Sexual dimorphism in skin immunity is mediated by an androgen-ILC2-dendritic cell axis

Males and females exhibit profound differences in immune responses and disease susceptibility. However, the factors responsible for sex differences in tissue immunity remain poorly understood. Here, we uncovered a dominant role for type 2 innate lymphoid cells (ILC2s) in shaping sexual immune dimorphism within the skin. Mechanistically, negative regulation of ILC2s by androgens leads to a reduction in dendritic cell accumulation and activation in males, along with reduced tissue immunity. Collectively, our results reveal a role for the androgen-ILC2-dendritic cell axis in controlling sexual immune dimorphism. Moreover, this work proposes that tissue immune set points are defined by the dual action of sex hormones and the microbiota, with sex hormones controlling the strength of local immunity and microbiota calibrating its tone.

TGF-β uncouples glycolysis and inflammation in macrophages and controls survival during sepsis

Changes in metabolism of macrophages are required to sustain macrophage activation in response to different stimuli. We showed that the cytokine TGF-β (transforming growth factor-β) regulates glycolysis in macrophages independently of inflammatory cytokine production and affects survival in mouse models of sepsis. During macrophage activation, TGF-β increased the expression and activity of the glycolytic enzyme PFKL (phosphofructokinase-1 liver type) and promoted glycolysis but suppressed the production of proinflammatory cytokines. The increase in glycolysis was mediated by an mTOR-c-MYC-dependent pathway, whereas the inhibition of cytokine production was due to activation of the transcriptional coactivator SMAD3 and suppression of the activity of the proinflammatory transcription factors AP-1, NF-κB, and STAT1. In mice with LPS-induced endotoxemia and experimentally induced sepsis, the TGF-β-induced enhancement in macrophage glycolysis led to decreased survival, which was associated with increased blood coagulation. Analysis of septic patient cohorts revealed that the expression of PFKL, TGFBRI (which encodes a TGF-β receptor), and F13A1 (which encodes a coagulation factor) in myeloid cells positively correlated with COVID-19 disease. Thus, these results suggest that TGF-β is a critical regulator of macrophage metabolism and could be a therapeutic target in patients with sepsis.

pDC-like cells are pre-DC2 and require KLF4 to control homeostatic CD4 T cells

Plasmacytoid dendritic cells (pDCs) have been shown to play an important role during immune responses, ranging from initial viral control through the production of type I interferons to antigen presentation. However, recent studies uncovered unexpected heterogeneity among pDCs. We identified a previously uncharacterized immune subset, referred to as pDC-like cells, that not only resembles pDCs but also shares conventional DC (cDC) features. We show that this subset is a circulating precursor distinct from common DC progenitors, with prominent cDC2 potential. Our findings from human CD2-iCre and CD300c-iCre lineage tracing mouse models suggest that a substantial fraction of cDC2s originates from pDC-like cells, which can therefore be referred to as pre-DC2. This precursor subset responds to homeostatic cytokines, such as macrophage colony stimulating factor, by expanding and differentiating into cDC2 that efficiently prime T helper 17 (TH17) cells. Development of pre-DC2 into CX3CR1+ ESAM- cDC2b but not CX3CR1- ESAM+ cDC2a requires the transcription factor KLF4. Last, we show that, under homeostatic conditions, this developmental pathway regulates the immune threshold at barrier sites by controlling the pool of TH17 cells within skin-draining lymph nodes.

Interferon-γ resistance and immune evasion in glioma develop via Notch-regulated co-evolution of malignant and immune cells

Immune surveillance is critical to prevent tumorigenesis. Gliomas evade immune attack, but the underlying mechanisms remain poorly understood. We show that glioma cells can sustain growth independent of immune system constraint by reducing Notch signaling. Loss of Notch activity in a mouse model of glioma impairs MHC-I and cytokine expression and curtails the recruitment of anti-tumor immune cell populations in favor of immunosuppressive tumor-associated microglia/macrophages (TAMs). Depletion of T cells simulates Notch inhibition and facilitates tumor initiation. Furthermore, Notch-depleted glioma cells acquire resistance to interferon-γ and TAMs re-educating therapy. Decreased interferon response and cytokine expression by human and mouse glioma cells correlate with low Notch activity. These effects are paralleled by upregulation of oncogenes and downregulation of quiescence genes. Hence, suppression of Notch signaling enables gliomas to evade immune surveillance and increases aggressiveness. Our findings provide insights into how brain tumor cells shape their microenvironment to evade immune niche control.

