Advancing cancer immunotherapy: the promise of self-photoconversion reporters for immune cell migration tracking

Immune cell migration holds great promise for the activation and coordination of immune responses against tumors. Cells such as dendritic cells capture tumor antigens and migrate to lymph nodes (LNs), where they activate T cells. This process initiates a cascade of immune responses leading to the generation of cytotoxic T lymphocytes that can recognize and eliminate cancer cells.¹ Understanding and manipulating immune cell migration is therefore crucial for developing effective immunotherapies to combat cancer and improve treatment outcomes.^2,3 However, evaluating immune cell migration poses challenges due to the complexity of the process and the limitations of current methods. In vitro assays, such as transwell migration tests, provide valuable insights into cell movement but lack the physiological context of in vivo migration.⁴ Live-cell imaging⁵ and flow cytometry⁶ offer dynamic visualization and quantitative analysis but may not capture the entire migratory process. In addition to in vitro evaluation, in vivo tracking techniques like intravital microscopy provide a holistic view of migration in living organisms but are often limited by tissue depth and accessibility.⁷ Recently, green-to-red photoconvertible proteins such as Kaede, KikGR, EosFP and Dendra2 have been developed that enable in vivo spatiotemporal imaging of immune cells of interest by site-specific photoconversion.⁸ Unfortunately, these photoconvertible proteins, which are activated by short wavelength light, require genetic engineering of the cells and can be rejected as foreign proteins in immunocompetent mice.

In a recent issue of eBioMedicine, Kobayashi and co-workers introduced a phototruncation-assisted cell tracking (PACT) approach and used it for in vivo monitoring of dynamic migration of immune cells between tumor and tumor-draining lymph nodes (TDLNs) after near-infrared photoimmunotherapy (NIR-PIT).⁹ PACT can noninvasively track cell migration without genetic modification by utilizing a photoconvertible technology sensitive to NIR light. Kobayashi and co-workers used 3′-OMe substituted DiR, an organic heptamethine cyanine (Cy7), as a cell-labeling probe, which can achieve cellular Cy7 to pentamethine cyanine (Cy5) conversion upon NIR laser irradiation. The labeling capability of PACT was confirmed by an ex vivo study in which 3′-OMe-DiR-stained splenocytes showed a high percentage of Cy5-labeled cells after 780 nm light irradiation. PACT allowed the detailed in vivo observation of dendritic cells and macrophages migrating from tumor sites to TDLNs after NIR-PIT upon intratumoral injection of Cy7, followed by an influx of CD8⁺ T cells back to the tumor upon injection of Cy7 into TDLNs, suggesting a robust immune response potentially beneficial for therapeutic outcomes. Strategic use of pharmacological inhibitors further confirmed the crucial roles of the Sphingosine-1-Phosphate (S1P) pathway and G-protein α subunit (Gαi) signaling in this migratory process, providing valuable insights into the underlying mechanisms of immune cell dynamics in cancer therapy.

These findings from this study suggest that PACT represent a powerful tool to explain how the migration of immune cells is optimized for ensuring immune efficacy. Despite these advances, questions remain regarding PACT's application across different cancer types and disease models, the specificity and efficiency of cell labeling and tracking, its integration into existing treatment protocols, and the long-term effects of the introduced dyes. Additionally, the impact of manipulating immune cell migration on systemic immunity and potential side effects warrant further investigation. Further studies combining this technique with intravital imaging will provide valuable insights into the migration, connection, function, and in vivo fate of various cell populations, as well as initial insights into off-target behavior and oncological events.

Contributors

Literature search: M.Z. and S.H.; Data collection: M.Z. and S.H.; Data interpretation: M.Z. and S.H.; Writing: M.Z., S.H. and H.T. All the authors read and approve the manuscript.

Declaration of interests

The authors declare no conflict of interest.