Pericytes limit tumor cell metastasis

Previously we observed that neural cell adhesion molecule (NCAM) deficiency in beta tumor cells facilitates metastasis into distant organs and local lymph nodes. Here, we show that NCAM-deficient beta cell tumors grew leaky blood vessels with perturbed pericyte-endothelial cell-cell interactions and deficient perivascular deposition of ECM components. Conversely, tumor cell expression of NCAM in a fibrosarcoma model (T241) improved pericyte recruitment and increased perivascular deposition of ECM molecules. Together, these findings suggest that NCAM may limit tumor cell metastasis by stabilizing the microvessel wall. To directly address whether pericyte dysfunction increases the metastatic potential of solid tumors, we studied beta cell tumorigenesis in primary pericyte-deficient Pdgfb(ret/ret) mice. This resulted in beta tumor cell metastases in distant organs and local lymph nodes, demonstrating a role for pericytes in limiting tumor cell metastasis. These data support a new model for how tumor cells trigger metastasis by perturbing pericyte-endothelial cell-cell interactions.     

Mice lacking expression of the chemokines CCL21-ser and CCL19 (plt mice) demonstrate delayed but enhanced T cell immune responses

The paucity of lymph node T cells (plt) mutation leads to a loss of CCL21 and CCL19 expression in secondary lymphoid organs. plt mice have defects in the migration of naive T cells and activated dendritic cells into the T cell zones of lymphoid organs, suggesting that they would have defects in T cell immune responses. We now demonstrate T cell responses in plt mice are delayed but ultimately enhanced. Responses to contact sensitization are decreased at day 2 after priming but increased at day 6. After subcutaneous immunization, antigen-specific T cell proliferation and cytokine production in plt mice are increased and remain markedly elevated for at least 8 wk. Compared with wild-type mice, a proportion of T cell response in plt mice are shifted to the spleen, and prior splenectomy reduces the T cell response in draining lymph nodes. After immunization of plt mice, T cells and dendritic cells colocalize in the superficial cortex of lymph nodes and in splenic bridging channels, but not in T cell zones. These results demonstrate that plt mice mount robust T cell responses despite the failure of naive T cells and activated dendritic cells to enter the thymus dependent areas of secondary lymphoid organs.     

L-selectin shedding does not regulate constitutive T cell trafficking but controls the migration pathways of antigen-activated T lymphocytes

L-selectin mediates rolling of lymphocytes in high endothelial venules (HEVs) of peripheral lymph nodes (PLNs). Cross-linking of L-selectin causes proteolytic shedding of its ectodomain, the physiological significance of which is unknown. To determine whether L-selectin shedding regulates lymphocyte migration, a mutant form that resists shedding (LdDeltaP-selectin) was engineered. Transgenic mice expressing either LDeltaP or wild-type (WT) L-selectin on T cells were crossed with L-selectin knockout (KO) mice. The cellularity and subset composition of secondary lymphoid organs did not differ between LDeltaP and WT mice, however, they were different from C57BL/6. Plasma levels of soluble L-selectin in LDeltaP mice were reduced to <5% of WT and C57BL/6 mice. The rolling properties of T lymphocytes from LDeltaP and WT mice on immobilized L-selectin ligands were similar. Furthermore, similar numbers of LDeltaP and WT T lymphocytes were recruited from the bloodstream into PLNs in mice, although LDeltaP T cells transmigrated HEVs more slowly. WT, but not LDeltaP-selectin, underwent rapid, metalloproteinase-dependent shedding after TCR engagement, and LDeltaP T cells retained the capacity to enter PLNs from the bloodstream. These results suggest that the ability to shed L-selectin is not required for T cell recirculation and homing to PLNs. However, L-selectin shedding from antigen-activated T cells prevents reentry into PLNs.     

