Covid-19 Interstitial Pneumonia: Histological and Immunohistochemical Features on Cryobiopsies

Abstract

Background: The pathogenetic steps leading to Covid-19 interstitial pneumonia remain to be clarified. Most postmortem studies to date reveal diffuse alveolar damage as the most relevant histologic pattern. Antemortem lung biopsy may however provide more precise data regarding the earlier stages of the disease, providing a basis for novel treatment approaches.

Objectives: To ascertain the morphological and immunohistochemical features of lung samples obtained in patients with moderate Covid-19 pneumonia.

Methods: Transbronchial lung cryobiopsy was carried out in 12 Covid-19 patients within 20 days of symptom onset.

Results: Histopathologic changes included spots of patchy acute lung injury with alveolar type II cell hyperplasia, with no evidence of hyaline membranes. Strong nuclear expression of phosphorylated STAT3 was observed in >50% of AECII. Interalveolar capillaries showed enlarged lumen and were in part arranged in superposed rows. Pulmonary venules were characterized by luminal enlargement, thickened walls, and perivascular CD4+ T-cell infiltration. A strong nuclear expression of phosphorylated STAT3, associated with PD-L1 and IDO expression, was observed in endothelial cells of venules and interstitial capillaries. Alveolar spaces macrophages exhibited a peculiar phenotype (CD68, CD11c, CD14, CD205, CD206, CD123/IL3AR, and PD-L1).

Conclusions: Morphologically distinct features were identified in early stages of Covid-19 pneumonia, with epithelial and endothelial cell abnormalities different from either classical interstitial lung diseases or diffuse alveolar damage. Alveolar type II cell hyperplasia was a prominent event in the majority of cases. Inflammatory cells expressed peculiar phenotypes. No evidence of hyaline membranes and endothelial changes characterized by IDO expression might in part explain the compliance and the characteristic pulmonary vasoplegia observed in less-advanced Covid-19 pneumonia.

Keywords: Acute respiratory distress syndrome; Coronavirus; Covid-19; Cryobiopsy; Indoleamine 2,3-dioxygenase-1; Lung biopsy; SARS-CoV-2; pSTAT-3.

Fig. 1

a, b H&E: parenchymal structure is variably altered by AECII hyperplasia, vascular enlargement, and interstitial thickening. c, d CK7: AECII form variable small nodules, aggregates, and pseudopapillary sprouts. e In situ demonstration of AECII infected by SARS-CoV-2: cytoplasmic (red) signals are evidenced in scattered cells recognized as AECII by morphology and location. f In situ analysis of IL-6 mRNA expression: strong signal is evidenced in scattered AECII. g pSTAT3 immunohistochemistry: strong signal demonstrated in most AECII. h Tubulin-beta-3 immunohistochemistry: strong signal in AECII; interstitial dilated spaces are negative. i Ki67 immunohistochemistry: elevated (>50%) proliferation in AECII. CK7, cytokeratin 7.

Fig. 2

Abnormal morphology and phenotype in enlarged vascular endothelial cells: H&E (a); CK7 (b, c). Lymphocyte infiltration of vascular walls: CD3 (d), CD4 (e), and CD8 (f): perivascular lymphocytes mostly exhibit a CD3+, CD4+, CD8-negative immunophenotype. Ph-STAT3 (g): strong nuclear expression in endothelial cells. PD-L1 (h, i), and IDO (j): strong expression in capillaries and venules. CD61 (k): occasional positive megakaryocytes within interstitial capillaries. Immunohistochemical profile of aggregates of alveolar mononuclear cells: CK7 negative (l), CD11c+ (m), CD4+ (n), CD14+ (o), CD123+ (p), CD206+ (q), CD303-negative (r), and PD-L1+ (s). CK7, cytokeratin 7.