The COVID-19 pandemic, caused by the novel coronavirus SARS-CoV-2, has gripped the world for several years. As the pandemic has evolved, our understanding of the virus and its consequences has deepened.
While much attention has been focused on acute cases of COVID-19 and the development of effective treatments and vaccines, another challenge has emerged: the management of long-term, lingering symptoms and complications that persist long after the resolution of the acute illness.
This condition is collectively known as Post-Acute Sequelae of SARS-CoV-2 (PASC). In this article, we delve into the world of PASC, with a particular focus on its pulmonary and extrapulmonary manifestations, the potential underlying mechanisms, and the prospects for therapeutic intervention.
Pulmonary PASC: A Lingering Threat
SARS-CoV-2 infection often inflicts extensive damage to the respiratory tract during its primary attack, making the lungs particularly vulnerable to long-term impairments. Many individuals recovering from COVID-19 continue to experience symptoms related to their respiratory system, even after the acute phase of the disease has passed.
For some unfortunate individuals, the consequences are even more severe. They develop a condition known as PASC pulmonary fibrosis (PASC-PF). In these cases, the lung tissue undergoes non-resolving fibroproliferative changes, leading to a progressive decline in lung function. Patients with PASC-PF often require ongoing oxygen supplementation and may eventually need lung transplantation to survive.
Unraveling the Mechanisms Behind PASC
Understanding the underlying mechanisms that drive the development of respiratory PASC has been a significant challenge. While there has been progress in identifying immune signatures associated with these conditions, the precise mechanistic roles of these immune responses in driving tissue damage and fibrosis remain unclear. One of the main obstacles has been the lack of “clinically relevant” animal models that mimic respiratory PASC in humans.
To address this gap in knowledge, researchers have sought to identify key cellular players in lung repair and regeneration. Among these players are type 2 alveolar epithelial (AT2) cells, which serve as facultative stem cells in the lungs.
These cells are responsible for self-renewal and can differentiate into type 1 alveolar (AT1) cells to replenish the damaged epithelial tissue after an injury. Recent research has shed light on the process of AT2 to AT1 trans-differentiation, identifying a transitional state characterized by high expression of cytokeratin 8 (Krt8hi).
Another significant aspect of lung repair and regeneration is the role of basal cell progenitors, marked by the presence of cytokeratin 5 (Krt5+). Severe alveolar damage can trigger the recruitment of these basal cell progenitors to the distal lung, where they may persist in an undifferentiated state or differentiate into upper airway cell fates, causing a phenomenon known as alveolar “bronchiolization.”
The accumulation of undifferentiated Krt8hi transitional cells and the emergence of ectopic Krt5+ pods are hallmark indicators of lung injury, and their prolonged presence has been associated with chronic diseases such as lung fibrosis.
A Unique Landscape in PASC-PF
Recent investigations have uncovered common histopathological features in respiratory PASC and PASC-PF patients. These include the persistent reduction in alveolar epithelial cells, the maintenance of Krt8hi and Krt5+ dysplastic progenitors, and the continued presence of various immune cell populations in the lungs.
While immune-derived cues have been shown to influence lung repair, their specific interactions with the alveolar epithelium and their roles in post-viral fibrosis have remained largely unexplored.
What sets PASC-PF apart from other lung conditions is the presence of spatially defined microenvironments composed of a dysregulated immune-epithelial progenitor niche. These unique niches are believed to underlie dysplastic lung repair and the development of tissue fibrosis following acute COVID-19. Importantly, these niches have not been observed in acute COVID-19 or idiopathic pulmonary fibrosis (IPF) lungs, suggesting that they are a distinctive feature of post-viral fibrosis.
Animal Models and Their Limitations
The quest to unravel the mechanisms of respiratory PASC has also encountered challenges in the realm of animal models. While the study of human PASC has provided valuable insights, researchers have sought to replicate these findings in animal models for further experimentation.
Unfortunately, attempts to induce severe alveolar pathology and fibrosis similar to PASC-PF in mice infected with SARS-CoV-2 MA-10 have fallen short.
Even aged BALB/c mice, which typically develop significant pulmonary inflammation and prolonged pathology following SARS-CoV-2 infection, have not exhibited the persistent Krt8hi and Krt5+ areas that are characteristic of human PASC-PF.
Moreover, the immune response in mice, particularly the CD8+ T cell response, differs significantly from that in humans. While CD8+ T cells are enriched in human PASC-PF lungs and may contribute to the maintenance of dysplastic areas, they do not show a substantial increase in BALB/c mice post SARS-CoV-2 infection. This divergence could be due to genetic differences or a bias towards TH2 immune responses in mice.
The need for clinically relevant animal models of SARS-CoV-2 post-viral fibrosis cannot be overstated. These models, validated through comparative analyses with human PASC, are crucial for uncovering the underlying mechanisms and identifying potential therapeutic targets.
A Ray of Hope: Insights from Influenza
While SARS-CoV-2 infection models in mice have proven challenging in replicating the full spectrum of PASC-PF, there have been significant insights gained from studying another respiratory virus: influenza.
