EFFICIENT INDUCTION OF IMMUNOGLOBULIN PRODUCTION IN NEONATAL NAIVE B-CELLS BY MEMORY CD4(-CELL SUBSET EXPRESSING HOMING RECEPTOR L-SELECTIN() T)

The humoral response in newborns is mainly restricted to IgM productio n, which may be attributable to the naive nature of both B and T cells at birth. In light of the current evidence that memory (CD45RO(+)) CD 4(+) T cells help B cell differentiation, the present study was undert aken to examine whether a specified population within memory CD4(+) T cells could induce the maturation of neonatal naive B cells. In the co nventional PWM-stimulated cultures, the generation of IgG- and IgA-pro ducing cells in addition to IgM production by neonatal B cells was sig nificantly enhanced by co-cultures with memory, but not naive, CD4(+) T cells. Memory CD4(+) T cells were further divided into two populatio ns based on expression of homing receptor L-selectin. These memory CD4 (+) T cell subpopulations appeared to behave in different fashions con cerning help for Ig production by naive (sIgD(+)) and mature (sIgD(-)) B cells. L-selectin-negative memory CD4(+) T cells exhibited helper f unction for Ig secretion by mature B cells. Intriguingly, Ig productio n by neonatal B cells as well as adult naive B cells, although less th an that by mature B cells, was efficiently promoted by L-selectin-posi tive memory CD4(+) T cells rather than L-selectin-negative ones. The r esults suggest that the capability of neonatal naive B cells to secret e IgG and IgA can be elicited by appropriate T-cell signals, especiall y from the L-selectin-positive population within memory CD4(+) T cells , seemingly indicating its possible role for isotype switching in B ce lls.