Delayed-type or nonimmediate drug hypersensitivity reactions often involve the activation of drug-specific T cells. As such, the molecular initiating event is an interaction between HLA proteins, HLA-binding peptides and the drug. For many years, the formation of covalently modified drug protein adducts was assumed to be a prerequisite for T-cell activation. The purpose of this article is to review recent studies using human PBMC, T-cell lines and clones, which show that drugs are in fact loaded onto HLA molecules in different forms to activate T cells.
We now know that protein-reactive drugs such as β-lactam antibiotics activate T cells via direct noncovalent interactions with HLA or HLA-binding peptides, direct covalent modification of HLA-binding peptides and covalent binding of non-HLA associated proteins. Adducts formed inside and outside of the cells undergo protein processing to generate HLA-binding peptides that are assumed to contain the drug modification. Studies using synthetic stable (e.g. oxypurinol) and reactive (e.g. nitroso sulfamethoxazole) metabolites show that metabolites activate T cells via the same pathways. A variety of drugs with different structural features have also been shown to activate T cells though a direct HLA-binding interaction. Of note, abacavir behaves in an unexpected way, binding deep in the peptide binding cleft of one HLA, selectively activating CD8+ T cells.
In-vitro studies have revealed that a number of drug HLA-binding interactions lead to the activation of T cells. These can be categorized according to two hypotheses, namely hapten and pharmacological interactions. As we move forward with the development of diagnostic and predictive T-cell assays, it is critical to reach a consensus that direct drug HLA binding and the formation of drug protein adducts are important events for T-cell activation.
aMRC Centre for Drug Safety Science, Department of Molecular and Clinical Pharmacology, University of Liverpool, Liverpool, United Kingdom
bDepartment of Rheumatology, Immunology and Allergology, University Hospital of Bern, Bern, Switzerland
Correspondence to Dean J. Naisbitt, The University of Liverpool, Liverpool, United Kingdom. Tel: +44 151 7945346; E-mail: firstname.lastname@example.org