Thymocyte emigration is mediated by active movement away from stroma-derived factors

Severe CD8+ T Lymphopenia in WHIM Syndrome Caused by Selective Sequestration in Primary Immune Organs

Warts, hypogammaglobulinemia, infections, and myelokathexis (WHIM) syndrome is an ultra-rare combined primary immunodeficiency disease caused by heterozygous gain-of-function mutations in the chemokine receptor CXCR4. WHIM patients typically present with recurrent acute infections associated with myelokathexis (severe neutropenia due to bone marrow retention of mature neutrophils). Severe lymphopenia is also common, but the only associated chronic opportunistic pathogen is human papillomavirus and mechanisms are not clearly defined. In this study, we show that WHIM mutations cause more severe CD8 than CD4 lymphopenia in WHIM patients and WHIM model mice. Mechanistic studies in mice revealed selective and WHIM allele dose-dependent accumulation of mature CD8 single-positive cells in thymus in a cell-intrinsic manner due to prolonged intrathymic residence, associated with increased CD8 single-positive thymocyte chemotactic responses in vitro toward the CXCR4 ligand CXCL12. In addition, mature WHIM CD8+ T cells preferentially home to and are retained in the bone marrow in mice in a cell-intrinsic manner. Administration of the specific CXCR4 antagonist AMD3100 (plerixafor) in mice rapidly and transiently corrected T cell lymphopenia and the CD4/CD8 ratio. After lymphocytic choriomeningitis virus infection, we found no difference in memory CD8+ T cell differentiation or viral load between wild-type and WHIM model mice. Thus, lymphopenia in WHIM syndrome may involve severe CXCR4-dependent CD8+ T cell deficiency resulting in part from sequestration in the primary lymphoid organs, thymus, and bone marrow.

The chemokine landscape: one system multiple shades

Leukocyte trafficking is mainly governed by chemokines, chemotactic cytokines, which can be concomitantly produced in tissues during homeostatic conditions or inflammation. After the discovery and characterization of the individual chemokines, we and others have shown that they present additional properties. The first discoveries demonstrated that some chemokines act as natural antagonists on chemokine receptors, and prevent infiltration of leukocyte subsets in tissues. Later on it was shown that they can exert a repulsive effect on selective cell types, or synergize with other chemokines and inflammatory mediators to enhance chemokine receptors activities. The relevance of the fine-tuning modulation has been demonstrated in vivo in a multitude of processes, spanning from chronic inflammation to tissue regeneration, while its role in the tumor microenvironment needs further investigation. Moreover, naturally occurring autoantibodies targeting chemokines were found in tumors and autoimmune diseases. More recently in SARS-CoV-2 infection, the presence of several autoantibodies neutralizing chemokine activities distinguished disease severity, and they were shown to be beneficial, protecting from long-term sequelae. Here, we review the additional properties of chemokines that influence cell recruitment and activities. We believe these features need to be taken into account when designing novel therapeutic strategies targeting immunological disorders.

CAR-T cell therapy in triple-negative breast cancer: Hunting the invisible devil

Triple-negative breast cancer (TNBC) is known as the most intricate and hard-to-treat subtype of breast cancer. TNBC cells do not express the well-known estrogen receptor, progesterone receptor, and human epidermal growth factor receptor 2 (HER2) expressed by other breast cancer subtypes. This phenomenon leaves no room for novel treatment approaches including endocrine and HER2-specific antibody therapies. To date, surgery, radiotherapy, and systemic chemotherapy remain the principal therapy options for TNBC treatment. However, in numerous cases, these approaches either result in minimal clinical benefit or are nonfunctional, resulting in disease recurrence and poor prognosis. Nowadays, chimeric antigen receptor T cell (CAR-T) therapy is becoming more established as an option for the treatment of various types of hematologic malignancies. CAR-Ts are genetically engineered T lymphocytes that employ the body’s immune system mechanisms to selectively recognize cancer cells expressing tumor-associated antigens (TAAs) of interest and efficiently eliminate them. However, despite the clinical triumph of CAR-T therapy in hematologic neoplasms, CAR-T therapy of solid tumors, including TNBC, has been much more challenging. In this review, we will discuss the success of CAR-T therapy in hematological neoplasms and its caveats in solid tumors, and then we summarize the potential CAR-T targetable TAAs in TNBC studied in different investigational stages.

CAR-T cell therapy in triple-negative breast cancer: Hunting the invisible devil

Triple-negative breast cancer (TNBC) is known as the most intricate and hard-to-treat subtype of breast cancer. TNBC cells do not express the well-known estrogen receptor, progesterone receptor, and human epidermal growth factor receptor 2 (HER2) expressed by other breast cancer subtypes. This phenomenon leaves no room for novel treatment approaches including endocrine and HER2-specific antibody therapies. To date, surgery, radiotherapy, and systemic chemotherapy remain the principal therapy options for TNBC treatment. However, in numerous cases, these approaches either result in minimal clinical benefit or are nonfunctional, resulting in disease recurrence and poor prognosis. Nowadays, chimeric antigen receptor T cell (CAR-T) therapy is becoming more established as an option for the treatment of various types of hematologic malignancies. CAR-Ts are genetically engineered T lymphocytes that employ the body's immune system mechanisms to selectively recognize cancer cells expressing tumor-associated antigens (TAAs) of interest and efficiently eliminate them. However, despite the clinical triumph of CAR-T therapy in hematologic neoplasms, CAR-T therapy of solid tumors, including TNBC, has been much more challenging. In this review, we will discuss the success of CAR-T therapy in hematological neoplasms and its caveats in solid tumors, and then we summarize the potential CAR-T targetable TAAs in TNBC studied in different investigational stages.

Recent Advances in Solid Tumor CAR-T Cell Therapy: Driving Tumor Cells From Hero to Zero?

