Phase I/II Trial of StemRegenin-1 Expanded Umbilical Cord Blood Hematopoietic Stem Cells Supports Testing as a Stand-Alone Graft

Clinical application of umbilical cord blood (UCB) as a source of hematopoietic stem cells for transplantation is limited by low CD34+ cell dose, increased risk of graft failure, and slow hematopoietic recovery. While the cell dose limitation is partially mitigated by using two UCB units, larger-dosed single units would be preferable. We have evaluated the feasibility and safety of StemRegenin-1 (SR-1), an aryl hydrocarbon receptor antagonist that expands CD34+ cells, by placing one of the two units in expansion culture. SR-1 produced a 330-fold increase in CD34+ cells and led to engraftment in 17/17 patients at a median of 15 days for neutrophils and 49 days for platelets, significantly faster than in patients treated with unmanipulated UCB. Taken together, the marked expansion, absence of graft failure, and enhanced hematopoietic recovery support testing of SR-1 expansion as a stand-alone graft and suggest it may ameliorate a limitation of UCB transplant.

Evidence that a neutrophil-keratinocyte crosstalk is an early target of IL-17A inhibition in psoriasis

The response of psoriasis to antibodies targeting the interleukin (IL)-23/IL-17A pathway suggests a prominent role of T-helper type-17 (Th17) cells in this disease. We examined the clinical and immunological response patterns of 100 subjects with moderate-to-severe psoriasis receiving 3 different intravenous dosing regimens of the anti-IL-17A antibody secukinumab (1 × 3 mg/kg or 1 × 10 mg/kg on Day 1, or 3 × 10 mg/kg on Days 1, 15 and 29) or placebo in a phase 2 trial. Baseline biopsies revealed typical features of active psoriasis, including epidermal accumulation of neutrophils and formation of microabscesses in >60% of cases. Neutrophils were the numerically largest fraction of infiltrating cells containing IL-17 and may store the cytokine preformed, as IL-17A mRNA was not detectable in neutrophils isolated from active plaques. Significant clinical responses to secukinumab were observed 2 weeks after a single infusion, associated with extensive clearance of cutaneous neutrophils parallel to the normalization of keratinocyte abnormalities and reduction of IL-17-inducible neutrophil chemoattractants (e.g. CXCL1, CXCL8); effects on numbers of T cells and CD11c-positive dendritic cells were more delayed. Histological and immunological improvements were generally dose dependent and not observed in the placebo group. In the lowest-dose group, a recurrence of neutrophils was seen in some subjects at Week 12; these subjects relapsed faster than those without microabscesses. Our findings are indicative of a neutrophil-keratinocyte axis in psoriasis that may involve neutrophil-derived IL-17 and is an early target of IL-17A-directed therapies such as secukinumab.

Conversion of the thymus into a bipotent lymphoid organ by replacement of FOXN1 with its paralog, FOXN4

The thymus is a lymphoid organ unique to vertebrates, and it provides a unique microenvironment that facilitates the differentiation of immature hematopoietic precursors into mature T cells. We subjected the evolutionary trajectory of the thymic microenvironment to experimental analysis. A hypothetical primordial form of the thymus was established in mice by replacing FOXN1, the vertebrate-specific master regulator of thymic epithelial cell function, with its metazoan ancestor, FOXN4, thereby resetting the regulatory and coding changes that have occurred since the divergence of these two paralogs. FOXN4 exhibited substantial thymopoietic activity. Unexpectedly, histological changes and a functional imbalance between the lymphopoietic cytokine IL7 and the T cell specification factor DLL4 within the reconstructed thymus resulted in coincident but spatially segregated T and B cell development. Our results identify an evolutionary mechanism underlying the conversion of a general lymphopoietic organ to a site of exclusive T cell generation.