Dntt expression reveals developmental hierarchy and lineage specification of hematopoietic progenitors

Intrinsic and extrinsic cues determine developmental trajectories of hematopoietic stem cells (HSCs) towards erythroid, myeloid and lymphoid lineages. To dissect early lymphoid specification events, we generated two transgenic mouse models reporting and tracing the expression of terminal deoxynucleotidyl transferase (TdT). Surprisingly, transient induction of TdT was detected on a newly identified multipotent progenitor (MPP) subset which lacked self-renewal capacity but maintained multilineage differentiation potential. TdT induction by MPPs reflected a transcriptionally dynamic but uncommitted stage characterized by low expression of lineage-associated genes. Further, analysis using single-cell CITE-Seq indicated that multipotency is associated with expression of Endothelial Cell-Selective Adhesion Molecule (ESAM). Developmental progression along all lineages is defined by downregulation of ESAM and by stable TdT expression within the lymphoid trajectory. Collectively, we propose a new developmental hierarchy of early hematopoietic progenitors for which we provide their transcriptional profile as well as their phenotypic and functional properties at single cell resolution.

Supported by NIH intramural funding 

Homeostatic IL-13 in healthy skin directs dendritic cell differentiation to promote TH2 and inhibit TH17 cell polarization

The signals driving the adaptation of type 2 dendritic cells (DC2s) to diverse peripheral environments remain mostly undefined. We show that differentiation of CD11blo migratory DC2s-a DC2 population unique to the dermis-required IL-13 signaling dependent on the transcription factors STAT6 and KLF4, whereas DC2s in lung and small intestine were STAT6-independent. Similarly, human DC2s in skin expressed an IL-4 and IL-13 gene signature that was not found in blood, spleen and lung DCs. In mice, IL-13 was secreted homeostatically by dermal innate lymphoid cells and was independent of microbiota, TSLP or IL-33. In the absence of IL-13 signaling, dermal DC2s were stable in number but remained CD11bhi and showed defective activation in response to allergens, with diminished ability to support the development of IL-4+GATA3+ helper T cells (TH), whereas antifungal IL-17+RORγt+ TH cells were increased. Therefore, homeostatic IL-13 fosters a noninflammatory skin environment that supports allergic sensitization.

Novel concepts in plasmacytoid dendritic cell (pDC) development and differentiation

Plasmacytoid dendritic cells (pDCs) are an immune subset specialized in the production of Type I Interferons (IFNs). They are characterized by co-expression of myeloid and lymphoid markers. Their developmental origin has been studied since their discovery and the identification of a myeloid progenitor capable of generating all dendritic cell (DC) subsets, including pDCs, led to their classification within the myeloid compartment. However, recent findings challenge this hypothesis and provide evidence for a lymphoid origin for the majority of pDCs ^46-48. In this review we discuss and present the original myeloid and the newer lymphoid developmental trajectories of pDCs.

An Important Role for CD4+ T Cells in Adaptive Immunity to Toxoplasma gondii in Mice Lacking the Transcription Factor Batf3

Immunity to Toxoplasma gondii at early stages of infection in C57BL/6 mice depends on gamma interferon (IFN-γ) production by NK cells, while at later stages it is primarily mediated by CD8 T cells. We decided to explore the requirement for CD4 T cells during T. gondii infection in Batf3-/- mice, which lack CD8α+ dendritic cells (DCs) that are necessary for cross-presentation of cell-associated antigens to CD8 T cells. We show that in this immunodeficient background on a BALB/c background, CD4 T cells become important effector cells and are able to protect Batf3-/- mice from infection with the avirulent strain RHΔku80Δrop5 Independently of the initial NK cell activation, CD4 T cells in wild-type and Batf3-/- mice were the major source of IFN-γ. Importantly, memory CD4 T cells were sufficient to provide protective immunity following transfer into Batf3-/- mice and secondary challenge with the virulent RHΔku80 strain. Collectively, these results show that under situations where CD8 cell responses are impaired, CD4 T cells provide an important alternative immune response to T. gondiiIMPORTANCEToxoplasma gondii is a widespread parasite of animals that causes zoonotic infections in humans. Although healthy individuals generally control the infection with only moderate symptoms, it causes serious illness in newborns and those with compromised immune systems such as HIV-infected AIDS patients. Because rodents are natural hosts for T. gondii, laboratory mice provide an excellent model for studying immune responses. Here, we used a combination of an attenuated mutant strain of the parasite that effectively vaccinates mice, with a defect in a transcriptional factor that impairs a critical subset of dendritic cells, to studying the immune response to infection. The findings reveal that in BALB/c mice, CD4 memory T cells play a dominant role in producing IFN-γ needed to control chronic infection. Hence, BALB/c mice may provide a more appropriate model for declining immunity seen in HIV-AIDS patients where loss of CD4 cells is associated with emergence of opportunistic infections.

