Complement receptor Mac-1 is an adaptor for NB1 (CD177)-mediated PR3-ANCA neutrophil activation

Folate Receptor β Regulates Integrin CD11b/CD18 Adhesion of a Macrophage Subset to Collagen

Folate, also known as vitamin B9, is necessary for essential cellular functions such as DNA synthesis, repair, and methylation. It is supplied to the cell via several transporters and receptors, including folate receptor (FR) β, a GPI-anchored protein belonging to the folate receptor family. As FRβ shows a restricted expression to cells of myeloid origin and only a subset of activated macrophages and placental cells have been shown to express functional FRβ, it represents a promising target for future therapeutic strategies. In this study, we performed affinity purification and mass spectrometric analysis of the protein microenvironment of FRβ in the plasma membrane of human FRβ(+) macrophages and FRβ-transduced monocytic THP-1 cells. In this manner, we identified a novel role of FRβ: that is, we report functional interactions of FRβ with receptors mediating cellular adhesion, in particular the CD11b/CD18 β2 integrin heterodimer complement receptor type 3/Mac-1. This interaction results in impeded adhesion of FRβ(+) human primary macrophages and THP-1 cells to collagen in comparison with their FRβ(-) counterparts. We further show that FRβ is only expressed by human macrophages when differentiated with M-CSF. These findings thus identify FRβ as a novel CD11b/CD18 regulator for trafficking and homing of a subset of macrophages on collagen.

Lessons from a double-transgenic neutrophil approach to induce antiproteinase 3 antibody-mediated vasculitis in mice

ANCA to either PR3 or MPO are found in patients with necrotizing vasculitis and glomerulonephritis. ANCA binding to their target antigens on neutrophils and subsequent neutrophil activation are pivotal disease mechanisms that lead to vascular inflammation and necrosis. ANCA interaction with PR3 is more complex than with MPO as the neutrophil-specific CD177 receptor is involved in PR3 surface expression and PR3-ANCA-induced neutrophil activation. Modeling human disease is important to clinical research. Highly successful mouse models of MPO-ANCA vasculitis exist; however, recapitulating PR3-ANCA vasculitis has not been successful. We generated double-transgenic (DT) mice that expressed human PR3 and CD177 under a myeloid-specific huMRP8 promoter in an attempt to model PR3-ANCA vasculitis. DT mice strongly expressed the human transgenes in and on murine neutrophils and bound murine and human anti-PR3 antibodies. Nevertheless, passive transfer of these antibodies into LPS-primed DT mice or immunization of C57BL/6 mice with human PR3 followed by irradiation and transplantation of DT bone marrow failed to induce glomerulonephritis. Further analyses revealed that anti-PR3 antibodies did not activate DT neutrophils as shown by superoxide generation. Moreover, we found that mice did not properly process human pro-PR3 into mature PR3 and, consequently, the signaling complex between PR3, CD177, and CD11b, which promotes neutrophil activation by anti-PR3 antibodies, failed to form. We conclude that important species differences in PR3 and CD177 exist between men and mice that prevented successful generation of a murine anti-PR3 antibody model.

The role of neutrophils in causing antineutrophil cytoplasmic autoantibody-associated vasculitis

PURPOSE OF REVIEW

Antineutrophil cytoplasmic antibody (ANCA)-activated phagocytes cause vasculitis and necrotizing crescentic glomerulonephritis. Experimental data support the notion that activation of neutrophils and monocytes by ANCA immunoglobulin G with generation of reactive oxygen species, degranulation of proteases, and formation of neutrophil extracellular traps play a role in tissue injury.

RECENT FINDINGS

We discuss novel findings regarding the expression of ANCA antigens and the mechanisms involved in myeloid cell activation by ANCA immunoglobulin G. The contribution of neutrophil serine proteases and their specific role in the generation of interleukin-1beta (IL-1β) is highlighted. ANCA-induced reactive oxygen species generation plays an important role in downregulating inflammation by inhibition of the inflammasome-dependent caspase-1 activation and subsequent IL-1β generation. Neutrophil extracellular trap generation by ANCA-activated neutrophils and their potential role in the pathogenesis of the disease will be discussed. Lastly, the pathogenic role of the complement system will be discussed.

