VLA-4 integrin concentrates at the peripheral supramolecular activation complex of the immune synapse and drives T helper 1 responses

The role of integrins in brain health and neurodegenerative diseases

Integrins are heterodimeric membrane proteins expressed on the surface of most cells. They mediate adhesion and signaling processes relevant for a wealth of physiological processes, including nervous system development and function. Interestingly, integrins are also recognized therapeutic targets for inflammatory diseases, such as multiple sclerosis. Here, we discuss the role of integrins in brain development and function, as well as in neurodegenerative diseases affecting the brain (Alzheimer's disease, multiple sclerosis, stroke). Furthermore, we discuss therapeutic targeting of these adhesion receptors in inflammatory diseases of the brain.

Human T-cell receptor triggering requires inactivation of Lim kinase-1 by Slingshot-1 phosphatase

Actin dynamics control early T-cell receptor (TCR) signalling during T-cell activation. However, the precise regulation of initial actin rearrangements is not completely understood. Here, we have investigated the regulatory role of the phosphatase Slingshot-1 (SSH1) in this process. Our data show that SSH1 rapidly polarises to nascent cognate synaptic contacts and later relocalises to peripheral F-actin networks organised at the mature immunological synapse. Knockdown of SSH1 expression by CRISPR/Cas9-mediated genome editing or small interfering RNA reveal a regulatory role for SSH1 in CD3ε conformational change, allowing Nck binding and proper downstream signalling and immunological synapse organisation. TCR triggering induces SSH1-mediated activation of actin dynamics through a mechanism mediated by Limk-1 inactivation. These data suggest that during early TCR activation, SSH1 is required for rapid F-actin rearrangements that mediate initial conformational changes of the TCR, integrin organisation and proximal signalling events for proper synapse organisation. Therefore, the SSH1 and Limk-1 axis is a key regulatory element for full T cell activation.

Induction of a memory-like CD4+ T-cell phenotype by airway smooth muscle cells

In asthma, CD4+ T-cell interaction with airway smooth muscle (ASM) may enhance its contractile properties and promote its proliferation. However, less is known about the effects of this interaction on T cells. To explore the consequences of interaction of CD4+ T cells with ASM we placed the cells in co-culture and analyzed the phenotypic and functional changes in the T cells. Effector status as well as cytokine expression was assessed by flow cytometry. An increase in CD45RA-CD45RO+ memory T cells was observed after co-culture; however, these cells were not more responsive to CD3/28 restimulation. A reduction in mitochondrial coupling and an increase in the production of mitochondrial reactive oxygen species by CD4+ T cells post-restimulation suggested altered mitochondrial metabolism after co-culture. RNA sequencing analysis of the T cells revealed characteristic downregulation of effector T-cell-associated genes, but a lack of upregulation of memory T-cell-associated genes. The results of this study demonstrate that ASM cells can induce a phenotypic shift in CD4+ T cells into memory-like T cells but with reduced capacity for activation.

Analysis of humoral and cellular immunity after SARS-CoV-2 vaccination in patients with multiple sclerosis treated with immunomodulatory drugs

We analyzed immune response to SARS-CoV-2 vaccination by measuring specific IgG titers and T-cell reactivity to different SARS-CoV-2 peptides in multiple sclerosis patients taking different disease-modifying treatments. Of the 88 patients included, 72 developed any kind of immune response after vaccination. Although DMTs such as fingolimod and anti-CD20+ treatments prevented patients from developing a robust humoral response to the vaccine, most of them were still able to develop a cellular response, which could be crucial for long-term immunity. It is probably advisable that all MS patients take additional/booster doses to increase their humoral and/or cellular immune response to SARS-CoV-2.

Integrin α3 promotes TH17 cell polarization and extravasation during autoimmune neuroinflammation

Multiple sclerosis (MS) is an autoimmune disease of the central nervous system (CNS) caused by CNS-infiltrating leukocytes, including TH17 cells that are critical mediators of disease pathogenesis. Although targeting leukocyte trafficking is effective in treating autoimmunity, there are currently no therapeutic interventions that specifically block encephalitogenic TH17 cell migration. Here, we report integrin α3 as a TH17 cell-selective determinant of pathogenicity in experimental autoimmune encephalomyelitis. CNS-infiltrating TH17 cells express high integrin α3, and its deletion in CD4+ T cells or Il17a fate-mapped cells attenuated disease severity. Mechanistically, integrin α3 enhanced the immunological synapse formation to promote the polarization and proliferation of TH17 cells. Moreover, the transmigration of TH17 cells into the CNS was dependent on integrin α3, and integrin α3 deficiency enhanced the retention of CD4+ T cells in the perivascular space of the blood-brain barrier. Integrin α3-dependent interactions continuously maintain TH17 cell identity and effector function. The requirement of integrin α3 in TH17 cell pathogenicity suggests integrin α3 as a therapeutic target for MS treatment.

Revisiting regulatory T cells as modulators of innate immune response and inflammatory diseases

Regulatory T cells (Treg) are known to be critical for the maintenance of immune homeostasis by suppressing the activation of auto- or allo-reactive effector T cells through a diverse repertoire of molecular mechanisms. Accordingly, therapeutic strategies aimed at enhancing Treg numbers or potency in the setting of autoimmunity and allogeneic transplants have been energetically pursued and are beginning to yield some encouraging outcomes in early phase clinical trials. Less well recognized from a translational perspective, however, has been the mounting body of evidence that Treg directly modulate most aspects of innate immune response under a range of different acute and chronic disease conditions. Recognizing this aspect of Treg immune modulatory function provides a bridge for the application of Treg-based therapies to common medical conditions in which organ and tissue damage is mediated primarily by inflammation involving myeloid cells (mononuclear phagocytes, granulocytes) and innate lymphocytes (NK cells, NKT cells, γδ T cells and ILCs). In this review, we comprehensively summarize pre-clinical and human research that has revealed diverse modulatory effects of Treg and specific Treg subpopulations on the range of innate immune cell types. In each case, we emphasize the key mechanistic insights and the evidence that Treg interactions with innate immune effectors can have significant impacts on disease severity or treatment. Finally, we discuss the opportunities and challenges that exist for the application of Treg-based therapeutic interventions to three globally impactful, inflammatory conditions: type 2 diabetes and its end-organ complications, ischemia reperfusion injury and atherosclerosis.

Feeding enhances fibronectin adherence of quiescent lymphocytes through non-canonical insulin signalling

Nutritional availability during fasting and refeeding affects the temporal redistribution of lymphoid and myeloid immune cells among the circulating and tissue-resident pools. Conversely, nutritional imbalance and impaired glucose metabolism are associated with chronic inflammation, aberrant immunity and anomalous leukocyte trafficking. Despite being exposed to periodic alterations in blood insulin levels upon fasting and feeding, studies exploring the physiological effects of these hormonal changes on quiescent immune cell function and trafficking are scanty. Here, we report that oral glucose load in mice and healthy men enhances the adherence of circulating peripheral blood mononuclear cells (PBMCs) and lymphocytes to fibronectin. Adherence to fibronectin is also observed upon regular intake of breakfast following overnight fasting in healthy subjects. This glucose load-induced phenomenon is abrogated in streptozotocin-injected mice that lack insulin. Intra-vital microscopy in mice demonstrated that oral glucose feeding enhances the homing of PBMCs to injured blood vessels in vivo. Furthermore, employing flow cytometry, Western blotting and adhesion assays for PBMCs and Jurkat-T cells, we elucidate that insulin enhances fibronectin adherence of quiescent lymphocytes through non-canonical signalling involving insulin-like growth factor-1 receptor (IGF-1R) autophosphorylation, phospholipase C gamma-1 (PLCγ-1) Tyr783 phosphorylation and inside-out activation of β-integrins respectively. Our findings uncover the physiological relevance of post-prandial insulin spikes in regulating the adherence and trafficking of circulating quiescent T-cells through fibronectin-integrin interaction.

