3.1 Total Effect of Immune Cells on Lung Cancer
Initially, 18,621 SNPs (immune cells; genome-wide locus significance level, p < 1 × 10^-5) were identified as instrumental variables (IVs) from the large-scale GWAS. The F-statistics of these IVs consistently exceeded the threshold of 10, indicating no indication of weak instrument bias. Subsequently, the IVW method was used to discern causal relationships between the five immune cell traits and traits associated with lung cancer from a multitude of immune cells (Table S1). The results of the IVW analyses indicated that genetic susceptibility to lung cancer increased for certain immune cell types: IgD- CD38dim B cell (OR = 1.050, 95% CI = 1.016–1.086, P = 0.0039) and CD25 on CD39 + CD4 + T cells (OR = 1.034, 95% CI = 1.009–1.059, P = 0.0062). Conversely, a decreased risk of lung cancer was observed for other immune cell fractions associated with genetically predicted abundance: CD39 + Treg cells (OR: 0.958, 95% CI: 0.931–0.985, p = 0.0023), IgD + B cells (OR: 0.900, 95% CI: 0.835–0.970, p = 0.0061), and CD45 on CD33br HLA DR + CD14dim cells (OR: 0.962, 95% CI: 0.937–0.988, p = 0.0039). The scatterplot and forest plot of the effect of CD39 + Treg cell-related SNPs on lung cancer are presented, demonstrating the strength of the association and effect value between these SNPs and lung cancer risk, respectively (Figs. 2 and S1). Cochran's Q-statistic and MR-Egger's intercept test indicated no heterogeneity or horizontal pleiotropy in the MR analysis (Table S2). Moreover, none of the SNPs severely violated the overall effect of immune cells on lung cancer in leave-one-out sensitivity analysis (Figure S2). In the funnel plot, the SNP dots were balanced in the results, suggesting no significant heterogeneity in the data (Figure S3).
3.2 Reverse MR Analysis
Previously, we identified five immune cell characteristics that were strongly associated with the risk of lung cancer. Subsequently, we conducted reverse MR analysis using lung cancer as the exposure factor and immune cells as the outcome. The results revealed that, except for IgD + B cells (OR = 0.850, 95% CI [0.725, 0.996], P = 0.045), the remaining four immune cell types did not show significant results. Therefore, we retained these four negative results for further analyses (Table S3).
3.3 Causal Effects of Blood Metabolites on Lung Cancer
As previously described, we screened 34,843 SNPs (blood metabolites; genome-wide significance level, p < 1 × 10 − 5). Subsequently, using the IVW method, we identified 20 causal relationships between the blood metabolite characteristics and lung cancer risk.
These include 13 blood metabolite characteristics associated with a relatively high risk of lung cancer: Adrenate (22:4n6) levels, Sphingomyelin (d18:1/18:1, d18:2/18:0) levels, Pantoate levels, Sphingomyelin (d18:2/14:0, d18:1/14:1) levels, Ethylparaben sulfate levels, Oleate/vaccenate (18:1) levels, 1-(1-enyl-stearoyl)-2-arachidonoyl-GPE (p-18:0/20:4) levels, 1-myristoyl-2-arachidonoyl-GPC (14:0/20:4) levels, Eicosapentaenoate (EPA; 20:5n3) levels, Two unknown metabolites (X-12407 levels, X-24556 levels), Arachidonate (20:4n6) to pyruvate ratio, Arachidonate (20:4n6) to linoleate (18:2n6) ratio, and 7 blood metabolite characteristics associated with a relatively low risk of lung cancer: 2-methoxyresorcinol sulfate levels, P-cresol glucuronide levels, Delta-CEHC levels, Creatine levels, Oleoyl-linoleoyl-glycerol (18:1 to 18:2) to linoleoyl-arachidonoyl-glycerol (18:2 to 20:4) ratio, Adenosine 5'-monophosphate (AMP) to glutamate ratio, and Retinol (Vitamin A) to linoleoyl-arachidonoyl-glycerol (18:2 to 20:4) ratio. The p-values for these blood metabolites were all less than 0.01, and the summary calculation results are shown in Table S4. In this section, we focus on showcasing scatter and forest plots that highlight the influence of SNPs associated with sphingomyelin (d18:2/14:0, d18:1/14:1) levels on lung cancer. These plots illustrate the strength of association and effect values between blood metabolite-related SNPs and the risk of lung cancer (Figs. 3 and S4).
3.4 Effects of Immune Cells on Blood Metabolites
Following the identification of four immune cells and 20 blood metabolite profiles critical for lung cancer, we investigated their causal roles. Using the IVW method in the MR analysis, we found significant correlations between immune cells and blood metabolites. Specifically, CD39 + Treg cells were highly correlated with sphingomyelin (d18:2/14:0, d18:1/14:1) levels (OR = 0.977, 95% CI [0.957–0.998], P = 0.035) and the adenosine 5'-monophosphate (AMP) to glutamate ratio (OR = 0.974, 95% CI [0.950–0.998], P = 0.033). Additionally, IgD - CD38dim B cells were significantly correlated with sphingomyelin (d18:1/18:1, d18:2/18:0) levels (OR = 1.026, 95% CI [1.000–1.053], P = 0.049). No heterogeneity or horizontal pleiotropy was observed in any of these results and there was no dominant effect of a particular SNP on the causal estimates (Table S5 and Figure S5). The funnel plot indicated balanced SNPs across results, suggesting no significant heterogeneity in the data (Figure S6). The scatterplot and forest plot illustrate the effect of CD39 + Treg cell-associated SNPs on Sphingomyelin (d18:2/14:0, d18:1/14:1) levels, demonstrating the association strength and effect value between immune cell-associated SNPs and blood metabolites (Figs. 4 and S7).
3.5 Immune cells mediate the impact of blood metabolites on the risk of lung cancer occurrence.
After analyzing the causal effects of immune cells on lung cancer and blood metabolites in our dataset, we conducted mediation analyses to characterize the mediating effects of two sphingomyelin levels and the adenosine 5'-monophosphate (AMP) to glutamate ratio on the association between immune cells and lung cancer. The mediation effect of sphingomyelin (d18:2/14:0, d18:1/14:1) levels in the causal pathway from CD39 + Treg cells to lung cancer was − -0.00369 (95% CI [-0.00716, -0.000218]; P = 0.037, Table S6), accounting for 8.52% of the total effect. Additionally, the mediation effect of the adenosine 5'-monophosphate (AMP) to glutamate ratio in the causal pathway from CD39 + Treg cells to lung cancer was 0.00466 (95% CI [0.000321, 0.00899]; P = 0.035, Table S6), accounting for − 10.8% of the total effect. However, Sphingomyelin (d18:1/18:1, d18:2/18:0) levels mediated 0.00368 (95% CI [0.0000423, 0.0074]; p = 0.052, Table S6) of IgD- CD38dim B cells, with a mediating effect validation resulting in a p-value greater than 0.05. Therefore, this was not further analyzed in this study. Here, we focus on showcasing a forest plot delineating the impact of CD39 + Treg cells on lung cancer occurrence via the modulation of sphingomyelin (d18:2/14:0, d18:1/14:1) levels. (Fig. 5).