GlyCAM-1, a physiologic ligand for L-selectin, activates beta 2 integrins on naive peripheral lymphocytes

Naive T cells are selectively recruited from the blood into peripheral lymph nodes during lymphocyte recirculation. L-selectin, a lectin-like receptor, mediates the initial attachment of lymphocytes to high endothelial venules (HEV) in lymph nodes. A subsequent step involving the activation of beta 2 integrins has been proposed to facilitate firm adhesion, but the activating signals are poorly understood. We report here that either antibody-mediated cross-linking of L-selectin on human lymphocytes or treatment of the cells with GlyCAM-1, an HEV-derived, secreted ligand for L-selectin, stimulates their binding to ICAM-1 through the beta 2 integrin pathway. Furthermore, GlyCAM-1 causes the rapid expression of a neoepitope on beta 2 integrins associated with a high-avidity state. Naive (CD45RA+), but not memory (CD45R0+) lymphocytes, respond to L-selectin cross-linking or GlyCAM-1 treatment. Thus, the complexing of L-selectin by specific ligands may provide key signals to naive lymphocytes, contributing to their selective recruitment into peripheral lymphoid organs.     

Lymphocyte-HEV interactions in lymph nodes of a sulfotransferase-deficient mouse

The interaction of L-selectin expressed on lymphocytes with sulfated sialomucin ligands such as CD34 and GlyCAM-1 on high endothelial venules (HEV) of lymph nodes results in lymphocyte rolling and is essential for lymphocyte recruitment. HEC-GlcNAc6ST-deficient mice lack an HEV-restricted sulfotransferase with selectivity for the C-6 position of N-acetylglucosamine (GlcNAc). HEC-GlcNAc6ST-/- animals exhibit faster lymphocyte rolling and reduced lymphocyte sticking in HEV, accounting for the diminished lymphocyte homing. Isolated CD34 and GlyCAM-1 from HEC-GlcNAc6ST-/- animals incorporate approximately 70% less sulfate than ligands from wild-type animals. Furthermore, these ligands exhibit a comparable reduction of the epitope recognized by MECA79, a function-blocking antibody that reacts with L-selectin ligands in a GlcNAc-6-sulfate-dependent manner. Whereas MECA79 dramatically inhibits lymphocyte rolling and homing to lymph nodes in wild-type mice, it has no effect on HEC-GlcNAc6ST-/- mice. In contrast, in vitro rolling on purified GlyCAM-1 from HEC-GlcNAc6ST-/- mice, although greatly diminished compared with that on the wild-type ligand, is inhibited by MECA79. Our results demonstrate that HEC-GlcNAc6ST contributes predominantly, but not exclusively, to the sulfation of HEV ligands for L-selectin and that alternative, non-MECA79-reactive ligands are present in the absence of HEC-GlcNAc6ST.     

Impaired primary T cell responses in L-selectin-deficient mice

L-selectin is a homing receptor that mediates the selective attachment of leukocytes to specialized high endothelial venules. To study the potential role of L-selectin in immune responses in intact mice, we generated L-selectin-deficient mice by gene targeting. L-selectin- deficient mice are defective in cutaneous delayed-type hypersensitivity (DTH) responses when tested after conventional intervals of immunization (4 d). Primary T cell proliferative responses and cytokine production (interleukin [IL] 2, IL-4, and interferon gamma) were also compromised when tested after 5 d of immunization, indicating that L- selectin is important for the immune response to antigens. In contrast, after more prolonged immunization protocols (9 d), normal responses were observed, suggesting that L-selectin-independent compensatory mechanisms exist. Interestingly, humoral responses of L-selectin- deficient mice to keyhole limpet hemocyanin are indistinguishable from wild-type control mice, implying that L-selectin plays no rate-limiting role in T cell help of B cell function. Thus, our results suggest that L-selectin plays an important role in the generation of primary T cell responses but may not be essential for humoral and memory T cell responses. L-selectin does not appear to be rate limiting for the events leading to antigen-driven neutrophil recruitment, since normal DTH responses are obtained at late time points after immunization.     

