Structural and functional studies with antibodies to the integrin β2 subunit

Abstract

To establish a structure and function map of the β2 integrin subunit, we mapped the epitopes of a panel of β2 monoclonal antibodies including function-blocking, non-blocking, and activating antibodies using human/mouse β2 subunit chimeras. Activating antibodies recognize the C-terminal half of the cysteine-rich region, residues 522-612. Antibodies that do not affect ligand binding map to residues 1-98 and residues 344-521. Monoclonal antibodies to epitopes within a predicted I-like domain (residues 104-341) strongly inhibit LFA-1-dependent adhesion. These function-blocking monoclonal antibodies were mapped to specific residues with human → mouse knock-out or mouse → human knock-in mutations. Combinatorial epitopes involving residues distant in the sequence provide support for a specific alignment between the β-subunit and I domains that was used to construct a three-dimensional model. Antigenic residues 133,332, and 339 are on the first and last predicted α-helices of the I-like domain, which are adjacent on its 'front'. Other antigenic residues in β2 and in other integrin β subunits are present on the front. No antigenic residues are present on the 'back' of the domain, which is predicted to be in an interface with other domains, such as the α subunit β-propeller domain. Most mutations in the β2 subunit in leukocyte adhesion deficiency are predicted to be buried in the β2 subunit I-like domain. Two long insertions are present relative to α-subunit I-domains. One is tied down to the back of the I-like domain by a disulfide bond. The other corresponds to the 'specificity-determining loop' defined in β1 and β3 integrins and contains the antigenic residue Glu175 in a disulfide-bonded loop located near the 'top' of the domain.