Secondary Lymphoid Compartments Research Articles

Naïve CD4+ T Cell Activation in the Nasal-Associated Lymphoid Tissue following Intranasal Immunization with a Flagellin-Based Subunit Vaccine

The nasal-associated lymphoid tissues (NALT) are generally accepted as an immune induction site, but the activation of naïve T-cells in that compartment has not been well-characterized. I wanted to determine if early events in naïve CD4+ T cell activation and the extent of antigen specific cell division are similar in NALT to that observed in other secondary lymphoid compartments. I performed antigen tracking experiments and analyzed the activation of naïve antigen-specific CD4+ T cells in the nasal-associated lymphoid tissues (NALT). I directly observed transepithelial transport of fluorescently labeled antigen from the lumen of the airway to the interior of the NALT two hours following immunization. One day following intranasal (i.n.) immunization with antigen and adjuvant, antigen-specific CD4+ T cells in the NALT associated as clusters, while antigen-specific CD4+ T cells in control mice immunized with adjuvant only remained dispersed. The antigen-specific CD4+ populations in the NALT and cranial deep cervical lymph nodes of immunized mice expanded significantly by day three following immunization. These findings are consistent with initial activation of naïve CD4+ T cells in the NALT and offer insight into adjuvant mechanism of flagellin in the upper respiratory compartment.

Robust engraftment of fetal nonhuman primate CD34-positive cells in immune-deficient mice.

Nonhuman primates (NHPs) represent one of the most important models for preclinical studies of novel biomedical interventions. In contrast with small animal models, however, widespread utilization of NHPs is restricted by cost, logistics, and availability. Therefore, we sought to develop a translational primatized mouse model, akin to a humanized mouse, to allow for high-throughput in vivo experimentation leveraged to inform large animal immunology-based studies. We found that adult rhesus macaque mobilized blood (AMb) CD34+-enriched hematopoietic stem and progenitor cells (HSPCs) engrafted at low but persistent levels in immune-deficient mice harboring transgenes for human (NHP cross-reactive) GM-CSF and IL3, but did not in mice with wild-type murine cytokines lacking NHP cross-reactivity. To enhance engraftment, fetal liver-derived HSPCs were selected as the infusion product based on an increased CD34hi fraction compared with AMb and bone marrow. Coupled with cotransplantation of rhesus fetal thymic fragments beneath the mouse kidney capsule, fetal liver-derived HSPC infusion in cytokine-transgenic mice yielded robust multilineage lymphohematopoietic engraftment. The emergent immune system recapitulated that of the fetal monkey, with similar relative frequencies of lymphocyte, granulocyte, and monocyte subsets within the thymic, secondary lymphoid, and peripheral compartments. Importantly, while exhibiting a predominantly naïve phenotype, in vitro functional assays demonstrated robust cellular activation in response to nonspecific and allogenic stimuli. This primatized mouse represents a viable and translatable model for the study of hematopoietic stem cell physiology, immune development, and functional immunology in NHPs. Summary Sentence: Engraftment of rhesus macaque hematopoietic tissues in immune-deficient mice yields a robust BLT/NeoThy-type primatized mouse model for studying nonhuman primate hematopoiesis and immune function in vivo.

Single-cell tracking of flavivirus RNA uncovers species-specific interactions with the immune system dictating disease outcome

Positive-sense RNA viruses pose increasing health and economic concerns worldwide. Our limited understanding of how these viruses interact with their host and how these processes lead to virulence and disease seriously hampers the development of anti-viral strategies. Here, we demonstrate the tracking of (+) and (−) sense viral RNA at single-cell resolution within complex subsets of the human and murine immune system in different mouse models. Our results provide insights into how a prototypic flavivirus, yellow fever virus (YFV-17D), differentially interacts with murine and human hematopoietic cells in these mouse models and how these dynamics influence distinct outcomes of infection. We detect (−) YFV-17D RNA in specific secondary lymphoid compartments and cell subsets not previously recognized as permissive for YFV replication, and we highlight potential virus–host interaction events that could be pivotal in regulating flavivirus virulence and attenuation.

