Blockade of CD2-LFA-3 interactions protects human skin allografts in immunodeficient mouse/human chimeras

CD58 Immunobiology at a Glance

The glycoprotein CD58, also known as lymphocyte-function antigen 3 (LFA-3), is a costimulatory receptor distributed on a broad range of human tissue cells. Its natural ligand CD2 is primarily expressed on the surface of T/NK cells. The CD2-CD58 interaction is an important component of the immunological synapse (IS) that induces activation and proliferation of T/NK cells and triggers a series of intracellular signaling in T/NK cells and target cells, respectively, in addition to promoting cell adhesion and recognition. Furthermore, a soluble form of CD58 (sCD58) is also present in cellular supernatant in vitro and in local tissues in vivo. The sCD58 is involved in T/NK cell-mediated immune responses as an immunosuppressive factor by affecting CD2-CD58 interaction. Altered accumulation of sCD58 may lead to immunosuppression of T/NK cells in the tumor microenvironment, allowing sCD58 as a novel immunotherapeutic target. Recently, the crucial roles of costimulatory molecule CD58 in immunomodulation seem to be reattracting the interests of investigators. In particular, the CD2-CD58 interaction is involved in the regulation of antiviral responses, inflammatory responses in autoimmune diseases, immune rejection of transplantation, and immune evasion of tumor cells. In this review, we provide a comprehensive summary of CD58 immunobiology.

Ex vivo pretreatment of human vessels with siRNA nanoparticles provides protein silencing in endothelial cells

Human endothelial cells are initiators and targets of the rejection response. Pre-operative modification of endothelial cells by small interfering RNA transfection could shape the nature of the host response post-transplantation. Ablation of endothelial cell class II major histocompatibility complex molecules by small interfering RNA targeting of class II transactivator can reduce the capacity of human endothelial cells to recruit and activate alloreactive T cells. Here, we report the development of small interfering RNA-releasing poly(amine-co-ester) nanoparticles, distinguished by their high content of a hydrophobic lactone. We show that a single transfection of small interfering RNA targeting class II transactivator attenuates major histocompatibility complex class II expression on endothelial cells for at least 4 to 6 weeks after transplantation into immunodeficient mouse hosts. Furthermore, silencing of major histocompatibility complex class II reduces allogeneic T-cell responses in vitro and in vivo. These data suggest that poly(amine-co-ester) nanoparticles, potentially administered during ex vivo normothermic machine perfusion of human organs, could be used to modify endothelial cells with a sustained effect after transplantation.

The use of gene silencing techniques in the treatment of post-transplantation host rejection is not long lasting and can have systemic effects. Here, the authors utilize a nanocarrier for siRNA for treatment of arteries ex vivo prior to implantation subsequently attenuating immune reaction in vivo.

Nurse-like cells promote CLL survival through LFA-3/CD2 interactions

In the tumoral micro-environment (TME) of chronic lymphocytic leukemia (CLL), nurse-like cells (NLC) are tumor-associated macrophages which play a critical role in the survival and chemoresistance of tumoral cells. This pro-survival activity is known to involve soluble factors, but few data are available on the relative role of cells cross-talk. Here, we used a transcriptome-based approach to systematically investigate the expression of various receptor/ligand pairs at the surface of NLC/CLL cells. Their relative contribution to CLL survival was assessed both by fluorescent microscopy to identify cellular interactions and by the use of functional tests to measure the impact of uncoupling these pairs with blocking monoclonal antibodies. We found for the first time that lymphocyte function-associated antigen 3 (LFA-3), expressed in CLL at significantly higher levels than in healthy donor B-cells, and CD2 expressed on NLC, were both key for the specific pro-survival signals delivered by NLC. Moreover, we found that NLC/CLL interactions induced the shedding of soluble LFA-3. Importantly, in an exploratory cohort of 60 CLL patients receiving frontline immunochemotherapy, increased levels of soluble LFA-3 were found to correlate with shorter overall survival. Altogether, these data suggest that LFA-3/CD2 interactions promote the survival of CLL cells in the tumor microenvironment.

Human allograft rejection in humanized mice: a historical perspective

Basic research in transplantation immunology has relied primarily on rodent models. Experimentation with rodents has laid the foundation for our basic understanding of the biological events that precipitate rejection of non-self or allogeneic tissue transplants and supported the development of novel strategies to specifically suppress allogeneic immune responses. However, translation of these studies to the clinic has met with limited success, emphasizing the need for new models that focus on human immune responses to allogeneic tissues. Humanized mouse models are an exciting alternative that permits investigation of the rejection of human tissues mediated by human immune cells without putting patients at risk. However, the use of humanized mice is complicated by a diversity of protocols and approaches, including the large number of immunodeficient mouse strains available, the choice of tissue to transplant and the specific human immune cell populations that can be engrafted. Here, we present a historical perspective on the study of allograft rejection in humanized mice and discuss the use of these novel model systems in transplant biology.

A new Hu-PBL model for the study of human islet alloreactivity based on NOD-scid mice bearing a targeted mutation in the IL-2 receptor gamma chain gene

Immunodeficient NOD-scid mice bearing a targeted mutation in the IL2 receptor common gamma chain (Il2rgamma(null)) readily engraft with human stem cells. Here we analyzed human peripheral blood mononuclear cells (PBMC) for their ability to engraft NOD-scid Il2rgamma(null) mice and established engraftment kinetics, optimal cell dose, and the influence of injection route. Even at low PBMC input, NOD-scid Il2rgamma(null) mice reproducibly support high human PBMC engraftment that plateaus within 3-4 weeks. In contrast to previous stocks of immunodeficient mice, we observed low intra- and inter-donor variability of engraftment. NOD-scid Il2rgamma(null) mice rendered hyperglycemic by streptozotocin treatment return to normoglycemia following transplantation with human islets. Interestingly, these human islet grafts are rejected following injection of HLA-mismatched human PBMC as evidenced by return to hyperglycemia and loss of human C-peptide. These data suggest that humanized NOD-scid Il2rgamma(null) mice may represent an important surrogate for investigating in vivo mechanisms of human islet allograft rejection.

Human effector memory CD4+ T cells directly recognize allogeneic endothelial cells in vitro and in vivo

The frequency of circulating alloreactive human memory T cells correlates with allograft rejection. Memory T cells may be divided into effector memory (T(EM)) and central memory (T(CM)) cell subsets, but their specific roles in allograft rejection are unknown. We report that CD4+ T(EM) (CD45RO+ CCR7- CD62L-) can be adoptively transferred readily into C.B-17 SCID/bg mice and mediate the destruction of human endothelial cells (EC) in vascularized human skin grafts allogeneic to the T cell donor. In contrast, CD4+ T(CM) (CD45RO+ CCR7+ CD62L+) are inefficiently transferred and do not mediate EC injury. In vitro, CD4+ T(EM) secrete more IFN-gamma within 48 h in response to allogeneic ECs than do T(CM). In contrast, T(EM) and T(CM) secrete comparable amounts of IFN-gamma in response to allogeneic monocytes (Mo). In the same cultures, both T(EM) and T(CM) produce IL-2 and proliferate in response to IFN-gamma-treated allogeneic human EC or Mo, but T(CM) respond more vigorously in both assays. Blockade of LFA-3 strongly inhibits both IL-2 and IFN-gamma secretion by CD4+ T(EM) cultured with allogeneic EC but only minimally inhibits responses to allogeneic Mo. Blockade of CD80 and CD86 strongly inhibits IL-2 but not IFN-gamma production by in response to allogeneic EC or Mo. Transduction of EC to express B7-2 enhances allogeneic T(EM) production of IL-2 but not IFN-gamma. We conclude that human CD4+ T(EM) directly recognize and respond to allogeneic EC in vitro by secreting IFN-gamma and that this response depends on CD2 but not CD28. Consistent with EC activation of effector functions, human CD4+ T(EM) can mediate allogeneic EC injury in vivo.

