Drug hypersensitivity and eosinophilia: The decisive role of p-i stimulation.

Eosinophilia is a common finding in drug hypersensitivity reactions (DHR). Its cause is unclear, as neither antigen/allergen-driven inflammation nor clonal expansion is involved. Most delayed-DHRs are due to p-i (pharmacologic interaction of drugs with immune receptors). These are off-target activities of drugs with immune receptors that result in various types of T-cell stimulation, some of which involve excessive IL-5 production. Functional and phenotypic studies of T-cell clones and their TCR-transfected hybridoma cell lines revealed that some p-i-induced drug stimulations occur without CD4/ CD8 co-receptor engagement. The CD4/CD8 co-receptors link Lck (lymphocyte-specific protein tyrosine kinase) and LAT (linker for activation of T cells) to the TCR. Alteration of Lck or LAT can result in a TCR signalosome with enhanced IL-5 production. Thus, if a more affine TCR-[drug/peptide/HLA] interaction allows bypassing the CD4 co-receptor, a modified Lck/LAT activation may lead to a TCR signalosome with elevated IL-5 production. This "IL-5-TCR-signalosome" hypothesis could also explain eosinophilia in superantigen or allo-stimulation (graft-versus-host disease), in which evasion of CD4/CD8 co-receptors has also been described. It may open new therapeutic possibilities in certain eosinophilic diseases by directly targeting the IL-5-TCR signalosome.

Viral infections and drug hypersensitivity.

Virus infections and T-cell-mediated drug hypersensitivity reactions (DHR) can influence each other. In most instances, systemic virus infections appear first. They may prime the reactivity to drugs in two ways: First, by virus-induced second signals: certain drugs like ß-lactam antibiotics are haptens and covalently bind to various soluble and tissue proteins, thereby forming novel antigens. Under homeostatic conditions, these neo-antigens do not induce an immune reaction, probably because co-stimulation is missing. During a virus infection, the hapten-modified peptides are presented in an immune-stimulatory environment with co-stimulation. A drug-specific immune reaction may develop and manifest as exanthema. Second, by increased pharmacological interactions with immune receptors (p-i): drugs tend to bind to proteins and may even bind to immune receptors. Without viral infections, this low affine binding may be insufficient to elicit T-cell activation. During a viral infection, immune receptors are more abundantly expressed and allow more interactions to occur. This increases the overall avidity of p-i reactions and may even be sufficient for T-cell activation and symptoms. There is a situation where the virus-DHR sequence of events is inversed: in drug reaction with eosinophilia and systemic symptoms (DRESS), a severe DHR can precede reactivation and viremia of various herpes viruses. One could explain this phenomenon by the massive p-i mediated immune stimulation during acute DRESS, which coincidentally activates many herpes virus-specific T cells. Through p-i stimulation, they develop a cytotoxic activity by killing herpes peptide-expressing cells and releasing herpes viruses. These concepts could explain the often transient nature of DHR occurring during viral infections and the often asymptomatic herpes-virus viraemia after DRESS.

Risk Assessment in Drug Hypersensitivity: Detecting Small Molecules Which Outsmart the Immune System.

Drug hypersensitivity (DH) reactions are clinically unusual because the underlying immune stimulations are not antigen-driven, but due to non-covalent drug-protein binding. The drugs may bind to immune receptors like HLA or TCR which elicits a strong T cell reaction (p-i concept), the binding may enhance the affinity of antibodies (enhanced affinity model), or drug binding may occur on soluble proteins which imitate a true antigen (fake antigen model). These novel models of DH could have a major impact on how to perform risk assessments in drug development. Herein, we discuss the difficulties of detecting such non-covalent, labile and reversible, but immunologically relevant drug-protein interactions early on in drug development. The enormous diversity of the immune system, varying interactions, and heterogeneous functional consequences make it to a challenging task. We propose that a realistic approach to detect clinically relevant non-covalent drug interactions for a new drug could be based on a combination of in vitro cell culture assays (using a panel of HLA typed donor cells) and functional analyses, supplemented by structural analysis (computational data) of the reactive cells/molecules. When drug-reactive cells/molecules with functional impact are detected in these risk assessments, a close clinical monitoring of the drug may reveal the true incidence of DH, as suppressing but also enhancing factors occurring in vivo can influence the clinical manifestation of a DH.

