Cellular and immune response in fatal COVID-19 pneumonia

Abstract

Introduction: the severity of COVID-19, causing fatal pneumonia, acute respiratory distress syndrome (ARDS), and thrombotic complications, is linked to intense inflammation. Elevated CD4+ and CD8+ cells in the lungs indicate harmful inflammation in severe cases. This study investigates immune responses in lung tissues of deceased patients across different stages of COVID-19 pneumonia.

Methods: lung tissues from 160 fatal COVID-19 cases, diagnosed via Real-Time RT-PCR, were histologically analyzed to identify pneumonia stages. Inflammatory cell counts were assessed immunohistochemically. Non-parametric tests analyzed categorical variables, while regression analysis evaluated relationships between continuous variables.

Results: the average patient age was 68.1 years (± 12.6). Microscopic analysis identified four pneumonia stages. CD4+, CD68 (macrophages), and IgG4 levels peaked by day 14, with notable elevation within seven days of symptom onset. CD4+ levels were significantly lower in DAD pneumonia (49.4% ± 15.7%) compared to ARDS (66.4% ± 19.3%) and thrombosis (70.2% ± 28.9%) (p < 0.05). Male patients had higher CD4+ values (68.5% ± 21.1%) than females (56.9% ± 22.4%) (p < 0.05). B cells (CD20) and NK cells were depleted across all stages. IgG4 expression reached 80-90% in acute phases but was nearly absent during organization and fibrosis stages.

Conclusion: a sharp decline in CD4+ and CD8+ during acute pneumonia and sepsis reflects immune exhaustion, while their elevation in ARDS and thrombosis likely triggers cytokine storms, causing severe lung damage. Elevated IgG4 levels in acute lung tissue correlate with fatal outcomes in severe COVID-19.

Keywords: CD4+; CD8+; COVID-19; IgG4; pneumonia.

Figure 1

stages in the development of COVID-19 pneumonia: A) ARDS, giant multinucleated cells and diffuse damage to alveolar septa, HEx20; B) DAD, formation of syncytial structures, HEx20; C) organization with fresh granulation tissue, HEx20; D) collagen proliferation and fibrosis, HEx20

Figure 2

immunohistochemical staining of CD3, CD4+, CD8+ cells in lung tissue: A, B) CD3 expression in 70-80% in the acute phase of the disease, x10; C, D) CD3 drops to 20-30% in the organizing phase and fibrosis, x20; E, C) CD4+ expression in 50-60% in the acute phase, x20; G, H) CD4+ lack of expression in the organizing phase and fibrosis, x20; I, J) CD8+ with 10-20% expression in the acute phase, x20; K, L) CD8+ reduced to single cells in the phase of organization and fibrosis, x40

Figure 3

immunohistochemical staining of CD68, CD20, IgG4 cells in lung tissue: A, B, C) CD68 expression in macrophages represented up to 80% in the phases of respiratory distress, acute pneumonia and early organization, x10, x20; D) in the fibrosis phase, macrophages are depleted x20; E, G) CD20, B-lymphocytes are almost completely exhausted in the ARDS stage, x10 F) DAD: in the stage of desquamative pneumonia, we observed a scant amount of B lymphocytes around small vessels with microthrombi, x20; G) CD20 complete absent in organization phase, x10; H) CD20 weakly expressed in the fibrotic stage, x20; J, G) IgG4 intense expression up to 80-90% in lung tissue in the acute phase, x40, x20; K, L) IgG4 almost complete depletion in the phases of organization and fibrosis, x20

Figure 4

dependence on CD4+ and CD8+ by complications; high mean CD4+ and CD8+ expression levels, except for the group with sepsis and DAD

Figure 5

significantly increased mean levels of CD4+ and CD8+ by the presence of diabetes mellitus