Fluorescent activated cell sorting: an effective approach to study dendritic cell subsets in human atherosclerotic plaques

Modeling human atherosclerotic lesions in the test tube: Are we there yet?

Atherosclerotic cardiovascular diseases remain a leading cause of morbidity and mortality worldwide. Atherogenesis is a slow and life-long process characterized by the accumulation of lipoproteins and immune cells within the arterial wall. Atherosclerosis has been successfully modeled in animals: However, there are economic, ethical, and translational concerns when using these models. There is also growing recognition of the need for robust human-based in vitro systems that can faithfully recapitulate key aspects of human atherosclerosis. Such systems may offer advantages in terms of scalability, reproducibility, and ability to manipulate specific variables, thereby facilitating a deeper understanding of disease mechanisms and accelerating the development of targeted therapeutics. Leveraging innovative in vitro platforms holds promise in complementing traditional animal models of atherosclerosis. In the present review, we discuss the advantages and disadvantages of recently developed models of atherosclerosis and propose ideas to be considered when developing future generations of models.

Applications of flow cytometry sorting in the pharmaceutical industry: A review

The article reviews applications of flow cytometry sorting in manufacturing of pharmaceuticals. Flow cytometry sorting is an extremely powerful tool for monitoring, screening and separating single cells based on any property that can be measured by flow cytometry. Different applications of flow cytometry sorting are classified into groups and discussed in separate sections as follows: (a) isolation of cell types, (b) high throughput screening, (c) cell surface display, (d) droplet fluorescent-activated cell sorting (FACS). Future opportunities are identified including: (a) sorting of particular fractions of the cell population based on a property of interest for generating inoculum that will result in improved outcomes of cell cultures and (b) the use of population balance models in combination with FACS to design and optimize cell cultures.

Dendritic Cells and T Cells, Partners in Atherogenesis and the Translating Road Ahead

Atherosclerosis is a chronic process associated with arterial inflammation, the accumulation of lipids, plaque formation in vessel walls, and thrombosis with late mortal complications such as myocardial infarction and ischemic stroke. Immune and inflammatory responses have significant effects on every phase of atherosclerosis. Increasing evidence has shown that both innate and adaptive “arms” of the immune system play important roles in regulating the progression of atherosclerosis. Accumulating evidence suggests that a unique type of innate immune cell, termed dendritic cells (DCs), play an important role as central instigators, whereas adaptive immune cells, called T lymphocytes, are crucial as active executors of the DC immunity in atherogenesis. These two important immune cell types work in pairs to establish pro-atherogenic or atheroprotective immune responses in vascular tissues. Therefore, understanding the role of DCs and T cells in atherosclerosis is extremely important. Here, in this review, we will present a complete overview, based on existing knowledge of these two cell types in the atherosclerotic microenvironment, and discuss some of the novel means of targeting DCs and T cells as therapeutic tactics for the treatment of atherosclerosis.

CD8+ T-cells contribute to lesion stabilization in advanced atherosclerosis by limiting macrophage content and CD4+ T-cell responses

AIMS

T lymphocytes play an important role in atherosclerosis development, but the role of the CD8+ T-cell remains debated, especially in the clinically relevant advanced stages of atherosclerosis development. Here, we set out to determine the role of CD8+ T-cells in advanced atherosclerosis.

METHODS AND RESULTS

Human endarterectomy samples analysed by flow cytometry showed a negative correlation between the percentage of CD8+ T-cells and macrophages, suggesting a possible protective role for these cells in lesion development. To further test this hypothesis, LDLr-/- mice were fed a western-type diet (WTD) for 10 weeks to induce atherosclerosis, after which they received CD8α-depleting or isotype control antibody for 6 weeks. Depletion of CD8+ T-cells in advanced atherosclerosis resulted in less stable lesions, with significantly reduced collagen content in the trivalve area, increased macrophage content and increased necrotic core area compared with controls. Mechanistically, we observed that CD8 depletion specifically increased the fraction of Th1 CD4+ T-cells in the lesions. Treatment of WTD-fed LDLr-/- mice with a FasL-neutralizing antibody resulted in similar changes in macrophages and CD4+ T-cell skewing as CD8+ T-cell depletion.