The important role of non-covalent drug-protein interactions in drug hypersensitivity reactions.

Drug hypersensitivity reactions (DHR) are heterogeneous and unusual immune reactions with rather unique clinical presentations. Accumulating evidence indicates that certain non-covalent drug-protein interactions are able to elicit exclusively effector functions of antibody reactions or complete T-cell reactions which contribute substantially to DHR. Here, we discuss three key interactions; (a) mimicry: whereby soluble, non-covalent drug-protein complexes ("fake antigens") mimic covalent drug-protein adducts; (b) increased antibody affinity: for example, in quinine-type immune thrombocytopenia where the drug gets trapped between antibody and membrane-bound glycoprotein; and (c) p-i-stimulation: where naïve and memory T cells are activated by direct binding of drugs to the human leukocyte antigen and/or T-cell receptors. This transient drug-immune receptor interaction initiates a polyclonal T-cell response with mild-to-severe DHR symptoms. Notable complications arising from p-i DHR can include viral reactivations, autoimmunity, and multiple drug hypersensitivity. In conclusion, DHR is characterized by abnormal immune stimulation driven by non-covalent drug-protein interactions. This contrasts DHR from "normal" immunity, which relies on antigen-formation by covalent hapten-protein adducts and predominantly results in asymptomatic immunity.

Highly specific and reliable in vitro diagnostic analysis of memory T and B lymphocytes in a Swiss cohort of COVID-19 patients.

The SARS-CoV-2 pandemic has claimed many lives and disrupted the quality of life of most individuals. Diagnostic tests not only serve to confirm past exposure but can provide information crucial for guiding healthcare options for patients. Current diagnostic tests for the presence of the SARS-CoV-2 virus or anti-spike protein antibodies do not address the question whether longer lasting cellular immunity is mounted in most individuals. Using an activation marker flow cytometric immunoassay (SARS-CoV-2 lymphocytes analysis), we showed that both CD4+/CD8+ T cell and B cell activation differ between naïve and infected individuals up to 11 months after infection. On the basis of the specificity of this diagnostic tool for detecting both SARS-CoV-2-experienced T and B cells, we propose that this assay could benefit immunocompromised individuals who are unable to mount sustained antibody responses, by determining cellular immunity as possible partial protection, and for studying immune correlates of protection - thereby increasing knowledge of COVID-19 in a wider range of patient groups.

Anaphylaxis to drugs: Overcoming mast cell unresponsiveness by fake antigens.

Our understanding of IgE-mediated drug allergy relies on the hapten concept, which is well established in inducing adaptive reactions of the immune system to small molecules like drugs. The role of hapten-carrier adducts in re-challenge reactions leading to mast cell degranulation and anaphylaxis is unclear. Based on clinical observations, the speed of adduct formation, skin and in vitro tests to inert drug molecules, a different explanation of IgE-mediated reactions to drugs is proposed: These are (a) A natural role of reduced mast cell (MC) reactivity in developing IgE-mediated reactions to drugs. This MC unresponsiveness is antigen-specific and covers the serum drug concentrations, but allows reactivity to locally higher concentrations. (b) Some non-covalent drug-protein complexes rely on rather affine bindings and have a similar appearance as covalent hapten-protein adducts. Such drug-protein complexes represent so-called "fake antigens," as they are unable to induce immunity, but may react with and cross-link preformed drug-specific IgE. As they are formed very rapidly and in high concentrations, they may cause fulminant MC degranulation and anaphylaxis. (c) The generation of covalent hapten-protein adducts requires hours, either because the formation of covalent bonds requires time or because first a metabolic step for forming a reactive metabolite is required. This slow process of stable adduct formation has the advantage that it may give time to desensitize mast cells, even in already sensitized individuals. The consequences of this new interpretation of IgE-mediated reactions to drugs are potentially wide-reaching for IgE-mediated drug allergy but also allergy in general.

