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The role of autoreactive T cells on the course of Coronavirus disease-19 (COVID-19) remains elusive. Type II pneumocytes represent the main target cells of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Autoimmune responses against antigens highly expressed in type II pneumocytes may influence the severity of COVID-19 disease. The aim of this study was to investigate autoreactive T cell responses against self-antigens highly expressed in type II pneumocytes in the blood of COVID-19 patients with severe and non-severe disease. We collected blood samples of COVID-19 patients with varying degrees of disease severity and of pre-pandemic controls. T cell stimulation assays with peptide pools of type II pneumocyte antigens were performed in two independent cohorts to analyze the autoimmune T cell responses in patients with non-severe and severe COVID-19 disease. Target cell lysis assays were performed with lung cancer cell lines to determine the extent of cell killing by type II PAA-specific T cells.Results: We identified autoreactive T cell responses against four recently described self-antigens highly expressed in type II pneumocytes, known as surfactant protein A, surfactant protein B, surfactant protein C and napsin A, in the blood of COVID-19 patients. These antigens were termed type II pneumocyte-associated antigens (type II PAAs). We found that patients with non-severe COVID-19 disease showed a significantly higher frequency of type II PAA-specific autoreactive T cells in the blood when compared to severely ill patients. The presence of high frequencies of type II PAA-specific T cells in the blood of non-severe COVID-19 patients was independent of their age. We also found that napsin A-specific T cells from convalescent COVID-19 patients could kill lung cancer cells, demonstrating the functional and cytotoxic role of these T cells. Our data suggest that autoreactive type II PAA-specific T cells have a protective role in SARS-CoV-2 infections and the presence of high frequencies of these autoreactive T cells indicates effective viral control in COVID-19 patients. Type II-PAA-specific T cells may therefore promote the killing of infected type II pneumocytes and viral clearance. Type II pneumocytes represent the main target cells of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and other respiratory viruses. The killing of infected type II pneumocytes leads to the release of not only viral antigens, but also of self-antigens, such as type II pneumocyte-associated antigens (type II PAAs). These self-antigens can lead to the priming and activation of CD8+ T cells. Patients with non-severe Coronavirus disease-19 (COVID-19) have an efficient and targeted viral clearance of SARS-CoV-2, suggesting a previous infection with other respiratory viruses, such as influenza. In the case of a subsequent infection with a virus like SARS-CoV-2, the swift T cell response by pre-existing autoreactive T cells could have a protective effect in these pre-immunized patients by promoting the killing of infected type II pneumocytes. This suggests that autoreactive type II PAA-specific and COVID-19-specific T cells may synergistically protect from severe COVID-19 infection, although this theory will require further studies for validation. On the other hand, patients that do not harbor such protective autoreactive T cells have an impaired viral clearance and develop severe COVID-19. [Display omitted] • Type II PAA-specific T cells are present in the blood of COVID-19 patients. • Patients with non-severe COVID-19 harbor increased numbers of type II PAA-specific T cells. • The protective effect of type II PAA-specific T cells is independent of patient age. • A T cell-mediated autoimmune response against type II PAAs may be protective in COVID-19. [ABSTRACT FROM AUTHOR]