Immunomodulatory properties of FK734, a humanized anti-CD28 monoclonal antibody with agonistic and antagonistic activities

BACKGROUND

We describe immunomodulatory effects of FK734, a humanized version of a mouse anti-human CD28 mAb (clone TN228), in vitro and in a chimeric human-mouse model of allograft rejection.

METHODS

Cytokine production and proliferation were assessed in a mixed lymphocyte reaction containing FK734, human T cells, and endothelial cells or monocytes. FK734 was also administered to SCID mice engrafted with human skin and adoptively transferred with human peripheral blood mononuclear cells allogeneic to the skin graft.

RESULTS

In vitro, FK734 enhanced secretion of interleukin-2 and interferon-gamma as well as proliferation of CD4+ and CD8+ T cells stimulated by allogeneic human leukocyte antigen (HLA)-DR+ human umbilical vein endothelial cells (which lack B7 molecules and FcgammaRs) or by blood monocytes (which express low levels of B7 molecules and FcgammaRs) compared with control mAb, but these effects were significantly smaller than those provided by mAb 28.2, a stimulatory mouse anti-human CD28 mAb, at comparable concentrations. However, FK734 generally inhibited cytokine secretion and T cell proliferation in cocultures with human umbilical vein endothelial cells transduced to express CD86. In vivo using SCID/beige mice bearing human skin with adoptively transferred peripheral blood mononuclear cells, administration of FK734 protected human endothelial cell-lined microvessels, significantly but incompletely reducing endothelial cell injury and T cell infiltration into the graft one or two weeks later.

CONCLUSIONS

FK734 is a partial agonist of CD28 signaling that can reduce human T cell alloresponses in the presence of strong costimulation by B7 molecules in vitro and can reduce T cell-mediated skin allograft rejection in vivo.

CCR5 usage by CCL5 induces a selective leukocyte recruitment in human skin xenografts in vivo

CCR5 is one of the major inflammatory chemokine receptors with potential therapeutical applications in humans. However, the redundancy of chemokines and their receptors, and the species specificity of chemokine receptor antagonists pose challenges to understanding of the role they play in pharmacological situations. To address this question, we used a humanized severe combined immunodeficient mouse model grafted with human skin and autologous leukocytes, and evaluated the effect of a blocking antibody against human CCR5, on CCL5-induced cutaneous leukocyte recruitment in vivo. At baseline, CCL5 induced a significant recruitment of T cells mainly of the memory phenotype, of monocytes/macrophages, eosinophils, and IFN-gamma(+) but not IL-4(+) and IL-5(+) cells. In vivo, anti-CCR5 antibody was able to almost completely inhibit the recruitment of monocytes/macrophages and T-helper (Th)1-type cells to inhibit partially the attraction of memory T cells, but had no effect on eosinophil infiltration, although all these cell types express other CCL5 binding chemokine receptors than CCR5. These results indicate that the in vivo environment regulates target cell specificity of CCL5 leading to differential cell recruitment, suggesting that antagonizing CCR5 receptor may be of therapeutic value in diseases such as acquired immuno deficiency syndrome, where CCL5/CCR5, monocytes, and Th1-type cells play a predominant role.

Structure-activity studies of peptides from the "hot-spot" region of human CD2 protein: development of peptides for immunomodulation

CD2 is a cell surface protein belonging to the immunoglobulin superfamily (IgSF) that plays a key role in mediating adhesion between human T-lymphocytes and target cells. The interaction between cell-adhesion molecules CD2 and CD58 is critical for immune response. Modulation or inhibition of these interactions has been shown to be therapeutically useful. Synthetic 12-mer linear and cyclic peptides and cyclic hexapeptides from the beta-turn and beta-strand region (hot spot) of human CD2 protein were designed to modulate CD2-CD58 interaction. The 12-amino acid synthetic cyclic peptides effectively blocked the interaction between CD2 and CD58 proteins as demonstrated by E-rosetting and heterotypic adhesion assays. NMR and molecular modeling studies indicated that these cyclic peptides exhibit beta-turn structure in solution and closely mimic the beta-turn structure of the surface epitopes of CD2 protein. The designed cyclic peptides with beta-turn structure have the ability to modulate CD2-CD58 interaction.

Memory T Cells and Their Costimulators in Human Allograft Injury

Both CD4(+) and CD8(+) human memory but not naive T cells respond to allogeneic human dermal microvascular endothelial cells (HDMEC) in vitro by secreting cytokines and by proliferating. Several recently identified costimulators, namely, 4-1BB ligand, ICOS ligand, and OX40 ligand, are up-regulated on cultured HDMEC in response to TNF or coculture with allogeneic T cells. Blockade of these costimulators each partially reduces IFN-gamma and IL-2 secretion and proliferation of previously resting memory T cells. The effects of these costimulators are overlapping but not identical. Memory but not naive T cells are the principal effectors of microvascular injury in human skin allografts following adoptive transfer into immunodeficient mice. Furthermore, blocking 4-1BB ligand, ICOS ligand, or OX40 ligand in this model reduces human skin allograft injury and T cell effector molecule expression. These data demonstrate that human memory T cells respond to microvascular endothelial cells and can injure allografts in vivo without priming. Furthermore, several recently described costimulators contribute to these processes.

Allogeneic versus xenogeneic immune reaction to bioengineered skin grafts

There are conflicting reports on the survival and immune reaction to allografts and xenografts of cultured skin substitutes (CSS). In this study, we investigated the allogeneic and xenogeneic responses to CSS of human keratinocytes and genetically engineered CSS expressing keratinocyte growth factor (KGF) that forms a hyperproliferative epidermis. CSS (control and KGF modified) and neonatal human foreskins were evaluated by immunohistochemistry for the expression of MHC class I and II. To study allograft rejection, grafts were transplanted to human peripheral blood mononuclear cell (huPBMC)-reconstituted SCID mice. To study xenograft rejection, grafts were transplanted to immunocompetent mice. Graft survival and immune reaction were assessed visually and microscopically. After transplantation, control CSS formed a normal differentiated epidermis, whereas KGF CSS formed a hyperproliferative epidermis. Control and KGF CSS expressed class I similar to neonatal foreskin, but did not express class II. In the allograft model, rejection of neonatal foreskins was between 5 and 9 days. In contrast, neither control nor KGF CSS was rejected by huPBMC-SCID mice. Histology showed dense mononuclear cell infiltration in human foreskins, with few, if any, mononuclear cells in control or KGF CSS. In contrast to the allogeneic reaction, CSS (control and KGF) were rejected in the xenograft model, but rejection was delayed (9-21 days) compared with neonatal skin (5-8 days). Humanized SCID mice rejected allografts of human neonatal foreskins, but did not reject control CSS or KGF CSS, even though the KGF CSS formed a hyperproliferative epidermis. Rejection of control and KGF CSS by immunocompetent mice in a xenograft model was comparable and their survival was significantly prolonged compared with neonatal skin. These results demonstrate that control CSS and hyperproliferative KGF CSS are less immunogenic than normal human skin and that sustained hyperproliferation of the epidermis does not accelerate rejection.

