In this MR study, 71 immunophenotypes were found to be potentially associated with UC. After FDR correction, 7 immunophenotypes were still strongly associated with UC, and 2 immunophenotypes were found to be significantly associated with UC disease after validation with external dataset. The two immunophenotypes are CX3CR1 on CD14 + CD16- monocytes and CX3CR1 on CD14 + CD16 + monocytes, both of which are risk factors for UC.
Abnormal mucosal immunity has been the mainstream theory of the pathogenesis of UC and is also an important therapeutic target of current research. Observational studies in UC patients have revealed a series of manifestations of immune cell colocalization in the inflammatory mucosa, including dendritic cells and macrophages[2.29−30], plasma cells[4.31], innate lymphoid cells[32–35] and several types of T cells[36–37]. Several biological agents, such as infliximab, adalimumab, and golimumab, which target TNF-α; vitolizumab, which targets α4β7 integrin; ertekinumab, which targets IL-12 and IL-23p40; and tofacitinib, which targets JAK1 and JAK3, have been used to treat patients with moderate to severe UC[38–39]. However, a very serious problem is that refractory UC patients lose their response after long-term medication and have to undergo colon resection. Therefore, there is an urgent need to explore the pathogenesis of UC and develop new targeted drugs.
Mononuclear macrophages residing in the human gut are derived from peripheral circulating monocytes, but their functions are significantly different from those of monocytes in circulation. Intestinal resident monocytes retain phagocytosis and antigen presentation functions but do not express innate responsive receptors, such as LPS receptor (CD14), FC-α receptor (CD89), FC-γ receptor (CD64, CD32, CD16), CR3 receptor (CD11b/CD18), CR4 receptor (CD11c/CD18), growth factor receptors IL-2 (CD25) and IL-3 (CD123), or integrin LFA-1 receptor (CD11a)/CD18) in the physiological state. Thus, intestinal monocytes show immune tolerance to intestinal flora antigens to maintain intestinal homeostasis[40–41]. However, observational studies have shown that one-third of mononuclear phagocytes in the intestinal mucosa are newly recruited from the peripheral circulation during active UC, and these cells lose their ability to further differentiate into resident monocytes and express reactive receptors on the surface in response to intestinal bacterial antigens, activating and amplifying intestinal inflammation[42].
CD14, which is present on the surface of monocytes and polymorphonuclear phagocytes, acts as a high-affinity receptor for LPS and LPS-binding protein complexes. Recent studies have shown that CD14 is involved in the immune response of intestinal monocytes and intestinal flora in UC patients. LPS binds to CD14 and then binds to toll receptor-4 (TLR4) to produce key proinflammatory factors, such as IL-1, IL-6, IL-8 and TNF-α, which can induce and aggravate intestinal inflammation[30]. The intestinal CD14 monocytes included CD14 + CD16 + intermediate monocytes and CD14 + CD16- classical monocytes. CD14 + CD16 + monocytes exhibit increased antigen-presenting ability and increased expression of inflammatory factors such as TNF and IL-12 due to the expression of HLA-DR, endothelial growth factor module-containing mucin-like receptor 2 (EMR2), Ig-like transcript 4 (ILT-4), CD43, and CD45RA on their surface[43]. Due to the proinflammatory nature of monocytes, eliminating monocytes was once considered a therapy for UC. A therapeutic study on granulocytes and monocytes by extracorporeal adsorption (GMA) revealed that the number of peripheral CD14 + CD16 + monocytes and the levels of TNF-α, IL-1b, IL-6, IL-8 and other inflammatory cytokines in UC patients were significantly decreased after GMA, which also proved the significance of CD14 + CD16 + monocytes in UC[44].
The migration and recruitment of circulating monocytes to the intestine are mainly mediated by chemokine gradients. According to their different cysteine sequences and homologous ligands, chemokines and chemokine receptors can be divided into C, CC, CXC and CX3C[45]. CX3CR1 is mainly expressed on NK cells, CD8 + T cells and CD14 + monocytes and is involved in immune cell migration and adhesion when it binds to CX3CL1/fractalkine (FKN) chemokines. CX3CR1 is closely related to the pathogenesis of autoimmune peripheral neuropathy[46], autoimmune encephalomyelitis[47], rheumatoid arthritis[48–49] and other autoimmune diseases. CX3CR1 also participates in the pathological process of UC through the fractalkine-CX3CR1 chemokine pathway to upregulate E-cadherin and mediate the adhesion of macrophages to the intestinal epithelium[45.50−51].
Previous research has shown that high expression of CX3CR1 on monocytes in active UC patients could promote the extravasation of CD14 + CD16 + monocytes into the intestinal mucosa[44]. Not only was the ability to recruit peripheral monocytes significantly enhanced, but the dysdifferentiation of newly recruited proinflammatory monocyte cells (CD11chighCCR2 + CX3CR1+) to intestinal tolerant monocytes (CD11c − CCR2 − CX3CR1−) was also aggravated, which caused the accumulation of proinflammatory monocytes in the intestinal mucosa[44.52]. Candia E et al. showed that CX3CR1 + cells can upregulate reactive receptors on the cell surface, such as TLR2, which can amplify the inflammatory response between monocytes and enterogenous antigens[53]. Therefore, decreasing CX3CR1 expression on the surface of monocytes or blocking the binding of CX3CR1 to CX3CL1/FKN could reduce the intestinal inflammatory response in UC patients.
Monocytes from healthy individuals or UC patients can be divided into three subsets—CX3CR1int, CX3CR1high, and CX3CR1- based on differences in the expression level of CX3CR1. CX3CR1 + cells include CX3CR1int and CX3CR1high cells, whose roles in the pathogenesis of UC remain inconclusive. A few studies have shown that CX3CR1high cells are dominant in the normal intestine; these cells do not respond to TLR stimulation and can produce IL-10. Thus, the CX3CR1high phenotype is an anti-inflammatory phenotype in the normal intestine[54–55]. The proportion of CX3CR1int cells in the normal colon is very low, and the half-life of CX3CR1int is short. However, in UC patients, CX3CR1int cells stop differentiating into CX3CR1high cells due to injury stimulation and instead produce the proinflammatory mediators IL-6, IL-23 and VEGF through TLR and NOD2 receptors, which sense bacterial antigens and differentiate into CX3CR1intLy6Clow cells to further support antigen presentation and migration, resulting in the accumulation of CX3CR1int cells in UC patients[16.56]. Therefore, these studies indicated that the CX3CR1high phenotype may be an anti-inflammatory phenotype, while the CX3CR1int phenotype is relevant to the pathogenesis of UC. Diehl GE et al[57] reported that CX3CR1high monocytes were the main cell group that caused key immune responses when intestinal noninvasive Salmonella bacteria were transferred to mesenteric lymph nodes and proposed that CX3CR1high monocytes were involved in the transfer of intestinal symbiotic bacteria to normal mucosal lymph nodes. Targeting CX3CR1high monocytes can improve UC. Overall, there is no consensus on the involvement of CX3CR1int or CX3CR1high cells in the pathogenesis of UC, but we revealed that CX3CR1 may be a potential therapeutic target for UC.
Although the results of this MR study identified potential therapeutic targets for UC, we must acknowledge that there is still limitations. First, the GWAS data used in this study were derived from a European population, so the results may not necessarily apply to UC patients worldwide. Second, we were unable to perform a population subgroup stratification analysis in the UC population due to the lack of detailed individual patient information. Third, while MR studies are very helpful in addressing the causal direction problem, biological mechanisms must also be considered when interpreting MR results, and stronger biological evidence is needed to support a substantial causal relationship between immune cell phenotypes and UC.