The antibody-binding site, through which an antibody binds to its epitope, is a complex structure formed by the folding together of six complementarity-determining regions (CDRs). However, certain peptides derived from CDR sequences retain antibody specificity and function; these are know as microantibodies (MicroAbs). For example, the F58 MicroAb is a 17 residue, cyclized peptide (CDLIYYDYEEDYYFDYC) derived from CDR-H3 of F58, an IgG1 specific for the gp120 envelope glycoprotein of human immunodeficiency virus type 1 (HIV-1). Both MicroAb and IgG recognize the same epitope in the V3 loop and, despite its small size, the MicroAb neutralizes the infectivity of HIV-1 IIIB only 32-fold less efficiently on a molar basis. The advantage of MicroAbs is that their small size facilitates structure-function analysis. Here, the F58 MicroAb was investigated using alanine scanning, mass spectroscopy and surface plasmon resonance. Neutralization of infectious IIIB was generally more sensitive to alanine substitution than binding to soluble gp120. There appeared to be a division of function within the MicroAb, with some residues involved in antigen binding (alanine substitution of 11D, 12Y or 13Y abrogated both binding and neutralization), whereas others were concerned solely with neutralization (substitution of 3L, 8Y or 14F abrogated neutralization, but not binding). The MicroAb is predominantly b-sheet and has strong conformational constraints that are probably essential for activity. The MicroAb and soluble gp120 formed a 1 : 1 complex, with an association rate that was threefold greater than that with IgG and a faster dissociation rate. Its equilibrium dissociation constant is 37?5-fold greater than that of IgG, in line with neutralization data. This study demonstrates how MicroAbs can make a useful contribution to the understanding of antigen-antibody interactions.
INTRODUCTIONAn antibody binds to its cognate epitope through its paratope, a domain formed from the six complementaritydetermining regions (CDRs) situated within the variable portions of the immunoglobulin light and heavy chains. Thus, a bivalently binding IgG can attach to antigen by the combined action of 12 CDRs. A microantibody (MicroAb) is the minimum recognition unit of an antibody and comprises a peptide derived from usually just one CDR. Its activity will depend on both its sequence and conformation. Some antiviral MicroAbs retain the virus binding and infectivity neutralizing activities of the parent antibody. The small size of MicroAbs should enable residues that are key to virus binding and neutralization activities to be easily identified. MicroAbs also have a potential advantage over IgG as therapeutic antivirals as they are less immunogenic and should be useful in situations where larger antibodies cannot easily penetrate.MicroAbs have proved difficult to identify. This may be because the potential MicroAb does not mimic the conformation of the CDR sufficiently well, because effective IgG binding requires more than one CDR or because the mass of ...