation-dependent "sticking" event required for lymphocyte arrest. This is the first demonstration that physiologic lymphocyte-endothelial interactions can involve sequential rolling, activation, and activation-dependent arrest, previously shown only for neutrophils. The inhibitory effect of the toxin is dependent on its G protein-modifying ADP-ribosyltransferase activity and can be reversed by phorbol myristic acetate, which bypasses cell surface receptors to trigger activation-dependent adhesion. Lymphocyte sticking can occur within 1-3 s after initiation of rolling. We conclude that a rapid receptor-mediated activation event involving G protein signaling can trigger stable lymphocyte attachment to HEV in vivo, and may play a critical role in regulating lymphocyte homing.

Two-step model of leukocyte-endothelial cell interaction in inflammation: distinct roles for LECAM-1 and the leukocyte beta 2 integrins in vivo.

The lectin homing receptor LECAM-1 (LAM-1, Leu8) and the beta 2 integrins, particularly Mac-1 (CD11b/CD18), participate in leukocyte-endothelial cell interactions in inflammation. LECAM-1 is rapidly shed while Mac-1 expression is dramatically increased upon neutrophil activation, suggesting functionally distinct roles for these molecules. Using intravital video microscopy, we have compared the effect of antibodies against LECAM-1 and CD18 on leukocyte interactions with rabbit mesenteric venules. Anti-LECAM-1 monoclonal antibody and its Fab fragments inhibited initial reversible leukocyte rolling along the vascular wall. Anti-CD18 monoclonal antibody had no effect on rolling but prevented subsequent firm attachment of leukocytes to venular endothelium. These results support a two-step model of leukocyte-endothelial cell interactions: reversible rolling mediated in part by LECAM-1 facilitates leukocyte recruitment by the local microenvironment and precedes activation-dependent firm attachment involving beta 2 integrins.

Expression of low levels of peripheral lymph node-associated vascular addressin in mucosal lymphoid tissues: possible relevance to the dissemination of passaged AKR lymphomas.

Lymphoid tumors display a wide variety of growth patterns in vivo, from that of a solitary extralymphoid tumor, to a general involvement of all lymphoid organs. Normal lymphocytes are uniquely mobile cells continuously recirculating between blood and lymph throughout much of their life cycle. Therefore, it is reasonable to propose that disseminating malignant lymphocytes may express recirculation characteristics or homing properties consistent with that of their normal lymphoid counterparts. Trafficking of lymphocytes involves the expression and recognition of both lymphocyte homing receptors and their opposing receptors on endothelium, the vascular addressins. These cell surface elements direct the tissue-selective localization of lymphocyte subsets in vivo into organized lymphoid organs and sites of chronic inflammation where specific binding events occur between lymphocytes and the endothelium of specialized high endothelial venules (HEV). In a recent murine study of 13 lymphoma lines, we found that lymphomas that bind well to high endothelial venules, in the Stamper-Woodruff in vitro assay (an assay of lymphocyte binding to venules in frozen sections of peripheral lymph nodes or Peyer's patches), spread hematogenously to all high endothelial venule bearing lymphoid organs, whereas non-binding lymphomas did not. In some cases lymphomas that bound with a high degree of selectivity to peripheral lymph node (PLN) high endothelial venules exhibited only limited organ preference of metastasis, involving the mucosal lymphoid organs Peyer's patches (PP) in addition to the peripheral lymph nodes of adoptive recipients. Here we demonstrate that Peyer's patch high endothelial venules express a low but functional level of peripheral lymph node addressin (MECA-79) that can be recognized by lymphomas expressing the peripheral lymph node homing receptor (MEL-14 antigen).(ABSTRACT TRUNCATED AT 250 WORDS)

Molecular cloning and expression of Pgp-1. The mouse homolog of the human H-CAM (Hermes) lymphocyte homing receptor.

Mouse phagocytic glycoprotein-1 (Pgp-1; Ly-24) is a 95-kDa glycoprotein of unknown function that has served as an important T cell/leukocyte differentiation marker. Recent work has suggested that it may be related to a human 85- to 95-kDa glycoprotein (termed variously the Hermes Ag/lymphocyte homing receptor, ECMRIII, P80, and CD44) that is involved in lymphocyte binding to high endothelial venules in the process of lymphocyte homing, and has been implicated in other cell adhesion events. The widespread expression of this molecular class in diverse organ systems suggests a broad role in cellular adhesion, and has led to the unifying designation homing-cellular adhesion molecule (H-CAM). By using human H-CAM cDNA probes, we have isolated a full-length cDNA for the mouse homolog. Comparison of the human and mouse sequences reveals that an N-terminal domain homologous to cartilage proteoglycan core and link proteins, as well as the C-terminal transmembrane and cytoplasmic sequences, are highly conserved (89% and 86% identity, respectively). In contrast, a proximal extracellular domain thought to serve as a target for O-glycosylation and chondroitin sulfate attachment has undergone substantial divergence (only 42% identity). Transient expression of the cDNA in CHO cells followed by immunologic staining confirms that this mouse H-CAM cDNA encodes Pgp-1.1, one of two known Pgp-1 alloantigens.

