Modulation of immunoglobulin M and G synthesis by monocytes and T lymphocytes in the newborn infant.

The role of peripheral blood T lymphocytes and monocytes in modulating Ig synthesis by B lymphocytes stimulated to become Ig-secreting plasma cells by pokeweed mitogen was studied using a sensitive enzyme-linked immunosorbent assay of IgM and IgG in healthy newborn infants and in adult controls. Neonates' B lymphocytes alone synthesized 20 to 30 times less Ig than adults' cells. Increasing proportions of autologous adults' T lymphocytes relative to B lymphocytes initially enhanced and then suppressed Ig synthesis, whereas autologous infants' T lymphocytes had no significant effect. Neonates' T lymphocytes cultured with adults' B lymphocytes enhanced and suppressed Ig synthesis comparably to adults' T lymphocytes, but neonates' B lymphocytes responded poorly to the helper effect of adults' T lymphocytes. Adults' monocytes diminished the helper and increased the suppressor effect of adults' and infants' T lymphocytes on adults' B lymphocytes, whereas neonates' monocytes cultured with neonates' T lymphocytes suppressed synthesis of IgG by neonates' B lymphocytes but had no effect on IgM. The findings suggest that the low serum Ig levels of the newborn period result primarily from immaturity of B lymphocytes rather than from T lymphocyte suppression of Ig synthesis. The normal sequential development of IgM followed by IgG may be a result of gradually increasing responsiveness by neonates' B lymphocytes to the helper effects of newborn T cells. Synthesis of IgG in the newborn infant also appears to be modestly suppressed by T lymphocytes and monocytes, which may contribute to the predominance of serum IgM.