The primary function of a B cell is to produce antibodies. As we have previously discussed, B cell development is linked to immunoglobulin (antibody) gene rearrangement, and development of (high affinity) antibody specificity is generated through affinity maturation. Affinity maturation takes place in the germinal center and the result of this process is the generation of B cell populations that produce antibodies and memory. In this article, we will explore what happens to activate memory and antibody production following the affinity maturation in the germinal center.
II. Review of the Germinal Center
Before we get to the topic of memory B cells and antibody production, let’s do a quick review of the germinal center (a more complete discussion was covered in a past blog). B cells, follicular T helper cells (Tfh), and follicular dendritic cells come together upon activation by antigen to form the germinal center (Figure 1). The environment generated by this association yields rounds of proliferation and somatic hypermutation (of immunoglobulin genes) in the antigen specific B cells, which allows for the generation and selection of high affinity antibodies. Class switching is also induced in the germinal center to generate IgG, IgA, or IgE isotypes based on the type of response generated by the pathogen or antigen.
The survival of the B cells in the germinal center is based on antigen stimulation and support from the Tfh. The B cells that cannot compete will die, resulting in the selection of multiple B cell clones with increased specificity and affinity toward the antigen. These germinal center B cells go on to become memory B cells or antibody producing plasma cells. It should be noted that for each immunogen many germinal centers will be generated in the spleen or lymph nodes (or other secondary lymphoid tissue) which will increase the diversity of antigen specific B cell clones.
III. Antibody Production and the Activation of Memory
It is believed that a vast number of the B cells selected in the germinal center go on to become plasmablasts which differentiate into antibody producing plasma cells1. Plasma cells are antibody factories that are responsible for producing all of the antibodies in circulation. Most of the plasma cells will remain locally (Figure 2) to produce antibodies for the recognition, targeting, and removal of the inducing pathogen1. These are typically referred to as short-lived plasma cells as they are only maintained while the pathogen is present and will die off once the infection is cleared. A few of the plasma cells will migrate to the bone marrow (Figure 2) and nestle into supportive niches where they are long-lived and may produce protective antibodies for decades2.
Memory is the hallmark of the acquired immunity, which allows for rapid expansion of T- and B-cell responses upon re-exposure to an immunogen. This rapid recall response is generated through the development of memory cells produced from the initial exposure. Only a few of the germinal center B cells go on to become memory B cells (Figure 2). Memory B cells; when appropriately activated by antigen; will rapidly proliferate, differentiate into plasma cells, and induce the generation of a germinal center3.
Human memory B cells have been more readily identified due to the expression of a cell surface marker (CD273) that is expressed on antigen experienced human B cells. Recently, mouse Memory B cells have been more directly identified through the expression of several cell surface markers4 (including PD-L2, CD73, & CD80). Interestingly, humans have more memory B cells than mice3,4, although this may be an artifact of mice being housed in pathogen-free facilities and therefore, are exposed to only a limited number of microbes and infectious agents. The evaluation of memory B cells has identified as many as five different populations of memory cells2,4,5.
What signals induce plasma cell versus memory differentiation is not clear. B cell receptor signaling has been implicated and the strength of the signal may be the determining factor3. While the short-term plasma cells remain locally to produce antibodies, memory cells are predominantly located in the spleen and bone marrow where they can be exposed to circulating antigens and rapidly respond if necessary. Memory cells need to be recruited to lymph nodes if the infectious agent induces local inflammation.
Antibody responses and the generation of memory are critical for the long-term protection of an individual. Therefore the germinal center “reaction” allows for the development of diversity and affinity and beyond the activation in the germinal center memory B cells and antibody producing plasma cells are generated for both short-term and long-term protection.
IV. Considerations for Monoclonal Antibody Development
Generating an appropriate antibody response toward a specific immunogen is always the initial critical step in the process of developing a monoclonal antibody. However, in order to capture that response in a single hybridoma clone, the appropriate specific B cells must be successfully fused with a myeloma cell line. While all mature B cell populations have the capacity to fuse with myeloma cells and generate antibody producing hybridoma cells; the memory B cells, the rapidly dividing activated (memory) B cells, and plasmablasts (and plasma cells), are the most effective fusion partners. As discussed above, the spleen is the richest source of memory B cells. Therefore, the final boost prior to generating hybridoma fusions is most effectively carried out such that ample delivery of the antigen to spleen allows for the rapid expansion of the memory population in the spleen. The appropriate delivery and timing of the final “fusion” boost are critical for effective monoclonal antibody development.
- Shenoy, G. N. et al. Recruitment of Memory B Cells to Lymph Nodes Remote from the Site of Immunization Requires an Inflammatory Stimulus. J. Immunol. 189, 521–528 (2012).
- Bergmann, B. et al. Memory B Cells in Mouse Models. Scand. J. Immunol. 78, 149–156 (2013).
- Tangye, S. G. & Tarlinton, D. M. Memory B cells: Effectors of long-lived immune responses. Eur. J. Immunol. 39, 2065–2075 (2009).
- Anderson, S. M., Tomayko, M. M., Ahuja, A., Haberman, A. M. & Shlomchik, M. J. New markers for murine memory B cells that define mutated and unmutated subsets. J. Exp. Med. 204, 2103–2114 (2007).
- Tomayko, M. M., Steinel, N. C., Anderson, S. M. & Shlomchik, M. J. Cutting Edge: Hierarchy of Maturity of Murine Memory B Cell Subsets. J. Immunol. 185, 7146–7150 (2010).