Distinct progenitor lineages contribute to the heterogeneity of plasmacytoid dendritic cells

Plasmacytoid dendritic cells (pDCs) are an immune subset devoted to the production of high amounts of type 1 interferons in response to viral infections. Whereas conventional dendritic cells (cDCs) originate mostly from a common dendritic cell progenitor (CDP), pDCs have been shown to develop from both CDPs and common lymphoid progenitors. Here, we found that pDCs developed predominantly from IL-7R⁺ lymphoid progenitor cells. Expression of SiglecH and Ly6D defined pDC lineage commitment along the lymphoid branch. Transcriptional characterization of SiglecH⁺Ly6D⁺ precursors indicated that pDC development requires high expression of the transcription factor IRF8, whereas pDC identity relies on TCF4. RNA sequencing of IL-7R⁺ lymphoid and CDP-derived pDCs mirrored the heterogeneity of mature pDCs observed in single-cell analysis. Both mature pDC subsets are able to secrete type 1 interferons, but only myeloid-derived pDCs share with cDCs their ability to process and present antigen.

Langerin+ DCs regulate innate IL-17 production in the oral mucosa during Candida albicans-mediated infection

The opportunistic fungal pathogen Candida albicans frequently causes diseases such as oropharyngeal candidiasis (OPC) in immunocompromised individuals. Although it is well appreciated that the cytokine IL-17 is crucial for protective immunity against OPC, the cellular source and the regulation of this cytokine during infection are still a matter of debate. Here, we directly visualized IL-17 production in the tongue of experimentally infected mice, thereby demonstrating that this key cytokine is expressed by three complementary subsets of CD90+ leukocytes: RAG-dependent αβ and γδ T cells, as well as RAG-independent ILCs. To determine the regulation of IL-17 production at the onset of OPC, we investigated in detail the myeloid compartment of the tongue and found a heterogeneous and dynamic mononuclear phagocyte (MNP) network in the infected tongue that consists of Zbtb46-Langerin- macrophages, Zbtb46+Langerin+ dendritic cells (DCs) and Ly6C+ inflammatory monocytes. Of those, the Langerin+ DC population stands out by its unique capacity to co-produce the cytokines IL-1β, IL-6 and IL-23, all of which promote IL-17 induction in response to C. albicans in the oral mucosa. The critical role of Langerin+ DCs for the innate IL-17 response was confirmed by depletion of this cellular subset in vivo, which compromised IL-17 induction during OPC. In conclusion, our work revealed key regulatory factors and their cellular sources of innate IL-17-dependent antifungal immunity in the oral mucosa.

Quality of TCR signaling determined by differential affinities of enhancers for the composite BATF-IRF4 transcription factor complex

Variable strengths of signaling via the T cell antigen receptor (TCR) can produce divergent outcomes, but the mechanism of this remains obscure. The abundance of the transcription factor IRF4 increases with TCR signal strength, but how this would induce distinct types of responses is unclear. We compared the expression of genes in the T_H2 subset of helper T cells to enhancer occupancy by the BATF-IRF4 transcription factor complex at varying strengths of TCR stimulation. Genes dependent on BATF-IRF4 clustered into groups with distinct TCR sensitivities. Enhancers exhibited a spectrum of occupancy by the BATF-IRF4 ternary complex that correlated with the sensitivity of gene expression to TCR signal strength. DNA sequences immediately flanking the previously defined AICE motif controlled the affinity of BATF-IRF4 for direct binding to DNA. Analysis by the chromatin immunoprecipitation-exonuclease (ChIP-exo) method allowed the identification of a previously unknown high-affinity AICE2 motif at a human single-nucleotide polymorphism (SNP) of the gene encoding the immunomodulatory receptor CTLA-4 that was associated with resistance to autoimmunity. Thus, the affinity of different enhancers for the BATF-IRF4 complex might underlie divergent signaling outcomes in response to various strengths of TCR signaling.

