Delayed drug hypersensitivity: models of T-cell stimulation

Abstract

Drug-induced hypersensitivity reactions can cause a variety of serious diseases by involving drug-specific T-cells. Many of these reactions have been explained by the hapten concept, which postulates that small chemical compounds need to bind covalently to proteins to be recognized by the immune system. Due to their chemical reactivity, haptens stimulate the innate immunity by binding covalently to endogenous proteins and form so called hapten-carrier complexes, which are antigenic and induce T-cell responses. In recent years, a new concept has been developed since drug-induced hypersensitivity reactions were also observed with chemically unreactive drugs. This concept implies direct and reversible interactions of the drug between T-cell receptors (TCR) and major histocompatability complex (MHC) molecules. Therefore it was termed pharmacological interactions with immune receptors (p-i concept). Early observations on drug reacting T-cell clones (TCC) let believe that drugs bind first to the T-cell receptor since HLA molecules could be exchanged without affecting the drug reactivity. However, MHC molecules were always required for full activation of TCC. According to its strong HLA-B*5701 association, recent data on abacavir suggest that a drug could first bind to the peptide binding groove of the MHC molecule. The thereby modified HLA molecule can then be recognized by specific T-cells. Consequently, two types of reactions based on the p-i mechanism may occur: on the one hand, drugs might preferentially bind directly to the TCR, whereas in defined cases with strong HLA association, drugs might bind directly to the MHC molecule.

Figure 1

Hapten and p-i mechanism of drug recognition by T-cells. (A) Hapten model: the MHC embedded peptide is covalently modified by the drug, creating a new antigenic determinant. (B) p-i concept for CD4⁺ T-cell stimulations: the drug (shown as SMX in this example) may bind to the TCR. Activation of the T-cell requires the interaction with MHC-class II molecules. No strict HLA-restriction can be demonstrated [39]. (C) p-i concept for HLA-allele restricted CD8⁺ T-cell responses: the drug (shown as abacavir in this example) binds avidly, but non-covalently to allele specific HLA-B determinants [45]. This binding may stabilize the HLA-complex even without peptide. A CD8⁺ T-cell reaction against this drug-HLA complex will be developed. It can only occur if the HLA molecule binds the drug with sufficient affinity, explaining the exquisite HLA-allele association of this reaction