The NSG – Human CLL Xenograft Model Recapitulates the Human Lymph Node Microenvironment in Regards to B-Cell Activation and Tumor Proliferation

Abstract 973Chronic lymphocytic leukemia (CLL) cells depend on signals from the tissue microenvironment for proliferation and survival. A comparative analysis of lymph node (LN) and peripheral blood (PB) derived CLL cells revealed dynamic B-cell receptor (BCR) and NF-kB activation in this microenvironment (Herishanu et al, Blood 2011). To dissect the pathogenic role of different signaling pathways in vivo, it is crucial to develop a model system that can reproduce the tumor biology encountered in the human LN. We hypothesized that secondary lymphoid compartments (such as the bone marrow (BM) or spleen) of mice may recapitulate the human LN microenvironment. In order to study the effect of the murine microenvironment on human CLL cells we chose to use the recently established NOD scid gamma null (NSG) - human CLL xenograft model (Bagnara et al., Blood 2011), with slight modifications: no adoptive transfer with normal human CD34 or CD14 positive cells was undertaken, and NSG mice were conditioned with 25 mg/kg busulfan instead of g-irradiation. 24 hours later, 1 × 108 CFSE labeled CLL PB mononuclear cells (PBMCs) were injected through the tail vein. Mice were bled weekly and sacrificed between two and six weeks post xenograft. The PB CLL cell count (evaluated as hCD45+, hCD19+, hCD5+ cells) one week after injection averaged 300 cells/μL but then rapidly declined and became virtually undetectable by week four. In contrast, CLL cells persisted in the spleen and BM. We next sought to determine if tumor proliferation was occurring in the xenograft model. We found that CLL proliferation (measured by the fraction of CFSE low cells) was barely apparent early (weeks 1–2) but increased significantly by weeks 3–4 (p=0.02). A similar trend was also observed for T-cells. There was no significant difference in the fraction of CFSE low cells among the three different compartments; however, the number of CLL cells in the spleen was significantly higher than in the PB or BM, suggesting increased homing to the spleen. We next assessed if the mouse spleen microenvironment would modulate the expression of CD38, CD69 and CD184 (CXCR4). Xenografted CLL cells extracted from murine spleens showed significantly increased expression of CD38 (p=0.01) and CD69 (p<0.001) and significantly decreased expression of CD184 (p<0.001) compared to PB cells, recapitulating findings previously reported in patients. We have previously shown that LN-resident CLL cells display gene expression signatures indicating BCR and NF-kB activation. We therefore sought to determine if the mouse spleen microenvironment would modulate gene expression in CLL cells. We compared patient derived PB-CLL cells to either matched patient derived LN-CLL cells or matched xenografted CLL cells isolated from the murine spleen and evaluated expression of BCR, NF-kB, and MYC target genes using quantitative RT-PCR (pre-designed Taqman Gene Expression assays). BCR (AICDA, EGR1, EGR3, GFI1, KLF10, OAS3) and NF-kB (CCL4, CCL3, RGS1, TNF, CCND2) target genes were upregulated in CLL cells from both the human LN and murine spleen. In addition, genes associated with proliferation (CDT1, PCNA, RRM2) were upregulated in CLL cells from both LN and murine spleen; consistent with the proliferation measurements using CFSE. However, MYC target genes (MYC, DUSP1 and HSPD1) were upregulated only in CLL cells from LN but not in the murine spleen. It has been hypothesized that blocking CXCR4 on tumor cells could inhibit their ability to home and interact with the tissue microenvironment. We therefore tested the effect of AMD3100 – a CXCR4 antagonist (5 mg/kg/day by continuous subcutaneous infusion) in this model. Unexpectedly, there was no effect on leukemic cell proliferation or on the CLL count in the PB of treated as compared to untreated mice. However, we found a dramatic increase in CXCR4 expression on CLL cells in the treated mice, indicating adequate drug delivery. Taken together, our results suggest that the murine spleen microenvironment can adequately recapitulate that of the human LN. These results provide further justification for the use of the NSG - human CLL xenograft model to study both the pathogenic mechanisms that contribute to disease progression in the tissue microenvironment and as a pre-clinical model for drug development and assessment.Supported by the Intramural Research Program of NHLBI Disclosures:No relevant conflicts of interest to declare.

