Ectodomain shedding and generation of two carboxy-terminal fragments of human complement receptor 2/CD21.

Numerous cell surface proteins are functionally regulated by a proteolytic cleavage event termed 'ectodomain shedding'. The complement receptor 2/CD21 extracellular domain (ectodomain) is constitutively released as a soluble form (sCD21), but its liberation can also be induced by various physiological and pharmacological stimuli. CD21-shedding modulates B cell activation, and sCD21 can activate other immune cells and allows transfer of immune complexes from marginal zone B cells to follicular dendritic cells. Deletion of the cytoplasmic domain of CD21 augments CD21-shedding, while removal of the extracellular membrane-adjacent short consensus repeat 16 abolishes shedding. Carboxy-terminal fragments (CTFs) and intracellular domains (ICDs) result from ectodomain shedding and regulated intramembrane cleavage (RIP) of CTFs, respectively. By modulating gene transcription, CTFs and ICDs can regulate cell function and homeostasis. Here, we demonstrate that two membrane-tethered CD21 CTFs of 8 and 16kDa are constitutively present in human B cells, while only the 8kDa CTF was detectable in murine B cells. Glutathione (GSH) regulates extracellular redox levels and is a known inducer of CD21-shedding. Interestingly, GSH-treatment of B lymphocytes only augmented sCD21 levels, but not CD21-CTF levels. In contrast, B cell activation led to increased CD21-CTF levels, suggesting a functional role for the CD21-CTFs in B cell activation and maintenance of B cell homeostasis.