Establishment of a scalable microfluidic assay for characterization of population-based neutrophil chemotaxis

Stable diffusion gradients in microfluidic conduits bounded by fluid walls

Assays mimicking in vitro the concentration gradients triggering biological responses like those involved in fighting infections and blood clotting are essential for biomedical research. Microfluidic assays prove especially attractive as they allow precise control of gradient shape allied to a reduction in scale. Conventional microfluidic devices are fabricated using solid plastics that prevent direct access to responding cells. Fluid-walled microfluidics allows the manufacture of circuits on standard Petri dishes in seconds, coupled to simple operating methods; cell-culture medium sitting in a standard dish is confined to circuits by fluid walls made of an immiscible fluorocarbon. We develop and experimentally validate an analytical model of diffusion between two or more aqueous streams flowing at different rates into a fluid-walled conduit with the cross-section of a circular segment. Unlike solid walls, fluid walls morph during flows as pressures fall, with wall shape changing down the conduit. The model is validated experimentally for Fourier numbers < 0.1 using fluorescein diffusing between laminar streams. It enables a priori prediction of concentration gradients throughout a conduit, so allowing rapid circuit design as well as providing bio-scientists with an accurate way of predicting local concentrations of bioactive molecules around responsive and non-responsive cells.

Mechanisms regulating neutrophil responses in immunity, allergy, and autoimmunity

Neutrophil granulocytes, or neutrophils, are the most abundant circulating leukocytes in humans and indispensable for antimicrobial immunity, as exemplified in patients with inborn and acquired defects of neutrophils. Neutrophils were long regarded as the foot soldiers of the immune system, solely destined to execute a set of effector functions against invading pathogens before undergoing apoptosis, the latter of which was ascribed to their short life span. This simplistic understanding of neutrophils has now been revised on the basis of insights gained from the use of mouse models and single-cell high-throughput techniques, revealing tissue- and context-specific roles of neutrophils in guiding immune responses. These studies also demonstrated that neutrophil responses were controlled by sophisticated feedback mechanisms, including directed chemotaxis of neutrophils to tissue-draining lymph nodes resulting in modulation of antimicrobial immunity and inflammation. Moreover, findings in mice and humans showed that neutrophil responses adapted to different deterministic cytokine signals, which controlled their migration and effector function as well as, notably, their biologic clock by affecting the kinetics of their aging. These mechanistic insights have important implications for health and disease in humans, particularly, in allergic diseases, such as atopic dermatitis and allergic asthma bronchiale, as well as in autoinflammatory and autoimmune diseases. Hence, our improved understanding of neutrophils sheds light on novel therapeutic avenues, focusing on molecularly defined biologic agents.

Simplified Cell Magnetic Isolation Assisted SC2 Chip to Realize "Sample in and Chemotaxis Out": Validated by Healthy and T2DM Patients' Neutrophils

Neutrophil migration in tissues critically regulates the human immune response and can either play a protective role in host defense or cause health problems. Microfluidic chips are increasingly applied to study neutrophil migration, attributing to their advantages of low reagent consumption, stable chemical gradients, visualized cell chemotaxis monitoring, and quantification. Most chemotaxis chips suffered from low throughput and fussy cell separation operations. We here reported a novel and simple "sample in and chemotaxis out" method for rapid neutrophils isolation from a small amount of whole blood based on a simplified magnetic method, followed by a chemotaxis assay on a microfluidic chip (SC² chip) consisting of six cell migration units and six-cell arrangement areas. The advantages of the "sample in and chemotaxis out" method included: less reagent consumption (10 μL of blood + 1 μL of magnetic beads + 1 μL of lysis buffer); less time (5 min of cell isolation + 15 min of chemotaxis testing); no ultracentrifugation; more convenient; higher efficiency; high throughput. We have successfully validated the approach by measuring neutrophil chemotaxis to frequently-used chemoattractant (i.e., fMLP). The effects of D-glucose and mannitol on neutrophil chemotaxis were also analyzed. In addition, we demonstrated the effectiveness of this approach for testing clinical samples from diabetes mellitus type 2 (T2DM) patients. We found neutrophils' migration speed was higher in the "well-control" T2DM than in the "poor-control" group. Pearson coefficient analysis further showed that the migration speed of T2DM was negatively correlated with physiological indicators, such as HbA1c (-0.44), triglyceride (-0.36), C-reactive protein (-0.28), and total cholesterol (-0.28). We are very confident that the developed "sample in and chemotaxis out" method was hoped to be an attractive model for analyzing the chemotaxis of healthy and disease-associated neutrophils.

