Integrin alpha 4 beta 7 co-stimulation of human peripheral blood T cell proliferation

The V2 loop of HIV gp120 delivers costimulatory signals to CD4+ T cells through Integrin α4β7 and promotes cellular activation and infection

Acute HIV infection is characterized by rapid viral seeding of immunologic inductive sites in the gut followed by the severe depletion of gut CD4⁺ T cells. Trafficking of α₄β₇-expressing lymphocytes to the gut is mediated by MAdCAM, the natural ligand of α₄β₇ that is expressed on gut endothelial cells. MAdCAM signaling through α₄β₇ costimulates CD4⁺ T cells and promotes HIV replication. Similar to MAdCAM, the V2 domain of the gp120 HIV envelope protein binds to α₄β₇ In this study, we report that gp120 V2 shares with MAdCAM the capacity to signal through α₄β₇ resulting in CD4⁺ T cell activation and proliferation. As with MAdCAM-mediated costimulation, cellular activation induced by gp120 V2 is inhibited by anti-α₄β₇ monoclonal antibodies (mAbs). It is also inhibited by anti-V2 domain antibodies including nonneutralizing mAbs that recognize an epitope in V2 that has been linked to reduced risk of acquisition in the RV144 vaccine trial. The capacity of the V2 domain of gp120 to mediate signaling through α₄β₇ likely impacts early events in HIV infection. The capacity of nonneutralizing V2 antibodies to block this activity reveals a previously unrecognized mechanism whereby such antibodies might impact HIV transmission and pathogenesis.

Controlling leukocyte trafficking in IBD

Inflammatory bowel disease (IBD) is characterized by the accumulation of immune cells, myeloid cells and lymphocytes in the inflamed intestine. The presence and persistence of these cells, together with the production of pro-inflammatory mediators, perpetuate intestinal inflammation in both ulcerative colitis and Crohn's disease. Thus, blockade of leukocyte migration to the intestine is a main strategy used to control the disease and alleviate symptoms. Vedolizumab is the only anti-integrin drug approved for the treatment of IBD but several other drugs also targeting integrins, chemokines or receptors involved in leukocyte intestinal trafficking are under development and investigated for their efficacy and safety in IBD. The challenge now is to better understand the specific mechanism of action underlying each drug and to identify biomarkers that would guide drug selection in the individual patient.

Cofilin hyperactivation in HIV infection and targeting the cofilin pathway using an anti-α4β7 integrin antibody

A functional HIV cure requires immune reconstitution for lasting viremia control. A major immune dysfunction persisting in HIV infection is the impairment of T helper cell migration and homing to lymphoid tissues such as GALTs (gut-associated lymphoid tissues). ART (antiretroviral therapy) does not fully restore T cell motility for tissue repopulation. The molecular mechanism dictating this persistent T cell dysfunction is not understood. Cofilin is an actin-depolymerizing factor that regulates actin dynamics for T cell migration. Here, we demonstrate that blood CD4 T cells from HIV-infected patients (n = 193), with or without ART, exhibit significantly lower levels of cofilin phosphorylation (hyperactivation) than those from healthy controls (n = 100; ratio, 1.1:2.3; P < 0.001); cofilin hyperactivation is also associated with poor CD4 T cell recovery following ART. These results suggest an HIV-mediated systemic dysregulation of T cell motility that cannot be repaired solely by ART. We further demonstrate that stimulating blood CD4 T cells with an anti-human α₄β₇ integrin antibody can trigger signal transduction and modulate the cofilin pathway, partially restoring T cell motility in vitro. However, we also observed that severe T cell motility defect caused by high degrees of cofilin hyperactivation was not repairable by the anti-integrin antibody, demonstrating a mechanistic hindrance to restore immune functions in vivo. Our study suggests that cofilin is a key molecule that may need to be therapeutically targeted early for T cell tissue repopulation, immune reconstitution, and immune control of viremia.

