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Immune cells in atherosclerosis include T, B, natural killer (NK) and NKT cells, macrophages, monocytes, dendritic cells (DCs), neutrophils, and mast cells. Advances in single-cell RNA sequencing (sRNA-Seq) have refined our understanding of immune cell subsets. Four recent studies have used scRNA-Seq of immune cells in human atherosclerotic lesions and peripheral blood mononuclear cells (PBMCs), some including cell surface phenotypes revealed by oligonucleotide-tagged antibodies, which confirmed known and identified new immune cell subsets and identified genes significantly up-regulated in PBMCs from HIV+ subjects with atherosclerosis compared to PBMCs from matched HIV+ subjects without atherosclerosis. The ability of scRNA-Seq to identify cell types is greatly augmented by adding cell surface phenotype using antibody sequencing. In this review, we summarize the latest data obtained by scRNA-Seq on plaques and human PBMCs in human subjects with atherosclerosis.

Cryopreserved peripheral blood mononuclear cells (PBMCs) are frequently collected and provide disease- and treatment-relevant data in clinical studies. Here, we developed combined protein (40 antibodies) and transcript single cell (sc)RNA sequencing in PBMCs. Among 31 participants in the WIHS Study, we sequenced 41,611 cells. Using Boolean gating followed by Seurat UMAPs and Louvain clustering, we identified 58 subsets among CD4 T, CD8 T, B, NK cells and monocytes. This resolution was superior to flow cytometry, mass cytometry or scRNA-sequencing without antibodies. Since the transcriptome was not needed for cell identification, combined protein and transcript scRNA-Seq allowed for the assessment of disease-related changes in transcriptomes and cell type proportion. As a proof-of-concept, we showed such differences between healthy and matched individuals living with HIV with and without cardiovascular disease. In conclusion, combined protein and transcript scRNA sequencing is a suitable and powerful method for clinical investigations using PBMCs.

Aims The role of CD4+ T cells in atherosclerosis has been shown to be dependent on cytokine cues that regulate lineage commitment into mature T helper sub-sets. In this study, we tested the roles of IL-1R1 and MyD88 signalling in CD4+ T cells in atherosclerosis. Methods and results We transferred apoe-/-myd88+/+ or apoe-/-myd88-/- CD4+ T cells to T- and B-cell-deficient rag1-/-apoe-/- mice fed high fat diet. Mice given apoe-/-myd88-/- CD4+ T cells exhibited reduced atherosclerosis compared with mice given apoe-/-myd88+/+ CD4+ T cells. CD4+ T cells from apoe-/-myd88-/- produced less IL-17 but similar levels of IFN-γ. Treatment of human CD4+ T cells with a MyD88 inhibitor inhibited IL-17 secretion in vitro. Transfer of il1r1-/- CD4+ T cells recapitulated the phenotype seen by transfer of myd88-/- CD4+ T cells with reduced lesion development and a reduction in Th17 and IL-17 production compared with wild type CD4+ T cell recipients. Relative collagen content of lesions was reduced in mice receiving il1r1-/- CD4+ T cells. Conclusion We demonstrate that both IL1R and MyD88 signalling in CD4+ T cells promote Th17 immunity, plaque growth and may regulate plaque collagen levels.

Background Oxidation of LDL particles entrapped in the extracellular matrix of the arterial wall is a key factor in the development of atherosclerosis. Lipid oxidation products, such as malondialdehyde (MDA), react with surrounding extracellular matrix proteins and cause modifications that are recognized by the immune system. MDA modification of collagen type IV is increased in carotid lesions from symptomatic patients and correlates with autoantibodies against MDA-modified collagen type IV in plasma. Objective The aim of this study was to determine whether autoantibodies against MDA-modified collagen type IV predict risk of development of myocardial infarction (MI). Methods Plasma levels of MDA-modified collagen type IV IgM and IgG antibodies were analysed by enzyme-linked immunosorbent assay in 385 subjects with incident MI during 13 years of follow-up and 410 age- and sex-matched controls in the Malmo Diet and Cancer study. Results MDA-modified collagen type IV IgG levels were higher in cases with incident MI than in controls. Subjects in the highest tertile of MDA-modified collagen type IV IgG had an increased risk of MI (hazard ratio 1.56, 95% confidence interval 1.22–2.00, P for trend 0.0004). This association remained significant after adjusting for factors included in the Framingham risk score and diabetes. High levels of MDA-collagen type IV IgG were associated with increased carotid intima-media thickness and elevated plasma levels of matrix metalloproteinase 10 and 12. Conclusions Immune responses against MDA-modified collagen type IV are associated with more severe carotid disease and increased risk of MI. These immune responses may reflect LDL oxidation in the artery wall, but could also affect the atherosclerotic disease process.

