Understanding and making use of human memory B cells

Abstract

The work of our laboratory has focused on the study of human memory B cells. Using an in vitro approach we dissected the triggering requirements of B cells and unveiled a distinct role for TLRs in the activation of naive versus memory B cells. Using an ex vivo approach we analyzed the dynamics of memory B cells and ASCs and the kinetics of serum antibodies during secondary immune responses and in steady state conditions and used these quantitative data to build up a model of serological memory. According to this model memory B cells behave as ;stem cells' capable of generating plasma cells and antibodies in an antigen-dependent as well as in an antigen-independent fashion. Finally we developed an efficient method to interrogate human memory B cells and to isolate human monoclonal antibodies. This method can be exploited for the production of neutralizing antibodies for serotherapy and for "analytic vaccinology".

Figure 1

Differential requirements for the activation of human naive and memory B cells. The activation of naive B cells (left) requires the integration of three signals: signal 1 delivered by antigen through B‐cell receptor (BCR), signal 2 delivered by antigen‐specific T‐helper cells via CD40 following cognate interaction, and signal 3 delivered by microbial products acting on Toll‐like receptors (TLRs). Note that naive B cells do not express TLRs, which are induced by signal 1. The activation of memory B cells (right) can be effectively triggered in the absence of BCR stimulation via TLRs or bystander T‐cell help.

Figure 2

Dynamics of memory B cells and antibody‐secreting cells (ASCs) in the presence or absence of antigen. Upon antigenic boosting (upper panels), specific memory B cells selectively proliferate and differentiate generating a burst of ASCs (measured by enzyme‐linked immunospot assay analysis) that peaks on day 6 and day 7. In the absence of antigen (lower panels), all memory B cells turn over at low rate and continually spill out ASCs of all memory specificities. In this condition, there is a good correlation between the frequency of memory B cells and serum antibody levels. Shown are tetanus toxoid (TT)‐specific ASCs 6 days and 10 years after a booster immunization.

Figure 3

Kinetics of memory B cells, antibody‐secreting cells (ASCs) and serum antibodies following a booster immunization. (A). In the absence of antigenic stimulation (1), memory B cells are in a dynamic equilibrium with plasma cells and antibodies. On day 6 and day 7 after booster immunization (2), large numbers of memory B cells are generated in an antigen‐dependent fashion (red arrows). Some of these cells enter the bone marrow in part by normal turnover or by displacing old plasma cells, while most die by day 10. On day 12 (3), there is a large population of long‐lived plasma cells that are rescued in the bone marrow. These cells slowly disappear until a new equilibrium is reached (4). The fraction of antigen‐specific B cells is shown in yellow, while the total B‐cell pool is in gray. Red and blue arrows indicate antigen‐dependent and antigen‐independent processes, respectively. Synthetic rate of IgG and serum antibody levels is indicated on the right side. (B). The experimental curve of antibody response following TT‐boost is shown in a dotted black line. The contributions of antigen‐induced short‐lived (1) and long‐lived plasma cells (2) are shown in red. The contribution of antigen‐independent mechanisms (3) is shown in blue.