We all know that when we’re sick, our immune system launches a response to help rid our bodies of the invading pathogen. But in addition to the cells generated for immediate pathogen elimination, our immune system also generates a set of cells that stick around for months to years; these cells are called memory cells1. These long-lasting memory cells that are generated during an infection are specific for a particular pathogen, so if that same pathogen tries to invade months later, your memory cells will immediately recognize it. Once the memory cell recognizes that pathogen it can mount a robust immune response, hopefully before you even begin to feel sick. These memory cells are what mediate the protection against pathogens that is generated by vaccination1.
There are two main types of memory cells: memory B cells and memory T cells. Memory T cells are derived from activated T cells during infection, which are responsible for cell-mediated immunity. T cells activate other immune cells upon infection, and kill cells that are infected with a pathogen. Memory B cells are derived from activated B cells, and are important for antibody secretion. Therefore B cells function as a vital part of humoral immunity, or immunity derived form macromolecules in fluid, in this case our bodily fluids. Antibodies bind pathogen to prevent it from entering your own cells, and to signal phagocytic cells (or “eater cells”) to destroy the pathogen by ingestion. When a memory B cell encounters its cognate pathogen upon secondary infection, it divides to form more B cells that begin to secrete antibodies to fight the pathogen.
There are many types of memory B cells, which differ in the type of antibody they produce. When a B cell is fighting an infection, it can undergo something called a class switch, which changes the type of antibodies it secretes. There are five main types of antibodies, aptly name isotypes, and each has a specific function. The IgG memory B cell, which secretes the IgG antibody isotype, has long been thought to be the primary contributor to our memory B cell populations. However in the past few years scientists have found that our memory B cell populations are actually more diverse than originally thought. In one recent study, scientists established an important role for another type of memory B cell generated after bacterial infection, the IgM B cell.
IgM B cells are the first B cells generated during an immune response for a specific pathogen. While some IgM B cells produce antibodies to begin to target the pathogen for destruction, other B cells begin to undergo class switching to produce other types of antibodies, like IgG. These B cells also undergo mutations in the DNA region that codes for the pathogen-binding domain on the antibody so that the pathogen can bind the antibody with a better fit; this is called affinity maturation. Therefore, it has long been thought that memory B cells that have undergone class switching and affinity maturation, like IgG memory B cells, are better suited for response to secondary infection since they bind the pathogen with higher affinity2. But a study published by Yates and colleagues showed that IgM memory B cells, which don’t undergo affinity maturation, are actually a large proportion of our memory B cells generated during a bacterial infection. Furthermore, these IgM cells are required for the generation of IgG responses during secondary antigen challenge.
The researchers first wanted to show that memory IgM B cells are truly generated during a bacterial infection. They infected mice with ehrlichiae, a tick-borne bacterium, and analyzed the immune cells that were generated in response to the ehrlichiae (E. muris) infection. Each type of immune cell has a unique set of cell surface proteins that distinguish it from another type of cell. Therefore the researchers analyzed the cell surface markers on the immune cells generated in response to E. muris infection to identify different populations of immune cells. They identified the cell surface markers by flow cytometry. In flow cytometry, cells are treated with a fluorescing molecule that binds to a cell surface molecule of interest. The cells are then run through a flow cytometer, which quantifies the amount of the fluorophore on the cell, indicating how much of the molecule of interest is present on the cell. Using flow cytometry, the researchers identified a population of cells with a unique cell marker, CD11c, which secretes IgM antibodies. They isolated these cells, and using flow cytometry established that the CD11c-positive cells also expressed known markers of memory B cells. These CD11c-positive IgM-secreting cells were further proven to be true memory cells by showing that like other memory cells, they undergo limited cell division, and contain mutations that are evidence for affinity maturation.
Once the researchers established that the CD11c-positive cells were IgM memory B cells, they wanted to determine where they are located in the immune system. They stained the spleen with a fluorescing antibody tag that binds CD11c-positive cells, and observed that they were located in the marginal zone of the spleen; this is consistent with previous studies that have shown that other types of memory B cells localize in the marginal zone of the spleen, where the cells contact flowing blood that may carry antigen.
The researchers then showed that when mice were infected for a second time with E. muris, there was a robust immune response from the memory cells specific for E. muris. So they wanted to determine if the CD11c-positive memory B cell population was directly responsible for the robust and rapid immunological response upon secondary infection, or if another memory B cell population was responsible. They investigated the role of the CD11c-positive memory B cells by re-infecting mice with E. muris 30 days after initial infection. In the experimental group of re-infected mice, the researchers depleted the CD11c-positive cell population by giving the mice a toxin that resulted in the depletion of the IgM memory B cells. They then analyzed the effect of IgM depletion by looking at IgG production in the control versus depleted mice. They found that mice that were depleted in IgM memory B cells had significantly lowered levels of IgG antibody. This suggests that that IgM memory cells undergo class switching upon secondary Ag challenge, and are responsible for the IgG response (generated by class switching).
Taking the data together, the researchers showed that IgM memory B cells are established in response to bacterial infection and that these IgM memory cells are responsible for IgG production during secondary challenge. But why is this important? Why does it matter that IgM memory B cells are now thought to play an important role in memory cell response to secondary infection? Yates and collegues’ finding help us to better understand memory cell generation. The findings suggest that different types of memory cells are best suited for different types of pathogens. While IgG memory B cells may be important for viral pathogens, this study shows that IgM memory cells may be the superior memory cell type for a bacterial infection.
While these findings are important, further research can be done to elucidate the functions of IgM memory cells. All of the experiments presented by Yates et al. were done in a mouse model. Research should be conducted to determine if IgM memory B cells are present in humans. If they exist, what characteristics do they share with the isolated IgM memory cells in mice, and what characteristics are different? In addition, the researchers only looked at one type of bacteria, E. muris. Future experiments can look for IgM memory cell production in response to different types of infections, in both humans and mice. Lastly, in future experiments researchers can look to see if IgM memory cells undergo class switching to generate other antibody isotypes upon secondary infection. Further investigation will help the scientific community to better understand immunological memory in response to different types of pathogens, and may help us to create better vaccines for specific types of pathogens.
Link to the article:
Yates, J.L., Racine, R., McBride, K.M., & Winslow, G.M. (2013). T Cell-Dependent IgM Memory B Cells Generated during Bacterial Infection Are Required for IgG Responses to Antigen Challenge. J. Immunol., 191, 1240-1249.
1. McHeyzer-Williams, M., Okitsu, S., Wang, N., & McHeyzer-Williams, L. (2012). Molecular programming of B cell memory. Nat. Rev. Immunol., 12, 24-34.
2. Good-Jacobson, K.L., & Tarlinton, D.M. (2012). Multiple routes to B-cell memory. Int. Immuunol., 24, 403-408.
Pillai, S., & Cariappa, A. (2009). The follicular versus marginal zone B lymphocyte cell fate decision. Nat. Rev. Immunol., 9, 767-777.
By Alexandra Doms