The anti-CD2 monoclonal antibody BTI-322 generates unresponsiveness by activation-associated T cell depletion

The antihuman CD2 MoAb BTI-322 (Lo-CD2a) effectively inhibits T cell responses in vitro to allogeneic cells, which is followed by unresponsiveness to the original stimulator in secondary stimulation. We studied the xenogeneic human antiporcine mixed lymphocyte reaction (MLR), and utilized anti-T cell receptor (TCR) Vbeta family antibody-induced cell proliferation to determine the specificity and mechanism. BTI-322 and its humanized version, MEDI-507, effectively inhibited the primary xenogeneic MLR. After suboptimal primary stimulation using lower numbers of xenogeneic stimulator cells, the unresponsiveness in secondary culture was apparent only for xenogeneic stimulator cells of the original SLA haplotype, and not for third-party stimulators or allogeneic cells. The inhibition of primary MLR was not observed for nylon-wool-purified T cells, but was seen after reconstitution of purified T cells with monocytes. Similarly, anti-Vbeta family-specific stimulation showed family-specific unresponsiveness in secondary culture. This required the presence of the whole BTI-322 molecule: a F(ab')2 fragment was not effective. T cells of a distinct Vbeta family were depleted after stimulation with an anti-Vbeta family-specific antibody and BTI-322. We conclude that the inhibition by BTI-322 of a primary xenogeneic MLR or the response to an anti-TCR Vbeta antibody is associated with unresponsiveness upon restimulation, due to activation-associated cell depletion. In this process, the interaction between monocytes and the Fc part of the antibody is involved. This unique characteristic of BTI-322 suggests the potential of the antibody for tolerance induction in vivo, besides the potential use as a T cell depleting agent.

Techniques: species' finest blend--humanized mouse models in inflammatory skin disease research

Differences between humans and mice often hamper the transfer of promising results from the bench to the clinic. For ethical reasons, research that involves patients is limited, and so there is an urgent need for models that mimic the human situation as closely as possible. In recent years, there has been considerable progress in generating humanized mouse models, and their application to drug discovery has proved fruitful. So, how can mice be humanized, and how can humanized mice be employed in immunology research and drug discovery? In this article, we answer these questions, focusing on T-cell-mediated skin diseases as an example.

A novel, rapid and sensitive heterotypic cell adhesion assay for CD2-CD58 interaction, and its application for testing inhibitory peptides

The immunoglobulin CD2 is a cell adhesion molecule that mediates T-cell activation by binding to its receptor CD58 on antigen-presenting cells (APCs). Modulation or inhibition of this interaction has been shown to be therapeutically useful. E-rosetting assay is usually applied in the study of the modulation of CD2-CD58 interaction. In this study, we demonstrated a novel, rapid and sensitive heterotypic cell adhesion assay for CD2-CD58 interaction. The CD2 expression on the surface of Jurkat cells and the CD58 expression on the Caco-2 cells were confirmed by flow cytometry and ELISA studies, respectively. Then Jurkat cells were fluorescent-labeled with 2 microM of BCECF-AM for 45 min at 37 degrees C before adding to confluent Caco-2 monolayers cultured in 96-well culture dishes. After 30 min, non-adherent Jurkat cells were removed by washing with PBS, while the monolayer-associated Jurkat cells were lysed with 0.5 ml of 2% Triton X-100 in 0.1 M NaOH. Fluorescence (FL) was quantitated using a microplate fluorescence analyzer with BCECF's excitation maximum of 485 nm and emission maximum of 535 nm. This method was successfully applied for testing inhibitory peptides to CD2-CD58 interaction.

Rejection of human islets and human HLA-A2.1 transgenic mouse islets by alloreactive human lymphocytes in immunodeficient NOD-scid and NOD-Rag1nullPrf1null mice

Immunodeficient NOD mice engrafted with human peripheral blood mononuclear cells (PBMCs) were used in two models of human islet allograft rejection. Model one: human PBMCs were engrafted into chemically diabetic NOD-scid mice bearing established subrenal human islet allografts. Inflammation and often complete islet allograft rejection were observed. Model 2 incorporated three key advances. First, we developed a new immunodeficient recipient, NOD-RagI(null)Prf1(null) mice. Second, graft-lymphocyte interactions were optimized by intrasplenic co-transplantation of islets and human PBMC. Third, NOD-scid islets expressing human HLA-A2.1 were used as allograft targets. Diabetic NOD-RagI(null)Prf1(null) recipients of HLA-A2.1 transgenic mouse islets, alone or co-engrafted with HLA-A2-positive human PBMC, exhibited durable graft survival and euglycemia. Contrastingly, co-transplantation with HLA-A2-negative human PBMC led to islet graft rejection without evidence of graft-vs.-host disease (GVHD). We propose that diabetic NOD-RagI(null)Prf1(null) mice co-engrafted with HLA-A2 mouse transgenic islets and allogeneic human PBMC provide an effective in vivo model of human islet allograft rejection.

A Role for CD2 Antibodies (BTI-322 and its Humanized Form) in the in vivo Elimination of Human T Lymphocytes Infiltrating an Allogeneic Human Skin Graft in SCID Mice: An Fcγ Receptor-Related Mechanism Involving Co-Injected Human NK Cells

BACKGROUND

Pilot clinical studies have shown that the rat anti-human-CD2 monoclonal antibody, LoCD2a/BTI-322, can efficiently prevent and treat acute kidney rejection. However, the in vivo mechanism by which it prevents allograft rejection has not been studied. BTI-322 and its humanized form have been shown to mediate in vitro antibody-dependent cell-mediated cytotoxicity (ADCC) against CD2 cells through the activation of monocytes or natural killer (NK) cells.

METHODS

Human fetal skin samples were grafted into severe combined immunodeficient/nonobese diabetic mice. Five weeks later (day 0), the mice were injected with human allogeneic peripheral blood lymphocytes (PBL). Either on day 0 or on day 14, mice were treated with BTI-322, hu-BTI-322, or their F(ab')2 fragments. Peripheral blood mononuclear cells (PBMC) thoroughly devoid of NK cells were also assayed.

RESULTS

After injection of PBL, the human skins became heavily infiltrated with activated human T lymphocytes, resulting in dermal microvascular injuries indicative of graft rejection. Early treatment with BTI-322 and hu-BTI-322 prevented all these events. These CD2 antibodies rapidly eliminated human T lymphocytes that had already infiltrated the grafts, with no evidence of recirculation toward the spleen. Their F(ab')2 fragments were, in contrast, ineffective. Elimination of NK cells from injected PBMC prevented the curative effect exerted by whole CD2 antibodies. It also abrogated their cytotoxicity potential against CD2 cells in ADCC assays.