Drug-related relapses in drug reaction with eosinophilia and systemic symptoms (DRESS).

BACKGROUND: A drug reaction with eosinophilia and systemic symptoms (DRESS) is a severe T cell mediated hypersensitivity reaction. Relapses of symptoms in the recovery phase are frequent and linked to the reduction of the corticosteroid treatment, to viral reactivations or to the exposure to new drugs. Here, we analyzed, how often the exposure to new drugs leads to new sensitization or drug-related relapses without detectable sensitization. METHODS: 46 patients with DRESS treated in the allergy division of the Inselspital, Bern University Hospital, were retrospectively assessed. Drug-related relapses were analyzed in terms of frequency and whether a possible sensitization evaluated by skin tests and/or lymphocyte transformation tests (LTT) to the new drugs was detectable. Furthermore, drug tolerance was evaluated in a subset of patients. RESULTS: 56 relapses were observed in 27 of 46 patients with DRESS (58.7%). 33 (58.9%) of these relapses were associated with the use of new drugs, 30 drug-related relapses were evaluated by patch test and/or lymphocyte transformation test. In 8/30 (26.7%) drug-related relapses, a sensitization to the new drug was demonstrated, suggesting the emergence of a multiple drug hypersensitivity syndrome (MDH). 14 patients experienced 22 drug-related relapses without any detectable sensitization and only 1/6 patients developed new symptoms upon reexposure. CONCLUSION: Patients with DRESS frequently suffered from drug related relapses. Half of the patients with drug-related relapses developed a MDH with proven sensitizations not only to the DRESS inducing drugs, but also to newly applied drugs. When not sensitized, drugs involved in drug related relapses could be reintroduced, if needed. Here, we propose a procedure for drug testing and future management of drug-related relapses in DRESS.

IgE-mediated chlorhexidine allergy-Cross-reactivity with other biguanide disinfectants.

BACKGROUND: Chlorhexidine (CHX) is a widely utilized disinfectant that can cause IgE-mediated urticaria/anaphylaxis. The cross-reactivity of patients with IgE-mediated CHX allergy with other disinfectants, which share structural similarities with CHX like polyhexanide (polyhexamethylene biguanide; PHMB), alexidine (ALX), or octenidine (OCT), is unknown. METHODS: Forty-four patients with anaphylaxis or urticaria upon CHX exposure and positive skin prick test (SPT) and/or positive CHX ImmunoCAP test (Phadia TFS, Uppsala, Sweden) were recruited. IgE to the biguanide and/or hexamethylene structure was investigated with PHMB ImmunoCAP (n = 32) and by basophil activation tests (BAT) with CHX and ALX (n = 37). Inhibition tests of CHX and PHMB ImmunoCAPs by CHX, ALX, PHMB, and OCT were performed. RESULTS: IgE reactivity to PHMB as surrogate marker for biguanide/hexamethylene reactivity was detected in 5/32 sera. Seven of 37 patients showed a positive BAT with ALX, but only under optimized conditions. Binding to CHX ImmunoCAP was inhibited by ALX in 1/32 sera, and binding to PHMB was blocked by ALX (1/5) and by OCT in another (1/5). In SPT, 9/10 patients were positive for CHX and 3 of them with ALX (only at highest concentration at 5 mg/mL). A further patient reacted primarily with OCT and showed IgE cross-reactivity with CHX, ALX, and PHMB. CONCLUSION: The IgE response to CHX seems polyclonal. The chloroguanide ending of CHX is the main epitope for the IgE and is suitable as screening assay to detect CHX reactivity. IgE-reactivities with the biguanide or hexamethylene components of other disinfectants (ALX, PHMB) can be detected by SPT, PHMB ImmunoCAP, and ALX-BAT in 15%-33% of CHX-allergic patients.

Efficacy of Omalizumab in Mastocytosis: Allusive Indication Obtained from a Prospective, Double-Blind, Multicenter Study (XOLMA Study).

