M E M O R Y B CELLS AT SUCCESSIVE STAGES OF DIFFERENTIATION Affinity Maturation and the Role of IgD Receptors* BY LEONORE A. HERZENBERG, SAMUEL J. BLACK,§ TAKESHI TOKUHISA, AND LEONARD

The ant igen-induced development o f memory B cell populat ions involves both an increase in the number of B cells capable of responding to an antigen and, especially with time and repeated antigen stimulation, a selective expansion of B cells commit ted to producing antibodies with higher combining affinities for the antigen (1). These two events, memory generation and affinity maturat ion, have generally been considered to be different manifestations of the same ongoing selection-expansion process; however, studies presented here demonstra te that they occur at different stages during memory development and can be regulated independently. In addition, these studies suggest that the presence or absence of surface IgD receptors may play a crucial role in determining the potential for affinity matura t ion in memory populations. We have shown that both IgD + and I g D 1 B cells from pr imed donors give rise to adoptive IgG memory responses and that IgD + memory cells are precursors of I g D memory cells (Black, S. J., T. Tokuhisa, L. A. Herzenberg, and L. A. Herzenberg. Memory B cells at successive stages of differentiation: expression of surface IgD and capaci ty for self renewal. Manuscr ip t in preparation.) (2, 3). In this paper, we further define the relationship between these memory populat ions by showing that IgD + memory cells taken from donors that were primed under a variety of conditions always give rise to p redominant ly low-affinity adopt ive responses, whereas their progeny, I g D memory cells, in the same donors, uniformly give rise to higher-affinity responses that are characteristic o f secondary responses to strong antigenic stimuli. These data, which support and extend conclusions from our earlier studies (2, 3), indicate that affinity matura t ion initiates with the emergence of the I g D memory populat ion and proceeds mainly, if not exclusively, within this populat ion thereafter. Thus, the transition from early (IgD +) to mature (IgD-) memory is required for the development of high-affinity response. Da ta presented here also bear on the roles played by helper T cells dur ing memory development. Earlier studies have shown that affinity matura t ion is regulated by the