SUMMARY

ANCA-induced activation of both neutrophils and monocytes is one of the main pathogenic mechanisms involved in disease induction. Therefore, a better understanding of the fundamental processes involved here are necessary. Specifically, the mechanisms involved in IL-1β generation have been recently identified and could lead to better targeted novel therapies.

Intermediate monocytes in ANCA vasculitis: increased surface expression of ANCA autoantigens and IL-1β secretion in response to anti-MPO antibodies

ANCA vasculitis encompasses several autoimmune conditions characterised by destruction of small vessels, inflammation of the respiratory tract and glomerulonephritis. Most patients harbour autoantibodies to myeloperoxidase (MPO) or proteinase 3 (PR3). Clinical and experimental data suggest that pathogenesis is driven by ANCA-mediated activation of neutrophils and monocytes. We investigated a potential role for distinct monocyte subsets. We found that the relative proportion of intermediate monocytes is increased in patients versus control individuals, and both MPO and PR3 are preferentially expressed on these cells. We demonstrate that MPO and PR3 are expressed independently of each other on monocytes and that PR3 is not associated with CD177. MPO expression correlates with that of Fc receptor CD16 on intermediate monocytes. Monocyte subsets respond differently to antibodies directed against MPO and PR3, with anti-MPO but not anti-PR3 leading to increased IL-1β, IL-6 and IL-8 production. In concordance with the observed higher surface expression of MPO on intermediate monocytes, this subset produces the highest quantity of IL-1β in response to anti-MPO stimulation. These data suggest that monocytes, specifically, the intermediate subset, may play a role in ANCA vasculitis, and also indicate that substantial differences exist between the effect of anti-MPO and anti-PR3 antibodies on these cells.

Genetic mechanism of human neutrophil antigen 2 deficiency and expression variations

Human neutrophil antigen 2 (HNA-2) deficiency is a common phenotype as 3-5% humans do not express HNA-2. HNA-2 is coded by CD177 gene that associates with human myeloproliferative disorders. HNA-2 deficient individuals are prone to produce HNA-2 alloantibodies that cause a number of disorders including transfusion-related acute lung injury and immune neutropenia. In addition, the percentages of HNA-2 positive neutrophils vary significantly among individuals and HNA-2 expression variations play a role in human diseases such as myelodysplastic syndrome, chronic myelogenous leukemia, and gastric cancer. The underlying genetic mechanism of HNA-2 deficiency and expression variations has remained a mystery. In this study, we identified a novel CD177 nonsense single nucleotide polymorphism (SNP 829A>T) that creates a stop codon within the CD177 coding region. We found that all 829TT homozygous individuals were HNA-2 deficient. In addition, the SNP 829A>T genotypes were significantly associated with the percentage of HNA-2 positive neutrophils. Transfection experiments confirmed that HNA-2 expression was absent on cells expressing the CD177 SNP 829T allele. Our data clearly demonstrate that the CD177 SNP 829A>T is the primary genetic determinant for HNA-2 deficiency and expression variations. The mechanistic delineation of HNA-2 genetics will enable the development of genetic tests for diagnosis and prognosis of HNA-2-related human diseases.

Characterization of a novel mouse model with genetic deletion of CD177

Neutrophils play an essential role in the innate immune response to infection. Neutrophils migrate from the vasculature into the tissue in response to infection. Recently, a neutrophil cell surface receptor, CD177, was shown to help mediate neutrophil migration across the endothelium through interactions with PECAM1. We examined a publicly available gene array dataset of CD177 expression from human neutrophils following pulmonary endotoxin instillation. Among all 22,214 genes examined, CD177 mRNA was the most upregulated following endotoxin exposure. The high level of CD177 expression is also maintained in airspace neutrophils, suggesting a potential involvement of CD177 in neutrophil infiltration under infectious diseases. To determine the role of CD177 in neutrophils in vivo, we constructed a CD177-genetic knockout mouse model. The mice with homozygous deletion of CD177 have no discernible phenotype and no significant change in immune cells, other than decreased neutrophil counts in peripheral blood. We examined the role of CD177 in neutrophil accumulation using a skin infection model with Staphylococcus aureus. CD177 deletion reduced neutrophil counts in inflammatory skin caused by S. aureus. Mechanistically we found that CD177 deletion in mouse neutrophils has no significant impact in CXCL1/KC- or fMLP-induced migration, but led to significant cell death. Herein we established a novel genetic mouse model to study the role of CD177 and found that CD177 plays an important role in neutrophils.