BACKGROUND: Patients with mastocytosis often suffer from a variety of symptoms caused by mast cell mediators where treatments remain difficult, showing various success rates. Omalizumab, a monoclonal anti-IgE antibody, has been postulated to have a positive impact on mastocytosis-associated symptoms such as flush, vertigo, gastrointestinal problems, or anaphylaxis. OBJECTIVE: To investigate the efficacy and safety of omalizumab in systemic mastocytosis. METHODS: Patients with histologically proven mastocytosis were investigated in a multicenter prospective double-blind placebo-controlled trial to receive either omalizumab or placebo, dosed according to IgE and body weight. The primary endpoint was change in the AFIRMM activity score after 6 months of treatment. Different laboratory parameters were analyzed. RESULTS: Sixteen patients were analyzed: 7 to omalizumab and 9 to placebo (mean age 47.7 ± 13.8 vs. 45.4 ± 8.8 years; 66.6 vs. 85.7% were female; mean disease duration 10.0 ± 5.1 vs. 4.5 ± 2.9 years, respectively). After 6 months the median AFIRMM score decreased 50% from 52.0 to 26.0 in the omalizumab group versus 104.0-102.0 in the placebo group (p = 0.286); however, the difference was not significant (p = 0.941). Secondary endpoints, including the number of allergic reactions, changes in major complaints, wheal-and-flare reaction due to mechanical irritation (Darier's sign), and frequency of the use of mastocytosis-specific drugs improved in the omalizumab group, but not significantly. Adverse events like urticaria, bronchospasm, and anaphylactic shock showed no significant difference between the groups. No severe adverse events occurred. FcεRI (Fc-epsilon receptor) expression on basophils decreased after receiving omalizumab versus placebo. CONCLUSION: Omalizumab was safe and showed a tendency to improve mastocytosis-related symptoms, in particular diarrhea, dizziness, flush, and anaphylactic reactions, including the AFIRMM score and secondary endpoints; however, the difference was not significant. Due to the small study size and difference at baseline between the study groups, further studies are required to confirm our findings.

Immune pathomechanism and classification of drug hypersensitivity.

Drug hypersensitivity reactions (DHR) are based on distinct mechanisms and are clinically heterogeneous. Taking into account that also off-target activities of drugs may lead to stimulations of immune or inflammatory cells, three forms of DHR were discriminated: the allergic-immune mechanism relies on the covalent binding of drugs/chemicals to proteins, which thereby form new antigens, to which a humoural and/or cellular immune response can develop. In IgE-mediated drug allergies, a possible tolerance mechanism to the drug during sensitization and the need of a covalent hapten-carrier link for initiation, but not for elicitation of IgE-mediated reactions is discussed. The p-i ("pharmacological interaction with immune receptor") concept represents an off-target activity of drugs with immune receptors (HLA or TCR), which can result in unorthodox, alloimmune-like stimulations of T cells. Some of these p-i stimulations occur only in carriers of certain HLA alleles and can result in clinically severe reactions. The third form of DHR ("pseudo-allergy") is represented by drug interactions with receptors or enzymes of inflammatory cells, which may lead to their direct activation or enhanced levels of inflammatory products. Specific IgE or T cells are not involved. This classification is based on the action of drugs and is clinically useful, as it can explain differences in sensitizations, unusual clinical symptoms, dependence on drug concentrations, predictability and immunological and pharmacological cross-reactivities in DHR.

Controversies in drug allergy: In vitro testing.

Despite their low frequency, drug hypersensitivity reactions (DHRs) can be serious and result in lifelong sequelae. The diagnosis is critical to avert future reactions and should identify the culprit drug or drugs and safe alternatives. However, making the diagnosis can be complex and challenging. Reliable in vitro tests can offer the potential to improve a diagnosis of DHR and influence medical decision making. Importantly, in vitro testing is frequently not performed as a test in isolation but rather as a component of a diagnostic algorithm along with additional tests. There are several in vitro approaches for the different endotypes of DHRs. However, only few are available for routine diagnosis, and many are restricted to research laboratories. In vitro tests exhibit varying sensitivity and specificity depending on the drug involved and the clinical phenotype. In vitro tests can complement skin tests, especially in patients with negative or equivocal skin test responses inconsistent with the clinical presentation and in severe reactions in which drug provocation tests are contraindicated. The main unmet need for many in vitro tests for the diagnosis of DHRs is validation in larger studies with standardized controls that could harmonize diagnostic management between the United States, European Union, and other regions of the world.