CONCLUSION

These findings demonstrate for the first time a local, protective role for CD8+ T-cells in advanced atherosclerosis, through limiting accumulation of Th1 cells and macrophages, identifying a novel regulatory mechanism for these cells in atherosclerosis.

B- and T-lymphocyte attenuator stimulation protects against atherosclerosis by regulating follicular B cells

AIMS

The immune system is strongly involved in atherosclerosis and immune regulation generally leads to attenuated atherosclerosis. B- and T-lymphocyte attenuator (BTLA) is a novel co-receptor that negatively regulates the activation of B and T cells; however, there have been no reports of BTLA and its function in atherosclerosis or cardiovascular disease (CVD). We aimed to assess the dominant BTLA expressing leucocyte in CVD patients and to investigate whether BTLA has a functional role in experimental atherosclerosis.

METHODS AND RESULTS

We show that BTLA is primarily expressed on B cells in CVD patients and follicular B2 cells in low-density lipoprotein receptor-deficient (Ldlr-/-) mice. We treated Ldlr-/- mice that were fed a western-type diet (WTD) with phosphate-buffered saline, an isotype antibody, or an agonistic BTLA antibody (3C10) for 6 weeks. We report here that the agonistic BTLA antibody significantly attenuated atherosclerosis. This was associated with a strong reduction in follicular B2 cells, while regulatory B and T cells were increased. The BTLA antibody showed similar immunomodulating effects in a progression study in which Ldlr-/- mice were fed a WTD for 10 weeks before receiving antibody treatment. Most importantly, BTLA stimulation enhanced collagen content, a feature of stable lesions, in pre-existing lesions.

CONCLUSION

Stimulation of the BTLA pathway in Ldlr-/- mice reduces initial lesion development and increases collagen content of established lesions, presumably by shifting the balance between atherogenic follicular B cells and atheroprotective B cells and directing CD4+ T cells towards regulatory T cells. We provide the first evidence that BTLA is a very promising target for the treatment of atherosclerosis.

Immune cells in carotid artery plaques: what can we learn from endarterectomy specimens?

INTRODUCTION

Endarterectomy specimens represent a unique opportunity to study atherosclerosis. This review aims to summarize the recent knowledge of atherogenesis from studies characterizing a cellular composition of carotid endarterectomy specimens.

EVIDENCE ACQUISITION

A non-systematic literature review was carried out to summarize recent knowledge regarding ex vivo analysis of carotid artery plaque composition. Upon evaluation of their relevance, and elaborate forward and backward search, 95 articles were included in the review.

EVIDENCE SYNTHESIS

Despite the significant advancement of in vivo imaging techniques, the stroke prediction based on carotid artery plaque morphology is not reliable. Besides analyses of plaque morphology, present studies focus on precise characterization of the different immune cell types and elucidation of their role in plaque development. Plaque content analyses revealed the presence of various immune cells in carotid artery plaques. Presence of different immune cells subpopulations can be connected to some undesirable changes in plaque stability.

CONCLUSIONS

Since the destabilization of the atherosclerotic plaque is a multifactorial process, a combination of various methods should be used to characterize the unstable plaques more accurately. In this context, studies characterizing plaque content from a cellular point of view could elucidate some processes underlying the plaque progression. Together with morphological evaluation, these analyses could enable more precise assessment of plaque stability.

CD39 identifies a microenvironment-specific anti-inflammatory CD8+ T-cell population in atherosclerotic lesions

BACKGROUND AND AIMS

CD8⁺ T-cells have been attributed both atherogenic and atheroprotective properties, but analysis of CD8⁺ T-cells has mostly been restricted to the circulation and secondary lymphoid organs. The atherosclerotic lesion, however, is a complex microenvironment containing a plethora of inflammatory signals, which may affect CD8⁺ T-cell activation. Here, we address how this environment affects the functionality of CD8⁺ T-cells.

