Epitope specificity of myeloperoxidase antibodies: identification of candidate human immunodominant epitopes (1)
Two major types of anti-neutrophil cytoplasmic antibodies: cytoplasmic ANCA (c-ANCA) and perinuclear ANCA (p-ANCA), have been found. The primary antigenic target of p-ANCA is the lysosomal enzyme myeloperoxidase (MPO). Animal experiments show that p-ANCA directed against MPO can cause vasculitis that resembles human vasculitic disease.
The 12 patients displayed significant reactivity to multiple sections of the protein, including seven major significant epitopes in overlapping decapeptides representing the MPO protein.
These major significant epitopes include epitope 1: GSASPMELLS (aa 91–100); epitope 2: WTPGVKRNGF (aa 213–222); epitope 3: SARIPCFLAG (aa 393–402); epitope 4: WDGERLYQEA (aa 437–446); epitope 5: YRSYNDSVDP (aa 479–488); epitope 6: RLDNRYQPMEPN (aa 511–522); and epitope 7: IFMSNSYPRD (aa 717–726). Epitopes 2 and 6 were bound by the highest percentage of patients, having been bound by 41·7% and 58·3% of tested patient sera.
In the 3D model, epitope 3 is the only epitope within close proximity to the active site of the protein (His261, Arg405 and Gln257). Both epitopes 6 and 7 share close proximity within the structural model of the protein, even though they are separated by 195 amino acids within the linear sequence. Interestingly, 11 of the 12 patients target one or both of these two epitopes, suggesting that this commonly targeted region of the protein could be an important feature in identifying immunodominant epitopes in the pathogenesis of AAV.
At least three indepen- dent T cell epitopes exist on the MPO molecule by using recombinant MPO fragments to detect autoreactive CD4+ T cells to multiple MPO epitopes. Crescentic glomerulone- phritis also correlates with a particular epitope (Ha epitope) of MPO-ANCA, recognizing the N terminus of the MPO heavy chain. The effects of MPO-ANCA are the result of interference with the active site of the protein, as the enzy- matic activity of MPO is mostly unaffected by the presence of MPO-ANCA. The titres of MPO-ANCA have also been shown not to reflect disease activity at all times.