Serum immunoglobulin M is associated with the severity of coronary artery disease in adults

Introduction

Coronary artery disease (CAD) is one of the most common chronic diseases and remains the leading cause of death worldwide (Laslett et al., 2012). Atherosclerosis, the primary underlying factor for CAD, is a chronic immune inflammatory condition that is characterized by the formation of plaques in the arteries, along with the accumulation of lipids and cells, such as leukocytes, endothelial cells, and foam cells, in intimal layers (Drosos, Tavridou & Kolios, 2015). Since atherosclerosis is almost irreversible, early identification, intervention of risk factors, and the identification of new biomarkers are integral to the prevention and treatment of CAD (Gilstrap & Wang, 2012).

Recently, numerous studies have shown that activation of the immune system is associated with the occurrence and development of CAD (Okabe et al., 2005; Tsiantoulas et al., 2014; Zhou et al., 2001). Inflammation and immune function play important roles in the development and progression of atherosclerosis, as well as plaque rupture and thrombosis (Tsiantoulas et al., 2014). A recent network-driven integrative analysis of data from genome-wide association studies identified that B-cell immune responses play a causative role in CAD (Huan et al., 2013). Generally, B cells are divided into innate-like B1 cells and B2 cells. Immunoglobulin M (IgM), mainly produced by B1 cells (Choi et al., 2012), is the largest type of immunoglobulin in the human body. It is the first isotype produced prior to class switching (Fellah et al., 1992). IgM effectively recognizes and eliminates pathogens in the early stage of immune defense (Boes, 2000). It is mainly found in the blood and has complement activation, sterilization, agglutination, cytotoxic and cytolytic activity, and immune regulation functions (Ehrenstein & Notley, 2010). Recent animal experiments and human studies have demonstrated the atheroprotective role of IgM. A previous study showed that the level of IgM antibodies against phosphorylcholine (IgM anti-PC) was negatively associated with the atherosclerotic disease burden in patients in the acute phase of ST-elevation myocardial infarction (STEMI) (Knudsen et al., 2019). However, the study was limited to the relationship between IgM anti-PC and atherosclerosis in STEMI patients. Currently, there is insufficient evidence of an association between IgM and CAD severity in humans. Our study aimed to investigate the relationship between IgM and CAD severity in Chinese patients undergoing coronary angiography (CAG).

Methods

Results

Associations between IgM and the severity of CAD

To analyse the relationship between IgM level and CAD severity, patients with CAD were classified into subgroups based on the number of regions with stenosis: 1–3 and >3 stenosed regions. IgM levels exhibited a significant decrease, and there was an increase in the number of stenosed regions (P_trend < 0.05). Then, the patients were divided into subgroups with one stenosed or >1 stenosed vessel. As shown in Fig. 1, the increase in the number of obstructed vessels (P_trend < 0.001) corresponded with a significant decrease in IgM levels. Additionally, the patients were further divided into subgroups according to tertiles of the Gensini scores. Similarly, the results showed that IgM gradually decreased as Gensini scores increased from tertile 1 to tertile 3 (P_trend < 0.05).

Stratified analysis of the IgM level with CAD

Stratified analysis of the association between serum IgM level and the severity of coronary artery lesions was conducted in different age, sex, BMI, smoking status, hypertension, dyslipidaemia, DM, history of stroke, and statin use history subgroups. The effect of interactions between IgM levels and these factors on CAD severity was analysed. As shown in Fig. 2, there was a negative association between IgM levels and CAD severity among people younger than 70 years of age, with a BMI greater than 24 kg/m², who had never smoked, had hypertension, did not have hyperlipidaemia or diabetes, had no history of stroke, and did not have a history of statin use. In addition, we observed that the association between IgM levels and CAD varied by age and statin use history (P_interaction = 0.029, P_interaction = 0.021). After adjusting for sex, BMI, smoking status, hypertension, dyslipidaemia, DM, history of stroke, and statin use history, we found that IgM levels were associated with CAD in people younger than 70 years of age (OR: 0.730, 95% CI [0.579–0.921], P = 0.008), but not in people older than 70 years of age (OR: 0.835, 95% CI [0.608–1.147], P = 0.266). Additionally, IgM levels were associated with CAD in people without a history of statin use (OR: 0.700, 95% CI [0.552–0.888], P = 0.003), but not in people with a history of statin use (OR: 1.064, 95% CI [0.746–1.519], P = 0.731) after adjusting for age, sex, BMI, smoking status, hypertension, dyslipidaemia, DM, and history of stroke (Fig. 2). We further analysed the relationship between IgM levels and CAD severity according to the Gensini score. IgM levels exhibited a negative association with the Gensini score across different groups: patients younger than 70 years of age, patients older than 70 years of age (Fig. 3), and patients both with and without a history of statin use (Fig. 4).