MAdCAM costimulation through Integrin-α4β7 promotes HIV replication

Human gut-associated lymphoid tissues (GALT) play a key role in the acute phase of HIV infection. The propensity of HIV to replicate in these tissues, however, is not fully understood. Access and migration of naive and memory CD4⁺ T cells to these sites is mediated by interactions between integrin α₄β₇, expressed on CD4⁺ T cells, and MAdCAM, expressed on high endothelial venules. We report here that MAdCAM delivers a potent costimulatory signal to naive and memory CD4⁺ T cells following ligation with α₄β₇. Such costimulation promotes high levels of HIV replication. An anti-α₄β₇ mAb that prevents mucosal transmission of SIV blocks MAdCAM signaling through α₄β₇ and MAdCAM-dependent viral replication. MAdCAM costimulation of memory CD4⁺ T cells is sufficient to drive cellular proliferation and the upregulation of CCR5, while naive CD4⁺ T cells require both MAdCAM and retinoic acid to achieve the same response. The pairing of MAdCAM and retinoic acid is unique to the GALT, leading us to propose that HIV replication in these sites is facilitated by MAdCAM-α₄β₇ interactions. Moreover, complete inhibition of MAdCAM signaling by an anti-α₄β₇ mAb, an analog of the clinically approved therapeutic vedolizumab, highlights the potential of such agents to control acute HIV infection.

Select gp120 V2 domain specific antibodies derived from HIV and SIV infection and vaccination inhibit gp120 binding to α4β7

The GI tract is preferentially targeted during acute/early HIV-1 infection. Consequent damage to the gut plays a central role in HIV pathogenesis. The basis for preferential targeting of gut tissues is not well defined. Recombinant proteins and synthetic peptides derived from HIV and SIV gp120 bind directly to integrin α4β7, a gut-homing receptor. Using both cell-surface expressed α4β7 and a soluble α4β7 heterodimer we demonstrate that its specific affinity for gp120 is similar to its affinity for MAdCAM (its natural ligand). The gp120 V2 domain preferentially engages extended forms of α4β7 in a cation -sensitive manner and is inhibited by soluble MAdCAM. Thus, V2 mimics MAdCAM in the way that it binds to α4β7, providing HIV a potential mechanism to discriminate between functionally distinct subsets of lymphocytes, including those with gut-homing potential. Furthermore, α4β7 antagonists developed for the treatment of inflammatory bowel diseases, block V2 binding to α4β7. A 15-amino acid V2 -derived peptide is sufficient to mediate binding to α4β7. It includes the canonical LDV/I α4β7 binding site, a cryptic epitope that lies 7-9 amino acids amino terminal to the LDV/I, and residues K169 and I181. These two residues were identified in a sieve analysis of the RV144 vaccine trial as sites of vaccine -mediated immune pressure. HIV and SIV V2 mAbs elicited by both vaccination and infection that recognize this peptide block V2-α4β7 interactions. These mAbs recognize conformations absent from the β- barrel presented in a stabilized HIV SOSIP gp120/41 trimer. The mimicry of MAdCAM-α4β7 interactions by V2 may influence early events in HIV infection, particularly the rapid seeding of gut tissues, and supports the view that HIV replication in gut tissue is a central feature of HIV pathogenesis.

Integrin α4β7 Blockade Preferentially Impacts CCR6+ Lymphocyte Subsets in Blood and Mucosal Tissues of Naive Rhesus Macaques

Infusion of a simianized anti-α₄β₇ mAb (Rh-α₄β₇) just before and following SIV infection protected rhesus macaques from developing AIDS and partially from vaginal SIV acquisition. Recently, short-term treatment with Rh-α₄β₇ in combination with cART was found to lead to prolonged viral suppression after withdrawal of all therapeutic interventions. The humanized form of Rh-α₄β₇, vedolizumab, is a highly effective treatment for inflammatory bowel disease. To clarify the mechanism of action of Rh-α₄β₇, naive macaques were infused with Rh-α₄β₇ and sampled in blood and tissues before and after treatment to monitor several immune cell subsets. In blood, Rh-α₄β₇ increased the CD4⁺ and CD8⁺ T cell counts, but not B cell counts, and preferentially increased CCR6⁺ subsets while decreasing CD103⁺ and CD69⁺ lymphocytes. In mucosal tissues, surprisingly, Rh-α₄β₇ did not impact integrin α₄⁺ cells, but decreased the frequencies of CCR6⁺ and CD69⁺ CD4⁺ T cells and, in the gut, Rh-α₄β₇ transiently decreased the frequency of memory and IgA⁺ B cells. In summary, even in the absence of inflammation, Rh-α₄β₇ impacted selected immune cell subsets in different tissues. These data provide new insights into the mechanisms by which Rh-α₄β₇ may mediate its effect in SIV-infected macaques with implications for understanding the effect of treatment with vedolizumab in patients with inflammatory bowel disease.