Multiple Drug Hypersensitivity.

Multiple drug hypersensitivity (MDH) is a syndrome that develops as a consequence of massive T-cell stimulations and is characterized by long-lasting drug hypersensitivity reactions (DHR) to different drugs. The initial symptoms are mostly severe exanthems or drug rash with eosinophilia and systemic symptoms (DRESS). Subsequent symptoms due to another drug often appear in the following weeks, overlapping with the first DHR, or months to years later after resolution of the initial presentation. The second DHR includes exanthema, erythroderma, DRESS, Stevens-Johnson syndrome/toxic epidermal necrolysis (SJS/TEN), hepatitis, and agranulocytosis. The eliciting drugs can be identified by positive skin or in vitro tests. The drugs involved in starting the MDH are the same as for DRESS, and they are usually given in rather high doses. Fixed drug combination therapies like sulfamethoxazole/trimethoprim or piperacillin/tazobactam are frequently involved in MDH, and 30-40% of patients with severe DHR to combination therapy show T-cell reactions to both components. The drug-induced T-cell stimulation appears to be due to the p-i mechanism. Importantly, a permanent T-cell activation characterized by PD-1+/CD38+ expression on CD4+/CD25low T cells can be found in the circulation of patients with MDH for many years. In conclusion, MDH is a drug-elicited syndrome characterized by a long-lasting hyperresponsiveness to multiple, structurally unrelated drugs with clinically diverse symptoms.

Cardiac involvement in DRESS syndrome.

OBJECTIVE: Cardiac involvement in drug rash with eosinophilia and systemic symptoms (DRESS) syndrome varies considerably between 4% and 21%. Here we present our case and review literatures for its diagnosis and management. An algorithm for diagnosis of cardiac involvement in DRESS syndrome is proposed in this article. DATA SOURCES: Data regarding DRESS-associated myocarditis and eosinophilic myocarditis were gather primarily from MEDLINE database. RESULTS: DRESS syndrome is a hypersensitivity reaction which is due to massive T cell stimulation resulting in cytotoxicity and eosinophil activation and recruitment. It is characterized by fever, morbilliform rash, and various systemic symptoms, in particular hepatitis. Hypersensitivity myocarditis (acute eosinophilic myocarditis) which is typically related to a drug reaction can lead to acute necrotizing eosinophilic myocarditis, cardiac thrombosis and fibrotic stage. Cardiac symptoms range from no symptoms to cardiogenic shock. Diagnosis is based on history, clinical findings, cardiac biomarkers and cardiac imaging techniques. Endomyocardial biopsy is done in a minority of patients for definite diagnosis. If suspected, drug discontinuation and suppression of immune reactions are the first therapies. Corticosteroids are the cornerstone of systemic treatments and should be initiated at the time of diagnosis of DRESS syndrome. Additional therapy and ventricular assist devices could be considered in refractory cases. CONCLUSIONS: According to its high morbidity and mortality, patients with DRESS syndrome should be carefully monitored or screened for cardiac involvement. Multidisciplinary care is important for a successful treatment outcome.

Classification of Drug Hypersensitivity into Allergic, p-i, and Pseudo-Allergic Forms.

Drug hypersensitivity reactions (DHR) are clinically and functionally heterogeneous. Different subclassifications based on timing of symptom appearance or type of immune mechanism have been proposed. Here, we show that the mode of action of drugs leading to immune/inflammatory cell stimulation is a further decisive factor in understanding and managing DHR. Three mechanisms can be delineated: (a) some drugs have or gain the ability to bind covalently to proteins, form new antigens, and thus elicit immune reactions to hapten-carrier complexes (allergic/immune reaction); (b) a substantial part of immune-mediated DHR is due to a typical off-target activity of drugs on immune receptors like HLA and TCR (pharmacological interaction with immune receptors, p-i reactions); such p-i reactions are linked to severe DHR; and (c) symptoms of DHR can also appear if the drug stimulates or inhibits receptors or enzymes of inflammatory cells (pseudo-allergy). These three distinct ways of stimulations of immune or inflammatory cells differ substantially in clinical manifestations, time of appearance, dose dependence, predictability, and cross-reactivity, and thus need to be differentiated.