Ltbetar signaling does not regulate Aire-dependent transcripts in medullary thymic epithelial cells

Thymic medullary epithelial cells (mTECs) play a major role in central tolerance induction by expressing tissue-specific Ags (TSAs). The expression of a subset of TSAs in mTECs is under the control of Aire (autoimmune regulator). Humans defective for AIRE develop a syndrome characterized by autoimmune disease in several endocrine glands. Aire has been proposed to be regulated by lymphotoxin beta receptor (Ltbetar) signaling and there is evidence that, additionally, Aire-independent transcripts may be regulated by this pathway. Given the potential clinical importance of Aire regulation in mTECs for the control of autoimmunity, we investigated the relation between Ltbetar signaling and TSA expression by whole genome transcriptome analysis. In this study, we show that the absence of Ltbetar has no effect on the expression of Aire and Aire-dependent TSAs. Also, the lack of Ltbetar signaling does not disturb regulatory T cells or the distribution of dendritic cells in the thymus. However, mTECs in Ltbetar-deficient mice show an aberrant distribution within the thymic medulla with disruption of their three-dimensional architecture. This is predicted to impair the interaction between mTECs and thymocytes as shown by the reduced surface uptake of MHCII by mature thymocytes in Ltbetar-deficient mice. We propose that the physiological medullary architecture ensures negative-selection by supporting lympho-epithelial interaction through a large epithelial cell surface distributed evenly across the medulla.

The stream of precursors that colonizes the thymus proceeds selectively through the early T lineage precursor stage of T cell development

T cell development in the thymus depends on continuous colonization by hematopoietic precursors. Several distinct T cell precursors have been identified, but whether one or several independent precursor cell types maintain thymopoiesis is unclear. We have used thymus transplantation and an inducible lineage-tracing system to identify the intrathymic precursor cells among previously described thymus-homing progenitors that give rise to the T cell lineage in the thymus. Extrathymic precursors were not investigated in these studies. Both approaches show that the stream of T cell lineage precursor cells, when entering the thymus, selectively passes through the early T lineage precursor (ETP) stage. Immigrating precursor cells do not exhibit characteristics of double-negative (DN) 1c, DN1d, or DN1e stages, or of populations containing the common lymphoid precursor 2 (CLP-2) or the thymic equivalent of circulating T cell progenitors (CTPs). It remains possible that an unknown hematopoietic precursor cell or previously described extrathymic precursors with a CLP, CLP-2, or CTP phenotype feed into T cell development by circumventing known intrathymic T cell lineage progenitor cells. However, it is clear that of the known intrathymic precursors, only the ETP population contributes significant numbers of T lineage precursors to T cell development.

The Foxn1-dependent transcripts PCOLCE2 and mPPP1R16B are not required for normal thymopoiesis

The adaptive immune system relies on the thymic microenvironment for the production of a diverse, self-tolerant T cell receptor repertoire. The central cellular organizer of the thymic microenvironment is the thymic epithelial cell (TEC). The development of TEC from endodermal precursor cells is under the control of the forkhead/winged helix transcription factor Foxn1 but the transcriptional program that leads to this unique epithelial differentiation has not been investigated functionally. Here, we show that expression of procollagen C-proteinase enhancer 2 (PCOLCE2) is absent in Foxn1-deficient TEC. In order to study the functional role of this gene in TEC differentiation, we have genetically inactivated PCOLCE2 and the gene encoding phosphatase 1 regulatory inhibitory subunit 16B (mPPP1R16B), another transcript lacking in Foxn1-deficient TEC. Mice deficient for either one or both of these transcripts presented a normal thymic microenvironment and undisturbed thymopoiesis. While there is no evidence for a functional role of PCOLCE2 and mPPP1R16B in thymus development, our results suggest that the lack of thymopoiesis in Foxn1-deficient mice is caused by multiple functional defects.

