Alveolar macrophages drive lung fibroblast function in cocultures of IPF and normal patient samples

Idiopathic pulmonary fibrosis (IPF) is characterized by increased collagen accumulation that is progressive and nonresolving. Although fibrosis progression may be regulated by fibroblasts and alveolar macrophage (AM) interactions, this cellular interplay has not been fully elucidated. To study AM-fibroblast interactions, cells were isolated from IPF and normal human lung tissue and cultured independently or together in direct 2-D coculture, direct 3-D coculture, indirect transwell, and in 3-D hydrogels. AM influence on fibroblast function was assessed by gene expression, cytokine/chemokine secretion, and hydrogel contractility. Normal AMs cultured in direct contact with fibroblasts downregulated extracellular matrix (ECM) gene expression whereas IPF AMs had little to no effect. Fibroblast contractility was assessed by encapsulating cocultures in 3-D collagen hydrogels and monitoring gel diameter over time. Both normal and IPF AMs reduced baseline contractility of normal fibroblasts but had little to no effect on IPF fibroblasts. When stimulated with Toll-like receptor (TLR) agonists, IPF AMs increased production of pro-inflammatory cytokines TNFα and IL-1β, compared with normal AMs. TLR ligand stimulation did not alter fibroblast contraction, but stimulation with exogenous TNFα and TGFβ did alter contraction. To determine if the observed changes required cell-to-cell contact, AM-conditioned media and transwell systems were utilized. Transwell culture showed decreased ECM gene expression changes compared with direct coculture and conditioned media from AMs did not alter fibroblast contraction regardless of disease state. Taken together, these data indicate that normal fibroblasts are more responsive to AM crosstalk, and that AM influence on fibroblast behavior depends on cell proximity.