Clinical features of hypersensitivity pneumonitis

Innate immunity and the pathogenicity of inhaled microbial particles

Non-infectious inhaled microbial particles can cause illness by triggering an inappropriate immunological response. From the pathogenic point of view these illnesses can be seen to be related to on one hand autoimmune diseases and on the other infectious diseases.

In this review three such illnesses are discussed in some detail. Hypersensitivity pneumonitis (HP) is the best known of these illnesses and it has also been widely studied in animal models and clinically. In contrast to HP Pulmonary mycotoxicosis (PM) is not considered to involve immunological memory, it is an acute self-limiting condition is caused by an immediate "toxic" effect. Damp building related illness (DBRI) is a controversial and from a diagnostic point poorly defined entity that is however causing, or attributed to cause, much more morbidity than the two other diseases.

In the recent decade there has been a shift in the focus of immunology from the lymphocyte centered, adaptive immunity towards innate immunity. The archetypal cell in innate immunity is the macrophage although many other cell types participate. Innate immunity relies on a limited number of germline coded receptors for the recognition of pathogens and signs of cellular damage. The focus on innate immunity has opened new paths for the understanding of many chronic inflammatory diseases. The purpose of this review is to discuss the impact of some recent studies, that include aspects concerning innate immunity, on our understanding of the pathogenesis of inflammatory diseases associated with exposure to inhaled microbial matter.

Hypersensitivity pneumonitis: current concepts and future questions

Hypersensitivity pneumonitis (extrinsic allergic alveolitis) caused by inhaled allergens can progress to disabling or even fatal end-stage lung disease. The only truly effective treatment is early recognition and control of exposure. Although patients produce antibody exuberantly, the immunopathogenesis involves cellular immunity--notably, CD8(+) cytotoxic lymphocytes, multinucleate giant cell granulomas, and ultimately interstitial fibrosis. Many causative agents have been recognized in occupational dusts or mists, but most current new cases arise from residential exposure to pet birds (pigeons and parakeets), contaminated humidifiers, and indoor molds. The symptoms and physical findings are nonspecific. Serum IgG containing high titers of specific antibody to the offending antigen is elevated. Pulmonary function tests show restrictive and diffusion defects with hypoxemia, especially after exercise. Occasionally, small airways disease causes obstruction. Radio-graphic changes vary according to the stage of the disease and are best evaluated by means of high-resolution computed tomography. In typical cases, the history of a known exposure and the presence of a characteristic interstitial lung disease with serologic confirmation of IgG antibody to the offending antigen suffice for diagnosis. In more obscure cases, observation of changes after a natural environmental exposure, bronchoalveolar lavage, and lung biopsy might be indicated. Among the many questions that remain are the following: What is the prevalence of hypersensitivity pneumonitis and how often is it the cause of chronic interstitial fibrosis? What is the long-term prognosis? Why do most individuals exposed to these antigens develop a vigorous antibody response whereas only a few develop the disease? How does exposure to endotoxin and cigarette smoking affect the disease? To answer these questions, standardized and validated clinical laboratory immunochemical tests are needed, in addition to a systematic approach to diagnosis, classification of disease severity, risk assessment, and management. This review is limited to the disease caused by airborne allergens and focuses on its immunopathogenesis, eliciting agents, clinical manifestations, diagnosis, management, and prognosis.

Summer-type hypersensitivity pneumonitis in a child

Summer-type hypersensitivity pneumonitis (HP) is a unique disease in Japan. The clinical features of this disease are as follows: 1) cough, fever and dyspnea as a clinical triad, 2) diffuse reticulonodular opacities on the chest X-ray film, 3) restrictive impairment and decrease in DLco, 4) hypoxia, 5) initiation in summer, 6) worsening of the condition when the patient returns home, 7) granuloma formation and alveolitis in the lung biopsy specimen, 8) familial clustering. The etiologic agent of this disease is debatable. In 1984 Ando et al reported that the etiologic agent was T. cutaneum. Now many people are pursuing the argument to its logical conclusion. We report a case of summer-type HP. It is uncommon in children, especially in a child in whose serum antibody to T. cutaneum can be demonstrated.

Hypersensitivity pneumonitis

Hypersensitivity pneumonitis or extrinsic allergic alveolitis is an immunologically mediated lung disease caused by repeated inhalations of organic antigens. The basic histologic lesion consists of a diffuse mononuclear cell infiltration of alveolar wall, alveoli, terminal bronchioles, and neighboring interstitium. The inflammation is often followed by granulomas, which then may progress to fibrosis. Although there is no single radiologic, physiologic, or immunologic test specific for hypersensitivity pneumonitis, the diagnosis can often be suspected on the basis of a compatible temporal relationship of pulmonary symptoms to the history of environmental or occupational exposure. Once the diagnosis is suspected, the presence of serum precipitating antibodies (IgG), suppressor cytotoxic lymphocytosis in bronchoalveolar lavage fluid, and granulomatous alveolitis in lung biopsy specimens are extremely helpful in confirming the diagnosis. In patients where the diagnosis is confirmed, avoidance of the causative antigen is the best therapy, although corticosteroids are used to suppress inflammation. Once the fibrosis is set in, the patient may gradually slide into respiratory failure, cor pulmonale, and death.

