Reiterating the Epitope Specificity of Prion-specific mAb 3F4

Abstract

I wish to commend Zou et al. (1) for an innovative study further characterizing a monoclonal antibody to human PrP (mAb 3F4) developed in our laboratory over 20 years ago. However, I must comment on the data interpretations and conclusions drawn in that study. The authors fail to reference our 1999 study (2) in which both epitope mapping using pepscan analysis and species specificity studies were performed. Pepscan analysis using 12-mer linear peptides identified the epitope of 3F4 to be TNMKHM, amino acids 107–112 of human PrP. Species-specific immunoreactivity confirmed this epitope. Species studies indicate that 3F4 immunoreacts with human, hamster, and feline PrP (TNMKHM) but not with rabbit PrP (TSMKHV), mouse or rat PrP (TNLKHV), or bovine or elk PrP (TNMKHV). Clearly these studies demonstrate the importance of both the 109 and 112 methionines relative to the 3F4 epitope. Close examination of the results of Zou et al. (1) fails to change our interpretation of our finding. The binding constant (Kd) shown for 3F4 is 100-fold greater for human compared to elk PrP (Kd of 1.3 for human PrP and approximately 130 for elk PrP). The Western blot sensitivity data also shown for 3F4 are at least 16-fold greater for human compared to elk PrP (1 ng for human PrP versus 16 ng for elk PrP). Zou et al. coined terminology to explain this difference; the 3F4 reactivity to human PrP is “species-specific” while the 3F4 reactivity to elk PrP is “species-preferred.” The authors also suggest changing the epitope specificity of 3F4 from its currently accepted TNMKHM (as described above) to the newly proposed KTNMK. When performing diagnostic testing where sensitivity and specificity are critical, one would hope that a “species-specific” antibody, like 3F4 for human PrP, would be the tool of choice. That sensitivity would be lost if 3F4 is used to diagnose prion disease in species other than human, hamster, or feline. Furthermore, the results of Zou et al. (1) do not warrant an alteration in the epitope specificity of our 3F4 monoclonal antibody.