A Small Molecule, Which Competes with MAdCAM-1, Activates Integrin α4β7 and Fails to Prevent Mucosal Transmission of SHIV-SF162P3

Mucosal HIV-1 transmission is inefficient. However, certain viral and host characteristics may play a role in facilitating HIV acquisition and systemic expansion. Cells expressing high levels of integrin α₄β₇ have been implicated in favoring the transmission process and the infusion of an anti-α₄β₇ mAb (RM-Act-1) prior to, and during a repeated low-dose vaginal challenge (RLDC) regimen with SIVmac251 reduced SIV acquisition and protected the gut-associated lymphoid tissues (GALT) in the macaques that acquired SIV. α₄β₇ expression is required for lymphocyte trafficking to the gut lamina propria and gut inductive sites. Several therapeutic strategies that target α₄β₇ have been shown to be effective in treating inflammatory conditions of the intestine, such as inflammatory bowel disease (IBD). To determine if blocking α₄β₇ with ELN, an orally available anti-α₄ small molecule, would inhibit SHIV-SF162P3 acquisition, we tested its ability to block MAdCAM-1 (α₄β₇ natural ligand) and HIV-gp120 binding in vitro. We studied the pharmacokinetic profile of ELN after oral and vaginal delivery in macaques. Twenty-six macaques were divided into 3 groups: 9 animals were treated with ELN orally, 9 orally and vaginally and 8 were used as controls. All animals were challenged intra-vaginally with SHIV-SF162P3 using the RLDC regimen. We found that ELN did not protect macaques from SHIV acquisition although it reduced the SHIV-induced inflammatory status during the acute phase of infection. Notably, integrins can exist in different activation states and, comparing the effect of ELN and the anti-α₄β₇ mAb RM-Act-1 that reduced susceptibility to SIV infection, we determined that ELN induces the active conformation of α₄β₇, while RM-Act-1 inhibits its activation through an allosteric mechanism. These results suggest that inhibition of α₄β₇ activation may be necessary to reduce susceptibility to SIV/SHIV infection and highlight the complexity of anti-integrins therapeutic approach in HIV as well as in IBD and other autoimmune diseases.

To successfully infect a new host through the sexual route, HIV needs to travel to anatomical sites distant from the mucosal site of exposure reaching draining lymph nodes and the gut, where it can expand and disseminate. The characteristics of the vaginal mucosal microenvironment that facilitate HIV acquisition are still unclear. Several lines of evidence suggest that the ability of HIV to infect cells expressing integrin α₄β₇, a receptor that normally guides immune cells to the gut, may constitute an advantage during transmission and blocking α₄β₇ with a laboratory-engineered antibody (mAb) was shown to reduce susceptibility to vaginal SIV infection. However, α₄β₇ can exist in different conformational states that can affect cell function and susceptibility to infection. Herein we show that while the anti-α₄β₇ mAb that reduced susceptibility to infection inhibits α₄β₇ activation, a drug that also binds to α₄β₇, but induces its activation does not decrease susceptibility to SHIV infection. Thus, our results suggest that not only α₄β₇ expression, but also its activation state may play a role in facilitating or inhibiting infection. Our study contributes to the understanding of mechanisms that facilitate HIV transmission, suggesting innovative ways to prevent it.

Retinoic acid imprints a mucosal-like phenotype on dendritic cells with an increased ability to fuel HIV-1 infection

