How would we design a “strep throat” vaccine to produce long-lasting immunity?
There are several first time cases that most people remember from childhood. His/her first bike, maybe a family pet, however, there is always one memory that almost every child has, which is the memory of getting “strep throat”. One of the most common infection in children, “strep throat,” causes unpleasant symptoms such as a sore throat, fever, and nausea. Nonetheless, most children will become infected with “strep throat” eventually. Therefore, this infection is common and can reoccur often for many children, which can be problematic.
“Strep throat” is caused by the bacterium Streptococcus pyogenes. This bacterium is a gram-positive bacterium that forms chains of circular cocci. The bacterium is classified by specific properties that exist with a protein found on the surface, the M protein. For our purposes, Group A S. pyogenes is what we will be focusing on because it can cause mild to severe symptoms. The bacterium releases 2 types of hemolytic, red-blood cell lysing, toxins: Streptolysin S and O.1) Both of the toxins can cause hemolysis and it is known that Streptolysin O triggers a very fast antibody response to S. pyogenes. Along with these two toxins, S. pyogenes also releases other toxins. The other toxins can cause fever and other immunological effects within humans, and are commonly referred to as exotoxins.2) Nonetheless, S. pyogenes can infect humans and cause harm, so why is controlling the infection important?
It has been shown that if the bacterium resides in the host system long enough, it is able to cause severe harm to the person. Not only will one get the sore throat, nausea, and fever, if the bacterium is within the system long enough it can cause the body to go into a toxic shock, as well as, cause the body to start attacking the tissue of heart valves, causing rheumatoid fever. Therefore, the bacterium needs to be controlled quickly and not allowed to persist within the body for long periods of time. The good news is that most Group A S. pyogenes are sensitive to penicillin, which is an antibiotic. A doctor would most commonly prescribe amoxicillin, and the bacteria would be eliminated.3) If the bacterium can be eliminated by antibiotics, then why should we care about alternative controls? Can we just give someone who has “strep throat” penicillin and be done with it? The answer is yes, but what if you can control getting the disease symptoms? This leads us to vaccination, which could possibly prevent severe infection, eliminating life threatening progression of infection and the painful symptoms.
The idea of vaccination is to produce immunity quickly and efficiently to a known pathogen that would normally cause symptoms within the body. To vaccinate against S. pyogenes, we would have to produce immunity to the bacterium specifically, as well as, produce immunity across mucosal membranes, where S. pyogenes infects. This idea of mucosal immunity is important due to only one type of antibody being able to cross the mucous membranes, IgA. However, and more importantly, if one is exposed to S. pyogenes on a regular basis, how can we prevent reoccurring infections? The immune system can produce memory cells within the body, that when a recognizable foreign invader is present, an immune response is initiated quickly and infection is either cleared much faster or slowed down compared to primary invasion. These memory cells are able to respond to a foreign invader very quickly and initiate an immune response. Thus, if a vaccine for S. pyogenes can be developed, memory cell production would be crucial for immunity. To understand how memory cell production against S. pyogenes could work, we must first understand what memory cells exist and how memory cell production is initiated and maintained.
Immunological memory is a crucial part of adaptive immunity. It allows reoccurring infections to be cleared faster and more efficiently. If infection does occur, it reduces the severity of the disease compared to previous exposure to the pathogen. Immunological memory is composed of both humoral and cell mediated immune (CMI) responses, which have differing effector functions. Humoral immune responses include pre-existing antibody circulating throughout the body, memory B cells (MBCs), and long-lived plasma cells (LLPCs). The CMI response includes the reactivation of CD8 memory T cells. 4)
The first line of defense against a reoccurring pathogen is the antibodies circulating in the body. They already have specific antigen binding regions, which would recognize the invader and neutralize the free floating pathogen. The LLPCs continue to produce this antibody that serves as the first line of defense against reoccurring or persistent infection. B cells (of the germinal center), on the other hand, develop a greater specificity for the antigen while waiting for the pathogen to invade again, which allows for mass secretion of more antigen specific antibodies if a reoccurring infection happens. Cell mediated immunity can be triggered by the re-occurrence of a pathogen, which would cause a faster response from the cytotoxic T cells. 5) The figure to the left shows the number of antibodies and T cells present during primary infection, and reoccurring infection.
