Cellular mechanisms of restricted immunoglobulin formation in the human neonate.

The functional capacity of human neonatal B lymphocytes has been investigated by in vitro methods using T lymphocyte-dependent (pokeweek mitogen, PWM) and -independent (Epstein-Barr virus, EBV) polyclonal B cell activators. B cell activation of single cells was detected by class-specific immunoglobulin (Ig) secretion using a reversed hemolytic plaque assay. It was found that neonatal B cells were triggered to secretion of IgM by EBV, with a magnitude comparable to adult levels, but that, in contrast to B cells from adults, they did not secret IgG. Cord lymphocytes did not secret Ig although they displayed a sizable DNA synthetic response to PWM. Using cell separation and culture experiments, it was shown that (allogeneic) adult T lymphocytes could restore cord B cell responsiveness to PWM and that cord T lymphocytes could not cooperate with adult B cells. In addition to this immaturity of cord T helper function for antibody synthesis, we found cells in the cord T cell-enriched fraction which inhibited the polyclonal response of adult lymphocytes to both PWM and EBV. These lymphocytes suppressed adult B lymphocytes directly but appeared ineffective against neonatal B lymphocytes themselves. The nature of these suppressing cells and their possible role in the fetal/maternal relationship are a matter of speculation.