What happens to the gut when we profoundly eliminate the B-cell compartment? CAR-T therapy has transformed multiple myeloma, but as BCMA-directed therapies expand, we are beginning to see consequences beyond the tumor. A new study characterizes CAR-T cell-induced enterocolitis (CAR-TEC) after cilta-cel and provides a striking look at what may be happening within the intestinal mucosa. Using single-cell transcriptomics, flow cytometry, and tissue imaging of intestinal biopsies, investigators compared patients with CAR-TEC, CAR-T–treated patients without enterocolitis, and healthy volunteers. They found a remarkably disrupted mucosal ecosystem: -Profound depletion of mucosal B cells and plasma cells -Expansion of highly cytotoxic CAR-T cells -Inflammatory remodeling across myeloid, stromal, and glial compartments -Telocyte-associated stromal niche dysfunction And importantly, interferon/JAK-STAT–associated reprogramming was detected across stromal, endothelial, and epithelial compartments. That observation provided a mechanistic rationale for targeting the pathway. In two patients, treatment with the selective JAK1 inhibitor upadacitinib was associated with clinical, endoscopic, and histologic improvement. The bigger story is that CAR-TEC may not simply represent “GI toxicity” from CAR-T. It may represent a multicellular failure of mucosal homeostasis arising from profound B-cell/plasma-cell depletion, persistent immune activation, and remodeling of the intestinal tissue environment. As B-cell–targeted therapies expand beyond oncology and into autoimmune disease, understanding what happens to the tissues that depend on these immune populations may become increasingly important. The next frontier of CAR-T safety may be understanding not only what happens to the tumor—but what happens to the immune ecosystems surrounding it. nature.com/articles/s41591-0… #CART #MultipleMyeloma #CellTherapy #Immunotherapy #CARTEC #Gastroenterology #TranslationalMedicine #CancerResearch