There are two types of B cell subsets that are responsible for humoral memory response, LLPCs and MBCs. Both cells are long-lived and express the high-affinity B-cell antigen receptor (BCR). These B cells are found in the germinal centers (GC) of the lymph nodes and spleen. The steps in the figure below correlate with the steps listed below. These steps tell us how humoral response is generated and its role in reoccurring infection.
The whole process begins with the activation of naive B cells by antigen in the presence of CD4+ T cells. On the naive B cell are the Ig molecules, specifically IgD and IgM that bind antigens of invaders. Some of the antigens of microbes that enter tissues or are present in the blood are transported to and concentrated in the B cell-rich follicles of the lymph nodes. B cells with specific Ig membrane receptors can recognize the antigen and bind it. However, this process requires the bringing together of two or more receptor molecules, which results in receptor cross-linking. Signals initiated by antigen receptor cross-linking are transferred by receptor-associated proteins. Because IgM and IgD antigen receptors of naive B cells have highly variable extracellular binding regions and short cytoplasmic domains, they require other molecules to help with signaling. IgM and IgD receptors are noncovalently attached to Igα and Igβ, forming the B cell receptor complex (BCR). When antigen is bound to the IgM or IgD molecules, this triggers signal transduction through the BCR, because the cytoplasmic domains of Igα and Igβ contain conserved immunoreceptor tyorsine-based activation motifs (ITAMs). These ITAM motifs play a role in signaling cascades in the cell. The net result of receptor-induced signaling in B cells is the activation of certain transcription factors that turn on specific genes that express products involved in B cell differentiation and proliferation. 6)
In addition, B cells that bind protein antigens by their BCR, then endocytose them, process them, and display them on the B cell surface with MHC II. T lymphocytes near the lymphoid follicle can then interact with the activated B cells, migrating outward to further stimulate activation. Helper T cells that recognize the antigen displayed by the B cells can bind the antigen with their TCR (T cell receptor), the MHC II with CD4+, and additional binding of the CD40 ligand (with CD40) expressed on the surface of B cells. The interaction of CD40 with the CD40 ligand delivers signals that further stimulates proliferation and synthesis of antibodies. Helper T cell signals also stimulate heavy chain isotype switching and affinity maturation. The binding of the antigen by the B cell and the activation by the helper T cell initially activates the naive B cell, which stimulates differentiation and proliferation.7)
The activated B cell then continues down one of two different pathways - short-lived plasma differentiation or the migration of active B cells to the germinal center (GC) to initiate germinal center reaction with the help of CD4+ T cells.
Selective expression of transcription factors B-lymphocyte-induced maturation protein 1 (Blimp-1) and X-box binding protein 1 (XBP-1) drive the differentiation of plasma cells, where CD40 signaling within the GC favors the generation of MBCs. The microphthalmia-associated transcription factor (MITF) also prevents plasma cell differentiation. When MITF is not present Blimp-1, XBP-1, and interferon regulatory factor 4 (IRF4) are induced, resulting in plasma cell differentiation. These plasma cells can then secrete mass amounts of antibody during primary infection. They are short lived and die. 8)
However, if MITF is present, the differentiation of plasma cells does not occur and the B cell retains its identity, which is defined by the paired box protein (PAX5). These memory B cells then migrate to the germinal center of lymphoid follicles to initiate GC reaction. A germinal center form from some of the progeny of activated B lymphocytes enter the lymphoid follicles. Within the GC, B cells undergo the GC reaction, which consists of somatic hypermutation, affinity maturation, and clonal selection, resulting in the generation of high-affinity MBCs or LLPCs (or precursors). 9)
Affinity maturation is a process by which the affinity of the antibody for the antigen is increased with prolonged or repeated exposure to the specific antigen. This allows the antibodies to bind to a microbe or antigen with greater affinity if the infection is reoccurring or not easily cleared. Affinity maturation occurs in the GC of lymphoid follicles and is a result of somatic hypermutation of Ig genes in dividing B cells followed by the selection of high-affinity antigen binding B cells. Inside the GC, B cells proliferate at a rapid rate. During proliferation, the Ig genes undergo numerous point mutations in the variable regions and specifically in the antigen binding hypervariable regions of the antibodies. This process is also known as somatic hypermutation. Activation-induced deaminase (AID) is an enzyme that plays a critical role in somatic mutation by changing the nucleotides of Ig genes and making them more susceptible to the mutational machinery with in the cell. The high-frequency mutations lead to the generation of different B cell clones whose Ig molecules have increased affinity for the antigen that triggered the response initially. 10)