METHODS AND RESULTS

We compared the cytokine production of CD8⁺ T-cells derived from spleens and enzymatically digested aortas of apoE^-/- mice with advanced atherosclerosis by flow cytometry. Aortic CD8⁺ T-cells produced decreased amounts of IFN-γ and TNF-α compared to their systemic counterparts. The observed dysfunctional phenotype of the lesion-derived CD8⁺ T-cells was not associated with classical exhaustion markers, but with increased expression of the ectonucleotidase CD39. Indeed, pharmacological inhibition of CD39 in apoE^-/- mice partly restored cytokine production by CD8⁺ T-cells. Using a bone-marrow transplantation approach, we show that TCR signaling is required to induce CD39 expression on CD8⁺ T-cells in atherosclerotic lesions. Importantly, analysis of human endarterectomy samples showed a strong microenvironment specific upregulation of CD39 on CD8⁺ T-cells in the plaques of human patients compared to matched blood samples.

CONCLUSIONS

Our results suggest that the continuous TCR signaling in the atherosclerotic environment in the vessel wall induces an immune regulatory CD8⁺ T-cell phenotype that is associated with decreased cytokine production through increased CD39 expression in both a murine atherosclerotic model and in atherosclerosis patients. This provides a new understanding of immune regulation by CD8⁺ T-cells in atherosclerosis.

Flow Cytometry-Based Characterization of Mast Cells in Human Atherosclerosis

The presence of mast cells in human atherosclerotic plaques has been associated with adverse cardiovascular events. Mast cell activation, through the classical antigen sensitized-IgE binding to their characteristic Fcε-receptor, causes the release of their cytoplasmic granules. These granules are filled with neutral proteases such as tryptase, but also with histamine and pro-inflammatory mediators. Mast cells accumulate in high numbers within human atherosclerotic tissue, particularly in the shoulder region of the plaque. These findings are largely based on immunohistochemistry, which does not allow for the extensive characterization of these mast cells and of the local mast cell activation mechanisms. In this study, we thus aimed to develop a new flow-cytometry based methodology in order to analyze mast cells in human atherosclerosis. We enzymatically digested 22 human plaque samples, collected after femoral and carotid endarterectomy surgery, after which we prepared a single cell suspension for flow cytometry. We were able to identify a specific mast cell population expressing both CD117 and the FcεR, and observed that most of the intraplaque mast cells were activated based on their CD63 protein expression. Furthermore, most of the activated mast cells had IgE fragments bound on their surface, while another fraction showed IgE-independent activation. In conclusion, we are able to distinguish a clear mast cell population in human atherosclerotic plaques, and this study establishes a strong relationship between the presence of IgE and the activation of mast cells in advanced atherosclerosis. Our data pave the way for potential therapeutic intervention through targeting IgE-mediated actions in human atherosclerosis.

Hypercholesterolemia Induces a Mast Cell-CD4+ T Cell Interaction in Atherosclerosis

Mast cells (MCs) are potent innate immune cells that aggravate atherosclerosis through the release of proinflammatory mediators inside atherosclerotic plaques. Similarly, CD4⁺ T cells are constituents of the adaptive immune response and accumulate within the plaques following lipid-specific activation by APCs. Recently it has been proposed that these two cell types can interact in a direct manner. However, no indication of such an interaction has been investigated in the context of atherosclerosis. In our study, we aimed to examine whether MCs can act as APCs in atherosclerosis, thereby modulating CD4⁺ T cell responses. We observed that MCs increased their MHC class II expression under hyperlipidemic conditions both in vivo and in vitro. Furthermore, we showed that MCs can present Ags in vivo via MHC class II molecules. Serum from high-fat diet-fed mice also enhanced the expression of the costimulatory molecule CD86 on cultured MCs, whereas OVA peptide-loaded MCs increased OT-II CD4⁺ T cell proliferation in vitro. The aortic CD4⁺ and T_H1 cell content of atherosclerotic mice that lack MCs was reduced as compared with their wild-type counterparts. Importantly, we identified MCs that express HLA-DR in advanced human atheromata, indicating that these cells are capable of Ag presentation within human atherosclerotic plaques. Therefore, in this artice, we show that MCs may directly modulate adaptive immunity by acting as APCs in atherosclerosis.

An update on clonality: what smooth muscle cell type makes up the atherosclerotic plaque?

