Cell-surface phenotyping identifies CD36 and CD97 as novel markers of fibroblast quiescence in lung fibrosis

Abstract

Fibroblasts play an important role in lung homeostasis and disease. In lung fibrosis, fibroblasts adopt a proliferative and migratory phenotype, with increased expression of α-smooth muscle actin (αSMA) and enhanced secretion of extracellular matrix components. Comprehensive profiling of fibroblast heterogeneity is limited, due to a lack of specific cell-surface markers. We have previously profiled the surface proteome of primary human lung fibroblasts. Here, we sought to define and quantify a panel of cluster of differentiation markers in primary human lung fibroblasts and IPF lung tissue, using immunofluorescence and FACS analysis. Fibroblast function was assessed by analysis of replicative senescence. We observed presence of distinct fibroblast phenotypes in vivo, characterized by various combinations of Desmin, αSMA, CD36, or CD97 expression. Most markers demonstrated stable expression over passages in vitro, but significant changes were observed for CD36, CD54, CD82, CD106, and CD140a. Replicative senescence of fibroblasts was observed from passage 10 onward. CD36- and CD97-positive, but αSMA-negative, cells were present in remodeled areas of IPF lungs. Transforming growth factor-β treatment induced αSMA and collagen I expression, but repressed CD36- and CD97 expression. We identified a panel of stable surface markers in human lung fibroblasts, applicable for positive cell isolation directly from lung tissue. TGF-β exposure represses CD36- and CD97 expression, while increasing αSMA expression; we therefore identified complex surface protein changes during fibroblast-myofibroblast activation. Co-existence of quiescence and activated fibroblast subtypes in the IPF lung suggests dynamic remodeling of fibroblast activation upon subtle changes to growth factor exposure in local microenvironmental niches.

CD36 and CD97 in Pancreatic Cancer versus Other Malignancies

“…There are few studies in literature that tackle the surface proteome profiling in different organs, to key cellular events in oncogenesis or metastasis development. Facts with reference to a potential common contribution of CD36 and CD97 were revealed firstly by Heinzelmann et al [95]. They studied expression changes in different CD markers profile in lung fibrosis, with a major emphasis on specific phenotypes during fibroblast-myofibroblast activation by TGFβ, known to express αSMA (α-smooth muscle actin).…”

“…It was also observed that CD36-and CD97-positive population decreased upon TGFβ stimulation and was part of a senescent population, as well, being significantly increased in high passages. The simultaneous presence of quiescent and activated fibroblasts could be mirrored by dynamic changes in surface markers; thus, different fibroblast phenotypes are characterized by various combinations of CD expression [95].…”

“…In light of these findings, the question whether CD36 and CD97 would complement each other remains open. Further studies are needed to gain insight into the concomitant impact of CD36 and CD97 on modulating fibroblasts phenotypes under different conditions [95], thus offering potential innovative therapeutic strategies to inactivate CAF and prevent aberrant tumor-stroma crosstalk in pancreatic cancer.…”

Origin of Myofibroblasts in Lung Fibrosis

“…Efforts have been made to better characterize the lung mesenchymal population and their heterogeneity, although the absence of a single, well-defined marker poses a challenge in quantifying their relative contributions to myofibroblasts [47][48][49]. In a study using signal transduction pathway readouts to define mesenchymal heterogeneity and lineages, Zepp et al identified several lineages of mesenchymal cells with distinct transcriptional and functional properties during homeostasis and following injury [45•].…”

Senescence in the lung: is this getting old?

“…Senescent cells have been reported in both human and animal models of pulmonary diseases (1,4,19,21,24,25,34,42,47,59,61,63). What is less clear is the extent to which phenotypic changes associated with normal chronological aging or in disease states are mechanistically attributable to cellular senescence or effects of senescent cells per se, i.e., how important are senescent cells in pathophysiology?…”