Expression and characterization of recombinant soluble human CD3 molecules: Presentation of antigenic epitopes defined on the native TCR-CD3 complex

Abstract

The TCR-CD3 complex consists of the clonotypic disulfide-linked TCRαβ or TCRδγ heterodimers, and the invariant CD3δ, ε, γ and ζ chains. We generated plasmid constructs expressing the extracellular domains of the CD3δ, ε or γ subunits fused to human IgG1 Fc. Recombinant fusion proteins consisting of individual CD3δ, ε or γ subunits reacted poorly with anti-CD3 mAb including G19-4, BC3, OKT3 and 64.1. Co-expression of the CD3ε-Ig with either the CD3δ-Ig (CD3εδ-Ig) or the CD3γ-Ig (CD3εγ-Ig) resulted in fusion proteins with much increased binding to G19-4. A brief acid treatment of the purified CD3εδ-Ig fusion protein substantially improved its binding to BC3, OKT3 and 64.1. Surface plasmon resonance analysis revealed that the dissociation constants for CD3εδ-Ig and anti-CD3 mAb ranged from 10-8 to 10-9 M. Based on these results, a single-chain (sc) construct encoding the CD3δ chain linked to the CD3ε chain with a flexible linker followed by human IgG1 Fc was expressed. The sc CD3δε-sclg reacted with anti-CD3 mAb without requiring acid treatment. Moreover, anti-CD3 mAb bound CD3εδ-Ig at a higher affinity than CD3εγ-Ig, suggesting potential structural differences between the CD3εδ and CD3εγ subunits. In summary, we report the expression of soluble recombinant CD3 proteins that demonstrate structural characteristics of the native CD3 complex expressed on the T cell surface. These CD3 fusion proteins can be used to further analyze the structure of the TCR-CD3 complex, and to identify molecules that can interfere with TCR-CD3-mediated signal transduction by disrupting the interaction between CD3 and TCR subunits.