Involvement of circulating fibrocytes in pathophysiology and prognosis of COVID-19 pneumonia – FIBROCO

Blood fibrocytes were quantified by flow cytometry as CD45+/CD15-/CD34+/Collagen-1+cells in patients hospitalized for COVID-19 pneumonia. In a subgroup of patients admitted in ICU, fibrocytes were quantified in blood and broncho-alveolar lavage (BAL). Serum amyloid P (SAP), TGF-beta, CXCL12, CCL2, and FGF2 serum concentration were measured in serum.

We included 57 patients in the Hospitalized group (median age 59 years [23-87]) and 16 Healthy controls. The median percentage of circulating fibrocytes was higher in patients compared to controls (3.6% [0.2-9.2] vs. 2.1% [0.9-5.1], p=0.04). Blood fibrocyte count was lower in the 6 patients who died compared to survivors (1.6% [0.2-4.4] vs. 3.7% [0.6-9.2], p=0.02). Initial fibrocyte count was higher in patients showing a complete lung CT resolution at 3 months. Circulating fibrocyte count was decreased in the ICU group (0.8% [0.1-2.0]) whereas BAL fibrocyte count was 6.7% [2.2-15.4]. Serum SAP and TGF-beta1 concentrations were increased in Hospitalized patients. SAP was also increased in ICU patients. CXCL12 and CCL2 were increased in ICU patients, and negatively correlated with circulating fibrocyte count. Fibrocytes were detected with immunohistochemistry (CD45+, vimentin+) in lung samples obtained at time of surgery in two patients one month after hospitalization for COVID-19 pneumonia.

Circulating fibrocytes are increased in patients hospitalized for COVID-19 pneumonia and a lower fibrocyte count was associated with an increased risk of death and a slower resolution of lung CT opacities.

Ghanem M, Homps-Legrand M, Garnier M, Morer L, Goletto T, Frija-Masson J, Wicky PH, Jaquet P, Bancal C, Hurtado-Nedelec M, de Chaisemartin L, Jaillet M, Mailleux A, Quesnel C, Poté N, Debray MP, de Montmollin E, Neukirch C, Borie R, Taillé C, Crestani B; French COVID Cohort Study Group. Blood fibrocytes are associated with severity and prognosis in COVID-19 pneumonia. Am J Physiol Lung Cell Mol Physiol. 2021 Nov 1;321(5): L847-L858.

Coronavirus disease 2019 (COVID-19) is caused by a new coronavirus, the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), and can be responsible of severe cases in 15-20% of patients, with bilateral pneumonia and acute respiratory distress syndrome (ARDS), characterized by a histological diffuse alveolar damage, and in more advanced disease, by fibroblasts proliferation and thickened alveolar septa, suggesting a fibrotic process. To date, only a few clinical and biological markers of severity have been identified, such as age, male gender, obesity, CRP or LDH levels, and lymphocytes count. Identifying biomarkers in order to help determining short and long-term prognosis would therefore be a major stake .

High circulating levels of fibrocytes are correlated to a poor outcome during acute respiratory distress syndrome (ARDS) and idiopathic pulmonary fibrosis. COVID-19 pneumonia shares common histological and inflammatory features with ARDS. We aimed to determine the involvement of circulating fibrocytes during the course of COVID-19 pneumonia, and to assess if fibrocytes could be a prognosis biomarker.

For this purpose, we will first assess the percentage of circulating fibrocytes, defined as CD45+/CD15-/CD34+/Coll-I+, in COVID-19 patients by flow cytometry. We will characterize circulating fibrocyte subpopulations by assessing the percentage of CXCR4+, CCR7+ and CCR2+ cells. We will look over to demonstrate an association between circulating fibrocyte percentage and prognosis during the acute phase of the disease. Then we will demonstrate that fibrocytes are recruited to the lung during COVID-19 by detecting alveolar fibrocytes by flow cytometry. We will procceed to the quantification of chemokines involved in fibrocytes recruitement (CCL2, CXCL12, CCL21, CCL3) in serum and bronchoalveolar lavage fluid (BALF) of COVID-19 patients by ELISA and Luminex. Patients will be re-evaluated by chest computed tomography after a 3 and 6-months follow-up, and we will try to determine whether initial circulating fibrocyte percentage could help predicting clinical outcome.Finally, we will try to better characterize the mechanisms involved in fibrocytes differentiation. For this, we will quantify Serum Amyloid Protein (SAP), the most potent inhibitor of fibrocytes differentiation during ARDS, in blood and BALF from COVID-19 patients. Then we will determine in vitro the ability of isolated blood monocytes of COVID-19 patients to differentiate into fibrocytes in the absence (basal condition), or in the presence of 10 ng/mL recombinant human (rh)-IL-4, a possible inducerof fibrocyte differentiation, or of 10-6 M dexamethasone, possible inhibitors of fibrocyte differentiation. We will try to determine whether alveolar environment is involved in monocyte-to-fibrocyte differentiation by stimulating isolated blood monocytes by BALF from COVID-19 patients, and we will determine if SAP contained in BALF of COVID-19 patients presents an inhibitory effect on fibrocytes differentiation.

This study will help to better understand severe COVID-19 pathophysiology by identifying fibrocytes as a possible prognostic biomarker, predicting unfavorable short-term outcome. Our preliminary data suggest that a low fibrocyte count could be associated with a slower resolution of lung injury and may be with the development of pulmonary fibrosis. If this result is confirmed, this would be of crucial interest to help understand the mechanisms of lung injury and development.