One simple question involves why scavenger receptor genetic deletion produced notable increases in PMN recruitment to the lung surface, especially if AMs were the primary affected cell type. Ozone generates focal lesions that occur predominately in the conducting airways (14
) and bronchoalveolar junctions. Most AMs encounter little or no ozone because inhaled ozone is removed by conducting airway wall reactions during proximal-to-distal longitudinal transport. However, ozone was administered continuously for at least 24 hours, which may have induced more diffuse effects and/or led to alterations in ELF unsaturated lipid homeostasis, thereby affecting precursor lipid availability. Aerosolized ROFA leachate and the instilled lipids were likely distributed throughout the lung, so that even with the shorter exposure and/or sampling times, the effects may have been more anatomically widespread. This is suggested by the data illustrating that the oxidized lipids did not upregulate macrophage inflammatory protein–2 (MIP-2) or increase inflammation in the scavenger receptor–competent animals (10
). It would be interesting to determine whether shorter ozone exposure times or analyses specific to the conducting airways would demonstrate equivalent effects with scavenger receptor genetic deletion.
Although it was reasonable to conclude that inhibiting PMN influx was attributable to scavenger receptor–mediated bioactive lipid removal, this would implicitly require that oxidized lipid removal rates exceeded their generation, effectively reducing bioactive lipid concentrations below critical thresholds. Furthermore, AMs would need to be localized near sites of oxidized lipid generation and/or diffusive or mucociliary transport. If bioactive lipid concentrations were low, then scavenger receptor–mediated clearance would not be a substantial challenge. However, because many inhaled agents initiate oxidant stress, which is further amplified by inflammation, a substantial total pool of lipid products might be achieved. This should also hold true for isoprostanes. Isoprostanes are not generated directly by ozone reaction; rather, they are caused by arachadonic acid autoxidation. The isoprostane measures reported in the current study (10
) represented net accumulation, not total yields or production rates. Even though scavenger receptor activity limited isoprostane accumulation, it remains equivocal whether this resulted from scavenger receptor–mediated clearance or reduced formation rates due to secondary AM-mediated effects on ROS/RNS production. The in vitro studies of AM lipid loading provided very useful initial insights, but further studies — to quantify oxidized lipid uptake versus degradation and the mass balance of generation versus removal — are needed to determine why scavenger receptor genetic deletion reduced intracellular lipid content and augmented inflammation. Furthermore, although inflammation is a hallmark ozone response, other biologic responses are not necessarily coupled and/or regulated via the same biologic pathways. It will be key in future studies to evaluate whether scavenger receptor expression specifically governs airway epithelial injury and influences other AM functions that modulate the observed responses. A critical question remains as to whether scavenger receptors contribute to suppressing inflammation by clearing ozone-generated proinflammatory lipids (15
), because many of the biological processes associated with cell injury and oxidative stress tend to potentiate lipid oxidation.
It will be interesting to see how scavenger receptor receptor activity influences more environmentally relevant exposure regimens and/or other stressors. Although it was beyond the scope of this initial study, it is not entirely clear how the chosen biologic readouts (i.e., levels of BAL neutrophils, protein, isoprostanes, and MIP-2) relate to what constitutes oxidant “susceptibility” versus “resistance.” Ultimately, what constitutes susceptibility versus resistance should be placed in the context of more specific measures of epithelial perturbation and injury. The contributions of cell-cell interactions and potential confounding influences of other cell types also warrant elucidation. In addition, other bioactive lipids (e.g., nitrolipids; refs. 16
) as well as electrophiles, operating through the electrophilic response element, have important and robust influences on signaling pathways. PMNs tend to amplify injury, but are critical for normal epithelial repair (18
). If scavenger receptor activities vary, influence on PMN trafficking could have notable effects on airway repair. Consequently, the long-term effects of diminished scavenger receptor activity on repair and airway remodeling also represent important areas of future investigation. These current and future studies have substantial potential relevance to human biology. MARCO is expressed and functions on human AMs (19
), and polymorphisms in both SR-AI/II and MARCO exist. Hence, further animal studies, linked to translational efforts, hold substantial promise for identifying the mechanisms responsible for known variations in human susceptibility to a wide variety of oxidative challenges.