Dimeric M315 is transported into mouse and rat milk in a degraded form

The controversial issue of serum to milk transport of IgA in rodents was addressed in experiments that evaluated the molecular integrity and antigen-binding ability of the dimeric IgA (dIgA) recovered in the stomachs of rat and mouse pups suckling dams which had been administered homogeneous, dimeric M315 intravenously (i.v.). Four rat and three mice dams were i.v. administered affinity-purified, 125I-dIgA anti-DNP (M315) of which greater than 83% could bind DNP after iodination, and of which greater than 82% could be captured by solid phase anti-M315. Dams were given the labelled dIgA either 1-day or 8-days postpartum. Twenty-three hours after i.v. administration, the radioactivity of each pup, its stomach contents and an aliquot of serum were analyzed for total radioactivity, anti-DNP activity and for the molecular integrity of the recovered I-125. Data showed that 11-43% of 125I-activity given to the dam could be recovered from the stomach contents and sera of the pups after this time. Immunoassay revealed that less than 2% of the recovered radioactivity could bind DNP, i.e. a loss of 98% of functional antibody. It was calculated from ultracentrifugational analyses that less than 0.7% of the 125I-dIgA had been transported intact to the suckling neonates. Analyses of stomach milk and neonatal sera by sucrose density gradient ultracentrifugation revealed that almost all recovered radioactivity was in the form of low mol. wt fragments. Data fail to support the concept of an active mechanism for the transport of intact IgA from serum to milk in rodents during early or mid-lactation.