Chimeric antigen receptor T-cells (CAR-Ts) are known as revolutionary living drugs that have turned the tables of conventional cancer treatments in certain hematologic malignancies such as B-cell acute lymphoblastic leukemia (B-ALL) and diffuse large B-cell lymphoma (DLBCL) by achieving US Food and Drug Administration (FDA) approval based on their successful clinical outcomes. However, this type of therapy has not seen the light of victory in the fight against solid tumors because of various restricting caveats including heterogeneous tumor antigen expression and the immunosuppressive tumor microenvironments (TME) that negatively affect the tumor-site accessibility, infiltration, stimulation, activation, and persistence of CAR-Ts. In this review, we explore strategic twists including boosting vaccines and designing implementations that can support CAR-T expansion, proliferation, and tumoricidal capacity. We also step further by underscoring novel strategies for triggering endogenous antitumor responses and overcoming the limitation of poor CAR-T tumor-tissue infiltration and the lack of definitive tumor-specific antigens. Ultimately, we highlight how these approaches can address the mentioned arduous hurdles.

Can we identify WHIM in infancy? Opportunities with the public newborn screening process

Newborn screening (NBS) for severe combined immunodeficiency (SCID) utilizing T-cell receptor excision circles (TRECs) has been implemented in all 50 states as of December 2018 and has been transformative for the clinical care of SCID patients. Though having high sensitivity for SCID, NBS-SCID has low specificity, therefore is able to detect other causes of lymphopenia in newborns including many inborn errors of immunity (IEIs). In a recent study, three of six newborns later diagnosed with Warts, Hypogammaglobulinemia, Infections, and Myelokathexis (WHIM) syndrome were found to have a low TRECs and lymphopenia at birth. This presents an opportunity to increase the detection and diagnosis of WHIM syndrome by NBS-SCID with immunological follow-up along with a combination of flow cytometry for immune cell subsets, absolute neutrophil count, and genetic testing, extending beyond the conventional bone marrow studies. Coupled with emerging technologies such as next-generation sequencing, transcriptomics and proteomics, dried blood spots used in NBS-SCID will promote earlier detection, diagnosis, and therefore treatment of IEIs such as WHIM syndrome.

Glycogen synthase kinase 3 drives thymocyte egress by suppressing β-catenin activation of Akt

Molecular pathways controlling emigration of mature thymocytes from thymus to the periphery remain incompletely understood. Here, we show that T cell–specific ablation of glycogen synthase kinase 3 (GSK3) led to severely impaired thymic egress. In the absence of GSK3, β-catenin accumulated in the cytoplasm, where it associated with and activated Akt, leading to phosphorylation and degradation of Foxo1 and downregulation of Klf2 and S1P₁ expression, thereby preventing emigration of thymocytes. A cytoplasmic membrane-localized β-catenin excluded from the nucleus promoted Akt activation, suggesting a new function of β-catenin independent of its role as a transcriptional activator. Furthermore, genetic ablation of β-catenin, retroviral expression of a dominant negative Akt mutant, and transgenic expression of a constitutively active Foxo1 restored emigration of GSK3-deficient thymocytes. Our findings establish an essential role for GSK3 in thymocyte egress and reveal a previously unidentified signaling function of β-catenin in the cytoplasm.

CAR T cells in solid tumors: challenges and opportunities

BACKGROUND

CARs are simulated receptors containing an extracellular single-chain variable fragment (scFv), a transmembrane domain, as well as an intracellular region of immunoreceptor tyrosine-based activation motifs (ITAMs) in association with a co-stimulatory signal.

MAIN BODY

Chimeric antigen receptor (CAR) T cells are genetically engineered T cells to express a receptor for the recognition of the particular surface marker that has given rise to advances in the treatment of blood disorders. The CAR T cells obtain supra-physiological properties and conduct as "living drugs" presenting both immediate and steady effects after expression in T cells surface. But, their efficacy in solid tumor treatment has not yet been supported. The pivotal challenges in the field of solid tumor CAR T cell therapy can be summarized in three major parts: recognition, trafficking, and surviving in the tumor. On the other hand, the immunosuppressive tumor microenvironment (TME) interferes with T cell activity in terms of differentiation and exhaustion, and as a result of the combined use of CARs and checkpoint blockade, as well as the suppression of other inhibitor factors in the microenvironment, very promising results were obtained from the reduction of T cell exhaustion.

CONCLUSION

Nowadays, identifying and defeating the mechanisms associated with CAR T cell dysfunction is crucial to establish CAR T cells that can proliferate and lyse tumor cells severely. In this review, we discuss the CAR signaling and efficacy T in solid tumors and evaluate the most significant barriers in this process and describe the most novel therapeutic methods aiming to the acquirement of the promising therapeutic outcome in non-hematologic malignancies.

CAR-T Cell Therapy in Cancer: Tribulations and Road Ahead

Chimeric antigen receptor- (CAR-) T cell therapy is one of the most recent innovative immunotherapies and is rapidly evolving. Like other technologies, CAR-T cell therapy has undergone a long development process, and persistent explorations of the actions of the intracellular signaling domain and make several improvements have led to the superior efficacy when anti-CD19 CAR-T cell treatments in B cell cancers. At present, CAR-T cell therapy is developing rapidly, and many clinical trials have been established on a global scale, which has great commercial potential. This review mainly describes the toxicity of CAR-T cell therapy and the challenges of CAR-T cells in the treatment of solid tumors, and looks forward to future development and opportunities for immunotherapy and reviews major breakthroughs in CAR-T cell therapy.