Early posttransplant reductions in club cell secretory protein associate with future risk for chronic allograft dysfunction in lung recipients: results from a multicenter study

BACKGROUND

Chronic lung allograft dysfunction (CLAD) increases morbidity and mortality for lung transplant recipients. Club cell secretory protein (CCSP), produced by airway club cells, is reduced in the bronchoalveolar lavage fluid (BALF) of lung recipients with CLAD. We sought to understand the relationship between BALF CCSP and early posttransplant allograft injury and determine if early posttransplant BALF CCSP reductions indicate later CLAD risk.

METHODS

We quantified CCSP and total protein in 1606 BALF samples collected over the first posttransplant year from 392 adult lung recipients at 5 centers. Generalized estimating equation models were used to examine the correlation of allograft histology or infection events with protein-normalized BALF CCSP. We performed multivariable Cox regression to determine the association between a time-dependent binary indicator of normalized BALF CCSP level below the median in the first posttransplant year and development of probable CLAD.

RESULTS

Normalized BALF CCSP concentrations were 19% to 48% lower among samples corresponding to histological allograft injury as compared with healthy samples. Patients who experienced any occurrence of a normalized BALF CCSP level below the median over the first posttransplant year had a significant increase in probable CLAD risk independent of other factors previously linked to CLAD (adjusted hazard ratio 1.95; p = 0.035).

CONCLUSIONS

We discovered a threshold for reduced BALF CCSP to discriminate future CLAD risk; supporting the utility of BALF CCSP as a tool for early posttransplant risk stratification. Additionally, our finding that low CCSP associates with future CLAD underscores a role for club cell injury in CLAD pathobiology.

Impact of disease-modifying therapies on humoral and cellular immune-responses following SARS-CoV-2 vaccination in MS patients

The impact of distinct disease‐modifying therapies (DMTs) on severe acute respiratory syndrome‐coronavirus 2 (SARS‐CoV‐2) vaccination efficacy in patients with multiple sclerosis (MS) is still enigmatic. In this prospective comparative study, we investigated humoral and cellular immune‐responses in patients with MS receiving interferon beta, natalizumab, and ocrelizumab pre‐vaccination and 6 weeks post second SARS‐CoV‐2 vaccination. Healthy individuals and interferon beta‐treated patients generated robust humoral and cellular immune‐responses. Although humoral immune responses were diminished in ocrelizumab‐treated patients, cellular immune‐responses were reduced in natalizumab‐treated patients. Thus, both humoral and cellular immune responses should be closely monitored in patients on DMTs. Whereas patients with a poor cellular immune‐response may benefit from additional vaccination cycles, patients with a diminished humoral immune‐response may benefit from a treatment with SARS‐CoV‐2 antibodies in case of an infection.

CD29 Enriches for Cytotoxic Human CD4+ T Cells

CD4⁺ T cells are key contributors in the induction of adaptive immune responses against pathogens. Even though CD4⁺ T cells are primarily classified as noncytotoxic helper T cells, it has become appreciated that a subset of CD4⁺ T cells is cytotoxic. However, tools to identify these cytotoxic CD4⁺ T cells are lacking. We recently showed that CD29 (integrin β1, ITGB1) expression on human CD8⁺ T cells enriches for the most potent cytotoxic T cells. In this study, we questioned whether CD29 expression also associates with cytotoxic CD4⁺ T cells. We show that human peripheral blood-derived CD29^hiCD4⁺ T cells display a cytotoxic gene expression profile, which closely resembles that of CD29^hi cytotoxic CD8⁺ T cells. This CD29^hi cytotoxic phenotype was observed ex vivo and was maintained in in vitro cultures. CD29 expression enriched for CD4⁺ T cells, which effectively produced the proinflammatory cytokines IFN-γ, IL-2, and TNF-α, and cytotoxic molecules. Lastly, CD29-expressing CD4⁺ T cells transduced with a MART1-specific TCR showed target cell killing in vitro. In conclusion, we demonstrate in this study that CD29 can be employed to enrich for cytotoxic human CD4⁺ T cells.

LFA-1 and kindlin-3 enable the collaborative transport of SLP-76 microclusters by myosin and dynein motors

Integrin engagement within the immune synapse enhances T cell activation, but our understanding of this process is incomplete. In response to T cell receptor (TCR) ligation, SLP-76 (LCP2), ADAP (FYB1) and SKAP55 (SKAP1) are recruited into microclusters and activate integrins via the effectors talin-1 and kindlin-3 (FERMT3). We postulated that integrins influence the centripetal transport and signaling of SLP-76 microclusters via these linkages. We show that contractile myosin filaments surround and are co-transported with SLP-76 microclusters, and that TCR ligand density governs the centripetal movement of both structures. Centripetal transport requires formin activity, actomyosin contraction, microtubule integrity and dynein motor function. Although immobilized VLA-4 (α4β1 integrin) and LFA-1 (αLβ2 integrin) ligands arrest the centripetal movement of SLP-76 microclusters and myosin filaments, VLA-4 acts distally, while LFA-1 acts in the lamellum. Integrin β2, kindlin-3 and zyxin are required for complete centripetal transport, while integrin β1 and talin-1 are not. CD69 upregulation is similarly dependent on integrin β2, kindlin-3 and zyxin, but not talin-1. These findings highlight the integration of cytoskeletal systems within the immune synapse and reveal extracellular ligand-independent roles for LFA-1 and kindlin-3. This article has an associated First Person interview with the first author of the paper.

Mucosal integrin α4β7 blockade fails to reduce the seeding and size of viral reservoirs in SIV-infected rhesus macaques

Cellular viral reservoirs are rapidly established in tissues upon HIV‐1/SIV infection, which persist throughout viral infection, even under long‐term antiretroviral therapy (ART). Specific integrins are involved in the homing of cells to gut‐associated lymphoid tissues (GALT) and inflamed tissues, which may promote the seeding and dissemination of HIV‐1/SIV to these tissue sites. In this study, we investigated the efficacy of prophylactic integrin blockade (α4β7 antibody or α4β7/α4β1 dual antagonist TR‐14035) on viral infection, as well as dissemination and seeding of viral reservoirs in systemic and lymphoid compartments post‐SIV inoculation. The results showed that blockade of α4β7/α4β1 did not decrease viral infection, replication, or reduce viral reservoir size in tissues of rhesus macaques after SIV infection, as indicated by equivalent levels of plasma viremia and cell‐associated SIV RNA/DNA to controls. Surprisingly, TR‐14035 administration in acute SIV infection resulted in consistently higher viremia and more rapid disease progression. These findings suggest that integrin blockade alone fails to effectively control viral infection, replication, dissemination, and reservoir establishment in HIV‐1/SIV infection. The use of integrin blockade for prevention or/and therapeutic strategies requires further investigation.

