Obesity is a disease characterized by chronic inflammation and the progressive loss in systemic insulin sensitivity. One of the more effective medications in the treatment of insulin resistance have been the thiazolidinediones (TZDs), which act through the nuclear receptor peroxisome proliferator-activated receptor gamma (PPARgamma ). Due to the many side-effects of TZDs, our laboratory sought out a natural phytochemical, abscisic acid (ABA), with chemical similarities to TZDs. Our first study demonstrated that ABA activates PPARgamma in vitro and significantly ameliorates white adipose tissue (WAT) inflammation and glucose tolerance in db/db mice. We next further examined the effect of ABA on the phenotype of adipose tissue macrophages (ATMs). In doing so, we discovered two separate ATM populations which differed in their expression of the macrophage surface glycoprotein and maturation marker F4/80 (F4/80hi vs F4/80lo). Dietary ABA-supplementation significantly reduced F4/80hiCCR2+ ATMs and had no effect on the F4/80lo population. Utilizing a tissue-specific knockout generated through Cre-lox recombination, we were able to determine that this effect was dependent on PPARgamma in immune cells. To further characterize the differences between the ATM subsets that were affected by ABA, we performed a multi-organ assessment (i.e., WAT, skeletal muscle and liver) of the effect of diet-induced obesity on the phenotype of infiltrating macrophages and T cells into metabolic organs. Based on our new data, we formulated a model by which F4/80hiCCR2hi ATMs infiltrate WAT and ultimately induce a CD11c+ pro-inflammatory phenotype in the resident F4/80loCCR2lo subset. Ultimately, our findings provide evidence that ABA has potential as an alternative preventive intervention, expound the role of PPARgamma in immune cells and, in general, expand our knowledge concerning the immunopathogenesis of obesity-induced insulin resistance. / Ph. D.

Subclinical endotoxemia (low levels of circulating bacterial endotoxin) has been observed in patients suffering from chronic inflammatory diseases such as atherosclerosis, diabetes, and obesity. However, the link between this condition and chronic inflammation is poorly understood. Previous work from our lab has shown that chronic exposure to super-low doses of bacterial endotoxin (LPS) aggravates atherosclerosis resulting in increased plaque size and instability in a macrophage-dependent manner in a mouse model of atherosclerosis. Further, we showed that super-low dose LPS (SLD-LPS) treatment was able to inhibit lysosomal fusion in immortalized macrophages. However, this was done under more acute treatment conditions. The aim of this project was to examine the molecular mechanisms by which chronic SLD-LPS may polarize macrophages to a non-resolving pro-inflammatory state consistent with chronic inflammation. This was carried out in two projects, the first a more broad phenotypic paper showing the disruption in homeostasis by chronic SLD-LPS in immortalized macrophages, while the second uses primary bone marrow-derived mouse macrophages to identify specific molecular signaling pathways used by chronic SLD-LPS. Here we show that chronic SLD-LPS led to the novel upregulation of pro-inflammatory mediators p62 and ccl2 with simultaneous downregulation of homeostatic mediators Nrf2 and slc40a1 in immortalized wild-type mouse macrophages. Further we showed this effect was reversed using the homeostatic restorative agent sodium phenylbutyrate (4-PBA), a newly reported activity for this reagent in mouse macrophages. This indicated that a disruption in homeostasis, possibly involving autophagy, may be responsible for the non-resolving pro-inflammatory polarization of macrophages. Therefore, in our second project, we further explored the effect of chronic SLD-LPS treatment on the homeostatic arm of the response by focusing on the Nrf2 inhibitor Keap1. Here we show that chronic SLD-LPS results in an