3.3 CD23 - structure and function on B cells
The low-affinity IgE-receptor FcεRII, also known as CD23, is a single
chain type II integral membrane protein of 45kD and belongs to the
C-type (Calcium-dependent) lectin superfamily 3,48-50.
The membrane-bound CD23 consists of three lectin ”head” domains spaced
from the membrane by a three α-helical coiled-coil ”stalk”. The lectin
head domains of CD23 in the human form contain the C-terminal tail
sequence. The stalk region is susceptible to proteolytic cleavage. Adam
10, a desintegrin and a metalloproteinase has been shown to release
soluble CD23 (sCD23). This proteolytic cleavage results in trimeric
fragments of CD23 (37kD) containing the stalk or monomeric fragments
(25kD, 16kD) lacking the stalk51. CD23 recognises the
protein rather than the carbohydrate moiety of IgE. A single lectin head
fragment can bind to the IgE-Fc portion with an affinity of ka
=106-107 M-1 while the avidity of
the trimeric CD23 to bind IgE-Fc results in the overall high-affinity
binding (Ka = 108 -109 M-1)51,52. Two isoforms of CD23 which differ by seven
(CD23a) or six amino acids (CD23b) have been defined. CD23a is
constitutively expressed on antigen-activated B cells, and IL-4 induces
CD23b expression in several cell types, including B cells and epithelial
cells3,53,54.