IsoLGs (Isolevuglandins) Drive Neutrophil Migration in Hypertension and Are Essential for the Formation of Neutrophil Extracellular Traps

BACKGROUND

IsoLGs (isolevuglandins) are electrophilic products of lipid peroxidation formed in the presence of reactive oxygen species. IsoLGs contribute to hypertension by an unknown mechanism. Studies have shown that reactive oxygen species production drives the formation of neutrophil extracellular traps (NETs) and that NETs accumulate within the aorta and kidneys of patients with hypertension. The purpose of this study was to determine the role of isoLGs in neutrophil migration and NET formation (NETosis) in hypertension.

METHODS

Mice were treated with Ang II (angiotensin II) and the specific isoLG scavenger 2-hydroxybenzylamine and examined for tissue neutrophil and NET accumulation by single-cell sequencing and flow cytometry. Isolated human neutrophils were studied to determine the role of isoLGs in NETosis and neutrophil chromatin expansion by immunofluorescence and live cell confocal microscopy.

RESULTS

Single-cell sequencing performed on sham, Ang II, and Ang II+2-hydroxybenzylamine treated mice revealed neutrophils as a primary target of 2-hydroxybenzylamine. Peripheral neutrophil migration, aortic NET accumulation, and renal NET accumulation is blocked with 2-hydroxybenzylamine treatment. In isolated human neutrophils, isoLGs accumulate during NETosis and scavenging of isoLGs prevents NETosis. IsoLGs drive neutrophil chromatin expansion during NETosis and disrupt nucleosome structure.

CONCLUSIONS

These observations identified a critical role of isoLGs in neutrophil migration and NETosis in hypertension and provide a potential therapy for NET-associated diseases including hypertension and associated end organ damage.

Immunotherapy discovery on tumor organoid-on-a-chip platforms that recapitulate the tumor microenvironment

Cancer immunotherapy has achieved remarkable success over the past decade by modulating patients' own immune systems and unleashing pre-existing immunity. However, only a minority of cancer patients across different cancer types are able to benefit from immunotherapy treatment; moreover, among those small portions of patients with response, intrinsic and acquired resistance remains a persistent challenge. Because the tumor microenvironment (TME) is well recognized to play a critical role in tumor initiation, progression, metastasis, and the suppression of the immune system and responses to immunotherapy, understanding the interactions between the TME and the immune system is a pivotal step in developing novel and efficient cancer immunotherapies. With unique features such as low reagent consumption, dynamic and precise fluid control, versatile structures and function designs, and 3D cell co-culture, microfluidic tumor organoid-on-a-chip platforms that recapitulate key factors of the TME and the immune contexture have emerged as innovative reliable tools to investigate how tumors regulate their TME to counteract antitumor immunity and the mechanism of tumor resistance to immunotherapy. In this comprehensive review, we focus on recent advances in tumor organoid-on-a-chip platforms for studying the interaction between the TME and the immune system. We first review different factors of the TME that recent microfluidic in vitro systems reproduce to generate advanced tools to imitate the crosstalk between the TME and the immune system. Then, we discuss their applications in the assessment of different immunotherapies' efficacy using tumor organoid-on-a-chip platforms. Finally, we present an overview and the outlook of engineered microfluidic platforms in investigating the interactions between cancer and immune systems, and the adoption of patient-on-a-chip models in clinical applications toward personalized immunotherapy.

