S1P Lyase Inhibition Increased Intestinal S1P, Disrupted the Intestinal Barrier and Aggravated DSS-Induced Colitis.

Background: Sphingosine-1-phospate (S1P) receptor agonists (eg, ozanimod) desensitize migrating lymphocytes by irreversibly binding to S1P receptors (S1PR) and triggering their proteasomal degradation. Desensitized lymphocytes cannot sense S1P, therefore, halting lymphocyte recirculation. The S1P lyase (SPL) irreversibly degrades S1P and its inhibition disrupts the S1P gradient. We previously found that systemic SPL inhibitors induce central immunosuppression. Here, we examined whether SPL inhibition may attenuate colitis without systemic immunotoxicity.

Methods: We first analyzed SPL expression and localization in mice using qRT-PCR and immunohistochemistry. SPL inhibitors 4-deoxypyridoxine hydrochloride (DOP) and 2-acetyl-4-(tetrahydroxybutyl) imidazole (THI) were used to inhibit SPL systemically, whereas a conditional intestinal epithelial cell (IEC)-specific SPL-deficient mouse was used to evaluate the effects of IEC-specific SPL inhibition on survival, disease activity, histological severity of dextran sulfate sodium-induced colitis, S1P levels, and intestinal permeability.

Results: Sgpl1 mRNA transcripts and protein were ubiquitously expressed in gastrointestinal (GI) tract leukocytes and IEC. Systemic SPL inhibitors did not induce colitis by themselves but depleted CD4+ and CD8+ T cells from blood. However, contrary to its therapeutic effects on ileitis, systemic inhibition reduced survival, accelerated weight loss, worsened histopathological inflammation indices, and tissue damage. We then examined the effects of IEC-specific inhibition on peripheral cell counts and severity of colitis. We found that while it spared peripheral immunity, it similarly hastened colitis. Finally, we examined whether colitis acceleration was due to epithelial barrier compromise after disruption of the S1P gradient. We found that not only systemic but also IEC-specific SPL inhibition increased local S1P levels and led to IEC barrier compromise.

Conclusion: Homeostatic intestinal S1P levels are critical for the regulation of IEC barrier function. Further studies using adaptive immunity-based inflammatory bowel diseases (IBD) models are required to assess the translational value of IEC-specific SPL inhibition as a therapeutic target for human IBD.