Active Immunization with Amyloid-β 1–42 Impairs Memory Performance through TLR2/4-Dependent Activation of the Innate Immune System

Active immunization with amyloid-β (Aβ) peptide 1-42 reverses amyloid plaque deposition in the CNS of patients with Alzheimer's disease and in amyloid precursor protein transgenic mice. However, this treatment may also cause severe, life-threatening meningoencephalitis. Physiological responses to immunization with Aβ(1-42) are poorly understood. In this study, we characterized cognitive and immunological consequences of Aβ(1-42)/CFA immunization in C57BL/6 mice. In contrast to mice immunized with myelin oligodendrocyte glycoprotein (MOG)(35-55)/CFA or CFA alone, Aβ(1-42)/CFA immunization resulted in impaired exploratory activity, habituation learning, and spatial-learning abilities in the open field. As morphological substrate of this neurocognitive phenotype, we identified a disseminated, nonfocal immune cell infiltrate in the CNS of Aβ(1-42)/CFA-immunized animals. In contrast to MOG(35-55)/CFA and PBS/CFA controls, the majority of infiltrating cells in Aβ(1-42)/CFA-immunized mice were CD11b(+)CD14(+) and CD45(high), indicating their blood-borne monocyte/macrophage origin. Immunization with Aβ(1-42)/CFA was significantly more potent than immunization with MOG(35-55)/CFA or CFA alone in activating macrophages in the secondary lymphoid compartment and peripheral tissues. Studies with TLR2/4-deficient mice revealed that the TLR2/4 pathway mediated the Aβ(1-42)-dependent proinflammatory cytokine release from cells of the innate immune system. In line with this, TLR2/4 knockout mice were protected from cognitive impairment upon immunization with Aβ(1-42)/CFA. Thus, this study identifies adjuvant effects of Aβ(1-42), which result in a clinically relevant neurocognitive phenotype highlighting potential risks of Aβ immunotherapy.

A novel pathway of haematopoiesis revealed after experimental malaria infection

A novel pathway of haematopoiesis revealed after experimental malaria infection

Rapid recruitment and activation of CD8+ T cells after herpes simplex virus type 1 skin infection

After localized infection, naive antigen-specific T cells must localize to those lymph nodes (LNs) draining the site of infection before engaging antigen-bearing dendritic cells. Given that naive precursors are initially distributed randomly throughout the secondary lymphoid compartment, it is unclear how long it takes most antigen-specific precursors to mobilize to draining LNs and become recruited into the primary T cell response. Here, we have examined the kinetics of these events, measuring the period over which naive precursors are recruited into the primary T cell response after cutaneous infection with herpes simplex virus type 1 (HSV-1). We show that despite prolonged MHC class-I-restricted antigen presentation, most naive HSV-specific precursors were recruited from the circulation in the first 4 days after inoculation. Furthermore, this prolonged presentation was also not essential for memory development, as truncating the period of antigen presentation to around 4 days did not affect the level of contraction, or long-term stability of the HSV-specific CD8(+) memory T cell pool. Thus, despite initially being dispersed throughout the entire circulation, the recruitment of naive precursors is achieved quite quickly, even when priming is restricted to a small number of draining LNs.

Uptake of CCR7 and acquisition of migratory properties by human KIR+ NK cells interacting with monocyte-derived DC or EBV cell lines: regulation by KIR/HLA-class I interaction