Eosinophil Responses at the Airway Epithelial Barrier during the Early Phase of Influenza A Virus Infection in C57BL/6 Mice

Eosinophils, previously considered terminally differentiated effector cells, have multifaceted functions in tissues. We previously found that allergic mice with eosinophil-rich inflammation were protected from severe influenza and discovered specialized antiviral effector functions for eosinophils including promoting cellular immunity during influenza. In this study, we hypothesized that eosinophil responses during the early phase of influenza contribute to host protection. Using in vitro and in vivo models, we found that eosinophils were rapidly and dynamically regulated upon influenza A virus (IAV) exposure to gain migratory capabilities to traffic to lymphoid organs after pulmonary infection. Eosinophils were capable of neutralizing virus upon contact and combinations of eosinophil granule proteins reduced virus infectivity through hemagglutinin inactivation. Bi-directional crosstalk between IAV-exposed epithelial cells and eosinophils occurred after IAV infection and cross-regulation promoted barrier responses to improve antiviral defenses in airway epithelial cells. Direct interactions between eosinophils and airway epithelial cells after IAV infection prevented virus-induced cytopathology in airway epithelial cells in vitro, and eosinophil recipient IAV-infected mice also maintained normal airway epithelial cell morphology. Our data suggest that eosinophils are important in the early phase of IAV infection providing immediate protection to the epithelial barrier until adaptive immune responses are deployed during influenza.

Dysregulation of Leukocyte Trafficking in Type 2 Diabetes: Mechanisms and Potential Therapeutic Avenues

Type 2 Diabetes Mellitus (T2DM) is a chronic inflammatory disorder that is characterized by chronic hyperglycemia and impaired insulin signaling which in addition to be caused by common metabolic dysregulations, have also been associated to changes in various immune cell number, function and activation phenotype. Obesity plays a central role in the development of T2DM. The inflammation originating from obese adipose tissue develops systemically and contributes to insulin resistance, beta cell dysfunction and hyperglycemia. Hyperglycemia can also contribute to chronic, low-grade inflammation resulting in compromised immune function. In this review, we explore how the trafficking of innate and adaptive immune cells under inflammatory condition is dysregulated in T2DM. We particularly highlight the obesity-related accumulation of leukocytes in the adipose tissue leading to insulin resistance and beta-cell dysfunction and resulting in hyperglycemia and consequent changes of adhesion and migratory behavior of leukocytes in different vascular beds. Thus, here we discuss how potential therapeutic targeting of leukocyte trafficking could be an efficient way to control inflammation as well as diabetes and its vascular complications.

VLA-4 as a Central Target for Modulating Neuroinflammatory Disorders

The complex steps leading to the central nervous system (CNS) inflammation and the progress to neuroinflammatory and neurodegenerative disorders have opened up new research and intervention avenues. This review focuses on the therapeutic targeting of the VLA-4 integrin to discuss the clear-cut effect on immune cell trafficking into brain tissues. Besides, we explore the possibility that blocking VLA-4 may have a relevant impact on nonmigratory activities of immune cells, such as antigen presentation and T-cell differentiation, during the neuroinflammatory process. Lastly, the recent refinement of computational techniques is highlighted as a way to increase specificity and to reduce the detrimental side effects of VLA-4 immunotherapies aiming at developing better clinical interventions.

Tetraspanin CD53: an overlooked regulator of immune cell function

Tetraspanins are membrane organizing proteins that play a role in organizing the cell surface through the formation of subcellular domains consisting of tetraspanins and their partner proteins. These complexes are referred to as tetraspanin enriched microdomains (TEMs) or the tetraspanin web. The formation of TEMs allows for the regulation of a variety of cellular processes such as adhesion, migration, signaling, and cell fusion. Tetraspanin CD53 is a member of the tetraspanin superfamily expressed exclusively within the immune compartment. Amongst others, B cells, CD4+ T cells, CD8+ T cells, dendritic cells, macrophages, and natural killer cells have all been found to express high levels of this protein on their surface. Almost three decades ago it was reported that patients who lacked CD53 suffered from an increased susceptibility to pathogens resulting in the clinical manifestation of recurrent viral, bacterial, and fungal infections. This clearly suggests a vital and non-redundant role for CD53 in immune function. Yet, despite this striking finding, the specific functional roles of CD53 within the immune system have remained elusive. This review aims to provide a concise overview of the published literature concerning CD53 and reflect on the underappreciated role of this protein in immune cell regulation and function.

Anti-αLβ2 antibodies reveal novel endocytotic cross-modulatory functionality

BACKGROUND AND PURPOSE

Antibodies targeting cell surface receptors are considered to enable highly selective therapeutic interventions for immune disorders and cancer. Their biological profiles are found, generally, to represent the net effects of antibody-target interactions. The former therapeutic anti-integrin αLβ2 antibody efalizumab seems to defeat this paradigm by eliciting, via mechanisms currently unknown, much broader effects than would be predicted based on its target specificity.

EXPERIMENTAL APPROACH

To elucidate the mechanisms behind these broad effects, we investigated in primary human lymphocytes in vitro the effects of anti-αLβ2 antibodies on the expression of αLβ2 as well as unrelated α4 integrins, in comparison to Fab fragments and small-molecule inhibitors.

KEY RESULTS

We demonstrate that anti-αLβ2 mAbs directly induce the internalization of α4 integrins. The endocytotic phenomenon is a direct consequence of their antibody nature. It is inhibited when monovalent Fab fragments or small-molecule inhibitors are used. It is independent of crosslinking via anti-Fc mAbs and of αLβ2 activation. The cross-modulatory effect is unidirectional and not observed in a similar fashion with the α4 integrin antibody natalizumab.

CONCLUSION AND IMPLICATIONS

The present study identifies endocytotic cross-modulation as a hitherto unknown non-canonical functionality of anti-αLβ2 antibodies. This cross-modulation has the potential to fundamentally alter an antibody's benefit risk profile, as evident with efalizumab. The newly described phenomenon may be of relevance to other therapeutic antibodies targeting cluster-forming receptors. Thus, pharmacologists should be cognizant of this action when investigating such antibodies.

Blockade of VLA4 sensitizes leukemic and myeloma tumor cells to CD3 redirection in the bone marrow microenvironment

Redirecting T cells to specifically kill malignant cells has been validated as an effective anti-cancer strategy in the clinic with the approval of blinatumomab for acute lymphoblastic leukemia. However, the immunosuppressive nature of the tumor microenvironment potentially poses a significant hurdle to T cell therapies. In hematological malignancies, the bone marrow (BM) niche is protective to leukemic stem cells and has minimized the efficacy of several anti-cancer drugs. In this study, we investigated the impact of the BM microenvironment on T cell redirection. Using bispecific antibodies targeting specific tumor antigens (CD123 and BCMA) and CD3, we observed that co-culture of acute myeloid leukemia or multiple myeloma cells with BM stromal cells protected tumor cells from bispecific antibody-T cell-mediated lysis in vitro and in vivo. Impaired CD3 redirection cytotoxicity was correlated with reduced T cell effector responses and cell-cell contact with stromal cells was implicated in reducing T cell activation and conferring protection of cancer cells. Finally, blocking the VLA4 adhesion pathway in combination with CD3 redirection reduced the stromal-mediated inhibition of cytotoxicity and T cell activation. Our results lend support to inhibiting VLA4 interactions along with administering CD3 redirection therapeutics as a novel combinatorial regimen for robust anti-cancer responses.

