Abstract
Immunoproteasomes (IP) are the main protein degradation system in immune cells. They have been shown to shape adaptive immune responses by generating epitopes for MHC I antigen presentation as well as by modulating inflammatory pathways and cytokine secretion. We have previously demonstrated the impact of the IP subunit LMP7 on alveolar macrophage (AM) M2 polarization in vitro indicating that IPs also participate in shaping innate immune responses. Here, we analyzed the role of IP subunit LMP2 in AM polarization.
We isolated primary AMs from LMP2-/- mice and treated them with LPS/IFNγ or with IL-4 for classical (M1) or alternative (M2) polarization, respectively. We did not observe any changes in M1 marker gene expression. This was confirmed using LMP2-/- NfκB reporter mice which did not reveal differential activation of NfκB-driven GFP expression. In contrast, LMP2 deficiency clearly augmented alternative AM activation: M2 marker genes were significantly increased in LMP2-/- AMs, which we were able to attribute to increased M2 signaling (STAT6 and AKT) as well as an increase in the key transcription factor IRF4 in response to IL-4 treatment. Microarray analyses confirmed altered IL-4 signatures upon M2 polarization. In contrast to wildtype and also LMP7-/-, LMP2-/- AMs displayed increased levels of MMP13 under basal and stimulated conditions suggesting that LMP2 has a unique function in these cells.
We are currently investigating the effect of LMP2-specific inhibition to scrutinize the therapeutic potential of LMP2 inhibitors for enhancing the pro-resolution capacity of M2 AMs in pulmonary diseases such as pneumonia- or sepsis-related acute respiratory distress syndrome.
- Copyright ©the authors 2016