Using MRI cell tracking to monitor immune cell recruitment in response to a peptide‐based cancer vaccine

Purpose: MRI cell tracking can be used to monitor immune cells involved in the immunotherapy response, providing insight into the mechanism of action, temporal progression of tumour growth and individual potency of therapies. To evaluate whether MRI could be used to track immune cell populations in response to immunotherapy, CD8+ cytotoxic T cells (CTLs), CD4+CD25+FoxP3+ regulatory T cells (Tregs) and myeloid derived suppressor cells (MDSCs) were labelled with superparamagnetic iron oxide (SPIO) particles. Methods: SPIO-labelled cells were injected into mice (one cell type/mouse) implanted with an HPV-based cervical cancer model. Half of these mice were also vaccinated with DepoVaxTM, a lipid-based vaccine platform that was developed to enhance the potency of peptide-based vaccines. Results: MRI visualization of CTLs, Tregs and MDSCs was apparent 24 hours post-injection, with hypointensities due to iron labelled cells clearing approximately 72 hours post-injection. Vaccination resulted in increased recruitment of CTLs and decreased recruitment of MDSCs and Tregs to the tumour. We also found that MDSC and Treg recruitment was positively correlated with final tumour volume. Conclusion: This type of analysis can be used to non-invasively study changes in immune cell recruitment in individual mice over time, potentially allowing improved application and combination of immunotherapies.