Lung-Restricted Macrophage Activation in the Pearl Mouse Model of Hermansky-Pudlak Syndrome

Abstract

Pulmonary inflammation, abnormalities in alveolar type II cell and macrophage morphology, and pulmonary fibrosis are features of Hermansky-Pudlak Syndrome (HPS). We used the naturally occurring “pearl” HPS2 mouse model to investigate the mechanisms of lung inflammation observed in HPS. Although baseline bronchoalveolar lavage (BAL) cell counts and differentials were similar in pearl and strain-matched wild-type (WT) mice, elevated levels of proinflammatory (MIP1γ) and counterregulatory (IL-12p40, soluble TNFr1/2) factors, but not TNF-α, were detected in BAL from pearl mice. After intranasal LPS challenge, BAL levels of TNF-α, MIP1α, KC, and MCP-1 were 2- to 3-fold greater in pearl than WT mice. At baseline, cultured pearl alveolar macrophages (AMs) had markedly increased production of inflammatory cytokines. Furthermore, pearl AMs had exaggerated TNF-α responses to TLR4, TLR2, and TLR3 ligands, as well as increased IFN-γ/LPS-induced NO production. After 24 h in culture, pearl AM LPS responses reverted to WT levels, and pearl AMs were appropriately refractory to continuous LPS exposure. In contrast, cultured pearl peritoneal macrophages and peripheral blood monocytes did not produce TNF-α at baseline and had LPS responses which were no different from WT controls. Exposure of WT AMs to heat- and protease-labile components of pearl BAL, but not WT BAL, resulted in robust TNF-α secretion. Similar abnormalities were identified in AMs and BAL from another HPS model, pale ear HPS1 mice. We conclude that the lungs of HPS mice exhibit hyperresponsiveness to LPS and constitutive and organ-specific macrophage activation.

Pulmonary Fibrosis in Hermansky-Pudlak Syndrome

“…Interestingly, murine models of HPS-1 (Pale ear) and HPS-2 (Pearl) show activation of alveolar macrophages in the lung, but not in the blood or peritoneum (47,48). Pale ear HPS-1 mice do not develop spontaneous fibrosis, but have higher baseline collagen deposition and show increased inflammation and collagen expression in response to silica challenge (47,49,50). In response to bleomycin, HPS-1 and HPS-2 mice developed fibrosis significantly earlier and to a greater extent than wild-type mice (48).…”

Early Alveolar Epithelial Dysfunction Promotes Lung Inflammation in a Mouse Model of Hermansky-Pudlak Syndrome

“…There has been increasing evidence that epithelial cell injury is an important factor in the development of HPS lung disease. Both Mahavadi and colleagues (8) and Young and colleagues (6) have reported evidence of epithelial cell injury, and prior work by Young and colleagues (5) suggests that the alveolar inflammation is a reactive and not a primary feature of HPS. Our data strongly support the alveolar type 2 cell as the initiator of lung disease in HPS.…”

“…The pale ear and pearl parent strains, although demonstrating lamellar body enlargement and an inflammatory cell infiltrate, did not develop fibrosis unless challenged with bleomycin (6). Single-mutation animals exhibited increased susceptibility to LPS challenge with more exuberant cytokine responses (5). Although macrophages from adult pale ear or pearl animals exhibited some evidence of activation, specifically increased tumor necrosis factor-a and IL-12p40, it was attributed to an abnormal alveolar milieu because these macrophages rapidly became quiescent when cultured in vitro, and cell-free BAL from these animals was able to activate otherwise normal alveolar macrophages.…”

“…Mouse models of HPS, including the mouse homologs of HPS1 (pale ear) and HPS2 (pearl), similarly display foamy degeneration of alveolar type 2 cells of adult animals. Adult pale ear and pearl animals exhibit a macrophage-predominant alveolar inflammation (4,5) and develop emphysema with aging but do not develop pulmonary fibrosis unless exposed to fibrogenic stimuli such as bleomycin, albeit at much lower doses than for littermate controls (6). Intercrossing HPS1/pale ear and HPS2/pearl strains results in a more severe phenotype (4) that more closely resembles the pulmonary manifestations of human HPS.…”

Epithelial-macrophage interactions determine pulmonary fibrosis susceptibility in Hermansky-Pudlak syndrome

“…To date, 10 genetic loci have been associated with HPS in humans, and pulmonary fibrosis has been reported in several of these subtypes, including HPS1 and HPS2 (1)(2)(3)(4)(5). Naturally occurring mutations in HPS mice reliably model important features of the human disease, including susceptibility to profibrotic stimuli, which is restricted to the same HPS subtypes that are associated with pulmonary fibrosis in humans (6)(7)(8). We have demonstrated that intrinsic defects in the alveolar epithelium due to HPS mutations underlie the fibrotic susceptibility in HPS mice.…”