ABSTRACT
Granulomatosis with polyangiitis (GPA), formerly Wegener's granulomatosis, is a small- and medium-vessel vasculitis with characteristic cutaneous morphologic presentation and systemic involvement. Most patients have palpable purpura at some point in their disease course, but this is not always the presenting manifestation. This autoimmune disorder can affect a range of organs, with the upper and lower respiratory tract, kidneys, and nervous system being commonly implicated, while gastrointestinal and cardiac involvement is less frequent. This is a 44-year-old female presenting to the emergency department (ED) with polyarthralgia and palpable purpura. Palpable purpura was distributed on the oral palate, elbow, and lower back, and a punch biopsy revealed leukocytoclastic vasculitis (LCV). While this was an atypical distribution for leukocytoclastic vasculitis, the skin biopsy provided the necessary evidence to diagnose GPA. This case characterizes non-specific and atypical signs and symptoms of GPA that all providers should be aware of in order to diagnose the condition early in its disease course.
ABSTRACT
The current model of antigen assembly with major histocompatibility complex (MHC) class I molecules posits that interactions between the tapasin N-terminal immunoglobulin (Ig)-like domain and the MHC class I peptide-binding groove permit tapasin to regulate antigen selection. Much less is known regarding interactions that might involve the tapasin C-terminal Ig-like domain. Additionally, the tapasin transmembrane/cytoplasmic region enables tapasin to bridge the MHC class I molecule to the transporter associated with antigen processing (TAP). In this investigation, we made use of two tapasin mutants to determine the relative contribution of the tapasin C-terminal Ig-like domain and the tapasin transmembrane/cytoplasmic region to the assembly of MHC class I molecules. Deletion of a loop within the tapasin C-terminal Ig-like domain (Δ334-342) prevented tapasin association with the MHC class I molecule K(d). Although tapasin Δ334-342 did not increase the efficiency of K(d) folding, K(d) surface expression was enhanced on cells expressing this mutant relative to tapasin-deficient cells. In contrast to tapasin Δ334-342, a soluble tapasin mutant lacking the transmembrane/cytoplasmic region retained the ability to bind to K(d) molecules, but did not facilitate K(d) surface expression. Furthermore, when soluble tapasin and tapasin Δ334-342 were co-expressed, soluble tapasin had a dominant negative effect on the folding and surface expression of not only K(d), but also D(b) and K(b). In addition, our molecular modeling of the MHC class I-tapasin interface revealed novel potential interactions involving tapasin residues 334-342. Together, these findings demonstrate that the tapasin C-terminal and transmembrane/cytoplasmic regions are critical to tapasin's capacity to associate effectively with the MHC class I molecule.
