Limitations of a hemolytic plaque assay for IgG-anti-IgG rheumatoid factor-producing cells

An attempt has been made to develop a hemolytic plaque assay capable of detecting homophile IgG rheumatoid factor (RF)-producing cells. Anti-immunoglobulin allotype-developing reagents were used to distinguish between target and effector IgG. The hemolytic assay has been used to demonstrate an apparently high level of homophile IgM and IgG RF-producing cells in the spleens and lymph nodes of mice stimulated by LPS. However, it appears that a large proportion of the plaques obtained in these assays are due to an artefact resulting from cross-linking of target and effector molecules by the developing reagents. In the case of IgM RF the artefact depends on the presence of a small contamination of the target IgG by IgM, allowing cross-linking of target and effector IgM by the anti-mu-specific developing reagent. With the IgG RF, cross-reactivity of the rabbit anti-Igh^b allotype-developing serum for the 'wrong' (Igh^a) allotype, normally undetectable, becomes sufficient to be biologically relevant when the developing antibody is complexed by being bound to its target (Igh^b) allotype. Nevertheless anti-allotype reagents may afford an accurate means of detecting homophile IgG RF producing cells using other assay systems.