Adaptive Immunodeficiency in WHIM Syndrome

Cysteine-X-cysteine chemokine receptor 4 (CXCR4) is a broadly expressed and multifunctional G protein-coupled chemokine receptor critical for organogenesis, hematopoiesis, and antimicrobial host defense. In the hematopoietic system, the binding of CXCR4 to its cognate chemokine ligand, CXCL12, mediates leukocyte trafficking, distribution, survival, activation, and proliferation. Warts, hypogammaglobulinemia, infections, and myelokathexis (WHIM) syndrome is a rare, autosomal dominant, combined immunodeficiency disorder caused by mutations in the C-terminus of CXCR4 that prevent receptor downregulation and therefore result in pathologically increased signaling. The “M” in the acronym WHIM refers to myelokathexis, the retention of neutrophils in the bone marrow resulting in neutropenia, which explains in part the increased susceptibility to bacterial infection. However, WHIM patients also present with B and T lymphopenia, which may explain the susceptibility to human papillomavirus (HPV), the cause of warts. The impact of WHIM mutations on lymphocytes and adaptive immunity has received less attention than myelokathexis and is the focus of this review.

A guide to chemokines and their receptors

The chemokines (or chemotactic cytokines) are a large family of small, secreted proteins that signal through cell surface G protein-coupled heptahelical chemokine receptors. They are best known for their ability to stimulate the migration of cells, most notably white blood cells (leukocytes). Consequently, chemokines play a central role in the development and homeostasis of the immune system, and are involved in all protective or destructive immune and inflammatory responses. Classically viewed as inducers of directed chemotactic migration, it is now clear that chemokines can stimulate a variety of other types of directed and undirected migratory behavior, such as haptotaxis, chemokinesis, and haptokinesis, in addition to inducing cell arrest or adhesion. However, chemokine receptors on leukocytes can do more than just direct migration, and these molecules can also be expressed on, and regulate the biology of, many nonleukocytic cell types. Chemokines are profoundly affected by post-translational modification, by interaction with the extracellular matrix (ECM), and by binding to heptahelical 'atypical' chemokine receptors that regulate chemokine localization and abundance. This guide gives a broad overview of the chemokine and chemokine receptor families; summarizes the complex physical interactions that occur in the chemokine network; and, using specific examples, discusses general principles of chemokine function, focusing particularly on their ability to direct leukocyte migration.

Chemotactic Cues for NOTCH1-Dependent Leukemia

The NOTCH signaling pathway is a conserved signaling cascade that regulates many aspects of development and homeostasis in multiple organ systems. Aberrant activity of this signaling pathway is linked to the initiation and progression of several hematological malignancies, exemplified by T-cell acute lymphoblastic leukemia (T-ALL). Interestingly, frequent non-mutational activation of NOTCH1 signaling has recently been demonstrated in B-cell chronic lymphocytic leukemia (B-CLL), significantly extending the pathogenic significance of this pathway in B-CLL. Leukemia patients often present with high-blood cell counts, diffuse disease with infiltration of the bone marrow, secondary lymphoid organs, and diffusion to the central nervous system (CNS). Chemokines are chemotactic cytokines that regulate migration of cells between tissues and the positioning and interactions of cells within tissue. Homeostatic chemokines and their receptors have been implicated in regulating organ-specific infiltration, but may also directly and indirectly modulate tumor growth. Recently, oncogenic NOTCH1 has been shown to regulate infiltration of leukemic cells into the CNS hijacking the CC-chemokine ligand 19/CC-chemokine receptor 7 chemokine axis. In addition, a crucial role for the homing receptor axis CXC-chemokine ligand 12/CXC-chemokine receptor 4 has been demonstrated in leukemia maintenance and progression. Moreover, the CCL25/CCR9 axis has been implicated in the homing of leukemic cells into the gut, particularly in the presence of phosphatase and tensin homolog tumor suppressor loss. In this review, we summarize the latest developments regarding the role of NOTCH signaling in regulating the chemotactic microenvironmental cues involved in the generation and progression of T-ALL and compare these findings to B-CLL.

T-cell egress from the thymus: Should I stay or should I go?

T‐cells bearing the αβTCR play a vital role in defending the host against foreign pathogens and malignant transformation of self. Importantly, T‐cells are required to remain tolerant to the host's own cells and tissues in order to prevent self‐reactive responses that can lead to autoimmune disease. T‐cells achieve the capacity for self/nonself discrimination by undergoing a highly selective and rigorous developmental program during their maturation in the thymus. This organ is unique in its ability to support a program of T‐cell development that ensures the establishment of a functionally diverse αβTCR repertoire within the peripheral T‐cell pool. The thymus achieves this by virtue of specialized stromal microenvironments that contain heterogeneous cell types, whose organization and function underpins their ability to educate, support, and screen different thymocyte subsets through various stages of development. These stages range from the entry of early T‐cell progenitors into the thymus, through to the positive and negative selection of the αβTCR repertoire. The importance of the thymus medulla as a site for T‐cell tolerance and the exit of newly generated T‐cells into the periphery is well established. In this review, we summarize current knowledge on the developmental pathways that take place during αβT‐cell development in the thymus. In addition, we focus on the mechanisms that regulate thymic egress and contribute to the seeding of peripheral tissues with newly selected self‐tolerant αβT‐cells.

Review on thymic microenvironments regulation of thymocyte maturation and egress of mature self‐tolerant T cells.

