A33-11 BCL6-Dependent Tissue-Resident Helper T Cells Promote Chronic Lung Sequelae Following Viral Pneumonia

Abstract Respiratory viral infections such as influenza and SARS-CoV-2 can lead to the development of chronic lung disease and impaired pulmonary function long after the recovery from acute illness. However, the mechanisms underlying these post-acute sequelae remain poorly understood. Using human lung samples from patients with post-acute sequelae of COVID-19 with pulmonary fibrosis (PASC-PF) and an aged mouse model of chronic lung disease following acute viral infection, we found that CD4+ T cells were enriched near the dysplastic Krt8Hi epithelial regions of the lung. Similarly, in aged mice, CD4+ T cells localized to Krt8Hi areas, and their spatial distribution correlated positively with the extent of epithelial dysplasia. Further analysis of bronchoalveolar lavage (BAL) fluid from COVID-19 convalescent individuals revealed a distinct population of FoxP3−PD-1HiCD4+ T cells highly expressing ICOS, CD69, and CD103 - markers characteristic of both T follicular helper (TFH) and tissue-resident memory (TRM) cells, which we have defined as tissue-resident helper (TRH) cells previously. Notably, the frequency of these TRH-like cells correlated positively with the extent of reticulate or consolidated regions on chest CT scans, indicative of lung fibrosis. Furthermore, the frequency of these cells was negatively associated with lung functional parameters including forced vital capacity (FVC). These findings suggest that PD-1HiCD4+ TRH-like cells expand in PASC and may contribute to chronic lung sequelae. Given that Bcl6 is required for TRH differentiation, we employed an aged, CD4-specific, inducible Bcl6-deficient (Bcl6iΔCD4) mouse model to evaluate the functional contribution of these cells to post viral lung fibrosis. Bcl6 deficiency led to a marked reduction in total and antigen-specific PD-1HiCD4+ TRH cells, as confirmed by flow cytometry and immunofluorescence. Strikingly, Bcl6 iΔCD4 mice displayed significantly decreased inflammatory and fibrotic lesions in the lung compared to wild-type controls. Moreover, Bcl6 iΔCD4 mice exhibited enhanced lung regeneration, evidenced by increased AT1 and AT2 epithelial cells and improved pulmonary function. Together, these findings identify Bcl6-dependent CD4+ TRH responses as key drivers of post-viral chronic lung sequelae. This study reveals a novel immunopathogenic mechanism linking persistent tissue helper memory T-cell responses to impaired lung repair and fibrosis, providing new insight into the development of post-viral lung disease and potential therapeutic targets for conditions such as respiratory long COVID. This abstract is funded by: American Lung Association, NIH