Macrophages are key innate immune cells which exist on a spectrum between pro-inflammatory (M1) and pro-resolutory (M2) states while retaining the ability to reprogram following exposure to new stimuli. Particulate matter (PM) exposure significantly alters macrophage function, leading to increasing respiratory infection morbidity and mortality. However, the influence of macrophage polarization on PM responses remains poorly understood. We hypothesized that human macrophages would demonstrate polarization state- and seasonality-specific responses to airborne PM collected from Xinxiang, China. CD14+CD16- monocytes were differentiated into macrophages and polarized using established methods with or without PM co-exposure followed by RNA sequencing (RNAseq). Macrophage expression profiles were primarily determined by inflammatory (M1) versus naive (M0) and alternative activation (M2), regardless of PM exposure. However, differential expression analysis using polarization state-specific reference groups uncovered distinct gene and pathway expression patterns. Directly polarized M0->M1 macrophages exhibited the fewest unique differentially expressed genes (DEGs) but displayed similar pathway level activity to reprogrammed M2->M1 cells. M2 macrophages showed the highest number of unique DEGs suggesting increased chemotactic pathway activity. Conversely, M0 cells exhibited greater expression of major inflammatory cytokines and chemokines. While polarization state was the primary driver of gene and pathway responses, expression levels varied depending on PM collection dates and correlated strongly with individual PM components in M0 and M2 cells. These findings highlight the need to consider both particle seasonality and macrophage polarization state when studying PM's impact on macrophages as these factors may contribute to the negative health outcomes associated with PM exposure during respiratory infections.
Smyth et al. (Thu,) studied this question.