CONCLUSION

F(ab')2 fragments of the CD2 antibodies could not prevent allograft rejection, whereas whole immunoglobulin G could, and human NK cells were required for the curative effect exerted by these antibodies. The results are consistent with an FcgammaR-dependent ADCC mechanism mediated in vivo by human NK cells.

Design, structure and biological activity of β-turn peptides of CD2 protein for inhibition of T-cell adhesion

The interaction between cell-adhesion molecules CD2 and CD58 is critical for an immune response. Modulation or inhibition of these interactions has been shown to be therapeutically useful. Synthetic 12-mer linear and cyclic peptides, and cyclic hexapeptides based on rat CD2 protein, were designed to modulate CD2-CD58 interaction. The synthetic peptides effectively blocked the interaction between CD2-CD58 proteins as demonstrated by antibody binding, E-rosetting and heterotypic adhesion assays. NMR and molecular modeling studies indicated that the synthetic cyclic peptides exhibit beta-turn structure in solution and closely mimic the beta-turn structure of the surface epitopes of the CD2 protein. Docking studies of CD2 peptides and CD58 protein revealed the possible binding sites of the cyclic peptides on CD58 protein. The designed cyclic peptides with beta-turn structure have the ability to modulate the CD2-CD58 interaction.

IL-11 protects human microvascular endothelium from alloinjury in vivo by induction of survivin expression

IL-11 can reduce tissue injury in animal models of inflammation but the mechanism(s) is unknown. When C.B-17 SCID/beige mice bearing human skin grafts are injected i.p. with human PBMC allogeneic to the donor skin, infiltrating T cells destroy human microvessels by day 21. Intradermal injection of human IL-11 (500 ng/day) delays the time course of graft microvessel loss without reducing the extent of T cell infiltration. Protective actions of IL-11 are most pronounced on day 15. IL-11 has no effect on T cell activation marker, effector molecule, cytokine expression, or endothelial ICAM-1 expression. IL-11 up-regulates the expression of survivin, a cytoprotective protein, in graft keratinocytes and endothelial cells. Topical application of survivin antisense oligonucleotide down-regulates survivin expression in both cell types and largely abrogates the protective effect of IL-11. We conclude that in this human transplant model, IL-11 exerts a cytoprotective rather than anti-inflammatory or immunomodulatory effect mediated through induction of survivin.

Alloimmune injury and rejection of human skin grafts on human peripheral blood lymphocyte-reconstituted non-obese diabetic severe combined immunodeficient beta2-microglobulin-null mice

Small animal models with the capacity to support engraftment of a functional human immune system are needed to facilitate studies of human alloimmunity. In the present investigation, non-obese diabetic (NOD) severe combined immunodeficient (scid) beta2-microglobulin-null (B2mnull) mice engrafted with human peripheral blood lymphocytes (hu-PBL-NOD-scid B2mnull mice) were used as in vivo models for studying human skin allograft rejection. Hu-PBL-NOD-scid B2mnull mice were established by injection of human spleen cells or PBLs and transplanted with full-thickness allogeneic human skin. Human cell engraftment was enhanced by injection of anti-mouse CD122 antibody. The respective contributions of human CD4+ and CD8+ cells in allograft rejection were determined using depleting antibodies. Human skin grafts on unmanipulated NOD-scid B2mnull mice uniformly survived but on chimeric hu-PBL-NOD-scid B2mnull mice exhibited severe immune-mediated injury that often progressed to complete rejection. The alloaggressive hu-PBLs did not require prior in vitro sensitization to elicit targeted effector cell activity. Extensive mononuclear cell infiltration directed towards human-origin endothelium was associated with thrombosis and fibrin necrosis. No evidence of graft-versus-host disease was detected. Either CD4+ or CD8+ T cells may mediate injury and alloimmune rejection of human skin grafts on hu-PBL-NOD-scid B2mnull mice. It is proposed that Hu-PBL-NOD-scid B2mnull mice engrafted with human skin will provide a useful model for analysis of interventions designed to modulate human allograft rejection.

Clinical trials of transplant tolerance: slow but steady progress

The search for tolerance therapies that would thwart the alloimmune response following organ transplantation while preserving a patient's protective immune response has been a formidable goal for clinical immunologists. Over the past few decades, a more detailed understanding of the molecular events associated with T-cell recognition and activation has demonstrated the feasibility of various tolerance approaches, such as costimulation blockade, in numerous animal models of both autoimmunity and transplantation. Yet, only a few promising new therapies have reached the early stages of human clinical development. In contrast, the use of T-cell depleting induction therapy has become widespread, and new trials have been designed with immunosuppressive drug withdrawal in mind. Furthermore, nonmyeloablative mixed chimeric approaches have allowed complete immunosuppressive withdrawal in some limited cases. In the course of these investigations, however, what has become increasingly clear is that the distinctions between immunosuppression and tolerance have been blurred as the success and durability of the therapies rely as much on the state of the organ and organism as they do the mechanism of action of the drug. In this review, we provide a summary of the progress and lessons in promoting clinical transplant tolerance and an overview of promising agents.

CCR3-blocking antibody inhibits allergen-induced eosinophil recruitment in human skin xenografts from allergic patients

Eosinophil, basophil, and T helper 2 (TH2) cell recruitment into tissues is a characteristic feature of allergic diseases. These cells have in common the expression of the chemokine receptor CCR3, which may represent a specific pathway for their accumulation in vivo. Although animal models of allergic reactions are available, findings cannot always be extrapolated to man. To overcome these limitations, we have developed a humanized mouse model of allergic cutaneous reaction using severe combined immunodeficiency mice engrafted with skin and autologous peripheral blood mononuclear cells from allergic donors. Intradermal injection of the relevant allergen into human skin xenografts from allergic individuals induced a significant recruitment of human CD4(+) T cells, basophils, and TH2-type cytokine mRNA-expressing cells, as well as murine eosinophils. Human skin xenografts, atopic status, and autologous peripheral blood mononuclear cell reconstitution were all mandatory to induce the allergic reaction. Next, we addressed the role of CCR3 in the endogenous mechanisms involved in the inflammatory cell recruitment in this experimental model of allergic cutaneous reaction. In vivo administration of an anti-human CCR3-blocking antibody selectively reduced accumulation of eosinophils but not that of CD4(+) cells, basophils, or cells expressing mRNA for TH2-type cytokines. These findings establish a new in vivo model of humanized allergic reaction and suggest that eosinophil migration is mediated mainly through CCR3. Finally, these results suggest that this model might be useful to test human-specific antiallergic modulators.