A silent chemokine receptor regulates steady-state leukocyte homing in vivo

The location of leukocytes in different microenvironments is intimately connected to their function and, in the case of leukocyte precursors, to the executed differentiation and maturation program. Leukocyte migration within lymphoid organs has been shown to be mediated by constitutively expressed chemokines, but how the bioavailability of these homeostatic chemokines is regulated remains unknown. Here, we report in vivo evidence for the role of a nonsignaling chemokine receptor in the migration of leukocytes under physiological, i.e., noninflammatory, conditions. We have studied the in vivo role of the silent chemokine receptor CCX-CKR1 by both loss- and gain-of-function approaches. CCX-CKR1 binds the constitutively expressed chemokines CC chemokine ligand (CCL)19, CCL21, and CCL25. We find that CCX-CKR1 is involved in the steady-state homing of CD11c(+)MHCII(high) dendritic cells to skin-draining lymph nodes, and it affects the homing of embryonic thymic precursors to the thymic anlage. These observations indicate that the silent chemokine receptor CCX-CKR1, which is exclusively expressed by stroma cells, but not hematopoietic cells themselves, regulates homeostatic leukocyte migration by controlling the availability of chemokines in the extracellular space. This finding adds another level of complexity to our understanding of leukocyte homeostatic migration.

The evolutionary history of lymphoid organs

Lymphoid organs are important regulators of lymphocyte development and immune responses. During vertebrate evolution, primary lymphoid organs appeared earlier than secondary lymphoid organs. Among the sites of primary lymphopoiesis during evolution and ontogeny, those for B cell differentiation have differed considerably, although they often have had myelolymphatic characteristics. In contrast, only a single site for T cell differentiation has occurred, exclusively the thymus. Based on those observations and the known features of variable-diversity-joining gene recombination, we propose a model for the successive specification of different lymphocyte lineages during vertebrate evolution. According to our model, T cells were the first lymphocytes to acquire variable-diversity-joining-type receptors, and the thymus was the first lymphoid organ to evolve in vertebrates to deal with potentially autoreactive, somatically diversified T cell receptors.

Bone marrow-derived hemopoietic precursors commit to the T cell lineage only after arrival in the thymic microenvironment

T lymphocytes develop in the thymus from hemopoietic precursors that commit to the T cell lineage under the influence of Notch signals. In this study, we show by single cell analyses that the most immature hemopoietic precursors in the adult mouse thymus are uncommitted and specify to the T cell lineage only after their arrival in the thymus. These precursors express high levels of surface Notch receptors and rapidly lose B cell potential upon the provision of Notch signals. Using a novel culture system with complexed, soluble Notch ligands that allows the titration of T cell lineage commitment, we find that these precursors are highly sensitive to both Delta and Jagged ligands. In contrast, their phenotypical and functional counterparts in the bone marrow are resistant to Notch signals that efficiently induce T cell lineage commitment in thymic precursors. Mechanistically, this is not due to differences in receptor expression, because early T lineage precursors, bone marrow lineage marker-negative, Sca-1-positive, c-Kit-positive and common lymphoid progenitor cells, express comparable amounts of surface Notch receptors. Our data demonstrate that the sensitivity to Notch-mediated T lineage commitment is stage-dependent and argue against the bone marrow as the site of T cell lineage commitment.

Thymus-homing precursors and the thymic microenvironment

T cells develop in the thymus from precursors that are generated in the bone marrow and continuously seed the thymus through the blood. During evolution, 'outsourcing' the development of one blood lineage, namely the T-cell lineage, to an anatomically distinct hematopoietic organ required the generation of migratory precursors in the bone marrow, their homing to specialized, precursor-retaining thymic niches and their subsequent differentiation. Niche building and precursor homing are therefore intricately linked and should be viewed in context. In this review, we discuss recent findings on the developmental and genetic events that prepare the thymic epithelial microenvironment for its complex tasks, and highlight recent progress in the definition of the thymus-settling cells and the homing process that leads them into the thymus.

