V. Sethi1, S. Kurtom1, I. Fernandez2, J. Pignac-Kobinger2, A. Ferrantella1, C. Jacob1, P. Roy1, P. Sharma1, M. T. Abreu2, S. Roy1, S. Ramakrishnan1, A. Saluja1, V. Dudeja1 1University Of Miami,Surgical Oncology,Miami, FL, USA 2University Of Miami,Gastroenterology,Miami, FL, USA
Introduction: Pancreatic ductal adenocarcinoma (PDAC) is characterized by a unique immunosuppressive milieu which makes it particularly resistant to modern checkpoint blocking drugs like anti-PD1 and anti-CTLA4. Like many other cancers, PDAC is associated with a changed ‘dysbiotic’ gut microbiota. Whether this dysbiotic state promotes cancer or is a neutral-bystander is unclear. We aimed to study as well as investigate the immunotherapeutic implications of this relationship in mice.
Methods: Wildtype mice-pups (guests) were co-housed with two different types of ‘host’ mice: cancer-bearing KPC (KrasG12D/+;Trp53R172H/+;Pdx-1-Cre) mice or with cancer-naïve mice. Due to coprophagy, the gut microbiota transferred horizontally from hosts to guests. After 2 months of co-caging, the guest mice were given subcutaneous KPC-PDAC and tumor progression was followed. In subsequent experiments, wildtype mice were depleted of their entire gut microbiome by oral antibiotics and growth of orthotopic, subcutaneous and metastatic PDAC was compared between them and controls. Experiments were repeated in various genetic backgrounds. Specific antibiotics were also combined with immune checkpoint-inhibitors in a therapeutic fashion. Pancreatic tumors and metastases were immunophenotyped and were probed for bacteria.
Results: Mice co-housed with KPC cagemates had more aggressive tumor-kinetics than mice co-caged with cancer-naïve mice. Eradicating the gut microbiota by oral antibiotics shrunk tumors in all models of PDAC. This effect disappeared in immunodeficient Rag1-/- mice but not in Tlr4-/- knockout mice. Antibiotics significantly enhanced the efficacy of anti-PD1 and anti-CTLA4 in decreasing tumor burden. Very interestingly, pancreatic tumors grew viable bacteria on culture medium and this was prevented by poorly-absorbable oral antibiotics. Similarly, 16S rRNA amplicon sequencing of PDAC metastases revealed the presence of a metastatic microbiome similar to the gut, thereby suggesting a gut-to-tumor translocation of bacteria. Flowcytometry results indicated that gut bacteria induced a myeloid cell-mediated immunosupression inside the tumor and antibiotics induced a Th1/Tc1 cell mediated anti-tumor immunity. Neutralizing various nodes of the gut bacteria-induced immunosuppressive axis by monoclonal antibodies abrogated the tumor-suppressing effects of antibiotics thereby, revealing a possible mechanism.
Conclusion: Our preclinical data presents evidence of the tumor-enhancing potential of gut dysbiosis in cancer-bearing hosts and highlights a potential role of oral antibiotics as novel immunotherapeutic agents against primary and metastatic pancreatic cancer.