Progressive Changes in CXCR4 Expression That Define Thymocyte Positive Selection Are Dispensable For Both Innate and Conventional αβT-cell Development

The ordered migration of immature thymocytes through thymic microenvironments generates both adaptive MHC restricted αβT-cells and innate CD1d-restricted iNKT-cells. While several chemokine receptors and ligands control multiple stages of this process, their involvement during early thymocyte development often precludes direct analysis of potential roles during later developmental stages. For example, because of early lethality of CXCR4-/- mice, and stage-specific requirements for CXCR4 in thymus colonisation and pre-TCR mediated selection, its role in thymic positive selection is unclear. Here we have examined CXCR4-CXCL12 interactions during the maturation of CD4+CD8+ thymocytes, including downstream stages of iNKT and αβT-cell development. We show CXCL12 expression is a common feature of cortical thymic epithelial cells, indicating widespread availability throughout the cortex. Moreover, CXCR4 expression by CD4+CD8+ pre-selection thymocytes is progressively downregulated following both MHC and CD1d-restricted thymic selection events. However, using CD4Cre-mediated deletion to bypass its involvement in CD4-CD8- thymocyte development, we show CXCR4 is dispensable for the maintenance and intrathymic positioning of CD4+CD8+ thymocytes, and their ability to generate mature αβT-cells and CD1d-restricted iNKT-cells. Collectively, our data define dynamic changes in CXCR4 expression as a marker for intrathymic selection events, and show its role in T-cell development is restricted to pre-CD4+CD8+ stages.

A dual-docking microfluidic cell migration assay (D2-Chip) for testing neutrophil chemotaxis and the memory effect

Chemotaxis is a classic mechanism for guiding cell migration and an important topic in both fundamental cell biology and health sciences. Neutrophils are a widely used model to study eukaryotic cell migration and neutrophil chemotaxis itself can lead to protective or harmful immune actions to the body. While much has been learnt from past research about how neutrophils effectively navigate through a chemoattractant gradient, many interesting questions remain unclear. For example, while it is tempting to model neutrophil chemotaxis using the well-established biased random walk theory, the experimental proof was challenged by the cell's highly persistent migrating nature. A special experimental design is required to test the key predictions from the random walk model. Another question that has interested the cell migration community for decades concerns the existence of chemotactic memory and its underlying mechanism. Although chemotactic memory has been suggested in various studies, a clear quantitative experimental demonstration will improve our understanding of the migratory memory effect. Motivated by these questions, we developed a microfluidic cell migration assay (so-called dual-docking chip or D2-Chip) that can test both the biased random walk model and the memory effect for neutrophil chemotaxis on a single chip enabled by multi-region gradient generation and dual-region cell alignment. Our results provide experimental support for the biased random walk model and chemotactic memory for neutrophil chemotaxis. Quantitative data analyses provide new insights into neutrophil chemotaxis and memory by making connections to entropic disorder, cell morphology and oscillating migratory response.

CCL19-CCR7-dependent reverse transendothelial migration of myeloid cells clears Chlamydia muridarum from the arterial intima

Regions of the normal arterial intima predisposed to atherosclerosis are sites of ongoing monocyte trafficking and also contain resident myeloid cells with features of dendritic cells. However, the pathophysiological roles of these cells are poorly understood. Here we found that intimal myeloid cells underwent reverse transendothelial migration (RTM) into the arterial circulation after systemic stimulation of pattern-recognition receptors (PRRs). This process was dependent on expression of the chemokine receptor CCR7 and its ligand CCL19 by intimal myeloid cells. In mice infected with the intracellular pathogen Chlamydia muridarum, blood monocytes disseminated infection to the intima. Subsequent CCL19-CCR7-dependent RTM was critical for the clearance of intimal C. muridarum. This process was inhibited by hypercholesterolemia. Thus, RTM protects the normal arterial intima, and compromised RTM during atherogenesis might contribute to the intracellular retention of pathogens in atherosclerotic lesions.

Sphingosine-1-Phosphate Induces Dose-Dependent Chemotaxis or Fugetaxis of T-ALL Blasts through S1P1 Activation

Sphingosine-1-phosphate (S1P) is a bioactive sphingolipid involved in several physiological processes including cell migration and differentiation. S1P signaling is mediated through five G protein-coupled receptors (S1P1-S1P5). S1P1 is crucial to the exit of T-lymphocytes from the thymus and peripheral lymphoid organs through a gradient of S1P. We have previously observed that T-ALL and T-LBL blasts express S1P1. Herein we analyzed the role of S1P receptors in the migratory pattern of human T-cell neoplastic blasts. S1P-triggered cell migration was directly related to S1P1 expression. T-ALL blasts expressing low levels of S1P1 mRNA (HPB-ALL) did not migrate toward S1P, whereas those expressing higher levels of S1P1 (MOLT-4, JURKAT and CEM) did migrate. The S1P ligand induced T-ALL cells chemotaxis in concentrations up to 500 nM and induced fugetaxis in higher concentrations (1000-10000 nM) through interactions with S1P1. When S1P1 was specifically blocked by the W146 compound, S1P-induced migration at lower concentrations was reduced, whereas higher concentrations induced cell migration. Furthermore, we observed that S1P/S1P1 interactions induced ERK and AKT phosphorylation, and modulation of Rac1 activity. Responding T-ALL blasts also expressed S1P3 mRNA but blockage of this receptor did not modify migratory responses. Our results indicate that S1P is involved in the migration of T-ALL/LBL blasts, which is dependent on S1P1 expression. Moreover, S1P concentrations in the given microenvironment might induce dose-dependent chemotaxis or fugetaxis of T-ALL blasts.

Microfluidic platform for the quantitative analysis of leukocyte migration signatures

Leukocyte migration into tissues is characteristic of inflammation. It is usually measured in vitro as the average displacement of populations of cells towards a chemokine gradient, not acknowledging other patterns of cell migration. Here, we designed and validated a microfluidic migration platform to simultaneously analyze four qualitative migration patterns: chemo-attraction, -repulsion, -kinesis and -inhibition, using single-cell quantitative metrics of direction, speed, persistence, and fraction of cells responding. We find that established chemokines C5a and IL-8 induce chemoattraction and repulsion in equal proportions, resulting in the dispersal of cells. These migration signatures are characterized by high persistence and speed and are independent of the chemokine dose or receptor expression. Furthermore, we find that twice as many T-lymphocytes migrate away than towards SDF-1 and their directional migration patterns are not persistent. Overall, our platform characterizes migratory signature responses and uncovers an avenue for precise characterization of leukocyte migration and therapeutic modulators.