accumulation of Keap1 in mouse bone marrow-derived macrophages, an effect specific to chronic SLD-LPS, as high doses of LPS failed to induce Keap1. We suggest that this effect may be related to a disruption in lysosomal fusion as evidenced by accumulation of autophagy flux markers MLKL and p62. Further, we show that these effects are dependent on the non-traditional TLR4 adaptor TRAM, suggesting an alternative dose-dependent signaling pathway for LPS. Together this work identifies novel signaling mechanisms involved in non-resolving pro-inflammatory polarization of murine macrophages, providing new insight behind how chronic super-low dose LPS exposure may lead to chronic inflammation. / Doctor of Philosophy / Inflammation is the body's natural response to injury or insult and can be beneficial in certain contexts such as pathogen clearance. However, left un-checked, chronic inflammation can exacerbate or even lead to disease pathology, such as is the case with modern diseases such as atherosclerosis, obesity, diabetes, etc. Despite the high prevalence of these diseases, effective treatments and therapies are still lacking. Recently it was discovered that many patients suffering from chronic inflammatory diseases had low levels bacterial endotoxin (LPS) in their circulation, a condition referred to as subclinical endotoxemia. However, possible links between this condition and chronic inflammatory disease remain poorly understood. Using a mouse model of atherosclerosis, previous research from our lab showed that persistent exposure to super-low doses of bacterial endotoxin (similar to those observed in humans) lead to aggravated atherosclerosis with both increased plaque size and instability. Further, we showed that this effect was primarily mediated by pro-inflammatory polarized immune cells called macrophages, but the molecular mechanism behind this polarization is still unclear. Further research into these molecular mechanisms may provide better targets for the development of future chronic inflammatory disease treatments. Here using a combination of mouse cell line and primary cell cultures, we discuss how chronic exposure to super-low doses of bacterial endotoxin leads to the chronic non-resolving pro-inflammatory polarization of macrophage immune cells, with particular emphasis on the distinct molecular signaling mechanisms induced by chronic super-low dose LPS.

Macrophage migration, an essential component of many biological processes and pathologic conditions, is mediated by integrated cellular signaling processes and cytoskeleton rearrangement. Recent advances indicate that the innate immunity signaling process plays a key role in the regulation of macrophage migration. Furthermore, our lab has provided evidence demonstrating the involvement of a key innate immunity signaling kinase, IRAK-1, as a critical modulator of murine macrophage migration. Macrophage migration induced by a potent PKC activator, phorbol 12-myristate 13-acetate (PMA), or lipopolysaccharide (LPS) was significantly decreased in IRAK-1-/- murine macrophages compared with wild type cells. Mechanistically, we first demonstrated that IRAK-1 works downstream of PKCε and directly binds to VASP, a cytoskeleton regulatory protein, to regulate PMA-induced macrophage migration. Secondly, we proved that IRAK-1 is required for LPS-induced macrophage migration and expression of MCP-1, a chemotactic cytokine for macrophages, via transcription factor C/EBPδ instead of NFκB. IRAK-1 binds directly to IKKε and inhibition or knock-down of IKKε results in a significant decrease in C/EBPδ expression and MCP-1 mRNA expression. Lastly, we identified the direct association between IRAK-1 and Rac1, a member of the Rac subfamily in the Rho family of GTPases. These finding further confirmed the essential role of IRAK-1 during macrophage migration. Our research provides a novel facet regarding the molecular signaling processes regulating macrophage migration. / Ph. D.