Human endothelial cell presentation of antigen and the homing of memory/effector T cells to skin

Dermal microvascular endothelial cells (ECs) form a continuous lining that normally bars blood-borne T lymphocytes from entering the skin, but as part of the response to foreign antigen, dermal ECs undergo alterations in their surface proteins so as to provide signals to circulating T cells that lead to their activation and recruitment. Several observations suggest that human dermal microvascular ECs may help initiate cutaneous immune reactions by presentation of cognate antigens to circulating T memory cells: (1) antigen-specific inflammatory responses in the skin, as in other organs, involve accumulation of memory and effector T cell populations that are enriched in cells specific for the eliciting antigen; (2) recall responses to intradermal protein antigens in the skin start very rapidly within two hours of challenge; (3) dermal microvascular ECs in humans and other large mammals basally display high levels of class I and class II MHC molecules, the only known purpose of which is to present antigenic peptides to lymphocytes; (4) the lumen of dermal capillaries are narrower than the diameter of circulating T cells, ensuring surface contact; and (5) cultured human ECs effectively present antigens to resting memory T cells isolated from the circulation. Upon contact with activated T cells or their secreted products (cytokines), dermal ECs themselves become activated, increasing their capacity to recruit memory and effector T cell populations in an antigen-independent manner. Specifically, activated ECs express inducible leukocyte adhesion molecules such as E-selectin, ICAM-1, and VCAM-1; and several lines of evidence, including neutralizing antibody experiments and gene knockouts, have supported a role of these molecules in T cell recruitment. Dermal ECs have unique expression patterns of adhesion molecules that can determine the subsets of memory T cells that are recruited into the skin. For example, slow internalization of E-selectin allows more persistent expression of this protein on the surface of dermal ECs, favoring interactions with CLA-1+ T cells. VCAM-1 expression, normally confined to venular EC may extend to capillaries within the dermal papillae and contribute to epidermal inflammation, recruiting alpha4beta7 integrin-expressing T cells that also express the cadherin-binding integrin alphaEbeta7. New models involving transplantation of normal and genetically modified human dermal ECs into immunodeficient mice may be used to further explore these properties.

Human myeloid dendritic cells transduced with an adenoviral interleukin-10 gene construct inhibit human skin graft rejection in humanized NOD-scid chimeric mice

Human myeloid DC were generated from peripheral blood mononuclear cells by monocyte adhesion and subsequent culture with rhGM-CSF and rhIL-4. We transduced immature (day 5 of culture) myeloid DC with an E1-deleted replication-deficient adenoviral vector encoding the cytokine IL-10 (AdV IL-10) and a control adenovirus MX-17 (AdV MX 17). Human DC transduced with AdV IL-10 showed inhibition of the mixed leukocyte culture, reduced cell surface expression of co-stimulatory molecules (CD80/CD86) and were unable to produce the potent allo-stimulatory cytokine, interleukin-12. In order to test the in vivo properties of these cells a humanized immunodeficient mouse skin transplantation model was developed. Immunodeficient NOD-scid mice were engrafted with human skin, reconstituted via intraperitoneal injection with allogeneic mononuclear cells (MNC) mixed with 1 x 10(6) DC that were autologous to the skin donor and that had been transduced with either AdV IL-10 or AdV MX-17. Skin grafts were removed at day 7 and 14 after reconstitution and studied histologically for evidence of rejection. In animals that received DC modified with AdV IL-10 there was reduced skin graft rejection as characterized by reduced mononuclear cell infiltration and less dermo-epidermal junction destruction compared with those animals that received DC modified with the control virus alone. Injection of equivalent numbers of donor-derived fibroblasts transduced with AdV IL-10 were ineffective at modifying rejection of skin grafts. Immunosuppressive cytokine gene therapy targeting human DC is a novel means of inhibition of the alloimmune response.

Human T cells infiltrate and injure pig coronary artery grafts with activated but not quiescent endothelium in immunodeficient mouse hosts

BACKGROUND

We have previously demonstrated that human artery grafts transplanted to immunodeficient mice are infiltrated and injured by unsensitized allogeneic human T cells. We extended our investigations to human anti-porcine xenoresponses in this model.

METHODS

Pig coronary artery segments were interposed into the infrarenal aorta of severe combined immunodeficiency/beige mice. After 7 days, certain recipients were reconstituted with human leukocytes and/or treated with proinflammatory cytokines. The grafts were harvested after 1-70 days and examined by histology, immunohistochemistry, and morphometry.

RESULTS

Pig artery grafts from untreated mice had no evidence of injury, leukocytic infiltrate, or endothelial cell activation up to 70 days postoperatively, despite deposition of murine complement. Host reconstitution with human peripheral blood mononuclear cells resulted in a discrete population of circulating T cells that did not infiltrate or injure the grafts up to 28 days after adoptive transfer. Administration of porcine interferon-gamma for up to 28 days sustained the expression of graft vascular cell adhesion molecule-1 and major histocompatibility complex antigens, but did not initiate recruitment of human leukocytes. In contrast, treatment with human tumor necrosis factor for 7 days induced the de novo expression of porcine E-selectin by graft endothelial cells and elicited human T cell infiltration and human peripheral blood mononuclear cell-dependent vascular injury.

CONCLUSIONS

The human peripheral blood mononuclear cell-severe combined immunodeficiency/beige mouse model identifies a significant difference between human T cell allogeneic and xenogeneic responses in vivo. Xenografts with quiescent endothelium are not infiltrated or injured by T cells under the same conditions in which allografts are rejected. Activation of pig coronary artery endothelial cells by human tumor necrosis factor, but not porcine interferon-gamma, elicits cellular xenoresponses.

Endothelial dysfunction and denudation in rat aortic allografts

Clinical evidence suggests that early endothelial cell (EC) dysfunction may predict the development of graft vascular disease. We wished to assess the early functional and morphological changes in the graft endothelium in a commonly used animal model of graft vascular disease, the rat aortic interposition allograft model. To assess graft EC function, regulation of vascular tone by ECs was monitored in aortic rings from grafts harvested at various times after transplantation (Tx). EC morphology was assessed by silver staining, which was followed by en face inspection of the luminal side of the grafts. Acetylcholine-induced EC-dependent vasorelaxation was reduced in allografts at post-Tx days 7 and 14, whereas in syngeneic grafts EC-dependent relaxation was unaffected at any time after Tx. In separate grafts collected at the same time points, massive leukocyte adhesion at post-Tx day 7 and EC denudation at days 14 and 28 were evident in allografts but not in syngeneic grafts. At post-Tx day 56 (a time at which vessel wall remodeling is pronounced in this model), an intact EC layer covered the grafts. EC dysfunction and morphological changes were prevented by immunosuppression of recipient rats with cyclosporine. Our study shows that Tx-induced EC dysfunction in rat aortic allografts can be observed within 1 week of Tx in rat aortic allografts and that this is occurring concomitantly with enhanced leukocyte adhesion to the graft ECs. These changes occur before any other morphological or functional changes described thus far in this model and appear to be immune-driven. Taken together, these results show that Tx-induced early EC dysfunction, as described in patients, may be studied in the model of rat aortic Tx.

