SIALYLATION REGULATES NEUTROPHIL TRANSEPITHELIAL MIGRATION, CD11B/CD18 ACTIVATION AND MUCOSAL INFLAMMATORY FUNCTION IN THE GUT

Background: Resolution of intestinal inflammation requires well-tuned communication between the immune compartment and the epithelium, carried out primarily via secreted mediators. The expression of these mediators is predominantly regulated by the transcription factor NF-κB. We have recently demonstrated that NF-κB can drive temporally and functionally distinct transcriptomes in the same cells, leading to divergent cell fate decisions. How these divergent transcriptomes and secretomes would affect resolution of inflammation is not known. Activation of NF-κB in inflammatory bowel diseases (IBD) directly correlates with the stage of inflammation. Although its role in leukocytes has been well characterized, its role remains controversial. Here we aimed to address the controversy by examining the contribution of epithelial NF-κB to disease progression.
Methods: We used transgenic NF-κB activity reporter mice, murine models with activated or repressed NF-κB selectively in intestinal epithelial cells, and human IBD single cell datasets to determine the contribution of epithelial NF-κB to acute inflammation and to resolution.
Results: Although NF-κB was detected in all epithelial cells, it was activated only in a subset of stem and enteroendocrine cells. Transcriptomes of these cells revealed expression of pro-apoptotic mediators. We therefore reasoned that selective suppression of NF-κB in epithelium would be protective in chemically induced colitis. To our surprise, low grade inflammation triggered by activated NF-κB in epithelium was protective in colitis, whereas suppression of NF-κB led to progressive destruction of epithelium and inability to resolve inflammation, concomitant with loss of regenerative capacity of stem cells. Current work focuses on confirmation of these NF-κB signatures in IBD patients.
Conclusion: NF-κB activation supports epithelial regeneration in colitis. The mechanism of regenerative NF-kB activity in epithelium might identify novel targets or protective mediators for IBD therapy.

Neutrophils (PMNs) are the first immune responders to infection/injury playing a critical role in clearing invading microbes and promoting tissue repair. However, dysregulated trafficking of PMNs across mucosal surfaces is a pathological hallmark of diseases characterized by persistent or intermittent bursts of inflammation including inflammatory bowel disease (IBD). The critical final step in PMN trafficking into mucosal lined organs (including the gut) involves transepithelial migration (TEpM). Recent studies have shown that terminal Fucose and GlcNAc glycans on the beta2 integrin CD11b/CD18 can be targeted to reduce PMN intestinal trafficking, highlighting the importance of glycosylation in regulating PMN inflammatory function. The most abundant terminal glycan on human and murine PMN is sialic acid (Sia) and previous studies have demonstrated increased surface sialidase activity in activated PMN. However the role of Sia in regulating PMN intestinal migration and inflammatory function is not well understood. Using human and murine models of PMN intestinal trafficking, we demonstrate that blocking sialidase- mediated removal of a2-3 linked sialic acid from CD11b/CD18 inhibits PMN TEpM in vitro and in vivo. Measurement of SOD inhibitable cytochrome C reduction revealed that sialidase inhibition reduced fMLF stimulated superoxide release by human and murine PMN. Flow cytometric analyses revealed that sialidase inhibition decreased fMLF mediated PMN degranulation responses and fMLF mediated CD11b/CD18 conformational activation. Finally, immunoblotting analyses demonstrated that de-sialylation activates spleen tyrosine kinase (Syk), a key intracellular mediator that signals downstream of CD11b/CD18 in human and murine PMN. Taken together, these data show that upon PMN activation, sialidases cleave Sia from CD11b/CD18 to facilitate conformational integrin activation and subsequent Syk dependent outside in signaling that drives PMN inflammatory effector functions including migration, degranulation, and superoxide release. Therefore, targeting CD11b/CD18 de-sialylation may represent a promising new therapeutic strategy for reducing dysregulated PMN influx and associated mucosal tissue damage in IBD and other chronic inflammatory disorders.