PURPOSE: Over-abundant fibroblast proliferation is one of the hallmarks of IPF. These fibroblasts are heterogeneous in their distribution and result in the ongoing deposition of excessive extra-cellular matrix leading to progressive pulmonary dysfunction. We sought to analyze the expression and distribution of proliferation marker, Proliferating Cell Nuclear Antigen (PCNA), and activated fibroblast marker, alpha-smooth muscle actin (alpha-SMA), by immunohistochemistry (IHC) within IPF lung tissue.
METHODS: IPF (LTRC) and normal lung tissue (Abnova) were analyzed by double-IHC using PCNA and alpha-SMA (Abcam) antibodies; detected by FITC and Texas-red secondary antibodies, respectively. Slides were counter stained with DAPI and H&E. Images were collected and processed by confocal microscopy and Adobe Photoshop 4.0.
RESULTS: IHC analysis revealed higher levels of PCNA expression in IPF compared to normal lung. Very low levels of PCNA were observed in the fibroblast foci and in areas of advanced fibrosis. However, increased PCNA staining was apparent in the parenchymal areas surrounding the fibroblast foci. A periphery of PCNA expressing cells was apparent around dense fibrotic regions of high alpha-SMA expression. A gradient of PCNA only expressing cells, to a few cells expressing both PCNA and alpha-SMA, to cells expressing only alpha-SMA; was apparent in the fibrotic core regions within the IPF lung.
CONCLUSION: There are higher rates of PCNA expression in the parenchyma, and regions surrounding fibrotic areas and foci in IPF lungs. In comparison, there are low levels of proliferation in fibroblast foci and advanced areas of fibrosis. We propose that fibrotic regions contain terminally differentiated quiescent cells within their fibrotic core, displaying high alpha-SMA expression. Therefore, it appears that the active fibroblast proliferation lies within the quasi-normal parenchyma, surrounding but outside the areas of fibroblast foci and advanced regions of fibrosis. Fibroblast migration and/or EMT may account for the presence of non-proliferating fibroblasts within the foci.
CLINICAL IMPLICATIONS: There exists a heterogeneous pattern of cell proliferation in IPF lung tissue.
DISCLOSURE: Mantej Chhina, No Financial Disclosure Information; No Product/Research Disclosure Information