Cell Adhesion Molecules and Their Roles and Regulation in the Immune and Tumor Microenvironment

The immune system and cancer have a complex relationship with the immune system playing a dual role in tumor development. The effector cells of the immune system can recognize and kill malignant cells while immune system-mediated inflammation can also promote tumor growth and regulatory cells suppress the anti-tumor responses. In the center of all anti-tumor responses is the ability of the immune cells to migrate to the tumor site and to interact with each other and with the malignant cells. Cell adhesion molecules including receptors of the immunoglobulin superfamily and integrins are of crucial importance in mediating these processes. Particularly integrins play a vital role in regulating all aspects of immune cell function including immune cell trafficking into tissues, effector cell activation and proliferation and the formation of the immunological synapse between immune cells or between immune cell and the target cell both during homeostasis and during inflammation and cancer. In this review we discuss the molecular mechanisms regulating integrin function and the role of integrins and other cell adhesion molecules in immune responses and in the tumor microenvironment. We also describe how malignant cells can utilize cell adhesion molecules to promote tumor growth and metastases and how these molecules could be targeted in cancer immunotherapy.

Regulatory T cells limit unconventional memory to preserve the capacity to mount protective CD8 memory responses to pathogens

Immunological memory exists so that following infection an expanded population of pathogen-specific lymphocytes can rapidly and efficiently control infection in the case of reexposure. However, in the case of CD8+ T lymphocytes, a population of unconventional CD44+CD122+ virtual memory T cells (T_VM) has been described that possesses many, though not all, features of "true memory" T cells, without the requirement of first encountering cognate antigen. Here, we demonstrate a role for regulatory T cell-mediated restraint of T_VM at least in part through limiting IL-15 trans-presentation by CD11b+ dendritic cells. Further, we show that keeping T_VM in check ensures development of functional, antigen-specific "true" memory phenotype CD8+ T cells that can assist in pathogen control upon reexposure.

Single-cell characterization and quantification of translation-competent viral reservoirs in treated and untreated HIV infection

The phenotypic characterization of the cells in which HIV persists during antiretroviral therapy (ART) remains technically challenging. We developed a simple flow cytometry-based assay to quantify and characterize infected cells producing HIV proteins during untreated and treated HIV infection. By combining two antibodies targeting the HIV capsid in a standard intracellular staining protocol, we demonstrate that p24-producing cells can be detected with high specificity and sensitivity in the blood from people living with HIV. In untreated individuals, the frequency of productively infected cells strongly correlated with plasma viral load. Infected cells preferentially displayed a transitional memory phenotype and were enriched in Th17, peripheral Tfh and regulatory T cells subsets. These cells also preferentially expressed activation markers (CD25, HLA-DR, Ki67), immune checkpoint molecules (PD-1, LAG-3, TIGIT, Tim-3) as well as the integrins α4β7 and α4β1. In virally suppressed individuals on ART, p24-producing cells were only detected upon stimulation (median frequency of 4.3 p24+ cells/10⁶ cells). These measures correlated with other assays assessing the size of the persistent reservoir including total and integrated HIV DNA, Tat/rev Induced Limiting Dilution Assay (TILDA) and quantitative viral outgrowth assay (QVOA). In ART-suppressed individuals, p24-producing cells preferentially displayed a transitional and effector memory phenotype, and expressed immune checkpoint molecules (PD-1, TIGIT) as well as the integrin α4β1. Remarkably, α4β1 was expressed by more than 70% of infected cells both in untreated and ART-suppressed individuals. Altogether, these results highlight a broad diversity in the phenotypes of HIV-infected cells in treated and untreated infection and suggest that strategies targeting multiple and phenotypically distinct cellular reservoirs will be needed to exert a significant impact on the size of the reservoir.

HIV persists in a small pool of infected CD4+ T cells during ART. A better characterization of these cells is a pre-requisite to the development of HIV eradication strategies. We developed a novel assay, named HIV-Flow, to simultaneously quantify and characterize reservoir cells in individuals receiving ART. With this assay, we found that a median of only 5 cells/million have the ability to produce the HIV protein Gag in individuals on suppressive ART. These frequencies correlated with other assays aimed at measuring HIV reservoirs. Importantly, we show that the HIV reservoir is phenotypically diverse, with numerous cell subsets contributing to the pool of persistently infected cells. Nonetheless, we identified several markers preferentially expressed at the surface or these rare reservoir cells, including immune checkpoint molecules and homing receptors. By combining these markers, we identified discrete cellular subsets highly enriched in HIV-infected cells. This novel assay will facilitate the identification of markers expressed by cellular HIV reservoirs.

Regulatory T cells mediate specific suppression by depleting peptide-MHC class II from dendritic cells

T regulatory cells (T_regs) can activate multiple suppressive mechanisms in vitro upon activation via the T cell receptor resulting in antigen-independent suppression. However, it remains unclear whether similar pathways operate in vivo. Here, we found that antigen-specific T_regs activated by dendritic cells (DCs) pulsed with two antigens suppressed T_naive specific for both cognate and non-cognate antigens in vitro, but only suppressed T_naive specific for cognate antigen in vivo. Antigen-specific T_regs formed strong interactions with DC resulting in selective inhibition of the binding of T_naive to cognate antigen, yet allowing bystander T_naive access. Strong binding resulted in removal of the cognate peptide-MHCII (pMHCII) from the DC surface reducing the capacity of the DC to present antigen. The enhanced binding of T_regs to DC coupled with their capacity to deplete pMHCII represents a novel pathway for T_reg-mediated suppression and may be a mechanism by which T_regs maintain immune homeostasis.

Lack of junctional adhesion molecule (JAM)-B ameliorates experimental autoimmune encephalomyelitis

In multiple sclerosis (MS) and its animal model experimental autoimmune encephalomyelitis (EAE) autoaggressive CD4⁺ T cells cross the blood-brain barrier (BBB) and cause neuroinflammation. Therapeutic targeting of CD4⁺ T-cell trafficking into the CNS by blocking α4-integrins has proven beneficial for the treatment of MS but comes with associated risks, probably due to blocking CD8⁺ T cell mediated CNS immune surveillance. Our recent observations show that CD8⁺ T cells also rely on α4β1-integrins to cross the BBB. Besides vascular cell adhesion molecule-1 (VCAM-1), we identified junctional adhesion molecule-B (JAM-B) as a novel vascular α4β1-integrin ligand involved in CD8⁺ T-cell migration across the BBB. This prompted us to investigate, if JAM-B also mediates CD4⁺ T-cell migration across the BBB. We first ensured that encephalitogenic T cells can bind to JAM-B in vitro and next compared EAE pathogenesis in JAM-B^-/- C57BL/6J mice and their wild-type littermates. Following immunization with MOG_aa35-55 peptide, JAM-B^-/- mice developed ameliorated EAE compared to their wild-type littermates. At the same time, we isolated higher numbers of CD45⁺ infiltrating immune cells from the CNS of JAM-B^-/- C57BL/6J mice suffering from EAE. Immunofluorescence staining revealed that the majority of CD45⁺ inflammatory cells accumulated in the leptomeningeal and perivascular spaces of the CNS behind the BBB but do not gain access to the CNS parenchyma. Trapping of CNS inflammatory cells was not due to increased inflammatory cell proliferation. Neither a loss of BBB integrity or BBB polarity potentially affecting local chemokine gradients nor a lack of focal gelatinase activation required for CNS parenchymal immune cell entry across the glia limitans could be detected in JAM-B^-/- mice. Lack of a role for JAM-B in the effector phase of EAE was supported by the observation that we did not detect any role for JAM-B in EAE pathogenesis, when EAE was elicited by in vitro activated MOG _aa35-55-specific CD4⁺ effector T cells. On the other hand, we also failed to demonstrate any role of JAM-B in in vivo priming, proliferation or polarization of MOG_aa35-55-specific CD4⁺ T cells in peripheral immune organs. Finally, our study excludes expression of and thus a role for JAM-B on peripheral and CNS infiltrating myeloid cells. Taken together, although endothelial JAM-B is not required for immune cell trafficking across the BBB in EAE, in its absence accumulation of inflammatory cells mainly in CNS leptomeningeal spaces leads to amelioration of EAE.

