Non-covalent presentation of sulfamethoxazole to human CD4+ T cells is independent of distinct human leucocyte antigen-bound peptides.

BACKGROUND: It has been shown that drugs comprise a group of non-peptide antigens that can be recognized by human T cells in the context of HLA class II and that this recognition is involved in allergic reactions. Recent studies have demonstrated a MHC-restricted but processing- and metabolism-independent pathway for the presentation of allergenic drugs such as lidocaine and sulfamethoxazole (SMX) to drug-specific T cells. However, there is little information so far on the precise molecular mechanisms of this non-covalent drug presentation.

OBJECTIVE: The aim of this study was to evaluate the requirements for a specific peptide occupying the groove of the MHC class II molecule for the efficient presentation of non-covalently bound drugs to CD4+ T cells.

METHODS: We analysed the effect of coincubation or prepulse of antigen presenting cells (APC) with different peptides on the proliferative responses of SMX-specific CD4+ T cell clones. In a second series of experiments, we eluted HLA-bound peptides from the surface of antigen presenting cells by mild acid treatment. Successful removal of peptides was tested directly using labelled peptides and functionally by monitoring activation and proliferation of peptide-specific T cell clones. Finally, the presentation of SMX to SMX-specific T cell clones before and after elution of MHC class II bound peptides was tested.

RESULTS: We found that neither peptide coincubation nor peptide prepulse of APC altered the proliferative response of SMX-specific T cells. APC treated with the acid for a short time retained cell viability, MHC class II expression and antigen presenting cell function. However, defined peptides could be eluted from surface MHC class II molecules nearly quantitatively. Nevertheless, the chemically non-reactive drug SMX could still be presented to specific T cells independent of the presence of distinct self-peptides.

CONCLUSION: Our data suggest that small molecules like drugs can bind to a multitude of HLA-bound peptides or that, similar to superantigens, they might bind directly to HLA.