In this study, causal relationship from ICs to RA was found through MR analysis based on large public GWAS data. Our results suggest a positive association and causal factor between 5 types of ICs and RA as well as 4 types of ICs with negative association significantly. In the reverse MR analysis, RA is associated with changes in two ICs of these.
First, dendritic cell absolute count and CD86 + myeloid dendritic cell absolute count increases the risk of RA. As a particular kind of antigen-presenting cells, dendritic cells (DCs) connect innate and adaptive immune responses. [19]. In RA, DC is thought to activate a special type of T cells (self-peptide-reactive inflammatory T cells) and B cells, which in turn produce autoantibodies [20, 21]. Currently, DCs are divided into two main functional cellular subpopulations: conventional myeloid DCs (cDCs) and plasmacytoid DCs (pDCs), which are distinguished by the expression of myeloid markers CD1c and CD141.[22]. When DCs recognize inflammatory stimuli (e.g., PAMP, DAMP, etc.), they start a signaling cascade that helps to activate the transcription factor pathway known as nuclear factor κB (NF-κB)., and in synovial and synovial fluid tissues of RA, these DCs are usually mature, with high concentrations and overexpression of NF-κB [23, 24]. DCs not only secrete chemokines CCL3, CCL17, CXCL10 and CXCL19 that allow T cells to be recruited in the RA synovium, but also secrete proinflammatory cytokines like: IL-1β, IL-6, IL-23 and IL-12 to promote the differentiation of CD4 T cells into T helper type 1 (Th1) and Th17, which play a role in RA disease.[25–27]. In addition, CD86 expression is considered as a sign of DC immaturity, and Moret's team found significantly high CD86 expression on myeloid DCs in RA synovial membranes, corroborating their strong T-cell stimulatory capacity. [25].
This is followed by the protective role of CD3 on effector memory CD4 + T cells and CD25 on CD4 + T cells in RA, which belong to mature and regulatory T cells (Treg), respectively. T cells control the stabilization or progression of the autoimmune stage during RA. The differentiation of naïve CD4 T cells into memory T cells that are highly proliferative, invasive, pro-inflammatory, or relatively quiescent has a significant effect on RA. [28, 29]. Furthermore, evidence for the protective role of mature and functioning CD4 T cells in RA is also provided by the shorter telomere sequences of naïve CD4 T cells and the mitochondrial malfunction of CD4 T cells in RA[30, 31]. Increased numbers or functional upregulation of Treg are thought to help treat autoimmune diseases, while the reverse can enhance immunity to tumors and chronic infection pathogens[32]. However, increased, unchanged and decreased Tregs were observed in RA patients[33–35]. Such differences mainly stem from the limitations of the way Treg is recognized.
Single-cell sequencing revealed that the fraction of IgD-CD27-B cells in the synovium of RA patients was much greater than that in peripheral blood, indicating its critical role in the RA pathogenesis[36]. B-cell accumulation in the synovium is highly associated with increased T-cell activation, while B-cells also secrete LTα and LTβ to maintain T-cell and B-cell infiltration [37, 38]. Unfortunately, there is a paucity of studies on the effects of different subpopulations of memory B cells on RA. Our positive and negative MR analyses showed that CD27 on memory B cell is a protective factor for RA. While CD25 on memory B cell was a risk factor for RA, the presence of RA was associated with a decrease in CD25 on memory B cell. To our knowledge, it is the first time to explore the association of CD25 + B memory cells with RA. Interestingly, patients with rheumatoid arthritis (RA) who had a reduced baseline percentage of CD27 + memory B cells exhibited more favorable results when treated with B-cell depletion therapy (BCDT). [39]. This may be related to the heterogeneity and difficulty in accurately identifying CD27-expressing memory B cells [40].
Finally, CD45 on CD33br HLA DR + CD14dim and CD11b on CD33br HLA DR + CD14dim are belong to the myeloid cell. Myeloid cells are major defenders of pathogens and take effect in facilitating the development of adaptive immunity, especially during acute infections [41]. However, in chronic inflammation or tumors, persistently low concentrations of myeloid growth factors ,combined with some inflammatory mediators, may bring about dysregulation of myeloid differentiation, which in turn leads to undermining of the immune response [42]. This seems to explain the fact that CD45 on CD33br HLA DR + CD14dim increases the risk of RA, while RA also leads to a decrease in its levels. But further mechanisms still need to be elucidated.
In this study, the causal relationship between ICs and RA was analyzed by applying MR, and several ICs associated with RA risk were identified based on the large GWAS data. There are also some limitations to this study: 1. Due to limitations in population selection, validation of this result's applicability to other populations is required.;2. Failure to observe horizontal pleiotropism because of method selection may bias the results.༛3. we have lowered the threshold, so false positive results may occur.