Immunomodulation with eicosapentaenoic acid supports the treatment of autoimmune small-vessel vasculitis

Small-vessel vasculitis is a life-threatening autoimmune disease that is frequently associated with anti-neutrophil cytoplasmic antibodies (ANCAs). Conventional immunotherapy including steroids and cyclophosphamide can cause serious adverse events, limiting the efficacy and safety of treatment. Eicosapentaenoic acid (EPA), a key component of fish oil, is an omega-3 polyunsaturated fatty acid widely known to be cardioprotective and beneficial for vascular function. We report two elderly patients with systemic ANCA-associated vasculitis (AAV) in whom the administration of EPA in concert with steroids safely induced and maintained remission, without the use of additioal immunosuppressants. To explore the mechanisms by which EPA enhances the treatment of AAV, we employed SCG/Kj mice as a spontaneous murine model of AAV. Dietary enrichment with EPA significantly delayed the onset of crescentic glomerulonephritis and prolonged the overall survival. EPA-derived anti-inflammatory lipid mediators and their precursors were present in the kidney, plasma, spleen, and lungs in the EPA-treated mice. Furthermore, a decrease in ANCA production and CD4/CD8-double negative T cells, and an increase in Foxp3(+) regulatory T cells in the lymph nodes of the kidney were observed in the EPA-treated mice. These clinical and experimental observations suggest that EPA can safely support and augment conventional therapy for treating autoimmune small-vessel vasculitis.

Anti-human neutrophil antigen-3a induced transfusion-related acute lung injury in mice by direct disturbance of lung endothelial cells

OBJECTIVE

Antibodies against human neutrophil antigen-3a (HNA-3a) located on choline transporter-like protein 2 induce severe transfusion-related acute lung injury (TRALI). This study aims to identify the mechanism implicated in anti-HNA-3a-mediated TRALI.

APPROACH AND RESULTS

Our analysis shows that anti-HNA-3a recognizes 2 choline transporter-like protein 2 isoforms (P1 and P2) on human microvascular endothelial cells from lung blood vessels but reacts only with the P1 isoform on neutrophils. Direct treatment of HNA-3a-positive endothelial cells with anti-HNA-3a, but not with anti-HNA-3b, leads to reactive oxygen species production, increased albumin influx, and decreased endothelial resistance associated with the formation of actin stress filaments and loosening of junctional vascular endothelium-cadherin. In a novel in vivo mouse model, TRALI was documented by significant increase in lung water content, albumin concentration, and neutrophil numbers in the bronchoalveolar lavage on injection of human anti-HNA-3a in lipopolysaccharides-treated, as well as nontreated mice. Interestingly, although neutrophil depletion alleviated severity of lung injury, it failed to prevent TRALI in this model. Infusion of anti-HNA-3a F(ab')2 fragments caused moderate TRALI. Finally, mice lacking nicotinamide adenine dinucleotide phosphate oxidase (NOX2(y/-)) were protected from anti-HNA-3a-mediated TRALI.

CONCLUSIONS

These data demonstrate the initiation of endothelial barrier dysfunction in vitro and in vivo by direct binding of anti-HNA-3a on endothelial cells. It seems, however, that the presence of neutrophils aggravates barrier dysfunction. This novel mechanism of TRALI primarily mediated by endothelial cell dysfunction via choline transporter-like protein 2 may help to define new treatment strategies to decrease TRALI-related mortality.

The human neutrophil subsets defined by the presence or absence of OLFM4 both transmigrate into tissue in vivo and give rise to distinct NETs in vitro

Neutrophil heterogeneity was described decades ago, but it could not be elucidated at the time whether the existence of different neutrophil subsets had any biological relevance. It has been corroborated in recent years that neutrophil subsets, defined by differential expression of various markers, are indeed present in human blood, calling for renewed attention to this question. The expression of the granule protein olfactomedin 4 (OLFM4) has been suggested to define two such neutrophil subsets. We confirm the simultaneous presence of one OLFM4-positive and one OLFM4-negative neutrophil subpopulation as well as the localization of the protein to specific granules. In vitro, these neutrophil subsets displayed equal tendency to undergo apoptosis and phagocytose bacteria. In addition, the subpopulations were recruited equally to inflammatory sites in vivo, and this was true both in an experimental model of acute inflammation and in naturally occurring pathological joint inflammation. In line with its subcellular localization, only limited OLFM4 release was seen upon in vivo transmigration, and release through conventional degranulation required strong secretagogues. However, extracellular release of OLFM4 could be achieved upon formation of neutrophil extracellular traps (NETs) where it was detected only in a subset of the NETs. Although we were unable to demonstrate any functional differences between the OLFM4-defined subsets, our data show that different neutrophil subsets are present in inflamed tissue in vivo. Furthermore, we demonstrate NETs characterized by different markers for the first time, and our results open up for functions of OLFM4 itself in the extracellular space through exposure in NETs.