In particular, influenza infection in aged C57BL/6 mice has induced chronic pulmonary sequelae that closely resemble the immunopathological features of human PASC-PF lungs.
This finding offers hope that studying influenza-induced lung sequelae may provide valuable insights into the mechanisms behind PASC-PF.
The Role of CD8+ T Cells in PASC-PF
One of the intriguing discoveries in the study of PASC-PF is the role of CD8+ T cells in impaired recovery and fibrotic remodeling. CD8+ T cells are an essential component of the immune response to viral infections. Following viral infection-mediated alveolar injury, lung-resident CD8+ T cells are recruited and maintained at sites of severe damage to protect against reinfection. This phenomenon has been termed “repair-associated memory depots.”
In individuals who successfully recover from acute COVID-19, pulmonary CD8+ T cells gradually decline as alveolar regeneration occurs. However, in cases of PASC-PF, as well as in aged mice infected with influenza, CD8+ T cells persist in the lungs over the long term. This prolonged presence of CD8+ T cells is associated with impaired lung recovery post-infection and the development of fibrotic lung disease.
A Link to Pulmonary Fibrosis
There is a growing body of evidence suggesting that PASC-PF may represent an intermediate state preceding the potential progression to idiopathic pulmonary fibrosis (IPF). IPF is a chronic and progressive lung disease characterized by the formation of scar tissue in the lungs, leading to impaired lung function and, ultimately, respiratory failure.
Whether CD8+ T cells play a central role in determining the balance between functional recovery and the development of PASC-PF or IPF is a critical question that remains unanswered.
The prolonged maintenance and activity of CD8+ T cells in the lungs could result from various factors. Excessive TGF-β signaling, reported to occur in PASC, may contribute to this phenomenon.
Chronic persistence of viral remnants or other independent mechanisms may also be at play. Further research is needed to unravel the intricate web of interactions that lead to the persistence of CD8+ T cells in the lungs and their impact on lung function and fibrotic remodeling.
The Key Role of Interleukin-1β
To shed light on the mechanisms driving respiratory sequelae post-viral infections, researchers have turned to advanced techniques such as imaging and spatial transcriptomics. These methods have revealed that chronic IL-1β signaling plays a pivotal role in the development of PASC-PF.
Spatially defined microenvironments within the lungs are characterized by aberrant interactions between CD8+ T cells and monocyte-derived macrophages, mediated by proinflammatory cytokines such as IFN-γ and TNF. These interactions perpetuate the chronic release of IL-1β, a potent proinflammatory cytokine, which impairs the trans-differentiation of AT2 cells into AT1 cells, hampering lung regeneration.
While in vitro studies have shown that chronic IL-1β impairs AT2 trans-differentiation, the logistical challenges of directly testing its effects on Krt8hi and Krt5+ progenitors in vivo using transgenic mice remain.
Therefore, it is plausible that IFN-γ and TNF, as well as IL-1β and their downstream mediators, may influence epithelial progenitor cell fate through actions on other lung immune and non-immune cells, ultimately resulting in fibrotic remodeling.
Hope on the Horizon: Therapeutic Interventions
The identification of key immune and cytokine players in the development of PASC-PF opens doors to potential therapeutic interventions. One of the most promising avenues involves neutralizing the activity of IFN-γ, TNF, or IL-1β in the post-acute phase of infection. These interventions have shown the potential to augment alveolar regeneration and dampen fibrotic sequelae in animal models.
Notably, the United States Food and Drug Administration (FDA) has granted emergency use authorization to drugs such as the IL-1 receptor antagonist Anakinra and the JAK inhibitor Baricitinib for the treatment of acute COVID-19. Given the observed benefits in both adult and pediatric patients with acute COVID-19, these drugs may hold promise as candidates for treating ongoing respiratory PASC in the clinical setting.
The landscape of COVID-19 research continues to evolve, with a growing focus on understanding and addressing the long-term consequences of SARS-CoV-2 infection. Post-Acute Sequelae of SARS-CoV-2 (PASC) poses a significant challenge, affecting millions of individuals worldwide.
Among the various manifestations of PASC, respiratory sequelae, particularly PASC pulmonary fibrosis (PASC-PF), have garnered attention for their debilitating impact on lung function and quality of life.
While much remains to be elucidated, recent research has uncovered crucial insights into the mechanisms behind PASC-PF. Spatially defined microenvironments, driven by interactions between CD8+ T cells, monocyte-derived macrophages, and cytokines like IL-1β, play a central role in perpetuating lung damage and fibrosis. Moreover, the potential for therapeutic interventions targeting these pathways offers hope for individuals grappling with long-term respiratory symptoms.
The road to understanding and managing PASC is a complex and challenging journey, but it is one that researchers and healthcare professionals are committed to undertaking. As our knowledge deepens and therapeutic strategies emerge, we move closer to helping the millions of individuals living with the enduring effects of COVID-19 find relief and recovery.
reference link : https://www.biorxiv.org/content/10.1101/2023.09.13.557622v1.full.pdf+html