Inflammatory Profiling of Peripheral Arterial Disease

A machine learning algorithm for peripheral artery disease prognosis using biomarker data

Peripheral artery disease (PAD) biomarkers have been studied in isolation; however, an algorithm that considers a protein panel to inform PAD prognosis may improve predictive accuracy. Biomarker-based prediction models were developed and evaluated using a model development (n = 270) and prospective validation cohort (n = 277). Plasma concentrations of 37 proteins were measured at baseline and the patients were followed for 2 years. The primary outcome was 2-year major adverse limb event (MALE; composite of vascular intervention or major amputation). Of the 37 proteins tested, 6 were differentially expressed in patients with vs. without PAD (ADAMTS13, ICAM-1, ANGPTL3, Alpha 1-microglobulin, GDF15, and endostatin). Using 10-fold cross-validation, we developed a random forest machine learning model that accurately predicts 2-year MALE in a prospective validation cohort of PAD patients using a 6-protein panel (AUROC 0.84). This algorithm can support PAD risk stratification, informing clinical decisions on further vascular evaluation and management.

A Systematic Review of Interleukins as Diagnostic and Prognostic Biomarkers for Peripheral Artery Disease

Background: Peripheral artery disease (PAD) involves atherosclerosis of the lower extremity arteries and is a major contributor to limb loss and death worldwide. Several studies have demonstrated that interleukins (ILs) play an important role in the development and progression of PAD; however, a comprehensive literature review has not been performed. Methods: A systematic review was conducted and reported according to PRISMA guidelines. MEDLINE was searched from inception to 5 December 2022, and all studies assessing the association between ILs and PAD were included. Results: We included 17 studies from a pool of 771 unique articles. Five pro-inflammatory ILs (IL-1β, IL-2, IL-5, IL-6, and IL-8) and one pro-atherogenic IL (IL-12) were positively correlated with PAD diagnosis and progression. In contrast, two anti-inflammatory ILs (IL-4 and IL-10) were protective against PAD diagnosis and adverse limb events. Specifically, IL-6 and IL-8 were the most strongly associated with PAD and can act as potential disease biomarkers to support the identification and treatment of PAD. Conclusions: Ongoing work to identify and validate diagnostic/prognostic inflammatory biomarkers for PAD has the potential to assist clinicians in identifying high-risk patients for further evaluation and management which could reduce the risk of adverse cardiovascular and limb events.

Sympathetic Nerve Activity and Blood Pressure Response to Exercise in Peripheral Artery Disease: From Molecular Mechanisms, Human Studies, to Intervention Strategy Development

Sympathetic nerve activity (SNA) regulates the contraction of vascular smooth muscle and leads to a change in arterial blood pressure (BP). It was observed that SNA, vascular contractility, and BP are heightened in patients with peripheral artery disease (PAD) during exercise. The exercise pressor reflex (EPR), a neural mechanism responsible for BP response to activation of muscle afferent nerve, is a determinant of the exaggerated exercise-induced BP rise in PAD. Based on recent results obtained from a series of studies in PAD patients and a rat model of PAD, this review will shed light on SNA-driven BP response and the underlying mechanisms by which receptors and molecular mediators in muscle afferent nerves mediate the abnormalities in autonomic activities of PAD. Intervention strategies, particularly non-pharmacological strategies, improving the deleterious exercise-induced SNA and BP in PAD, and enhancing tolerance and performance during exercise will also be discussed.

Endothelial cell-derived pro-fibrotic factors increase TGF-β1 expression by smooth muscle cells in response to cycles of hypoxia-hyperoxia

BACKGROUND

The vascular pathology of peripheral artery disease (PAD) encompasses abnormal microvascular architecture and fibrosis in response to ischemia-reperfusion (I/R) cycles. We aimed to investigate the mechanisms by which pathological changes in the microvasculature direct fibrosis in the context of I/R.

