Postnatal liver B cell precursors contribute to the establishment of a mature B cell pool in secondary lymphoid organs in mice.

The process in which the neonatal liver reorganizes its structure and functions to transit between the different fetal and adult periods is poorly explored in the literature. We know that there is a significant change in the cellular immune profile of the liver throughout this postnatal development, where the predominance of myeloid cells in neonates is replaced by lymphoid cells in adults. B lymphocytes are among these cells that change in number. But, despite their great importance in many processes, there are few studies on B cells in the hepatic environment and even fewer studies on B cells in the neonatal liver. For these reasons, we asked ourselves: how does the liver B cell population change in neonates until it is established in the mature adult organ? To answer this question, mice in the fetal, neonatal, and adult stages of development were studied. We observed by intravital microscopy that B cells are located within the hepatic sinusoids in neonates and adults and seem to patrol the vasculature. One-week-old neonates also showed clusters of cells in the parenchyma. In neonates, larger, more active B cells with major morphological alterations predominate. On the other hand, in adults, cells with a classic rounded morphology and less movement predominate. Through flow cytometry, we observed that different subpopulations of B cells are present in the development phases. Precursor cells still predominate in the liver up to two weeks after birth and the profile of B cell subpopulations as characterized in adults are established in the organs only in the fourth week. B cell establishment is not yet complete at birth and continues into the neonatal period. In this context, the neonatal liver still plays an important role with potential in the formation of mature B cells that can migrate to colonize other organs. Finally, the immaturity profile of B cells in the neonatal phase is reflected in the reduced production of antibodies in neonates, which is supplied by the supply of IgG in breast milk.