T-cell-mediated drug hypersensitivity: immune mechanisms and their clinical relevance.

T-cell-mediated drug hypersensitivity represents a significant proportion of immune mediated drug hypersensitivity reactions. In the recent years, there has been an increase in understanding the immune mechanisms behind T-cell-mediated drug hypersensitivity. According to hapten mechanism, drug specific T-cell response is stimulated by drug-protein conjugate presented on major histocompatibility complex (MHC) as it is presented as a new antigenic determinant. On the other hand, p-i concept suggests that a drug can stimulate T cells via noncovalent direct interaction with T-cell receptor and/or peptide-MHC. The drug binding site is quite variable and this leads to several different mechanisms within p-i concept. Altered peptide repertoire can be regarded as an 'atypical' subset of p-i concept since the mode of the drug binding and the binding site are essentially identical to p-i concept. However, the intracellular binding of abacavir to HLA-B(*)57:01 additionally results in alteration in peptide repertoire. Furthermore the T-cell response to altered peptide repertoire model is only shown for abacavir and HLA-B(*)57:01 and therefore it may not be generalised to other drug hypersensitivity. Danger hypothesis has been postulated to play an important role in drug hypersensitivity by providing signal 2 but its experimental data is lacking at this point in time. Furthermore, the recently described allo-immune response suggests that danger signal may be unnecessary. Finally, in view of these new understanding, the classification and the definition of type B adverse drug reaction should be revised.

Drug Hypersensitivity: How Drugs Stimulate T Cells via Pharmacological Interaction with Immune Receptors.

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.

Report from the National Institute of Allergy and Infectious Diseases workshop on drug allergy.

Allergic reactions to drugs are a serious public health concern. In 2013, the Division of Allergy, Immunology, and Transplantation of the National Institute of Allergy and Infectious Diseases sponsored a workshop on drug allergy. International experts in the field of drug allergy with backgrounds in allergy, immunology, infectious diseases, dermatology, clinical pharmacology, and pharmacogenomics discussed the current state of drug allergy research. These experts were joined by representatives from several National Institutes of Health institutes and the US Food and Drug Administration. The participants identified important advances that make new research directions feasible and made suggestions for research priorities and for development of infrastructure to advance our knowledge of the mechanisms, diagnosis, management, and prevention of drug allergy. The workshop summary and recommendations are presented herein.

Abacavir induced T cell reactivity from drug naïve individuals shares features of allo-immune responses.

Abacavir hypersensitivity is a severe hypersensitivity reaction which occurs exclusively in carriers of the HLA-B*57∶01 allele. In vitro culture of PBMC with abacavir results in the outgrowth of abacavir-reacting CD8+ T cells, which release IFNγ and are cytotoxic. How this immune response is induced and what is recognized by these T cells is still a matter of debate. We analyzed the conditions required to develop an abacavir-dependent T cell response in vitro. The abacavir reactivity was independent of co-stimulatory signals, as neither DC maturation nor release of inflammatory cytokines were observed upon abacavir exposure. Abacavir induced T cells arose in the absence of professional APC and stemmed from naïve and memory compartments. These features are reminiscent of allo-reactivity. Screening for allo-reactivity revealed that about 5% of generated T cell clones (n = 136) from three donors were allo-reactive exclusively to the related HLA-B*58∶01. The addition of peptides which can bind to the HLA-B*57∶01-abacavir complex and to HLA-B*58∶01 during the induction phase increased the proportion of HLA-B*58∶01 allo-reactive T cell clones from 5% to 42%. In conclusion, abacavir can alter the HLA-B*57∶01-peptide complex in a way that mimics an allo-allele ('altered self-allele') and create the potential for robust T cell responses.