Characterization of Monoclonal Antibodies to the αIIbβ3 Integrin on Bovine Platelets

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Abstract
A set of monoclonal antibodies (MoAbs) to leucocyte antigens is an essential tool to identify different cell types and functional membrane molecules involved in immune responses. Since no MoAbs existed to bovine integrins, except against the β2 subfamily, we generated MoAbs to β3 integrin after the immunization of mice with bovine platelets. Two MoAbs, IL‐A164 (IgG2a) and IL‐A166 (IgG1), were selected that reacted specifically with bovine platelets and detected the same membrane molecule. The antigen was a heterodimer of two polypeptide chains of 122 kDa and 95 kDa as resolved by SDS‐PAGE under reducing conditions. Although the Mr of the smaller subunit is identical to that of β2 integrin, pre‐absorption with an antibody to β2 (or CD18) did not remove the bovine antigen. Comparing the molecular masses of the two subunits in reduced and non‐reduced forms showed a pattern that was similar to that of human GPIIb/IIIa (also called αIIbβ3 or CD41a). Reduction of the bovine molecule increased the apparent Mr of the light chain from 76 kDa to 95 kDa, while the heavy subunit changed from 136 kDa to 122 kDa. As with human GPIIb, the decrease in Mr of the α‐subunit is probably a result of a small disulphide‐linked polypeptide, although no additional evidence for this was detected for the bovine integrin. Sequencing of the N‐terminal amino acids of both bovine polypeptides showed identity of the bovine integrin with human GPIIb/IIIa.