Quality of TCR signaling determined by differential affinities of enhancers for the composite BATF-IRF4 transcription factor complex

Variable strengths of T cell receptor (TCR) signaling can produce divergent outcomes, but the mechanism remains obscure. The abundance of the transcription factor IRF4 increases with TCR signal strength, but how this would induce distinct types of responses is unclear. We compared T_H2 gene expression with BATF/IRF4 enhancer occupancy at varying strengths of TCR stimulation. BATF/IRF4-dependent genes clustered into distinct TCR-sensitivities. Enhancers exhibited a spectrum of occupancy by BATF/IRF4 ternary complex that correlated with TCR-sensitivity of gene expression. DNA sequences immediately flanking the previously defined AICE motif controlled the affinity for BATF/IRF4 for direct binding to DNA. ChIP-exo analysis allowed identification of a novel high-affinity AICE2 motif at a human SNP of CTLA4 associated with resistance to autoimmunity. Thus, the affinity of different enhancers for the BATF-IRF4 complex may underlie divergent signaling outcomes in response to various strengths of TCR signaling.

RAB43 facilitates cross-presentation of cell-associated antigens by CD8α+ dendritic cells

RAB43 is a vesicular transport protein unique to CD8α⁺ DCs that is localized to the Golgi. Kretzer et al. show that RAB43 is necessary for optimal cross-presentation of cell-associated antigens by CD8α⁺ DCs in vitro and in vivo but that it is dispensable for cross-presentation by in vitro monocyte-derived DCs.

In this study, to examine cross-presentation by classical dendritic cells (DCs; cDCs), we evaluated the role of RAB43, a protein found to be selectively expressed by Batf3-dependent CD8α⁺ and CD103⁺ compared with other DC subsets and immune lineages. Using a specific monoclonal antibody, we localized RAB43 expression to the Golgi apparatus and LAMP1⁻ cytoplasmic vesicles. Mice with germline or conditional deletion of Rab43 are viable and fertile and have normal development of cDCs but show a defect for in vivo and in vitro cross-presentation of cell-associated antigen. This defect is specific to cDCs, as Rab43-deficient monocyte-derived DCs showed no defect in cross-presentation of cell-associated antigen. These results suggest that RAB43 provides a specialized activity used in cross-presentation selectively by CD8α⁺ DCs but not other antigen-presenting cells.

Rab43 facilitates cross-presentation of cell-associated antigens by CD8a+ dendritic cells

Batf3-dependent classical dendritic cells (cDCs) are essential for cross-presentation and clearance of intracellular pathogens through priming of CD8 T cells. By analyzing differences in gene expression between cDC subsets we identified vesicular transport molecules unique to cross-presenting dendritic cell subsets. RAB43, a GTPase that is localized to the golgi, is selectively expressed in Batf3-dependent CD8a+ and CD103+ cDCs and was highly conserved between mouse and human. Mice with germline and conditional Rab43 deletions are viable and fertile with normal hematopoiesis. However, mice lacking Rab43 showed reduced cross-presentation of cell-associated, but not soluble, antigens both in vitro and in vivo. Bone-marrow derived DCs (BMDCs) from Rab43 deficient mice show normal cross-presentation efficiency, suggesting that Rab43 is involved in optimizing cross-presentation in vivo through a pathway unique to CD8a+ DCs.

Migratory CD103+ dendritic cells suppress helminth-driven type 2 immunity through constitutive expression of IL-12

Everts et al. use Batf3^−/− mice to examine the role of Batf3-dependent CD8α⁺ and CD103⁺ DCs in Th2 immunity in response to helminth infection. Loss of Batf3-dependent DCs resulted in rapid control of normally chronic infection with Heligmosomoides polygyrus, whereas liver fibrosis was exacerbated with Schistosoma mansoni infection. Mechanistically, steady-state IL-12 production by migratory CD103⁺ DCs was found to antagonize Th2 responses.