Formation of a functional thymus initiated by a postnatal epithelial progenitor cell

The thymus is essential for the generation of self-tolerant effector and regulatory T cells. Intrathymic T-cell development requires an intact stromal microenvironment, of which thymic epithelial cells (TECs) constitute a major part. For instance, cell-autonomous genetic defects of forkhead box N1 (Foxn1) and autoimmune regulator (Aire) in thymic epithelial cells cause primary immunodeficiency and autoimmunity, respectively. During development, the thymic epithelial rudiment gives rise to two major compartments, the cortex and medulla. Cortical TECs positively select T cells, whereas medullary TECs are involved in negative selection of potentially autoreactive T cells. It has long been unclear whether these two morphologically and functionally distinct types of epithelial cells arise from a common bi-potent progenitor cell and whether such progenitors are still present in the postnatal period. Here, using in vivo cell lineage analysis in mice, we demonstrate the presence of a common progenitor of cortical and medullary TECs after birth. To probe the function of postnatal progenitors, a conditional mutant allele of Foxn1 was reverted to wild-type function in single epithelial cells in vivo. This led to the formation of small thymic lobules containing both cortical and medullary areas that supported normal thymopoiesis. Thus, single epithelial progenitor cells can give rise to a complete and functional thymic microenvironment, suggesting that cell-based therapies could be developed for thymus disorders.

Evidence for a functional second thymus in mice

The thymus organ supports the development of T cells and is located in the thorax. Here, we report the existence of a second thymus in the mouse neck, which develops after birth and grows to the size of a small lymph node. The cervical thymus had a typical medulla-cortex structure, was found to support T cell development, and could correct T cell deficiency in athymic nude mice upon transplantation. The identification of a regular second thymus in the mouse may provide evolutionary links to thymus organogenesis in other vertebrates and suggests a need to reconsider the effect of thoracic thymectomy on de novo T cell production.

BMP signaling is required for normal thymus development

The microenvironment of the thymus fosters the generation of a diverse and self-tolerant T cell repertoire from a pool of essentially random specificities. Epithelial as well as mesenchymal cells contribute to the thymic stroma, but little is known about the factors that allow for communication between the two cells types that shape the thymic microenvironment. In this study, we investigated the role of bone morphogenetic protein (BMP) signaling in thymus development. Transgenic expression of the BMP antagonist Noggin in thymic epithelial cells under the control of a Foxn1 promoter in the mouse leads to dysplastic thymic lobes of drastically reduced size that are ectopically located in the neck at the level of the hyoid bone. Interestingly, the small number of thymocytes in these thymic lobes develops with normal kinetics and shows a wild-type phenotype. Organ initiation of the embryonic thymic anlage in these Noggin transgenic mice occurs as in wild-type mice, but the tight temporal and spatial regulation of BMP4 expression is abrogated in subsequent differentiation stages. We show that transgenic Noggin blocks BMP signaling in epithelial as well as mesenchymal cells of the thymic anlage. Our data demonstrate that BMP signaling is crucial for thymus development and that it is the thymic stroma rather than developing thymocytes that depends on BMP signals.

A multipotent precursor in the thymus maps to the branching point of the T versus B lineage decision

Hematopoietic precursors continuously colonize the thymus where they give rise mainly to T cells, but also to B and dendritic cells. The lineage relationship between these three cell types is unclear, and it remains to be determined if precursors in the thymus are multipotent, oligopotent, or lineage restricted. Resolution of this question necessitates the determination of the clonal differentiation potential of the most immature precursors in the thymus. Using a CC chemokine receptor 9–enhanced green fluorescent protein knock-in allele like a surface marker of unknown function, we identify a multipotent precursor present in bone marrow, blood, and thymus. Single cells of this precursor give rise to T, B, and dendritic cells. A more differentiated stage of this multipotent precursor in the thymus has lost the capacity to generate B but not T, dendritic, and myeloid cells. Thus, the newly identified precursor maps to the branching point of the T versus B lineage decision in the hematopoietic lineage hierarchy.