In the germinal center, B cells interact more frequently with T cells and follicular dendritic cells (FDCs). These cells are the limiting factors of survival or death of the GC B cells, and selection of high-affinity B cells (clonal selection). 11) GC B cells die by apoptosis unless they recognize a specific antigen or helper T cells send them signals to prolong life. While somatic hypermutation is occurring in the GC, antibody that was secreted earlier binds the antigen which triggered response. This forms an antigen-antibody complex, which can activate complement. Follicular dendritic cells (FDCs) that reside in the germinal center, can then bind the Fc (constant) region of the antibody (of the antigen-antibody complex) using Fc receptors expressed on their surface. They can also bind products of compliment, which would also help display the antigen-antibody complex. The B cells that have undergone somatic hypermutation can then bind the antigen presented on the FDCs, which sends signals that saves the B cell from programmed cell death. B cells can also bind free floating antigen, process it through endocytosis, and display it on their surface MHC II. Therefore, they can present the antigen to helper T cells in the lymph nodes, which can bind the antigen-MHC II complex and send signals to the B cells that trigger survival. Therefore, the B cells that survive are capable of binding antigen at low concentrations. This increases competition for binding the antigen by the B cells, so only the B cells that can bind the antigen really well are capable of surviving (hence clonal selection). Therefore, this selects for higher-affinity antigen binding B cells, which can secrete antibodies with a higher affinity for binding the antigen. The cells that are capable of surviving can then proliferate and differentiate into MBCs or LLPCs. The figure to the left shows how selection occurs based on the binding of specific antigens with high affinity. 12)
The differentiation of GC B cells into memory B cells or long-lived plasma cells depends on helper signals from CD40 ligand (CD40L) on T cells. When GC B cells have a CD40-CD40L interaction with helper T cells, they differentiate into memory B cells as seen in Step 3 of the figure above. However, if this interaction does not occur, GC B cells tend to differentiate into long-lived plasma cells. As discussed above with short-lived plasma cell differentiation, the same transcriptional factors need to be present to repress gene expression so that plasma differentiation can occur. For example, if MITF is present, which CD40 signaling might induce the expression for, plasma cell differentiation cannot occur and the B cell preserves its identity, differentiating into a memory B cell. However, if MITF is not present then plasma cell differentiation can occur. The exact mechanism of differentiation is unknown; however, there is substantial evidence that leads us to believe that these interactions trigger one type of differentiation vs. the other. 13)
The function of long-lived plasma cells is to secrete antibody at low levels after the exposure and clearing of a specific pathogen for which the precursor to the LLPC was exposed to. The long-lived plasma cells secrete isotype-switched, high-affinity antibodies, which can bind the antigen more effectively. These antibodies serve as the first line of defense against a reoccurring invader. These cells are found in the bone marrow and have no BCR receptors, therefore, they cannot respond to an invader or bind antigens. Their only function is to secrete antibodies at a low level. They are also terminally differentiated, which means they cannot undergo further differentiation. Their state of being and function is set until they die. The picture to the right shows a plasma cell. 14)
The function of memory B cells is to recognize the antigen from a reoccurring pathogen. This antigen binds to the high-affinity antibody receptor on the surface or the memory B cells. This recognition triggers signal transduction and the differentiation and proliferation of the memory B cells into either more memory B cells or plasma cells, which will secrete antibodies with high-affinity for the antigen at very high levels, much higher than the levels of secreted antibodies done by the LLPCs. This process is known as the rapid recall response, which is shown in Step 4 of the correlating figure. The memory B cells are primarily found in the lymph nodes of the body. 15)
Both MBCs and LLPCs have estimated half-lives of more than 100 days. The survival of MBCs and LLPCs does not seem to require cytokine signals like in the case of CD8 memory T cell survival.