Inflammation-induced endothelial cell adhesion to lymphocytes, neutrophils, and monocytes. Role of homing receptors and other adhesion molecules.

Adhesion to the vascular endothelium precedes or is a necessary prelude to leukocyte migration into the underlying tissue. Constitutive lymphocyte trafficking through lymphoid organs is controlled by tissue-specific interactions between molecules expressed on the surface of the lymphocyte (homing receptors) and ligands (vascular addressins) expressed on endothelial cells (HEV) within lymphoid tissues. Preliminary evidence suggests that lymphocytes may employ related but distinct interactions in their entry into some chronic sites of inflammation. Other leukocytes, such as neutrophils and monocytes, express molecules related or identical to lymphocyte homing receptors, and these molecules are exquisitely regulated by chemotactic factors and appear to be involved in the homing of these cells to inflamed tissues. In addition, inflammation in vivo induces increased endothelial cell adhesiveness for leukocytes that undoubtedly plays a key role in regulating leukocyte extravasation. Tissue- and inflammation-specific leukocyte/endothelial cell adhesion molecules constitute attractive targets for suppression or manipulation of the early stages of tissue inflammation.

A human lymphocyte homing receptor, the hermes antigen, is related to cartilage proteoglycan core and link proteins.

Lymphocyte interactions with high endothelial venules (HEV) during extravasation into lymphoid tissues involve an 85-95 kd class of lymphocyte surface glycoprotein(s), gp90Hermes (CD44). We report here the cloning of cDNA for gp90Hermes expressed in a mucosal HEV-binding B lymphoblastoid cell line, KCA. Northern hybridization revealed the presence of three invariant RNA bands at 1.5, 2.2, and 4.5 kb in mucosal HEV-, lymph node HEV-, or dual-binding cells. The deduced amino acid sequence predicts a mature protein with a C-terminal cytoplasmic tail, a hydrophobic transmembrane domain of 23 amino acids, and an N-terminal extracellular region of 248 amino acids. A proximal extracellular domain is the probable region of O-glycosylation and chondroitin sulfate linkage and displays at least two of the three immunodominant epitope clusters of native gp90Hermes. A distal region contains the majority of potential N-glycosylation sites and cysteines, and exhibits a striking homology to tandemly repeated domains of the cartilage link and proteoglycan core proteins. No significant similarities were found to the immunoglobulin, integrin, or cadherin gene families. Thus gp90Hermes represents a novel class of integral membrane protein involved in lymphocyte-endothelial cell interactions and lymphocyte homing.

A tissue-specific endothelial cell molecule involved in lymphocyte homing.

An endothelial cell surface molecule that is selectively expressed in mucosal organs is required for lymphocyte homing to mucosal lymphoid tissues. This 'vascular addressin' appears to function as a tissue-specific marker or address signal for recognition by lymphocytes circulating in the blood.

High endothelial venule binding as a predictor of the dissemination of passaged murine lymphomas.

It has long been postulated that normal lymphocyte homing mechanisms help determine the metastatic spread of lymphoid neoplasms. The traffic of normal lymphocytes is controlled in part by the regulated expression of surface receptors for high endothelial venules (HEV), specialized venules that mediate the extravasation of circulating lymphocytes from the blood into lymphoid organs and sites of chronic inflammation. Here we have compared the in vivo growth patterns of HEV-binding vs. nonbinding murine lymphomas passaged intramuscularly into syngeneic recipients. We report that lymphomas that bind well to HEV (as assessed in a quantitative in vitro assay) disseminate widely via the blood, involving all lymph node groups symmetrically. Although both HEV-binding and nonbinding lymphomas gain access to the blood, gross involvement of lymph nodes by nonbinding lymphomas is limited to nodes draining local tumor at the site of injection, a prominent feature of these lymphomas; distant lymph nodes are not enlarged. The results suggest that the expression of functional receptors for HEV either controls the hematogenous dissemination of malignant lymphocyte populations to HEV-bearing organs, or is coregulated with factors determining this metastatic behavior. The findings support the concept that normal lymphocyte homing mechanisms are important to the spread of leukemias and lymphomas.

Lymphocyte recognition of high endothelium: antibodies to distinct epitopes of an 85-95-kD glycoprotein antigen differentially inhibit lymphocyte binding to lymph node, mucosal, or synovial endothelial cells.