ATG4B/autophagin-1 regulates intestinal homeostasis and protects mice from experimental colitis

The identification of inflammatory bowel disease (IBD) susceptibility genes by genome-wide association has linked this pathology to autophagy, a lysosomal degradation pathway that is crucial for cell and tissue homeostasis. Here, we describe autophagy-related 4B, cysteine peptidase/autophagin-1 (ATG4B) as an essential protein in the control of inflammatory response during experimental colitis. In this pathological condition, ATG4B protein levels increase in parallel with the induction of autophagy. Moreover, ATG4B expression is significantly reduced in affected areas of the colon from IBD patients. Consistently, atg4b (-/-) mice present Paneth cell abnormalities, as well as an increased susceptibility to DSS-induced colitis. atg4b-deficient mice exhibit significant alterations in proinflammatory cytokines and mediators of the immune response to bacterial infections, which are reminiscent of those found in patients with Crohn disease or ulcerative colitis. Additionally, antibiotic treatments and bone marrow transplantation from wild-type mice reduced colitis in atg4b (-/-) mice. Taken together, these results provided additional evidence for the importance of autophagy in intestinal pathologies and describe ATG4B as a novel protective protein in inflammatory colitis. Finally, we propose that atg4b-null mice are a suitable model for in vivo studies aimed at testing new therapeutic strategies for intestinal diseases associated with autophagy deficiency.

Ly6 family proteins in neutrophil biology

The murine Ly6 complex was identified 35 years ago using antisera to lymphocytes. With advances in mAb development, molecular cloning, and genome sequencing, >20 structurally related genes have been identified within this complex on chromosome 15. All members of the Ly6 family and their human homologues share the highly conserved LU domain and most also possess a GPI anchor. Interestingly, many Ly6 proteins are expressed in a lineage-specific fashion, and their expression often correlates with stages of differentiation. As a result, Ly6 proteins are frequently used as surface markers for leukocyte subset identification and targets for antibody-mediated depletion. Murine neutrophils display prominent surface expression of several Ly6 proteins, including Ly6B, Ly6C, and Ly6G. Although the physiology of most Ly6 proteins is not well understood, a role in neutrophil functions, such as migration, is recognized increasingly. In this review, we will provide an overview of the Ly6 complex and discuss, in detail, the specific Ly6 proteins implicated in neutrophil biology.

Extracellular protein disulfide isomerase regulates ligand-binding activity of αMβ2 integrin and neutrophil recruitment during vascular inflammation

β2 integrins play a crucial role during neutrophil recruitment into the site of vascular inflammation. However, it remains unknown how ligand-binding activity of the integrin is regulated. Using fluorescence intravital microscopy in mice generated by crossing protein disulfide isomerase (PDI) floxed mice with lysozyme-Cre transgenic mice, we demonstrate that neutrophil PDI is required for neutrophil adhesion and crawling during tumor necrosis factor-α-induced vascular inflammation in vivo. Rescue experiments show that the isomerase activity of extracellular PDI is critical for its regulatory effect on neutrophil recruitment. Studies with blocking anti-PDI antibodies and αLβ2 or αMβ2 null mice suggest that extracellular PDI regulates αMβ2 integrin-mediated adhesive function of neutrophils during vascular inflammation. Consistently, we show that neutrophil surface PDI is important for αMβ2 integrin-mediated adhesion of human neutrophils under shear and static conditions and for binding of soluble fibrinogen to activated αMβ2 integrin. Confocal microscopy and biochemical studies reveal that neutrophil surface PDI interacts with αMβ2 integrin in lipid rafts of stimulated neutrophils and regulates αMβ2 integrin clustering, presumably by changing the redox state of the integrin. Thus, our results provide the first evidence that extracellular PDI could be a novel therapeutic target for preventing and treating inappropriate neutrophil sequestration.