Oxidation of low-density lipoprotein (LDL) in the arterial extracellular matrix results in malondialdehyde (MDA)-modifications of surrounding matrix proteins. We have recently demonstrated an association between high levels of autoantibodies against MDA-modified collagen type IV and risk for development of myocardial infarction. Collagen type IV is an important component of the endothelial basement membrane and influences smooth muscle cell function. We hypothesized that immune responses against collagen type IV could contribute to vascular injury affecting the development of atherosclerosis. To investigate this possibility, we induced an antibody-response against collagen type IV in apolipoprotein E (Apo E)-deficient mice. Female ApoE−/− mice on C57BL/6 background were immunized with α1α2 type IV collagen chain peptides linked to the immune-enhancer PADRE, PADRE alone or PBS at 12 weeks of age with three subsequent booster injections before the mice were killed at 23 weeks of age. Immunization of PADRE alone induced autoantibodies against PADRE, increased IL-4 secretion from splenocytes and reduced SMC content in the subvalvular plaques. Immunization with peptides of α1α2 type IV collagen chains induced a strong IgG1antibody response against collagen type IV peptides without affecting the distribution of T cell populations, plasma cytokine or lipid levels. There were no differences in atherosclerotic plaque development between collagen α1α2(IV)-PADRE immunized mice and control mice. Our findings demonstrate that the presence of antibodies against the basement membrane component collagen type IV does not affect atherosclerosis development in ApoE−/− mice. This suggests that the association between autoantibodies against collagen type IV and risk for myocardial infarction found in humans does not reflect a pathogenic role of these autoantibodies.

The mechanisms responsible for macrovascular complications in diabetes remain to be fully understood. Recent studies have identified impaired vascular repair as a possible cause of plaque vulnerability in diabetes. This notion is supported by observations of a reduced content of fibrous proteins and smooth muscle cell mitogens in carotid endarterectomy from diabetic patients along with findings of decreased circulating levels of endothelial progenitor cells. In the present study we used a diabetic mouse model to characterize how hyperglycemia affects arterial repair responses. We induced atherosclerotic plaque formation in ApoE-deficient (ApoE−/−) and heterozygous glucokinase knockout ApoE-deficient mice (ApoE−/− GK+/−) mice with a shear stress-modifying cast. There were no differences in cholesterol or triglyceride levels between the ApoE−/− and ApoE−/− GK+/− mice. Hyperglycemia did not affect the size of the formed atherosclerotic plaques, and no effects were seen on activation of cell proliferation, smooth muscle cell content or on the expression and localization of collagen, elastin and several other extracellular matrix proteins. The present study demonstrates that hyperglycemia per se has no significant effects on tissue repair processes in injured mouse carotid arteries, suggesting that other mechanisms are involved in diabetic plaque vulnerability.

Background The absence of interleukin-25 (IL-25) favors the induction of Th1 and Th17 immune responses in mice. Th1 immune responses have been associated with the pathology of atherosclerosis, a lipid and inflammation driven disease of the arterial wall. Purpose of research To evaluate the effect of IL-25 on human peripheral blood mononuclear cells (hPBMCs) in the presence and absence of oxidized low density lipoprotein (oxLDL), a key player in atherosclerosis development. Principal results Human PBMCs were incubated with recombinant human IL-25 (rhIL-25) in the presence and absence of oxLDL and analyzed with flow cytometry while cytokine secretion was measured in cell culture supernatants. The IL-25 receptor, IL-17RB, was mostly expressed on T cells. Incubation of hPBMCs with IL-25 reduced the frequency of Th17 cells. Furthermore, IL-25 inhibited the release of the Th17-inducing cytokine IL-6 from dendritic cells isolated from hPBMCs indicating that the IL-25 mediated Th17 suppression may be indirect. Moreover, IL-25 reduced the secretion of the proinflammatory cytokine IFNγ from hPBMCs. OxLDL decreased IFNγ release from hPBMCs regardless of the presence or absence of IL-25. Conclusions IL-25 reduces Th1 and Th17 immune responses in hPBMCs raising the interesting possibility that IL-25 could have a protective role in human atherosclerosis.

Increased glucose levels are associated with the generation of advanced glycation endproduct (AGE) modifications. Interaction between AGE-modified plaque components and immune cells is believed to have an important role in the development of vascular complications in diabetes. Methylglyoxal (MGO) is one type of reactive aldehyde that gives rise to AGE modification. The present study analyzed whether autoantibodies against MGO-modified epitopes of the low-density lipoprotein apolipoprotein B (apoB) 100 predict cardiovascular events. A library consisting of 302 peptides comprising the complete apoB100 molecule was screened to identify peptides targeted by MGO-specific autoantibodies. Peptide (p) 220 (apoB amino acids 3286-3305) was identified as a major target. Baseline IgM and IgG against MGO–peptide 220 (p220) were measured in 700 individuals from the Malmö Diet and Cancer Cohort. A total of 139 cardiovascular events were registered during the 15-y follow-up period. Controlling for major cardiovascular risk factors demonstrated that subjects in the lowest tertile of MGO-p220 IgM had an increased risk for cardiovascular events (hazard ratio [95% confidence interval]: 2.07 [1.22–3.50]; ptrend = 0.004). Interestingly, the association between MGO-p220 IgM and cardiovascular events remained and even tended to become stronger when subjects with prevalent diabetes were excluded from the analysis (2.51 [1.37-4.61]; ptrend = 0.002). MGO-p220 IgM was inversely associated with blood glucose, but not with oxidized low-density lipoprotein. Finally, we demonstrate that anti-MGO-p220 IgM is produced by B1 cells. These data show that subjects with low levels of IgM recognizing MGO-modified p220 in apoB have an increased risk to develop cardiovascular events and that this association is present in nondiabetic subjects.