Histologic features suggesting connective tissue disease in idiopathic pulmonary fibrosis

Immune mechanisms in fibrotic interstitial lung disease

Fibrotic interstitial lung diseases (fILDs) have poor survival rates and lack effective therapies. Despite evidence for immune mechanisms in lung fibrosis, immunotherapies have been unsuccessful for major types of fILD. Here, we review immunological mechanisms in lung fibrosis that have the potential to impact clinical practice. We first examine innate immunity, which is broadly involved across fILD subtypes. We illustrate how innate immunity in fILD involves a complex interplay of multiple cell subpopulations and molecular pathways. We then review the growing evidence for adaptive immunity in lung fibrosis to provoke a re-examination of its role in clinical fILD. We close with future directions to address key knowledge gaps in fILD pathobiology: (1) longitudinal studies emphasizing early-stage clinical disease, (2) immune mechanisms of acute exacerbations, and (3) next-generation immunophenotyping integrating spatial, genetic, and single-cell approaches. Advances in these areas are essential for the future of precision medicine and immunotherapy in fILD.

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

Lung involvement in autoimmune diseases (AID) is uncommon, but may precede other organ manifestations. A diagnostic problem is chronicity presenting with lung fibrosis. A new category of interstitial pneumonia with autoimmune features for patients with clinical symptoms of AID and presenting with usual interstitial pneumonia (UIP) enables antifibrotic treatment for these patients. Hypersensitivity pneumonia (HP) and other forms of lung fibrosis were not included into this category. As these diseases based on adverse immune reactions often present with unspecific clinical symptoms, a specified pathological diagnosis will assist the clinical evaluation. We aimed to establish etiology-relevant differences of patterns associated with AID or HP combined with lung fibrosis. We retrospectively evaluated 51 cases of AID, and 29 cases of HP with lung fibrosis, and compared these to 24 cases of idiopathic pulmonary fibrosis (UIP/IPF). Subacute AID and HP most often presented with organizing pneumonia (OP), whereas chronicity was associated with UIP. Unspecified fibrosis was seen in a few cases, whereas NSIP pattern was rare. In 9 cases, the underlying etiology could not be defined. Statistically significant features differentiating chronic AID or HP from UIP/IPF are lymphocytic infiltrations into myofibroblastic/fibroblastic foci. Other features significantly associated with AID and HP were granulomas, isolated Langhans giant cells, and protein deposits, but seen in only a minority of cases. A combination of UIP with one of these features enabled a specific etiology-based diagnosis. Besides the antifibrotic drug regimen, additional therapies might be considered.

Interstitial Lung Disease in Connective Tissue Disease: A Common Lesion With Heterogeneous Mechanisms and Treatment Considerations

Connective tissue disease (CTD) related interstitial lung disease (CTD-ILD) is one of the leading causes of morbidity and mortality of CTD. Clinically, CTD-ILD is highly heterogenous and involves rheumatic immunity and multiple manifestations of respiratory complications affecting the airways, vessels, lung parenchyma, pleura, and respiratory muscles. The major pathological features of CTD are chronic inflammation of blood vessels and connective tissues, which can affect any organ leading to multi-system damage. The human lung is particularly vulnerable to such damage because anatomically it is abundant with collagen and blood vessels. The complex etiology of CTD-ILD includes genetic risks, epigenetic changes, and dysregulated immunity, which interact leading to disease under various ill-defined environmental triggers. CTD-ILD exhibits a broad spectra of clinical manifestations: from asymptomatic to severe dyspnea; from single-organ respiratory system involvement to multi-organ involvement. The disease course is also featured by remissions and relapses. It can range from stability or slow progression over several years to rapid deterioration. It can also present clinically as highly progressive from the initial onset of disease. Currently, the diagnosis of CTD-ILD is primarily based on distinct pathology subtype(s), imaging, as well as related CTD and autoantibodies profiles. Meticulous comprehensive clinical and laboratory assessment to improve the diagnostic process and management strategies are much needed. In this review, we focus on examining the pathogenesis of CTD-ILD with respect to genetics, environmental factors, and immunological factors. We also discuss the current state of knowledge and elaborate on the clinical characteristics of CTD-ILD, distinct pathohistological subtypes, imaging features, and related autoantibodies. Furthermore, we comment on the identification of high-risk patients and address how to stratify patients for precision medicine management approaches.