Lysosomal cathepsin creates chimeric epitopes for diabetogenic CD4 T cells via transpeptidation

Abstract

The identification of the peptide epitopes presented by major histocompatibility complex class II (MHCII) molecules that drive the CD4 T cell component of autoimmune diseases has presented a formidable challenge over several decades. In type 1 diabetes (T1D), recent insight into this problem has come from the realization that several of the important epitopes are not directly processed from a protein source, but rather pieced together by fusion of different peptide fragments of secretory granule proteins to create new chimeric epitopes. We have proposed that this fusion is performed by a reverse proteolysis reaction called transpeptidation, occurring during the catabolic turnover of pancreatic proteins when secretory granules fuse with lysosomes (crinophagy). Here, we demonstrate several highly antigenic chimeric epitopes for diabetogenic CD4 T cells that are produced by digestion of the appropriate inactive fragments of the granule proteins with the lysosomal protease cathepsin L (Cat-L). This pathway has implications for how self-tolerance can be broken peripherally in T1D and other autoimmune diseases.

Potential Mimicry of Viral and Pancreatic β Cell Antigens Through Non-Spliced and cis-Spliced Zwitter Epitope Candidates in Type 1 Diabetes

“…The KCNK16 17-21/11-14 cis-spliced peptide was predicted to efficiently bind HLA-A*02:01 complex (predicted IC 50 = 93 nM; Table S5). It partially overlapped with the HLA-A*03:01-restricted non-spliced peptide KCNK16 [13][14][15][16][17][18][19][20][21] [RVLPLLLAY] (Figure 5J), which was identified in HLA-I immunopeptidomes of human ECN90 pancreatic b cell line upon IFN-g stimulation (19).…”

“…A handful of known HIPs can trigger a CD4 + T cell response both in NOD mice and in T1D patients (7)(8)(9)(10)(11). The enzymes (or biochemical reactions) catalyzing their production are not fully understood, although pioneer studies suggest that HIPs or HIPs' precursors might be produced in b cell's insulin crinosomes or professional antigen presenting cell (APC)'s lysosomes (9,12,13). Their identification, however, is still controversial, and the employment of different mass spectrometry data analysis strategies has led to contradictory results in HIPs' identification (12,14).…”

Neoepitopes in Type 1 Diabetes: Etiological Insights, Biomarkers and Therapeutic Targets

“…Increased reactivity to several HIPs was shown in peripheral blood of T1D patients (53,54), Furthermore, in genetically at risk individuals, HIPs were detectable, and were shown to have a predominantly pro-inflammatory profile in those that progressed to developing disease (55), making them interesting candidates for novel biomarkers. Finally, a recent report suggests that transpeptidation of beta-cell antigens, mediated by cathepsin L, generates chimeric epitopes through fusion of secretory granule proteins with WE14, for diabetogenic CD4 T-cells (56). In regard to HLA class I epitopes, spliced peptides, generated in the proteasome through a process referred to as transpeptidation by which two different regions of a protein or of two different proteins are fused (57,58), have been identified in a human beta-cell line by HLA-peptidomics (21,25) and were recognized by circulating and pancreas-infiltrating CD8+ T-cells from T1D donors.…”

Inherent Beta Cell Dysfunction Contributes to Autoimmune Susceptibility

“…One example are hybrid insulin peptides (HIPs) that are attracting considerable attention as pathogenic targets of CD4+ T cells [ 74 ]. These peptides are not genetically encoded, but instead are derived by fusion of two distinct protein fragments via transpeptidation [ 75 ], and generated mainly in beta cell autophagosomes and/or crinophagic bodies during turnover of damaged or defunct secretory granules [ 75 , 76 , 77 ]. In healthy cells newly formed HIPs will likely be relatively short-lived due to the actions of lysosomal amino- and/or carboxypeptidases also present in the same compartments.…”