Discussion

In this study, we observed that serum IgM concentration was associated with the traditional risk factors and severity of CAD. IgM levels gradually decreased with an increase in both the number of obstructed vessels and the Gensini score. After adjusting for age and sex in the logistic regression models, higher IgM levels exhibited a significant negative association with the severity of CAD, as assessed by the Gensini score. The relationship was still significant after further adjusting for smoking status, hypertension, dyslipidaemia, diabetes, stroke, and statin use history.

We investigated whether IgM levels were associated with CAD in a Chinese coronary angiography cohort. First, we analysed the relationship between serum IgM concentrations and traditional CAD risk factors. The results revealed a strong association between IgM levels and the concentrations of TC, TG, and LDL-C, as well as a positive association between IgM concentration and TP, HDL-C and ALB. Both high TC and high LDL-C levels play an important role in CAD (Mostaza et al., 2000; O’Donoghue et al., 2014). We found positive associations between IgM and TG, TC and LDL-C (Table 2), which could possibly be related to statin use. Logistic regression analysis showed that dyslipidaemia was not associated with CAD severity. The relationship between the clinical diagnosis of dyslipidaemia and CAD was not significant due to statin use.

Atherosclerosis is the primary cause of cardiovascular deaths related to heart attacks and strokes (Kyaw et al., 2011). It is considered to be a lipid-driven chronic inflammatory disease that begins with the accumulation of translocated lipids in arterial intimal layers. Recent studies have found that plaque development, progression, and rupture are modulated by the immune system (Kyaw et al., 2012). The immune inflammatory response is considered to be the most important pro-atherosclerotic factor. Although total serum immunoglobulins (Igs) are not normally considered relevant to clinical cardiovascular (CV) disease, there is substantial evidence in the preclinical literature suggesting that IgM is associated with atherosclerosis. The earliest evidence supporting an atheroprotective role of natural IgM is that sIgM−/− Ldlr−/− mice, which cannot secrete IgM, develop significantly more extensive atherosclerosis than Ldlr−/− controls (Lewis et al., 2009). Most serum IgM is natural IgM, which is produced spontaneously by a distinct subset of B cells and does not require any stimulation by foreign antigens or microorganisms (Nguyen & Baumgarth, 2016). Studies have found that natural IgM exerts a protective effect by interfering with the interaction between ox-LDL and macrophages, thereby preventing the formation of foam cells and limiting the proinflammatory effects of ox-LDL (Binder et al., 2003; Hörkkö et al., 1999). Likewise, natural IgM promotes IgM deposits and reduces necrotic core size in atherosclerotic lesions, suggesting that IgM Abs has a protective effect against atherosclerosis (Kyaw et al., 2011). In addition, natural IgM may enhance the clearance of oxidized phospholipid-bearing apoptotic cells that accumulate in atherosclerotic plaques (Thorp et al., 2008). Serum IgM also contains a small amount of antigen-specific IgM produced by B1b cells when they are activated by both nonantigenic and antigen-dependent stimuli. Rosenfeld et al. (2015) recently found that B1b cells could also produce atheroprotective OSE-reactive IgM antibodies, which protect against atherosclerosis in mice. This finding suggests that similar mechanisms may occur in humans (Rosenfeld et al., 2015).

