The present study provided comprehensive epidemiologic insights into the relationship between new metrics for assessing an individual's systemic inflammatory activity on the impact of CKD progression and prognostic outcomes. Our findings suggested that elevated levels of SII and SIRI were significantly associated with CKD morbidity in the general population, as well as mortality in patients with CKD.
As indicators for assessing an individual's systemic inflammatory activity, the higher the values of SII and SIRI, the more active the body's immune system and inflammatory state [9]. A study showed that high levels of SII increased all-cause and cardiovascular mortality in patients with CKD, with more than half of the deaths in patients attributed to cardiovascular diseases[17]. The authors suggest that a possible explanation is that inflammation accelerates the progression of atherosclerosis in patients with CKD, leading to coronary artery damage, which exacerbates myocardial ischemia and strikes a blow to patients' cardiac function, ultimately leading to death.
Despite the lack of systematic studies on the association of SII and SIRI with CKD, several previous studies have demonstrated that inflammation and inflammatory responses can contribute to the progression of CKD by altering or interfering with intrarenal microcirculatory regulation and perfusion distribution, causing renal damage[18–20]. Inflammatory cells such as monocytes, neutrophils, lymphocytes, and inflammatory cytokines such as interleukin-1 (IL-1) and tumor necrosis factor-a (TNF-a) are involved in the progression of CKD by a variety of mechanisms, including an increase in pro-inflammatory cytokines, activation of oxidative stress, chronic and recurrent infections, altered adipose tissue metabolism, and dysregulation of the gut microbiota and many other mechanisms[21]. Previous studies have shown that monocytes play an essential role in the progression of CKD and that NLRP3 inflammasomes associated with renal disease when stimulated by substances such as reactive oxygen species (ROS), promote the release of IL-1 from monocytes, which in turn activates the classical pro-inflammatory signaling pathways of the intrinsic immune system, NF-kB (nuclear factor-kB) and AP-1 (activator protein-1), destroying renal units and leading to intrarenal microcirculatory dysregulation[18, 22]. In addition, neutrophils contribute to the progression of CKD through migration, production of ROS, secretion of neutrophil serine proteases (NSPs), and release of neutrophil extracellular traps (NETs)[22, 23]. Patients with CKD also experience premature aging of their T lymphocytes, which express high levels of pro-inflammatory cytokines that interact with the chronic inflammatory milieu of the patients with CKD, exacerbating the progression of the CKD and increasing the patient's susceptibility to atherosclerosis and ischemic organ damage[24–26]. In addition, the decrease in lymphocyte count also reflects patients' impaired immune resistance, making them more prone to infections and deterioration of renal function[21]. It has been demonstrated in animal studies that T lymphocytes infiltrating the kidneys increase free radicals to participate in the development of CKD and hypertension[27]. This finding is supported by observational studies showing that higher monocytes, neutrophils, and low lymphocytes are associated with a higher risk of CKD[28]. Meanwhile, platelets are involved in thrombus formation at the site of vascular injury, and changes in the PLT index reflect the severity of endothelial injury in renal vessels to some extent[29].
All cause mortality and CVD mortality in patients younger than 60 years were more susceptible to SII levels. It may result from more pronounced accelerated atherosclerosis triggered by chronic inflammation-induced vascular injury in younger individuals[26, 30]. In gender stratification, male patients were shown to have a higher risk of cardiovascular mortality and hypertension mortality. The protective, anti-inflammatory effects of estrogen on podocytes, as well as estrogen's ability to protect the renal endothelial barrier, may account for the gender differences[31, 32]. Notably, these differences were not found in the SIRI analysis. In addition, we found that high BMI increased all cause mortality and cardiovascular mortality in patients. This may be because abdominal fat deposition that may accompany obesity increases the level of inflammation and brings higher mortality in patients with CKD[33]. Our results showed that smoking interacted with SII and SIRI and increased risk of death. It may be because smoking-induced endothelial dysfunction may modulate immune and inflammatory cell responses, resulting in elevated levels of inflammation[34]. In addition, smoking negatively affects many processes closely related to the development of renal fibrosis [35].
In addition, we observed that SII and SIRI levels were associated with mortality in patients with different underlying diseases, including diabetes, hypertension, and CVD. The results of our analysis are consistent with previous studies: diabetes is a significant risk factor for cardiovascular death and death from hypertension[36, 37]. Previous studies have described several potential pathways of glycemic status that lead to effects on vascular outcomes associated with oxidative stress, late glycosylation processes, and epigenetic mechanisms, which in turn affect mortality in patients with CKD[38, 39]. In our analysis, the non-hypertensive population was more sensitive than patients with hypertension in the effect of SII on mortality, possibly because hypertension is usually accompanied by additional inflammation, and biomarkers of inflammation are elevated in patients with hypertension[40]. Thus, the altered mortality is more pronounced under a marginal inflammatory burden. The results of our subgroup analysis showed that cardiovascular diseases increased the risk of mortality in patients with CKD. As stated previously, most patients with CKD have cardiovascular disease. They are more likely to die from cardiovascular disease than from end-stage renal disease, which may be associated with impaired vascular reactivity, endothelial dysfunction, and increased arterial stiffness in patients with CKD[41]. However, the increase in SII was more significant for increasing the risk of death in patients without cardiovascular diseases. This may be due to the fact that SII is an index associated with platelet count, and platelet elevation leads to thrombosis and vascular endothelial changes. This effect is more pronounced in people who themselves have intact endothelial function[42].
Our study has several strengths. First, this study is based on the large sample size and appropriate covariates adjustment, enhancing the reliability and representativeness of our findings. In addition, we performed sensitivity analyses to assess the robustness of our results. To the best of our knowledge, this is the first study to systematically report the association between these inflammatory indices and the incidence and prognosis of CKD.
We also note some limitations of this study. First, the blood cell-based test was performed once, and the concentrations of these blood cells may change during the follow-up period, not fully reflecting the long-term inflammatory index levels in the population. In addition, single measurements of blood cell counts may be affected by other factors, which may lead to residual confounding and require attention when interpreting results.