AbstractC-C chemokine receptor type 7 (CCR7) is a chemokine receptor playing a pivotal role in the induction of human natural killer (NK)–cell migration to lymph nodes. We show that “licensed” peripheral blood killer immunoglobulin-like receptor–positive (KIR+) NK-cell populations, as well as KIR+ NK-cell clones, de novo express CCR7 upon coculture with mature dendritic cells (mDCs) or Epstein-Barr virus (EBV)–transformed lymphoblastoid cell lines. As a consequence, they become capable of migrating in response to the CCR7-specific chemokines C-C chemokine ligand (CCL)–19 and/or CCL21. The acquisition of CCR7 by NK cells requires direct cell-to-cell contact, is detectable within a few minutes, and is due to receptor uptake from CCR7+ cells. This mechanism is tightly regulated by KIR-mediated recognition of human leukocyte antigen (HLA) class I as well as by adhesion molecules including leukocyte function-associated antigen 1 (LFA-1) and CD2. Analysis of NK-cell clones revealed that alloreactive (KIR-ligand mismatched) but not autologous NK cells acquire CCR7. These data have important implications in haploidentical hematopoietic stem cell transplantation (HSCT), in which alloreactive NK cells may acquire the ability to migrate to secondary lymphoid compartments (SLCs), where they can kill recipient antigen-presenting cells (APCs) and T cells thus preventing graft-versus-host (and host-versus-graft) reactions.

Impact of Mammalian Target of Rapamycin Inhibition on Lymphoid Homing and Tolerogenic Function of Nanoparticle-Labeled Dendritic Cells following Allogeneic Hematopoietic Cell Transplantation

Dendritic cells (DC) play a major role in the pathogenesis of graft-vs-host disease (GvHD). Directed modification of surface molecules on DC that provide instructive signals for T cells may create a tolerogenic DC phenotype that affects GvHD severity. To investigate the impact of the mammalian target of rapamycin (mTOR) inhibitor rapamycin (RAPA) on in vivo migratory capacities, tolerogenic function, and B7 superfamily surface expression on DC following allogeneic hematopoietic cell transplantation (aHCT), we generated a platform for magnetic resonance imaging and bioluminescence imaging based cell trafficking studies. Luciferase transgenic DC were labeled with superparamagnetic iron oxide nanoparticles bound to a murine IgG Ab that allowed for Fc-gammaR-mediated endocytosis. Locally injected luc(+) DC could be tracked within their anatomical context by bioluminescence imaging and magnetic resonance imaging after aHCT, based on stable intracellular localization of superparamagnetic iron oxide-IgG complexes. RAPA preconditioned DC (DC-R) displayed reduced expression of MHC class II, B7-1 (CD80), and B7-2 (CD86) but not B7-H4 whose ligation of T cells has a profound inhibitory effect on their proliferation and cytokine secretion. DC-R of recipient genotype reduced GvHD severity that is compatible with their tolerogenic phenotype. CCR5, CCR7, and CD62L expression was not affected by mTOR inhibition, which allowed for DC-R in vivo trafficking to secondary lymphoid compartments where immunregulation is required. This study is the first to delineate the impact of RAPA on DC migration and tolerogenic function after aHCT. Modification of the DC phenotype by mTOR inhibition may have therapeutic potential in an attempt to reduce GvHD following aHCT.

The acute phase protein haptoglobin regulates host immunity

The contribution of acute phase plasma proteins to host immune responses remains poorly characterized. To better understand the role of the acute phase reactant and major hemoglobin-binding protein haptoglobin (Hp) on the function of immune cells, we generated Hp-deficient C57BL/6J mice. These mice exhibit stunted development of lymphoid organs associated with lower counts of mature T and B cells in the blood and secondary lymphoid compartments. Moreover, these mice show markedly reduced adaptive immune responses as represented by reduced accumulation of IgG antibody after immunization with adjuvant and nominal antigen, abrogation of Th1-dominated delayed-type hypersensitivity reaction, loss of mitogenic responses mounted by T cells, and reduced T cell responses conveyed by APCs. Collectively, these defects are in agreement with the observations that Hp-deficient mice are not capable of generating a recall response or deterring a Salmonella infection as well as failing to generate tumor antigen-specific responses. The administration of Hp to lymphocytes in tissue culture partially ameliorates these functional defects, lending further support to our contention that the acute phase response protein Hp has the ability to regulate immune cell responses and host immunity. The phenotype of Hp-deficient mice suggests a major regulatory activity for Hp in supporting proliferation and functional differentiation of B and T cells as part of homeostasis and in response to antigen stimulation.

