Immunohistochemical identification and characterization of smooth muscle-like cells in idiopathic pulmonary fibrosis.

The interstitium of the fibrotic lung possesses a contractile capability that is unusual for nonmuscle tissue. An abundance of actin filament-laden cells have been demonstrated in animal and human studies of fibrotic lung tissue and have frequently been termed myofibroblasts. The origin and significance of these cells remain unclear. Proliferation of cells with the capability to contract and thereby generate force within the parenchyma is potentially a significant contribution to the increased lung elastic recoil of advanced pulmonary fibrosis. In the present study, we immunohistochemically examined these intermediate phenotypes of filament-laden cells with a focus on those expressing smooth muscle-associated isoforms of actin. The monoclonal antibody HHF35 was used to study the presence and distribution of cells expressing alpha and gamma smooth muscle actin in idiopathic pulmonary fibrosis (IPF). Adjacent sections of tissue from open lung biopsies of eight patients with IPF were stained with a pentachrome stain and with multiple antibodies (HHF35, polyclonal anti-actin, anti-vimentin, anti-keratin, anti-procollagen I, and anti-von Willebrand Factor VIII) to identify specific cell types. In addition, anti-laminin antibody was used to stain basement membrane. Many tightly packed, HHF35-reactive cells were found to be architecturally dissociated from airways and blood vessels in all eight patients with IPF. Some HHF35-reactive bundles were composed of loosely associated cells, and single smooth muscle cell types (SMC) were distributed in the fibrotic interstitium. Interestingly, some of the SMC were distinctly negative for anti-laminin and stained atypically with pentachrome. Moreover, some single SMC were found to be anti-procollagen type I reactive with double staining technique.(ABSTRACT TRUNCATED AT 250 WORDS)