Pulmonary fibroproliferative response in old mice that survive acute lung injury by mouse-adapted sars-cov-2

ABSTRACT

RATIONALE While SARS-CoV-2 viral infection, acute lung injury, and inflammation resolve in a timely manner in most individuals, there is growing clinical evidence of long-term sequelae of COVID-19 in some patients, particularly in vulnerable populations. We established a mouse model of SARS-CoV-2 infection using a mouseadapted virus and a standard laboratory strain of mice that displays age-dependent disease severity. In comparison to young BALB/c mice, old BALB/c mice display increased morbidity and mortality when infected with SARS-CoV-2 MA-10, and most succumb to acute infection or reach the criteria for humane euthanasia within 7 days of infection. We examined the lung pathology of older BALB/c mice that survive acute infection to determine the potential long-term pulmonary manifestations of COVID-19 in vulnerable populations such as the elderly.METHODS Mice were randomized and assigned to specific harvest days spanning 30 days prior to the start of the experiment. BALB/cAnNHsd mice were intranasally infected with SARS-CoV-2 MA-10 and clinical signs of disease were monitored, including weight loss and lung function via whole body plethysmography. At each time point, animals were sacrificed and lung lobes were collected for viral titer and histopathological analyses. Lung viral titers of the caudal right lobe were determined by plaque assay. Histopathology of the left lobe was assessed utilizing formalin-fixed, paraffin-embedded specimens.RESULTSIn comparison to 10-week-old BALB/c mice, 1-year-old BALB/c mice were highly susceptible to SARS-CoV-2 MA-10, displaying high morbidity and mortality, even requiring a lower viral dose than young BALB/c mice to yield similar kinetics of weight loss and clinical signs. In some experiments, survival of older mice was low as ∼15% at day 7. For older mice surviving to days 15 and 30 post infection, acute lung injury resolved but there were regionally extensive consolidated areas containing proliferative smooth muscle actin-positive fibroblasts, collagen accumulation, and admixed immune cells with formation of tertiary lymphoid organs. Mice displaying this pulmonary fibroproliferative response did not have detectable levels of virus in the lung.CONCLUSIONSThis mouse adapted SARS-CoV-2 model reveals a response in older mice surviving acute lung injury with robust chronic inflammation and tissue remodeling resulting in pulmonary fibrosis despite viral clearance of the tissue. This offers a model to investigate mediators driving the fibroproliferative and inflammatory responses that may be a COVID-19 sequela and cause persistent pulmonary dysfunction in some vulnerable patients such as the elderly.