CD83 Expression Is a Sensitive Marker of Activation Required for B Cell and CD4+ T Cell Longevity In Vivo

CD83 is a surface marker that differentiates immature and mature human dendritic cell populations. Thymic epithelial cell expression of CD83 is also necessary for efficient CD4+ T cell development in mice. The altered phenotypes of peripheral B and CD4+ T cells, and the reduction of peripheral CD4+ T cells in CD83-/- mice, suggest additional functions for CD83. To assess this, a panel of mAbs was generated to characterize mouse CD83 expression by peripheral leukocytes. As in humans, activation of conventional and plasmacytoid murine dendritic cell subsets led to rapid up-regulation of CD83 surface expression in mice. In primary and secondary lymphoid compartments, a subset of B cells expressed low-level CD83, while CD83 was not detected on resting T cells. However, CD83 was prominently up-regulated on the majority of spleen B and T cells within hours of activation in vitro. In vivo, a low dose of hen egg lysozyme (1 microg) induced significant CD83 but not CD69 expression by Ag-specific B cells within 4 h of Ag challenge. Although B cell development appeared normal in CD83-/- mice, B and CD4+ T cell expression of CD83 was required for lymphocyte longevity in adoptive transfer experiments. Thus, the restricted expression pattern of CD83, its rapid induction following B cell and T cell activation, and its requirement for B cell and CD4+ T cell longevity demonstrate that CD83 is a functionally significant and sensitive marker of early lymphocyte activation in vivo.

CD4+CD25+ Regulatory T Cells Enhance Immune Reconstitution Following Allogeneic Hematopoietic Cell Transplantation by Protecting Thymic and Lymphoid Compartments from Graft-Versus-Host Disease Damage without Impacting T Cell Repertoire Development.

Regulatory T cells (Treg) protect from acute graft-versus-host disease (GvHD) in murine models of major-MHC mismatched hematopoietic cell transplantation (HCT) presumably by dampening the proliferation of mature effector T cells. It is unclear whether the effect of Treg on effector T cells is a selective or nonselective process or if Treg regulate the process of intrathymic and peripheral T cell maturation and selection following HCT, particularly given the intrinsic link of GvHD and immune reconstitution. We previously showed that Treg improved the quantitative and functional lymphoid reconstitution in a murine model of HCT. In the current study, we hypothesize that Treg prevent thymic and lymphoid damage from GvHD, leading to enhanced lymphoid reconstitution. Lethally-irradiated adult thymectomized Balb/c (H2d) recipients were transplanted with wild-type FVB (H2q) T cell depleted bone marrow (TCD-BM) cells and CD4+/CD8+ cells (Tcon), the latter to induce GvHD, with or without donor Treg given at a 1:1 dose ratio with Tcon. At day 30, when all groups had reached full donor chimerism, transplant recipients were challenged with murine CMV (5×105 pfu/mouse) intraperitoneally. At day 90, survival for thymectomized groups with TCD-BM alone, with Tcon, or with Tcon+Treg was 78%, 0%, and 45%, respectively, compared to 100%, 0%, and 86% survival in their respective euthymic infected counterparts (p<0.05 for thymectomized vs euthymic Treg groups). Elispot for Interferon-γ showed CMV-specific donor responses in all infected groups. CMV viral titers in the liver and kidney 2 weeks after infection was lower in recipients that received Treg compared to animals that received Tcon alone. Compared to euthymic transplant controls, thymectomized animals had higher viral titers in the liver, lungs, and kidneys in all groups. Uninfected thymectomized mice in the respective groups served as controls to separate the effect of CMV infection and GvHD on survival. All animals, infected or uninfected, that received Treg had no evidence of clinical GvHD while animals that received Tcon alone had significant GvHD. In euthymic recipients, gross and histologic examination confirmed the general preservation of thymic integrity and architecture in animals that received Treg compared with smaller involuted thymuses partially replaced by adipose tissue in animals that received Tcon alone. T cell repertoire assessed by V-beta TCR screening with FACS analysis showed a polyclonal distribution in animals with or without Treg. Spectratyping at day 30 post-transplantation showed that Treg had no significant impact on the TCR repertoire diversity in animals which received Tcon. Based on survival of a subset of infected thymectomized animals that received Treg, we evaluated the impact of Treg on secondary lymphoid organs following HCT. Animals without transferred Treg had significant splenic and lymph node fibrosis and hypoplasia with a reduction in T cell numbers due to GvHD. Our findings indicate that Treg indirectly enhance immune reconstitution by protecting the thymic and secondary lymphoid compartments from GvHD damage, allowing the generation and peripheral expansion of lymphoid cells without impacting the diversity of T cell repertoire.