METHODS

Primary human aortic endothelial cells (ECs) were cultured under cycles of normoxia-hypoxia (NH) or normoxia-hypoxia-hyperoxia (NHH) to mimic I/R. Primary human aortic smooth muscle cells (SMCs) were cultured and treated with media from the ECs.

FINDINGS

The mRNA and protein expression of the pro-fibrotic factors platelet derived growth factor (PDGF)-BB and connective tissue growth factor (CTGF) were significantly upregulated in ECs undergoing NH or NHH cycles. Treatment of SMCs with media from ECs undergoing NH or NHH cycles led to significant increases in TGF-β1, TGF-β pathway signaling intermediates, and collagen expression. Addition of neutralizing antibodies against PDGF-BB and CTGF to the media blunted the increases in TGF-β1 and collagen expression. Treatment of SMCs with PAD patient-derived serum also led to increased TGF-β1 levels.

INTERPRETATION

In an in-vitro model of I/R, which recapitulates the pathophysiology of PAD, increased secretion of PDGF-BB and CTGF by ECs was shown to be predominantly driving TGF-β1-mediated expression by SMCs. These cell culture experiments help elucidate the mechanism and interaction between ECs and SMCs in microvascular fibrosis associated with I/R. Thus, targeting these pro-fibrotic factors may be an effective strategy to combat fibrosis in response to cycles of I/R.

FUNDING

National Institute on Aging at the National Institutes of Health grant number R01AG064420.

RESEARCH IN CONTEXT

Evidence before this study: Previous studies in gastrocnemius biopsies from peripheral artery disease (PAD) patients showed that transforming growth factor beta 1 (TGF-β1), the most potent inducer of pathological fibrosis, is increased in the vasculature of PAD patients and correlated with collagen deposition. However, the exact cellular source of TGF-β1 remained unclear. Added value of this study: Exposing cells to cycles of normoxia-hypoxia-hyperoxia (NHH) resulted in pathological changes that are consistent with human PAD. This supports the idea that the use of NHH may be a reliable, novel in vitro model of PAD useful for studying associated pathophysiological mechanisms. Furthermore, pro-fibrotic factors (PDGF-BB and CTGF) released from endothelial cells were shown to induce a fibrotic phenotype in smooth muscle cells. This suggests a potential interaction between these cell types in the microvasculature that drives increased TGF-β1 expression and collagen deposition. Thus, targeting these pro-fibrotic factors may be an effective strategy to combat fibrosis in response to cycles of ischemia-reperfusion.

IL-6 signaling pathway contributes to exercise pressor reflex in rats with femoral artery occlusion in association with Kv4 activity in muscle afferent nerves

Interleukin-6 (IL-6) via trans-signaling pathway plays a role in modifying muscle sensory nerve-exaggerated exercise pressor reflex in rats with ligated femoral arteries, but the underlying mechanisms are poorly understood. It is known that voltage-gated potassium channel subfamily member Kv4 channels contribute to the excitabilities of sensory neurons and neuronal signaling transduction. Thus, in this study, we determined that 1) IL-6 regulates the exaggerated exercise pressor reflex in rats with peripheral artery disease (PAD) induced by femoral artery ligation and 2) Kv4 channels in muscle dorsal root ganglion (DRG) neurons are engaged in the role played by IL-6 trans-signaling pathway. We found that the protein levels of IL-6 and its receptor IL-6R expression were increased in the DRGs of PAD rats with 3-day of femoral artery occlusion. Inhibition of muscle afferents' IL-6 trans-signaling pathway (gp130) by intra-arterial administration of SC144, a gp130 inhibitor, into the hindlimb muscles of PAD rats alleviated blood pressure response to static muscle contraction. On the other hand, we found that 3-day femoral occlusion decreased amplitude of Kv4 currents in rat muscle DRG neurons. The homo IL-6/IL-6Rα fusion protein (H. IL-6/6Rα), but not IL-6 alone significantly inhibited Kv4 currents in muscle DRG neurons; and the effect of H. IL-6/6Rα was largely reverted by SC144. In conclusion, our data suggest that via trans-signaling pathway upregulated IL-6 in muscle afferent nerves by ischemic hindlimb muscles inhibits the activity of Kv4 channels and thus likely leads to adjustments of the exercise pressor reflex in PAD.