L-selectin-deficient mice have impaired leukocyte recruitment into inflammatory sites

L-selectin, a cell surface adhesion molecule that is expressed by most leukocytes, mediates leukocyte rolling along vascular endothelium at sites of inflammation. The contribution of L-selectin to leukocyte migration in models of chronic inflammation was assessed by using mice that lack cell surface L-selectin expression. Significant inhibition of neutrophil (56-62%), lymphocyte (70-75%), and monocyte (72-78%) migration into an inflamed peritoneum was observed 24 and 48 h after administration of thioglycollate, an inflammatory stimulus. L-selectin- deficient mice were also significantly impaired in delayed-type hypersensitivity reactions. Footpad swelling in response to sheep red blood cell challenge was reduced 75% in L-selectin-deficient mice compared with wild-type mice. Ear swelling in a model of contact hypersensitivity induced by oxazolone challenge was also reduced by 69% compared to wild-type mice. Consistent with L-selectin-mediating leukocyte migration into diverse vascular beds during inflammation, L- selectin-deficient mice were significantly resistant to death resulting from lipopolysaccharide (LPS)-induced toxic shock. LPS administration resulted in a 90% mortality rate in control mice after 24 h, while there was a 90% survival rate in L-selectin-deficient mice. These results demonstrate that L-selectin plays a prominent role in leukocyte homing to nonlymphoid tissues during inflammation and that blocking this process can be beneficial during pathological inflammatory responses.     

Aspirin reduces lung cancer metastasis to regional lymph nodes

Background

Lung cancer is the main cause of cancer-related death worldwide. The high mortality is probably attributable to early metastasis; however, the mechanism underlying metastasis to regional lymph nodes is still unknown. Cyclooxygenase (COX)-derived prostaglandin E₂ (PGE₂) induces tumor growth and metastasis and is associated with a poor prognosis. The present study investigated the effect of an authentic COX inhibitor, aspirin, on regional lymph node metastasis during the development of lung cancer in mice.

Methods

An orthotopic intrapulmonary implantation model based on male C57BL/6 (6–8-weeks-old) mice was used. The lungs were injected with a solution containing Lewis lung carcinoma (LLC) cells overexpressing green fluorescent protein (GFP) and BD Matrigel^®. The effect of aspirin on mediastinal lymph node metastasis of LCC cells from the primary injection sites was then examined.

Results

The implantation process took approximately 30 s per mouse and operative mortality was 10%. Single pulmonary nodules developed at the implanted site in 95% of animals, and regional mediastinal lymph node metastasis was observed at 14 days post-LLC-GFP cell injection in all mice that formed a primary lung tumor. The mean survival time of mice injected with LLC-GFP cells was 15 ± 3 days (range, 12–22 days). Histopathological analysis revealed that no metastatic tumors developed in the regional mediastinal lymph nodes by Day 10–12 post-LLC-GFP cell injection and no metastasis to distant organs or distant lymph nodes was observed by Day 21 post-injection. Oral administration of aspirin (100 mg/kg, twice a day) after LLC-GFP cell injection inhibited metastasis to the regional lymph nodes, with no significant suppression of primary tumor growth in the lungs. Aspirin treatment led to a significant reduction in mortality (P < 0.0001).

Conclusions

The present lymph node metastasis model is useful for evaluating the efficacy of agents that inhibit tumor metastasis to the regional lymph nodes. Aspirin reduced the metastasis of LLC-GFP cells injection to the regional lymph nodes, with a significant reduction in mortality. These findings suggested that COX inhibitors have potential for preventing lymph node metastasis.     

Hematopoietic Progenitor Cell Rolling in Bone Marrow Microvessels: Parallel Contributions by Endothelial Selectins and Vascular Cell Adhesion Molecule 1

We have used intravital microscopy to study physiologically perfused microvessels in murine bone marrow (BM). BM sinusoids and venules, but not adjacent bone vessels, supported rolling interactions of hematopoietic progenitor cells. Rolling did not involve L-selectin, but was partially reduced in wild-type mice treated with antibodies to P- or E-selectin and in mice that were deficient in these two selectins. Selectin-independent rolling was mediated by α4 integrins, which interacted with endothelial vascular cell adhesion molecule (VCAM)-1. Parallel contribution of the endothelial selectins and VCAM-1 is not known to direct blood cell trafficking to other noninflamed tissues. This combination of constitutively expressed adhesion molecules may thus constitute a BM-specific recruitment pathway for progenitor cells analogous to the vascular addressins that direct selective lymphocyte homing to lymphoid organs.     