Several studies on human subjects have shown that serum IgM antibodies to oxidation-specific epitopes (OSEs) are inversely correlated with CAD. For example, serum IgM antibodies to copper-oxidized low-density lipoprotein (CuOx-LDL) and malondialdehyde-modified low-density lipoprotein (MDA-LDL) are inversely correlated with carotid intima media thickness and the risk of developing a >50%-diameter stenosis in the coronary arteries (Karvonen et al., 2003; Tsimikas et al., 2007, 2012). Similarly, IgM titres to OSE phosphocholine have been reported to be inversely correlated with the incidence of heart attack and CAD risk in patients with lupus (Anania et al., 2010; Grönlund et al., 2009; Grönwall et al., 2012). In addition, serum IgM is negatively associated with baPWV in women and negativelya ssociated with a lower risk of arterial stiffness, which is also an index of subclinical atherosclerosis (Liu et al., 2019). Inflammation and autoimmune responses play important roles in CAD, yet little is known about whether serum IgM levels are reduced in CAD.

Stratified analysis results showed that IgM levels were associated with CAD severity based on the number of affected regions, affected vessels, and Gensini score tertiles (Fig. 1). The Gensini score was used to evaluate the severity of coronary artery disease. Because the Gensini score included different percentages of stenosis, it was more indicative of the degree of atherosclerosis than the Syntax score. Khamis et al. (2016) also found that total serum IgM levels were lower in patients with CAD and stroke than in controls, which is consistent with our results. In addition, logistic regression analysis revealed that after adjusting for traditional risk factors in CAD patients, serum IgM levels were still negatively and independently associated with the Gensini score. Additional stratified analyses showed a strong association between IgM levels and CAD in people younger than 70 years old and in those without a history of statin use (Fig. 2). These results indicated that IgM may be involved in the progression of atherosclerosis, especially in people younger than 70 years old and those without a history of statin use. Palmieri et al. (2021) found that IgM antibodies against MDA bound to human albumin were significantly decreased in 60-year-olds who developed CAD within a 5-year follow-up, linking IgM anti-MDA levels above the 66th percentile to a reduced risk of CAD. This was independent of other traditional risk markers. Our study also demonstrated that serum IgM concentrations decreased with age, and the protective effect of IgM on atherosclerosis was particularly pronounced in people younger than 70 years old. We propose that CAD is independently associated with serum IgM in the coronary angiography population, which may enable IgM to serve as a biomarker for the severity of CAD. Existing studies generally report that IgM has a protective effect against atherosclerosis. IgM may act by neutralizing the proinflammatory properties of oxidized low-density lipoprotein (ox-LDL), inhibiting the uptake of ox-LDL by macrophages, and promoting apoptotic cell clearance (Rosenfeld et al., 2015; Tsiantoulas et al., 2014). Additionally, the role of specific antibodies as biomarkers of atherosclerosis has been extensively studied, especially antibodies to epitopes induced by oxidative modification of low-density lipoprotein (LDL). Tsimikas et al. (2012) found that IgG Cu-OxLDLs were associated with a higher risk of CVD, while IgM MDA-LDLs were associated with a lower risk of CVD. In addition, Khamis et al. (2016) found that while IgM anti-MDA-LDL antibodies were significantly associated with freedom from CV events, these antibodies were also significantly associated with serum IgM levels. Therefore, these researchers suggested that in hypertensive patients, total serum IgM levels are independent predictors of the absence of CAD in general. Total serum Ig levels significantly improve the risk classification of cardiovascular events (Khamis et al., 2016). Thus, comparing only one form of IgM anti-MDA levels may overlook the predictive power of other specific antibodies. In fact, total serum IgM is a more favourable biological indicator.

Our study still has some limitations. First, as this was a cross-sectional study, it was difficult to determine the causal association between IgM and CAD severity in Chinese patients, so our findings need to be confirmed in further prospective studies. Second, the control subjects who had a <50% narrowing of coronary arteries were not truly healthy individuals, which may have caused an underestimation of the association between IgM and CAD and limited the power of this study.

Conclusion

In conclusion, we analysed the relationships between serum IgM levels and CAD severity in Chinese patients. The results showed that reduced IgM levels were associated with a higher severity of CAD, especially in people younger than 70 years old and those without a history of statin use.

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