Remarkable Differences in Cellular Activation State and Migratory and Proliferative Potential among Clonal Cells Derived from Different Tissues of Chronic Lymphocytic Leukemia Patients.

An antigen-independent process that takes place in the bone marrow (BM) leads to the birth of B cells from bone marrow precursors. Cross-talk between components of the microenvironment of BM or secondary lymphoid compartment(s), eg. spleen (SP), nurtures the subsequent evolution of B cells and governs in large part the natural history of normal B cells and B cell malignancies including chronic lymphocytic leukemia (B-CLL). Interaction of B cells with stromal elements confers upon them features enabling their transient sequestration, proliferation and extended survival. In this report we have compared the characteristics of clonal B-CLL cells obtained from paired specimens (peripheral blood, PB with corresponding BM or SP obtained within an interval of less than 1 month of each other) from 17 individual untreated B-CLL patients. These cells were tested by surface immunofluorescence and flow cytometry for their expression of a panel of chemokine receptors (CCR −1, −2, −4, and −7 and CXCR−1, −2, −3, and −4) and markers of cellular activation (CD23, CD62L, CD69, CD71 and HLA-DR). The relative age of the B cells (telomere length) and their ability to maintain telomere length (telomerase activity) were studied in paired BM/SP and PB samples from 15 of these cases. Although PB-, BM- and SP-derived B cells expressed activation markers, specifically, the percentages of cells expressing ZAP-70 and Ki-67 were significantly higher in BM- and SP-derived B cells than those expressed by corresponding PB-derived B cells (p<0.01). Increase in extent of CD38-positivity among members of the clone correlates with poor prognosis in B-CLL. Interestingly, in cases with low CD38 expression (<30% cells expressing CD38), the percentage of B-CLL cells expressing CXCR3 and CCR7 was significantly higher among PB-B cells, than BM/SP-derived B cells. No such differences existed in cases with high CD38 expression. This suggests a role for these receptors and their ligands in maintaining homeostasis of B-CLL cells in this subset of cases (that does not show remarkable progression of disease). In each of the 15 cases studied BM and SP-B cells had significantly higher telomerase activity (p<0.01) than those in PB, although telomere lengths of B cells from both sources were comparable. These findings highlight important differences in cellular kinetics among lymphoid compartments in B-CLL.

Bone marrow‐derived cells expand memory CD8+ T cells in response to viral infections of the lung and skin

While naive CD8(+) T cells have been shown to require bone marrow-derived dendritic cells (DC) to initiate immunity, such a requirement for memory CD8(+) T cells has had limited assessment. By generating bone marrow chimeras that express the appropriate antigen-presenting molecules on either radiation-sensitive bone marrow-derived or radiation-resistant non-bone marrow-derived compartments, we showed that both primary and secondary immune responses to influenza virus infection of the lung were initiated in the draining LN. This required cells of bone marrow origin, most likely DC, for optimal expansion within the secondary lymphoid compartment. This was similarly the case with HSV-1 infection of the skin. As Langerhans cells are radioresistant, unlike other DC populations, these studies also demonstrate that the radiosensitive DC responsible for secondary expansion of HSV-specific memory are not Langerhans cells.

Mesenteric B cells centrally inhibit CD4+ T cell colitis through interaction with regulatory T cell subsets.

