We previously found a significant association of numerous sequence homologies to bacterial species found in the gut microbiome with IL-10-inducing Class II epitopes to tumor antigens and hypothesized that those microbial/tumor antigen-specific T-cells will prevent an anti-tumor immune response from developing. Aim 1 is 90% complete. We have demonstrated here that T-cells specific for Pseudomonas aeruginosa and the>50% homologous tumor antigen, YB1, can traffic to an TgMMTV-neu tumor implant. The bacterial and tumor antigen cross-reactive T-cells subsequently promoted tumor growth. These results speak to a potential mechanism as to why whole protein vaccines have been unsuccessful in demonstrating anti-tumor activity in Phase III clinical trials.