Sulfation-dependent recognition of high endothelial venules (HEV)- ligands by L-selectin and MECA 79, and adhesion-blocking monoclonal antibody

L-selectin is a lectin-like receptor that mediates the attachment of lymphocytes to high endothelial venules (HEV) of lymph nodes during the process of lymphocyte recirculation. Two sulfated, mucin-like glycoproteins known as Sgp50/GlyCAM-1 and Sgp90/CD34 have previously been identified as HEV-associated ligands for L-selectin. These proteins were originally detected with an L-selectin/Ig chimera called LEC-IgG. GlyCAM-1 and CD34 are also recognized by an antiperipheral node addressin (PNAd) mAb called MECA 79, which blocks L-selectin- dependent adhesion and selectively stains lymph node HEV. The present study compares the requirements for the binding of MECA 79 and LEC-IgG to HEV-ligands. Whereas desialylation of GlyCAM-1 and CD34 drastically reduced binding to LEC-IgG, this treatment enhanced the binding of GlyCAM-1 to MECA 79. In contrast, the binding of both MECA 79 and LEC- IgG to GlyCAM-1 and CD34 was greatly decreased when the sulfation of these ligands was reduced with chlorate, a metabolic inhibitor of sulfation. Because MECA 79 stains HEV-like vessels at various sites of inflammation, recognition by L-selectin of ligands outside of secondary lymphoid organs may depend on sulfation. In addition to their reactivity with GlyCAM-1 and CD34, both MECA 79 and LEC-IgG recognize an independent molecule of approximately 200 kD in a sulfate-dependent manner. Thus, this molecule, which we designate Sgp200, is an additional ligand for L-selectin.     

The difference in immunological properties between lymph node metastatic and non-metastatic cell lines of MCA-induced fibrosarcoma of C4W mice.

To pursue the process of lymph node metastasis, i.e, the preferential tumor growth in lymph node, we have established the non-metastatic M2B cell line which was derived from 3-methylcholanthrene-induced fibrosarcoma of C4W mouse and a metastatic cell line, M2BLN-M+ which was obtained from metastatic lymph nodes of irradiated C4W mouse which was subcutaneously implanted with cultured tumor cells, because implanted tumor cells were derived from the spontaneous metastatic lymph node of the parental M2B tumor, but regressed in naive C4W mouse. We examined the characteristics of both tumor cell lines in terms of the immunological cellular interactions. M2BLN-M+ showed unexpectedly to be more susceptible to cytotoxicity of immune effectors (NK cell, macrophage and cytotoxic T lymphocyte) than M2B did. When cultured both tumor cells with these effector cells, the growth inhibition of M2BLN-M+ was greater than that of M2B. The regional lymph node of tumor-bearer, however, showed no effective cytotoxic activity as reported by others. On the contrary, when cultured both tumor cells with non-immune lymph node cells, to be surprised, the proliferation of M2B was markedly suppressed, while that of M2BLN-M+ was slightly inhibited. The lymph node cells of M2B-bearing mice showed stronger cytostatic activity to M2B. The results suggest that the cytostatic activity of lymph node cells will be a pivotal factor, concerning the establishment of lymph node metastasis.     