Human TNF can induce nonspecific inflammatory and human immune-mediated microvascular injury of pig skin xenografts in immunodeficient mouse hosts

TNF activates endothelial cells to express cell surface molecules that are necessary to recruit a local infiltrate of leukocytes. Because the actions of this proinflammatory cytokine are not species restricted, we investigated whether human TNF can up-regulate porcine endothelial adhesion molecules to elicit human T cell infiltration and damage of pig skin xenografts in a chimeric immunodeficient mouse model. We have previously demonstrated the vigorous rejection of human skin allografts and the absence of injury to porcine skin xenografts in human PBMC-SCID/beige mice. Intradermal administration of human TNF at high doses (600 or 2000 ng) caused nonspecific inflammatory damage of pig skin grafts, whereas low concentrations of TNF (60 or 200 ng) resulted in human PBMC-dependent injury of porcine endothelial cells. There was a strong correlation among pig skin xenograft damage, human T cell infiltration, and the TNF-induced up-regulation of swine MHC class I and class II molecules, VCAM-1, and, in particular, the de novo expression of porcine E-selectin. The microvascular damage and leukocytic infiltration elicited by TNF were enhanced by porcine IFN-gamma, suggesting that xenografts may be less prone to cytokine-mediated injury due to the species-restricted effects of recipient IFN-gamma. Our results indicate that maintenance of a quiescent endothelium, which does not express E-selectin or other activation-dependent adhesion molecules, is important in preventing human anti-porcine T cell xenoresponses in vivo and that TNF signaling molecules and TNF-responsive gene products are appropriate therapeutic targets to protect against human T cell-mediated rejection of pig xenografts.

LFA-3 (CD58) mediates T-lymphocyte adhesion in chronic inflammatory infiltrates

Previous studies have suggested that LFA-3 has an important role in a number of chronic inflammatory pathologies, although an active role for LFA-3 within in vivo inflammatory reactions has not previously been directly observed in humans. To assess the importance of LFA-3 in this process, this study used an adaptation of the Stamper-Woodruff lymphocyte adhesion assay to measure the binding of exogenous activated lymphocytes to the T-cell-dominated chronic inflammatory infiltrate of oral lichen planus. Antibody blockade experiments showed that anti-LFA-3 monoclonal antibody reduced lymphocyte adhesion by approximately 29%, while anti-ICAM-1 produced a reduction of 26%. These results thus suggest that both LFA-3 and ICAM-1 are likely to mediate cell-cell interactions within lesional tissues in vivo. Moreover, these findings are also the first to directly demonstrate that LFA-3-mediated adhesion, like that of ICAM-1, is functionally important in the molecular pathology of inflammatory mucosal disease.

Immunobiology of human vascular endothelium

The author's laboratory studies interactions between human T lymphocytes and vascular endothelial cells (EC). Our work is organized around three hypotheses. First, we propose that vascular EC can initiate secondary (i.e., recall) immune reactions by presenting antigenic peptide-major histocompatibility complex (MHC) complexes to those circulating memory T cells whose cognate antigen is locally present within a peripheral tissue, e.g., as a consequence of infection or allogeneic transplantation. In this way, EC can increase the efficiency of immune surveillance. Second, we propose that T cell signals, both secreted (e.g., cytokines) and contact-dependent (e.g., CD40 ligand), activate new gene expression in EC that induce the capacity to perform new effector functions, such as leukocyte recruitment and activation or initiation of intravascular coagulation. In this way, EC can participate as effector cells for cell-mediated immune reactions. Third, we propose that EC are major targets of immune-mediated injury. Consequently, increasing resistance of endothelium to immune effector mechanisms may protect tissues from damage, e.g., in allograft rejection. These three hypotheses are explored through in vitro experiments, through analyses of human tissue specimens, and through in vivo studies employing novel human-mouse chimeric animals.

Human Endothelial Cells Augment Early CD40 Ligand Expression in Activated CD4+ T Cells Through LFA-3-Mediated Stabilization of mRNA

Human endothelial cells (EC) augment CD40 ligand (CD40L) expression on PHA-activated CD4+ T cells at early times (e.g., 4–6 h). Fixed EC, devoid of mRNA, are comparable to living EC in their capacity to augment early CD40L expression on CD4+ T cells. Fixed EC increase T cell mRNA expression of both IL-2 and CD40L compared with PHA alone at 6 h. EC are unable to increase the rate of transcription of CD40L compared with PHA alone as measured with a promoter-reporter gene, although they do increase transcription of an IL-2 promoter-reporter gene. Fixed EC prolong the half-life of CD40L mRNA >2-fold. Inclusion of anti-human LFA-3 (CD58) mAb or pretreatment of EC with an LFA-3 antisense oligonucleotide blocks EC-induced increases in CD40L expression, whereas mAb to ICAM-1 or pretreatment with ICAM-1 antisense oligonucleotide does not. Moreover, mAb to LFA-3 reverses the capacity of EC to prolong the half-life of CD40L mRNA, whereas mAb to ICAM-1, even in combination with mAb to ICAM-2, does not. We conclude that EC use LFA-3 to increase early CD40L protein expression on newly activated CD4+ T cells by stabilizing CD40L mRNA.

Gene therapy in transplantation

Facilitation of solid organ and cell transplantation depends on metabolic and immunologic factors that can be manipulated ex vivo and in vivo using gene transfer technology. Vectors have been developed which can optimally transfer relevant genes to various tissues and organs. Interventions aimed at promoting tissue preservation before transplantation, prevention of oxidative stress and immunological rejection have recently become attractive options using viral and nonviral gene delivery vehicles. Further understanding of the mechanisms involved in tolerance induction as well as the facilitation of xenogeneic engraftment have made possible a variety of avenues that can be exploited using gene transfer technology.

Endothelial cells modify the costimulatory capacity of transmigrating leukocytes and promote CD28-mediated CD4(+) T cell alloactivation

Activated vascular endothelial cells (ECs) express major histocompatibility complex (MHC) class II molecules in vitro and in vivo in acute and chronic allograft rejection. However, human ECs may be limited in their ability to effectively activate CD4(+) T cells, because they do not express members of the B7 family (CD80 and CD86) of costimulatory molecules. In this study, we show that ECs promote the full activation of CD4(+) T cells via trans-costimulatory interactions. By reverse transcriptase polymerase chain reaction, Western blot, and FACS((R)) analysis, we could not detect the expression of CD80 and CD86 on activated ECs and found minimal expression on purified CD4(+) T cells. In contrast, both CD80 and CD86 were expressed in allogeneic CD4(+) T cell-EC cocultures. Expression of CD86 peaked at early times between 12 and 24 h after coculture, whereas CD80 was not expressed until 72 h. Addition of anti-CD86 but not anti-CD80 monoclonal antibodies to cocultures inhibited IL-2 production and the proliferation of CD4(+) T cells to allogeneic donor human umbilical vein ECs (HUVECs), as well as to skin and lung microvascular ECs. Furthermore, we found that interferon gamma-activated ECs but not untreated ECs induced mRNA and cell surface expression of CD80 and CD86 on CD4(+) T cells, and these T cells were functional to provide a trans-costimulatory signal to autologous CD4(+) T cells. Blockade of MHC class II and lymphocyte function-associated antigen 3 but not other EC cell surface molecules on IFN-gamma-activated ECs inhibited the induction of CD86 on CD4(+) T cells. Transmigration of purified populations of monocytes across EC monolayers similarly resulted in the induction of functional CD86, but also induced the de novo expression of the cytokines interleukin (IL)-1alpha and IL-12. In addition, EC-modified monocytes supported enhanced proliferation of allogeneic and autologous CD4(+) T cells. Taken together, these data define the ability of the endothelium to modify CD4(+) T cells and monocytes for trans-costimulatory events. This unique function of the endothelium in alloimmune T cell activation has functional consequences for the direct and the indirect pathways of allorecognition.

