Abacavir induces loading of novel self-peptides into HLA-B*57:01: an autoimmune model for HLA-associated drug hypersensitivity

Background: Abacavir drug hypersensitivity in HIV-treated patients is associated with HLA-B� 57:01 expression. To understand the immunochemistry of abacavir drug reactions, we investigated the effects of abacavir on 1) HLA-B� 57:01 epitope-binding in vitro and 2) the quality and quantity of self-peptides presented by HLA-B� 57:01 from abacavir-treated cells. Design and methods: An HLA-B� 57:01 specific epitope binding assay was developed to test for effects of abacavir, didanosine or flucloxacillin on self-peptide binding. To examine whether abacavir alters the peptide repertoire in HLA-B� 57:01, a B-cell line secreting soluble HLA (sHLA) was cultured in the presence or absence of abacavir, peptides were eluted from purified HLA, and the peptide epitopes comparatively mapped by mass spectroscopy to identify drug-unique peptides. Results: Abacavir, but not didansosine or flucloxacillin, enhanced binding of the FITC labeled self-peptide LF9 to HLA-B� 57:01 in a dose dependent manner. Endogenous peptides isolated from abacavir-treated HLA-B� 57:01 B-cells showed amino acid sequence differences compared with peptides from untreated cells. Novel druginduced (DI)-peptides lacked typical carboxyl(C)-terminal amino-acids characteristic of the HLA-B� 57:01 peptide motif and instead contained predominantly Isoleucine or Leucine residues. DI-peptides bind to soluble HLA-B� 57:01 with high affinity that was not altered by abacavir addition. Conclusion: Our results support a model of drug-induced autoimmunity in which abacaviraltersthequantity andquality of self-peptideloadinginto HLA-B� 57:01.Druginduced loading of novel self-peptides into HLA, possibly by abacavir either altering the binding cleft or modifying the peptide loading complex, generates an array of neo