Becoming the bull: Structural insights and engineering strategies for ultralong CDR-H3 paratopes from cattle

Cattle produce a subset of antibodies (Abs) with unique architecture. These immunoglobulins feature an ultralong CDR-H3 (uCDR-H3) composed of up to about 70 amino acids that protrudes from the scaffold and folds into a distinct stalk-and-knob structure. The knob is a disulfide-rich mini-domain that contains all paratope residues for antigen recognition. Due to their vast structural diversity and low molecular weight, cattle-derived knob paratopes have emerged as promising building blocks for engineering next-generation Abs. In this review, we discuss the structural features of uCDR-H3 Abs alongside engineering strategies of cattle-derived knob architectures, including stand-alone knob peptides and knob engraftments to generate bispecific antibodies (bsAbs).