The SPPL3-defined glycosphingolipid repertoire regulates immune responses by improving HLA class I access

Summary HLA class I (HLA-I) drives immune responses by presenting antigen-derived peptides to cognate CD8 + T cells. This process is often hijacked by tumors and pathogens for immune evasion. Since therapeutic options for restoring HLA-I antigen presentation are limited, we aimed to identify new HLA-I pathway targets. By iterative genome-wide screens we uncovered that the cell surface glycosphingolipid (GSL) repertoire determines effective HLA-I antigen presentation. We show that absence of the protease SPPL3 augments B3GNT5 enzyme activity, resulting in upregulated levels of surface (neo)lacto-series GSLs. These GSLs sterically impede molecular interactions with HLA-I and diminish CD8 + T cell activation. In accordance, a disturbed SPPL3-B3GNT5 pathway in glioma associates with decreased patient survival. Importantly, we show that this immunomodulatory effect can be reversed through GSL synthesis inhibition using clinically approved drugs. Overall, our study identifies a GSL signature that functionally inhibits antigen presentation and represents a potential therapeutic target in cancer, infection and autoimmunity.