Distinct microbiota composition and dendritic cell activation in the appendix microenvironment of ulcerative colitis patients

Abstract

Appendectomy has been associated with reduced risk of developing ulcerative colitis (UC) or experiencing flares after diagnosis, suggesting the appendix may play a role in UC pathogenesis. Given its function in microbial homeostasis and gut immunity, we investigated the relationship between mucosal microbiota and immune environment of the appendix in UC. Appendix tissue was collected from 85 patients undergoing surgery for UC, acute appendicitis (APA) or colon cancer (CC). Immunophenotyping of dendritic cells (DC), macrophages and T lymphocytes was performed using flow cytometry. Microbiota composition was analyzed via 16S rRNA gene amplicon sequencing. Although alpha diversity did not differ between UC and non-UC appendices, beta diversity indicated significant compositional differences. Five bacterial species (Actinomyces hyovaginalis, Mogibacterium sp. Fusobacterium sp. Pseudoramibacter eubacterium, and Streptococcus anginosus) were significantly reduced in the UC appendix compared to APA. However, no species were associated with UC disease activity. In contrast, UC patients showed a significantly higher frequency of activated DCs (CD1a⁺ HLAdr⁺ CD86⁺). DC activation levels correlated with daily stool frequency and T-cell activation. These findings suggest that the appendix may contribute to UC pathogenesis through immune, rather than microbial, mechanisms - supporting a role for dendritic cell-mediated T-cell priming in colonic inflammation.

Keywords: Appendix; dendritic cells; microbiota; ulcerative colitis.

Figure 1.

Microbiota in the appendix of patients with APA, CC and UC. (A) Bar plot showing family distribution in the appendix of APA, CC and UC patients. The top seven most abundant families per group are shown; remaining families are grouped as “other.” (B) Box plots of α-diversity between APA, CC and UC patients according to richness, Pielou eveness and Shannon Index. (C) PCoA plots of Bray-Curtis and Weighted UniFrac distance metrics according to APA, CC and UC patients.

Figure 2.

Comparison of microbial profiles in the appendix of patients with APA, CC and UC. (A) Heatmap of five differentially abundant species in the appendix of APA, CC and UC patients. (B) Relative abundance of five differentially abundant species in the appendix of APA, CC and UC patients.

Figure 3.

Immune microenvironment in the appendix of patients with APA, CC and UC. Flow cytometry analysis of (A) CD1a+ (B) CD1a+HLAdr+CD86+ (C) CD163+ (D) CD8a+ (E) CD4+ cells in the appendix of patients with APA, CC and UC.

Figure 4.

Activated dendritic cells in the appendix of patients with UC. Direct correlation between activated DC and M2 macrophages and (A) Number of daily stools. (B) Correlation between activated DC and activated M2 macrophages in the appendix of patients with APA, CC and UC. (C) Correlation between activated DC and activated CD8a+ lymphocytes mean fluorescence intensity in the appendix of patients with APA, CC and UC. (D) Correlation between M2 macrophages and activated CD4+ lymphocytes in the appendix of patients with APA, CC and UC.