Inflammatory bowel disease reflects an aberrant mucosal CD4+ T cell response to commensal enteric bacteria. In addition to regulatory T cell subsets, recent studies have revealed a protective role of B cells in murine CD4+ T cell colitis, but the relationship of their action to T cell immunoregulation is unknown. Here we report that mesenteric lymph node (MLN) B cells protect mice from colitis induced by Galphai2-/- CD4+ T cells. Protection required the transfer of both B cells and CD8alpha+ T cells; neither cell type alone was sufficient to inhibit CD4+ T cell-mediated colitis. Similar results were also observed in colitis induced by CD4+CD45RBhi T cells. Immunoregulation was associated with localization of B cells and expansion of CD4-CD8- CD3+NK1.1+ T cells in the secondary lymphoid compartment, as well as expansion of CD4+CD8alpha+ T cells in the intestinal intraepithelial compartment. MLN B cells from Galphai2-/- mice were deficient in a phenotypic subset and failed to provide cotransfer colitis protection. These findings indicate that protective action of B cells is a selective trait of MLN B cells acquired through a Galphai2-dependent developmental process and link B cells with the formation of regulatory T cells associated with mucosal immune homeostasis.

The small subset of CD56brightCD16- natural killer cells is selectively responsible for both cell proliferation and interferon-gamma production upon interaction with dendritic cells.

The encounter of NK cells with dendritic cells (DC) undergoing maturation may result in the induction of NK cell proliferation. Whether such proliferation involves most NK cells or just a subset has yet to be determined. In the present study we analyzed the nature of such proliferating NK cells by combining carboxyfluorescein succinimidyl ester staining and double-fluorescence cytofluorimetric analysis. Freshly isolated peripheral blood NK cells cultured with LPS and immature DC underwent proliferation; however, proliferating cells were confined to a minor NK cell subset. This subset is characterized by the CD56(bright)CD16(-)NKG2A(+)KIR(-) surface phenotype (KIR, killer Ig-like receptor). This was further confirmed by the fact that, after cell sorting, only the CD56(bright) NK cells were able to proliferate in response to the DC stimulus, whereas the CD56(dull) were not. We also provide evidence that the CD56(bright) subset is the main source of IFN-gamma-producing NK cells, upon interaction with DC. The CD56(bright)CD16(-) NK cells express a panel of surface molecules including CD62L, CCR7 and CXCR3 that may allow their homing either to secondary lymphoid compartments or to inflamed tissues. This implies that, in vivo, the interactions between DC undergoing maturation and CD56(bright) NK cells may occur in different tissues and have different functional implications.

Regulation of B-cell tolerance by complement C4

Systemic lupus erythematosus (SLE) is a B-cell-dependent autoimmune disease characterized by autoantibodies specific for nuclear antigens such as dsDNA and histones. Apoptotic blebs released from dying cells are thought to represent a major source of autoantigen. The cellular and molecular basis for SLE is not known but a major risk factor is deficiency in products of natural immunity. For example, individuals deficient in serum complement proteins C1q or C4 almost always develop lupus. This presents a paradox as the complement system is known to mediate pathogenesis in SLE. In addition to its known role in inflammation, the complement system participates in humoral antibody responses via enhancement of B-lymphocyte activation, germinal center survival and maintenance of long-term effector responses. Two nonmutually exclusive hypotheses for a protective role of complement are termed the clearance hypothesis and the B-cell tolerance hypothesis. The former proposes that complement (and natural immunity) protects by clearance of apoptotic blebs and sequesters them from the adaptive immune system. The tolerance hypothesis proposes that complement and natural immunity act in concert to localize lupus-antigens to sites within the primary and secondary lymphoid compartments where developing B cells undergo negative selection. To examine directly a role for complement C4 in protection from self-reactive B cells, we have bred the deficient mice with mice bearing rearranged immunoglobulin heavy chain and light chain anti-chromatin transgenes (Tg) (termed 564) specific for chromatin. On a normal C4-sufficient background, the self-reactive Tg B cells are blocked at the immature or transitional stage of development. By contrast, in the absence of C4, the self-reactive B cells continue to develop and enter the mature compartment. Moreover, they appear to secrete self-reactive antibody of both IgM and IgG isotypes. These results identify a functional role for C4 in regulation of self-reactive B cells and support the B-cell tolerance hypothesis.