Type 2 immune predisposition results in accelerated neutrophil aging causing susceptibility to bacterial infection

Atopic individuals show enhanced type 2 immune cell responses and a susceptibility to infections with certain bacteria and viruses. Although patients with allergic diseases harbor normal counts of circulating neutrophils, these cells exert deficient effector functions. However, the underlying mechanism of this dysregulation of neutrophils remains ill defined. Here, we find that development, aging, and elimination of neutrophils are accelerated in mice with a predisposition to type 2 immunity, which, in turn, causes susceptibility to infection with several bacteria. Neutrophil-mediated immunity to bacterial infection was greatly decreased in mice with a genetic or induced bias to type 2 immunity. Abrogation of interleukin-4 (IL-4) receptor signaling in these animals fully restored their antibacterial defense, which largely depended on Ly6G⁺ neutrophils. IL-4 signals accelerated the maturation of neutrophils in the bone marrow and caused their rapid release to the circulation and periphery. IL-4-stimulated neutrophils aged more rapidly in the periphery, as evidenced by their phenotypic and functional changes, including their decreased phagocytosis of bacterial particles. Moreover, neutrophils from type 2 immune predisposed mice were eliminated at a higher rate by apoptosis and phagocytosis by macrophages and dendritic cells. Collectively, IL-4 signaling-mediated neutrophil aging constitutes an important adaptive deficiency in type 2 inflammation, contributing to recurrent bacterial infections.

Patients with systemic sclerosis show phenotypic and functional defects in neutrophils

BACKGROUND

Systemic sclerosis (SSc) is a multiorgan autoimmune disease characterized by inflammation, vascular modification, and progressive fibrosis of the skin and several visceral organs. Innate and adaptive immune cells, including myeloid, B and T cells, are believed to be central to the pathogenesis of SSc. However, the role and functional state of neutrophil granulocytes (neutrophils) are ill-defined in SSc.

METHODS

We performed a prospective study of neutrophils freshly isolated from SSc patients and healthy donors (HD) by measuring in these neutrophils (i) functional cell surface markers, including CD16, CD62L, CD66b, CD66c, CXCR1, CXCR2, and CXCR4; (ii) cytokine-activated intracellular signal transducer and activator of transcription (STAT) pathways, such as phosphorylated STAT3 (pSTAT3), pSTAT5, and pSTAT6; (iii) production of neutrophil extracellular traps (NET) and intracellular myeloperoxidase (MPO); and (iv) phagocytosis of bacteria by the neutrophils.

RESULTS

Neutrophils of SSc patients expressed lower CD16 and CD62L and higher pSTAT3 and pSTAT6 compared to HD. Moreover, neutrophils of SSc patients lacked CXCR1 and CXCR2, the receptors responding to the potent neutrophil chemoattractant CXCL8. Neutrophils of SSc patients were also deficient in MPO levels, NET formation, and phagocytosis of bacteria.

CONCLUSIONS

Neutrophils of patients with SSc display several functional defects affecting cell migration, NET formation, and phagocytosis of bacteria.

CCR7-guided neutrophil redirection to skin-draining lymph nodes regulates cutaneous inflammation and infection

Neutrophils are the first nonresident effector immune cells that migrate to a site of infection or inflammation; however, improper control of neutrophil responses can cause considerable tissue damage. Here, we found that neutrophil responses in inflamed or infected skin were regulated by CCR7-dependent migration and phagocytosis of neutrophils in draining lymph nodes (dLNs). In mouse models of Toll-like receptor-induced skin inflammation and cutaneous Staphylococcus aureus infection, neutrophils migrated from the skin to the dLNs via lymphatic vessels in a CCR7-mediated manner. In the dLNs, these neutrophils were phagocytosed by lymph node-resident type 1 and type 2 conventional dendritic cells. CCR7 up-regulation on neutrophils was a conserved mechanism across different tissues and was induced by a broad range of microbial stimuli. In the context of cutaneous immune responses, disruption of CCR7 interactions by selective CCR7 deficiency of neutrophils resulted in increased antistaphylococcal immunity and aggravated skin inflammation. Thus, neutrophil homing to and clearance in skin-dLNs affects cutaneous immunity versus pathology.