Homing of immature thymocytes to the subcapsular microenvironment within the thymus is not an absolute requirement for T?cell development

T cell development is thought to occur in distinct microenvironments within the thymus. Namely, the subcapsular zone, the cortex and the medulla have been described to support expansion of the immature thymocyte pool, positive selection of useful specificities and elimination of potentially self-reactive specificities, respectively. Consistent with this model, thymocytes show a highly ordered migration pattern and move into these niches in the expected sequence. Here we show that the chemokine receptor CCR9 plays a nonredundant role in the homing of immature thymocytes to the subcapsular zone. In CCR9-deficient mice, T cells in early stages of development do not accumulate in their physiological microenvironment underneath the thymic capsule and are instead homogeneously distributed across the thymic cortex. Remarkably, this abnormality does not result in a detectable defect in T cell development in CCR9-deficient mice, suggesting that the transit of immature thymocytes through the subcapsular microenvironment is not an absolute requirement for proper T cell development.

Genetic dissection of thymus development in mouse and zebrafish

Lymphoid organs represent a specialized microenvironment for interaction of stromal and lymphoid cells. In primary lymphoid organs, these interactions are required to establish a self-tolerant repertoire of lymphocytes. While detailed information is available about the genes that control lymphocyte differentiation, little is known about the genes that direct the establishment and differentiation of principal components of such microenvironments. Here, we discuss genetic studies addressing the role of thymic epithelial cells (TECs) during thymopoiesis. We have identified an evolutionarily conserved key regulator of TEC differentiation, Foxn1, that is required for the immigration of prothymocytes into the thymic primordium. Because Foxn1 specifies the prospective endodermal domain that gives rise to thymic epithelial cells, it can be used to identify the evolutionary origins of this specialized cell type. In the course of these studies, we have found that early steps of thymus development in zebrafish are very similar to those in mice. Subsequently, we have used chemical mutagenesis to derive zebrafish lines with aberrant thymus development. Strengths and weaknesses of mouse and zebrafish models are largely complementary such that genetic analysis of mouse and zebrafish mutants may lead to a better understanding of thymus development.

Thymic medullary epithelial cell differentiation, thymocyte emigration, and the control of autoimmunity require lympho-epithelial cross talk via LTbetaR

Thymocytes depend on the interaction with thymic epithelial cells for the generation of a diverse, nonautoreactive T cell repertoire. In turn, thymic epithelial cells acquire their three-dimensional cellular organization via instructive signals from developing thymocytes. The nature of these signals has been elusive so far. We show that thymocytes and medullary epithelial cells (MECs) communicate via the lymphotoxin beta receptor (LTbetaR) signaling axis. Normal differentiation of thymic MECs requires LTbetaR ligand on thymocytes and LTbetaR together with nuclear factor-kappaB-inducing kinase (Nik) in thymic epithelial cells. Impaired lympho-epithelial cross talk in the absence of the LTbetaR causes aberrant differentiation and reduced numbers of thymic MECs, leads to the retention of mature T lymphocytes, and is associated with autoimmune phenomena, suggesting an unexpected role for LTbetaR signaling in central tolerance induction.

Laser capture microdissection-based expression profiling identifies PD1-ligand as a target of the nude locus gene product

T cell development requires the interaction of developing thymocytes with thymic epithelial cells. Thymic epithelial cells acquire their unique phenotype under the control of the winged-helix transcription factor Whn, which is lacking in the nude mouse. Whn-dependent genes may therefore be important regulators of lympho-epithelial interactions. To identify Whn target genes we isolated RNA populations of wild-type and nude thymic anlagen from embryonic day 12.5 embryos by laser capture microdissection and compared them by gene expression profiling on microarrays representing 22,000 transcripts. All cDNA with expression differences confirmed by quantitative RT-PCR using RNA from individual anlagen and by in situ hybridization were found to be present in wild-type and absent in nude samples. Three of eight confirmed transcripts were of hematopoietic origin; these transcripts emanate from hematopoietic precursors which have just entered the thymic anlage. Five transcripts were of epithelial origin; one of these corresponds to the recently identified PD-1 ligand (PD-L1), the receptor of which is known to modulate positive selection and to play a role in the control of autoimmunity, and the remaining transcripts code for novel genes. The presented results support our prediction that this systematic approach by gene expression profiling yields regulators of thymopoiesis.

