HDL has been recognized as a traditional protective factor against atherosclerosis. Early epidemiological studies have consistently shown that HDL-C is inversely correlated with cardiovascular risk . However, subsequent attempts for drug therapies that aim to raise HDL-C levels with niacin  or CETP inhibitor  have both been in vain. HDL-mediated RCT is the key protective function of HDL. CEC, a metric defined by ex vivo experiments which reflects the first and rate-limiting step of RCT, has been demonstrated to be more valuable than HDL-C in predicting CAD risks [9–12, 14, 15]. However, CEC is susceptible to impairment in many disease states.
Our results showed that compared to non-CAD controls, CEC was lower in CAD patients and was not significantly related to HDL-C. The reduced CEC in CAD patients has already been well described in a high-quality study  but the correlation between HDL-C and CEC was inconsistent in previous reports. For example, the correlation coefficient between HDL-C and CEC was 0.51 (p < 0.0001) in the study by Khera et al.  (combining 442 CAD patients and 351 controls) but it was − 0.09 in the study by Zhang et al. (313 CAD patients) . The discordance could result from the different study populations, as most of the patients in Zhang et al.’s study and our study had ACS, while Khera et al.’s study excluded ACS patients. In addition to CAD, the CEC of HDL is also impaired in other diseases. Our previous observation found that patients with end-stage renal disease exhibited significantly decreased CEC compared to controls . HIV infection has been reported to induce structural and functional changes in HDL particles . A similar impairment of CEC was also found in patients with type 1  or type 2 diabetes  and autoimmune diseases .
A common feature of the above diseases is that inflammation is a major part of their pathophysiological mechanism. The inflammatory process can influence HDL components, shifting HDL proteome to inflammatory profile , and some of these uninvited guests can be detrimental to CEC. For example, the enrichment of serum amyloid A (SAA) or apolipoprotein C-III (ApoC-III) in HDL can impair HDL-mediated CEC [19, 31]. In addition to the change of HDL components, inflammation can also lead to modification of apoA-I, the major protein of HDL. Our early experimental study has found that myeloperoxidase (MPO)-mediated tryptophan oxidation of apoA-I was associated with decreased cholesterol efflux . Modified apoA-I was also abundant in the plasma recovered from patients with CAD . Although we did not measure the level of serum MPO or modified apoA-I in this study, correlation analysis revealed that the level of apoA-I (determined by NMR) was not correlated with CEC in CAD patients, suggesting that apoA-I modification could be a reason for HDL dysfunction in this study.
Our study showed that hsCRP was inversely correlated with CEC in CAD patients. The correlation remained significant after adjusting other conventional risk factors, HDL-C levels, and HDL subclasses. This independent role of hsCRP to CEC has not been reported in patients with CAD, although univariate negative correlation has been noted before . Inflammation in patients with CAD mainly derives from the activation of inflammatory pathways by various atherogenic risk factors. IL-1-IL-6-CRP axis is one of the most critical inflammatory pathways, and its activation can explain our findings. Cells in the atheroma (e.g., endothelial cells, macrophages, smooth muscle cells, etc.) produce IL-1β when exposed to stimuli (e.g., oxidized lipoproteins, disturbed blood flow, etc.). IL-1β strongly augments the production of IL-6 by various cell types. IL-6 induces liver to produce acute-phase proteins, such as CRP and SAA [35, 36]. We speculated that the activation of the IL-1-IL-6-CRP axis increased the level of hsCRP, and at the same time, led to the SAA-mediated CEC impairment. Inflammation was the possible reason behind the negative correlation between hsCRP and CEC.
hsCRP is a valuable marker of CAD. In people without a history of vascular disease, the risk ratios for CAD per 1-SD higher of logeCRP concentration were 1.37 (1.27–1.48) after adjustment for conventional risk factors . In patients with CAD, it has been shown that patients with higher hsCRP baseline level (usually ≥ 2 mg/L) have significantly higher risk of major adverse cardiovascular events (MACE) compared to those with lower hsCRP baseline level (usually < 2 mg/L) [38, 39]. hsCRP is not only a risk marker but can also predict the clinical benefit of antiatherogenic agents. Results from the JUPITER trial showed that participants without dyslipidemia but who had hsCRP levels higher than 2 mg/L can benefit more from rosuvastatin treatment . In the CANTOS trial, participants with canakinumab, a monoclonal antibody targeting IL-1β, who achieved an on-treatment hsCRP level < 2 mg/L had a 25% reduction in the risk of MACE, while no significant benefit was observed in participants whose on-treatment hsCRP concentration was 2 mg/L or above . Recent results from the COLCOT trial also showed that the non-specific anti-inflammation agent colchicine reduced stroke and coronary revascularization in patients with myocardial infarction, accompanied by a large reduction (> 65%) of the hsCRP level . On the other hand, treatment with another anti-inflammation medicine, methotrexate, did not reduce the risk of MACE, as there was no decrease in the hsCRP levels following treatment . Results from these high-quality studies and the current study consistently revealed the potential role of IL-1-IL-6-CRP axis in CAD and underscored the importance of hsCRP testing in patients with CAD.
In our study, the HDL subclass distribution was also determined by NMR spectroscopy. Our results showed that CAD patients with above median hsCRP levels exhibited a different HDL subclass distribution compared to patients with hsCRP levels below the median, namely the increased large HDL particles (HDL1) and the decreased small HDL particles (HDL4). Partial correlation analysis revealed that even after controlling for hsTnT levels, age, LDL-C levels, and diabetes, hsCRP levels remained positively correlated with HDL1-cholesterol levels, suggesting that the systematic inflammatory status of CAD was the underlying reason for HDL subfraction-distribution abnormality. HDL is a heterogeneous population of particles with sizes ranging from 7 nm-14 nm in diameter , and HDL particles are known to shift toward larger ones as a result of inflammation [20, 46]. The mechanism by which inflammation affects HDL remodeling is still not fully understood. In our study, the underlying reason, as mentioned before, could be the increased SAA levels  or the altered activity of plasma enzymes related to HDL metabolism, such as CETP .
In conclusion, we found that HDL-C levels could not reflect the functional status of HDL in patients with CAD, and the impaired correlation between HDL-C levels and CEC was possibly due to inflammation-induced HDL-subclass remodeling. The inflammatory marker, hsCRP, had an independent and negative relationship with CEC in CAD patients. As a relatively small group of patients was taken into account, further studies with larger cohorts including stable CAD patients, ACS patients, and mechanistic studies will need to be undertaken to further verify our conclusions and to elucidate the underlying mechanism for CEC impairment and HDL remodeling.