The immunological synapse as a pharmacological target

The development of T cell mediated immunity relies on the assembly of a highly specialized interface between T cell and antigen presenting cell (APC), known as the immunological synapse (IS). IS assembly is triggered when the T cell receptor (TCR) binds to specific peptide antigen presented in association to the major histocompatibility complex (MHC) by the APC, and is followed by the spatiotemporal dynamic redistribution of TCR, integrins, co-stimulatory receptors and signaling molecules, allowing for the fine-tuning and integration of the signals that lead to T cell activation. The knowledge acquired to date about the mechanisms of IS assembly underscores this structure as a robust pharmacological target. The activity of molecules involved in IS assembly and function can be targeted by specific compounds to modulate the immune response in a number of disorders, including cancers and autoimmune diseases, or in transplanted patients. Here, we will review the state-of-the art of the current therapies which exploit the IS to modulate the immune response.

Tetraspanin CD9: A Key Regulator of Cell Adhesion in the Immune System

The tetraspanin CD9 is expressed by all the major subsets of leukocytes (B cells, CD4⁺ T cells, CD8⁺ T cells, natural killer cells, granulocytes, monocytes and macrophages, and immature and mature dendritic cells) and also at a high level by endothelial cells. As a typical member of the tetraspanin superfamily, a prominent feature of CD9 is its propensity to engage in a multitude of interactions with other tetraspanins as well as with different transmembrane and intracellular proteins within the context of defined membranal domains termed tetraspanin-enriched microdomains (TEMs). Through these associations, CD9 influences many cellular activities in the different subtypes of leukocytes and in endothelial cells, including intracellular signaling, proliferation, activation, survival, migration, invasion, adhesion, and diapedesis. Several excellent reviews have already covered the topic of how tetraspanins, including CD9, regulate these cellular processes in the different cells of the immune system. In this mini-review, however, we will focus particularly on describing and discussing the regulatory effects exerted by CD9 on different adhesion molecules that play pivotal roles in the physiology of leukocytes and endothelial cells, with a particular emphasis in the regulation of adhesion molecules of the integrin and immunoglobulin superfamilies.

Integrins in T Cell Physiology

From the thymus to the peripheral lymph nodes, integrin-mediated interactions with neighbor cells and the extracellular matrix tune T cell behavior by organizing cytoskeletal remodeling and modulating receptor signaling. LFA-1 (αLβ2 integrin) and VLA-4 (α4β1 integrin) play a key role throughout the T cell lifecycle from thymocyte differentiation to lymphocyte extravasation and finally play a fundamental role in organizing immune synapse, providing an essential costimulatory signal for the T cell receptor. Apart from tuning T cell signaling, integrins also contribute to homing to specific target organs as exemplified by the importance of α4β7 in maintaining the gut immune system. However, apart from those well-characterized examples, the physiological significance of the other integrin dimers expressed by T cells is far less understood. Thus, integrin-mediated cell-to-cell and cell-to-matrix interactions during the T cell lifespan still represent an open field of research.

Not Just an Adhesion Molecule: LFA-1 Contact Tunes the T Lymphocyte Program

The α_Lβ₂ integrin LFA-1 is known to play a key role in T lymphocyte migration, which is necessary to mount a local immune response, and is also the main driver of autoimmune diseases. This migration-triggering signaling process in T cells is tightly regulated to permit an immune response that is appropriate to the local trigger, as well as to prevent deleterious tissue-damaging bystander effects. Emerging evidence shows that, in addition to prompting a diverse range of downstream signaling cascades, LFA-1 stimulation in T lymphocytes modulates gene-transcription programs, including genetic signatures of TGF-β and Notch pathways, with multifactorial biological outcomes. This review highlights recent findings and discusses molecular mechanisms by which LFA-1 signaling influence T lymphocyte differentiation into the effector subsets Th1, Th17, and induced regulatory T cells. We argue that LFA-1 contact with a cognate ligand, such as ICAM-1, independent of the immune synapse activates a late divergence in T cells' effector phenotypes, hence fine-tuning their functioning.

Role of Drebrin at the Immunological Synapse

Although drebrin was first described in neurons, it is also expressed in cells of the immune system, such as T lymphocytes and mast cells. Another member of the drebrin family of proteins, mammalian actin-binding protein 1 (mAbp-1) is more widely expressed and plays important roles in the function of macrophages, polymorphonuclear neutrophils, and B lymphocytes. We will briefly discuss on the function of mAbp-1 and drebrin in immune cells with emphasis on T cells. Specifically, drebrin enables the immune responses of CD4⁺ T lymphocytes. T cells are activated after the recognition of an antigen presented by antigen-presenting cells through cognate cell-cell contacts called immunological synapses (IS). In CD4⁺ T cells, drebrin associates with the chemokine receptor CXCR4, and both molecules redistribute to the IS displaying similar dynamics. Through its interaction with CXCR4 and the actin cytoskeleton, drebrin regulates T cell activation. CD4⁺ T cells are one of the main targets for the human immunodeficiency virus (HIV)-1. This virus utilizes the IS structure to be transmitted to uninfected cells, forming cell-cell contacts called virological synapses (VS). Interestingly, drebrin negatively regulates HIV-1 infection of CD4⁺ T lymphocytes, by regulating actin polymerization at the VS.

Diacylglycerol Kinases: Shaping Diacylglycerol and Phosphatidic Acid Gradients to Control Cell Polarity

Diacylglycerol kinases (DGKs) terminate diacylglycerol (DAG) signaling and promote phosphatidic acid (PA) production. Isoform specific regulation of DGKs activity and localization allows DGKs to shape the DAG and PA gradients. The capacity of DGKs to constrain the areas of DAG signaling is exemplified by their role in defining the contact interface between T cells and antigen presenting cells: the immune synapse. Upon T cell receptor engagement, both DGK α and ζ metabolize DAG at the immune synapse thus constraining DAG signaling. Interestingly, their activity and localization are not fully redundant because DGKζ activity metabolizes the bulk of DAG in the cell, whereas DGKα limits the DAG signaling area localizing specifically at the periphery of the immune synapse. When DGKs terminate DAG signaling, the local PA production defines a new signaling domain, where PA recruits and activates a second wave of effector proteins. The best-characterized example is the role of DGKs in protrusion elongation and cell migration. Indeed, upon growth factor stimulation, several DGK isoforms, such as α, ζ, and γ, are recruited and activated at the plasma membrane. Here, local PA production controls cell migration by finely modulating cytoskeletal remodeling and integrin recycling. Interestingly, DGK-produced PA also controls the localization and activity of key players in cell polarity such as aPKC, Par3, and integrin β1. Thus, T cell polarization and directional migration may be just two instances of the general contribution of DGKs to the definition of cell polarity by local specification of membrane identity signaling.