The tissue microenvironment shapes the characteristics and functions of dendritic cells (DCs), which are important players in HIV infection and dissemination. Notably, DCs in the gut have the daunting task of orchestrating the balance between immune response and tolerance. They produce retinoic acid (RA), which imprints a gut-homing phenotype and influences surrounding DCs. To investigate how the gut microenvironment impacts the ability of DCs to drive HIV infection, we conditioned human immature monocyte-derived DCs (moDCs) with RA (RA-DCs), before pulsing them with HIV and mixing them with autologous T cells. RA-DCs showed a semimature, mucosal-like phenotype and released higher amounts of TGF-β1 and CCL2. Using flow cytometry, Western blot, and microscopy, we determined that moDCs express the cell adhesion molecule mucosal vascular addressin cell adhesion molecule-1 (MAdCAM-1) and that RA increases its expression. MAdCAM-1 was also detected on a small population of DCs in rhesus macaque (Macaca mulata) mesenteric lymph node. RA-DCs formed more DC-T cell conjugates and promoted significantly higher HIV replication in DC-T cell mixtures compared with moDCs. This correlated with the increase in MAdCAM-1 expression. Blocking MAdCAM-1 partially inhibited the enhanced HIV replication. In summary, RA influences DC phenotype, increasing their ability to exacerbate HIV infection. We describe a previously unknown mechanism that may contribute to rapid HIV spread in the gut, a major site of HIV replication after mucosal exposure.

Preliminary in vivo efficacy studies of a recombinant rhesus anti-alpha(4)beta(7) monoclonal antibody

Recent findings established that primary targets of HIV/SIV are lymphoid cells within the gastrointestinal (GI) tract. Focus has therefore shifted to T-cells expressing alpha(4)beta(7) integrin which facilitates trafficking to the GI tract via binding to MAdCAM-1. Approaches to better understand the role of alpha(4)beta(7)+ T-cells in HIV/SIV pathogenesis include their depletion or blockade of their synthesis, binding and/or homing capabilities in vivo. Such studies can ideally be conducted in rhesus macaques (RM), the non-human primate model of AIDS. Characterization of alpha(4)beta(7) expression on cell lineages in RM blood and GI tissues reveal low densities of expression by NK cells, B-cells, naïve and TEM (effector memory) T-cells. High densities were observed on TCM (central memory) T-cells. Intravenous administration of a single 50mg/kg dose of recombinant rhesus alpha(4)beta(7) antibody resulted in significant initial decline of alpha(4)beta(7)+ lymphocytes and sustained coating of the alpha(4)beta(7) receptor in both the periphery and GI tissues.

Mechanisms of leucocyte recruitment to the inflamed large intestine: redundancy in integrin and addressin usage

The caecal-dwelling nematode Trichuris muris provides a natural model of human whipworm infection. Resistance to T. muris is dependent on a host Th2 response, and CD4+Th2 cells migrate to the gut-associated lymphoid tissue (GALT) to elicit parasite expulsion. Thus, CD4+T cells infiltrate the caecal lamina propria during infection, along with other leucocyte subsets that are not critical for parasite expulsion, such as eosinophils. Trafficking of leucocytes to the GALT has been shown to be dependent on the alpha4beta7/MAdCAM-1 integrin-addressin interaction. However, where inflammation is present, such as during T. muris infection, redundant mechanisms of leucocyte recruitment may also occur in addition to traditional gut-homing interactions. We utilized an anti-integrin/addressin antibody treatment regime to investigate this redundancy in resistant, T. muris-infected C57BL/6 mice. Where only the alpha4beta7/MAdCAM-1 interaction was blocked, mice remained resistant to T. muris infection, making a Th2 response and both CD4+T cells and eosinophils infiltrated the site of infection. However, in the absence of available alpha4beta7 and alpha4beta1, mice became chronically infected with T. muris and mounted a more Th1-biased immune response. Interestingly, CD4+T cells, but not eosinophils, were able to infiltrate the caecum, showing different levels of redundancy between leucocyte subsets during infection.

CD45RA T-cell activation without proliferation by a partial agonist monoclonal antibody to beta1 integrin

CD45RA T cells are fully co-activated by natural beta1 integrin ligands fibronectin (FN) and VCAM-1, as well as monoclonal antibody (mAb) 19H8, which binds a combinatorial epitope of the alpha4beta1 heterodimer. These integrin ligands stimulate CD3-dependent proliferation and the upregulation of early activation markers CD25 and CD69. However, beta1-specific antibody 33B6, which binds to a similar range of the predominant T-cell integrins as natural ligands FN (alpha4beta1 and alpha5beta1) and VCAM-1 (alpha4beta1), failed to costimulate proliferation in the CD45RA subset, while retaining the ability to costimulate early activation markers CD25 and CD69. After addition of exogenous human interleukin-2 to the culture media, 33B6 costimulation of proliferation is restored. These data provide evidence that a branch of the alpha4beta1 integrin-signaling pathway in CD45RA T cells can be independently regulated and exploited through the use of partial agonist ligands, including mAbs to the integrin heterodimer.