Exercise training in intermittent claudication: Effects on antioxidant genes, inflammatory mediators and proangiogenic progenitor cells

Exercise training remains a therapy of choice in intermittent claudication (IC). However, too exhaustive exercise may cause ischaemic injury and inflammatory response. We tested the impact of three-month treadmill training and single treadmill exercise on antioxidant gene expressions, cytokine concentrations and number of marrow-derived proangiogenic progenitor cells (PPC) in the blood of IC patients. Blood samples of 12 patients were collected before and after training, before and 1, 3 and 6 hours after the single exercise. PPCs were analysed with flow cytometry, cytokine concentrations were checked with Milliplex MAP, while expression of mRNAs and miRNAs was evaluated with qRT-PCR. Treadmill training improved pain-free walking time (from 144 ± 44 seconds [s] to 311 ± 134 s, p=0.02) and maximum walking time (from 578 ± 293 s to 859 ± 423 s, p=0.01) in IC patients. Before, but not after training, the single treadmill exercise increased the number of circulating CD45dimCD34+CD133-KDR+ PPCs (p=0.048), decreased expression of HMOX1 (p=0.04) in circulating leukocytes, reduced tumour necrosis factor-α (p=0.03) and tended to elevate myeloperoxidase (p=0.06) concentrations in plasma. In contrast, total plasminogen activator inhibitor-1 was decreased by single exercise only after, but not before training (p=0.02). Both before and after training the single exercise decreased monocyte chemoattractant protein (MCP)-1 (p=0.006 and p=0.03) concentration and increased SOD1 (p=0.001 and p=0.01) expression. Patients after training had also less interleukin-6 (p=0.03), but more MCP-1 (p=0.04) in the blood. In conclusion, treadmill training improves walking performance of IC patients, attenuates the single exercise-induced changes in gene expressions or PPC mobilisation, but may also lead to higher production of some proinflammatory cytokines.

Prevalent peripheral arterial disease and inflammatory burden

Background

Strong evidence implicates inflammation in the development of atherosclerotic heart disease but less is known about peripheral arterial disease (PAD). Our objective was to test the hypothesis that a composite index of inflammatory burden is associated with PAD.

Methods

Cross-sectional analysis of a randomly-selected group of 903 community-dwelling men in the MrOS cohort recruited between 2000 and 2002. Using blood samples, we measured seven cytokines and related these levels to prevalent PAD (ankle-brachial index (ABI) <0.9) both individually and as part of an “inflammatory burden score” (a composite sum of the number of pro-inflammatory cytokines in the highest quartile).

Results

Overall, 6.75% of men had ABI <0.9. The odds of prevalent PAD were higher in men with the highest quartile (Q4) levels of interleukin-6 multivariable (MV) adjusted (odds ratio (OR) =3.95 (95% CI, 1.4–11.3), tumor necrosis factor alpha OR = 4.44 (95% confidence interval (CI), 1.5–12.8), and C-reactive protein OR = 3.63 (95% CI, 1.4–9.4) compared to men in Q1. The magnitude of the association of these cytokines with PAD was similar to the effect of being 10 years older, OR = 2.41 (95% CI, 1.16–3.7). These significant effects persisted after additional MV adjustment for smoking except for CRP. Men with the highest inflammatory burden score (≥3) had 3.6 (95% CI, 1.5–8.7) increased odds of PAD, p trend = 0.03. After smoking adjustment the linear trend was borderline statistically significant (p trend = 0.10).

Conclusion

Inflammatory burden is associated with prevalent PAD, an association similar to aging 10 years. The inflammatory effects of smoking contributes to the underlying association between inflammation and PAD.