Semaphorin 3F and neuropilin-2 control the migration of human T-cell precursors

Neuropilins and semaphorins are known as modulators of axon guidance, angiogenesis, and organogenesis in the developing nervous system, but have been recently evidenced as also playing a role in the immune system. Here we describe the expression and role of semaphorin 3F (SEMA3F) and its receptor neuropilin-2 (NRP2) in human T cell precursors. NRP2 and SEMA3F are expressed in the human thymus, in both lymphoid and non-lymphoid compartments. SEMA3F have a repulsive effect on thymocyte migration and inhibited CXCL12- and sphingosine-1-phosphate (S1P)-induced thymocyte migration by inhibiting cytoskeleton reorganization prior to stimuli. Moreover, NRP2 and SEMA3F are expressed in human T-cell acute lymphoblastic leukemia/lymphoma primary cells. In these tumor cells, SEMA3F also blocks their migration induced by CXCL12 and S1P. Our data show that SEMA3F and NRP2 are further regulators of human thymocyte migration in physiological and pathological conditions.

Maturation and migration of murine CD4 single positive thymocytes and thymic emigrants

T lymphopoiesis in the thymus was thought to be completed once they reach the single positive (SP) stage, when they are “fully mature” and wait to be exported at random or follow a “first in-first out” manner. Recently, accumulating evidence has revealed that newly generated SP thymocytes undergo further maturation in the thymic medulla before they follow a tightly regulated emigrating process to become recent thymic emigrants (RTEs). RTEs in the periphery then experience a post-thymic maturation and peripheral tolerance and eventually become licensed as mature naïve T cells. This review summarizes the recent progress in the late stage T cell development in and outside of the thymus. The regulation of this developmental process is also discussed.

Complex chemoattractive and chemorepellent Kit signals revealed by direct imaging of murine mast cells in microfluidic gradient chambers

Besides its cooperating effects on stem cell proliferation and survival, Kit ligand (KL) is a potent chemotactic protein. While transwell assays permit studies of the frequency of migrating cells, the lack of direct visualization precludes dynamic chemotaxis studies. In response, we utilize microfluidic chambers that enable direct observation of murine bone marrow-derived mast cells (BMMC) within stable KL gradients. Using this system, individual Kit+ BMMC were quantitatively analyzed for migration speed and directionality during KL-induced chemotaxis. Our results indicated a minimum activating threshold of ~3 ng ml(-1) for chemoattraction. Analysis of cells at KL concentrations below 3 ng ml(-1) revealed a paradoxical chemorepulsion, which has not been described previously. Unlike chemoattraction, which occurred continuously after an initial time lag, chemorepulsion occurred only during the first 90 minutes of observation. Both chemoattraction and chemorepulsion required the action of G-protein coupled receptors (GPCR), as treatment with pertussis toxin abrogated directed migration. These results differ from previous studies of GPCR-mediated chemotaxis, where chemorepulsion occurred at high ligand concentrations. These data indicate that Kit-mediated chemotaxis is more complex than previously understood, with the involvement of GPCRs in addition to the Kit receptor tyrosine kinase and the presence of both chemoattractive and chemorepellent phases.

CXCL12/CXCR4 signal axis plays an important role in mediating bone morphogenetic protein 9-induced osteogenic differentiation of mesenchymal stem cells

Mesenchymal progenitor stem cells (MPCs) are a group of bone marrow stromal progenitor cells processing osteogenic, chondrogenic, adipogenic and myogenic lineages differentiations. Previous studies have demonstrated that bone morphogeneic protein 9(BMP9) is one of the most osteogenic BMPs both in vitro and in vivo, however, the underlying molecular mechanism of osteogenesis induced by BMP9 is needed to be deep explored. Here, we used the recombinant adenoviruses assay to introduce BMP9 into C3H10T1/2 mesenchymal stem cells to elucidate the role of CXCL12/CXCR4 signal axis during BMP9-incuced osteogenic differentiation. The results showed that CXCL12 and CXCR4 expressions were down-regulated at the stage of BMP9-induced osteogenic differentiation, in a dose- and time-dependent. Pretreatment of C3H10T1/2 cells with CXCL12/CXCR4 could significantly affect the early and mid osteogenic markers alkaline phosphatase (ALP), osteocalcin (OCN), the transcription factors of Runx2, Osx, Plzf and Dlx5 expression, through activating the Smad, MAPK signaling pathway. Addition of exogenous CXCL12 did not affect the changes of the late osteogenic marker calcium deposition. Thus, our findings suggest a co-requirement of the CXCL12/CXCR4 signal axis in BMP9-induced the early- and mid-process of osteogenic differentiation of MSCs.

Opposing chemokine gradients control human thymocyte migration in situ

The ordered migration of thymocytes from the cortex to the medulla is critical for the appropriate selection of the mature T cell repertoire. Most studies of thymocyte migration rely on mouse models, but we know relatively little about how human thymocytes find their appropriate anatomical niches within the thymus. Moreover, the signals that retain CD4+CD8+ double-positive (DP) thymocytes in the cortex and prevent them from entering the medulla prior to positive selection have not been identified in mice or humans. Here, we examined the intrathymic migration of human thymocytes in both mouse and human thymic stroma and found that human thymocyte subsets localized appropriately to the cortex on mouse thymic stroma and that MHC-dependent interactions between human thymocytes and mouse stroma could maintain the activation and motility of DP cells. We also showed that CXCR4 was required to retain human DP thymocytes in the cortex, whereas CCR7 promoted migration of mature human thymocytes to the medulla. Thus, 2 opposing chemokine gradients control the migration of thymocytes from the cortex to the medulla. These findings point to significant interspecies conservation in thymocyte-stroma interactions and provide the first evidence that chemokines not only attract mature thymocytes to the medulla, but also play an active role in retaining DP thymocytes in the cortex prior to positive selection.

