Multiple sclerosis (MS) is one of the most common chronic inflammatory demyelinated disorders affecting the central nervous system (1). It frequently leads to high rates of disability and recurrent episodes among young people, causing impairments in sensation, mobility, and cognitive function (2). Currently, MS affects 2.8 million people worldwide and is diagnosed in one person at an average age of 32 every five minutes (3). Unfortunately, the available evidence is insufficient to establish the effectiveness of drug treatment in halting or reversing the progression of MS, primarily due to the limited understanding of the underlying mechanisms driving its pathogenesis.

Immunological mechanisms play a crucial role in the progression of MS, leading to neurological damage. Aberrant activation of lymphocytes, specifically CD8⁺ and CD4⁺ T cells, contributes to the destruction of oligodendrocytes and neurons in active MS lesions (4). Additionally, MS patients often exhibit increased peripheral blood Th17 cells and elevated levels of Th17-related cytokines in their serum (5). B cells are also involved in the formation of lesions throughout various stages of the disease, as indicated by the presence of oligoclonal bands in MS (6). Given these findings, many current clinical efforts are focused on immune-based therapies, such as the inhibition of auto-reactive T cells, activation of regulatory T cells (Tregs), and regulation of B cell activity to prevent the relapse of this category of disorders (7, 8). However, it is important to note that the current understanding is limited as the study only examined a few commonly studied subsets of immune cells. The causal relationship between MS and peripheral immune cells remains unclear and requires further investigation.

Although the randomized controlled trial is widely regarded as the most reliable and rigorous method for generating clinical evidence, it can be challenging due to the need for a large sample size and significant human and financial resources. Mendelian randomization (MR) is an alternative approach that uses instrumental variables (IVs) assessed in exposure-related genome-wide association studies (GWAS) to evaluate the causal relationship between exposure and outcome, which needs to meet three key assumptions that IVs should be strongly related to exposure, independent of any confounding factors, and conditional independence with the outcome conditional on the exposure (9). Based on the random allocation of gametes during meiosis, MR has the ability to reduce confounding factors and strengthen causal inference (9). In this study, instrumental variables derived from large-scale genome-wide association studies (GWAS) of hematological traits were utilized to evaluate the causal relationship between peripheral immune cell counts and MS risk and to investigate their impact on MS severity using the MR approach.

In recent years, GWAS have greatly advanced our understanding of the genetic factors involved in MS, with over 200 associated loci identified (16). Although the precise mechanism is complex and not fully understood, there is increasing evidence suggesting that the immune response, including immune cell recruitment, activation, and cytokine release, plays a role in the progression of MS (22, 23). In this study, a causal relationship between peripheral immune cell counts and the risk of MS, as well as MS severity, was assessed through the MR approach. We revealed that peripheral white blood cell count and lymphocyte count were closely associated with the risk of MS. These findings align with previous research indicating that MS patients often exhibit elevated levels of these immune cells (24, 25). Meanwhile, current studies reveal that the ratios between peripheral immune cells, such as neutrophil-lymphocyte ratio (NLR) and monocyte-lymphocyte ratio (MLR), are robustly associated with MS severity (26) and risk of MS recurrence within two years (27). Elevated NLR at the outset of relapsing-remitting multiple sclerosis (RRMS) is linked to disease activity (28), suggesting that it could be a reliable inflammatory biomarker for predicting disease activity in MS.

Considering that the overall effects can be different based on specific cell subtypes, our study further investigated immune cell subsets. It identified five subtypes of immune cells that were protective against MS and four that were detrimental. Specifically, higher absolute counts of total T cell and CD4⁺ T cell were found to have a detrimental effect on MS. This could be attributed to the persistent migration of autoreactive T cells and other inflammatory immune cells from peripheral regions into the brain and spinal cord, resulting in demyelination of the neural axons (29). In normal immune responses, CD4⁺ T cells show significant plasticity in terms of differentiation (30). In MS, there is an expansion of Th1/Thl7 cells, a decrease in Th2/Treg cells, and elevated levels of interferon γ levels (31). The primary drive of autoimmunity is Th17 cells, which have a stronger proliferation ability than Th1 cells. Additionally, Th17 cells inhibit the differentiation of other lymphocyte subtypes, particularly through the production of cytokines such as transforming growth factor beta (TGF-β) (32). Given that there are many subsets of T cells, subsequent further discussions of their roles are necessary.