CD8α⁺ and CD103⁺ dendritic cells (DCs) play a central role in the development of type 1 immune responses. However, their role in type 2 immunity remains unclear. We examined this issue using Batf3^−/− mice, in which both of these DC subsets are missing. We found that Th2 cell responses, and related events such as eosinophilia, alternative macrophage activation, and immunoglobulin class switching to IgG1, were enhanced in Batf3^−/− mice responding to helminth parasites. This had beneficial or detrimental consequences depending on the context. For example, Batf3 deficiency converted a normally chronic intestinal infection with Heligmosomoides polygyrus into an infection that was rapidly controlled. However, liver fibrosis, an IL-13–mediated pathological consequence of wound healing in chronic schistosomiasis, was exacerbated in Batf3^−/− mice infected with Schistosoma mansoni. Mechanistically, steady-state production of IL-12 by migratory CD103⁺ DCs, independent of signals from commensals or TLR-initiated events, was necessary and sufficient to exert the suppressive effects on Th2 response development. These findings identify a previously unrecognized role for migratory CD103⁺ DCs in antagonizing type 2 immune responses.

Transcriptional Control of Dendritic Cell Development

The dendritic cells (DCs) of the immune system function in innate and adaptive responses by directing activity of various effector cells rather than serving as effectors themselves. DCs and closely related myeloid lineages share expression of many surface receptors, presenting a challenge in distinguishing their unique in vivo functions. Recent work has taken advantage of unique transcriptional programs to identify and manipulate murine DCs in vivo. This work has assigned several nonredundant in vivo functions to distinct DC lineages, consisting of plasmacytoid DCs and several subsets of classical DCs that promote different immune effector modules in response to pathogens. In parallel, a correspondence between human and murine DC subsets has emerged, underlying structural similarities for the DC lineages between these species. Recent work has begun to unravel the transcriptional circuitry that controls the development and diversification of DCs from common progenitors in the bone marrow.

Transcriptional Regulation of Mononuclear Phagocyte Development

Mononuclear phagocytes (MP) are a quite unique subset of hematopoietic cells, which comprise dendritic cells (DC), monocytes as well as monocyte-derived and tissue-resident macrophages. These cells are extremely diverse with regard to their origin, their phenotype as well as their function. Developmentally, DC and monocytes are constantly replenished from a bone marrow hematopoietic progenitor. The ontogeny of macrophages is more complex and is temporally linked and specified by the organ where they reside, occurring early during embryonic or perinatal life. The functional heterogeneity of MPs is certainly a consequence of the tissue of residence and also reflects the diverse ontogeny of the subsets. In this review, we will highlight the developmental pathways of murine MP, with a particular emphasis on the transcriptional factors that regulate their development and function. Finally, we will discuss and point out open questions in the field.

Batf3 maintains autoactivation of Irf8 for commitment of a CD8α(+) conventional DC clonogenic progenitor

The transcription factors Batf3 and IRF8 are required for the development of CD8α(+) conventional dendritic cells (cDCs), but the basis for their actions has remained unclear. Here we identified two progenitor cells positive for the transcription factor Zbtb46 that separately generated CD8α(+) cDCs and CD4(+) cDCs and arose directly from the common DC progenitor (CDP). Irf8 expression in CDPs required prior autoactivation of Irf8 that was dependent on the transcription factor PU.1. Specification of the clonogenic progenitor of CD8α(+) cDCs (the pre-CD8 DC) required IRF8 but not Batf3. However, after specification of pre-CD8 DCs, autoactivation of Irf8 became Batf3 dependent at a CD8α(+) cDC-specific enhancer with multiple transcription factor AP1-IRF composite elements (AICEs) within the Irf8 superenhancer. CDPs from Batf3(-/-) mice that were specified toward development into pre-CD8 DCs failed to complete their development into CD8α(+) cDCs due to decay of Irf8 autoactivation and diverted to the CD4(+) cDC lineage.

CRTAM controls residency of gut CD4+CD8+ T cells in the steady state and maintenance of gut CD4+ Th17 during parasitic infection

Interactions between cell adhesion molecules CRTAM and Cadm1 regulate the residency and maintenance of CD4⁺CD8⁺ and CD4⁺ T cells in the gut that can influence the immune response to infection.