The tissue-specific homing of lymphocytes is directed by specialized high endothelial venules (HEV). At least three functionally independent lymphocyte/HEV recognition systems exist, controlling the extravasation of circulating lymphocytes into peripheral lymph nodes, mucosal lymphoid tissues (Peyer's patches or appendix), and the synovium of inflamed joints. We report here that antibodies capable of inhibiting human lymphocyte binding to one or more HEV types recognize a common 85-95-kD lymphocyte surface glycoprotein antigen, defined by the non-blocking monoclonal antibody, Hermes-1. We demonstrate that MEL-14, a monoclonal antibody against putative lymph node "homing receptors" in the mouse, functionally inhibits human lymphocyte binding to lymph node HEV but not to mucosal or synovial HEV, and cross-reacts with the 85-95-kD Hermes-1 antigen. Furthermore, we show that Hermes-3, a novel antibody produced by immunization with Hermes-1 antigen isolated from a mucosal HEV-specific cell line, selectively blocks lymphocyte binding to mucosal HEV. Such tissue specificity of inhibition suggests that MEL-14 and Hermes-3 block the function of specific lymphocyte recognition elements for lymph node and mucosal HEV, respectively. Recognition of synovial HEV also involves the 85-95-kD Hermes-1 antigen, in that a polyclonal antiserum produced against the isolated antigen blocks all three classes of lymphocyte-HEV interaction. From these studies, it is likely that the Hermes-1-defined 85-95-kD glycoprotein class either comprises a family of related but functionally independent receptors for HEV, or associates both physically and functionally with such receptors. The findings imply that related molecular mechanisms are involved in several functionally independent cell-cell recognition events that direct lymphocyte traffic.

Leukocyte-endothelial cell recognition: evidence of a common molecular mechanism shared by neutrophils, lymphocytes, and other leukocytes.

The interaction of leukocytes with endothelial cells is intrinsic to the process of leukocyte extravasation, whether during the entry of blood polymorphonuclear leukocytes and monocytes into sites of acute and chronic inflammation, or during the homing of lymphocytes to lymphoid organs. A lymphocyte surface glycoprotein, defined by monoclonal antibody MEL-14, has been described that appears to mediate lymphocyte recognition of postcapillary venules in peripheral lymph nodes, and to control the migration of lymphocytes from the blood into these lymphoid organs. We now report that the antigenic determinant recognized by MEL-14 is present at high levels on other leukocytes as well, including neutrophils, monocytes, and eosinophils; and we demonstrate involvement of the MEL-14 antigen in neutrophil-endothelial cell interactions. MEL-14 immunoprecipitates a neutrophil surface protein of Mr approximately 100,000, similar in m.w. to the 80,000 to 90,000 dalton lymphocyte surface MEL-14 antigen, and it blocks the interaction of neutrophils with endothelial cells in an in vitro model of adhesion to postcapillary venules in lymph node frozen sections. Neutrophil binding to lymph node venules is also inhibited by PPME, a mannose-6-phosphate-rich yeast polysaccharide that is thought to mimic the endothelial cell ligand for the MEL-14-defined lymphocyte receptor. Interestingly, neither MEL-14 nor PPME exhibit a major effect on neutrophil binding to postcapillary venules in Peyer's patches, suggesting that as for lymphocytes, the neutrophil MEL-14 antigen is involved in recognition of tissue-specific endothelial determinants. Finally, we show that MEL-14 inhibits the capacity of neutrophils to migrate from the blood into sites of acute inflammation in the skin. These observations lead us to propose that receptors for tissue-specific endothelial determinants are utilized by neutrophils and lymphocytes and probably other leukocytes during the physiologic process of leukocyte extravasation in vivo.

Lymphoid tissue- and inflammation-specific endothelial cell differentiation defined by monoclonal antibodies.

Endothelial cells play an essential role in immune responses by regulating the entry of leukocytes into lymphoid tissues and sites of inflammation. As an initial approach to analyzing endothelial cell specialization in relation to such immune function, we have produced monoclonal antibodies (MAB) against mouse lymph node endothelium. Three antibodies were selected: MECA-20, recognizing the endothelium of all blood vessels in lymphoid as well as non-lymphoid organs; MECA-217, which stains the endothelium lining large elastic arteries, but among small vessels is specific for post-capillary venules within lymphoid organs and tissues exposed to exogenous antigen, such as skin and uterus; and MECA-325, an antibody that demonstrates specificity for the specialized high endothelial venules (HEV) that control lymphocyte homing into lymph nodes and Peyer's patches. MECA-325 failed to stain vessels in any non-lymphoid organs tested. Immunoperoxidase studies of HEV in lymph node frozen sections, and of isolated high endothelial cells in suspensions, demonstrated that the antigens recognized by all three antibodies are expressed at the cell surface; those defined by MECA-20 and MECA-325 are also present in the cytoplasm. To study the regulation of the antigens defined by these MAB in relation to extra-lymphoid immune reactions, we assessed their expression in induced s.c. granulomas as a model for chronic inflammation. Small vessels in the granulomas were already stained by MECA-217 in the first days of development. In contrast MECA-325 detected postcapillary venules (which frequently displayed the morphologic characteristics of HEV) only from approximately 1 wk, in parallel with the development of a persistent mononuclear cell infiltrate including numerous lymphocytes. The selective appearance of the MECA-325 antigen on vascular endothelium supporting lymphocyte traffic in both lymphoid and extra-lymphoid sites suggests that this antigen may play an important role in the process of lymphocyte extravasation. The demonstration of lymphoid organ- and inflammation-specific microvascular antigens offers direct evidence for a complex specialization of endothelium in relation to immune stimuli, and supports the concept that microvascular differentiation may play an important role in local immune responses.