Differential Immune Modulation With Carbon-Ion Versus Photon Therapy

Radiation therapy (RT) modulates the immune characteristics of the tumor microenvironment (TME). It is not known whether these effects are dependent on the type of RT used.

We evaluated the immunomodulatory effects of carbon-ion therapy (CiRT) compared with biologically equivalent doses of photon therapy (PhRT) on solid tumors. Orthotopic 4T1 mammary tumors in immunocompetent hosts were treated with CiRT or biologically equivalent doses of PhRT. Seventy-two hours after RT, tumors were harvested and the immune characteristics of the TME were quantified by flow cytometry and multiplex cytokine analyses.

PhRT decreased the abundance of CD4+ and CD8+ T cells in the TME at all doses tested, with compensatory increases in proliferation. By contrast, CiRT did not significantly alter CD8+ T-cell infiltration. High-dose CiRT increased secretion of proinflammatory cytokines by tumor-infiltrating CD8+ T cells, including granzyme B, IL-2, and TNF-α, with no change in IFN-γ. Conversely, high-dose PhRT increased CD8+ T-cell secretion of IFN-γ only. At most of the doses studied, PhRT increased proliferation of immunosuppressive regulatory T cells; this was only seen with high-dose CiRT. Cytokine analyses of bulk dissociated tumors showed that CiRT significantly increased levels of IFN-γ, IL-2, and IL-1β, whereas PhRT increased IL-6 levels alone.

At low doses, lymphocytes differ in their sensitivity to CiRT compared with PhRT. Unlike PhRT, low-dose CiRT is generally lymphocyte-sparing. At higher doses, CiRT is a more potent inducer of proinflammatory cytokines and merits further study as a modulator of the immunologic characteristics of the TME.