Background: Abscopal responses require radiation-induced immune priming and may vary with tumor immunogenicity. Whether FLASH preserves priming across tumors with contrasting immunogenicity is unclear. We compared matched-dose FLASH and conventional radiotherapy (CONV) in bilateral CT-26 and LL2 models.
Methods: Poorly immunogenic LL2 tumors were established in C57BL/6 mice and relatively immunogenic CT-26 tumors in BALB/c mice, each with an irradiated leg tumor and a nonirradiated flank tumor. Film-verified doses were matched between FLASH (mean 118 Gy/s) and CONV (≤0.067 Gy/s). LL2 tumors received 15 Gy once with or without anti-PD-1. CT-26 tumors received 12 Gy in two fractions with or without intratumoral immature dendritic cells (iDCs; 1 × 10⁶ after each fraction). Tumor growth and survival were monitored; CT-26 apoptosis and tumor-cell surface markers were evaluated through 72 hours.
Results: FLASH and CONV produced comparable suppression of irradiated tumors in both models. In LL2, no significant abscopal response was detected, and anti-PD-1 provided no clear additional benefit. In CT-26, all radiotherapy-containing regimens suppressed irradiated tumors versus control (all p<0.001), without differences among regimens. A significant reduction in nonirradiated tumor growth was detected only with CONV+iDC versus control (p=0.002); FLASH+iDC did not reach significance (p=0.150). Apoptosis was comparable between modalities. At 72 hours, significant increases from baseline encompassed CD86, CD80, PD-L1, I-A/I-E, and H-2K^d after CONV, but only CD86 and I-A/I-E after FLASH, indicating a broader pattern of late immune-relevant surface-marker upregulation after CONV.
Conclusions: Despite comparable local control, an abscopal signal was observed only in relatively immunogenic CT-26 under CONV+iDC, whereas none was detected in poorly immunogenic LL2. This model-dependent rescue-like pattern, together with broader late surface-marker upregulation after CONV, supports the hypothesis that primary-tumor immune-priming competence shapes systemic responses. Ongoing matched iDC studies and analyses of tumor-derived immune signals and dendritic-cell activation will directly test this mechanism.