The inducible CXCR3 ligands control plasmacytoid dendritic cell responsiveness to the constitutive chemokine stromal cell-derived factor 1 (SDF-1)/CXCL12

The recruitment of selected dendritic cell (DC) subtypes conditions the class of the immune response. Here we show that the migration of human plasmacytoid DCs (pDCs), the blood natural interferon alpha-producing cells, is induced upon the collective action of inducible and constitutive chemokines. Despite expression of very high levels of CXCR3, pDCs do not respond efficiently to CXCR3 ligands. However, they migrate in response to the constitutive chemokine stromal cell-derived factor 1 (SDF-1)/CXCL12 and CXCR3 ligands synergize with SDF-1/CXCL12 to induce pDC migration. This synergy reflects a sensitizing effect of CXCR3 ligands, which, independently of a gradient and chemoattraction, decrease by 20-50-fold the threshold of sensitivity to SDF-1/CXCL12. Thus, the ability of the constitutive chemokine SDF-1/CXCL12 to induce pDC recruitment might be controlled by CXCR3 ligands released during inflammation such as in virus infection. SDF-1/CXCL12 and the CXCR3 ligands Mig/CXCL9 and ITAC/CXCL1 display adjacent expression both in secondary lymphoid organs and in inflamed epithelium from virus-induced pathologic lesions. Because pDCs express both the lymph node homing molecule l-selectin and the cutaneous homing molecule cutaneous lymphocyte antigen, the cooperation between inducible CXCR3 ligands and constitutive SDF-1/CXCL12 may regulate recruitment of pDCs either in lymph nodes or at peripheral sites of inflammation.     

CXCL12 mediates CCR7-independent homing of central memory cells, but not naive T cells, in peripheral lymph nodes

Central memory CD8(+) T cells (T(CM)) confer superior protective immunity against infections compared with other T cell subsets. T(CM) recirculate mainly through secondary lymphoid organs, including peripheral lymph nodes (PLNs). Here, we report that T(CM), unlike naive T cells, can home to PLNs in both a CCR7-dependent and -independent manner. Homing experiments in paucity of lymph node T cells (plt/plt) mice, which do not express CCR7 ligands in secondary lymphoid organs, revealed that T(CM) migrate to PLNs at approximately 20% of wild-type (WT) levels, whereas homing of naive T cells was reduced by 95%. Accordingly, a large fraction of endogenous CD8(+) T cells in plt/plt PLNs displayed a T(CM) phenotype. Intravital microscopy of plt/plt subiliac lymph nodes showed that T(CM) rolled and firmly adhered (sticking) in high endothelial venules (HEVs), whereas naive T cells were incapable of sticking. Sticking of T(CM) in plt/plt HEVs was pertussis toxin sensitive and was blocked by anti-CXCL12 (SDF-1alpha). Anti-CXCL12 also reduced homing of T(CM) to PLNs in WT animals by 20%, indicating a nonredundant role for this chemokine in the presence of physiologic CCR7 agonists. Together, these data distinguish naive T cells from T(CM), whereby only the latter display greater migratory flexibility by virtue of their increased responsiveness to both CCR7 ligands and CXCL12 during homing to PLN.     

VEGF-A induces tumor and sentinel lymph node lymphangiogenesis and promotes lymphatic metastasis

The mechanisms of tumor metastasis to the sentinel lymph nodes are poorly understood. Vascular endothelial growth factor (VEGF)-A plays a principle role in tumor progression and angiogenesis; however, its role in tumor-associated lymphangiogenesis and lymphatic metastasis has remained unclear. We created transgenic mice that overexpress VEGF-A and green fluorescent protein specifically in the skin, and subjected them to a standard chemically-induced skin carcinogenesis regimen. We found that VEGF-A not only strongly promotes multistep skin carcinogenesis, but also induces active proliferation of VEGF receptor-2-expressing tumor-associated lymphatic vessels as well as tumor metastasis to the sentinel and distant lymph nodes. The lymphangiogenic activity of VEGF-A-expressing tumor cells was maintained within metastasis-containing lymph nodes. The most surprising finding of our study was that even before metastasizing, VEGF-A-overexpressing primary tumors induced sentinel lymph node lymphangiogenesis. This suggests that primary tumors might begin preparing their future metastatic site by producing lymphangiogenic factors that mediate their efficient transport to sentinel lymph nodes. This newly identified mechanism of inducing lymph node lymphangiogenesis likely contributes to tumor metastasis, and therefore, represents a new therapeutic target for advanced cancer and/or for the prevention of metastasis.     

