Serious challenges remain in immunotherapy, such as to effectively deliver antigens and non-invasively monitor biological processes. Therefore, we aim to design a useful nanocarrier for enhancing antigen-specific immune response and performing functional molecular imaging guided immunotherapy. To achieve this goal, ε-caprolactone modified polyethylenimine (PEI-CL) was prepared based on the ring-opening reaction. This novel macromolecule served as an optimal vehicle that efficiently bound ovalbumin (OVA) antigen forming a controlled nanostructure through electronic self-assembly and resulted in a substantially increased cellular uptake. In vivo, PEI-CL/OVA nanovaccine stimulated inflammatory cytokine production and promoted proliferation of OVA-specific T cells. Moreover, due to the unique chemical and physical properties, PEI-CL was demonstrated as a chemical exchange saturation transfer contrast agent for magnetic resonance imaging. In conclusion, the PEI-CL has synergistic advantages in the effective delivery of antigen and non-invasive imaging for tracking immunotherapeutic cells.