Physical association and functional interaction between beta1 integrin and CD98 on human T lymphocytes

CD98 is a cell surface protein previously characterized as a cell activation marker, an amino acid transporter, and has recently been implicated in integrin-related functions. Integrins are cell surface proteins, important for homotypic cell aggregation, cell adhesion, and coactivation of T lymphocytes. We have previously shown that the anti-CD98 mAb 80A10, when coimmobilized with anti-CD3 mAb OKT3, is able to mediate human T cell coactivation that is inhibited by anti-beta1 integrin specific mAb 18D3. These results indicated a functional association of CD98 and beta1 integrin signaling but left open the question of a physical association. We now show the induction of homotypic aggregation through CD98 among human T cells and this aggregation was inhibited by anti-beta1 integrin mAb. Therefore, CD98-dependent lymphocyte proliferation and adhesion may involve integrins. Competitive binding assays and fluorescence colocalization analysis suggested that CD98 and beta1 integrin were physically associated. Differential extraction techniques and immunoprecipitations provided the first evidence that the alpha4beta1 integrin and CD98 are specifically associated on human T lymphocytes.

Monoclonal antibody against beta7 integrins, but not beta7 deficiency, attenuates intestinal allograft rejection in mice

BACKGROUND

beta7 integrins mediate homing and retention of lymphocytes to the normal and inflamed small bowel in a tissue-selective fashion. In the present study, we investigated the expression of beta7 integrins after small bowel transplantation (SBT) and tested the effects of blocking beta7-mediated pathways by using monoclonal antibody (mAb) or knockout mice.

METHODS

Heterotopic SBT from BALB/c to C57BL/6 (B6) was used as a surgical model. Expression of beta7 integrins was measured on recipient lymphocytes (CD4 and CD8 ) in spleen, blood, and mesenteric lymph nodes (MLN) using flow cytometry. To analyze the effects of blocking beta7 integrins on graft survival, either beta7-deficient B6 or wild-type B6 mice that were treated with mAb against beta7 were studied.

RESULTS

After allogeneic SBT, there were markedly increased levels of alpha4beta7 recipient CD8 lymphocytes in MLN, blood, and spleen as early as 3 days postoperatively. In contrast, alpha4beta7 integrin levels in isograft recipients were similar to those of normal mice. Mean survival time of intestinal allografts was not affected by beta7 deficiency (7.3+/-1 days) compared with wild-type mice (7.5+/-0.8 days). However, mAb against beta7 integrins significantly prolonged graft survival (12.8+/-1 days) compared with treatment with control mAb (7.3+/-1 days, <0.001). Histologic changes of SBT rejection were significantly attenuated when mice were given mAb against beta7.

CONCLUSION

As indicated by the increased levels of alpha4beta7 CD8 lymphocytes, activation of this integrin contributes to the immune response in SBT rejection. Furthermore, blocking beta7 integrins with mAb provides a suitable target for immunosuppressive therapy. The discrepancy in survival data obtained by mAb and beta7 deficiency may be a result of the more rapid activation of compensatory mechanisms in the knockout mice.

Adenosine suppresses alpha(4)beta(7) integrin-mediated adhesion of T lymphocytes to colon adenocarcinoma cells

The interaction of T lymphocytes with tumor cells, a key step in the antitumor immune response, is suppressed by adenosine, a nucleoside produced at increased levels within the hypoxic tumor environment. We have explored the mechanism by which adenosine interferes with the lymphocyte:tumor cell interaction. The adhesion of anti-CD3-stimulated T cells to syngeneic MCA-38 mouse colon adenocarcinoma cells did not involve LFA-1 (alpha(L)beta(2)) or VLA-5 (alpha(5)beta(1)). However, antibodies against either lymphocyte alpha(4) or beta(7) (but not beta(1)) integrin subunits, or against VCAM-1 on the tumor cells, significantly suppressed adhesion, showing that the recognition of MCA-38 cells by T cells is strongly dependent upon the association of alpha(4)beta(7) on the effector cells with VCAM-1 on the tumor targets. This association is modulated by adenosine: The ability of adenosine to suppress T cell adhesion to MCA-38 cells was lost if alpha(4)beta(7) was functionally blocked with anti-alpha(4) antibodies (i) prior to or (ii) during the adhesion assay or if (iii) alpha(+)(4) cells were depleted from the T lymphocyte population. The binding of T cells to fibronectin through alpha(4)beta(1) was not suppressed by adenosine. We conclude that adenosine partially inhibits the interaction of T lymphocytes with tumor cells by blocking the function of integrin alpha(4)beta(7).