Chemokines define distinct microenvironments in the developing thymus

During thymus development, prothymocytes home to the thymus where they migrate as maturing thymocytes from the cortex to the medulla. Chemotaxis assays show that developing T cells of newborn mice respond to certain chemokines depending on their differentiation state. In situ expression analyses indicate that the same chemokines are expressed in distinct microenvironments within the thymic stroma. Expression of chemokines is regulated temporally during embryogenesis; in the alymphoid early thymic anlage, only TECK, SDF-1 and SLC but not ELC, MDC or TARC are expressed. Fetal blood prothymocytes destined to colonize the thymus respond to the embryonic chemokines TECK and SDF-1 in chemotaxis assays with high efficacy. The in vivo significance of this finding is demonstrated by studies in the nude mouse where the thymic anlage lacks TECK and SDF-1 expression and prothymocytes home to the parathyroid anlage rather than to the thymic anlage. Developing thymocytes respond to chemokines expressed in distinct microenvironments within the thymic stroma in a way that correlates well with the previously observed migration pattern from cortex to medulla. The complexity of these chemokine-defined microenvironments increases as the thymic anlage develops to a mature thymus.

B lymphocyte chemotaxis regulated in association with microanatomic localization, differentiation state, and B cell receptor engagement

Migration of mature B lymphocytes within secondary lymphoid organs and recirculation between these sites are thought to allow B cells to obtain T cell help, to undergo somatic hypermutation, to differentiate into effector cells, and to home to sites of antibody production. The mechanisms that direct migration of B lymphocytes are unknown, but there is evidence that G protein-coupled receptors, and possibly chemokine receptors, may be involved. Stromal cell- derived factor (SDF)-1alpha is a CXC chemokine previously characterized as an efficacious chemoattractant for T lymphocytes and monocytes in peripheral blood. Here we show with purified tonsillar B cells that SDF-1alpha also attracts naive and memory, but not germinal center (GC) B lymphocytes. Furthermore, GC B cells could be converted to respond to SDF-1alpha by in vitro differentiation into memory B lymphocytes. Conversely, the migratory response in naive and memory B cells was significantly reduced after B cell receptor engagement and CD40 signaling. The receptor for SDF-1, CXC chemokine receptor 4 (CXCR4), was found to be expressed on responsive as well as unresponsive B cell subsets, but was more rapidly downregulated on responsive cells by ligand. Finally, messenger RNA for SDF-1 was detected by in situ hybridization in a layer of cells surrounding the GC. These findings show that responsiveness to the chemoattractant SDF-1alpha is regulated during B lymphocyte activation, and correlates with positioning of B lymphocytes within a secondary lymphoid organ.

Functional expression of the CXC-chemokine receptor-4/fusin on mouse microglial cells and astrocytes

The mRNA for the seven-transmembrane-spanning G protein-coupled receptor fusin/CXCR-4 is expressed in primary mouse astrocyte cultures and the transformed mouse microglial cell line, N9. Cell surface expression of fusin in these cells was confirmed by staining with a polyclonal anti-fusin Ab. The functional capacity of this chemokine receptor was examined by evaluating the calcium responses following stimulation of glial cells with the CXC-chemokine, stromal-derived cell factor-1alpha (SDF-1alpha). Both astrocytes and microglial cells mobilized calcium following stimulation with chemically synthesized SDF-1alpha. SDF-1alpha- and carbachol-mediated calcium responses of astrocytes were partially inhibited by treatment with pertussis toxin (PTx), suggesting receptor coupling to a combination of G alpha(i) and other G proteins. In contrast, the calcium responses of microglial cells to SDF-1alpha were completely PTx sensitive, while responses to carbachol stimulation were PTx resistant. The ability of SDF-1alpha to induce glial cell migration was also examined. Synthetic SDF-1alpha was a potent chemoattractant for mouse microglial cells at ligand concentrations of 10 to 500 ng/ml; peak responses were noted at 100 ng/ml. In contrast, astrocytes did not migrate toward a gradient of SDF-1alpha. The failure of SDF-1alpha to induce astrocyte migration was specific, as another chemokine, macrophage inflammatory protein-1alpha, triggered astrocyte chemotaxis.

