Humans are daily exposed to chemicals from various sources, including cosmetics, jewelry, clothes, and hair dyes, which can result in the occurrence of contact allergy and subsequent allergic contact dermatitis (ACD), a type IV delayed hypersensitivity reaction. ACD is characterized by inflammation and eczema at the site of exposure, and no definitive cure for this condition has been identified to date, with only symptomatic treatment options involving corticosteroids being available.

The research presented in this thesis is centered around mass spectrometry (MS) strategies aimed at enhancing our comprehension of events that occur during the early stages of the development of contact allergy. Special emphasis is given to characterizing various contact allergens (haptens) and their interactions with endogenous proteins, as these interactions are considered crucial in the initiation of contact allergy. Moreover, the thesis endeavors to explore the activation of prehaptens and prohaptens, which are non-reactive compounds capable of transforming into haptens outside or inside the skin, respectively.

In Paper I, a bottom-up proteomics approach was employed to investigate the adductome of two major blood proteins, human serum albumin (HSA) and hemoglobin (Hb). The study aimed to identify the most reactive sites on these proteins upon exposure to different haptens with varying sensitization potencies. Highly susceptible sites on HSA and Hb were identified as the most likely targets for in vivo modification. This study is the first investigation of the Hb adductome in the context of contact allergy and may contribute to the development of improved diagnostic tools using blood samples. With Hb on focus, Paper II evaluated three different MS-based methods, including bottom-up proteomics, detachment of N-terminal adducts by FIRE, and limited proteolysis (LiP), to determine the most suitable approach for assessing exposure through this protein. The three methods showed different strengths and limitations depending on the nature of the hapten. In Paper III, the research conducted revealed the presence of a hapten-protein conjugate in blood samples mice treated with the synthetic hapten tetramethyl rhodamine isothiocyanate (TRITC) topically. The identified protein was the macrophage migration inhibitory factor (MIF), marking the first instance of such a conjugate being detected in blood samples after topical hapten application. The study also indicated that MIF could potentially be modified by other contact allergens, suggesting its potential as a biomarker for the condition. In Paper IV, contact allergy to propolis, a by-product of honey used in biocosmetics, was investigated. Air oxidation experiments with a model peptide and MS detection, revealed that quinones formed from the oxidation of major propolis components are responsible for adduct formation. The identified adducts are likely the cause of contact allergy to propolis, providing valuable insights into the underlying mechanisms of propolis contact allergy and potential implications for clinical diagnosis. In Paper V, the bioactivation of cinnamic alcohol, a common ingredient in many cosmetic products, was investigated using in vitro systems and a targeted MS approach. Two metabolites, namely pOH-cinnamic alcohol and pOH-cinnamic aldehyde, were identified as of particular interest and their sensitizing potency was evaluated, with the latter categorized as a moderate sensitizer.

In summary, this doctoral thesis employed MS techniques to characterize contact allergens and their protein conjugates, yielding valuable insights into the molecular mechanisms underlying contact allergy development. The findings have potential implications for improving diagnostic tools and strategies for preventing and treating contact allergy.