Dissecting infant and maternal antibody repertoires exposes the early onset of infant humoral immunity.

Objectives: The early development of humoral immunity is important for long-term protection of the newborn. Here, we set out to discern these infant-produced antibodies from the vast background of maternal antibodies, which is challenging but essential to shed light on the infant-produced repertoire. Methods: Using IgA1 and IgG1 antibody clonal repertoire analysis by mass spectrometry, we compared matched maternal serum, maternal milk, and infant serum samples, both at birth (T1) and at 7-11 weeks after delivery (T2) in four mother-infant dyads. Results: We observed for both IgA1 and IgG1 unique infant-produced antibody repertoires at T2. For IgA1 at T2, no substantial clonal overlap was found between infant serum and breastmilk. The serum IgG1 clonal repertoires were highly alike at birth for mother and infant, but at T2, the contribution of the maternal clonal population in the infant had been drastically reduced, and a large portion of the T2 IgG1 repertoire originated from the infant. Conclusions: Newborns produce their own antibody repertoires as early as a few months after birth. From this small study, no convincing evidence is found for transfer of milk antibodies into the infant circulation.