Orchestrating Lymphocyte Polarity in Cognate Immune Cell-Cell Interactions

The immune synapse (IS) is a specialized structure established between different immune cells that fulfills several functions, including a role as a communication bridge. This intimate contact between a T cell and an antigen-presenting cell promotes the proliferation and differentiation of lymphocytes involved in the contact. T-cell activation requires the specific triggering of the T-cell receptor (TCR), which promotes the activation of different signaling pathways inducing the polarization of the T cell. During this process, different adhesion and signaling receptors reorganize at specialized membrane domains, concomitantly to the polarization of the tubulin and actin cytoskeletons, forming stable polarization platforms. The centrosome also moves toward the IS, driving the movement of different organelles, such as the biosynthetic, secretory, degrading machinery, and mitochondria, to sustain T-cell activation. A proper orchestration of all these events is essential for T-cell effector functions and the accomplishment of a complete immune response.

Cytomegalovirus-Associated CD4(+) CD28(null) Cells in NKG2D-Dependent Glomerular Endothelial Injury and Kidney Allograft Dysfunction

Emerging data suggest that expansion of a circulating population of atypical, cytotoxic CD4(+) T cells lacking costimulatory CD28 (CD4(+) CD28(null) cells) is associated with latent cytomegalovirus (CMV) infection. The purpose of the current study was to increase the understanding of the relevance of these cells in 100 unselected kidney transplant recipients followed prospectively for a median of 54 months. Multicolor flow cytometry of peripheral blood mononuclear cells before transplantation and serially posttransplantation was undertaken. CD4(+) CD28(null) cells were found predominantly in CMV-seropositive patients and expanded in the posttransplantation period. These cells were predominantly effector-memory phenotype and expressed markers of endothelial homing (CX3CR1) and cytotoxicity (NKG2D and perforin). Isolated CD4(+) CD27(-) CD28(null) cells proliferated in response to peripheral blood mononuclear cells previously exposed to CMV-derived (but not HLA-derived) antigens and following such priming incubation with glomerular endothelium resulted in signs of endothelial damage and apoptosis (release of fractalkine and von Willebrand factor; increased caspase 3 expression). This effect was mitigated by NKG2D-blocking antibody. Increased CD4(+) CD28(null) cell frequencies were associated with delayed graft function and lower estimated glomerular filtration rate at end follow-up. This study suggests an important role for this atypical cytotoxic CD4(+) CD28(null) cell subset in kidney transplantation and points to strategies that may minimize the impact on clinical outcomes.

Function and Dynamics of Tetraspanins during Antigen Recognition and Immunological Synapse Formation

Tetraspanin-enriched microdomains (TEMs) are specialized membrane platforms driven by protein–protein interactions that integrate membrane receptors and adhesion molecules. Tetraspanins participate in antigen recognition and presentation by antigen-­presenting cells (APCs) through the organization of pattern-recognition receptors (PRRs) and their downstream-induced signaling, as well as the regulation of MHC-II–peptide trafficking. T lymphocyte activation is triggered upon specific recognition of antigens present on the APC surface during immunological synapse (IS) formation. This dynamic process is characterized by a defined spatial organization involving the compartmentalization of receptors and adhesion molecules in specialized membrane domains that are connected to the underlying cytoskeleton and signaling molecules. Tetraspanins contribute to the spatial organization and maturation of the IS by controlling receptor clustering and local accumulation of adhesion receptors and integrins, their downstream signaling, and linkage to the actin cytoskeleton. This review offers a perspective on the important role of TEMs in the regulation of antigen recognition and presentation and in the dynamics of IS architectural organization.

Integrin α4β1 controls G9a activity that regulates epigenetic changes and nuclear properties required for lymphocyte migration

The mechanical properties of the cell nucleus change to allow cells to migrate, but how chromatin modifications contribute to nuclear deformability has not been defined. Here, we demonstrate that a major factor in this process involves epigenetic changes that underpin nuclear structure. We investigated the link between cell adhesion and epigenetic changes in T-cells, and demonstrate that T-cell adhesion to VCAM1 via α4β1 integrin drives histone H3 methylation (H3K9me2/3) through the methyltransferase G9a. In this process, active G9a is recruited to the nuclear envelope and interacts with lamin B1 during T-cell adhesion through α4β1 integrin. G9a activity not only reorganises the chromatin structure in T-cells, but also affects the stiffness and viscoelastic properties of the nucleus. Moreover, we further demonstrated that these epigenetic changes were linked to lymphocyte movement, as depletion or inhibition of G9a blocks T-cell migration in both 2D and 3D environments. Thus, our results identify a novel mechanism in T-cells by which α4β1 integrin signaling drives specific chromatin modifications, which alter the physical properties of the nucleus and thereby enable T-cell migration.

The interaction affinity between vascular cell adhesion molecule-1 (VCAM-1) and very late antigen-4 (VLA-4) analyzed by quantitative FRET

Very late antigen-4 (VLA-4), a member of integrin superfamily, interacts with its major counter ligand vascular cell adhesion molecule-1 (VCAM-1) and plays an important role in leukocyte adhesion to vascular endothelium and immunological synapse formation. However, irregular expressions of these proteins may also lead to several autoimmune diseases and metastasis cancer. Thus, quantifying the interaction affinity of the VCAM-1/VLA-4 interaction is of fundamental importance in further understanding the nature of this interaction and drug discovery. In this study, we report an ‘in solution’ steady state organic fluorophore based quantitative fluorescence resonance energy transfer (FRET) assay to quantify this interaction in terms of the dissociation constant (K_d). We have used, in our FRET assay, the Alexa Fluor 488-VLA-4 conjugate as the donor, and Alexa Fluor 546-VCAM-1 as the acceptor. From the FRET signal analysis, K_d of this interaction was determined to be 41.82 ± 2.36 nM. To further confirm our estimation, we have employed surface plasmon resonance (SPR) technique to obtain K_d = 39.60 ± 1.78 nM, which is in good agreement with the result obtained by FRET. This is the first reported work which applies organic fluorophore based ‘in solution’ simple quantitative FRET assay to obtain the dissociation constant of the VCAM-1/VLA-4 interaction, and is also the first quantification of this interaction. Moreover, the value of K_d can serve as an indicator of abnormal protein-protein interactions; hence, this assay can potentially be further developed into a drug screening platform of VLA-4/VCAM-1 as well as other protein-ligand interactions.

Integrin alpha L controls the homing of regulatory T cells during CNS autoimmunity in the absence of integrin alpha 4

Experimental autoimmune encephalomyelitis (EAE), the animal model of multiple sclerosis (MS), results from an autoimmune attack of the central nervous system (CNS) by effector T helper (Th) 1 and Th17 cells. Regulatory T cells (Treg) can control effector T cells and limit the progression of CNS autoimmunity. Integrin alpha 4 (Itga4) is critical for the entry of Th1 but not Th17 cells into the CNS during EAE. Whether Itga4 controls the homing of Tregs in the CNS and whether Tregs can limit Th17-mediated EAE has, however, not been addressed. Through selective elimination of Itga4 in Foxp3-expressing cells, we show here that Tregs can suppress Th17-mediated EAE and enter into the CNS independently of Itga4. Furthermore, similarly to Th17 cells and in contrast to Th1 cells, Tregs depend on LFA-1 for their entry into the CNS in the absence of Itga4. Therefore, these data suggest that the efficacy of Itga4 neutralization on MS progression may be associated with the prevention of Th1 cells and the maintenance of Tregs migration into the CNS.

