Leukocyte Trafficking via Lymphatic Vessels in Atherosclerosis

In recent years, lymphatic vessels have received increasing attention and our understanding of their development and functional roles in health and diseases has greatly improved. It has become clear that lymphatic vessels are critically involved in acute and chronic inflammation and its resolution by supporting the transport of immune cells, fluid, and macromolecules. As we will discuss in this review, the involvement of lymphatic vessels has been uncovered in atherosclerosis, a chronic inflammatory disease of medium- and large-sized arteries causing deadly cardiovascular complications worldwide. The progression of atherosclerosis is associated with morphological and functional alterations in lymphatic vessels draining the diseased artery. These defects in the lymphatic vasculature impact the inflammatory response in atherosclerosis by affecting immune cell trafficking, lymphoid neogenesis, and clearance of macromolecules in the arterial wall. Based on these new findings, we propose that targeting lymphatic function could be considered in conjunction with existing drugs as a treatment option for atherosclerosis.

Efficient aortic lymphatic drainage is necessary for atherosclerosis regression induced by ezetimibe

A functional lymphatic vasculature is essential for tissue fluid homeostasis, immunity, and lipid clearance. Although atherosclerosis has been linked to adventitial lymphangiogenesis, the functionality of aortic lymphatic vessels draining the diseased aorta has never been assessed and the role of lymphatic drainage in atherogenesis is not well understood. We develop a method to measure aortic lymphatic transport of macromolecules and show that it is impaired during atherosclerosis progression, whereas it is ameliorated during lesion regression induced by ezetimibe. Disruption of aortic lymph flow by lymphatic ligation promotes adventitial inflammation and development of atherosclerotic plaque in hypercholesterolemic mice and inhibits ezetimibe-induced atherosclerosis regression. Thus, progression of atherosclerotic plaques may result not only from increased entry of atherogenic factors into the arterial wall but also from reduced lymphatic clearance of these factors as a result of aortic lymph stasis. Our findings suggest that promoting lymphatic drainage might be effective for treating atherosclerosis.

Hyaluronan Receptor LYVE-1-Expressing Macrophages Maintain Arterial Tone through Hyaluronan-Mediated Regulation of Smooth Muscle Cell Collagen

The maintenance of appropriate arterial tone is critically important for normal physiological arterial function. However, the cellular and molecular mechanisms remain poorly defined. Here, we have shown that in the mouse aorta, resident macrophages prevented arterial stiffness and collagen deposition in the steady state. Using phenotyping, transcriptional profiling, and targeted deletion of Csf1r, we have demonstrated that these macrophages-which are a feature of blood vessels invested with smooth muscle cells (SMCs) in both mouse and human tissues-expressed the hyaluronan (HA) receptor LYVE-l. Furthermore, we have shown they possessed the unique ability to modulate collagen expression in SMCs by matrix metalloproteinase MMP-9-dependent proteolysis through engagement of LYVE-1 with the HA pericellular matrix of SMCs. Our study has unveiled a hitherto unknown homeostatic contribution of arterial LYVE-1⁺ macrophages through the control of collagen production by SMCs and has identified a function of LYVE-1 in leukocytes.

Bidirectional Crosstalk between Lymphatic Endothelial Cell and T Cell and Its Implications in Tumor Immunity

Lymphatic vessels have been traditionally considered as passive transporters of fluid and lipids. However, it is apparent from recent literature that the function of lymphatic vessels is not only restricted to fluid balance homeostasis but also extends to regulation of immune cell trafficking, antigen presentation, tolerance, and immunity, all which may impact the progression of inflammatory responses and diseases such as cancer. The lymphatic system and the immune system are intimately connected, and there is emergent evidence for a crosstalk between T cell and lymphatic endothelial cell (LEC). This review describes how LECs in lymph nodes can affect multiple functional properties of T cells and the impact of these LEC-driven effects on adaptive immunity and, conversely, how T cells can modulate LEC growth. The significance of such crosstalk between T cells and LECs in cancer will also be discussed.

