The clinical success of immune checkpoint therapies such as PD-1, PD-L1, and CTLA-4 inhibitory antibodies has stimulated resurgent interest in a wide range of approaches to cancer immunotherapy. To support the development of these new modalities it is essential to have robust, well characterized preclinical animal models to evaluate efficacy and identify toxicities. Preclinical efficacy assessments of novel immune-oncology therapies requires a functional immune system which limits the usefulness of traditional xenograft models and drive the use of humanized (human immune cell engrafted animals) and syngeneic model systems. The syngeneic MC38 mouse colon cancer model is popular for efficacy assessment studies due to its responsiveness to typical immune checkpoint inhibitors (ICI). We endeavored to characterize the kinetic immune response to checkpoint inhibitors (anti-PD-1 + anti-CTLA-4) in MC38 tumors using flow cytometry and gene expression analysis. Gene expression analysis provided a signature of gene changes that correlate to immune driven changes in tumor growth and may be used in preclinical pharmacodynamics studies as evidence of mode of action for other novel immune-oncology therapies. In addition, gene overrepresentation analysis highlighted the presence and involvement of B cell populations in the MC38 tumor environment which was supported by IHC data that show distinctive B cell staining in what resemble tumor associated tertiary lymphoid structures. The in depth molecular and cellular characterization associated with ICI responses in the MC38 model serves as a template for evaluating the mechanism of action and therapeutic activity of other novel immuno-oncology therapies.