Hypersensitivity pneumonitis

The lung is constantly exposed to a wide variety of environmental insults. In its defense against these environmental challenges, however, the lung responds through a limited number of pathophysiologic mechanisms. This is well illustrated by a group of diseases which are collectively referred to as hypersensitivity pneumonitis. This syndrome includes a very large number of different diseases. However, in the United States, only farmer's lung, bird-breeder's lung, and ventilation hypersensitivity pneumonitis occur with any significant frequency. Each of these is characterized by flu-like symptoms, in conjunction with a pneumonitis consisting of lymphocytic granulomatous infiltration of the alveoli and terminal bronchioles. This disease is caused by the inhalation of antigenic material which usually originates from the dusts of organic material. A host of different dusts and antigens have been described in conjunction with hypersensitivity pneumonitis but each leads to the same characteristic clinical syndrome. Thus, each of the diseases shares similar clinical features but differs primarily with respect to the nature of the exposure and causative antigens. The clinical features, pathogenesis, course, prognosis, and treatment of these related diseases are reviewed.

Organic dust toxic syndrome: an acute febrile reaction to organic dust exposure distinct from hypersensitivity pneumonitis

Organic dust toxic syndrome is a term recently coined to describe a noninfectious, febrile illness associated with chills, malaise, myalgia, a dry cough, dyspnea, headache and nausea which occurs after heavy organic dust exposure. Organic dust toxic syndrome shares many clinical features with acute farmer's lung and other forms of hypersensitivity pneumonitis, including the presence of increased numbers of neutrophils in bronchoalveolar lavage. However, organic dust toxic syndrome differs from acute hypersensitivity pneumonitis in several respects: the chest X-ray does not show infiltrates, severe hypoxemia does not occur, prior sensitization to antigens in the organic dust is not required and there are no known sequelae of physiological significance, such as the recurrent attacks and the pulmonary fibrosis which may be seen with chronic hypersensitivity pneumonitis. Organic dust toxic syndrome is thought to be much more common than farmer's lung. It is important for clinical and investigational purposes that organic dust toxic syndrome be distinguished from acute farmer's lung.

Role of mast cells in the pathogenesis of hypersensitivity pneumonitis

To examine the possibility that mast cells have a central role in the pathogenesis of hypersensitivity pneumonitis, 20 patients with this disease were studied with the aim of seeking evidence for mast cell degranulation. The number of mast cells recovered by bronchoalveolar lavage from patients with hypersensitivity pneumonitis was more than 1,000 times greater than those recovered from normal individuals. Furthermore, discontinuation of antigen exposure resulted in an increase in the number of mast cells observed, consistent with the possibility that antigen exposure had induced mast cell degranulation. Cessation of antigen exposure also resulted in a rapid decrease in the number of neutrophils and eosinophils recovered by lavage, followed by an increase in the number of T8+ T lymphocytes present. In each case the time course of the changes was consistent with the possibility that mast cell degranulation had been important in regulating the number of the immune and inflammatory cells present in the lung. Histamine was present in lavage fluid supernatant from patients with hypersensitivity pneumonitis. The amount of histamine present was, however, closely correlated with the number of mast cells present and not with the interval since last antigen exposure. Delay in separating cells from lavage fluid supernatant resulted in an increase in histamine content. These results suggest that the free histamine in lavage fluid resulted from the degranulation of mast cells induced by the lavage procedure as histamine released in vivo has a short half life. We suggest that hypersensitivity pneumonitis results from a "late phase reaction" initiated by antigen induced mast cell degranulation.

Interstitial pulmonary fibrosis

The study of animal models of IPF has demonstrated that there is a stereotyped response of the respiratory airspace walls to a wide variety of injuries. Inflammatory and immune effector cells play a major and complex role in the fibrosing process. They may contribute to the injury of the lung beyond the original insult. These cells secrete substances that play an important role in determining cell traffic in the lungs and in controlling the connective tissue-producing cells. Products derived from the inflammatory response may interfere with protection of normal lung matrix, although injury to lung matrix itself does not lead to fibrosis. Injury to endothelial cells and especially type I epithelial cells appears to play a major role in the fibrogenic response. Further understanding of the factors that injure these cells, the development of methods of protecting them from injury, and a clear understanding of their role in the fibrogenic process appear to be key to developing better methods of preventing and treating interstitial pulmonary fibrosis.