STEVOR is a Plasmodium falciparum erythrocyte binding protein that mediates merozoite invasion and rosetting

Variant surface antigens play an important role in Plasmodium falciparum malaria pathogenesis and in immune evasion by the parasite. Although most work to date has focused on P. falciparum Erythrocyte Membrane Protein 1 (PfEMP1), two other multigene families encoding STEVOR and RIFIN are expressed in invasive merozoites and on the infected erythrocyte surface. However, their role during parasite infection remains to be clarified. Here we report that STEVOR functions as an erythrocyte-binding protein that recognizes Glycophorin C (GPC) on the red blood cell (RBC) surface and that its binding correlates with the level of GPC on the RBC surface. STEVOR expression on the RBC leads to PfEMP1-independent binding of infected RBCs to uninfected RBCs (rosette formation), while antibodies targeting STEVOR in the merozoite can effectively inhibit invasion. Our results suggest a PfEMP1-independent role for STEVOR in enabling infected erythrocytes at the schizont stage to form rosettes and in promoting merozoite invasion.

Lymphatic vessels are essential for the removal of cholesterol from peripheral tissues by SR-BI-mediated transport of HDL

Removal of cholesterol from peripheral tissues to the bloodstream via reverse cholesterol transport (RCT) is a process of major biological importance. Here we demonstrate that lymphatic drainage is required for RCT. We have previously shown that hypercholesterolemia in mice is associated with impaired lymphatic drainage and increased lipid accumulation in peripheral tissues. We now show that restoration of lymphatic drainage in these mice significantly improves cholesterol clearance. Conversely, obstruction of lymphatic vessels in wild-type mice significantly impairs RCT. Finally, we demonstrate using silencing RNA interference, neutralizing antibody, and transgenic mice that removal of cholesterol by lymphatic vessels is dependent on the uptake and transcytosis of HDL by scavenger receptor class B type I expressed on lymphatic endothelium. Collectively, this study challenges the current view that lymphatic endothelium is a passive exchange barrier for cholesterol transport and provides further evidence for its interplay with lipid biology in health and disease.

Expansion of cortical and medullary sinuses restrains lymph node hypertrophy during prolonged inflammation

During inflammation, accumulation of immune cells in activated lymph nodes (LNs), coupled with a transient shutdown in lymphocyte exit, results in dramatic cellular expansion. Counter-regulatory measures to restrain LN expansion must exist and may include re-establishment of lymphocyte egress to steady-state levels. Indeed, we show in a murine model that egress of lymphocytes from LNs was returned to steady-state levels during prolonged inflammation following initial retention. This restoration in lymphocyte egress was supported by a preferential expansion of cortical and medullary sinuses during late inflammation. Cortical and medullary sinus remodeling during late inflammation was dependent on temporal and spatial changes in vascular endothelial growth factor-A distribution. Specifically, its expression was restricted to the subcapsular space of the LN during early inflammation, whereas its expression was concentrated in the paracortical and medullary regions of the LN at later stages. We next showed that this process was mostly driven by the synergistic cross-talk between fibroblastic reticular cells and interstitial flow. Our data shed new light on the biological significance of LN lymphangiogenesis during prolonged inflammation and further underscore the collaborative roles of stromal cells, immune cells, and interstitial flow in modulating LN plasticity and function.

Subcutaneous Infection with Non-mouse Adapted Dengue Virus D2Y98P Strain Induces Systemic Vascular Leakage in AG129 Mice

Introduction: Dengue (DEN) is a mosquito-borne viral disease which has become an increasing economic and health burden for the tropical and subtropical world. Plasma leakage is the most life threatening condition of DEN and may lead to hypovolaemic shock if not properly managed. Materials and Methods: We recently reported a unique dengue virus strain (D2Y98P) which upon intraperitoneal (IP) administration to immunocompromised mice led to systemic viral dissemination, intestine damage, liver dysfunction, and increased vascular permeability, hallmarks of severe DEN in patients (Tan et al, PLoS Negl Trop Dis 2010;4:e672). Results: Here we report the clinical manifestations and features observed in mice subcutaneously (SC) infected with D2Y98P, which is a route of administration closer to natural infection. Similar to the IP route, increased vascular permeability, intestine damage, liver dysfunction, transient lymphopenia (but no thrombocytopenia) were observed in the SC infected mice. Furthermore, the SC route of infection was found more potent than the IP route whereby higher viral titers and earlier time-of-death rates were measured. In addition, various staining approaches revealed structurally intact blood vessels in the moribund animals despite pronounced systemic vascular leakage, as reported in dengue hemorrhagic fever/dengue shock syndrome (DHF/DSS) patients. Interestingly, measurement of soluble mediators involved in vascular permeability indicated that vascular leakage may occur at an early stage of the disease, as proposed in DEN patients. Conclusion: We believe that this novel mouse model of DEN-associated vascular leakage will contribute to a better understanding of DEN pathogenesis and represents a relevant platform for testing novel therapeutic treatments and interventions. Key words: Dengue shock syndrome, Dengue hemorrhagic fever, Capillary leakage

