Jan Fric

NFAT control of innate immunity

The calcineurin/nuclear factor of activated T cells (NFAT) signaling pathway mediates multiple adaptive T-cell functions, but recent studies have shown that calcineurin/NFAT signaling also contributes to innate immunity and regulates the homeostasis of innate cells. Myeloid cells, including granulocytes and dendritic cells, can promote inflammation, regulate adaptive immunity, and are essential mediators of early responses to pathogens. Microbial ligation of pattern-recognition receptors, such as TLR4, CD14, and dectin 1, is now known to induce the activation of calcineurin/NFAT signaling in myeloid cells, a finding that has provided new insights into the molecular pathways that regulate host protection. Inhibitors of calcineurin/NFAT binding, such as cyclosporine A and FK506, are broadly used in organ transplantation and can act as potent immunosuppressive drugs in a variety of different disorders. There is increasing evidence that these agents influence innate responses as well as inhibiting adaptive T-cell functions. This review focuses on the role of calcineurin/NFAT signaling in myeloid cells, which may contribute to the various unexplained effects of immunosuppressive drugs already being used in the clinic.

Use of Human Monoclonal Antibodies to Treat Chikungunya Virus Infection

Chikungunya virus (CHIKV) is an alphavirus prevalent in tropical regions. It causes an acute febrile disease that, in elderly individuals and newborns, is often associated with severe complications. We previously reported the isolation and characterization of 2 human monoclonal antibodies neutralizing CHIKV in vitro: 5F10 and 8B10. Here, we tested their efficacy in vivo as prophylactic and therapeutic treatments of CHIKV infection in AGR129 mice. In both settings, 5F10 and 8B10 were able to significantly delay CHIKV-driven lethality. Our results support the development of prophylactic and therapeutic treatments for CHIKV infection, using a combination of 5F10 and 8B10.

CD103+ Dendritic Cells Control Th17 Cell Function in the Lung

Th17 cells express diverse functional programs while retaining their Th17 identity, in some cases exhibiting a stem-cell-like phenotype. Whereas the importance of Th17 cell regulation in autoimmune and infectious diseases is firmly established, the signaling pathways controlling their plasticity are undefined. Using a mouse model of invasive pulmonary aspergillosis, we found that lung CD103(+) dendritic cells (DCs) would produce IL-2, dependent on NFAT signaling, leading to an optimally protective Th17 response. The absence of IL-2 in DCs caused unrestrained production of IL-23 and fatal hyperinflammation, which was characterized by strong Th17 polarization and the emergence of a Th17 stem-cell-like population. Although several cell types may be affected by deficient IL-2 production in DCs, our findings identify the balance between IL-2 and IL-23 productions by lung DCs as an important regulator of the local inflammatory response to infection.

Calcineurin/NFAT signalling inhibits myeloid haematopoiesis

Nuclear factor of activated T cells (NFAT) comprises a family of transcription factors that regulate T cell development, activation and differentiation. NFAT signalling can also mediate granulocyte and dendritic cell (DC) activation, but it is unknown whether NFAT influences their development from progenitors. Here, we report a novel role for calcineurin/NFAT signalling as a negative regulator of myeloid haematopoiesis. Reconstituting lethally irradiated mice with haematopoietic stem cells expressing an NFAT‐inhibitory peptide resulted in enhanced development of the myeloid compartment. Culturing bone marrow cells in media supplemented with Flt3‐L in the presence of the calcineurin/NFAT inhibitor Cyclosporin A increased numbers of differentiated DC. Global gene expression analysis of untreated DC and NFAT‐inhibited DC revealed differential expression of transcripts that regulate cell cycle and apoptosis. In conclusion, these results provide evidence that calcineurin/NFAT signalling negatively regulates myeloid lineage development. The finding that inhibition of NFAT enhances myeloid development provides a novel insight into understanding how the treatment with drugs targeting calcineurin/NFAT signalling influence the homeostasis of the innate immune system.