Neutrophils come of age in chronic inflammation

Neutrophils have long been known to participate in acute inflammation, but a role in chronic inflammatory and autoimmune diseases is now emerging. These cells are key players in the recognition and elimination of pathogens, but they also sense self components, including nucleic acids and products of sterile tissue damage. While this normally contributes to tissue repair, it can also lead to the release of highly immunogenic products that can trigger and/or amplify autoimmune pathogenic loops. Understanding the mechanisms that underlie neutrophil activation, migration, survival and their various forms of death in health and disease might provide us with new approaches to treat chronic inflammatory conditions.

Endothelium-neutrophil interactions in ANCA-associated diseases

The two salient features of ANCA-associated vasculitis (AAV) are the restricted microvessel localization and the mechanism of inflammatory damage, independent of vascular immune deposits. The microvessel localization of the disease is due to the ANCA antigen accessibility, which is restricted to the membrane of neutrophils engaged in β2-integrin-mediated adhesion, while these antigens are cytoplasmic and inaccessible in resting neutrophils. The inflammatory vascular damage is the consequence of maximal proinflammatory responses of neutrophils, which face cumulative stimulations by TNF-α, β2-integrin engagement, C5a, and ANCA by the FcγRII receptor. This results in the premature intravascular explosive release by adherent neutrophils of all of their available weapons, normally designed to kill IgG-opsonized bacteria after migration in infected tissues.

How anti-neutrophil cytoplasmic autoantibodies activate neutrophils

Neutrophils are pivotal to host defence during infectious diseases. However, activated neutrophils may also cause undesired tissue damage. Ample examples include small-vessel inflammatory diseases (vasculitis) that are associated with anti-neutrophil cytoplasmic autoantibodies (ANCA) residing in the patients' plasma. In addition to being an important diagnostic tool, convincing evidence shows that ANCA are pathogenic. ANCA-neutrophil interactions induce important cellular responses that result in highly inflammatory necrotizing vascular damage. The interaction begins with ANCA binding to their target antigens on primed neutrophils, proceeds by recruiting transmembrane molecules to initiate intracellular signal transduction and culminates in activation of effector functions that ultimately mediate the tissue damage.

Ly6G ligation blocks recruitment of neutrophils via a β2-integrin-dependent mechanism

Ly6G is a glycosylphosphatidylinositol (GPI)-anchored protein of unknown function that is commonly targeted to induce experimental neutrophil depletion in mice. In the present study, we found that doses of anti-Ly6G Abs too low to produce sustained neutropenia remained capable of inhibiting experimental arthritis, leaving joint tissues free of infiltrating neutrophils. Thioglycollate-stimulated peritonitis was also attenuated. No alteration in neutrophil apoptosis was observed, implicating impaired recruitment. Indeed, Ly6G ligation abrogated neutrophil migration toward LTB(4) and other chemoattractants in a transwell system. Exploring the basis for this blockade, we identified colocalization of Ly6G and β2-integrins by confocal microscopy and confirmed close association by both coimmunoprecipitation and fluorescence lifetime imaging microscopy. Anti-Ly6G Ab impaired surface expression of β2-integrins in LTB(4)-stimulated neutrophils and mimicked CD11a blockade in inhibiting both ICAM-1 binding and firm adhesion to activated endothelium under flow conditions. Correspondingly, migration of β2-integrin-deficient neutrophils was no longer inhibited by anti-Ly6G. These results demonstrate that experimental targeting of Ly6G has functional effects on the neutrophil population and identify a previously unappreciated role for Ly6G as a modulator of neutrophil migration to sites of inflammation via a β2-integrin-dependent mechanism.

Basic and translational concepts of immune-mediated glomerular diseases

Genetically modified immune responses to infections and self-antigens initiate most forms of GN by generating pathogen- and danger-associated molecular patterns that stimulate Toll-like receptors and complement. These innate immune responses activate circulating monocytes and resident glomerular cells to release inflammatory mediators and initiate adaptive, antigen-specific immune responses that collectively damage glomerular structures. CD4 T cells are needed for B cell-driven antibody production that leads to immune complex formation in glomeruli, complement activation, and injury induced by both circulating inflammatory and resident glomerular effector cells. Th17 cells can also induce glomerular injury directly. In this review, information derived from studies in vitro, well characterized experimental models, and humans summarize and update likely pathogenic mechanisms involved in human diseases presenting as nephritis (postinfectious GN, IgA nephropathy, antiglomerular basement membrane and antineutrophil cytoplasmic antibody-mediated crescentic GN, lupus nephritis, type I membranoproliferative GN), and nephrotic syndrome (minimal change/FSGS, membranous nephropathy, and C3 glomerulopathies). Advances in understanding the immunopathogenesis of each of these entities offer many opportunities for future therapeutic interventions.

