Probiotics and postbiotics: Lactobacillus gallinarum produces indole-3-carboxylic acid (ICA), which inhibits IDO1-mediated kynurenine production and blocks aryl hydrocarbon receptor (AHR)-dependent Treg differentiation, thereby restoring CD8+ T cell cytotoxicity and sensitizing CRC to PD-1 inhibitors (123). Here, CD8A is linked to colorectal carcinoma.