A dichotomy in cortical actin and chemotactic actin activity between human memory and naive T cells contributes to their differential susceptibility to HIV-1 infection

Human memory and naive CD4 T cells can mainly be identified by the reciprocal expression of the CD45RO or CD45RA isoforms. In HIV-1 infection, blood CD45RO memory CD4 T cells are preferentially infected and serve as a major viral reservoir. The molecular mechanism dictating this differential susceptibility to HIV-1 remains largely obscure. Here, we report that the different susceptibility of memory and naive T cells to HIV is not determined by restriction factors such as Apobec3G or BST2. However, we observed a phenotypic distinction between human CD45RO and CD45RA resting CD4 T cells in their cortical actin density and actin dynamics. CD45RO CD4 T cells possess a higher cortical actin density and can be distinguished as CD45RO(+)Actin(high). In contrast, CD45RA T cells are phenotypically CD45RA(+)Actin(low). In addition, the cortical actin in CD45RO memory CD4 T cells is more dynamic and can respond to low dosages of chemotactic induction by SDF-1, whereas that of naive cells cannot, despite a similar level of the chemokine receptor CXCR4 present on both cells. We further demonstrate that this difference in the cortical actin contributes to their differential susceptibility to HIV-1; resting memory but not naive T cells are highly responsive to HIV-mediated actin dynamics that promote higher levels of viral entry and early DNA synthesis in resting memory CD4 T cells. Furthermore, transient induction of actin dynamics in resting naive T cells rescues HIV latent infection following CD3/CD28 stimulation. These results suggest a key role of chemotactic actin activity in facilitating HIV-1 latent infection of these T cell subsets.

Combinatorial guidance by CCR7 ligands for T lymphocytes migration in co-existing chemokine fields

Chemokines mediate the trafficking and positioning of lymphocytes in lymphoid tissues that is crucial for immune surveillance and immune responses. In particular, a CCR7 ligand, CCL21, plays important roles in recruiting T cells to secondary lymphoid tissues (SLT). Furthermore, CCL21 together with another CCR7 ligand, CCL19, direct the navigation and compartmentation of T cells within SLT. However, the distinct roles of these two chemokines for regulating cell trafficking and positioning are not clear. In this study, we explore the effect of co-existing CCL19 and CCL21 concentration fields on guiding T cell migration. Using microfluidic devices that can configure single and superimposed chemokine fields we show that under physiological gradient conditions, human peripheral blood T cells chemotax to CCL21 but not CCL19. Furthermore, T cells migrate away from the CCL19 gradient in a uniform background of CCL21. This repulsive migratory response is predicted by mathematical modeling based on the competition of CCL19 and CCL21 for CCR7 signaling and the differential ability of the two chemokines for desensitizing CCR7. These results suggest a new combinatorial guiding mechanism by CCL19 and CCL21 for the migration and trafficking of CCR7 expressing leukocytes.

Wound Healing Versus Regeneration: Role of the Tissue Environment in Regenerative Medicine

One of the major challenges in the field of regenerative medicine is how to optimize tissue regeneration in the body by therapeutically manipulating its natural ability to form scar at the time of injury or disease. It is often the balance between tissue regeneration, a process that is activated at the onset of disease, and scar formation, which develops as a result of the disease process that determines the ability of the tissue or organ to be functional. Using biomaterials as scaffolds often can provide a "bridge" for normal tissue edges to regenerate over small distances, usually up to 1 cm. Larger tissue defect gaps typically require both scaffolds and cells for normal tissue regeneration to occur without scar formation. Various strategies can help to modulate the scar response and can potentially enhance tissue regeneration. Understanding the mechanistic basis of such multivariate interactions as the scar microenvironment, the immune system, extracellular matrix, and inflammatory cytokines may enable the design of tissue engineering and wound healing strategies that directly modulate the healing response in a manner favorable to regeneration.

Dynamic alterations in chemokine gradients induce transendothelial shuttling of human T cells under physiologic shear conditions

The active movement of cells from subendothelial compartments into the bloodstream (intravasation) has been recognized for several decades by histologic and physiologic studies, yet the molecular effectors of this process are relatively uncharacterized. For extravasation, studies based predominantly on static transwell assays support a general model, whereby transendothelial migration (TEM) occurs via chemoattraction toward increasing chemokine concentrations. However, this model of chemotaxis cannot readily reconcile how chemokines influence intravasation, as shear forces of blood flow would likely abrogate luminal chemokine gradient(s). Thus, to analyze how T cells integrate perivascular chemokine signals under physiologic flow, we developed a novel transwell-based flow chamber allowing for real-time modulation of chemokine levels above (luminal/apical compartment) and below (abluminal/subendothelial compartment) HUVEC monolayers. We routinely observed human T cell TEM across HUVEC monolayers with the combination of luminal CXCL12 and abluminal CCL5. With increasing concentrations of CXCL12 in the luminal compartment, transmigrated T cells did not undergo retrograde transendothelial migration (retro-TEM). However, when exposedto abluminal CXCL12, transmigrated T cells underwent striking retro-TEM and re-entered the flow stream [corrected]. This CXCL12 fugetactic (chemorepellant) effect was concentration-dependent, augmented by apical flow, blocked by antibodies to integrins, and reduced by AMD3100 in a dose-dependent manner. Moreover, CXCL12-induced retro-TEM was inhibited by PI3K antagonism and cAMP agonism. These findings broaden our understanding of chemokine biology and support a novel paradigm by which temporospatial modulations in subendothelial chemokine display drive cell migration from interstitial compartments into the bloodstream.