Osteoarthritis (OA) is the leading cause of musculoskeletal disability in people and horses, and is characterized by progressive joint degeneration. There is a critical need for a better understanding of disease processes leading to OA in order to develop more efficient therapies. A shared feature among different arthritic conditions is chronic synovitis. Macrophages are the main drivers of synovitis and can display pro-inflammatory (M1) or pro-resolving responses (M2). Macrophages promote joint health through phagocytic and secretory activities; however, when these functions are overwhelmed, macrophages upregulate inflammation, recruiting more cells to counteract damage. Once cell recruitment is efficiently accomplished, macrophages coordinate tissue repair and further resolution of inflammation. Bone marrow mononuclear cells (BMNC) are a source of macrophages used to treat inflammation and produce essential molecules for cartilage metabolism; however, little information exists regarding their use in joints. The studies presented in this dissertation focus on understanding the dual role of macrophages in driving and resolving synovitis and how to harness their therapeutic potential. In the first study, patterns of macrophage phenotypes (M1:M2) in healthy and osteoarthritic equine synovium were compared and correlated with gross pathology, histology, and synovial fluid cytokines. M1 and M2 markers were co-expressed in normal and osteoarthritic joints, varying in intensity of expression according to degree of inflammation. Concentrations of synovial fluid IL-10, a macrophage-produced cytokine that is vital for chondrocyte recovery from injury, was lower in OA joints. The combined findings of this study suggest homeostatic mechanisms from synovial macrophages in OA may be overwhelmed, preventing inflammation resolution. In the second study we investigated the response of BMNC to normal (SF) and inflamed synovial fluid (ISF). BMNC cultured in autologous SF or ISF developed into macrophage cultures that were more confluent in ISF (~100%) than SF (~25%), and exhibited phenotypes that were ultimately similar to cells native to normal joints. BMNC cultured in SF or ISF were neither M1 nor M2, but exhibited aspects of both phenotypes and a regulatory response, characterized by increasing counts of IL-10+ macrophages, decreasing concentrations of IL-1β, and progressively increasing concentrations of IL-10 and IGF-1, all more marked in ISF. These findings suggest that homeostatic mechanisms were preserved over time, and potentially favored by macrophage proliferation. Our data suggest that BMNC therapy could potentiate the macrophage- and IL-10-associated mechanisms of joint homeostasis lost in OA. Finally, using an equine model of synovitis, the last study investigated the response of normal and inflamed joints to autologous BMNC injection. Inflamed joints treated with BMNC showed gross and analytical improvements in synovial fluid and synovial membrane, with increasing numbers of regulatory macrophages and synovial fluid concentrations of IL-10, not observed in saline-treated controls. Autologous BMNC are readily available, downregulate synovitis through macrophage-associated effects, and can benefit thousands of patients with OA. Combined, the results of these studies support the role of macrophage-driven synovial homeostasis and identified a therapeutic way to recover homeostatic mechanisms of synovial macrophages lost during chronic inflammation. Our findings also uncover new research directions and methods for future studies targeting modulation of joint inflammation. / Doctor of Philosophy / Osteoarthritis (OA) is a common cause of joint deterioration in people and horses. Current treatments provide limited recovery of joint function, creating an urgent need for more efficient therapies; however, development of new treatments requires better understanding of the mechanism causing OA. A shared characteristic among many arthritic conditions is long-standing inflammation. Cells called macrophages are the main drivers of joint inflammation and can exert pro- and anti-inflammatory effects. Macrophages promote joint health by clearing aggressor agents and secreting molecules required for optimal joint function. However, when these housekeeping functions are overwhelmed by damage, macrophages drive inflammation recruiting more cells to cope with increased demands for repair. If this process is efficiently accomplished, macrophages then resolve inflammation, recovering joint health. Macrophages in the bone marrow (BMNC - bone marrow mononuclear cells) are used to treat inflammation in several tissues and are known to produce molecules essential for joint health. Although little information exists regarding their use in joints, studies treating different organs suggest it can provide high rewards. The studies presented in this dissertation focused on understanding the dual function of macrophages in driving and controlling joint inflammation, and harnessed their therapeutic potential. In the first study, macrophages were investigated in normal and OA-affected joints, and curiously exhibited a hybrid