Usual Interstitial Pneumonia (UIP)
This is the most common variant of the pathology, characterized by severe remodeling of the pulmonary interstitium.
Morphological Interstitial Changes:
- Thickening and reduplication of endothelial and epithelial basement membranes occur.
- A sharp increase in poorly degradable type IV and V collagens is observed.
- The blood-air barrier is infiltrated by inflammatory cells (histiocytes, lymphocytes), as well as fibroblasts and myofibroblasts.
Role of Fibroblasts: Fibroblasts localize not only within the barrier zone but also penetrate between thickened membranes (the "interposition" phenomenon) and extend into capillary and alveolar lumens. This leads to microvascular obliteration, impaired gas exchange, and the development of pulmonary hypertension.
Intra-alveolar and Epithelial Changes: Proteinaceous exudate organizes within the alveolar spaces, forming Masson bodies and leading to carnification. In late stages, normal type I alveolar epithelial cells are massively replaced by type II alveolar cells. The latter show cellular atypia and contain immature osmiophilic bodies, resulting in impaired surfactant production, bronchiolar obliteration, and areas of atelectasis.
The disease is frequently complicated by cor pulmonale and progressive cardiopulmonary failure.
Non-Specific Interstitial Pneumonia (NSIP)
This form is challenging to diagnose because it lacks strictly specific morphological features and can occur in various clinical contexts.
Clinical Presentation: Slowly progressive respiratory failure and worsening exertional dyspnea predominate. Chest imaging reveals bilateral accentuation of lung markings, predominantly in the basal zones.
Pathomorphology:
- The process has a patchy (mosaic) pattern: affected areas alternate with preserved tissue zones.
- Unlike UIP, inflammatory changes (lymphohistiocytic infiltration) predominate here.
- Microvascular changes are prominent, ranging from destructive vasculitis to fibrinoid necrosis.
Early stages feature serous inflammation and edema, which are later replaced by moderate fibrosis. Overall, NSIP is characterized by a relatively favorable clinical course and high patient survival rate.
Desquamative Interstitial Pneumonia (DIP)
The disease onset is gradual, with a mean patient age of about 43 years. The key distinction of DIP is that the primary pathogenetic event is alveolar epithelial injury, while the pulmonary interstitium is involved secondarily.
Morphogenetic Stages:
- Early Stage: Alveolar lumens accumulate activated macrophages containing dark pigment granules. The alveolar epithelium undergoes hyperplasia, injury, and shedding (desquamation). The interstitium shows only moderate fibrosis and infiltration by lymphocytes (occasionally eosinophils).
- Late Stage: Interstitial sclerosis becomes more pronounced. Desquamated cells and macrophages accumulate in the alveoli. Gradually, only type II pneumocytes persist, and squamous cell metaplasia of the epithelium develops in 70% of cases.
In later stages, tissues actively express markers such as pancytokeratins, TGF-β, FGFb, and Ki67.
Cryptogenic Organizing Pneumonia (COP)
Cryptogenic organizing pneumonia is a disorder of undetermined etiology that clinically and radiologically mimics standard infectious pneumonia. In literature, it is often referred to by the synonym BOOP (bronchiolitis obliterans organizing pneumonia).
Manifestations:
- Subacute onset, accompanied by a prominent dry cough.
- Fever is observed in 60% of patients.
- Chest radiography demonstrates persistent focal patchy opacities that do not resolve spontaneously.