MBCs are thought to be more dependent on antigen stimulation. Studies have shown that MBCs can survive for several months without continuous stimulation from binding a specific antigen. So how do MBCs persist for a life-time? A proposal done by Lanzavecchia and group proposed that MBCs may be maintained through homeostatic proliferation. This occurs through the binding of TLR and CD40. In response to ligands of TLR and CD40, MBCs proliferate and differentiate into plasma cells without BCR signaling. Therefore, TLR signaling may be a key part of slow cell division and terminal differentiation of MBCs into LLPCs. This proposal would explain how both MBCs and LLPCs are replenished over time, however, the exact mechanism is unknown. Lanzavecchia's group also proposed that MBCs behave in a similar way to stem cells by sustaining their numbers by very slowly producing LLPCs. Gene expression profiling has shown that B cells have several genes that stem cell express. Again, the exact mechanism i unknown. 16)
LLPCs do not express enough BCR on their surface, therefore, their survival does not depend on their ability to bind antigen. The precursors for LLPCs (GC B cells or re-stimulated MBCs) tend to migrate toward the bone marrow into regions known as the survival niches. They do this via chemotaxis of CXCL-12 chemokine. These cells are then continuously stimulated with signals that induce “pro-survival”, IL-5 and IL-6 ligands for CD44, TFN, CXCL-12 and ligands for B cell maturation. These signals came from surrounding stromal cells. This makes an ideal environment for survival and secretion of antibodies, which are the main functions of LLPCs. 17)
If re-exposure to a specific pathogen does occur or the infection is chronic (or latent), then the body can respond much faster and effectively to the pathogen. The first line of defense against a chronic or re-invading pathogen are the pre-existing antibodies, which are produced by the long-lived plasma cells. These antibodies could effectively neutralize the pathogen so that little or no infection occurs. However, if the levels of pre-existing antibodies are not sufficient for neutralizing the pathogen, the BCR on the memory B cell surface can bind the antigens of the pathogen, which induces differentiation of memory B cells into plasma cells. This response will be much faster and more effective because the freshly differentiated plasma cells will secrete very high levels of antigen-specific high-affinity antibodies. These antibodies will bind the antigen at a much higher affinity than previously because the B cells had undergone somatic hypermutation and affinity maturation. This process is different than the antigenic stimulation of naive B cells described in Step 1. Nevertheless additional stimulation of cell mediated response would more effectively clear an infection, especially, if the microbe is invasive. Therefore, T cells and memory T cells also play a crucial role in eliminating a reoccurring pathogen. 18)
Having effector T cells constantly activated could result in immunopathological damage. Therefore, it is crucial to be able to deactivate T cells do they do not continue to trigger response, continue to kill host cells, and can be preserved for later use. This is exemplified by chronic infections. To combat this, T cells differentiate into memory T cells, which wait around in an inactive state and can potentially become active or proliferate into effector T cells rapidly when the antigen is present from a reoccurring pathogen.19)
The pathway for the differentiation of memory T cells is unknown. However, there are two pathways that are probable for memory T cells differentiation: the linear differentiation pathway and the divergent differentiation pathway.20)
In the linear differentiation pathway, memory T cells develop from already existing effector T cells. The effector T cells differentiate into memory T cells after the antigen has disappeared. The memory T cell can then later differentiate back into an effector cell if it encounter antigens of the reoccurring pathogen. A variation of this pathway is the decreasing potential hypothesis. This hypothesis follows the same idea that memory cells arise from effector cells. However, in this model the early effector cells can differentiate into one type of a memory cell when no antigen is present(TCM) and the late effector cells can differentiate into another type of a memory cell when no antigen is present (TEM). These memory cells can later differentiate into the memory cells that result from the early effector T cells (TCM). Both of these pathways are shown in the figure to the right, letters b and c.21)
In the divergent differentiation pathway, activated T cells can follow one of two paths. In early stages of infection, the occurrence of antigen in addition to costimulation in the presence of inflammatory signals, type-1 interferon and IL-21. These signals trigger the differentiation of effector T cells. On the other hand, the presence of antigen and additional costimulation in the absence of inflammation may lead to the differentiation of memory T cells. If inflammation is present, the activated T cell will become an effector T cell, but if there is no inflammation, the activated T cell will become a memory T cell. This is also shown in the figure to the right, letter a.22)