Natalizumab restores aberrant miRNA expression profile in multiple sclerosis and reveals a critical role for miR-20b

Objective

To identify microRNAs (miRNAs) regulated by anti-α4 integrin monoclonal antibody therapy (natalizumab) in the peripheral blood of patients with relapsing-remitting (RR) multiple sclerosis (MS) and to confirm their role in experimental settings in vivo.

Methods

In a longitudinal study of 17 RR-MS patients, we investigated blood miRNA expression profiles at baseline and after 1 year of natalizumab therapy by microarray technique and quantitative PCR validation. We compared the baseline expression profiles of these patients to those of 18 age- and sex-matched healthy controls. We confirmed the contribution of resulting candidate miRNAs in an animal model of MS, experimental autoimmune encephalomyelitis (EAE) induced by adoptive transfer of proteolipid protein (PLP)_139–151-activated lymphocytes in SJL/J mice or by active immunization of miR-106a∼363-deficient C57BL/6 mice (or wildtype litter mates) with myelin oligodendrocyte glycoprotein (MOG)_35–55.

Results

Our longitudinal analysis revealed that miR-18a, miR-20b, miR-29a, and miR-103 were upregulated and predominantly expressed by CD4⁺ T cells, whereas miR-326 was downregulated upon natalizumab treatment. A comparison of untreated RR-MS patients at baseline with healthy controls revealed that the four natalizumab-upregulated targets were initially downregulated in MS. All confirmed targets showed disease-dependent expression in splenocytes of mice suffering from EAE. Genetic deletion of the miRNA cluster miR-106a∼363 (containing natalizumab-regulated miR-20b) resulted in a more severe EAE course and an in vivo upregulation of the miR-20b target genes rorgt, stat3, and vegfa.

Interpretation

Our study indicates that natalizumab restores dysregulated miRNA patterns in MS and reveals the contribution of miR-20b in autoimmune demyelination in vivo.

Tetraspanins CD9 and CD151 at the immune synapse support T-cell integrin signaling

Understanding how the immune response is activated and amplified requires detailed knowledge of the stages in the formation of the immunological synapse (IS) between T lymphocytes and antigen-presenting cells (APCs). We show that tetraspanins CD9 and CD151 congregate at the T-cell side of the IS. Silencing of CD9 or CD151 blunts the IL-2 secretion and expression of the activation marker CD69 by APC-conjugated T lymphocytes, but does not affect the accumulation of CD3 or actin to the IS, or the translocation of the microtubule-organizing center toward the T-B contact area. CD9 or CD151 silencing diminishes the relocalization of α4β1 integrin to the IS and reduces the accumulation of high-affinity β1 integrins at the cell-cell contact. These changes are accompanied by diminished phosphorylation of the integrin downstream targets FAK and ERK1/2. Our results suggest that CD9 and CD151 support integrin-mediated signaling at the IS.

Integrin modulators: a patent review

INTRODUCTION

Integrins are heterodimeric cell surface receptors, which enable adhesion, proliferation, and migration of cells by recognizing binding motifs in extracellular matrix (ECM) proteins. As transmembrane linkers between the cytoskeleton and the ECM, they are able to recruit a huge variety of proteins and to influence signaling pathways bidirectionally, thereby regulating gene expression and cell survival. Hence, integrins play a key role in various physiological as well as pathological processes, which has turned them into an attractive target for pharmaceutical research.

AREAS COVERED

In this review, the latest therapeutic developments of drug candidates and recently patented integrin ligands are summarized.

EXPERT OPINION

Integrins have been proven to be valuable therapeutic targets in the treatment of several inflammatory and autoimmune diseases, where leukocyte adhesion processes are regulated by them. Furthermore, they play an important role in pathological angiogenesis and tumor metastasis, being a promising target for cancer therapy.

The N terminus of SKAP55 enables T cell adhesion to TCR and integrin ligands via distinct mechanisms

The T cell receptor (TCR) triggers the assembly of "SLP-76 microclusters," which mediate signals required for T cell activation. In addition to regulating integrin activation, we show that Src kinase-associated phosphoprotein of 55 kD (SKAP55) is required for microcluster persistence and movement, junctional stabilization, and integrin-independent adhesion via the TCR. These functions require the dimerization of SKAP55 and its interaction with the adaptor adhesion and degranulation-promoting adaptor protein (ADAP). A "tandem dimer" containing two ADAP-binding SKAP55 Src homology 3 (SH3) domains stabilized SLP-76 microclusters and promoted T cell adhesion via the TCR, but could not support adhesion to integrin ligands. Finally, the SKAP55 dimerization motif (DM) enabled the coimmunoprecipitation of the Rap1-dependent integrin regulator Rap1-GTP-interacting adaptor molecule (RIAM), the recruitment of talin into TCR-induced adhesive junctions, and "inside-out" signaling to β1 integrins. Our data indicate that SKAP55 dimers stabilize SLP-76 microclusters, couple SLP-76 to the force-generating systems responsible for microcluster movement, and enable adhesion via the TCR by mechanisms independent of RIAM, talin, and β1 integrins.

Integrated genomic and prospective clinical studies show the importance of modular pleiotropy for disease susceptibility, diagnosis and treatment

Background

Translational research typically aims to identify and functionally validate individual, disease-specific genes. However, reaching this aim is complicated by the involvement of thousands of genes in common diseases, and that many of those genes are pleiotropic, that is, shared by several diseases.

Methods

We integrated genomic meta-analyses with prospective clinical studies to systematically investigate the pathogenic, diagnostic and therapeutic roles of pleiotropic genes. In a novel approach, we first used pathway analysis of all published genome-wide association studies (GWAS) to find a cell type common to many diseases.

Results

The analysis showed over-representation of the T helper cell differentiation pathway, which is expressed in T cells. This led us to focus on expression profiling of CD4⁺ T cells from highly diverse inflammatory and malignant diseases. We found that pleiotropic genes were highly interconnected and formed a pleiotropic module, which was enriched for inflammatory, metabolic and proliferative pathways. The general relevance of this module was supported by highly significant enrichment of genetic variants identified by all GWAS and cancer studies, as well as known diagnostic and therapeutic targets. Prospective clinical studies of multiple sclerosis and allergy showed the importance of both pleiotropic and disease specific modules for clinical stratification.

Conclusions

In summary, this translational genomics study identified a pleiotropic module, which has key pathogenic, diagnostic and therapeutic roles.

CD11c⁺ cells primed with unrelated antigens facilitate an accelerated immune response to influenza virus in mice

Recent evidence suggests that an individual's unique history and sequence of exposures to pathogens and antigens may dictate downstream immune responses to disparate antigens. We show that the i.n. delivery of nonreplicative virus-like particles (VLPs), which bear structural but no antigenic similarities to respiratory pathogens, acts to prime the lungs of both C56BL/6 and BALB/c mice, facilitating heightened and accelerated primary immune responses to high-dose influenza challenge, thus providing a nonpathogenic model of innate imprinting. These responses correspond closely to those observed following natural infection with the opportunistic fungus, Pneumocystis murina, and are characterized by accelerated antigen processing by DCs and alveolar macrophages, an enhanced influx of cells to the local tracheobronchial lymph node, and early upregulation of T-cell co-stimulatory/adhesion molecules. CD11c⁺ cells, which have been directly exposed to VLPs or Pneumocystis are necessary in facilitating enhanced clearance of influenza virus, and the repopulation of the lung by Ly-6C⁺ precursors relies on CCR2 expression. Thus, immune imprinting 72 h after VLP-priming, or 2 weeks after Pneumocystis-priming is CCR2-mediated and results from the enhanced antigen processing, maturation, and trafficking abilities of DCs and alveolar macrophages, which cause accelerated influenza-specific primary immune responses and result in superior viral clearance.

