Antibody repertoire development in fetal and neonatal piglets. XXIV. Hypothesis: The ileal Peyer patches (IPP) are the major source of primary, undiversified IgA antibodies in newborn piglets

The ileal Peyers patches (IPP) of newborn germfree (GF) piglets were isolated into blind loops and the piglets colonized with a defined probiotic microflora. After 5 weeks, IgA levels in the intestinal lavage (IL) of loop piglets remained at GF levels and IgM comprised ∼70% while in controls, IgA levels were elevated 5-fold and comprised ∼70% of total Igs. Loop piglets also had reduced serum IgA levels suggesting the source of serum IgA had been interrupted. The isotype profile for loop contents was intermediate between that in the IL of GF and probiotic controls. Surprisingly, colonization alone did not result in repertoire diversification in the IPP. Rather, colonization promoted pronounced proliferation of fully switched IgA+IgM— B cells in the IPP that supply early, non-diversified “natural” SIgA antibodies to the gut lumen and a primary IgA response in serum. •Ileal Peyers patches (IPP) could be isolated in blind loops under germfree conditions.•IPP are responsible for the initial non-diversified secretory IgA (SIgA) in the neonatal gut.•Colonization with a probiotic flora failed to induce repertoire diversification in the IPP.•Despite their isolation, lymphocytes in the IPP of isolated loops were activated.•“Natural” SIgA provides first responder anti-bacterial defense for the neonatal gut.