Differential immunoregulation by human surfactant protein A variants determines severity of SARS-CoV-2-induced lung disease

Introduction: COVID-19 remains a significant threat to public health globally. Infection in some susceptible individuals causes life-threatening acute lung injury (ALI/ARDS) and/or death. Human surfactant protein A (SP-A) is a C-type lectin expressed in the lung and other mucosal tissues, and it plays a critical role in host defense against various pathogens. The human SP-A genes (SFTPA1 and SFTPA2) are highly polymorphic and comprise several common genetic variants, i.e., SP-A1 (variants 6A², 6A⁴) and SP-A2 (variants 1A⁰, 1A³). Here, we elucidated the differential antiviral and immunoregulatory roles of SP-A variants in response to SARS-CoV-2 infection in vivo. Methods: Six genetically-modified mouse lines, expressing both hACE2 (SARS-CoV-2 receptor) and individual SP-A variants: (hACE2/6A² (6A²), hACE2/6A⁴ (6A⁴), hACE2/1A⁰ (1A⁰), and hACE2/1A³ (1A³), one SP-A knockout (hACE2/SP-A KO (KO) and one hACE2/mouse SP-A (K18) mice, were challenged intranasally with 10³ PFU SARS-CoV-2 or MEM medium (Sham). Results: Infected KO and 1A⁰ mice had more weight loss and mortality compared to other mouse lines. Relative to other infected mouse lines, a more severe ALI was observed in KO, 1A⁰, and 6A² mice. Reduced viral titers were generally observed in the lungs of infected SP-A mice relative to KO mice. Transcriptomic analysis revealed an upregulation in genes that play central roles in immune responses such as MyD88, Stat3, IL-18, and Jak2 in the lungs of KO and 1A⁰ mice. However, Mapk1 was significantly downregulated in 6A² versus 1A⁰ mice. Analysis of biological pathways identified those involved in lung host defense and innate immunity, including pathogen-induced cytokine, NOD1/2, and Trem1 signaling pathways. Consistent with the transcriptomic data, levels of cytokines and chemokines such as G-CSF, IL-6, and IL-1β were comparatively higher in the lungs and sera of KO and 1A⁰ mice with the highest mortality rate. Furthermore, we observed the complexity of COVID-19, such as the difference between lung and systemic immune response to viral infection and of viral load and mortality among SP-A variants in this model. Conclusion: These findings demonstrate that human SP-A variants differentially modulate SARS-CoV-2-induced lung injury and disease severity by differentially inhibiting viral infectivity and regulating immune-related gene expressions.