Accelerated tumor growth in mice deficient in DNAM-1 receptor

Since the identification of ligands for human and mouse DNAM-1, emerging evidence has suggested that DNAM-1 plays an important role in the T cell– and natural killer (NK) cell–mediated recognition and lysis of tumor cells. However, it remains undetermined whether DNAM-1 is involved in tumor immune surveillance in vivo. We addressed this question by using DNAM-1–deficient mice. DNAM-1–deficient cytotoxic T lymphocyte (CTL) and NK cells showed significantly less cytotoxic activity against DNAM-1 ligand-expressing tumors in vitro than wild-type (WT) cells. The methylcholanthrene (MCA)-induced fibrosarcoma cell line Meth A expressed the DNAM-1 ligand CD155, and DNAM-1–deficient mice showed increased tumor development and mortality after transplantation of Meth A cells. Moreover, the DNAM-1–deficient mice developed significantly more DNAM-1 ligand-expressing fibrosarcoma and papilloma cells in response to the chemical carcinogens MCA and 7,12-dimethylbenz[a]anthracene (DMBA), respectively, than did WT mice. These results indicate that DNAM-1 plays an important role in immune surveillance of tumor development.     

Tumors induced by murine sarcoma virus contain precursor cells capable of generating tumor-specific cytolytic T lymphocytes

Leukocyte fractions extracted from the tumor mass and the lymphoid organs of C57BL/6 (B6) mice carrying murine sarcoma virus-induced tumors contained primed cytolytic T-lymphocyte (CTL) precursor cells, in addition to active cytotoxic T cells. These leukocyte fractions gave a secondary response when stimulated in vitro with syngeneic tumor cells, generating large numbers of specific CTL. The activity of these CTL (H-2b) was apparently H-2-restricted, because it was ineffective on tumor targets bearing strongly cross-reacting tumor-specific antigens but with the H-2d haplotype. Furthermore, only H-2b cells bearing the Friend, Moloney, Rauscher-associated antigen, such as Rauscher leukemia virus-induced RBL-5 cells and Friend leukemia virus-induced HFL/b cells, were lysed efficiently. B male GV cells (H-2b cells induced by Gross leukemia virus) were not affected by the same CTL. We propose the existence of a dynamic state involving the migration of primed CTL precursor cells between the lymphoid organs and the tumor mass, as well as the differentiation of these precursor cells within the tumor mass into highly specific CTL.     

A new model of rectal cancer with regional lymph node metastasis allowing in vivo evaluation by imaging biomarkers

Object

The work is aimed to develop a murine model of rectal cancer, which could be used to monitor lymph node metastasis development by magnetic resonance imaging (MRI) and optical imaging (OI) techniques.

Subjects and methods

Ht-29 cancer cells were directly injected into the submucosal layer of the rectum of athymic nude mice using trans-anal rectal cancer cell injection (TARCI). Thirty-six mice were inoculated with 10 × 10⁵ cells and five mice were treated with sterile phosphate buffer solution. One to 4 weeks after cell injection, tumor growth was evaluated in vivo using T2-weighted MRI at 4.7T. A further group of animal (n = 6) treated with ht-29_luc cells, with the same protocol, was monitored by optical imaging. In both groups, the presence of the primary tumor and of lymph nodes metastasis was confirmed by histology.

Results

In all animals, primary tumors were detectable by MRI, 1 week from TARCI. After 4 weeks primary tumors showed a mean longitudinal diameter of about 2 cm. All animals developed regional lymph node metastases. Others organs (e.g. lung or liver) were not affected. In fat-suppressed, T2-weighted MRI, lymph nodes appeared as small areas characterized by hyper-intense signal compared to muscle. OI permitted evaluation of the primary tumor growth in perineal region.

Conclusions

TARCI of ht-29 cells into the rectum of nude mice is a feasible way to obtain a easily reproducible model of regional lymph node metastases could be monitored by magnetic resonance and optical imaging techniques.     

