Route of Uptake of Palmitoylated Encephalitogenic Peptides of Myelin Proteolipid Protein by Antigen-Presenting Cells: Importance of the Type of Bond between Lipid Chain and Peptide and Relevance to Autoimmunity

Abstract

Previously, we have shown that thiopalmitoylation of peptides of myelin proteolipid protein, as occurs naturally in vivo, increases their ability to induce experimental autoimmune encephalomyelitis, the animal model of multiple sclerosis, and skews the autoimmune response toward a CD4+-mediated response. In contrast, the same peptide, when synthesized with a stable amide bond between peptide and lipid, inhibits experimental autoimmune encephalomyelitis and skews the response toward a CD8+ response. The aim of the current study was to determine the mechanisms responsible for these observations. We show that proteolipid protein lipopeptides, when synthesized with a thioester bond between the lipid and the peptide, are taken up into APCs via an actin-independent endocytic route, the thioester bond is cleaved in the endosome, and the peptide is subsequently displayed on the surface of the APC in the context of MHC class II. The same peptide, when synthesized with the lipid attached via a stable amide bond, rapidly enters into the cytoplasm of the APC and forms micelles; however, the bond between peptide and lipid is not cleaved, and the micelles travel via the endoplasmic reticulum to complex with MHC class I. These findings have implications for vaccine development and for the development of MHC class II-restricted autoimmune diseases, as many human autoantigens thus far identified are thioacylated.

Thiopalmitoylation of Altered Peptide Ligands Enhances Their Protective Effects in an Animal Model of Multiple Sclerosis

“…The complementary data obtained from S-palmQ144 show that the enhancement of the protective effects afforded by thiopalmitoylation is not exclusive to one APL but can be generalized to other APLs. These results support the idea that thiopalmitoylation of a peptide enhances its innate properties; thus, thiopalmitoylation of a peptide that has the potential to be pathogenic can enhance pathogenicity (as we have previously shown in EAE (2,5) and experimental autoimmune neuritis (3)), whereas thiopalmitoylation of a peptide that has the potential to protect against disease can enhance the efficacy of the therapeutic effect. This could have potential applicability in vaccine development, providing a simple means to increase MHC class II-restricted T cell responses.…”

“…5G). These results confirm that the thioester bond between lipid and peptide helps to channel the peptide into the MHC class II presentation pathway, because of the presence of thioesterases in these organelles, thereby inducing a CD4 + T cell response, as we have previously found with encepalitogenic peptides in macrophages (5).…”

“…Previously, we have found that thiopalmitoylated encephalitogenic peptides are not only taken up more efficiently by another type of APC (macrophages) than are the same nonpalmitoylated peptides, but are also channeled into the MHC class II presentation pathway, and we have proposed that this underlies their improved immunogenicity and the enhancement of CD4 + T cell responses (4,5). In this study, we sought to confirm whether S-palmAPL was taken up more efficiently than nonpalmitoylated APL by DCs.…”

“…We have previously demonstrated with encephalitogenic thiopalmitoylated peptides that the thioester bond between the lipid and peptide can be broken down in endosomes and lysosomes (part of the MHC class II presentation pathway and which contain thioesterases), thereby effectively "stranding" the peptide in these organelles, and making it much more likely that they will be presented by MHC class II molecules (5). To assess whether this same mechanism was likely to be occurring with the S-palmA188 peptide, we investigated whether there was colocalization of biotinylated peptide and MHC class II in DC that has been incubated with the biotinylated peptide for 60 min.…”

“…Similarly, thiopalmitoylation of peptides from P 0 , a major protein of peripheral nervous system myelin, enhances their neuritogenic potential in experimental autoimmune neuritis (3). The enhanced pathogenicity of the thiopalmitoylated peptides appears to be because of their increased uptake (compared with nonpalmitoylated peptide) into the MHC class II presentation pathway (4,5). One question arising from the above studies is whether the thiopalmitoylation is merely enhancing the innate potential of these peptides to be pathogenic or whether thiopalmitoylation of peptides that have little or no disease-inducing potential can actually make them pathogenic.…”