Thymic emigration: when and how T cells leave home

The thymus supports the differentiation of multiple distinct T cell subsets that play unique roles in the immune system. CD4 and CD8 alpha/beta T cells, gamma/delta T cells, NKT cells, regulatory T cells, and intraepithelial lymphocytes all develop in the thymus and must leave it to provide their functions elsewhere in the body. This article will review recent research indicating differences in the time and migration patterns of T cell subsets found in the thymus. Additionally, we review current understanding of the molecules involved in thymocyte emigration, including the sphingolipid receptor S1P(1) and its regulation by the Krüppel-like transcription factor KLF2.

Exogenous insulin-like growth factor 1 enhances thymopoiesis predominantly through thymic epithelial cell expansion

Insulin-like growth factor 1 (IGF-1) enhances thymopoiesis but given the broad distribution of IGF-1 receptors (IGF-1Rs), its mechanism of action has remained unclear. To identify points of thymic regulation by IGF-1, we examined its effects on T-cell precursors, thymocytes, and thymic epithelial cells (TECs) in normal and genetically altered mice. In thymus-intact but not thymectomized mice, IGF-1 administration increased peripheral naive and recent thymic emigrant (RTE) populations, demonstrating its effect on T-cell production, not peripheral expansion. IGF-1 administration increased bone marrow LSK (lineage(-), Sca-1(+), c-kit(+)) precursor proliferation and peripheral LSK populations, increased thymocyte populations in a sequential wave of expansion, and proportionately expanded TEC subpopulations and enhanced their chemokine expression. To separate IGF-1's effects on thymocytes and TECs, we generated mice lacking IGF-1R on thymocytes and T cells. Thymocyte and RTE numbers were decreased in these mice, but IGF-1 treatment produced comparable thymocyte numbers to similarly treated wild-type mice. We additionally separated thymic- from LSK-specific effects by demonstrating that IGF-1 increased thymocyte numbers despite impaired early thymic progenitor (ETP) importation in PSGL-1KO mice. These results indicate the critical point thymic function regulation by IGF-1 involves TEC expansion regulating thymocyte precursor entry and facilitating thymocyte development.

S100A8 triggers oxidation-sensitive repulsion of neutrophils

The inflammatory response to tissue injury is a multi-faceted process. During this process, neutrophils migrate in the extravascular spaces, directed to the site of injury by chemical gradients generated by chemotactic molecules. S100A8, a protein associated with a wide variety of inflammatory conditions, is heavily over-expressed in association with inflammation. We hypothesized that human S100A8 possesses neutrophil-repelling properties that result in an anti-inflammatory effect in vivo. The chemotactic activity of S100A8 on neutrophils was tested in Transwell chemotaxis assays. Analysis of the data indicates that S100A8 causes a repulsion of peripheral neutrophils, an activity that S100A8 loses upon its oxidation. Using a mutant of S100A8 resistant to oxidation and consistent with the in vitro findings, we demonstrated that S100A8 causes a strong anti-inflammatory effect in the rat air-pouch model of inflammation in vivo. These data highlight a naturally occurring novel anti-inflammatory pathway and provide potential molecular targets for the development of novel anti-inflammatory therapeutics. Abbrevations: ethylene diamine tetraacetic acid (EDTA); limulus amoebocyte lysate assay (LAL); pertussis toxin (PTX); forward scatter (FSC); Interleukin-8 (IL-8); formyl-Met-Leu-Phe (fMLP); monocyte chemotactic protein 1 (MCP1).

Shear flow-dependent integration of apical and subendothelial chemokines in T-cell transmigration: implications for locomotion and the multistep paradigm

Lymphocyte extravasation requires that emigrating cells process chemoattractant signals, typically mediated by chemokines, encountered on endothelial surface (apical) and subendothelial (basal) compartments. These signals are delivered under conditions of hemodynamic shear, a fundamental feature of all physiologic leukocyte-endothelial interactions. To analyze lymphocyte responsiveness to spatially distributed chemokines and their effects on transendothelial migration (TEM) under hydrodynamic shear, we constructed a transwell-based flow assay. We observed that the inflammatory chemokine CCL5 (RANTES) induces negligible human T-cell migration across inflamed human umbilical vascular endothelial cells (HUVECs) when displayed alone in the subendothelial compartment under static or hemodynamic shear conditions or when combined with apical CXCL12 (SDF-1alpha) under static conditions. However, under shear stress, T cells encountering apically presented CXCL12 were primed to undergo robust LFA-1-dependent TEM toward subendothelial CCL5. Notably, locomotive T cells arriving at endothelial junctions were retained and extended pseudopodia into and through the junctions, thereby increasing sensitivity to subendothelial CCL5. These findings provide the first evidence that lymphocytes integrate, conditional to shear forces, permissive apical chemokine deposits, and integrin engagement signals, resulting in morphologic changes and amplified chemotaxis to an otherwise weak subendothelial chemokine signal.