Retention of lymphocytes in the intestinal mucosa requires specialized chemokine receptors and adhesion molecules. We find that both CD4⁺CD8⁺ and CD4⁺ T cells in the intestinal epithelium, as well as CD8⁺ T cells in the intestinal mucosa and mesenteric lymph nodes, express the cell adhesion molecule class I–restricted T cell–associated molecule (Crtam) upon activation, whereas the ligand of Crtam, cell adhesion molecule 1 (Cadm1), is expressed on gut CD103⁺DCs. Lack of Crtam–Cadm1 interactions in Crtam^−/− and Cadm1^−/− mice results in loss of CD4⁺CD8⁺ T cells, which arise from mucosal CD4⁺ T cells that acquire a CD8 lineage expression profile. After acute oral infection with Toxoplasma gondii, both WT and Crtam^−/− mice mounted a robust TH1 response, but markedly fewer TH17 cells were present in the intestinal mucosa of Crtam^−/− mice. The almost exclusive TH1 response in Crtam^−/− mice resulted in more efficient control of intestinal T. gondii infection. Thus, Crtam–Cadm1 interactions have a major impact on the residency and maintenance of CD4⁺CD8⁺ T cells in the gut mucosa in the steady state. During pathogenic infection, Crtam–Cadm1 interactions regulate the dynamic equilibrium between newly formed CD4⁺ T cells and their retention in the gut, thereby shaping representation of disparate CD4⁺ T cell subsets and the overall quality of the CD4⁺ T cell response.

BAFF-R expression correlates with positive selection of immature B cells

The interaction between BAFF and BAFF-R is crucial for the development of mature B cells. Here, we report that the expression of BAFF-R is first detectable on a fraction of mouse CD19(+) CD93(+) IgM(+) CD23(-) and human CD19(+) CD10(+) IgM(+) BM B cells. This BAFF-R(+) BM B-cell population shows higher levels of surface IgM expression and decreased RAG-2 transcripts than BAFF-R(-) immature B cells. When cultured, mouse BAFF-R(-), but not BAFF-R(+) immature B cells spontaneously undergo B-cell receptor editing. However, BAFF-R(+) immature B cells cultured in the presence of an anti-κ light chain antibody are induced to undergo receptor editing. This receptor editing correlates with down-modulation of surface BAFF-R expression and the up-regulation of RAG-2 at the RNA level. B-cell receptor (BCR) cross-linking on splenic T1 B cells results in down-modulation of the BAFF-R, and receptor editing and RAG-2 up-regulation in a minor fraction of B cells. BCR cross-linking on splenic T2/3 B cells results in partly down and partly up-modulation of BAFF-R expression and no evidence for receptor editing. Overall, our data indicate that BAFF-R expression is tightly regulated during B-cell development in mouse and human and its expression is correlated with positive selection.

The preTCR-dependent DN3 to DP transition requires Notch signaling, is improved by CXCL12 signaling and is inhibited by IL-7 signaling

The requirement for Notch signaling during T-cell development has been extensively studied. Nevertheless, the developmental stage at which it is required and whether additional signaling pathways are needed are still poorly understood. By using a stromal-cell-free culture system, we show that sorted double-negative 3 (DN3) thymocytes only require a Delta-like-4-induced Notch signal to differentiate into double-positive (DP) cells. This differentiation process is preTCR-α dependent. DN3 cells undergo 4-5 proliferation cycles, and the addition of the chemokine CXCL12 improves proliferation. IL-7 blocks the differentiation of DN3 cells to DP cells but not the Notch-induced proliferation of cultured DN3 cells. The impaired differentiation correlates with an inhibition of Rag-2 up-regulation. Overall, the in vitro stromal-cell-free culture system presented here also provides a powerful and unique tool for studying the mechanisms involved in the positive and negative selection of T cells.

Tolerance checkpoints in B-cell development: Johnny B good

B-cell development up to the immature B-cell stage takes place in the bone marrow, while final maturation into mature B cells occurs in the spleen. During differentiation, the precursor and immature B cells have to pass several checkpoints, including those in which they are censored for being auto-reactive, and therefore being potentially dangerous. Numerous studies have shown that the immature B-cell stage in the bone marrow and the transitional B-cell stages in the spleen comprise distinct checkpoints at which auto-reactivity is censored. Recently, evidence has been provided that the large pre-BII stage in the bone marrow, at which the pre-BCR is expressed, is yet another B-cell tolerance checkpoint. Here, we review these findings and speculate on directions for possible further experimentation.

