Early appearance of functional plasma cells (CD138+CD98+) in non-immunized neonate mice

Plasma cells (PCs) are terminally differentiated antibody-secreting cells, derived from activated B-lymphocytes in response to either T-independent or T-dependent antigens. The plasma cell population is scarce in circulation in non-immunized individuals. It is established that neonates are incapable of mounting an efficient immune response due to the immaturity of the immune system. However, this disadvantage is well overcome through the antibodies neonates receive from breastmilk. This implies that neonates will be only protected against antigens the mother had previously encountered. Thus, the child might be potentially susceptible to new antigens. This issue prompted us to seek for the presence of PCs in non-immunized neonate mice. We found a PC population identified as CD138+/CD98+ cells since day one after birth. These PCs were positive for Ki67 and expressed Blimp-1, B220, and CD19, which suggests the populations are plasmablasts and PCs with heterogeneous phenotype. These PCs were also determined to secrete antibodies, although mainly isotype IgM. Altogether, the results indicated that neonate PCs can produce antibodies against antigens they encounter in the first weeks of life, most likely coming from food, colonizing microbiota, or the environment.