Phagocytosis of Particulate Antigens – All Roads Lead to Calcineurin/NFAT Signaling Pathway

Antigen-presenting cells (APC) possess multiple cell surface receptors that recognize common microbe-associated antigens as well as immune complexes and inert particles. Upon encountering such antigens these receptors must cooperate to achieve phagocytosis and trigger signaling cascades that initiate the innate immune response. While the stimuli initiating these signaling cascades are diverse, recent data have revealed that their effects in APC and particularly in dendritic cells (DC), all have something in common: downstream activation of nuclear factor of activated T cells (NFAT). NFAT is a family of transcription factors that has emerged as a key mediator of the initiation of immune responses by APCs, and specifically of IL-2 production by DC as reviewed (1). Intriguingly NFAT activation now seems to be the shared endpoint of several signaling pathways that all begin with uptake of particulate antigens. Notably, the NFAT family members appeared at the origin of vertebrates whereas nearly all the other signaling pathways, including NFκB pathway, are very ancient and present in all invertebrate’s species. NFAT signaling plays essential roles in vertebrate organogenesis and development but also in the formation of adaptive immunity. In addition, G. R. Crabtree has suggested that “NFAT may have contributed to the evolutionary adaptation of innate immunity: e.g., minimize the costs of inflammation by collaboration with adaptive immunity” (2). It is likely that the ability of DC to link innate with adaptive immunity might be the result of DC’s ability to couple phagocytic functions to NFAT activation, leading to extensive gene reprograming. This is the latest in a series of new hypothesis to better understand of the true complexity of the process of pathogen sensing, uptake, and response in APC and in particular in DC. But as is so often the case, with new hypotheses and knowledge has come new questions: how can such diverse stimuli all converge on similar pathways of immune activation? How do APCs integrate signaling from multiple immune uptake receptors? And how can we explain the difference in APC responses to soluble and particulate antigens? In this article we will review the recent steps forward in our understanding of the intricate cross-talk between pathways of phagocytosis and immune signaling in APC, and the evidence that NFAT activation is a unique hallmark of this process.

The need to identify myeloid dendritic cell progenitors in human blood

Dendritic cells (DC) are professional phagocytes possessing a unique ability to sense perturbations in the tissue microenvironment and promote adaptive immune responses, whilst maintaining immunological tolerance. Mouse myeloid DC progenitors with the ability to migrate through the blood and replenish the DC pool have been identified in bone marrow but the ontogeny of human DC is poorly understood. Access to lymphoid tissues for human DC isolation is severely limited and researchers have resorted to the use of in vitro derivation systems in attempts to understand DC development, which may result in misleading conclusions. The identification of a human DC progenitor in blood would greatly enhance the understanding of DC homeostasis and their role in pathogenesis.

Impaired calcineurin signaling in myeloid cells results in downregulation of pentraxin-3 and increased susceptibility to aspergillosis.

Treatment of post-transplant patients with immunosuppressive drugs targeting the calcineurin–nuclear factor of activated T cells (NFAT) pathway, including cyclosporine A or tacrolimus, is commonly associated with a higher incidence of opportunistic infections, such as Aspergillus fumigatus, which can lead to severe life-threatening conditions. A component of the A. fumigatus cell wall, β-glucan, is recognized by dendritic cells (DCs) via the Dectin-1 receptor, triggering downstream signaling that leads to calcineurin–NFAT binding, NFAT translocation, and transcription of NFAT-regulated genes. Here, we address the question of whether calcineurin signaling in CD11c-expressing cells, such as DCs, has a specific role in the innate control of A. fumigatus. Impairment of calcineurin in CD11c-expressing cells (CD11ccrecnb1loxP) significantly increased susceptibility to systemic A. fumigatus infection and to intranasal infection in irradiated mice undergoing bone marrow transplant. Global expression profiling of bone marrow-derived DCs identified calcineurin-regulated processes in the immune response to infection, including expression of pentraxin-3, an important antifungal defense protein. These results suggest that calcineurin inhibition directly impairs important immunoprotective functions of myeloid cells, as shown by the higher susceptibility of CD11ccrecnbloxP mice in models of systemic and invasive pulmonary aspergillosis, including after allogeneic bone marrow transplantation. These findings are relevant to the clinical management of transplant patients with severe Aspergillus infections.

