As in many viral infections, the acute phase of SARS-CoV-2 infection is characterized, in symptomatic patients, by more or less severe lymphopenia. In March 2020, by analyzing chest CT-scans of patients hospitalized in ICU for acute respiratory distress syndrome associated with COVID-19, radiologists at the Clinique Ambroise Paré in Neuilly observed that most presented with thymic hyperplasia. Its intensity seemed to be associated with greater lung damage but also with a better prognosis. Following this observation, we analyzed thymic function in about twenty of these patients and showed that it was stronger than that of the control subjects hospitalized in the same ICU, whatever the age of the patients (Cuvelier et al. 2021).
This study clearly indicates that elevated thymic function is beneficial in patients with a severe form of COVID-19, especially in older subjects. In addition, we have shown that the extent of thymic export in patients correlates with thymus hyperplasia, lymphopenia and IL-7 plasma concentration, the essential cytokine for T-cell development in the thymus and peripheral T-cell homeostasis, that we have also identified as part of the danger signals produced rapidly during mucosal viral infections (Beq et al. 2009; Ponte et al. 2017). Our data suggest that a strong capacity to produce new T cells during the acute phase of SARS-CoV-2 infection would increase, in some patients, the capacity to mount an effective adaptive immune response and fight lung inflammation. However, the disparity in thymic function in these patients, reflected in a weak correlation between the intensity of thymopoiesis and age, suggests that other factors, possibly genetic, could contribute to the protection against the most severe forms of disease.
In the continuation of this project, we are now questioning the causes and consequences of the exacerbated thymic function observed in patients hospitalized for severe COVID-19.
Our expertise in measuring thymic function has also been used for many collaborative projects both in France and abroad. We have contributed to exploring the importance of this function in patients after transplantation of hematopoietic stem cells, bone marrow, heart or thymus, in subjects thymectomized in childhood, in children with chronic granulomatous disease or suffering from congenital immunodeficiency (CD4 lymphocytopenia, growth hormone deficiency, FOXN1-deficiency, DiGeorge syndrome, orphan diseases, etc.) and in patients infected with HIV-1 or HIV-2.
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