Inhibition of HLA antibody cytotoxicity by intravenous immunoglobulin G F(ab′) 2 dimers, monomers, and monovalent F(ab)

Abstract

IVIgG preparations are clinically relevant to sensitized transplant candidates because they inhibit HLA alloantibody in vitro and in vivo. We hypothesized that IVIgG F(ab′) 2 idiotypic-antiidiotypic dimers may possess a greater immunomodulatory capacity when remonomerized than non-dimerizable F(ab′) 2 monomers in IVIgG. We reasoned that when 60–75% of potential antiidiotypic IVIgG monomers fail to bind to IVIgG molecules in a large pool of plasma donors (>10,000), IVIgG monomers may fail to inhibit HLA idiotypic antibodies of sensitized transplant candidates. In the first series of AHG T cell crossmatches, non-fractionated IVIgG F(ab′) 2 was found to inhibit titered HLA antibodies in 13 out of 29 (45%) crossmatch combinations. Crossmatch inhibition was incomplete, i.e., a particular titered HLA antibody specificity was not always inhibited by IVIgG F(ab′) 2 in every HLA antigen-matched target cell crossmatch. Next, the IVIgG F(ab′) 2 product was fractionated into F(ab′) 2 dimers, F(ab′) 2 monomers and Fab monovalent components by size exclusion high pressure liquid chromatography (HPLC) and retested in crossmatches which previously demonstrated inhibition. The percent of crossmatches that were inhibited by HPLC F(ab′) 2 IVIgG fractions in three separate experiments was statistically similar for pH 4.0 remonomerized dimers, 82%; pH 6.0 dimers, 50%; monomers, 64%; and monovalent Fab, 64% ( p = 0.50) . Soluble class I HLA antigen was undetectable in IVIgG F(ab′) 2 by an ELISA assay. In conclusion, IVIgG dimers and monomers appear to have similar immunomodulatory capacities, and separation of whole IVIgG products into dimer and monomer fractions does not appear to be warranted. Further, IVIgG products should be tested for optimal HLA antibody inhibition in vitro prior to in vivo therapy.