Functional expression of the CXC-chemokine receptor-4/fusin on mouse microglial cells and astrocytes

The mRNA for the seven-transmembrane-spanning G protein-coupled receptor fusin/CXCR-4 is expressed in primary mouse astrocyte cultures and the transformed mouse microglial cell line, N9. Cell surface expression of fusin in these cells was confirmed by staining with a polyclonal anti-fusin Ab. The functional capacity of this chemokine receptor was examined by evaluating the calcium responses following stimulation of glial cells with the CXC-chemokine, stromal-derived cell factor-1alpha (SDF-1alpha). Both astrocytes and microglial cells mobilized calcium following stimulation with chemically synthesized SDF-1alpha. SDF-1alpha- and carbachol-mediated calcium responses of astrocytes were partially inhibited by treatment with pertussis toxin (PTx), suggesting receptor coupling to a combination of G alpha(i) and other G proteins. In contrast, the calcium responses of microglial cells to SDF-1alpha were completely PTx sensitive, while responses to carbachol stimulation were PTx resistant. The ability of SDF-1alpha to induce glial cell migration was also examined. Synthetic SDF-1alpha was a potent chemoattractant for mouse microglial cells at ligand concentrations of 10 to 500 ng/ml; peak responses were noted at 100 ng/ml. In contrast, astrocytes did not migrate toward a gradient of SDF-1alpha. The failure of SDF-1alpha to induce astrocyte migration was specific, as another chemokine, macrophage inflammatory protein-1alpha, triggered astrocyte chemotaxis.

The HIV coreceptors CXCR4 and CCR5 are differentially expressed and regulated on human T lymphocytes

The chemokine receptors CXCR4 and CCR5 function as coreceptors for HIV-1 entry into CD4+ cells. During the early stages of HIV infection, viral isolates tend to use CCR5 for viral entry, while later isolates tend to use CXCR4. The pattern of expression of these chemokine receptors on T cell subsets and their regulation has important implications for AIDS pathogenesis and lymphocyte recirculation. A mAb to CXCR4, 12G5, showed partial inhibition of chemotaxis and calcium influx induced by SDF-1, the natural ligand of CXCR4. 12G5 stained predominantly the naive, unactivated CD26(low) CD45RA+ CD45R0- T lymphocyte subset of peripheral blood lymphocytes. In contrast, a mAb specific for CCR5, 5C7, stained CD26(high) CD45RA(low) CD45R0+ T lymphocytes, a subset thought to represent previously activated/memory cells. CXCR4 expression was rapidly up-regulated on peripheral blood mononuclear cells during phytohemagglutinin stimulation and interleukin 2 priming, and responsiveness to SDF-1 increased simultaneously. CCR5 expression, however, showed only a gradual increase over 12 days of culture with interleukin 2, while T cell activation with phytohemagglutinin was ineffective. Taken together, the data suggest distinct functions for the two receptors and their ligands in the migration of lymphocyte subsets through lymphoid and nonlymphoid tissues. Furthermore, the largely reciprocal expression of CXCR4 and CCR5 among peripheral blood T cells implies distinct susceptibility of T cell subsets to viral entry by T cell line-tropic versus macrophage-tropic strains during the course of HIV infection.

Molecular cloning and characterization of a murine pre-B-cell growth-stimulating factor/stromal cell-derived factor 1 receptor, a murine homolog of the human immunodeficiency virus 1 entry coreceptor fusin

