CD4 T cells facilitate the RT-induced abscopal effect by promoting antigen cross-presentation to CD8 T cells at unirradiated tumor sites.

[BACKGROUND] The local effect of radiotherapy (RT) is enhanced by CD8 T-cell responses elicited through dendritic cell (DC)-mediated cross-presentation of tumor antigens, facilitated by RT-induced damage-associated molecular patterns. The abscopal effect-regression of non-irradiated tumors-has been observed clinically, particularly in combination with immune checkpoint blockade, although it remains uncommon. To better understand how to enhance this effect, we investigated two RT/α-programmed death 1 (PD-1)-based triple combinations incorporating either α-cytotoxic T lymphocyte-associated protein 4 (CTLA-4) or CD122-targeted interleukin (IL)-2 complexes (IL-2c).

[METHODS] We tested these regimens in B16 melanoma and C51 colon carcinoma models in mice with one irradiated and one non-irradiated tumor on opposite flanks.

[RESULTS] In both models, RT/αPD-1/αCTLA-4 elicited a stronger abscopal response than RT/αPD-1/IL-2c. In the C51 model, RT/αPD-1/αCTLA-4 achieved a 61.5% abscopal cure rate, dependent on both CD8 and CD4 T cells. In contrast, the less effective RT/αPD-1/IL-2c response required only CD8 T cells. The enhanced abscopal effect with RT/αPD-1/αCTLA-4 was associated with increased numbers, effector function, and reduced exhaustion of tumor-specific CD8 tumor-infiltrating lymphocytes (TILs) and of CD4 TILs, along with elevated CD80CD86 DCs in abscopal tumors, as shown by flow cytometry; immunofluorescence confirmed increased T-cell infiltration. CD4 T-cell depletion during RT/αPD-1/αCTLA-4 treatment impaired abscopal but not irradiated tumor control, reducing infiltration of tumor-specific CD8 T cells and conventional (c) DC1s, and diminishing cDC1-mediated cross-presentation in abscopal tumors. Activated CD4 T cells upregulated CD80/CD86 on cDC1s and enhanced cross-presentation, partly via interferon-γ and tumor necrosis factor. Adoptively transferred tumor-specific CD8 T cells from tumor-irradiated donors localized to unirradiated tumors and draining lymph nodes in αPD-1/αCTLA-4-treated recipients, but not in untreated or CD4 T cell-depleted mice.

[CONCLUSIONS] These results demonstrate that an RT-based combination therapy that robustly induces CD4 T cells alongside CD8 T cells can elicit a strong abscopal response and suggest that CD4 effector T cells act at abscopal sites by promoting DC-mediated cross-presentation of tumor antigens to CD8 T cells originating from the irradiated tumor.