To determine whether expression of class II major histocompatibility complex antigens on alveolar epithelium is relevant to the pathogenesis of idiopathic pulmonary fibrosis (IPF) lung biopsy specimens were investigated from nine patients with IPF with or without connective tissue disease, four patients with sarcoidosis, eight patients with lung disease of presumably infectious origin, and five controls. The alveolar epithelium stained strongly with anti-Ia (HLA-DR) or Leu 10 (HLA-DS) monoclonal antibodies, in eight of nine biopsy specimens from patients with IPF, in three of four biopsy specimens from patients with sarcoidosis, in all six biopsy specimens from patients with presumably viral, mycobacterial, or pneumocystic lung disease, but not in control lung tissue, nor in two biopsy specimens from patients with bacterial pneumonia. Mononuclear cell infiltrates consisted of T4 positive (helper/inducer) lymphocytes, predominantly present in a nodular arrangement in the interstitium, and T8 positive (cytotoxic/suppressor) cells, distributed equally in the interstitium and subepithelially or intraepithelially. T8 cells outnumbered T4 cells in six of nine biopsy specimens from patients with IPF, but in none of the biopsy specimens from patients with sarcoidosis or interstitial lung disease of infectious origin. Although the expression of class II antigens on the alveolar epithelium which is infiltrated by T8 cells in IPF is consistent with local presentation of autoantigens and an ensuing local immune response, class II expression is also present in interstitial lung disease of sarcoidosis and microbial infections: its role in the pathogenesis of IPF must therefore remain speculative.
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