Antibody-bearing liposomes improve agglutination of latex particles used in clinical diagnostic assays

Ligand-bearing liposomes are used to enhance the agglutination 'signal' of a typical latex assay for the detection of human rheumatoid factor. Heat-denatured IgG, the antigen to which rheumatoid factor binds naturally, was covalently attached to latex spheres. The liposomes were covalently coated with a 'second ligand' which also recognizes rheumatoid factor, anti-human IgM Fab′ fragments. In the present test configuration, rheumatoid factor present in a patient's serum binds to the IgG attached to the latex particles. The liposomes, in turn, bind rapidly to rheumatoid factor-sensitized latex, via the second ligand, promoting the formation of large, clearly visible latex aggregates. When latex spheres bearing the same type and density of second ligand were used to replace the liposomes they failed to improve agglutination, suggesting that multivalent presentation of the second ligand is not sufficient to insure the improvement. These results suggest that fluidity of the liposomal membrane is a requirement for the effect. Sensitivity as well as 'readability' are improved by the liposomes while specificity remains unaffected. The principle of using ligand-bearing liposomes to enhance particle agglutination is applicable to a wide range of other diagnostic assays.