A selective reversible azapeptide inhibitor of human neutrophil proteinase 3 derived from a high affinity FRET substrate

The biological functions of human neutrophil proteinase 3 (PR3) remain unclear because of its close structural resemblance to neutrophil elastase and its apparent functional redundancy with the latter. Thus, all natural inhibitors of PR3 preferentially target neutrophil elastase. We have designed a selective PR3 inhibitor based on the sequence of one of its specific, sensitive FRET substrates. This azapeptide, azapro-3, inhibits free PR3 in solution, PR3 bound to neutrophil membranes, and the PR3 found in crude lung secretions from patients with chronic inflammatory pulmonary diseases. But it does not inhibit significantly neutrophil elastase or cathepsin G. Unlike most of azapeptides, this inhibitor does not form a stable acyl-enzyme complex; it is a reversible competitive inhibitor with a K(i) comparable to the K(m) of the parent substrate. Low concentrations (60 μM) of azapro-3 totally inhibited the PR3 secreted by triggered human neutrophils (200,000 cells/100 μL) and the PR3 in neutrophil homogenates and in lung secretions of patients with lung inflammation for hours. Azapro-3 also resisted proteolysis by all proteases contained in these samples for at least 2h.

Phenotypic and functional plasticity of cells of innate immunity: macrophages, mast cells and neutrophils

Hematopoietic cells, including lymphoid and myeloid cells, can develop into phenotypically distinct 'subpopulations' with different functions. However, evidence indicates that some of these subpopulations can manifest substantial plasticity (that is, undergo changes in their phenotype and function). Here we focus on the occurrence of phenotypically distinct subpopulations in three lineages of myeloid cells with important roles in innate and acquired immunity: macrophages, mast cells and neutrophils. Cytokine signals, epigenetic modifications and other microenvironmental factors can substantially and, in some cases, rapidly and reversibly alter the phenotype of these cells and influence their function. This suggests that regulation of the phenotype and function of differentiated hematopoietic cells by microenvironmental factors, including those generated during immune responses, represents a common mechanism for modulating innate or adaptive immunity.

Pathogenesis of antineutrophil cytoplasmic autoantibody vasculitis

PURPOSE OF REVIEW

Antineutrophil cytoplasmic autoantibodies (ANCAs) are associated with vasculitis. Current therapy involves administration of toxic therapy that is not optimally effective. This review will summarize evidence for the pathogenicity of ANCAs, which will suggest possible strategies for improving treatment.

RECENT FINDINGS

Pauci-immune small vessel vasculitis is associated with antibodies against myeloperoxidase (MPO-ANCA) and proteinase 3 (PR3-ANCA). One research group has reported a high frequency of autoantibodies against lysosomal-associated membrane protein 2 (LAMP-2) in patients with MPO-ANCA or PR3-ANCA. Epigenetic dysregulation appears to be the basis for increased MPO and PR3 neutrophil gene expression in ANCA disease. Release of neutrophil extracellular traps may be involved in initiating the ANCA autoimmune response and causing vessel injury. Generation of C5a by alternative pathway activation is involved in pathogenesis in mouse models. Intervention strategies in mice that target antigens, antibodies and inflammatory signaling pathways may translate into novel therapies. Animal models of LAMP-ANCA and PR3-ANCA disease have been proposed. Molecular mimicry and responses to complementary peptides may be initiating events for ANCAs. T cells, including regulatory T cells, have been implicated in the origin and modulation of the ANCAs, as well as in the induction of tissue injury.

SUMMARY

Our basic understanding of the origins and pathogenesis of ANCA disease is advancing. This deeper understanding already has spawned novel therapies that are being investigated in clinical trials. This brief review shows that there are more questions than answers, and new questions are emerging faster than existing questions are being answered.