Effects of oestrogen and exercise on caspase-3 activity in primary and secondary lymphoid compartments in ovariectomized mice.

This study investigated the effect of oestrogen exposure and exercise on caspase-3 activity, a measure of apoptosis, in lymphocytes from the thymus, spleen, and lymph nodes in ovariectomized mice. Fifty-nine female B6D2F1 mice were randomized to hormone and exercise conditions. Hormone treatment consisted of implantation with oestradiol pellets (0.72 mg oestradiol) or placebo pellets (0 mg) for 21 days following bilateral ovariectomy (OVX). Exercise consisted of a single treadmill exercise bout (26 m min(-1), 6 degrees slope, 90-min) or sedentary condition. Mice were killed and the thymus, spleen and lymph nodes were removed for the determination of caspase-3 expression by enzyme-linked immunosorbent assay (ELISA), serum oestrogen levels by RIA, and tissue weights. Body weights were monitored throughout the study. In the thymus, oestrogen exposure, exercise and both treatments together were associated with higher caspase-3 activity (P < 0.05) and lower thymus weights (P < 0.05). In contrast, oestrogen exposure and exercise treatment were not associated with greater caspase-3 activity or change in tissue weight in secondary lymphoid tissues (spleen, lymph nodes). Oestrogen-replaced OVX mice had a higher concentration of plasma oestradiol than placebo OVX mice (P < 0.05). The results suggest that oestrogen and treadmill exercise are associated with greater apoptosis, as measured by caspase-3 activity, in the thymus but not in the spleen or lymph nodes. Clinical studies will be necessary to determine if women who take oestrogen have higher rates of apoptosis in primary lymphoid tissues and the significance of thymocyte apoptosis for maintenance of cellular immune function during the post-menopausal years.

The Immune Modulator FTY720 Targets Sphingosine 1-Phosphate Receptors

Immunosuppressant drugs such as cyclosporin have allowed widespread organ transplantation, but their utility remains limited by toxicities, and they are ineffective in chronic management of autoimmune diseases such as multiple sclerosis. In contrast, the immune modulating drug FTY720 is efficacious in a variety of transplant and autoimmune models without inducing a generalized immunosuppressed state and is effective in human kidney transplantation. FTY720 elicits a lymphopenia resulting from a reversible redistribution of lymphocytes from circulation to secondary lymphoid tissues by unknown mechanisms. Using FTY720 and several analogs, we show now that FTY720 is phosphorylated by sphingosine kinase; the phosphorylated compound is a potent agonist at four sphingosine 1-phosphate receptors and represents the therapeutic principle in a rodent model of multiple sclerosis. Our results suggest that FTY720, after phosphorylation, acts through sphingosine 1-phosphate signaling pathways to modulate chemotactic responses and lymphocyte trafficking.

B cells directly tolerize CD8(+) T cells.

This report investigates the response of CD8(+) T cells to antigens presented by B cells. When C57BL/6 mice were injected with syngeneic B cells coated with the Kb-restricted ovalbumin (OVA) determinant OVA257-264, OVA-specific cytotoxic T lymphocyte (CTL) tolerance was observed. To investigate the mechanism of tolerance induction, in vitro-activated CD8(+) T cells from the Kb-restricted, OVA-specific T cell receptor transgenic line OT-I (OT-I cells) were cultured for 15 h with antigen-bearing B cells, and their survival was determined. Antigen recognition led to the killing of the B cells and, surprisingly, to the death of a large proportion of the OT-I CTLs. T cell death involved Fas (CD95), since OT-I cells deficient in CD95 molecules showed preferential survival after recognition of antigen on B cells. To investigate the tolerance mechanism in vivo, naive OT-I T cells were adoptively transferred into normal mice, and these mice were coinjected with antigen-bearing B cells. In this case, OT-I cells proliferated transiently and were then lost from the secondary lymphoid compartment. These data provide the first demonstration that B cells can directly tolerize CD8(+) T cells, and suggest that this occurs via CD95-mediated, activation-induced deletion.