Almost 50 years ago, Earl Benditt and his son John described the clonality of the atherosclerotic plaque. This led Benditt to propose that the atherosclerotic lesion was a smooth muscle neoplasm, similar to the leiomyomata seen in the uterus of most women. Although the observation of clonality has been confirmed many times, interest in the idea that atherosclerosis might be a form of neoplasia waned because of the clinical success of treatments for hyperlipemia and because animal models have made great progress in understanding how lipid accumulates in the plaque and may lead to plaque rupture. Four advances have made it important to reconsider Benditt's observations. First, we now know that clonality is a property of normal tissue development. Second, this is even true in the vessel wall, where we now know that formation of clonal patches in that wall is part of the development of smooth muscle cells that make up the tunica media of arteries. Third, we know that the intima, the "soil" for development of the human atherosclerotic lesion, develops before the fatty lesions appear. Fourth, while the cells comprising this intima have been called "smooth muscle cells", we do not have a clear definition of cell type nor do we know if the initial accumulation is clonal. As a result, Benditt's hypothesis needs to be revisited in terms of changes in how we define smooth muscle cells and the quite distinct developmental origins of the cells that comprise the muscular coats of all arterial walls. Finally, since clonality of the lesions is real, the obvious questions are do these human tumors precede the development of atherosclerosis, how do the clones develop, what cell type gives rise to the clones, and in what ways do the clones provide the soil for development and natural history of atherosclerosis?

The cathelicidin protein CRAMP is a potential atherosclerosis self-antigen in ApoE(-/-) mice

Auto-immunity is believed to contribute to inflammation in atherosclerosis. The antimicrobial peptide LL-37, a fragment of the cathelicidin protein precursor hCAP18, was previously identified as an autoantigen in psoriasis. Given the reported link between psoriasis and coronary artery disease, the biological relevance of the autoantigen to atherosclerosis was tested in vitro using a truncated (t) form of the mouse homolog of hCAP18, CRAMP, on splenocytes from athero-prone ApoE(-/-) mice. Stimulation with tCRAMP resulted in increased CD8+ T cells with Central Memory and Effector Memory phenotypes in ApoE(-/-) mice, differentially activated by feeding with normal chow or high fat diet. Immunization of ApoE(-/-) with different doses of the shortened peptide (Cramp) resulted in differential outcomes with a lower dose reducing atherosclerosis whereas a higher dose exacerbating the disease with increased neutrophil infiltration of the atherosclerotic plaques. Low dose Cramp immunization also resulted in increased splenic CD8+ T cell degranulation and reduced CD11b+CD11c+ conventional dendritic cells (cDCs), whereas high dose increased CD11b+CD11c+ cDCs. Our results identified CRAMP, the mouse homolog of hCAP-18, as a potential self-antigen involved in the immune response to atherosclerosis in the ApoE(-/-) mouse model.

Characterization of Human Monocyte-derived Dendritic Cells by Imaging Flow Cytometry: A Comparison between Two Monocyte Isolation Protocols

Dendritic cells (DCs) are antigen presenting cells of the immune system that play a crucial role in lymphocyte responses, host defense mechanisms, and pathogenesis of inflammation. Isolation and study of DCs have been important in biological research because of their distinctive features. Although they are essential key mediators of the immune system, DCs are very rare in blood, accounting for approximately 0.1 - 1% of total blood mononuclear cells. Therefore, alternatives for isolation methods rely on the differentiation of DCs from monocytes isolated from peripheral blood mononuclear cells (PBMCs). The utilization of proper isolation techniques that combine simplicity, affordability, high purity, and high yield of cells is imperative to consider. In the current study, two distinct methods for the generation of DCs will be compared. Monocytes were selected by adherence or negatively enriched using magnetic separation procedure followed by differentiation into DCs with IL-4 and GM-CSF. Monocyte and MDDC viability, proliferation, and phenotype were assessed using viability dyes, MTT assay, and CD11c/ CD14 surface marker analysis by imaging flow cytometry. Although the magnetic separation method yielded a significant higher percentage of monocytes with higher proliferative capacity when compared to the adhesion method, the findings have demonstrated the ability of both techniques to simultaneously generate monocytes that are capable of proliferating and differentiating into viable CD11c+ MDDCs after seven days in culture. Both methods yielded > 70% CD11c+ MDDCs. Therefore, our results provide insights that contribute to the development of reliable methods for isolation and characterization of human DCs.