Role played by interleukin-6 in evoking the exercise pressor reflex in decerebrate rats: effect of femoral artery ligation

IL-6 signaling via the soluble IL-6 receptor (sIL-6r) has been shown to increase primary afferent responsiveness to noxious stimuli. This finding prompted us to test the hypothesis that IL-6 and sIL-6r would increase the exercise pressor reflex in decerebrate rats with freely perfused femoral arteries. We also tested the hypothesis that soluble glycoprotein (sgp)130, an inhibitor of IL-6/sIL-6r signaling, would decrease the exaggerated exercise pressor reflex that is found in decerebrate rats with ligated femoral arteries. In rats with freely perfused femoral arteries, coinjection of 50 ng of IL-6 and sIL-6r into the arterial supply of the hindlimb significantly increased the peak pressor response to static (control: 14 ± 3 mmHg and IL-6/sIL-6r: 17 ± 2 mmHg, P = 0.03) and intermittent isometric (control: 10 ± 2 mmHg and IL-6/sIL-6r: 15 ± 4 mmHg, P = 0.03) hindlimb muscle contraction. In rats with ligated femoral arteries, injection of 50 ng of sgp130 into the arterial supply of the hindlimb reduced the peak pressor response to static (control: 24 ± 2 mmHg and sgp130: 16 ± 3 mmHg, P = 0.01) and intermittent isometric (control: 16 ± 2 mmHg and sgp130: 13 ± 2 mmHg, P = 0.04) hindlimb muscle contraction, whereas there was no effect of sgp130 on the exercise pressor reflex in rats with freely perfused femoral arteries. We conclude that coinjection of exogenous IL-6 and sIL-6r increased the exercise pressor reflex in rats with freely perfused femoral arteries. More importantly, we also conclude that IL-6 and sIL-6r play an endogenous role in evoking the exercise pressor reflex in rats with ligated femoral arteries but not in rats with freely perfused femoral arteries.

Low levels of a natural IgM antibody are associated with vein graft stenosis and failure

BACKGROUND

All humans have natural, protective antibodies directed against phosphorylcholine (PC) epitopes, a common inflammatory danger signal appearing at sites of cell injury, oxidative stress, and on bacterial capsules. In large human cohorts, low levels of anti-PC IgM were associated with a significantly increased risk of stroke or myocardial infarction. However, it is not known if these antibodies protect against the premature closure of arterial reconstructions.

METHODS

A prospective, observational study of patients undergoing elective, infrainguinal, autogenous vein bypasses for atherosclerotic occlusive disease of the legs was conducted. Clinical data were recorded prospectively, and preoperative levels of anti-PC IgM measured with the CVDefine kit from Athera Biotechnologies (Solna, Sweden). The principal clinical end point was the loss of primary patency (loss of graft flow, or any intervention for stenosis). Patients were followed regularly by duplex ultrasound at 1, 3, 6, 12, 18 months, and yearly thereafter.

RESULTS

Fifty-six patients were studied, for an average of 1.3 years. Indications for surgery were claudication (33.9%), ischemic rest pain (17.9%), and ischemia with ulceration or gangrene (48.2%). Seventeen (30.4%) patients experienced loss of primary patency (10 graft occlusions, seven surgical or endovascular revisions of graft stenoses). Kaplan-Meier survival analysis showed that the quartile of patients with the lowest anti-PC IgM levels had significantly worse primary graft patency (log-rank test, P = .0085). Uni- and multivariate Cox proportional hazards analysis revealed that the preoperative anti-PC IgM level was an important predictor of graft failure. Patients with IgM values in the lowest quartile had a 3.6-fold increased risk of graft failure (95% confidence interval: 1.1-12.1), even after accounting for other significant clinical or technical factors such as indication for surgery, site of distal anastomosis, or vein graft diameter.

CONCLUSIONS

A naturally occurring IgM antibody directed against the proinflammatory epitope PC may be protective against vein graft stenosis and failure, through anti-inflammatory mechanisms. Measurement of this antibody may be a useful prognostic indicator, although larger studies of more diverse populations will be needed to confirm these results. The biological actions of anti-PC IgM suggest it may be useful in developing immunotherapies to improve bypass longevity.