Thrombospondin-1 inhibits TCR-mediated T lymphocyte early activation

Biological activities of the matrix glycoprotein thrombospondin-1 (TSP1) are cell type specific and depend on the relative expression or activation of several TSP1 receptors. Although engaging individual TSP1 receptors in T lymphocytes can elicit costimulating signals, in this study we show that intact TSP1 inhibits TCR-mediated T cell activation, assessed globally using cDNA microarrays. TSP1 signaling suppressed expression of several genes induced in Jurkat T cells, including the T cell activation markers CD69, early growth response gene-1 (Egr-1), and phosphatase of activated cells (PAC-1). TCR-stimulated and CD47-costimulated IL-2 secretion and cell surface CD69 expression were also inhibited by TSP1. The specific inhibitory effect of TSP1 was verified in freshly isolated human PBMCs. TSP1 inhibited TCR-mediated but not protein kinase C-mediated T cell activation. Using CD69 expression as a marker, we demonstrated that the inhibitory activity of TSP1 depended on two TSP1 receptors, CD47 and integrin-associated protein heparan sulfate proteoglycans. Signals from these receptors inhibited TCR signaling downstream of ZAP70, but upstream of NF-AT. Therefore, the expression of TSP1 induced during wound repair and in tumor stroma may limit T cell activation at these sites.

Convergence between CD98 and integrin-mediated T-lymphocyte co-stimulation

CD98 is a widely expressed cell surface heterodimeric glycoprotein, which is rapidly up-regulated upon activation of T lymphocytes. Monoclonal antibody (mAb) 80A10 recognizes an epitope on CD98 and in combination with CD3 antibody causes proliferation of peripheral blood T lymphocytes. CD98 co-stimulatory activity, mediated by either mAb 80A10 or 4F2, a well-characterized CD98-specific mAb, is blocked in the presence of the soluble beta1 integrin antibody 18D3. Previously we have reported that co-stimulatory activity of antibodies to integrins alpha4beta1, alpha5beta1, alphaLbeta2 and alpha4beta7 is inhibited by 18D3, whereas co-stimulation mediated by non-integrins was unaffected. Thus the non-integrin CD98 is uniquely sensitive to the inhibitory effects of beta1 integrin-blocking antibodies, which may reflect convergent signalling mechanisms between integrins and CD98. This is consistent with recent reports suggesting that CD98 may regulate integrin-mediated adhesive events.

Intracellular analysis of interleukin-2 induction provides direct evidence at the single cell level of differential coactivation requirements for CD45RA+ and CD45RO+ T cell subsets

Through measurements of intracellular cytokine production, evidence is provided at the single cell level that triggering different cell surface molecules preferentially activates discrete human peripheral blood (PB) T cell subsets. T cell costimulation due to cross-linking a variety of individual molecules (beta1, beta2, and beta7 integrins, CD26, CD43, or CD44), in conjunction with the CD3/TCR complex, preferentially activated CD45RO+ PB T lymphocytes. CD28, however, costimulated interleukin-2 (IL-2) production in both CD45RO+ and CD45RA+ subpopulations. The amount of soluble IL-2 produced by CD28 coactivation was 15-30-fold higher than that due to integrin or CD26-dependent coactivation, although even the lowest amount of soluble IL-2 produced was in the range of the high-affinity IL-2 receptor. The overall proliferative responses were similar among all costimulatory settings. This was in part due to the uniform upregulation of IL-2 receptor-alpha (IL-2R alpha) (CD25) expression on the entire T cell population activated under each of the different costimulatory conditions. The data provide direct evidence on a single cell level that activation of human CD45RA+ (naive) T cells is stringently controlled and, in these studies, limited to CD28 costimulation for induction of IL-2 production. In contrast, coactivation of CD45RO+ (memory) T lymphocytes can proceed by a variety of different PB T cell surface molecules.