Angiogenesis in the huPBL-SCID model of human transplant rejection

BACKGROUND

Angiogenesis is characteristic of chronic inflammatory reactions. The process of angiogenesis is reported to be proinflammatory in part due to enhanced adhesion events and in part due to increased perfusion and permeability to sites of inflammation. However, little is known about the association between angiogenesis and rejection.

METHODS

Severe combined immune deficient mice are permissive for the growth of human skin allografts and human peripheral blood mononuclear cells (PBMC). Human PBMC were injected into mice by intravenous or intraperitoneal injection. The infiltration of cells and the associated angiogenesis reactions in the skin allografts were analyzed temporally by videomicroscopy and spatially by immunohistochemistry.

RESULTS

Human alloreactive mononuclear cells migrated to human skin but not mouse skin within hours after the intravenous infusion of PBMC. Within 3 days, areas of angiogenesis were observed in the skin grafts at the sites of infiltrates. The vessel densities in skin grafts were 24+/-6 vessels per calibrated grid at baseline on the day of the infusion and increased to 55+/-16 vessels per calibrated field by day 10. Skin grafts harvested from humanized severe combined immune deficient mice 7-14 days after the intraperitoneal infusion of human PBMC showed a similar increased density of vessels that were spatially associated with mononuclear cell infiltrates.

CONCLUSIONS

A significant angiogenesis response was associated with the cell infiltrates in the human skin allografts. The onset of angiogenesis appeared after the initial development of localized infiltrates and preceded the development of microvascular destruction. These findings suggest that alloreactive T cells and/or monocytes mediate the angiogenesis response in skin allografts.

The allogeneic response to cultured human skin equivalent in the hu-PBL-SCID mouse model of skin rejection

BACKGROUND

Engineered tissues have been proposed for the treatment of a variety of conditions including the partial or complete replacement of human organs. To determine the basis for the rejection of these tissues, we analyzed the immune response to allogeneic human skin equivalent (HSE, also called Apligraf) in the humanized SCID mouse (hu-PBL-SCID).

METHODS

Two models of hu-PBL-SCID were used for these studies. In one model, human skin or HSE was transplanted onto humanized mice so that graft survival could be analyzed. In the other model, skin grafts were allowed to heal on naive mice before humanization. This model was used to analyze the immunologic response to the vascularized skin allograft. Humanization was performed by adoptive transfer of human PBL into SCID mice by i.p. injection.

RESULTS

Both human foreskin and HSE successfully engrafted onto naive SCID mice and remained stable for more than 6 months. In contrast, human foreskin was rejected by 21 days posttransplant in hu-PBL-SCID, whereas HSE consistently engrafted for more than 28 days. Treatment of HSE grafts with interferon-y for 5 days to induce maximal MHC class II molecule expression before grafting failed to induce rejection. HSE also engrafted onto hu-PBL-SCID mice that were exposed to alloantigen by prior injection with interferon-gamma-treated keratinocytes identical to those used to generate the HSE. In addition, we determined that humanization of SCID mice following engraftment and vascularization of human foreskin resulted in marked CD3+ T cell infiltrates and a lymphocyte-induced vasculitis. In contrast, the response in vascularized HSE was associated with minimal CD3+ T cell infiltration in the absence of vasculitis or morphological features of rejection.

CONCLUSION

These results support the use of HSE and other allogeneic engineered tissues in humans provided that such tissues are limited in their antigen presenting capabilities. In addition, our findings suggest a critical function for the donor endothelial cell in rejection.

Functional glycan-free adhesion domain of human cell surface receptor CD58: design, production and NMR studies

A general strategy is presented here for producing glycan-free forms of glycoproteins without loss of function by employing apolar-to-polar mutations of surface residues in functionally irrelevant epitopes. The success of this structure-based approach was demonstrated through the expression in Escherichia coli of a soluble 11 kDa adhesion domain extracted from the heavily glycosylated 55 kDa human CD58 ectodomain. The solution structure was subsequently determined and binding to its counter-receptor CD2 studied by NMR. This mutant adhesion domain is functional as determined by several experimental methods, and the size of its binding site has been probed by chemical shift perturbations in NMR titration experiments. The new structural information supports a 'hand-shake' model of CD2-CD58 interaction involving the GFCC'C" faces of both CD2 and CD58 adhesion domains. The region responsible for binding specificity is most likely localized on the C, C' and C" strands and the C-C' and C'-C" loops on CD58.

Polarized expression and function of the costimulatory molecule CD58 on human intestinal epithelial cells

BACKGROUND & AIMS

Intestinal epithelial cells (IECs) can process foreign protein antigens and display antigenic peptides to CD4(+) T lymphocytes via HLA class II molecules. The purpose of this study was to determine the nature of the second, or costimulatory, signal provided by IECs.

METHODS

We investigated surface expression of the costimulatory molecules CD58 (LFA-3), CD80 (B7-1), and CD86 (B7-2) by using flow cytometry, confocal microscopy, and vectorial biotinylation. Antibodies specific for CD58, CD80, and CD86 were used in blocking experiments to assess the role of these molecules in providing a costimulatory signal to CD4(+) T cells by IECs.

RESULTS

CD58, but not CD80 or CD86, was observed to be expressed constitutively on both native IECs and in the IEC lines T84 and HT-29. The surface expression of CD58 was highly polarized and restricted to the basolateral surface of the cell. Antibodies against CD58, but not CD80 or CD86, inhibited the stimulation of CD4(+) T-cell proliferation mediated by IECs.

CONCLUSIONS

CD58 is expressed by polarized IECs in a topologically restricted manner at the region of T-cell contact and can function as a costimulatory molecule in HLA class II-mediated antigen presentation.

Human CD4+ T Cells Mediate Rejection of Porcine Xenografts

It has previously been demonstrated that xenograft rejection in rodents is dependent on CD4+ T cells. However, because of the lack of an appropriate in vivo model, little is known about the cellular basis of human T cell-mediated rejection of xenografts. In this study, we have evaluated the ability of human T cells to mediate rejection of porcine skin grafts in a novel in vivo experimental system using immunodeficient mice as recipients. Recombinase-activating gene-1-deficient mice (R−) lacking mature B and T cells were grafted with porcine skin and received human lymphocytes stimulated in vitro with irradiated porcine PBMC. Skin grafts on mice given either unseparated, activated human lymphocytes, or NK cell-depleted lymphocyte populations were rejected within 18 days after adoptive cell transfer. In contrast, skin grafts on mice given T cell-depleted human lymphocytes or saline showed no gross or histologic evidence of rejection up to 100 days after adoptive transfer. Purified CD4+ T cells were also able to mediate rejection of porcine skin grafts. These data suggest that human CD4+ T cells are sufficient to induce rejection of porcine xenografts. Thus, strategies directed toward CD4+ T cells may effectively prevent cellular rejection of porcine xenografts in humans.