pro- and anti-inflammatory identity in both groups. The indicators of this mixed identity were more markedly expressed in arthritic joints showing gross inflammation. Low levels of a macrophage-derived anti-inflammatory protein called IL-10 were detected in OA joints. The results of this study suggest that anti-inflammatory mechanisms from macrophages may be overwhelmed in OA-affected joints, preventing inflammation to be resolved, and that recovering this anti-inflammatory function may aid in the treatment of OA. In the second study we investigated how the incubation of BMNC in fluid from normal and inflamed joints affects the response of macrophages. Similar to what we observed in the first study, BMNC incubated in both normal and inflamed joint fluid induced macrophages to develop a hybrid identity that was ultimately similar to native cells from normal joints. Macrophages proliferated more when incubated in fluid from inflamed joints. Macrophages in both groups produced anti-inflammatory effects with high levels of IL-10 that were highest in ISF cultures. These observations suggest that higher proliferation of macrophages in inflamed joint fluid helped preserve anti-inflammatory mechanisms. Therefore, our study suggests that joint injection with BMNC could maximize macrophage- and IL-10-associated mechanisms required to resolve joint inflammation. The third and final study investigated the response of normal and inflamed joints to BMNC injection using a model of joint inflammation in horses. Inflamed joints treated with BMNC showed visual and laboratorial markers of improvement, with increasing numbers of macrophages and concentrations of IL-10 in the joint fluid, which remained lower in joints treated with placebo. BMNC provide means to recover macrophage-associated effects required to control joint inflammation and can benefit thousands of patients with OA. Together, the results of these studies show that macrophages are biased promoters of joint health, leading to inflammation when their anti-inflammatory mechanisms are overwhelmed. Replenishing inflamed joints with healthy macrophages maximizes their anti-inflammatory effects, favoring the recovery of a healthy articular environment.

Chez les métazoaires, la différenciation terminale est généralement accompagnée par une sortie définitive du cycle cellulaire. Cependant, les macrophages et très peu d'autres types cellulaires rompent avec ce dogme. En effet, il est maintenant admis que les macrophages conservent la capacité de s'auto-renouveler indépendamment des cellules souches ou progénitrices. À cet égard, nous avons démontré que la double déficience en facteurs Maf dans les macrophages (Maf-DKO) leur confère la capacité de s'auto-renouveler indéfiniment en culture sans se dé-différencier ou devenir tumorigènes. Ce phénotype d'auto-renouvellement semble être médié par un réseau transcriptionnel de de gènes régissant l'auto-renouvellement qui sont également actifs dans les cellules souches embryonnaires, parmi lesquels Myc et Klf4. Ces deux facteurs sont activés et nécessaires pour l'auto-renouvellement des Maf-DKO. De façon intéressante, l'expression de Myc seul induit une prolifération illimitée des macrophages, mais provoque une transformation tumorale. Nous avons donc cherché à décrypter les mécanismes grâce auxquels Myc et Klf4 induisent l'auto-renouvellement des macrophages, en comparaison à la transformation cellulaire causée par l'expression de Myc uniquement. En outre, nous nous sommes concentrés sur l'identification de gènes candidats permettant un auto-renouvellement illimité des macrophages, tout en les protégeant de la transformation cancéreuse. Notre objectif est de contribuer à l'identification du programme transcriptionnel régulant l'auto-renouvellement non tumoral des macrophages. / In metazoan, terminal differentiation is generally accompanied by permanent exit from the cell cycle. Yet, macrophages and very few other examples break with this dogma. Indeed, it has become evident that macrophages retain the ability to self-renew independently of stem or progenitor cells. In this regard, we have previously shown that MafB/c-Maf double deficient (Maf-DKO) macrophages are able to self-renew indefinitely in vitro without dedifferentiating or becoming tumorigenic. This self-renewal phenotype appears to be mediated by a transcriptional network of self-renewal genes also active in embryonic stem cells, among which Myc and Klf4. Interestingly, these two factors are activated and required for Maf-DKO self-renewal. By contrast, Myc alone induces an unlimited proliferation of macrophages but causes malignant transformation. We aimed to decipher the mechanisms by which Myc and Klf4 induce stem cell-like self-renewal in macrophages, in comparison to cellular transformation caused by the expression of Myc alone. Additionally, we focused on identifying candidate genes allowing an unlimited self-renewal of macrophages while protecting them from tumorigenic transformation or aberrant proliferation. Our objective is to contribute to the identification of the transcriptional program regulating non-tumorigenic self-renewal in macrophages.

