Reactions among IgM antibodies derived from normal, neonatal mice

Attempting to obtain a representative sample of the "natural antibody" repertoire in the developing immune system, we have derived IgM-secreting hybridomas from 4 normal untreated BALB/c mice of the same litter on day 6 after birth. Partially purified IgM preparations obtained in the supernatants of 70 such clones were each screened in binding assays for reactivity with a panel of 9 IgM antibodies, randomly selected from the same collection. Five of these 9 IgM antibodies were found to react with a considerable number of other IgM in the collection, while the other 4 showed only sporadic reactivity. On the other hand, more than half of the 70 antibodies were found to bind specifically to at least one of these five. With a few exceptions, these reactions showed quantitative levels ranging from 5 to 20% of those observed between either of the two interacting IgM and monoclonal rat anti-mu antibodies. The selectivity of these reactions indicated V-region specificity, which was confirmed by analyzing in some detail the reaction between 2 IgM antibodies isolated from the same mouse.