Pre-B-cell growth-stimulating factor/ stromal cell-derived factor 1 (PBSF/SDF-1) is a member of the CXC group of chemokines that is initially identified as a bone marrow stromal cell-derived factor and as a pre-B-cell stimulatory factor. Although most chemokines are thought to be inducible inflammatory mediators, PBSF/SDF-1 is essential for perinatal viability,. B lymphopoiesis, bone marrow myelopoiesis, and cardiac ventricular septal formation, and it has chemotactic activities on resting lymphocytes and monocytes. In this paper, we have isolated a cDNA that encodes a seven transmembrane-spanning-domain receptor, designated pre-B-cell-derived chemokine receptor (PB-CKR) from a murine pre-B-cell clone, DW34. The deduced amino acid sequence has 90% identity with that of a HUMSTSR/fusin, a human immunodeficiency virus 1 (HIV-1) entry coreceptor. However, the second extracellular region has lower identity (67%) compared with HUMSTSR/fusin. PB-CKR is expressed during embryo genesis and in many organs and T cells of adult mice. Murine PBSF/SDF-1 induced an increase in intracellular free Ca2+ in DW34 cells and PB-CKR-transfected Chinese hamster ovary (CHO) cells, suggesting that PB-CKR is a functional receptor for murine PBSF/SDF-1. Murine PBSF/ SDF-1 also induced Ca2+ influx in fusin-transfected CHO cells. On the other hand, considering previous results that HIV-1 does not enter murine T cells that expressed human CD4, PB-CKR may not support HIV-1 infection. Thus, PB-CKR will be an important tool for functional mapping of HIV-1 entry coreceptor fusin and for understanding the function of PBSF/SDF-1 further.

A highly efficacious lymphocyte chemoattractant, stromal cell-derived factor 1 (SDF-1)

Chemotactic factors are postulated to direct emigration of lymphocytes from the blood stream into sites of inflammation. Members of a family of chemotactic cytokines, termed chemokines, have been shown to attract lymphocytes but efficacy, i.e., the maximal percentage of attracted cells, has been low. We have identified a highly efficacious lymphocyte chemotactic activity in the supernatants of the murine bone marrow stroma cell line MS-5 which attracts 10-fold more lymphocytes in vitro than currently described lymphocyte chemoattractants. Purification of this chemotactic activity revealed identity to stromal cell-derived factor 1 (SDF-1). SDF-1 acts on lymphocytes and monocytes but not neutrophils in vitro and is both a highly efficacious and highly potent mononuclear cell attractant in vivo. In addition, SDF-1 induces intracellular actin polymerization in lymphocytes, a process that is thought to be a prerequisite for cell motility. Since SDF-1 is expressed constitutively in a broad range of tissues it may have a role in immune surveillance and in basal extravasation of lymphocytes and monocytes rather than in inflammation.

The lymphocyte chemoattractant SDF-1 is a ligand for LESTR/fusin and blocks HIV-1 entry

Chemokines are chemotactic cytokines that activate and direct the migration of leukocytes. There are two subfamilies, the CXC and the CC chemokines. We recently found that the CXC-chemokine stromal cell-derived factor-1 (SDF-1) is a highly efficacious lymphocyte chemoattractant. Chemokines act on responsive leukocyte subsets through G-protein-coupled seven-transmembrane receptors, which are also used by distinct strains of HIV-1 as cofactors for viral entry. Laboratory-adapted and some T-cell-line-tropic (T-tropic) primary viruses use the orphan chemokine receptor LESTR/fusin (also known as fusin), whereas macrophage-tropic primary HIV-1 isolates use CCR-5 and CCR-3 (refs 7-11), which are receptors for known CC chemokines. Testing of potential receptors demonstrated that SDF-1 signalled through, and hence 'adopted', the orphan receptor LESTR, which we therefore designate CXC-chemokine receptor-4 (CXCR-4). SDF-1 induced an increase in intracellular free Ca2+ and chemotaxis in CXCR-4-transfected cells. Because SDF-1 is a biological ligand for the HIV-1 entry cofactor LESTR, we tested whether it inhibited HIV-1. SDF-1 inhibited infection by T-tropic HIV-1 of HeLa-CD4 cells, CXCR-4 transfectants, and peripheral blood mononuclear cells (PBMCs), but did not affect CCR-5-mediated infection by macrophage-tropic (M-tropic) and dual-tropic primary HIV-1.