Lung in chronic autoimmune diseases and hypersensitivity – how to separate these from idiopathic pulmonary fibrosis

Background Lung involvement in autoimmune disease (AID) is not uncommon, and may precede other organ manifestations. The histologic pattern is variable and difficult to interpret. Another problem was recently encountered: chronicity presenting with fibrosing pneumonia. This resulted that patients with chronic AID and usual interstitial pneumonia (UIP) were excluded from antifibrotic therapy. Creating a new category of UIP with autoimmune features (IPAF) for patients with AID enabled treatment for these patients. This however raised the opinion, that a pathological investigation for the underlying disease is not necessary. Results We retrospectively evaluated 66 cases of AID, 31 cases of hypersensitivity pneumonias (HP), and 10 clinically confirmed usual interstitial pneumonia/idiopathic pulmonary fibrosis (UIP/IPF) cases. 12 additional cases could not be assigned into any of these categories. Acute AID presented with lymphocytic interstitial pneumonia, immune complex deposition, cytotoxicity for specific cell compartments, and alveolar hemorrhage. Subacute patterns most often presented with organizing pneumonia, whereas chronic pattern most often presented as UIP. Granulomas were present in some patients. UIP pattern was the most common presentation in chronic AID and HP, whereas NSIP was rather rare. Conclusion The most important, statistically significant feature differentiating chronic AID or HP, from UIP/IPF are lymphocytic infiltrations into myofibroblastic foci. Other features such as granulomas, Langhans giant cells, and protein deposits were significantly associated with AID and HP, but were not encountered in all cases. This study demonstrated that the morphological analysis can uncover the possible etiology in many cases, and a more specific diagnosis can be provided to the clinicians.