Objective: To assess the causal association between specific oral microbiota and the risk of oral leukoplakia (OLK) using a Mendelian randomization (MR) approach, and to elucidate the potential mediating role of immune cells. Methods: Summary statistics from genome-wide association studies (GWAS) of the oral microbiome, GWAS data for immune cell phenotypes, and GWAS summary statistics for OLK from FinnGen were used. The inverse variance weighted (IVW) method was adopted as the primary approach, and it was supplemented by MR Egger regression, simple mode, weighted median, and weighted mode methods for additional analyses, to investigate the causal relationship between 3 117 types of tongue coating and salivary microbiota, as well as 731 immune cell traits, and OLK. Furthermore, a two-step MR approach was applied to explore the potential mediating role of immune cells in the association between oral microbiota and OLK. Results: IVW analysis revealed causal associations between 15 oral microbial genera and OLK. Among these, Streptococcus, Neisseria, and Catonella were associated with a reduced risk of OLK, with Fusobacterium showing the most significant protective effect (OR = 0.41, P = 0.023). In contrast, genera, including Microbacterium, Campylobacter, and Haemophilus_A, were linked to an increased risk of OLK, with Lancefieldella exhibiting the strongest risk effect (OR = 2.66, P = 0.006). Eleven immune cell phenotypes with potential causal associations with OLK were identified, including four protective and seven risk-increasing factors. Mediation analysis further identified four key mediating pathways: pathogenic genera, particularly Campylobacter_A and Lancefieldella, may promote the development and progression of OLK by upregulating highly activated pro-inflammatory immune subsets such as activated monocytes, B cells, and myeloid cells. Conversely, the potentially protective genus Catonella appeared to exert inhibitory effects on OLK by significantly downregulating dendritic cell subsets. Conclusion This study is the first to reveal, at the genetic level, causal pathways through which specific oral microbial genera influence the risk of OLK by mediating immune cell responses. These findings provide novel insights into the immunopathological mechanisms underlying OLK and offer potential targets for intervention strategies aimed at modulating specific microbial genera or immune cell subsets.

Objetive:To investigate whether the methylation patterns of the interleukin-4 (IL-4) gene promoter changed and whether environmental factors affected the methylation level of IL-4 gene in the peripheral blood of patients with recurrent aphthous ulcer (RAU).Methods:Totally 20 patients, who were diagnosed with RAU, were recruited from May 2018 to May 2019 in the Department of Stomatology， First Hospital of Shanxi Medical University in the study (RAU group), including 12 females and 8 males, with mean age of 16-35 years. During the same period, 20 healthy volunteers matching the age and gender of the RAU group were selected from the medical personnel of the same hospital as the healty control group, including 11 females and 9 males, with mean age of 15-35 years. Peripheral blood samples of two groups were collected and the methylation levels of the IL-4 promoter were detected by bisulfite sequencing PCR (BSP). The IL-4 promoter methylation level of each sample was analyzed by direct sequencing and the IL-4 mRNA level was detected by real-time quantitative PCR. The data obtained were statistically analyzed.Results:The IL-4 gene promoter fragment contained 10 CPG sites from -1400 to -1625 bp. The methylation rates of CPG -1556, CPG -1483, CPG -1479and 10 CPG sites were significantly higher in RAU group ［(32.0±19.9)%, (53.0±13.4)%, (46.0±19.8)% and (39.3±12.4)%］ than in healthy control group ［(20.0±3.2)%, (35.5±12.3)%, (28.0±14.4)% and (32.6±5.8)%］, with statistically significant differences ( P<0.05). The relative expression of IL-4 mRNA in the peripheral blood of RAU patients (1.0±0.1) was significantly lower than that of the healthy control group (1.5±0.2) ( P<0.01). There was a significant negative correlation between the overall methylation rate of IL-4 gene promoter and the relative expression level of IL-4 mRNA in RAU group ( r=-0.494, P<0.05). In the multivariate analysis, smoking, vitamin B12 and folic acid in the RAU group were significantly correlated with the overall methylation rate of the IL-4 gene promoter ( P<0.01). Conclusions:The hypermethylation of IL-4 promoter in RAU patients may be related to the reduction of IL-4 gene transcription. Vitamin B12, folic acid and smoking may affect IL-4 gene methylation in peripheral blood of RAU patients.