Clinical, imaging, serological, and histopathological features of pulmonary post-acute sequelae after mild COVID-19 (PASC)

Abstract

Background: A significant proportion of patients experience prolonged pulmonary, cardiocirculatory or neuropsychiatric symptoms after Coronavirus disease 2019 (COVID-19), termed post-acute sequelae of COVID (PASC). Lung manifestations of PASC include cough, dyspnea on exertion and persistent radiologic abnormalities and have been linked to viral persistence, ongoing inflammation and immune dysregulation. So far, there is limited data on lung histopathology and tissue-based immune cell subtyping in PASC. Methods: 51 unvaccinated patients (median age, 40 years; 43% female) with a median of 17 weeks (range, 2-55 weeks) after mild SARS-CoV-2 infection (without hospitalization) underwent full clinical evaluation including high-resolution computed tomography (HR-CT) and transbronchial biopsy. We used RT-PCR/FISH and immunohistochemistry (nucleocapsid/spike/CD3/CD4/CD8) for residual SARS-CoV-2 detection and T lymphocyte subtyping, respectively. We assessed interstitial fibrosis and macrophage profiles by transmission electron microscopy (TEM) and immunofluorescence multiplex staining, while cytokine profiling in broncho-alveolar lavage (BAL) fluid was performed by legendplex immunoassay. Results: Dyspnea on exertion was the leading symptom of pulmonary PASC in our cohort. In 16% and 42.9% of patients, FEV1 and MEF50 were ≤80% and 35.3% showed low attenuation volume (LAV) in >5% of lung area, in line with airflow obstruction. There was a significant correlation between oxygen pulse and time since COVID (p=0.009). Histopathologically, PASC manifested as organizing pneumonia (OP), fibrinous alveolitis and increased CD4+ T cell infiltrate predominantly around airways (bronchiolitis), while the residual virus components were detectable in only a single PASC patient (2%). T cell infiltrates around small airways were inversely correlated with time since COVID, however, this trend failed to reach statistical significance. We identified discrete interstitial fibrosis and a pro-fibrotic macrophage subtype (CD68/CD163/S100A9) as well as significantly elevated interleukin 1β in BAL fluid from PASC patients (p=0.01), but H-scores for fibrotic macrophage population did not correlate with severity of clinical symptoms or T cell infiltration. Interpretation: We show decreased FEV1/MEF50 and increased LAV in line with obstructive lung disease due to CD4+ T cell-predominant bronchiolitis as well as evidence of pro-fibrotic signaling in a subset of unvaccinated PASC patients. Since our results point towards self-limiting inflammation of small airways without detectable viral reservoirs, it remains unclear whether pulmonary symptoms in PASC are SARS-CoV-2-specific or represent a general response to viral infection. Still, evidence of pro-fibrotic signaling should warrant clincal follow-up and further research into possible long-time fibrotic remodeling in PASC patients.

Single-cell analysis of bronchoalveolar cells in inflammatory and fibrotic post-COVID lung disease

“…T cell infiltration has been a consistent finding in recent studies which have examined the post-COVID-19 airspaces; almost exclusively within 3-6 months of the acute insult, when radiological inflammation is more common than fibrosis (Cheon et al, 2021;Gagiannis et al, 2023;Ravaglia et al, 2022;Vijayakumar et al, 2022). The cellular proportions of our fibrotic PCLD samples, harvested at 9-12 months after acute illness, were akin to those reported in BAL samples from healthy individuals, with macrophages comprising greater than 80% and lymphocytes less than 15% of cells (Mould et al, 2021;The BAL Cooperative Group Steering Committee, 1990).…”

“…Minimal interstitial fibrosis and the presence of pro-fibrotic macrophages have also been reported in transbronchial lung biopsies from a subset of individuals sampled at least 12 weeks after mild COVID-19 (Gagiannis et al, 2023). However, there was no difference between the frequency at which mild fibrotic features were detected in the PCLD group and pre-pandemic autopsy samples from individuals who had died of non-respiratory causes (Gagiannis et al, 2023). In our study, which encompassed a broad range of severity of acute disease, including several individuals with COVID-19 ARDS, the phenotype of myeloid cells in both radiological groups was consistent with the spectrum of macrophages and monocytes found in healthy airspaces (Mould et al, 2021); with no convincing evidence for exaggerated activity of pro-fibrogenic pathways.…”

“…Pro-fibrotic macrophages have been implicated in the pathogenesis of severe COVID-19 ARDS, which has been associated with rapid onset pulmonary fibrosis, which improves over time (Wendisch et al, 2021). Minimal interstitial fibrosis and the presence of pro-fibrotic macrophages have also been reported in transbronchial lung biopsies from a subset of individuals sampled at least 12 weeks after mild COVID-19 (Gagiannis et al, 2023). However, there was no difference between the frequency at which mild fibrotic features were detected in the PCLD group and pre-pandemic autopsy samples from individuals who had died of non-respiratory causes (Gagiannis et al, 2023).…”

Proximal immune-epithelial progenitor interactions drive chronic tissue sequelae post COVID-19

“…Given the extensive damage to the respiratory tract during primary infection, the lungs are particularly susceptible to sustained impairments including dyspnea, compromised lung function, and radiological abnormalities which persist up to 2 years post infection in contrast to the majority of extrapulmonary sequelae (1,2,4). Some individuals also develop a non-resolving fibroproliferative response -PASC pulmonary fibrosis (PASC-PF) and typically require persistent oxygen supplementation and eventual lung transplantation (4)(5)(6)(7)(8).…”

Effect of mRNA vaccination on pulmonary sequelae after mild COVID-19

“…Evaluation of PASC can be hampered by preexisting lung disease or severe COVID-19 with scarring or post-ventilaton change [5]. In a a prospective cohort of 51 previously healthy pulmonary PASC patients after mild COVID-19, we have previously shown pulmonary function impairment in a subset of patients, including a reduction in maximal expiratory flow at 50% of FVC (MEF50) and a reduction in FEV1 together with CD4+T cell-mediated bronchiolitis, indicating obstructive change [6,7]. To our best knowledge, this was the first study to use verifiable tissue-based criteria to assess lung involvement in PASC in previously healthy patients after mild COVID-19.…”