Anti-CD43 Inhibition of T Cell Homing

The homing of lymphocytes from the blood is controlled by specialized processes of lymphocyte–endothelial cell interaction. Interference with these processes offers the potential to manipulate lymphocyte traffic, and thus to modulate normal and pathologic immune and inflammatory responses. We selected antilymphocyte monoclonal antibodies (mAbs) for inhibition of lymphocyte binding in vitro to lymph node high endothelial venules (HEV), specialized vessels that support lymphocyte recruitment into lymph nodes. mAb L11 blocks T cell binding to lymph node and Peyer's patch HEV and inhibits T cell extravasation from the blood into organized secondary lymphoid tissues. In contrast, L11 has no effect on lymphocyte binding to purified vascular ligands for L-selectin, α4β7, or LFA-1, suggesting that it inhibits by a novel mechanism. The L11 antigen is CD43, a sialomucin implicated in vitro in regulation of lymphocyte activation, whose expression is often dysregulated in the Wiskott-Aldrich syndrome. CD43 represents a novel target for experimental and therapeutic manipulation of lymphocyte traffic and may help regulate T cell distribution in vivo.     

P-selectin glycoprotein ligand-1 mediates L-selectin-dependent leukocyte rolling in venules

Leukocyte rolling in postcapillary venules of inflamed tissues is reduced in L-selectin-deficient mice and mice treated with L-selectin blocking antibodies, but the glycoprotein ligand for L-selectin in inflamed venules is unknown. Here, we show that L-selectin-dependent rolling after P-selectin blockade is completely absent in P-selectin glycoprotein ligand-1 (PSGL-1)-/- mice or wild-type mice treated with a PSGL-1 blocking monoclonal antibody. Immunohistochemistry and flow cytometry failed to show PSGL-1 expression on resting or inflamed endothelium or on platelets. To investigate whether leukocyte-expressed PSGL-1 is mediating L-selectin-dependent rolling, we reconstituted lethally irradiated wild-type mice with PSGL-1-/- bone marrow cells. These chimeric mice showed no L-selectin-dependent rolling, suggesting that leukocyte-expressed PSGL-1 mediates L-selectin-dependent rolling. Frame-to-frame video analysis of L-selectin-dependent rolling in wild-type mice showed that the majority of observed L-selectin-dependent leukocyte rolling was between free flowing leukocytes and already adherent leukocytes or possibly leukocyte fragments, followed by E-selectin-dependent leukocyte rolling along the endothelium. Leukocyte rolling was significantly slower for leukocyte-endothelial than leukocyte-leukocyte interactions. We conclude that leukocyte-expressed PSGL-1 serves as the main L-selectin ligand in inflamed postcapillary venules. L-selectin binding to PSGL-1 initiates tethering events that enable L-selectin-independent leukocyte-endothelial interactions. These findings provide a molecular mechanism for the inflammatory defects seen in L-selectin-deficient mice.     

CC chemokine receptor 7-dependent and -independent pathways for lymphocyte homing: modulation by FTY720.

Cognate interaction of chemokine receptor CCR7 on lymphocytes with its ligands CCL19 and CCL21 expressed on high endothelial venules (HEVs) is essential for effective migration of T and B cells across HEVs into secondary lymphoid organs. Plt mice, which lack expression of CCL19 and CCL21-ser, both ligands for CCR7 on HEVs, as well as CCR7-deficient mice, have a defective cell migration and reduced homing of lymphocytes. FTY720, a novel immunosuppressant, causes a reduction of lymphocytes in peripheral blood and tissues and their sequestration into lymphoid tissues. In this study we demonstrate that FTY720 rescues the homing defect in both CCR7(-/-) mice and plt mice. After FTY720 treatment, the number of CD4(+) and CD8(+) T cells as well as B cells in peripheral blood is reduced while pertussis toxin-sensitive homing into peripheral lymph nodes, mesenteric lymph node, and Peyer's patches is increased. Immunohistology demonstrates that FTY720 enables these cells to enter lymphoid tissue through HEVs. Thus, our data suggest an alternative G-alpha(i)-dependent, CCR7-CCL19/CCL21-independent mechanism for lymphocyte homing through HEVs which is strongly augmented in the presence of FTY720.