Crucial role for BAFF-BAFF-R signaling in the survival and maintenance of mature B cells

Defects in the expression of either BAFF (B cell activating factor) or BAFF-R impairs B cell development beyond the immature, transitional type-1 stage and thus, prevents the formation of follicular and marginal zone B cells, whereas B-1 B cells remain unaffected. The expression of BAFF-R on all mature B cells might suggest a role for BAFF-R signaling also for their in vivo maintenance. Here, we show that, 14 days following a single injection of an anti-BAFF-R mAb that prevents BAFF binding, both follicular and marginal zone B cell numbers are drastically reduced, whereas B-1 cells are not affected. Injection of control, isotype-matched but non-blocking anti-BAFF-R mAbs does not result in B cell depletion. We also show that this depletion is neither due to antibody-dependent cellular cytotoxicity nor to complement-mediated lysis. Moreover, prevention of BAFF binding leads to a decrease in the size of the B cell follicles, an impairment of a T cell dependent humoral immune response and a reduction in the formation of memory B cells. Collectively, these results establish a central role for BAFF-BAFF-R signaling in the in vivo survival and maintenance of both follicular and marginal zone B cell pools.

CD34+ cord blood cell-transplanted Rag2-/- gamma(c)-/- mice as a model for Epstein-Barr virus infection

Recent studies suggest that Epstein-Barr virus (EBV) can infect naïve B cells, driving them to differentiate into resting memory B cells via the germinal center reaction. This hypothesis has been inferred from parallels with the biology of normal B cells but has never been proven experimentally. Rag2(-/-) gamma(c)(-/-) mice that were transplanted with human CD34(+) cord blood cells as newborns were recently shown to develop human B, T, and dendritic cells, constituting lymphoid organs in situ. Here we used this model to better define the strategy of EBV infection of human B cells in vivo and to compare this model system with different conditions of EBV infection in humans. Our results support the model of EBV persistence in vivo in cases that were characterized by follicular hyperplasia and a relatively normal CD4(+) and CD8(+) T-cell distribution. Intriguingly, in cases that were characterized by nodular and diffuse proliferation with a preponderance of CD8(+) T cells, similar to infectious mononucleosis, EBV still infects naïve B cells but also induces clonal expansion and ongoing somatic mutations without germinal center reactions. Our results reveal different strategies of EBV infection in B cells that possibly result from variations in the host immune response. Future experiments might allow understanding of the mechanisms responsible for persistent EBV infection and provide targets for more highly tailored therapeutic interventions.

Human adaptive immune system Rag2-/-gamma(c)-/- mice

Although many biologic principles are conserved in mice and humans, species-specific differences exist, for example, in susceptibility and response to pathogens, that often do not allow direct implementation of findings in experimental mice to humans. Research in humans, however, for ethical and practical reasons, is largely restricted to in vitro assays that lack components and the complexity of a living organism. To nevertheless study the human hematopoietic and immune system in vivo, xenotransplantation assays have been developed that substitute human components to small animals. Here, we summarize our recent findings that transplantation of human cord blood CD34(+) cells to newborn Rag2(-/-)gamma(c)(-/-) mice leads to de novo development of major functional components of the human adaptive immune system. These human adaptive immune system Rag2(-/-)gamma(c)(-/-) (huAIS-RG) mice can now be used as a technically straightforward preclinical model to evaluate in vivo human adaptive immune system development as well as immune responses, for example, to vaccines or live infectious pathogens.

Disseminated and sustained HIV infection in CD34+ cord blood cell-transplanted Rag2-/-gamma c-/- mice

Because of species selectivity, HIV research is largely restricted to in vitro or clinical studies, both limited in their ability to rapidly assess new strategies to fight the virus. To prospectively study some aspects of HIV in vivo, immunodeficient mice, transplanted with either human peripheral blood leukocytes or human fetal tissues, have been developed. Although these are susceptible to HIV infection, xenoreactivity, and short infection spans, resource and ethical constraints, as well as biased HIV coreceptor tropic strain infection, pose substantial problems in their use. Rag2(-/-)gamma(c)(-/-) mice, transplanted as newborns with human CD34(+) cells, were recently shown to develop human B, T, and dendritic cells, constituting lymphoid organs in situ. Here we tested these mice as a model system for HIV-1 infection. HIV RNA levels peaked to up to 2 x 10(6) copies per milliliter of plasma early after infection, and viremia was observed for up to 190 days, the longest time followed. A marked relative CD4(+) T cell depletion in peripheral blood occurred in CXCR4-tropic strain-infected mice, whereas this was less pronounced in CCR5-tropic strain-infected animals. Thymus infection was almost exclusively observed in CXCR4-tropic strain-infected mice, whereas spleen and lymph node HIV infection occurred irrespective of coreceptor selectivity, consistent with respective coreceptor expression on human CD4(+) T cells. Thus, this straightforward to generate and cost-effective in vivo model closely resembles HIV infection in man and therefore should be valuable to study virus-induced pathology and to rapidly evaluate new approaches aiming to prevent or treat HIV infection.

