Two-state selection of conformation-specific antibodies.

We present a general strategy for identification of conformation-specific antibodies using phage display. Different covalent probes were used to trap caspase-1 into 2 alternative conformations, termed the on-form and the off-form. These conformation-trapped forms of the protease were used as antigens in alternating rounds of selection and antiselection for antibody antigen-binding fragments (Fabs) displayed on phage. After affinity maturation, 2 Fabs were isolated with K(D) values ranging from 2 to 5 nM, and each bound to their cognate conformer 20- to 500-fold more tightly than their noncognate conformer. Kinetic analysis of the Fabs indicated that binding was conformation dependent, and that the wild-type caspase-1 sits much closer to the off-form than the on-form. Bivalent IgG forms of the Fabs were used to localize the different states in cells and revealed the activated caspase-1 is concentrated in a central structure in the cytosol, similar to what has been described as the pyroptosome. These studies demonstrate a general strategy for producing conformation-selective antibodies and show their utility for probing the distribution of caspase-1 conformational states in vitro and in cells.