Human allogeneic vascular rejection after arterial transplantation and peripheral lymphoid reconstitution in severe combined immunodeficient mice

BACKGROUND

Interspecies differences create important shortcomings in existing animal models used to describe in vivo events responsible for allograft rejection. Alloimmune destruction of human dermal microvessels, histologically consistent with rejection, has been demonstrated in human skin-grafted severe combined immunodeficient (SCID) mice receiving allogeneic human peripheral blood mononuclear cells (PBMC). We have now documented human alloimmune injury in a vascularized, SCID-human arterial transplantation model.

METHODS

Fresh human artery was used to replace the CB.17 SCID/beige mouse infrarenal aorta. Seven days later, 3x10(8) human PBMC were administered intraperitoneally, and lymphocyte engraftment was considered successful when circulating human CD3+ cells were later identified in peripheral blood.

RESULTS

Forty-six of 49 (94%) mice undergoing transplantation survived, including 14 controls with arterial grafts receiving no PBMC. Twenty-eight of 32 mice demonstrated circulating human CD3+ cells, 14 days after PBMC administration. Animals were killed at 14, 21, or 28 days after receiving allogeneic PBMC, and arteries were recovered for histology and immunohistology. All 14 control mice had patent transplanted grafts with normal vascular histology and no lymphoid infiltration. Damage to transplanted arteries among lymphocyte-engrafted mice was apparent by 14 and 21 days in some animals, whereas 16 of 22 exhibited moderate to severe intimal, medial, and/or adventitial lymphocytic infiltration with intimal expansion by day 28. The infiltrate consisted of HLA-A, -B, -C+, and -DR+, human CD3+ cells, approximately equally distributed as CD4+ and CD8+ subsets. Some infiltrating lymphocytes were cytolytic cells as demonstrated by perforin staining. The endothelium of transplanted human arteries exhibited endothelialitis, and the endothelial cells stained intensely with anti-HLA-A, -B, -C and anti-HLA-DR antibodies. The expanded intima was predominantly smooth muscle cells, staining positively for smooth muscle alpha-actin, HLA-A, -B, -C and HLA-DR. Medial necrosis was not observed.

CONCLUSION

The results provide evidence of alloimmune-mediated vascular rejection in this human arterial transplantation model.

Endothelial antigen presentation: stimulation of previously activated but not naïve TCR-transgenic mouse T cells

In vitro experiments have shown that endothelial cells can function as antigen-presenting cells to CD4(+) T lymphocytes. The studies presented here address the question of whether naïve versus previously activated CD4(+) helper T cells differ in their responses to endothelial antigen presentation. TCR-transgenic mice were used as a source of naive T cells of defined antigen specificity. These cells were stimulated in vitro with antigen and splenic antigen-presenting cells to generate populations of T lymphocytes with a previously activated/memory phenotype. Two different types of mouse endothelial cells were used as antigen-presenting cells, including the SVEC4-10 line derived from lymph node endothelium and primary murine pulmonary microvascular endothelium. Monolayer cultures of both types of endothelium were capable of antigen-dependent stimulation of previously activated TCR-transgenic CD4(+) cells. In contrast, neither endothelial type could activate naïve CD4(+) T cells. When costimulatory signals were provided in trans by the addition of MHC-mismatched mouse spleen cells, activation of naïve T cells by endothelial antigen presentation could be demonstrated. The expression of ICAM-1 or VCAM-1 on the endothelial cells was not sufficient to activate naïve T cells. Furthermore, the mouse lung endothelium constitutively expresses B7-1, and therefore, the inability of endothelium to stimulate naïve T cells could not be attributed to a lack of CD28-ligands. These studies suggest that the potential role of endothelial antigen presentation in immune responses is restricted to promoting responses by T cells which have previously encountered antigen presented by other antigen-presenting cells.

Interactions between T lymphocytes and endothelial cells in allograft rejection

Vascular endothelial cells participate in the process of allograft rejection by promoting both the recruitment and the activation of alloreactive T cells. There have been three major recent advances in the field of interactions between T cells and endothelial cells that are of direct relevance to the process of cell-mediated responses to allografts: first, endothelial cells mediate selective recruitment of CD4+ T cell subsets, including naive and memory T cells and T cell subsets of the Th1 and Th2 phenotypes; second, endothelial cells co-stimulate the production of effector cytokines by helper T cells; and third, endothelial cells regulate T cell apoptosis.

Dermal microvascular injury in the human peripheral blood lymphocyte reconstituted-severe combined immunodeficient (HuPBL-SCID) mouse/skin allograft model is T cell mediated and inhibited by a combination of cyclosporine and rapamycin

We have analyzed the mechanism of human endothelial injury in a human peripheral blood lymphocyte-severe combined immunodeficient (huPBL-SCID) mouse/human skin graft model of allograft injury and examined the effect of immunosuppressive drugs on this process. In this model, split-thickness human skin containing the superficial dermal microvessels was grafted onto immunodeficient C.B-17 SCID or SCID/beige mice and allowed to heal. Human peripheral blood mononuclear cells (PBMCs) allogeneic to the skin, when subsequently introduced by intraperitoneal injection, caused destruction of the human dermal microvasculature by day 16, evident as endothelial cell sloughing and thrombosis. In the same specimens, mouse microvessels that invaded the human skin graft were uninjured. Human microvascular cell injury was accompanied by a mononuclear cell infiltrate consisting of approximately equal numbers of human CD4+ and CD8+ T cells, some of which contained perforin-positive granules. We found no evidence of human natural killer cells and noted occasional human, but not mouse, macrophages at a frequency indistinguishable from that resident in skin on animals not receiving human PBMCs. These human T cell infiltrates did not extend into adjacent mouse skin. Human immunoglobulin G antibody was detected in the blood and was diffusely present throughout mouse and human tissues in SCID mice receiving PBMCs. Mouse C3 was detected on human dermal vessels in both unreconstituted control animals and those that received PBMCs. Blood and tissues from mice injected with PBMCs depleted of B cells showed no human immunoglobulin, but circulating CD3+ cells were detected by flow cytometry at levels comparable with those of animals receiving whole PBMCs. Significantly, skin graft infiltration by human T cells and human dermal microvascular injury were equivalent in the B cell-depleted and whole-PBMC-reconstituted mice. Mice inoculated with PBMCs depleted of CD8+ T cells developed microvascular injury and infiltrates containing perforin-expressing CD4+ T cells. These data suggested a cytolytic T cell-dependent mechanism of microvessel injury. We then tested the ability of T cell immunosuppressants, cyclosporine and rapamycin, to attenuate vessel damage. Neither cyclosporine nor rapamycin alone effectively reduced either mononuclear cell infiltration or vascular injury. However, a combination of the two agents reduced both parameters. We conclude that the huPBL-SCID/skin allograft model may be used both to study cytolytic T cell-mediated rejection and to test the effect of immunosuppressive drug strategies in vivo in a small-animal model of human immune responses.