Fugetaxis: active movement of leukocytes away from a chemokinetic agent

Chemotaxis or active movement of leukocytes toward a stimulus has been shown to occur in response to chemokinetic agents including members of the recently identified superfamily of proteins called chemokines. Leukocyte chemotaxis is thought to play a central role in a wide range of physiological and pathological processes including the homing of immune cells to lymph nodes and the accumulation of these cells at sites of tissue injury and pathogen or antigen challenge. We have recently identified a novel biological mechanism, which we term fugetaxis (fugere, to flee from; taxis, movement) or chemorepulsion, which describes the active movement of leukocytes away from chemokinetic agents including the chemokine, stromal cell derived factor-1, and the HIV-1 envelope protein, gp120. In this article, we review the evidence that supports the observation that leukocyte fugetaxis occurs in vitro and in vivo and suggestions that this novel mechanism can be exploited to modulate the immune response. We propose that leukocyte fugetaxis plays a critical role in both physiological and pathological processes in which leukocytes are either excluded or actively repelled from specific sites in vivo including thymic emigration, the establishment of immune privileged sites and immune evasion by viruses and cancer. We believe that current data support the thesis that a greater understanding of leukocyte fugetaxis will lead to the development of novel therapeutic approaches for a wide range of human diseases.

Plasma chemokine levels correlate with the outcome of antiviral therapy in patients with hepatitis C

Chronic infection with the hepatitis C virus (HCV) is associated with failures of T-cell-mediated immune clearance and with abnormal B-cell growth and activation. We examined the levels of chemokines that bind to CXC chemokine receptor 3 (CXCR3) to determine whether such chemokines might play a role in the failure of the immune system to clear HCV infection. Elevations in CXC ligand 9 (CXCL9), CXCL10, and CXCL11 were observed in all patients with HCV. CXCR3 expression was increased significantly on peripheral blood B lymphocytes, but not T lymphocytes, from individuals with HCV infection. Chemokine levels were measured in samples collected before, during, and after antiviral therapy from a group of 29 patients infected with HCV genotypes 1a (24 patients) and 1b (5 patients). Levels of CXCL10 and CXCL9 decreased following successful antiviral therapy; CXCL11 did not decline significantly during or in the first 6 months after therapy. The baseline level of CXCL10 (measured before the start of antiviral treatment) was greatest in patients with HCV who subsequently became nonresponders to therapy. These results suggest that plasma concentrations of immunoreactive CXCL10 may be a predictor of responsiveness or nonresponsiveness to antiviral therapy with pegylated interferon (IFN) with or without ribavirin. This observation has implications for understanding the pathogenesis of HCV infection.

The ins and outs of CCR7 in the thymus

Although it is widely supposed that chemokines play a role in the thymus, most existing evidence is circumstantial. In this issue, two groups provide direct evidence that the chemokine receptor CCR7 is required for normal thymocyte migration (Ueno, T., F. Saito, D. Gray, S. Kuse, K. Hieshima, H. Nakano, T. Kakiuchi, M. Lipp, R. Boyd, and Y. Takahama. 2004. J. Exp. Med. 200:493-505; Misslitz, A., O. Pabst, G. Hintzen, L. Ohl, E. Kremmer, H. T. Petrie, and R. Forster. 2004. J. Exp. Med. 200:481-491). The two papers focus on distinct and opposite migration events, an early outward migration and a later inward migration. Together these papers provide a fascinating picture of the complex role of CCR7 in orchestrating thymocyte migration.

Migration of antigen-specific T cells away from CXCR4-binding human immunodeficiency virus type 1 gp120

Cell-mediated immunity depends in part on appropriate migration and localization of cytotoxic T lymphocytes (CTL), a process regulated by chemokines and adhesion molecules. Many viruses, including human immunodeficiency virus type 1 (HIV-1), encode chemotactically active proteins, suggesting that dysregulation of immune cell trafficking may be a strategy for immune evasion. HIV-1 gp120, a retroviral envelope protein, has been shown to act as a T-cell chemoattractant via binding to the chemokine receptor and HIV-1 coreceptor CXCR4. We have previously shown that T cells move away from the chemokine stromal cell-derived factor 1 (SDF-1) in a concentration-dependent and CXCR4 receptor-mediated manner. Here, we demonstrate that CXCR4-binding HIV-1 X4 gp120 causes the movement of T cells, including HIV-specific CTL, away from high concentrations of the viral protein. This migratory response is CD4 independent and inhibited by anti-CXCR4 antibodies and pertussis toxin. Additionally, the expression of X4 gp120 by target cells reduces CTL efficacy in an in vitro system designed to account for the effect of cell migration on the ability of CTL to kill their target cells. Recombinant X4 gp120 also significantly reduced antigen-specific T-cell infiltration at a site of antigen challenge in vivo. The repellant activity of HIV-1 gp120 on immune cells in vitro and in vivo was shown to be dependent on the V2 and V3 loops of HIV-1 gp120. These data suggest that the active movement of T cells away from CXCR4-binding HIV-1 gp120, which we previously termed fugetaxis, may provide a novel mechanism by which HIV-1 evades challenge by immune effector cells in vivo.

Rapid induction of medullary thymocyte phenotypic maturation and egress inhibition by nanomolar sphingosine 1-phosphate receptor agonist

Only a small number of T cells generated in the thymus each day are selected to replenish the peripheral T cell pool. Much is known about thymic selection; however, little is known of the mechanisms regulating medullary maturation and the release of mature T cells into the blood. Here we demonstrate a rapid acceleration of medullary thymocyte phenotypic maturation through loss of CD69 induced by sphingosine 1-phosphate (S1P) receptor agonist. Low nanomolar agonist concentrations selectively induce changes in CD69(int) CD62L(high) single positive T cells, resulting in down-modulation of CD69 within 2 h. While CD69 loss is accelerated, egress of mature T cells into blood is inhibited >95% within 2 h. Both processes exhibit parallel sensitivities and dose-responses. Together, these data reveal a potent means for rapidly regulating thymic export where S1P receptor agonism alters both phenotypic maturation and egress of thymocytes into blood during late thymic maturation. The S1P system is now shown to acutely regulate both thymic and lymph node egress. Inhibition of lymphocyte egress from thymus and lymph node can contribute synergistically to clinically useful immunosupression by disrupting recirculation of peripheral T cells.