Elevated circulating CD16+ monocytes and TLR4+ monocytes in older adults with multiple cardiometabolic disease risk factors
Introduction
Biomarkers of inflammation are linked to cardiometabolic diseases, for example type 2 diabetes and cardiovascular disease, such that people with cardiometabolic diseases have higher pro-inflammatory biomarkers than healthy individuals (Shai et al., 2005; Rohde et al., 1999; Cesari et al., 2003a; Cesari et al., 2003b; Hu et al., 2004). The hyper-inflammation may be both a product of the cardiometabolic diseases and a critical component of the development pathways for cardiometabolic diseases. For example, inflammation plays a role in the development of both insulin resistance and plaque development (Fenyo and Gafencu, 2013; Nackiewicz et al., 2014; Wolf and Ley, 2019). The link between inflammation and cardiometabolic disease is further supported by the observation that many of the medications to treat these diseases, such as metformin (Saisho, 2015; Hattori et al., 2006), glitazones (Heliövaara et al., 2007), and statins (Shin et al., 2017) have potent anti-inflammatory actions. Therefore, it is difficult to determine the proportion of these medication's therapeutic actions that can be attributed to the reduction in inflammation or to other mechanisms.
There appear to be multiple pathways for the development of atherosclerosis. Once linked strongly to hyperlipidemia, pathways to atherogenesis now include the immune system in its pathology. In the lipid-centric model, lipids deposited in the arterial walls lead to upregulation of endothelium adhesion molecules (Wang et al., 2007). These adhesion molecules promote monocyte attachment and transmigration, and the monocytes morph into pro-inflammatory foam cells that promote the vessel progression from fatty streak to atherosclerosis (Fenyo and Gafencu, 2013). In contrast, the inflammation-initiated model features adhesion molecule-laden monocytes being recruited to the surface of the endothelium—in the absence of accumulated LDL. In this pathway, monocyte recruitment is believed to be facilitated by pro-inflammatory cytokines and pathogen-associated molecular patterns (PAMPs) or damage-associated molecular patterns (DAMPs), which initiate pro-inflammatory signaling (Edfeldt et al., 2002; Li and Sun, 2007; Roshan et al., 2016).
It is clear that the lipid-centric model has dominated the attention of medical and pharmaceutical science, with effective medications available to lower LDL. However, since cholesterol lowering by itself does not preclude cardiac events, it is clear that the immune system's role in these processes needs additional attention as it may also provide an as yet, undetermined therapeutic target (Geovanini and Libby, 2018; Fernández-Friera et al., 2017).
Central to both the lipid-centric and inflammation-centric models of atherosclerosis development is the involvement of monocytes, with mononuclear phagocytes playing a role in every step of the process. For example, significant murine inflammatory monocyte (Ly-6Chi) expansion was reported after high fat feeding, resulting in hypercholesterolemia, and these Ly-6Chi cells were selectively accumulated in atherosclerotic lesions (Swirski et al., 2007). Following statin treatment, the Ly-6Chi population contracted (Swirski et al., 2007). Inflammatory monocytes also have a role in cardiovascular disease in humans. Specifically, circulating inflammatory monocyte (CD14++CD16+) count at study enrollment accurately predicted cardiovascular events during the follow-up period (Rogacev et al., 2012).
Monocytes are a phagocytotic cell of the innate immune system with a broad array of cell-surface receptors, including adhesion molecules (VCAM), CD36 (oxLDL phagocytosis), and toll-like receptors, one of which, TLR4, confers LPS recognition on the monocyte (Janeway, 2005). However, there is evidence that high levels of TLR4 expression may be detrimental to health. For example, high expression of TLR4 is linked to poor health outcomes in a variety of conditions, including hepatitis C (Mohamed et al., 2017), atrial fibrillation (Gurses et al., 2016), inflammatory bowel disease (Cario and Podolsky, 2000), and cancer. (Chung and Kim, 2016; Hao et al., 2018) Specific to cancer, higher TLR4 expression on cancer cells is linked to reduced overall patient survival (Hao et al., 2018) and TLR4 influencing cancer cell survival after chemotherapy (Chung and Kim, 2016). Further, it appears clear that TLR4 is implicated in the immune-centric model of atherosclerosis development (Roshan et al., 2016). Elevated monocyte cell-surface TLR4 in people at an elevated risk for cardiometabolic diseases could further support an immune-centric model of atherosclerosis development, as well as a potential source of increased inflammation often reported in people with or at risk for cardiometabolic diseases. Collectively, this supports that TLR4 may provide a useful target for new disease therapies.