Activation of the Flt3 signal transduction cascade rescues and enhances type I interferon-producing and dendritic cell development

Flt3 ligand (Flt3L) is a nonredundant cytokine in type I interferon–producing cell (IPC) and dendritic cell (DC) development, and IPC and DC differentiation potential is confined to Flt3⁺ hematopoietic progenitor cells. Here, we show that overexpression of human Flt3 in Flt3⁻ (Flt3⁻Lin⁻IL-7Rα⁻Thy1.1⁻c-Kit⁺) and Flt3⁺ (Flt3⁺Lin⁻IL-7Rα⁻Thy1.1⁻c-Kit⁺) hematopoietic progenitors rescues and enhances their IPC and DC differentiation potential, respectively. In defined hematopoietic cell populations, such as Flt3⁻ megakaryocyte/erythrocyte-restricted progenitors (MEPs), enforced Flt3 signaling induces transcription of IPC, DC, and granulocyte/macrophage (GM) development–affiliated genes, including STAT3, PU.1, and G-/M-/GM-CSFR, and activates differentiation capacities to these lineages. Moreover, ectopic expression of Flt3 downstream transcription factors STAT3 or PU.1 in Flt3⁻ MEPs evokes Flt3 receptor expression and instructs differentiation into IPCs, DCs, and myelomonocytic cells, whereas GATA-1 expression and consecutive megakaryocyte/erythrocyte development is suppressed. Based on these data, we propose a demand-regulated, cytokine-driven DC and IPC regeneration model, in which high Flt3L levels initiate a self-sustaining, Flt3-STAT3– and Flt3-PU.1–mediated IPC and DC differentiation program in Flt3⁺ hematopoietic progenitor cells.

Inhibition of natural type I IFN-producing and dendritic cell development by a small molecule receptor tyrosine kinase inhibitor with Flt3 affinity

In vivo steady-state type I natural IFN-producing and dendritic cell (DC) development is largely dependent on Flt3 signaling. Natural IFN-producing and DC progenitors and their respective downstream cell populations express the flt3 receptor, and Flt3 ligand (Flt3L)(-/-) mice have reduced while Flt3L-injected mice develop markedly increased numbers of both cell types. In the present study, we show that SU11657, a small multitargeted receptor tyrosine kinase inhibitor with Flt3 affinity, suppressed in vitro natural IFN-producing and DC development in Flt3L-supplemented mouse whole bone marrow cell cultures in a dose-dependant manner, while DC development in GM-CSF-supplemented cultures was not affected. In vivo SU11657 application led to a significant decrease of both natural IFN-producing and DCs, comparable to the reduction observed in Flt3L(-/-) mice. Conversely, Flt3L plasma levels increased massively in inhibitor-treated animals, likely via a regulatory feedback loop, without being able to compensate for pharmacological Flt3 inhibition. No obvious toxicity was observed, and hemopoietic progenitor cell and stem cell function remained intact as assessed by myeloid colony-forming unit activity and in vivo bone marrow repopulation assays. Furthermore, upon treatment discontinuation, IFN-producing and DCs recovered to normal levels, proving that treatment effects were transient. Given the importance of IFN-producing and DCs in regulation of immune responses, these findings might lead to new pharmacological strategies in prevention and treatment of autoimmune diseases and complications of organ or blood cell transplantation.

Identification of preleukemic precursors of hyperdiploid acute lymphoblastic leukemia in cord blood

Previous studies involving identical twins with concordant leukemia and retrospective scrutiny of archived neonatal blood spots have shown that common chromosome translocations of pediatric leukemia frequently arise before birth. The IGH/TCR clonotypic sequences used as surrogate molecular markers suggest this is also likely to be true for hyperdiploid acute lymphoblastic leukemia (ALL). Yet evidence that hyperdiploidy itself is an early or initiating event occurring prenatally has been limited. Now, however, we can provide direct evidence of this from our identification of CD34+/CD19+ B-lineage progenitor cells with triploid chromosomes in the stored cord blood of an individual who subsequently developed hyperdiploid ALL.