After lasso screening, IL-10, ESR, C3, IL-4, GFR, and PLT were found to be predictive factors for ACD in patients with SLE. All of these factors appear to be interconnected with the pathogenesis of ACD.
In patients with SLE, IL-10 is primarily produced by B cells through autocrine secretion and by macrophages through paracrine secretion. There is a positive correlation between IL-10 titers and SLEDAI and anti-dsDNA antibody titers in the serum of patients with SLE[14,15]. IL-10 secretion is also associated with tissue injury caused by lupus inflammation[14]. A key role played by IL-10 in humoral immunity is to stimulate the production of antibodies by activated B cells[16]. Meanwhile, IL-10 exerts an anti-inflammatory effect by reducing the production of anti-inflammatory factors and free radicals [17]. In inflammatory bowel disease, patients who received IL-10 treatment developed anemia, which gradually improved without further clinical intervention following the discontinuation of IL-10 treatment. Ferritin concentrations increased dose-dependently in patients treated with IL-10, suggesting that IL-10 may induce anemia by stimulating ferritin production[18]. In previous studies, it was found that IL-10 promotes heme catabolism by inducing the expression of heme oxygenase-1 (HO-1) via the p38 mitogen-activated protein kinase pathway[19]. Furthermore, IL-10 inhibits erythroid progenitor cells directly[20]. The Stat3-IL-10-IL-6 pathway has been found to promote macrophage cytokinesis and phenotypic transformation in liver damage conditions[21]. ACD patients had higher levels of IL-10 and IL-6 serum titers than their non-ACD counterparts. Although IL-6 was eventually omitted from the model, it is undeniable that it also plays a crucial role in the mechanism of ACD. IL-6 stimulates hepatocytes to produce hepcidin under inflammatory conditions. Specifically, hepcidin acts on phagocytes in the intestinal epithelium, liver, and reticuloendothelial cells, anchoring ferroprotein (FP1) and disassembling it. Iron is transported from macrophages to plasma by FP1. The decomposition of FP1 by hepcidin results in a lower level of plasma iron, which inhibits the production of hemoglobin[22]. Our hypothesis is that, during the inflammatory phase of SLE, IL-10 induces IL-6 production through the stat3-IL-10-IL-6 pathway, thus aggravating ACD. It has been demonstrated that anti-IL-10 therapy is an effective treatment for SLE. In mouse models, anti-IL-10 antibodies have been shown to reduce kidney damage[23]. Patients with SLE treated with anti-IL-10 antibodies showed significant improvements in their skin and joint symptoms[24].
Similar to IL-10, IL-4 facilitates the proliferation of activated B cells and promotes their transformation into plasma cells, hence being called the B cell stimulating factor[25]. Through its interaction with receptors, IL-4 activates tyrosine kinases. Both myeloid and erythroid cells express IL-4 receptors[26]. However, the function of IL-4 in normal erythropoiesis remains controversial. A study conducted by I GREEN et al.[27] concluded that IL-4 and EPO were synergistic in stimulating the production of erythroid progenitor cells[27]. As reported in another study, IL-4 inhibited IL-3-supported erythroid burst forming units, while having little or no effect on EPO-supported erythrogenic burst units[28]. In this regard, IL-4 may have differential effects on erythropoiesis in different environments in the body. There is an increase in IL-4 production when chronic inflammation is exacerbated in SLE [29].Upon analyzing the data, we found that the SLEDAI score was higher in the ACD group than in the non-ACD group, and the IL-4 level was also higher. The increased levels of IL-4 during SLE active periods could have intensified the inhibitory effect on erythroid progenitor cells.
In the body, the complement system is instrumental in eliminating pathogens, apoptotic cells, and cellular debris. A deficiency in complement impairs the body's ability to eliminate antigens, leading to the accumulation of autoantibodies and ultimately contributing to the onset of SLE [30]. A decrease in C3 and/or C4 was included as an indicator of SLE activity in SLEDAI-2000. As shown in Table 1, C3 and C4 decreased more in the ACD group than in the non-ACD group, but C3 decreased significantly more. During complement activation, C4 is only involved in the classical pathway, whereas C3 additionally participates in the bypass pathway and the lectin-activated pathway. Upon recognizing C3 fragments on the surface of apoptotic cells, phagocytes bind and phagocytose them. In the case of abnormally activated C3, erythrocytes are destroyed and extravascular hemolysis is induced, resulting in paroxysmal hemoglobinuria[31]. Presently, anti-C3 medications are utilized in the treatment of paroxysmal hemoglobinuria[32].
The ESR is a commonly used indicator of inflammation as well as an effective predictor of disease progression. It is elevated in acute inflammatory disorders, connective tissue diseases, severe anemia, and malignancies while decreasing in erythrocytosis and dehydration. In spite of not being included in the SLEDAI as an indicator of lupus activity, an elevated ESR is still considered as a criterion for lupus activity in the European Consensus Lupus Activity Measure (ECLAM), SLE Index Score (SIS). Plasma fibrinogen concentration and the number and morphology of erythrocytes affect ESR[33]. Even though fibrinogen increases during the active phase of SLE, this elevation is relatively moderate for individuals with SLE, where anemia is a common symptom. Therefore, ESR may represent the erythrocyte state of SLE patients to some extent[34].
In general, PLT values range between 100 and 300 grams per liter. The PLT levels of the majority of patients in the ACD group were within the normal range, but they were still lower than those in the non-ACD group. As PLT aid in blood coagulation, the relatively low concentration of PLT in the ACD group increases the risk of bleeding, contributing to anemia.
GFR and uric acid can both be utilized to evaluate renal function. Approximately 50% of SLE patients suffer from variable degrees of kidney injury, resulting in impaired renal function[35]. Compared to the non-ACD group, the GFR of the ACD group was lower and the uric acid level was higher, indicating a relatively impaired renal function. It is critical to note that the kidney is the major organ for EPO secretion and that decreased renal function will result in decreased EPO production, which will culminate in a decrease in erythropoiesis[9]. Meanwhile, anti-EPO antibodies are detectable in patients with SLE and serum EPO levels are lower than normal [36]. SLE patients with ACD are more likely to have anti-EPO antibodies, leading to insensitivity to EPO[37].
To date, regarding medications for SLE, hydroxychloroquine is advised for people without contraindications, while immune suppressants are recommended for those who are suffering flare-ups[38]. Hydroxychloroquine has been shown to improve dermatological symptoms and immunosuppressants are effective in treating refractory SLE and reducing glucocorticoids [39]. However, SLE patients are prone to ACD, which may be exacerbated by the overuse of antimalarials and immunosuppressants [40]. This model is clinically useful since it is able to predict the risk of ACD among patients, thus assisting in the prevention of further hematological harm caused by medication usage.
There are still some limitations. The first limitation of this study is that it only included patients from one hospital. A multicenter, multiethnic, and larger clinical cohort is required in the future in order to improve the accuracy of the model. Furthermore, additional variables such as serum EPO titer, anti-EPO antibody test, and renal pathology should be included as conditions permit.