In the study, CD25⁺⁺CD4⁺ T cells, a subset of CD4⁺T cells, were also demonstrated to reduce MS susceptibility. CD25 is an interleukin (IL)-2 receptor α as a surface marker for Tregs. Tregs play a crucial role in maintaining immune homeostasis, avoiding autoimmune disorders, and minimizing transplant rejection, by the suppression of the immune response of various cell types, including effector CD4⁺ T cells (33). Tregs exert their inhibitory effects on effector T cells through direct binding to specific receptors on target cells, such as cytotoxic T lymphocyte-associated antigen 4 (CTLA-4), TGF-β, and anti-glucocorticoid-induced tumor necrosis factor receptor (GITR) (34, 35). As a result, effector T cells exhibit reduced responsiveness to IL-2 due to decreased expression of the IL-2 receptor chain (36). In MS patients, Tregs are deficient or defective in the blood, brains, and cerebrospinal fluid (CSF), characterized by reduced expression of the Forkhead box P3 (Foxp3) protein (37), which impairs their ability to adequately suppress the immune response of T cells towards the myelin basic protein (38). Interferon-β, the preferred immunomodulatory drug for RRMS, increases the frequency of Tregs and restores their inhibitory function by upregulating Foxp3 expression levels (39). Our MR research corroborated the causative relationship between CD4⁺ Tregs and MS, supporting the point that individuals with more CD4⁺ Tregs are less likely to suffer from MS. Our study provided suggestive evidence that the resting Tregs and a higher proportion of resting Tregs in CD4⁺ Tregs might decrease the MS severity though they cannot pass the FDR adjustment. And an observational study indicated that the frequency of Tregs in MS patients reduced compared with healthy controls, and there is a negative connection between Tregs frequency and EDSS in MS (40). Therefore, it is necessary to further study the possibility of Tregs becoming a target for alleviating the severity of MS.

In addition to the natural CD4⁺ Tregs, another class of induced Tregs, including CD8⁺ Tregs, deserves attention. Though the phenotypic characteristics of CD8⁺ Tregs resemble those of naturally generated CD4⁺ Tregs, featuring higher levels of Foxp3, GITR, and CTLA-4, and reduced levels of CD127 (41), the functionality of CD4⁺ Tregs has been extensively researched, while there has been relatively little attention given to the research of CD8⁺ Tregs. Our study also found that a higher percentage of CD25⁺⁺CD8^br T cell in T cell and a higher percentage of CD25⁺⁺CD8^br T cell in CD8^br T cell decreased the risk of MS and have the potential to alleviate MS severity, respectively. According to a previous study, CD8⁺ Tregs are less frequent in the blood and show impaired function in MS patients (42). In MS and other experimental autoimmune disorders, researchers have discovered the beneficial effect of T cell vaccination, which stimulates the occurrence of CD8⁺ Tregs, and results in the selective downregulation of self-reactive CD4⁺ T cells (41). Given the inhibitory effect of CD8⁺ Tregs on self-response, it may become a candidate for future intervention in the occurrence and development of autoimmune diseases.

Additionally, IgD^-CD38^dim B cells, belonging to Switched Memory B cells, were discovered to decrease the risk in MS. Class-switched memory B cells had a higher frequency in the CSF than in the blood sample, suggesting that class-switched memory B cells may divert from the periphery into the CSF region (43). Another B cell subpopulations-IgD⁺CD38^- B cells (Unswitched Memory B cells) decreased in RRMS patients when compared with healthy controls (44). Unswitched B memory cells, defined as mainly virgin naive cells, are highly reactive and phenotypically comparable to Switched Memory B cells (45). The study has shown that RRMS and secondary progressive multiple sclerosis have higher levels of memory B cells in their peripheral blood (46). When stimulated by B cell receptor and CD40, memory B cells in MS patients can effectively present neuronal antigens to T cells, activate T cells, and secrete Tumor necrosis factor-α (47). The administration of Atacicept, which enhances memory B cell responses, has been observed to promote MS recurrence, indicating the significant involvement of memory B cells in MS relapse (48). Cladribine, which potentially depletes memory B cells, has been approved for the treatment of relapsing MS (49). Therefore, further research should be devoted to investigating the potential pathways of how IgD^-CD38^dim B cells impact MS, offering prospective therapeutic avenues for clinical application.