The purpose of this study was to examine the relationship between cardiometabolic disease risk factors, monocyte phenotype, and monocyte TLR4 expression in healthy older adults. We hypothesized that older adults with three or more commonly measured cardiometabolic disease risk factors would have higher percentage of circulating monocytes and a greater proportion of monocytes positive for TLR4.
Section snippets
Participants
As previously described (Markofski et al., 2014), men and women between the ages of 55–75 yrs. were recruited for this study (females, n = 29; males, n = 16). Participants were free from chronic diseases (diabetes, osteoporosis, active cancer, cardiovascular disease with the exception of controlled blood pressure) known to impact study variables and from contraindications to exercise. Additionally, participants were not taking medication known to affect the immune system and/or the inflammatory
Results
Participants in the low cardiovascular disease risk factors group had lower waist circumference (p < 0.0001), CRP (p = 0.0002), fat-free weight (p < 0.0001), and higher (p = 0.0019) 02max than participants in the elevated cardiovascular disease risk factors group (Table 1). There were no differences (p > 0.05) between groups for age, BMI, cholesterol, glucose, fat free weight, or leg press 1RM (Table 1).
Participants in the elevated cardiovascular disease risk group had a lower (p = 0.0231)
Discussion
In this secondary analysis we have assessed the importance of established cardiometabolic risk factors for determining immune biomarkers associated with cardiometabolic risk. Older, overweight non-tobacco users with an elevated risk for cardiovascular disease had higher percentages of circulating monocytes associated with increased inflammation and disease risk. Specifically, the elevated risk group had higher percentages of circulating inflammatory (CD14 + CD16+) monocytes and of circulating
Conclusion
In summary, older men and women with fewer cardiometabolic risk factors had lower serum and cellular markers of inflammation and higher aerobic capacity. Specifically, the participants with fewer cardiometabolic risk factors had a lower percentage of circulating inflammatory CD14 + CD16+ monocytes, and a lower number of circulating classical CD14 + CD16- monocytes positive for cell surface TLR4. These results strengthen the growing support for the role of inflammation and TLR4 in chronic
Funding
MMM: Support for this work was provided by the National Cancer Institute of the National Institutes of Health via P20CA221697 to the University of Houston and P20CA221696 and P20CA221696-02S1 to the University of Texas, MD Anderson Cancer Center. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health.
CRediT authorship contribution statement
Melissa M Markofski: conceptualization, data curation, formal analyses, funding acquisition, investigation, methodology, project administration, writing – original draft, writing – reviewing and editing. Michael G Flynn: conceptualization, data curation, funding acquisition, methodology, resources, supervision, writing – original draft, writing – reviewing and editing.
Declaration of competing interest
The authors have no financial or other relationships that may lead to a conflict of interest.
Acknowledgements
MGF: This research was supported (in whole or in part) by HCA and/or an HCA affiliated entity. The views expressed in this publication represent those of the author(s) and do not necessarily represent the official views of HCA or any of its affiliated entities.
References (39)
Inflammatory markers and cardiovascular disease (The health, aging and body composition [Health ABC] Study)
Am. J. Cardiol.
(2003)- et al.
The involvement of the monocytes/macrophages in chronic inflammation associated with atherosclerosis
Immunobiology
(2013) Normal LDL-cholesterol levels are associated with subclinical atherosclerosis in the absence of risk factors
J. Am. Coll. Cardiol.
(2017)Monocyte toll-like receptor expression in patients with atrial fibrillation
Am. J. Cardiol.
(2016)A standardized randomized 6-month aerobic exercise-training down-regulated pro-inflammatory genes, but up-regulated anti-inflammatory, neuron survival and axon growth-related genes
Exp. Gerontol.
(2015)CD14 CD16 monocytes independently predict cardiovascular events: a cohort study of 951 patients referred for elective coronary angiography
J. Am. Coll. Cardiol.
(2012)- et al.
Survey of C-reactive protein and cardiovascular risk factors in apparently healthy men
Am. J. Cardiol.
(1999) Influence of exercise training and age on CD14 cell-surface expression of toll-like receptor 2 and 4
Brain Behav. Immun.
(2005)TLR4/NF-?B signaling induces GSDMD-related pyroptosis in tubular cells in diabetic kidney disease
Front. Endocrinol. (Lausanne)
(2019)ACSM's Guidelines for Exercise Testing and Pescription
(2018)