To better understand the role of sphingolipids in the multifactorial process of inflammatory bowel disease (IBD), we elucidated the role of CerS4 in colitis and colitis-associated cancer (CAC). For this, we utilized the azoxymethane/dextran sodium sulphate (AOM/DSS)-induced colitis model in global CerS4 knockout (CerS4 KO), intestinal epithelial (CerS4 Vil/Cre), or T-cell restricted knockout (CerS4 LCK/Cre) mice. CerS4 KO mice were highly sensitive to the toxic effect of AOM/DSS, leading to a high mortality rate. CerS4 Vil/Cre mice had smaller tumors than WT mice. In contrast, CerS4 LCK/Cre mice frequently suffered from pancolitis and developed more colon tumors. In vitro, CerS4-depleted CD8+ T-cells isolated from the thymi of CerS4 LCK/Cre mice showed impaired proliferation and prolonged cytokine production after stimulation in comparison with T-cells from WT mice. Depletion of CerS4 in human Jurkat T-cells led to a constitutively activated T-cell receptor and NF-κB signaling pathway. In conclusion, the deficiency of CerS4 in T-cells led to an enduring active status of these cells and prevents the resolution of inflammation, leading to a higher tumor burden in the CAC mouse model. In contrast, CerS4 deficiency in epithelial cells resulted in smaller colon tumors and seemed to be beneficial. The higher tumor incidence in CerS4 LCK/Cre mice and the toxic effect of AOM/DSS in CerS4 KO mice exhibited the importance of CerS4 in other tissues and revealed the complexity of general targeting CerS4.

The concept of proper resolution of inflammation rather than counteracting it, gained a lot of attention in the past few years. Re-assembly of tissue and cell homeostasis as well as establishment of adaptive immunity after inflammatory processes are the key events of resolution. Neutrophiles and macrophages are well described as promotors of resolution, but the role of T cells is poorly reviewed. It is also broadly known that sphingolipids and their imbalance influence membrane fluidity and cell signalling pathways resulting in inflammation associated diseases like inflammatory bowel disease (IBD), atherosclerosis or diabetes. In this review we highlight the role of sphingolipids in T cells in the context of resolution of inflammation to create an insight into new possible therapeutical approaches.

Ceramide synthase 5 is one of six enzymes that catalyze the production of ceramides from sphingosine or sphinganine. Ceramides are important components of cell membranes and act as signaling molecules. Previously it has been shown that ceramide synthase 6 and 2 influence colitis in several animal models with sometimes opposite effects. Here, we investigated the disease course of dextran sodium sulfate-induced acute colitis and azoxymethane/dextran sodium sulfate-induced colitis-associated colon cancer in mice with global ceramide synthase 5 knockout (CerS5-ko) or with ceramide synthase 5 knockout restricted to the colon epithelium (CerS5fl/fl VilCre). We monitored disease development and analyzed colon barrier function as well as the immune cell status in these mice. CerS5-ko mice but not CerS5fl/fl-VilCre mice were more susceptible to acute and chronic inflammation. However, the cell barrier function of colon epithelial cells was not disturbed by downregulation of ceramide synthase 5. Instead, untreated CerS5-ko mice displayed reduced numbers of CD3+ immune cells in the spleen, colon, and blood, especially of intraepithelial CD8+ T-cells, which was not obvious in CerS5fl/fl Vil Cre mice. Reduced T-cell number in colon tissue of CerS5-ko mice was accompanied by a reduced expression of IL-1β, IFNγ, and IL-4. In vitro investigations revealed that knockdown of ceramide synthase 5 in T-cells impaired T-cell activation. In summary, we show that CerS5-ko mice were more susceptible to dextran sodium sulfate-induced colitis and azoxymethane/dextran sodium sulfate-induced colitis-associated colon cancer. A reduced number of T-cells in the colon epithelium that was already the case in untreated CerS5-ko mice might have contributed to this effect.