A Novel Microfluidic Device for the Neutrophil Functional Phenotype Analysis: Effects of Glucose and Its Derivatives AGEs

Neutrophil dysfunction is closely related to the pathophysiology of patients with diabetes mellitus, but existing immunoassays are difficult to implement in clinical applications, and neutrophil’s chemotaxis as a functional biomarker for diabetes mellitus prognostic remains largely unexplored. Herein, a novel microfluidic device consisted of four independent test units with four cell docking structures was developed to study the neutrophil chemotaxis, which allowed multiple cell migration observations under a single field of view (FOV) and guaranteed more reliable results. In vitro studies, the chemotaxis of healthy neutrophils to N-Formyl-Met-Leu-Phe (fMLP) gradient (0, 10, 100, and 1000 nM) was concentration-dependent. The distinct promotion or suppression in the chemotaxis of metformin or pravastatin pretreated cells were observed after exposure to 100 nM fMLP gradient, indicating the feasibility and efficiency of this novel microfluidic device for clinically relevant evaluation of neutrophil functional phenotype. Further, the chemotaxis of neutrophils pretreated with 25, 50, or 70 mM of glucose was quantitatively lower than that of the control groups (i.e., 5 mM normal serum level). Neutrophils exposed to highly concentrated advanced glycation end products (AGEs) (0.2, 0.5, or 1.0 μM; 0.13 μM normal serum AGEs level), a product of prolonged hyperglycemia, showed that the higher the AGEs concentration was, the weaker the migration speed became. Specifically, neutrophils exposed to high concentrations of glucose or AGEs also showed a stronger drifting along with the flow, further demonstrating the change of neutrophil chemotaxis. Interestingly, adding the N-benzyl-4-chloro-N-cyclohexylbenzamide (FPS-ZM1) (i.e., high-affinity RAGE inhibitor) into the migration medium with AGEs could hinder the binding between AGEs and AGE receptor (RAGE) located on the neutrophil, thereby keeping the normal chemotaxis of neutrophils than the ones incubated with AGEs alone. These results revealed the negative effects of high concentrations of glucose and AGEs on the neutrophil chemotaxis, suggesting that patients with diabetes should manage serum AGEs and also pay attention to blood glucose indexes. Overall, this novel microfluidic device could significantly characterize the chemotaxis of neutrophils and have the potential to be further improved into a tool for risk stratification of diabetes mellitus.

Group-2 innate lymphoid cell-dependent regulation of tissue neutrophil migration by alternatively activated macrophage-secreted Ear11

Type-2 immunity is characterised by interleukin (IL)-4, IL-5 and IL-13, eosinophilia, mucus production, IgE, and alternatively activated macrophages (AAM). However, despite the lack of neutrophil chemoattractants such as CXCL1, neutrophils, a feature of type-1 immunity, are observed in type-2 responses. Consequently, alternative mechanisms must exist to ensure that neutrophils can contribute to type-2 immune reactions without escalation of deleterious inflammation. We now demonstrate that type-2 immune-associated neutrophil infiltration is regulated by the mouse RNase A homologue, eosinophil-associated ribonuclease 11 (Ear11), which is secreted by AAM downstream of IL-25-stimulated ILC2. Transgenic overexpression of Ear11 resulted in tissue neutrophilia, whereas Ear11-deficient mice have fewer resting tissue neutrophils, whilst other type-2 immune responses are not impaired. Notably, administration of recombinant mouse Ear11 increases neutrophil motility and recruitment. Thus, Ear11 helps maintain tissue neutrophils at homoeostasis and during type-2 reactions when chemokine-producing classically activated macrophages are infrequently elicited.

Microfluidic devices for neutrophil chemotaxis studies

Neutrophil chemotaxis plays a vital role in human immune system. Compared with traditional cell migration assays, the emergence of microfluidics provides a new research platform of cell chemotaxis study due to the advantages of visualization, precise control of chemical gradient, and small consumption of reagents. A series of microfluidic devices have been fabricated to study the behavior of neutrophils exposed on controlled, stable, and complex profiles of chemical concentration gradients. In addition, microfluidic technology offers a promising way to integrate the other functions, such as cell culture, separation and analysis into a single chip. Therefore, an overview of recent developments in microfluidic-based neutrophil chemotaxis studies is presented. Meanwhile, the strength and drawbacks of these devices are compared.