Subcutaneous infection with non-mouse adapted Dengue virus D2Y98P strain induces systemic vascular leakage in AG129 mice

INTRODUCTION

Dengue (DEN) is a mosquito-borne viral disease which has become an increasing economic and health burden for the tropical and subtropical world. Plasma leakage is the most life threatening condition of DEN and may lead to hypovolaemic shock if not properly managed.

MATERIALS AND METHODS

We recently reported a unique dengue virus strain (D2Y98P) which upon intraperitoneal (IP) administration to immunocompromised mice led to systemic viral dissemination, intestine damage, liver dysfunction, and increased vascular permeability, hallmarks of severe DEN in patients (Tan et al, PLoS Negl Trop Dis 2010;4:e672).

RESULTS

Here we report the clinical manifestations and features observed in mice subcutaneously (SC) infected with D2Y98P, which is a route of administration closer to natural infection. Similar to the IP route, increased vascular permeability, intestine damage, liver dysfunction, transient lymphopenia (but no thrombocytopenia) were observed in the SC infected mice. Furthermore, the SC route of infection was found more potent than the IP route whereby higher viral titers and earlier time-of-death rates were measured. In addition, various staining approaches revealed structurally intact blood vessels in the moribund animals despite pronounced systemic vascular leakage, as reported in dengue hemorrhagic fever/dengue shock syndrome (DHF/DSS) patients. Interestingly, measurement of soluble mediators involved in vascular permeability indicated that vascular leakage may occur at an early stage of the disease, as proposed in DEN patients.

CONCLUSION

We believe that this novel mouse model of DEN-associated vascular leakage will contribute to a better understanding of DEN pathogenesis and represents a relevant platform for testing novel therapeutic treatments and interventions.

Temperature sensitivity of human perforin mutants unmasks subtotal loss of cytotoxicity, delayed FHL, and a predisposition to cancer

The pore-forming protein perforin is critical for defense against many human pathogens and for preventing a catastrophic collapse of immune homeostasis, manifested in infancy as Type 2 familial hemophagocytic lymphohistiocytosis (FHL). However, no evidence has yet linked defective perforin cytotoxicity with cancer susceptibility in humans. Here, we examined perforin function in every patient reported in the literature who lived to at least 10 years of age without developing FHL despite inheriting mutations in both of their perforin (PRF1) alleles. Our analysis showed that almost 50% of these patients developed at least 1 hematological malignancy in childhood or adolescence. The broad range of pathologies argued strongly against a common environmental or viral cause for the extraordinary cancer incidence. Functionally, what distinguished these patients was their inheritance of PRF1 alleles encoding temperature-sensitive missense mutations. By contrast, truly null missense mutations with no rescue at the permissive temperature were associated with the more common severe presentation with FHL in early infancy. Our study provides the first mechanistic evidence for a link between defective perforin-mediated cytotoxicity and cancer susceptibility in humans and establishes the paradigm that temperature sensitivity of perforin function is a predictor of FHL severity.

Antibodies targeting the PfRH1 binding domain inhibit invasion of Plasmodium falciparum merozoites

Invasion by the malaria merozoite depends on recognition of specific erythrocyte surface receptors by parasite ligands. Plasmodium falciparum uses multiple ligands, including at least two gene families, reticulocyte binding protein homologues (RBLs) and erythrocyte binding proteins/ligands (EBLs). The combination of different RBLs and EBLs expressed in a merozoite defines the invasion pathway utilized and could also play a role in parasite virulence. The binding regions of EBLs lie in a conserved cysteine-rich domain while the binding domain of RBL is still not well characterized. Here, we identify the erythrocyte binding region of the P. falciparum reticulocyte binding protein homologue 1 (PfRH1) and show that antibodies raised against the functional binding region efficiently inhibit invasion. In addition, we directly demonstrate that changes in the expression of RBLs can constitute an immune evasion mechanism of the malaria merozoite.