LAUSR

Antigen combining sites of the camelid heavy chain antibody variable domain (VHH) are constructed by three complementarity determining regions (CDR1, CDR2, and CDR3). We prepared cDNA using mRNA extracted from peripheral lymphocytes of alpacas that had been nonimmunized or immunized with human serum albumin (HSA). The VHH gene fragments encoding the amino-terminal half-containing CDR1 as well as CDR2 and the carboxyl-terminal half-containing CDR3 were amplified independently by PCR, and then full-length VHH gene fragments were generated by overlap extension PCR and cloned into the phagemid vector. This protocol, referred to as CDR shuffling, allowed us to construct an alpaca VHH phage display library possessing repertoires different from those naturally occurring in animals. We asked, first, whether this library was able to provide the functional VHH fragments against HSA, an immunized antigen, and obtained 29 anti-HSA VHH clones, 41% possessed KD values of lower than 10 -8 M, 5 of which had KD values of 10 -10 M. We also obtained VHH clones against nonimmunized protein antigens such as cardiac troponin T and I, Ebola virus glycoprotein 1, and human immunoglobulin G by biopanning. We compared the amino acid sequences and affinities and found that 43% of VHHs had KD values of less than 10 -8 M, although those having KD values of 10 -10 M were unavailable. These results suggested that the CDR-shuffled VHH phage display library could potentially provide VHHs against nonimmunized protein antigens with similar levels of affinities to those against immunized antigens.