CD15 The role of lipid-metabolizing enzymes in CD1a-associated cutaneous inflammation
Authors: Ye John , Ng Jessica , Singh Randeep , Nahler Janina , Naidoo Karmella , Hardman Clare , Gao Fei , Gileadi Uzi , Chen Yi-Ling , Ogg Graham 
Published: June 2024

Abstract
Atopic dermatitis (AD) is a common, relapsing–remitting inflammatory skin disease that is associated with other disorders such as food allergy, allergic asthma and rhinitis, leading to increased rates of morbidity and mortality. The discovery of CD1a involvement in AD pathogenesis through the presentation of permissive neolipid antigens generated by house dust mite phospholipase A2 (PLA2) enhanced our understanding of the complex network of T-cell targets that contribute to disease development. As well as defining agonist lipids, it is now known that very long-chain sphingomyelins can act as natural lipid antagonists of CD1a–T-cell receptor (TCR) engagement. We sought to determine whether bacterial sphingomyelinases (SMases) could also activate CD1a-reactive T cells and help explain the known bacterial infection-associated manifestations of AD. An isoelectric focusing system was used to evaluate the hydrolytic capacity of bacterial SMases on very long-chain CD1a-preferred sphingomyelins. Additionally, this approach permitted examination of the impact of cobalt and nickel ions on bacterial SMase activity. Furthermore, using primary human T-cell clones and TCR transfer with CRISPR Homology Directed Repair, we evaluated SMase-reactive CD1a-dependent T-cell responses. Moreover, we examined both lesional and nonlesional skin from patients with AD, patients with psoriasis and healthy controls using RNA-Seq. Bacterial SMases were able to digest CD1a-selected sphingomyelins, and the enzymatic activity was potentiated by the presence of divalent metal ions, which are known contact dermatitis allergens. Furthermore, bacterial SMase-reactive CD1a-dependent T-cell responses were identified through markedly increased detection of AD-associated inflammatory cytokines and elevated activation marker expression. Consistency of responses between the primary T-cell clones and HDR T cells confirmed that these responses were indeed TCR specific. Skin transcriptomic changes included altered lipid processing pathways, in particular, mammalian SMase and PLA2-mediated pathways, activated in lesional tissue. We showed that bacterial SMases can effectively degrade very long-chain antagonist sphingomyelins and potentiate CD1a-reactive T-cell responses. In addition, our bioinformatics analysis revealed enhanced expression of mammalian SMases in lesional AD and psoriasis, suggesting potential broad involvement of this enzyme class in skin inflammation. This study extends our understanding of the role of lipid-metabolizing enzymes to CD1a-dependent T-cell activation in AD pathology, as well as defining a hitherto unknown pathway of contact dermatitis. The mechanisms provide insights towards novel targets for therapeutic intervention. The principal investigator has received relevant research collaboration support from UCB and Janssen. The principal investigator and some members of the group are on patents filed on CD1a.