Lessons from Mycobacterium avium complex-associated pneumonitis: a case report
© Zota et al; licensee BioMed Central Ltd. 2008
Received: 21 August 2007
Accepted: 13 May 2008
Published: 13 May 2008
Mycobacterium avium complex (MAC) is an increasingly recognized cause of pulmonary disease in immunocompetent individuals. An acute form of MAC lung disease, MAC-associated pneumonitis, has generally been associated with the use of hot tubs. There is controversy in the literature about whether MAC-associated pneumonitis is a classic hypersensitivity pneumonitis or is a direct manifestation of mycobacterial infection.
We report the second case in the literature of MAC-associated pneumonitis not related to the use of hot tubs. The source of MAC in a 52-year-old immunocompetent patient was an intrapulmonary cyst containing numerous acid-fast bacilli. The patient developed disseminated miliary nodules throughout both lung fields. Histological examination of resected lung tissue revealed well-formed, acid-fast negative granulomas composed predominantly of CD4+ T-cells and CD68+ histiocytes. The granulomas were strongly positive for tumor necrosis factor-α, a pro-inflammatory cytokine.
The attempt to classify MAC-associated pneumonitis as either a classic hypersensitivity pneumonitis or a direct manifestation of mycobacterial infection is not particularly useful. Our case demonstrates that MAC-associated pneumonitis is characterized by a vigorous T-helper 1-like, pro-inflammatory, immune response to pulmonary mycobacterial infection. The immunopathology provides a rationale for clinical studies of anti-MAC therapy with the addition of anti-inflammatory agents (for example, corticosteroids) to hasten the resolution of infection and symptoms.
Mycobacterium avium complex (MAC)-associated pneumonitis has typically been reported in immunocompetent persons exposed to aerosolized MAC from indoor or outdoor hot tubs (termed "hot tub lung") [1–3]. There is controversy in the literature about whether the illness is a classic hypersensitivity pneumonitis or is a direct manifestation of mycobacterial infection. Histopathologic and bronchoscopic differences have been described between classic hypersensitivity pneumonitis and MAC-associated pneumonitis [2, 4].
We present here the second reported case of MAC-associated pneumonitis that was not associated with the use of hot tubs . The pathological features of this case of MAC-associated hypersensitivity pneumonitis stress the differences between 'classic' hypersensitivity pneumonitis and MAC-associated pneumonitis. These differences have important implications for the diagnosis as well as for the treatment of MAC-associated pneumonitis. Our report also highlights the clinical importance of alternative sources of MAC that can lead to a hypersensitivity pneumonitis.
His past medical history was unremarkable. He did not take any immunosuppressive medications and had no exposure to hot tubs. Physical examination revealed mild hypoxemia and coarse rales over the left lower lung field. White blood cell count was 12,400/mm3. HIV serology was negative, as was a tuberculin skin test. Blood and urine cultures were negative. A pre-operative HRCT showed a new diffuse pattern of miliary nodules throughout both lung fields (Figure 1b). No ground glass opacities were seen. Because of the central location of the lesion a left lower lobectomy was performed. Examination of the resected lobe revealed a cyst filled with thick yellow fluid that was AFB positive on a smear. Cultures from the cyst fluid eventually grew MAC; bacterial and fungal cultures were negative.
The patient was initially placed on a four-drug anti-tuberculosis regimen. After identification of MAC in the culture, his antibiotic regimen was changed to azithromycin, ethambutol, and rifampin. His respiratory symptoms resolved over the ensuing three months, with disappearance of the miliary nodules on chest HRCT (Figure 1c).
MAC infection activated a vigorous immune response in our patient that, together with the antimycobacterial regimen, likely contributed to the resolution of disease. The antimycobacterial immune response was characterized by well-formed granulomas with a low organism load, increased CD4/CD8 ratio in the granulomas, and marked TNF-α production. This T-helper 1-like immune response is considered pivotal for the eradication of mycobacterial pathogens. It is also somewhat different than what is reported in classic hypersensitivity pneumonitis. The histologic appearance of classic hypersensitivity pneumonitis is characterized by poorly formed, non-necrotizing granulomas, and patchy interstitial pneumonitis. It is described as a combination of IgG-mediated and CD8+ > CD4+ T-cell mediated hypersensitivity reactions . One possible exception is farmer's lung, which is often characterized by increased CD4/CD8 ratios .
In our patient, the TNF-α+ CD4+ T-cell dominant granulomatous immune response produced clinical symptoms and diffuse miliary nodules. The MAC organism load was high in the cyst fluid, but low in the diffuse parenchymal granulomas. This may suggest that heavy or continuous exposure to MAC (for example, in hot tubs or, in this case, from a heavily colonized cyst) promotes the development of antimycobacterial T-helper 1-like immune responses in the lung. This case is also a reminder that pulmonary structural abnormalities may be a reservoir for triggering MAC-associated pneumonitis.
The attempt to classify MAC-associated pneumonitis as either a classic hypersensitivity pneumonitis or a direct manifestation of mycobacterial infection is not particularly useful. Our case demonstrates that MAC-associated pneumonitis is caused by a vigorous T-helper 1-like immune response to pulmonary MAC infection. The T-helper 1-like antimycobacterial immune response is consistent with the observation that MAC-associated pneumonitis is generally a self-limited disease . Corticosteroids were not used in this case. However, the observed immunopathology provides a rationale for clinical studies of the use of anti-MAC therapy and anti-inflammatory agents (for example, corticosteroids) to hasten the resolution of infection and symptoms.
Written informed consent was obtained from the patient for publication of this case report and any accompanying images. A copy of the written consent is available for review by the Editor-in-Chief of this journal.
high-resolution computed tomography
Mycobacterium avium complex
tumor necrosis factor-α.
The authors would like to thank Karen Dresser for performing the double immunostaining. This work was performed at the University of Massachusetts Medical School, Worcester, MA, USA and supported in part by the National Institutes of Health (NIH R21 AI060791). The contents of this publication are solely the responsibility of the authors and do not necessarily represent the official views of the National Institutes of Health.
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