Furtherly, we demonstrated that the proportion of HLA DR⁺⁺ monocyte of total monocyte and of total leukocyte increased the risk of MS, and HLA DR⁺⁺ monocyte might be associated with higher severity of MS. HLA-DR represents a class II major histocompatibility complex antigen that is prominently present on the outer membrane of B lymphocytes, monocytes, and macrophages. Its primary role is to facilitate the process of antigen presentation to CD4⁺ T cells. Previous studies have identified HLA-DRB1*15:01 as a significant genetic factor contributing to the development of MS, increasing the risk by threefold (50). It is worth mentioning that monocytes play anti-inflammatory and pro-inflammatory functions in the early phases of the disease (51), suggesting the necessity of investigating the function of multiple monocyte subtypes in MS onset. And in our study, CD14⁺CD16⁺ monocyte, which represents the pro-inflammation phenotype, and the proportion of CD14⁺CD16⁺ monocyte in monocyte showed a suggestive causal association with increased risk in MS. Current study has shown that patients with RRMS have a higher frequency of CD14⁺CD16⁺ monocyte compared to healthy controls (44), which is consistent with our study and provide its potential as a biomarkers for MS. Moreover, the effect of myeloid-derived suppressor cells (MDSCs) in regulating the immune response, as well as its immunosuppressive properties in autoimmune diseases, prompt that its role in MS cannot be ignored. In the experimental autoimmune encephalomyelitis (EAE) model, MDSCs can trigger T cell apoptosis to suppress inflammation and boost recovery of EAE symptoms (52). Similarly, the abundance of Monocytic MDSC in blood samples from untreated MS patients at the time of first relapse had a negative correlation with the EDSS at baseline and after one-year follow-up (53), underscoring the need for dynamic MDSC monitoring in the development of MS.

Finally, we have discovered that interleukin-2 receptor antagonist (IL-2Ra) increased susceptibility to MS. IL-2 is known to enhance the survival and proliferation of activated T cells, as well as the differentiation of effector T cells, by binding to the IL-2 receptor (54). IL-2Ra blocks the downstream signaling of the IL-2 receptor by specifically antagonizing its α chain (55). However, previous studies have shown that IL-2Ra has the capacity to form complexes with IL-2, such as sIL-6R-IL-6 (56) and sIL-15R-IL-15 complexes (57), thereby enhancing signaling. It should be noted, though, that this occurrence is less likely. Therefore, further in-depth and comprehensive research is required to explore the exact role of immune factors on MS.

The reliability of our research findings was strengthened by the utilization of recently published high-quality GWAS. The study was designed meticulously with reliable data sources utilized, resulting in a substantial level of statistical power. However, it is necessary to acknowledge the limitations of the MR method discussed in the literature review. Our study has some limitations as follows: firstly, the research primarily focuses on participants of European ancestry. Therefore, the research results cannot be generalized to other ethnic groups. It is essential to replicate the study in other populations. Secondly, analyzing immune cell subtypes based on small sample sizes may result in insufficient ability to detect the impact of immune cell subtypes on the risk and severity of MS. Thirdly, various transient variables, such as age and lifestyle factors, could influence an individual’s immunological characteristics at a given moment, which may not correspond to the immune features defined by coding genes throughout an individual’s lifetime.

In conclusion, our study has identified the causal relationship between peripheral immunity and MS through Mendelian randomization. These findings have the potential to serve as biomarkers for predicting the onset and development of MS and offer new insights into the underlying mechanisms involved in MS. Further research is needed to better understand the specific relationship between peripheral immune cells and MS.

The original contributions presented in the study are included in the article/Supplementary Material. Further inquiries can be directed to the corresponding authors.

LC: Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Software, Validation, Visualization, Writing – original draft. L-FZ: Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Software, Validation, Visualization, Writing – original draft. L-YZ: Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Writing – original draft. Y-HC: Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Writing – original draft. M-HD: Formal analysis, Investigation, Methodology, Writing – original draft. X-WP: Formal analysis, Investigation, Methodology, Writing – original draft. SY: Software, Validation, Visualization, Writing – original draft. L-QZ: Software, Validation, Visualization, Writing – original draft. KS: Software, Validation, Visualization, Writing – review & editing. JX: Software, Validation, Visualization, Writing – review & editing. WW: Funding acquisition, Writing – review & editing. CQ: Conceptualization, Data curation, Funding acquisition, Writing – review & editing. D-ST: Conceptualization, Data curation, Funding acquisition, Writing – review & editing.