INCREASE IN MK2 ACTIVITY IN CROHN’S DISEASE: ROLE IN THE INFLAMMATORY ACTIVATION OF FIBROBLASTS

Mesenchymal cells known as myo-/fibroblasts (MFs) are critical immunosuppressors under gut mucosal homeostasis. Expression of immune checkpoint PD-L1 by MFs plays a key role in the control of T cell inflammatory responses. In Crohn’s disease (CD), MFs switch their activity from immunosuppressive to pro-inflammatory, where they are also known as Inflammatory Fibroblasts. However, the mechanisms responsible for these pathological changes in MF activity are unknown. Map-kinase-activated protein kinase 2 (MK2) is a major regulator of inflammation in the gut. MK2 is downstream of p38 signaling, it evokes a sub-pathway that directly regulates the production of key inflammatory cytokines implicated in CD (such as TNF-α, IL-1, and IL-6). Thus, we hypothesized that activation of MK2 signaling is critical to the pathological changes in MFs during the immunopathogenesis of CD.

Human normal and CD tissues and derived MFs, as well as animal models relevant to CD, were used in this study. MF signaling/activity was analyzed using RNAseq, qRT-PCR, western blot (WB), cytokine/chemokine multiplex arrays, and confocal microscopy.

In situ analysis demonstrated an increase in MK2 activity within the inflamed compared to the non-inflamed CD and healthy control intestinal tissues, which was confirmed by WB and multiplex signaling array analysis. In situ increase in MK2 activity in CD intestinal mucosa was greatly associated with mesenchymal stromal cells that bear a “myofibroblast” phenotype (positive for α-SMA expression). An increase in MK2 activity was also observed in primary MF cultures isolated from CD (CD-MFs) when compared to normal (N-) MFs. MK2 activity within CD-MFs was also associated with a significant decrease in the expression of the immunosuppressive checkpoint PD-L1 and an increase in the expression of inflammatory CCL2 and IL-6. Inhibition of MK2 activity within CD-MFs through using the MK2-specific inhibitor PF-3644022 (10 μM) reversed the inflammatory activity of MFs. Remarkably, we observed a differential role of p38 and MK2 in the regulation of PD-L1 expression in MFs: while p38 was required for basal expression of PD-L1, activation of MK2 downregulates PD-L1 expression. These data indicate a unique role of MK2 activation in pathological reprogramming of MFs in CD. Use of MK2 inhibitor in a therapeutic modality in chronic DSS and IL-10 KO murine models of CD also significantly reduce MF-linked inflammatory responses in vivo.

Our data suggest that an increase in MK2 activity in CD is critical to the reprogramming of the MF from immunosuppressive toward pathological Inflammatory fibroblasts. Targeting MK2 activity within MFs could be a desirable strategy for improving the efficacy of current IBD therapeutic approaches.