The IL-17F/IL-17RC Axis Promotes Respiratory Allergy in the Proximal Airways

The interleukin 17 (IL-17) cytokine and receptor family is central to antimicrobial resistance and inflammation in the lung. Mice lacking IL-17A, IL-17F, or the IL-17RA subunit were compared with wild-type mice for susceptibility to airway inflammation in models of infection and allergy. Signaling through IL-17RA was required for efficient microbial clearance and prevention of allergy; in the absence of IL-17RA, signaling through IL-17RC on epithelial cells, predominantly by IL-17F, significantly exacerbated lower airway Aspergillus or Pseudomonas infection and allergic airway inflammation. In contrast, following infection with the upper respiratory pathogen Staphylococcus aureus, the IL-17F/IL-17RC axis mediated protection. Thus, IL-17A and IL-17F exert distinct biological effects during pulmonary infection; the IL-17F/IL-17RC signaling axis has the potential to significantly worsen pathogen-associated inflammation of the lower respiratory tract in particular, and should be investigated further as a therapeutic target for treating pathological inflammation in the lung.

Chronic Inflammation in Immune Aging: Role of Pattern Recognition Receptor Crosstalk with the Telomere Complex?

Age-related decline in immunity is characterized by stem cell exhaustion, telomere shortening, and disruption of cell-to-cell communication, leading to increased patient risk of disease. Recent data have demonstrated that chronic inflammation exerts a strong influence on immune aging and is closely correlated with telomere length in a range of major pathologies. The current review discusses the impact of inflammation on immune aging, the likely molecular mediators of this process, and the various disease states that have been linked with immunosenescence. Emerging findings implicate NF-κB, the major driver of inflammatory signaling, in several processes that regulate telomere maintenance and/or telomerase activity. While prolonged triggering of pattern recognition receptors is now known to promote immunosenescence, it remains unclear how this process is linked with the telomere complex or telomerase activity. Indeed, enzymatic control of telomere length has been studied for many decades, but alternative roles of telomerase and potential influences on inflammatory responses are only now beginning to emerge. Crosstalk between these pathways may prove to be a key molecular mechanism of immunosenescence. Understanding how components of immune aging interact and modify host protection against pathogens and tumors will be essential for the design of new vaccines and therapies for a wide range of clinical scenarios.

Calcium and calcineurin-NFAT signaling regulate granulocyte-monocyte progenitor cell cycle via Flt3-L.

Maintenance of myeloid progenitor cells is controlled by complex regulatory mechanisms and is orchestrated by multiple different transcription factors. Here, we report that the activation of the transcription factor nuclear factor of activated T cells (NFAT) by calcium‐sensing protein calcineurin inhibits the proliferation of myeloid granulocyte–monocyte progenitors (GMPs). Myeloid progenitor subtypes exhibit variable sensitivity to induced Ca2+ entry and consequently display differential engagement of the calcineurin‐NFAT pathway. This study shows that inhibition of the calcineurin‐NFAT pathway enhances the proliferation of GMPs both in vitro and in vivo and demonstrates that calcineurin‐NFAT signaling in GMPs is initiated by Flt3‐L. Inhibition of the calcineurin‐NFAT pathway modified expression of the cell cycle regulation genes Cdk4, Cdk6, and Cdkn1a (p21), thus enabling rapid cell cycle progression specifically in GMPs. NFAT inhibitor drugs are extensively used in the clinic to restrict the pathological activation of lymphoid cells, and our data reveal for the first time that these therapies also exert potent effects on maintenance of the myeloid cell compartment through specific regulation of GMP proliferation. Stem Cells 2014;32:3232–3244