Gut microbiota constituents may affect hypertrophic scarring risk through interaction with specific immune cells in a two-step, two-sample Mendelian randomization study

Abstract

Hypertrophic scars (HS), classified as abnormal scarring, result from an overactive tissue response during wound healing following dermal trauma. Nonetheless, the precise mechanistic pathway underlying its occurrence remains elusive. The principal aim of this study is to elucidate the causal relationship among gut microbiota (GM), immune cell function, and hypertrophic scarring in a European demographic. Leveraging the genome-wide association analysis (GWAS) database, we conducted a two-sample Mendelian randomization (MR) study on gut microbiota (GM), immune cells, and HS. To ascertain the causality between GM, immune cells, and HS, we utilized the inverse variance weighted (IVW) method while employing multiple approaches to negate the effects of pleiotropy and heterogeneity. Furthermore, we quantitatively evaluated the influences of immune cells-mediated GM on hypertrophic scar through a two-step MR analysis. The two-sample MR analysis demonstrated a potential causality between 5 genera of gut microbiotas and 23 immune cell traits with respect to hypertrophic scarring. Further, our results showed that the causal pathway from the genus Subdoligranulum to hypertrophic scar (HS) was mediated by B cell-activating factor receptor (BAFF-R) on CD20- CD38- B cell, with a beta value of 0.034 (95% CI [0.002, 0.066]; P = 0.004), contributing to 7.60% of the total effect of Subdoligranulum on HS. Similarly, CD24 on IgD + CD38 + B cell exhibited a causal impact in the pathway from genus Coprococcus 1 to HS, with a beta value of -0.015 (95% CI [-0.029, -0.001]; P = 0.023), constituting 6.70% of the total effect of Coprococcus 1 on HS. Additionally, the CD8 + T cell %T cell mediated the causal pathway from the genus Adlercreutzia to HS with a beta value of 0.075 (95% CI [0.017, 0.133]; P = 0.024), contributing to 10.10% of the total effect of Adlercreutzia on HS. Our study indicates that the development of hypertrophic scars might be influenced by specific gut microbiota and immune cells. We highlight the possible role of two distinct immune cell genotypes as mediators in this relationship. However, most statistical significance of these findings was not maintained after FDR correction, suggesting our results should be viewed as preliminary and interpreted with caution. Further research is needed to confirm these associations.

Keywords: Gut microbiota; Hypertrophic scar; Immune cells; Mendelian randomization; Two-step analysis.

Fig. 1

Overview of the Mendelian randomization analytical framework. This figure illustrates the two - sample MR approach used to investigate the causal relationships between gut microbiota (GM), immune cells, and hypertrophic scar (HS). The diagram outlines the flow of analysis starting from the selection of genetic instruments from GWAS datasets for GM and immune cells, through to the estimation of their effects on hypertrophic scar risk. Key steps include the use of instrumental variables (IVs) to assess causality and the implementation of various statistical methods to address pleiotropy and heterogeneity. The parameters and symbols in the figure are explained as follows: β_direct represents the direct effect of gut microbiota on hypertrophic scars, without the mediating variable of immune cells; β1 represents the effect of the exposure (gut microbiota) on the mediator (immune cells); β2 represents the effect of the mediator (immune cells) on the outcome (hypertrophic scar); β3^total represents the total effect of the exposure (gut microbiota) on the outcome (hypertrophic scar), calculated as β3^total = β_direct + β1 × ββ2; Crosses (×): Indicate the exclusion of confounding factors in the analysis. These symbols show that the genetic instruments are not directly affected by confounding factors, ensuring the validity of the MR analysis.

Fig. 2

Study design and data sources. This figure illustrates the data sources and analysis methods used in our study. The first part shows the GWAS summary data for the exposure (gut microbiota), mediator (immune cells), and outcome (hypertrophic scar). The second part outlines the MR analysis methods used, including IVW, Weighted Median, Simple Mode, Weighted Mode, and MR-Egger. The third part details the sensitivity analysis and mediation effect calculation. Specifically, the sensitivity analysis includes Cochran’s Q for heterogeneity testing, MR-Egger intercept for horizontal pleiotropy testing, and Leave-one-out for pleiotropic and outlier testing. The mediation effect calculation includes the indirect effect and mediation proportion.

Fig. 3

Associations between GM and HS Risk. This figure displays the positive and reverse results from the inverse variance weighted (IVW) MR analysis and MR Egger. (A) Shows the associations of selected gut microbiota taxa on the risk of developing HS. (B) Shows the associations of HS on the selected gut microbiota taxa. Each point represents a different microbial taxon, with their respective effect sizes (odds ratios) and 95% confidence intervals plotted on the x-axis.

Fig. 4

Impact of immune cell genotypes on hypertrophic scar before FDR correction. This visualizes the findings from the MR analysis regarding the role of various immune cell genotypes in modifying the risk of hypertrophic scar, the effect sizes and confidence intervals are plotted for each immune cell genotypes that elevate the risk of hypertrophic scar are marked in red, whereas those that mitigate the risk are shown in blue. This figure underscores the complex interplay between immune regulation and hypertrophic scar pathogenesis.

Fig. 5

Impact of different GMs on immune cell genotypes. Each point represents a different microbial taxon, with their respective effect sizes (odds ratios) and 95% confidence intervals plotted on the x-axis.