Selon les facteurs solubles de l’environnement (cytokines), les monocytes peuvent se différencier en macrophages (Mφ) ou en cellules dendritiques. Mes travaux de thèse montrent que l’IL-34, un second ligand du M-CSF-R (c-fms ou CD115), induit la différenciation de monocytes humains en macrophages CD14+ CD163+ (IL-34-Mφ), phénotypiquement et fonctionnellement similaires aux M-CSF-Mφ et aux macrophages associés aux tumeurs (TAM) isolés du cancer de l’ovaire. Ils possèdent des propriétés immunosuppressives et diminuent la prolifération des lymphocytes T CD4+ activés. Comme les Mφ orchestrent la réponse immunitaire, j’ai évalué la capacité des M-CSF-Mφ, IL-34-Mφ et des TAM à polariser les LT CD4+ mémoires. Les résultats montrent qu’ils polarisent des LT non-Th17 mémoires en LTh17 CCR4+ CCR6+ CD161+ conventionnels exprimant ou non de l’IFN. Ce processus est médié par l’expression constitutive de l’IL-1α membranaire exprimées sur ces macrophages.Dans le but d’identifier des stratégies pour prévenir l’accumulation de ces Mφ dans le cancer, j’ai montré que (i) l’IFNγ et le GM-CSF inhibent la différenciation des M-CSF-Mφ et IL-34-Mφ et que (ii) l’IFNγ permet la réversion des M-CSF-Mφ et IL-34-Mφ en Mφ immunostimulants. En conclusion, mes travaux montrent que les M-CSF-Mφ, IL-34-Mφ et les TAM, considérés initialement comme anti-inflammatoires, induisent la génération de lymphocytes Th17 via une expression constitutive de l’IL-1α. Ce processus pourrait contribuer à maintenir, localement, une inflammation modérée nécessaire au développement tumoral. / According to the soluble factors in the environment (cytokines), monocytes differentiate into macrophages (Mϕ) or dendritic cells. This project demonstrates that IL-34, a second ligand of the M-CSF receptor (c-fms or CD115), induces the differentiation of human monocytes into CD14high CD163 high Mφ (IL-34-Mφ), phenotypically and functionally similar to M-CSF-Mφ and to tumorassociated macrophages (TAM) isolated from the ovarian cancer. They exhibit potent immunosuppressive properties and decrease the proliferation of stimulated Tcells. As Mφ orchestrate the immune response, I have evaluated the capacity of M-CSF-Mφ, IL-34-Mφ and TAM to polarize human memory CD4+ T cells. Unexpectedly, results showed that they switch non- Th17 memory CD4+ T cells into conventional CCR4+ CCR6+ CD161+ Th17 cells, expressing or not IFNγ. This process is mediated by the constitutive expression of membrane IL-1α on these Mϕ subsets. In an attempt to identify strategies to prevent an accumulation of immunomodulatory Mϕ in cancer, I have shown that (i) IFNγ and GM-CSF prevent M-CSFandIL-34-induced monocyte differentiation into immunosuppressive Mφ and (ii) that IFNγ switchesestablished M-CSF-Mφ and IL-34-Mφ into immunostimulatory Mφ. In conclusion, this study demonstrates that human MCSF- Mφ, IL-34-Mφ and TAM initially considered as antiinflammatorycells, induce in vitro Th17 cell generation via a constitutive expression of membrane IL-1α. This process may contribute to maintain locally a restrained and smoldering inflammation required for angiogenesis and metastasis in tumors.

