CD8A and colorectal carcinoma: For instance, indole‐3‐carboxaldehyde, a metabolite derived from Lactobacillus gallinarum, improves anti‐PD‐1 therapy efficacy in CRC by inhibiting the differentiation of CD4+ Tregs and modulating the IDO1/Kyn/AhR axis to enhance CD8+ T‐cell function [97].