PROJECT SUMMARY/ABSTRACT In recent years, the field of cancer immunotherapy has seen a renaissance – with the use of monoclonal antibodies that target immune checkpoints to activate anticancer immune responses demonstrating unparalleled clinical success. Several of these therapies have now gained FDA approval and are part of routine treatment regimens for several malignancies. Despite the overall success of immunotherapeutic regimens, existing modalities present complications that current research efforts seek to overcome: (1) systemic delivery of checkpoint blockade monoclonal antibodies lead to diverse and unpredictable immune-related adverse events (irAE), (2) boosting responses from the endogenous antitumor repertoire often relies on the existence of pre- primed antitumor T cells, which in the case of highly immunosuppressive tumors or those with low mutational burden may be extremely rare, and (3) attempts to combine immunotherapies to additively boost T cell responses demonstrate increased on-target, off-tumor toxicity. Thus, to circumvent toxicity and immunosuppression, contemporary strategies focus on developing methods to deliver potent immunostimulants directly into a tumor, locally priming antitumor T cells to attack disseminated metastases exhibiting a similar antigenic profile. Bridging these observations, the goals of this proposal are to engineer probiotic strains of bacteria that selectively colonize colorectal cancer (CRC) and locally release immune checkpoint blockade. We hypothesize that this approach will result in more robust and diversified antitumor T cell immunity and promote the clearance of colonized primary and metastatic colorectal cancer lesions and systemically growing CRC-derived foci. The primary innovations of this proposal are in engineering probiotics as an immunotherapeutic delivery vector to locally release high-dose immune checkpoint blockade. Specifically, the proposed system has several advantages over current therapeutic strategies, including: (1) tumor-specific production of immunotherapies and LPS adjuvant, (2) bacteria lysis, leading to effective release of novel immunotherapeutics and LPS, (3) local delivery of novel immunotherapeutics that are toxic to deliver systemically, and (4) oral delivery of probiotics that selectively colonize CRC metastases. This work seeks to shift current research and clinical practice paradigms to overcome current limitations of immunotherapies, by providing a unique vehicle to locally deliver immunotherapies that stimulate antitumor immunity while preventing systemic toxicity and mitigating irAE.