MAdCAM-1 costimulates T cell proliferation exclusively through integrin alpha4beta7, whereas VCAM-1 and CS-1 peptide use alpha4beta1: evidence for "remote" costimulation and induction of hyperresponsiveness to B7 molecules

We have analyzed the effects of the alpha4 integrin ligands mucosal addressin cell adhesion molecule-1 (MAdCAM-1), vascular cell adhesion molecule-1 (VCAM-1), and the fibronectin CS-1 splice variant on T cell activation. Immobilized MAdCAM-1 and VCAM-1 IgG-Fc chimeras and a fibronectin CS-1 peptide efficiently costimulate T cell proliferation when antigen presentation is mimicked by anti-CD3 antibody. VCAM-1-Fc and fibronectin CS-1, which are adhesive ligands for both the alpha4beta1 and alpha4beta7 integrins, medicate T cell costimulation exclusively through integrin alpha4beta1, but not through alpha4beta7. The inability of VCAM-1-Fc to costimulate via alpha4beta7 suggests that cell adhesion per se is insufficient, and that exquisite recognition and activation events must be triggered. MAdCAM-1-Fc mediates costimulation exclusively via alpha4beta7, and can both synergize with and induce hyperresponsiveness to the classical costimulator B7-2. MAdCAM-1-Fc and VCAM-1-Fc, but not B7-2, effectively costimulate when immobilized on sites spatially distant from the anti-CD3 antibody ("remote" costimulation). In vitro, the relative potencies of the CAM were VCAM-1-Fc> ICAM-1-Fc> MAdCAM-1-Fc > B7-Fc, except at high concentrations where ICAM-1 was the most potent. Features of costimulatory CAM revealed by this study have important implications for the design of immunotherapeutic vaccine strategies to combat cancer and infection.

The alpha4 subunit of integrin is important for neural crest cell migration

We identify the alpha4 subunit of integrin as a predominant integrin expressed by neural crest cells in both avian and murine embryos. Using degenerate primers, we obtained a PCR fragment of the chick integrin alpha4 subunit that was subsequently used to clone the full-length subunit with a predicted amino acid sequence 60% identical to human and mouse alpha4 subunits. In situ hybridization demonstrates that chick integrin alpha4 mRNA is expressed at high levels by migrating neural crest cells and neural crest-derived ganglia at both cranial and trunk levels. An antibody against the murine alpha4 subunit revealed similar distribution patterns in mouse to chick. In addition to neural crest cells, the integrin alpha4 subunit was later observed on the muscle masses of the limb, the apical ectodermal ridge, and the developing liver. To examine the functional role of the integrin alpha4 subunit in neural crest cell migration, we used an explant preparation that allows visualization of neural crest cells in their normal environment with or without perturbing reagents. In the presence of a blocking antibody against the mouse integrin alpha4 subunit, there was a profound abrogation of neural crest cell migration at trunk and hindbrain levels. Both the numbers of migrating neural crest cells and the total distance traversed were markedly reduced. Similarly, avian embryos injected with synthetic peptides that contain the integrin alpha4 binding site in fibronectin displayed abnormal neural crest cell migration. Our results suggest that the integrin alpha4 subunit is important for normal neural crest cell migration and may be one of the primary alpha subunits used for neural crest cell migration in vivo. Furthermore, the integrin alpha4 subunit represents a useful neural crest marker in the mouse.

Mucosal addressin cell adhesion molecule-1 (MAdCAM-1). Its binding motif for alpha 4 beta 7 and role in experimental colitis

The integrin alpha 4 beta 7 and mucosal addressin cell adhesion molecule-1 (MAdCAM-1) are molecules involved in the normal recirculation of lymphocytes between the blood and the gastrointestinal tract. These molecules may play a complementary and significant role in animal models of colitis. We have investigated the structural interaction between alpha 4 beta 7 and MAdCAM-1. Site-directed mutagenesis studies of the MAdCAM-1 molecule has led to the identification of the amino acid residue (LDT) in the loop between beta strands C and D of the Ig-superfamily-like folds being involved in the adhesive and cell activation functions of MAdCAM-1 with alpha 4 beta 7.