Evaluating the Immunomodulatory Profile and Treatment Efficacy of Whole Lung Low-Dose Radiation Therapy (LDRT) in a Preclinical Model of Severe Viral Pneumonia

PURPOSE/OBJECTIVE(S): Whole Lung Low-Dose Radiation Therapy (LDRT) is currently being tested as a treatment for SARS-CoV-2. Whether LDRT alters the natural history of viral infection, lung microenvironment or host immunity is currently unknown. We developed a preclinical model of severe viral pneumonia using pandemic-strain influenza A virus (IAV) to evaluate potential immunomodulation and treatment efficacy of LDRT. MATERIALS/METHODS: Female BALB/c mice were infected with an intranasal challenge of 2 × 10(2)–2 × 10(4) PFU of 2009/H1N1 IAV. LDRT was administered at 0.5 Gy or 1.5 Gy x 1 (or sham) 72 hours after IAV challenge. Mice were serially monitored for survival and morbidity (raw lung weight, % weight loss, disease severity index). Acute changes in the lung microenvironment were assayed 72hrs post-LDRT (day 6) with profiling of myeloid/T-cell subsets by cytometry, gene expression by qRT-PCR and histological evaluation of H&E-stained lung sections. RESULTS: LDRT did not extend survival in lethally challenged (2 × 10(4) PFU) mice relative to non-irradiated controls [median survival(days): 7 IAV/sham vs 8 IAV/0.5 Gy vs 8 IAV/1.5 Gy; P = 0.24], with 100% mortality in all IAV groups. In a less virulent model (2 × 10(3) PFU), LDRT did not extend survival with significantly shorter survival observed in the IAV/1.5 Gy group [median survival (% mortality): NR (30%) IAV/sham vs 10d (53%) IAV/0.5 Gy vs 9d (87%) IAV/1.5 Gy; P < 0.001]. Raw lung weight was ∼2-fold higher in IAV mice [mean weight: 0.17g PBS/sham vs 0.31g IAV/sham vs 0.30g IAV/0.5 Gy vs 0.31g IAV/1.5 Gy; P < 0.01 all IAV groups]. Percentage change in body weight was -20.8% (IAV/sham), -21.7% (IAV/0.5 Gy) and -22.5% (IAV/1.5 Gy) compared to PBS-control mice on day 6. Immune profiling demonstrated monocytic and neutrophilic lung infiltrate in response to IAV, with significant increases in CD11b+Ly6G+ neutrophils (P = 0.02) and Ly6C+CD11b+ classical monocytes (P = 0.02). Relative to PBS-challenge, bulk RNA analysis demonstrated robust interferon expression (Ifnb1, Ifng) and upregulation of myeloid/T-cell chemotaxis (Ccl2, Cxcl10) [P < 0.05 in all IAV groups]. A mixed inflammatory response was noted with significant increase in pro- and anti-inflammatory cytokines (Il6, Il10) and M1 markers (Inos2, Cd80) [P < 0.05 in all IAV groups]. Arg1 expression was increased in IAV mice treated with 1.5 Gy LDRT relative to IAV/0.5 Gy (P = 0.02) and IAV/sham (P = 0.02). Histological evidence of alveolar septum rupture, peri-bronchial infiltration, lung parenchyma destruction and vascular congestion was consistent with severe acute lung injury; similar changes were observed in LDRT and non-irradiated lungs of IAV mice. CONCLUSION: In this preclinical IAV model of severe viral pneumonia we did not observe a therapeutic effect of LDRT on survival and morbidity. LDRT did not appear to consistently reduce or reverse IAV-induced inflammatory changes in the lung microenvironment.