Natalizumab and progressive multifocal leukoencephalopathy: migrating towards safe adhesion molecule therapy in multiple sclerosis

Natalizumab, a humanized monoclonal antibody against alpha4beta1 integrin, was shown in clinical trials to dramatically reduce the relapse rate, development of new magnetic resonance imaging (MRI) lesions and progression of disability in patients with relapsing multiple sclerosis. Following its expedited approval, sales of the drug were discontinued owing to the emergence of two cases of progressive multifocal leukoencephalopathy (PML), a rare but deadly viral infection of the central nervous system (CNS) associated with immunosuppression. Owing to the effect of natalizumab on central nervous system leukocyte recruitment, the emergence of PML has been attributed to diminished immunosurveillance. The lack of additional opportunistic or CNS infections among natalizumab-treated patients, however, suggests that alternate mechanisms may contribute to the infectious risk. This review examines how the inhibition of alpha4beta1-mediated adhesion might establish a unique milieu for the development of PML and how future approaches to selective adhesion molecule therapy in multiple sclerosis might avoid a similar fate.

Increased B cell and cytotoxic NK cell proportions and increased T cell responsiveness in blood of natalizumab-treated multiple sclerosis patients

Background

Changes in the blood lymphocyte composition probably both mediate and reflect the effects of natalizumab treatment in multiple sclerosis, with implications for treatment benefits and risks.

Methods

A broad panel of markers for lymphocyte populations, including states of activation and co-stimulation, as well as functional T cell responses to recall antigens and mitogens, were assessed by flow cytometry in 40 patients with relapsing multiple sclerosis before and after one-year natalizumab treatment.

Results

Absolute numbers of all major lymphocyte populations increased after treatment, most markedly for NK and B cells. The fraction of both memory and presumed regulatory B cell subsets increased, as did CD3^-CD56^dim cytotoxic NK cells, whereas CD3^-CD56^bright regulatory NK cells decreased. The increase in cell numbers was further associated with a restored T cell responsiveness to recall antigens and mitogens in functional assays.

Conclusions

Our data confirms that natalizumab treatment increases the number of lymphocytes in blood, likely mirroring the expression of VLA-4 being highest on NK and B cells. This finding supports reduction of lymphocyte extravasation as a main mode of action, although the differential effects on subpopulation composition suggests that cell-signalling may also be affected. The systemic increase in T cell responsiveness reflects the increase in numbers, and while augmenting anti-infectious responses systemically, localized responses may become correspondingly decreased.

Location, location, location: the impact of migratory heterogeneity on T cell function

T cell migration is crucial for an effective adaptive immune response to invading pathogens. Naive and memory T cells encounter pathogen antigens, become activated, and differentiate into effector cells in secondary lymphoid tissues, and then migrate to the site(s) of infection where they exert effector activities that control and eliminate pathogens. To achieve activation, efficient effector function, and good memory formation, T cells must traffic between lymphoid and non-lymphoid tissues within the body. This complex process is facilitated by chemokine receptors, selectins, CD44, and integrins that mediate the interactions of T cells with the environment. The expression patterns of these migration receptors (MR) dictate the tissues into which the effector T cells migrate and enable them to occupy specific niches within the tissue. While MR have been considered primarily to facilitate cell movement, we highlight how the heterogeneity of signaling through these receptors influences the function and fate of T cells in situ. We explore what drives MR expression heterogeneity, how this affects migration, and how this impacts T cell effector function and memory formation.

Postthymic expansion in human CD4 naive T cells defined by expression of functional high-affinity IL-2 receptors

As the thymus involutes with age, the maintenance of peripheral naive T cells in humans becomes strongly dependent on peripheral cell division. However, mechanisms that orchestrate homeostatic division remain unclear. In this study we present evidence that the frequency of naive CD4 T cells that express CD25 (IL-2 receptor α-chain) increases with age on subsets of both CD31(+) and CD31(-) naive CD4 T cells. Analyses of TCR excision circles from sorted subsets indicate that CD25(+) naive CD4 T cells have undergone more rounds of homeostatic proliferation than their CD25(-) counterparts in both the CD31(+) and CD31(-) subsets, indicating that CD25 is a marker of naive CD4 T cells that have preferentially responded to survival signals from self-Ags or cytokines. CD25 expression on CD25(-) naive CD4 T cells can be induced by IL-7 in vitro in the absence of TCR activation. Although CD25(+) naive T cells respond to lower concentrations of IL-2 as compared with their CD25(-) counterparts, IL-2 responsiveness is further increased in CD31(-) naive T cells by their expression of the signaling IL-2 receptor β-chain CD122, forming with common γ-chain functional high-affinity IL-2 receptors. CD25 plays a role during activation: CD25(+) naive T cells stimulated in an APC-dependent manner were shown to produce increased levels of IL-2 as compared with their CD25(-) counterparts. This study establishes CD25(+) naive CD4 T cells, which are further delineated by CD31 expression, as a major functionally distinct immune cell subset in humans that warrants further characterization in health and disease.

Natalizumab exerts direct signaling capacity and supports a pro-inflammatory phenotype in some patients with multiple sclerosis

Natalizumab is a recombinant monoclonal antibody raised against integrin alpha-4 (CD49d). It is approved for the treatment of patients with multiple sclerosis (MS), a chronic inflammatory autoimmune disease of the CNS. While having shown high therapeutic efficacy, treatment by natalizumab has been linked to progressive multifocal leukoencephalopathy (PML) as a serious adverse effect. Furthermore, drug cessation sometimes induces rebound disease activity of unknown etiology. Here we investigated whether binding of this adhesion-blocking antibody to T lymphocytes could modulate their phenotype by direct induction of intracellular signaling events. Primary CD4⁺ T lymphocytes either from healthy donors and treated with natalizumab in vitro or from MS patients receiving their very first dose of natalizumab were analyzed. Natalizumab induced a mild upregulation of IL-2, IFN-γ and IL-17 expression in activated primary human CD4⁺ T cells propagated ex vivo from healthy donors, consistent with a pro-inflammatory costimulatory effect on lymphokine expression. Along with this, natalizumab binding triggered rapid MAPK/ERK phosphorylation. Furthermore, it decreased CD49d surface expression on effector cells within a few hours. Sustained CD49d downregulation could be attributed to integrin internalization and degradation. Importantly, also CD4⁺ T cells from some MS patients receiving their very first dose of natalizumab produced more IL-2, IFN-γ and IL-17 already 24 h after infusion. Together these data indicate that in addition to its adhesion-blocking mode of action natalizumab possesses mild direct signaling capacities, which can support a pro-inflammatory phenotype of peripheral blood T lymphocytes. This might explain why a rebound of disease activity or IRIS is observed in some MS patients after natalizumab cessation.