Temporal variations in the fine specificity of IgM anti‐fluorescyl antibodies

This study compares the fine specificities of the primary and secondary fluorescein (FITC)‐specific immunoglobulin M (IgM) repertoires in BALB/c mouse serum and monoclonal antibodies (MoAb) and has found reproducible, immunization‐dependent differences. FITC and four of its homologues: iodoacetamido fluorescein (IAF), dichlorotriazinyl aminofluorescein (DTAF), substituted rhodamine isothiocyanate (XRITC) and tetramethyl rhodamine isothiocyanate (TRITC), each conjugated to bovine serum albumin (BSA), were used to determine reactivity patterns of serum IgM from mice immunized once or twice with FITC‐haemocyanin (FITC‐Hy), Reactivity patterns were also obtained for 20 IgM MoAb, eight of which were produced by fusions of SP2/0 myeloma cells with splenocytes from mice immunized once (primary) and 12 from mice immunized twice (secondary) with FITC‐Hy. Each MoAb exhibited a unique line specificity pattern, evidence of extensive heterogeneity in the FITC‐specific repertoire. Reactivities of IgM MoAb with certain homologues were found to be more characteristic of either the primary or secondary response. Polyclonal serum IgM also showed reproducible immunization‐dependent variations in fine specificity. Such a pattern could result from idiotypic suppression of primary antibodies, from the expansion of subsets of IgM memory cells utilizing novel genes and/or from somatic mutation absent in primary IgM antibodies.