High affinity antibodies to the Protective Antigen (PA) component of the Bacillus anthracis toxin are in late stage testing for prophylaxis and therapy against anthrax intoxication. However, recent studies (Nowakowski, A et al.) indicate that treatment with multiple antibodies simultaneously can confer a synergistic protection to toxin challenge. To explore multi-antibody therapeutic approaches, antibodies toward the protective antigen (PA) component of the B. anthracis exotoxin were isolated from immune libraries using two variations of the E. coli based protein library screening technology, Anchored Periplasmic Expression (APEx) (Harvey, B.R. et al., Mazor, Y. et al.). Full length antibodies and scFv antibodies with nanomolar affinities were isolated as early as the second round of flow cytometric screening. To compare the systems, the isolated scFvs were converted to full-length antibodies and the isolated full-length antibodies were converted to scFvs and compared using APEx. In general, all the full-length antibodies gave higher relative FACS signals as compared to their scFv counterparts. Further, conversion from full-length antibodies to soluble scFvs often resulted in a significant decrease in affinity. These results indicate the E¬-clonal system is better suited than the original APEx system for the isolation of a diverse array of antibodies with a wide affinity range.

Nowakowski, A. et al. (2002) Proceedings of the National Academy of Sciences of USA 99(17): 11346

Harvey, B.R. et al. (2004) Proceedings of the National Academy of Sciences of USA 101(25): 9193

Mazor, Y, et al. (2007) Nature Biotechnology 25(5): 563

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