In previous observational research, associations between peripheral immune cell counts and IBD have been described [10, 20, 21]. Our present study is the first study to employ the bi-directional two-sample MR analysis to reveal the causalities of peripheral immune cell counts on IBD and vice versa, which may provide potential biomarkers and therapeutic targets for IBD.
IBD is a chronic inflammatory disease caused by multiple factors. Although its pathogenesis has been robustly studied, the etiology remains unclear. Recent studies have emphasized the significant impact of immune cells on IBD incidence, progression, and risk [22]. Growing evidence suggests that neutrophils are implicated in the pathogenesis of chronic inflammatory diseases such as IBD [23] and CD [24], and that effectively modulating neutrophil functions and limiting their pro-inflammatory potential can reduce mucosal inflammation in IBD [23]. Our findings that increased peripheral neutrophils may be a causal risk factor for IBD and CD were consistent with these findings. Neutrophils are essential components of the innate immune system. Typically, when enhanced intestinal mucosal permeability and barrier dysfunction allow bacteria to infiltrate the intestinal mucosa, neutrophils are activated and recruited [25] and then migrate to sites of bacterial infection to initiate the defense against microbes [24, 26]. However, in patients with IBD, increased peripheral neutrophils are excessively recruited and accumulate in the intestinal mucosa with delayed resolution, leading to sustained inflammatory activation and mucosal injury [27, 28], which contributes to the pathogenesis of IBD. Conversely, impaired migration of peripheral neutrophils to sites of bacterial infection results in reduced bacterial clearance, leading to chronic inflammation and potentially driving the development of CD [24].
In addition, disrupted regulation of intestinal immunity leads to chronic, relapsing immune activation and the development of gastrointestinal pathologies [29]. Macrophages are important for the maintenance of intestinal immune balance. Generally, macrophages are differentiated from peripheral monocytes that are recruited to the intestine and participate in bacterial clearance and re-establishment of the epithelial barrier [30]. Several studies have shown reduced classical blood monocytes in CD patients [37, 38], suggesting their migration to the intestine and involvement in inflammatory processes. Previous studies suggest that altered monocyte-macrophage differentiation is associated with impaired resolution of intestinal inflammation, resulting in reduced bacterial clearance [31, 32] and excessive cytokine secretion [33, 34], which may play a crucial role in the development of IBD [35, 36]. In summary, robust functional studies have shown that monocytes are involved in the development of IBD, and our MR analysis further supports the immune pathogenesis of IBD.
Peripheral eosinophils also play a critical role in mediating immune mechanisms, though eosinophils constitute only < 5% of peripheral blood leukocytes in humans. Recent studies have described that the number of peripheral eosinophils can predict disease course and response to therapy [37–39]. Peripheral eosinophilia count has been observed associated with aggressive disease phenotype characterized by frequent use of corticosteroids, biological use, hospitalizations, and UC-related surgery [8, 9, 40]. Our findings corroborate these observations, suggesting that elevated peripheral eosinophil count may be a causal risk factor for UC. Eosinophils contribute to various homeostatic processes in the gastrointestinal tract, including maintaining the epithelial barrier integrity, supporting tissue structure, regulating immune cell populations, and modulating local immune responses [41–44]. In response to bacterial cues, peripheral eosinophils are recruited to the intestinal mucosa, where they exert protective effects through their antibacterial and immunomodulatory functions. Dysfunction in the recruitment of peripheral eosinophils to the inflamed colon worsens inflammation and may promote tissue damage through the secretion of pro-inflammatory cytokines in chronic inflammatory conditions [45, 46]. The findings of the current study contribute to a deeper understanding of the immunopathological mechanisms underlying IBD, highlighting the potential role of specific immune cell populations.
Although alterations of specific peripheral immune cell counts may be causal risk factors for IBD and its subtypes, an elevation in peripheral eosinophil [47] and basophil counts [48, 49] has been consistently documented in the circulation of patients suffering from chronic inflammatory conditions such as IBD. Our reverse MR results provide further support for the hypothesis that these chronic inflammatory diseases may affect the level of peripheral immune cells in turn. Patients with IBD are characterized by the production of substantial amounts of cytokines, such as interleukin (IL) -3, IL-4, IL-5, and IL-13 [50]. These cytokines have been previously associated with the enhancement of peripheral blood immune cell proliferation and differentiation, including both eosinophils [47] and basophils [50].
Peripheral lymphocyte count is significantly lower in patients with IBD compared to health controls [51, 52]. the reduced peripheral lymphocyte count may be explained by the migration of lymphocytes to the inflamed mucosa [52]. Additionally, a significantly elevated incidence of apoptosis in circulating lymphocytes among IBD patients during flare could account for the reduced number of peripheral lymphocytes as well [53]. Furthermore, cytokines produced during intestinal inflammation are upregulated in IBD patients [50], such as tumor necrosis factor-alpha (TNF-α) [54], which may adversely impact lymphocyte survival, thereby leading to a decrease in their quantity. Yao et al. have described CD patients with higher levels of peripheral neutrophil cell counts [17]. Our study is consistent with this clinical phenomenon by observing CD causally associated with higher levels of peripheral neutrophil cell counts. It is believed that impaired innate immune responses contribute to CD. The migration of neutrophils in patients with CD was significantly lower than in control, suggesting that this impairment may contribute to the increased neutrophil counts observed in circulating blood [26]. However, additional functional studies are required to confirm this hypothesis.
A strength of this study is that it is, to our knowledge, the first to examine the causal associations between peripheral immune cells and IBD, including CD and UC subgroups using a two-sample bi-directional MR analysis. The large sample size in GWAS increased the measurement precision, and the rigorous selection of IVs greatly improved the confidence of our results. Additionally, the multivariable MR analysis had a major strength that further estimated the causal effect of each peripheral immune cell on IBD. Nevertheless, this study has several limitations. First, although there was no evidence of horizontal pleiotropy by sensitivity analysis, the association mediated via other causal pathways could not be entirely excluded. Additionally, as our cohorts included mostly European ancestry, the findings of this study may be limited in their generalization to non-European populations. Finally, as the study was analyzed at the genetic level, it is crucial to interpret these results with caution.