Allopurinol (ALP) hypersensitivity is a major cause of severe cutaneous adverse reactions and is strongly associated with the HLA-B*58:01 allele. However, it can occur in the absence of this allele with identical clinical manifestations. The immune mechanism of ALP-induced severe cutaneous adverse reactions is poorly understood, and the T cell–reactivity pattern in patients with or without the HLA-B*58:01 allele is not known. To understand the interactions among the drug, HLA, and TCR, we generated T cell lines that react to ALP or its metabolite oxypurinol (OXP) from HLA-B*58:01+ and HLA-B*58:01− donors and assessed their reactivity. ALP/OXP-specific T cells reacted immediately to the addition of the drugs and bypassed intracellular Ag processing, which is consistent with the “pharmacological interaction with immune receptors” (p-i) concept. This direct activation occurred regardless of HLA-B*58:01 status. Although most OXP-specific T cells from HLA-B*58:01+ donors were restricted by the HLA-B*58:01 molecule for drug recognition, ALP-specific T cells also were restricted to other MHC class I molecules. This can be explained by in silico docking data that suggest that OXP binds to the peptide-binding groove of HLA-B*58:01 with higher affinity. The ensuing T cell responses elicited by ALP or OXP were not limited to particular TCR Vβ repertoires. We conclude that the drug-specific T cells are activated by OXP bound to HLA-B*58:01 through the p-i mechanism.

Small chemicals like drugs tend to bind to proteins via noncovalent bonds, e.g. hydrogen bonds, salt bridges or electrostatic interactions. Some chemicals interact with other molecules than the actual target ligand, representing so-called ‘off-target' activities of drugs. Such interactions are a main cause of adverse side effects to drugs and are normally classified as predictable type A reactions. Detailed analysis of drug-induced immune reactions revealed that off-target activities also affect immune receptors, such as highly polymorphic human leukocyte antigens (HLA) or T cell receptors (TCR). Such drug interactions with immune receptors may lead to T cell stimulation, resulting in clinical symptoms of delayed-type hypersensitivity. They are assigned the ‘pharmacological interaction with immune receptors' (p-i) concept. Analysis of p-i has revealed that drugs bind preferentially or exclusively to distinct HLA molecules (p-i HLA) or to distinct TCR (p-i TCR). P-i reactions differ from ‘conventional' off-target drug reactions as the outcome is not due to the effect on the drug-modified cells themselves, but is the consequence of reactive T cells. Hence, the complex and diverse clinical manifestations of delayed-type hypersensitivity are caused by the functional heterogeneity of T cells. In the abacavir model of p-i HLA, the drug binding to HLA may result in alteration of the presenting peptides. More importantly, the drug binding to HLA generates a drug-modified HLA, which stimulates T cells directly, like an allo-HLA. In the sulfamethoxazole model of p-i TCR, responsive T cells likely require costimulation for full T cell activation. These findings may explain the similarity of delayed-type hypersensitivity reactions to graft-versus-host disease, and how systemic viral infections increase the risk of delayed-type hypersensitivity reactions.

The antiretroviral drug abacavir (abc) elicits severe drug hypersensitivity reactions in HLA-B*5701 + individuals. To understand the abc-specific activation of CD8 + T cells, we generated abc-specific T-cell clones (abc-TCCs). Abc reactivity could not be linked to the metabolism and/or processing of the drug, since abc metabolizing enzymes were not expressed in immune cells and inhibition of the proteasome in APCs did not affect TCC reactivity. Ca 2+ influx assays revealed different reactivity patterns of abc-TCCs. While all TCCs reacted to abc presented on HLA-B*5701 molecules, a minority also reacted immediately to abc in solution. Titration experiments showed that the ability to react immediately to abc correlated significantly with the TCR avidity of the T cells. Modifications of soluble abc concentrations revealed that the reactivity patterns of abc-TCCs were not fixed but dynamic. When TCCs with an intermediate TCR avidity were stimulated with increasing abc concentrations, they showed an accelerated activation kinetic. Thus, they reacted immediately to the drug, similar to the reaction of TCCs of high avidity. The observed immediate activation and the noninvolvement of the proteasome suggest that, in contrast to haptens, abc-specific T-cell stimulation does not require the formation of covalent bonds to produce a neo-antigenic determinant.Keywords: Abacavir hypersensitivity r HLA-B*5701 r TCR avidity Supporting Information available online IntroductionHypersensitivity reactions to drugs can lead to a variety of clinical symptoms and these involve different immune mechanisms [1]. Some of these reactions depend on genetic factors, among which HLA molecules play a particularly important role [2][3][4]. A striking example of such a genetic association is found in hypersensitivity reactions to the antiretroviral drug abacavir (abc), whereby it is strongly associated with the HLA-B*5701 allele [5]. Similar to other severe drug reactions [1], abc hypersensitivity Correspondence: Prof. Werner J. Pichler e-mail: wernerjoseph.pichler@insel.ch reactions involve drug-reacting T cells. This was illustrated by the presence of CD8 + T cells in skin biopsies of hypersensitive patients [6]. Moreover, a population of CD8 + T cells from HLA-B*5701 + individuals secrets IFN-γ in response to abc in vitro, irrespective of previous exposure to abc [7]. However, how abc is presented and subsequently stimulates T cells is still unclear.Currently, two models account for the stimulation of T cells by drugs. According to the hapten model, compounds bind to certain amino acids via covalent bonds, with or without previous metabolism of the drug. These hapten-modified proteins are then processed into antigenic peptides and are loaded onto MHC molecules of APCs. In this instance, the haptenization of molecules is important for the activation of the innate immune system [8]. If this was the case in abc hypersensitivity, these hapten complexes C

This study establishes the important synergistic role of drug concentration and HLA-B*58:01 allele in the allopurinol or oxypurinol-specific T cell responses. Despite the prevailing dogma that Type B adverse drug reactions are dose independent, allopurinol hypersensitivity is primarily driven by oxypurinol-specific T cell response in a dose-dependent manner, particular in the presence of HLA-B*58:01 allele.

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.