Post-operative ileus (POI) is the functional inhibition of propulsive intestinal motility which is a frequent occurrence following abdominal surgery in the horse and in humans. Rodent and human-derived data have shown that manipulation-induced activation of the resident muscularis externa (ME) macrophages in the intestine contributes to the pathophysiology of the disease. Most studies of the disease, specifically in the horse, have focussed on identification of risk factors, descriptive studies of the disease or the assessment of the efficacy of various therapeutic and prophylactic interventions. As a result, the proposed pathogenesis of equine POI is largely reliant on the translation of data from rodent models. The aims of this thesis were to identify macrophage populations in the normal equine gastrointestinal tract (GIT) and to study equine macrophage activation by stimulating equine bone marrow-derived macrophages (eqBMDMs) with lipopolysaccharide (LPS) as a model for intestinal macrophage activation. Firstly, the normal population of macrophages in the equine GIT was determined. Using CD163 as an immunohistochemical marker for macrophages. CD163+ve cells were present in all tissue layers of the equine intestine: mucosa, submucosa, ME and serosa. CD163+ve cells were regularly distributed within the ME, with accumulations adjacent to the myenteric plexus, and therefore to intestinal motility effector cells such as neurons and the Interstitial Cells of Cajal. The differentiation and survival of intestinal macrophages depends upon signals from the macrophage colony-stimulating factor (CSF-1) receptor. LPS translocation from the gut lumen is thought to be a key activator of ME macrophages. To provide a model for gut macrophages, a protocol was optimised to produce pure populations of equine bone marrow-derived macrophages (eqBMDMs) by cultivation of equine bone marrow in CSF-1. Macrophage functionality was assessed using microscopy, flow cytometry and phagocytosis assays. EqBMDMs responded to LPS stimulation with increases in expression of positive control genes, tumour necrosis factor alpha (TNF-α) and Indoleamine 2,3-dioxygenase (IDO1). The same mRNA was subjected to transcriptomic (RNA-Seq) analysis. Differential gene expression and network cluster analysis demonstrated an inflammatory response characterised by the production of pro-inflammatory cytokines such as interleukin 1 beta (IL-1β) and interleukin 6 (IL-6). However, in contrast to rodent macrophages, eqBMDMs failed to produce nitric oxide in response to LPS, showing species-specific variation in innate immune biology. Using these data, we compared gene expression in normal equine intestine and in intestine from horses undergoing abdominal surgery for colic (abdominal pain). Horses undergoing abdominal surgery showed evidence of increased expression of IL-1β, IL-6 and TNF-α in the mucosa and ME when compared to control tissue. Horses with post-operative reflux (POR), a clinical sign of POI, had increased gene expression of IL-1β, IL-6 and TNF-α compared to horses that did not develop POR following abdominal surgery. These preliminary data suggest that there is macrophage activation within the ME of the intestine during abdominal surgery in the horse, and that a greater activation state is present in horses that subsequently develop POR. The final part of this study was to investigate the effect of a long-acting form of CSF- 1, an Fc fusion protein (CSF1-Fc), as a potential treatment for POI using a mouse model. This work, performed in collaboration with another research group, found that mice lacking the C-C chemokine receptor type 2 (CCR2) gene, which is required for monocyte recruitment into tissues, had a longer recovery period following intestinal manipulation (IM) than wild type (WT) mice. With the administration of CSF1-Fc, infiltration of neutrophils to the ME was reduced and the number of macrophages in the ME was increased in both WT and CCR2-/- mice following IM. Administration of CSF1-Fc in CCR2-/- mice improved recovery of gastrointestinal transit three days following IM, to the same extent as WT mice. Network cluster analysis and RT-qPCR of the ME revealed clusters of genes induced and downregulated by CSF1-Fc, with increased expression of anti-inflammatory and pro-resolving genes after IM in WT and CCR2-/- mice following treatment with CSF1-Fc.

