MEMORY, BUT NOT NAIVE, PERIPHERAL-BLOOD B-LYMPHOCYTES DIFFERENTIATE INTO IG-SECRETING CELLS AFTER CD40 LIGATION AND COSTIMULATION WITH IL-4AND THE DIFFERENTIATION FACTORS IL-2, IL-10, AND IL-3

The ligation of CD40 on B lymphocytes by CD40 ligand, transiently expr essed on activated Th cells, provides a key activation signal required for the germinal center B cell response. In vitro, human B cell activ ation has been investigated extensively by coculturing tonsillar B cel ls with CD32-transfected fibroblasts coated with anti-CD40 Abs, in the presence of cytokines (the CD40 system). When tonsillar IgD(+) B cell s are cultured in the CD40 system with IL-4, cells proliferate and swi tch to IgG, but they display a block of differentiation illustrated by the persistence of IgD expression on cycling B cells. In this study, rue analyzed the responses of peripheral blood B lymphocyte fractions, which may contain fewer in vivo activated cells than those from tonsi ls. While the differentiation block was confirmed with peripheral naiv e B cells cultured in the CD40 system with IL-4, it was also observed with the combination of IL-2, IL-10, and IL-3 alone or together with I L-4 (persistence of >90% IgD(+) cells, including 24-60% IgD(+), IgG(+) cells, and <6% IgD(+), IgA(+) cells after 8 days). IgD(+), IgG(-), an d IgA(-)(naive) B cells secreted 70-fold less Ig than IgG(+), IgA(+) ( memory) 8 cells in response to anti-CD40 plus IL-2, IL-10, and IL-3. I gG(-), IgA(-) B cells, or IgD(-), IgM(+), which should include IgM(+) memory cells, strongly secreted IgM, but no IgG. in conclusion, only m emory B cells secreted Ig; like memory T cells, their activation requi rements to differentiate into effector cells seem less stringent than those of the naive cells.