Tumor-specific antibodies elicited by engineered bacteria promote bladder cancer immunotherapy in preclinical mouse models.

The intratumoral microbiome has recently emerged as a potential hallmark of cancer, with implications for response or resistance to therapy. Bacteria can either promote or inhibit cancer growth. However, intratumoral bacteria can also be engineered using synthetic biology to remodel the tumor microenvironment. Here, we engineered the probiotic bacterium Escherichia coli Nissle 1917 (EcN) to express the human chemokine CXCL13 (C-X-C motif chemokine ligand 13), a critical component of germinal center (GC) formation. Antibody affinity maturation and class switching are fundamental aspects of adaptive immune response. Both occur primarily in the GCs of secondary lymphoid organs for defense against pathogens. Immune checkpoint blockade (ICB) efficacy is primarily driven by T cells; however, recent studies in mice and humans have shown that humoral immune responses act as critical partners for ICB-mediated antitumor activity. Using orthotopic models of bladder cancer, intravesically delivered engineered CXCL13-expressing EcN colonized bladder tumors and elicited GC responses in bladder tumor-draining lymph nodes after intravesical delivery. When combined with programmed cell death protein 1 (PD-1) blockade, engineered EcN improved antitumor activity in two aggressive, fast-growing, and immunologically cold orthotopic mouse models of bladder cancer. Mechanistically, this antitumor effect was dependent on the presence of CD8+ T cells and CD4+ T follicular helper cells; combination therapy increased tumor-specific antibody responses and promoted long-term survival and protective immunity upon tumor rechallenge. Thus, we demonstrate that synthetically engineered CXCL13-expressing EcN can enhance the efficacy of PD-1 checkpoint blockade immunotherapy by amplifying tumor-specific humoral immunity.