T cell responses to the putative dominant autoepitope in primary biliary cirrhosis (PBC)

Abstract

PBC is characterized by T cell-mediated destruction of the biliary epithelial cells lining the small intrahepatic bile ducts. The E2 and E3 binding protein (E3BP (protein X)) components of pyruvate dehydrogenase complex (PDC) are disease-specific autoantigens in PBC. Attempts to localize the T cell autoepitopes within PDC-E2 have, however, generated contradictory results. One study has suggested the presence of T cell epitopes throughout PDC-E2, whilst another has identified a single dominant 14 amino acid T cell epitope (p163) spanning the lipoic acid binding lysine residue in the inner lipoyl domain (ILD) of PDC-E2. The aim of the current study was to determine the prevalence of T cell responses to p163 and PDC-E2 ILD, and the role played by lipoylation of these antigens in their immunogenicity, in a UK PBC population. We found that the majority of the PBC patients showing a 6-day peripheral blood T cell proliferative response to native human PDC also responded, in a MHC class II-restricted fashion, to biochemically purified PDC-E2 and E3BP (which co-purify) (9/10 positive (SI > 2.76), mean SI 5.74 +/- 5.04 (PDC-E2/E3BP) versus 6.67 +/- 3.84 (PDC), P = NS), implying that the important PBC-specific T cell epitopes are contained within the PDC-E2 or E3BP components of PDC. Only a minority of patients responsive to PDC, however, responded to either lipoylated recombinant PDC-E2 ILD (4/10 positive, mean SI 1.98 +/- 1.24, P < 0.005 versus PDC response) or lipoylated p163 (4/12 positive, mean SI 1.90 +/- 1.58, P < 0.001). The lipoylation state did not affect the T cell response to either ILD or p163. Our findings suggest that in some UK patients with PBC there are immunodominant T cell autoepitopes within PDC-E2/E3BP which are outside the ILD of PDC-E2.

Fig 1

Schematic representation of the structure of human (a) PDC-E2 and (b) E3BP. The sequences of peptide p163 from PDC-E2, and the equivalent peptide from E3BP, and their localization within the lipoyl domains are shown. The lipoic acid binding lysine residue is marked K. A high level of sequence identity is observed between the inner lipoyl domain of PDC-E2 and the single lipoyl domain of E3BP. Within p163 and the homologous peptide from E3BP sequence identity is represented by the boxed residues, sequence similarity by *. The three hatched residues, identical in the two sequences and containing the lipoic acid binding residue in each, are the residues identified by Shimoda et al. as the minimal conserved residues for the PDC-E2-derived p163 epitope [13].

Fig 2

Comparative peripheral blood T cell proliferative responses to whole human PDC (PDC), biochemically purified human PDC-E2/E3BP (E2/E3BP) and lipoylated recombinant human PDC-E2 inner lipoyl domain (recom) in a series of PBC patients responding to whole human PDC. The patient who responded to whole PDC but not PDC-E2/E3BP did not respond to recombinant inner lipoyl domain.

Fig 3

Blocking effects of addition of anti-class I and class II MoAbs on the peripheral blood T cell proliferative response to PDC-E2/E3BP in a representative PBC patient.

Fig 4

Comparative peripheral blood T cell responses to human PDC (PDC) and lipoylated p163 in a series of PBC patients responding to PDC.

Fig 5

Correlation between the response to lipoylated and unlipoylated recombinant PDC-E2 inner lipoyl domain in PBC patients responsive to PDC.

Fig 6

Correlation between the response to lipoylated and unlipoylated p163 in PBC patients responsive to PDC. A significant correlation is seen.