Antigenic changes in human albumin caused by reactivity with the occupational allergen diphenylmethane diisocyanate.

Diphenylmethane diisocyanate (MDI), the chemical commonly used as a cross-linking agent in commercial polyurethane production, is a well-recognized cause of asthma. Reaction products between MDI and "self" proteins are hypothesized to act as antigens capable of inducing airway inflammation and asthma; however, such MDI antigens remain incompletely understood. We used a variety of analytical methods to characterize the range of MDI-albumin reaction products that form under physiological conditions. Sites of MDI conjugation on antigenic MDI-albumin products, as defined by serum immunoglobulin G (IgG) from MDI-exposed workers, were determined by high-performance liquid chromatography (HPLC) followed by tandem mass spectrometry (MS/MS). The data identified 14 MDI conjugation sites (12 lysines and 2 asparagines) on human albumin and highlight reaction specificity for the second lysine in dilysine (KK) motifs, and this may be a common characteristic of "immune-sensitizing" chemicals. Several of the MDI conjugation sites are not conserved in albumin from other species, and this may suggest species differences in epitope specificity for self protein (albumin)-isocyanate conjugates. The study also describes new applications of contemporary proteomic methodology for characterizing and standardizing MDI-albumin conjugates destined for use in clinical research.