Provided are methods for sensitizing gastrin-associated tumors and/or cancers in subjects to inducers of humoral and cellular immune responses. In some embodiments, the methods relate to administering compositions that have anti-gastrin antibodies, gastrin peptides, and/or nucleic acids that inhibit expression of gastrin gene products to subjects. Also provided are methods for preventing, reducing, and/or eliminating the formation of fibroses associated with tumors and/or cancers, and methods for treating gastrin-associated tumors and/or cancers that include administering to subjects in need thereof a first agent that provides and/or induces an anti-gastrin humoral or cellular immune response in the subject and a second agent that includes one or more stimulators of cellular immune responses against the tumors and/or cancers.