Structural studies of human CD23 and its complexes

Abstract

IgE plays a central role in the pathogenesis of immediate hypersensitivity reactions
through interacting with its receptors, in particular the high affinity receptor FcεRI. The
low-affinity IgE receptor, CD23, affects IgE-dependent immune responses by regulating
the synthesis of IgE, facilitating allergen presentation to the immune system, and
influencing the activation and differentiation of B- and T-cells. Surprisingly, CD23 is
different from other Ig receptors and belongs to the C-type (calcium-dependent) lectin
family. Calcium binding to CD23 affects IgE binding to CD23, but previous NMR and
crystal structures of CD23 gave conflicting results concerning the calcium binding sites.
Investigation of calcium binding site(s) in CD23 and its complexes may assist CD23-
targeted drug design.
DerCD23 (a fragment of human CD23 that consists of the lectin domain and part of the
C-terminal tail) and derCD23 mutants designed to remove each of the two potential
calcium binding sites, were expressed and purified. The crystal structures of Ca2+-bound
wild type derCD23, the complex with the Fcε3-4 sub-fragment of IgE-Fc, and four
putative calcium binding site mutants of derCD23 were solved. Binding affinities of
derCD23 and its mutants for calcium and for IgE-Fc were measured with ITC and SPR.
The results indicate a loop of derCD23 (loop 4) is stabilized upon calcium binding to
“site 2” and thereby contributes to the increased binding affinity for IgE. In addition, a
residue (D258) in the non-conserved “site 1” is observed to bind IgE directly in the
Ca2+-bound derCD23/Fcε3-4 complex. Thus, Ca2+ bound to site 2 in CD23 stabilizes
loop 4 for IgE binding, whereas site 1 has evolved to bind IgE directly.
Clinical studies of IDEC152 (also known as Lumiliximab), a primatized IgG1, anti-
CD23 monoclonal antibody (IDEC Pharmaceuticals, San Diego, CA) show positive
clinical effects in patients with allergic asthma and chronic lymphocytic leukemia. The
Fab fragment of IDEC152 was prepared by enzymatic digestion from IDEC152 and
crystals of the complex of derCD23 with IDEC152-Fab were grown, which diffracted to
2.4 Å resolution.

Date of Award	2012
Original language	English
Awarding Institution	King's College London
Supervisor	Brian Sutton (Supervisor) & Andrew Beavil (Supervisor)