Macrophage-derived inflammation contributes to chronic inflammation in adipose tissue in obesity and is also linked to the development of skeletal muscle (SM) insulin resistance. The long-chain n-3 PUFA have been shown to modulate cytokine secretion from macrophages, though subsequent effects on SM inflammation and function are unknown. A model of macrophage conditioned media (MCM) was used to examine effects of n-3 PUFA on macrophage inflammation and consequent effects on SM cells. Treatment of RAW 264.7 macrophages with long-chain n-3 PUFA decreased LPS-induced MCP-1 and IL-6 gene expression and MCP-1 secreted protein. In turn, MCM from n-3 PUFA-treated macrophages decreased TNF-α and IL-6 gene expression in LPS-stimulated L6 SM cells, but did not affect insulin-stimulated pAkt content. Long-chain n-3 PUFA did not affect gene expression of inflammatory signaling intermediates NF-κB and TLR4. Overall this thesis suggests that long-chain n-3 PUFA are important nutritional strategies for reducing macrophage-derived inflammation, with ensuing benefits in SM inflammation. / NSERC-CGS, Ontario Graduate Scholarship

During our lifetime, our bones constantly go through remodelling to maintain the skeletal system. This is done by osteoblasts that deposit new bone tissue, osteoclasts that remove the bone matrix and mechanosensing osteocytes. In case of bone implants, increased resorption by osteoclasts due to inflammation (inflammatory osteolysis) leads to aseptic implant loosening. This study focuses on how to detect these inflammatory resorbing cells at an early stage and prevent their activity with appropriate medication. To achieve this, we differentiated classical monocytes into macrophage-like cells, osteoclasts(OCs) and foreign body giant cells (FBGC) and their secretome was studied to identify specific biomarkers. Previously, tartrate resistant acid phosphatase (TRAP) was studied as an important biomarker for OCs and macrophages. An ELISA to separate and quantitate the two TRAP isoforms was used to distinguish the resorbing OCs from inflammatory FBGCs on the basis of the isoform ratio. This assay gave high levels of 5b isoform for osteoclastic stimulation and high 5a levels for the inflammatory stimulation. Also, different aminothiazole inhibitors were tested which were shown to be efficient drugs in inhibiting inflammatory osteolysis by reducing osteoclast formation and resorption in sub-micromolar concentration. Further to apply this study to patient samples, an immunoassay panel can be developed which will help detect TRAP and multiple biomarkers like CTX specific to aseptic loosening simultaneously. This will help in early, efficient and accurate diagnosis of inflammatory osteolytic bone loss and provide us with an accurate diagnosis and sufficient time for appropriate treatment.

Protozoa of the Leishmania genus are transmitted between mammalian hosts by the sandfly and cause the neglected tropical disease leishmaniasis. Upon injection into the mammalian host by the sandfly promastigote-form parasites are phagocytosed by macrophages, where they differentiate into amastigotes. Although many virulence factors are known to modulate macrophage signalling pathways to favour infection, the delivery mechanisms are largely unknown. During differentiation to amastigotes the promastigote flagellum shortens dramatically and the fate of the excess flagellar membrane is unknown. Here we investigate the possibility that during Leishmania mexicana differentiation, shedding of the flagellar membrane is a source of extracellular vesicles (EVs) which provide a virulence factor delivery mechanism. The kinetics and structural mechanisms of EV release from promastigotes were investigated by live cell imaging and by measuring the concentration of shed EVs. Isolated EVs from a differentiating parasite culture or a control promastigote parasite culture were analysed by fluorescence and electron microscopy and mass spectrometry. To study the biological effects of EVs, macrophages were exposed to isolated EVs or live promastigotes and cytokine secretion was quantified by ELISA. An LPG1 null mutant was used to assess the contribution of virulence factor lipophosphoglycan (LPG) to the observed effects. Known protein virulence factors and LPG are present in L. mexicana EV fractions as well as known flagellar proteins. We show that there is a link between L. mexicana flagellar shortening and EV release, which is a recently discovered phenomenon in Chlamydomonas and mammalian cell research. We find that isolated EVs and live promastigotes induce changes in secreted cytokine levels from human and murine macrophages, including a substantial and previously unreported suppression of CXCL10, a chemokine which plays a protective role in Leishmania infection. LPG contributes to the effects observed on cytokine production, and EVs may be an important delivery mechanism for LPG.