A Numerical Simulation of Cell Separation by Simplified Asymmetric Pinched Flow Fractionation

As a typical microfluidic cell sorting technique, the size-dependent cell sorting has attracted much interest in recent years. In this paper, a size-dependent cell sorting scheme is presented based on a controllable asymmetric pinched flow by employing an immersed boundary-lattice Boltzmann method (IB-LBM). The geometry of channels consists of 2 upstream branches, 1 transitional channel, and 4 downstream branches (D-branches). Simulations are conducted by varying inlet flow ratio, the cell size, and the ratio of flux of outlet 4 to the total flux. It is found that, after being randomly released in one upstream branch, the cells are aligned in a line close to one sidewall of the transitional channel due to the hydrodynamic forces of the asymmetric pinched flow. Cells with different sizes can be fed into different downstream D-branches just by regulating the flux of one D-branch. A principle governing D-branch choice of a cell is obtained, with which a series of numerical cases are performed to sort the cell mixture involving two, three, or four classes of diameters. Results show that, for each case, an adaptive regulating flux can be determined to sort the cell mixture effectively.

Linking CD11b (+) Dendritic Cells and Natural Killer T Cells to Plaque Inflammation in Atherosclerosis

Atherosclerosis remains the leading cause of death and disability in our Western society. To investigate whether the dynamics of leukocyte (sub)populations could be predictive for plaque inflammation during atherosclerosis, we analyzed innate and adaptive immune cell distributions in blood, plaques, and lymphoid tissue reservoirs in apolipoprotein E-deficient (ApoE^−/−) mice and in blood and plaques from patients undergoing endarterectomy. Firstly, there was predominance of the CD11b⁺ conventional dendritic cell (cDC) subset in the plaque. Secondly, a strong inverse correlation was observed between CD11b⁺ cDC or natural killer T (NKT) cells in blood and markers of inflammation in the plaque (including CD3, T-bet, CCR5, and CCR7). This indicates that circulating CD11b⁺ cDC and NKT cells show great potential to reflect the inflammatory status in the atherosclerotic plaque. Our results suggest that distinct changes in inflammatory cell dynamics may carry biomarker potential reflecting atherosclerotic lesion progression. This not only is crucial for a better understanding of the immunopathogenesis but also bares therapeutic potential, since immune cell-based therapies are emerging as a promising novel strategy in the battle against atherosclerosis and its associated comorbidities. The cDC-NKT cell interaction in atherosclerosis serves as a good candidate for future investigations.

Isolation of dendritic cells from umbilical cord blood using magnetic activated cell sorting or adherence

Dendritic cells (DCs) are a highly specialized type of antigen-presenting cell. The present study describes and compares two methods for preparing DCs from umbilical cord blood. The first method involves the isolation of DCs by magnetic activated cell sorting (MACS). This technique isolates CD34⁺ cells from cord blood and induces the formation of DCs by the addition of cytokines, granulocyte macrophage colony-stimulating factor and interleukin-4. The second method involves the generation of large numbers of DCs from cord blood using an adherent method, which isolates umbilical cord blood mononuclear cells and induces DCs in the same conditions as those used in MACS. The DCs were harvested following 7 days of incubation and observed with an inverted microscope. The phenotype of the cells was then analyzed by flow cytometry. The results revealed that, subsequent to 7 days of incubation, the differentiated DCs obtained using the adherent method were more mature than those isolated using MACS. However, these cells were unable to be maintained in culture for more than 9-10 days. By contrast, the DCs derived from CD34⁺ cells by MACS were phenotypically stable and could be maintained for up to 3 weeks in culture. Either method produced DCs from cord blood. However, the DCs isolated using the MACS method demonstrated higher homogeneity, yield and viability than those obtained using the adherent method. Due to the various compositions of the monocyte subsets isolated, isolation methods affect the phenotypes and functions of the resultant DCs.