Memory T cells are not able to kill or activate other cells like effector T cells can. Generally, compared to effector T cells, memory T cells have had their DNA reprogrammed (by methylating different portions of DNA, transcribing different transcription factors, etc.) so that they are in an inactive state. Memory T cells can be activated when they re-encounter an antigen and this activation causes the cellular changes to allow the T cells to perform its effector functions once again.23) For example, in CD8 T cells, the change from an effector T cell to a memory T cell is characterized by a change from an active state to a resting state. This change occurs due to lower production of granzyme B and higher production of Bcl-2.24)
There are two types of memory T cells: central memory T cells (TCM) and effector memory T cells (TEM). The difference between these two types of cells is that TCM cells are in the lymphatic system and TEM cells are out in the body, not in the lymphatic system. This difference is due to the amount of CD62L and CCR7 on their membranes. TCM cells have high amount of CD62L and CCR7 on their membranes compared to TEM cells. This lack of CD62L and CCR7 is what causes the TEM cells to be able to leave the lymphatic system. Another difference between these two cells types is their functions. TCM cells do not regain their cytotoxic abilities when they are re-stimulated. They simply proliferate and then redifferentiate into cytotoxic T cells. TCM cells do this by retaining their ability to produce Interleukin-2 (IL-2).25) IL-2 is a cytokine which stimulates T cells to grow and differentiate by expressing certain genes.26). TEM cells, on the other hand, do regain their cytotoxic abilities when they are restimulated by an antigen. TEM cells do this by retaining the ability to produce INF-γ and TNF-α.27) INF-γ is an interferon which is used by T cells to stimulate macrophages. When the memory T cell is “activated” by a once-again-present antigen, it can release INF-γ to activate macrophages in the area.28) TNF-α is a cytokine that causes apoptosis in cells.29) When the TEM is stimulated, the TNF-α genes are able to be turned on again and the cell can induce apoptosis in infected cells, like a normal cytotoxic T cell.30)
Memory T cells are maintained by a process called homeostatic turnover. This is a slow, but steady cell division process that replaces the memory T cells as they die. This process is kept up by IL-7 (promotes survival) and IL-15 (promotes division).31) Specifically, CD4 cells only need IL-7 to maintain their memory T cells, while CD8 cells need both IL-7 and IL-15, although IL-7 is still the main player.32) It is also believed that TLR signaling might play a role in homeostatic proliferation of memory T cells. It is well known that CD4 T cells are necessary for maintaining functionality of CD8 T cell memory. This is because memory cells generated in the absence of CD4 T cells tend to be unstable, have reduced cytokine production, and undergoes TNF-related apoptosis inducing ligand (TRAIL)-mediated death. CD4 T cells bind CD8 T cells via CD40-CD40L interaction. Signals are then exchanged, which code for survival and proliferation. CD4 memory T cells can survive a long time without the presence of antigen. However, their ability to divide and secrete cytokines is drastically lowered compared to CD4 memory cells in the presence of antigen. Therefore, the presence of antigen plays a significant role in the maintenance of memory T cells. 33)
Chronic infections cause the formation of memory T cells to not occur. T cells become dysfunctional and eventually die without differentiating into memory T cells. This is not surprising seeing that it is the disappearance of the antigen that causes the T cells to differentiate into memory T cells. If the antigen persists due to chronic infection, differentiation does not occur, and so T cells will just die without any memory T cells being produced.34) This might not be thought of as problematic since memory cells are not needed while the infection is on-going, and would presumably be made once the infection starts receding, however it is a problem when it comes to long lasting immunity and vaccine efforts. Without the production of memory T cells we will not have a faster immune response if the pathogen invades again because no memory cells are produced. Therefore, we lack any kind of immunity that should be produced during primary infection. We also have to be careful not to create a vaccine that will keep the immune system “turned on” or we will not produce any memory T cells to create lasting immunity.
How do Vaccines Illicit Different Memory Cell production Responses
Different vaccines can produce different memory cell production within the immune system. One of the most effective vaccine strategies for memory cell production is a live, attenuated virus infection or bacterium infection. A live, attenuated virus is a form of the virus of choice that is less virulent than the normal virus. This means the virus has been modified, either by random mutations or engineered, to reduce the virulence. We can do genetic modifications to bacteria to produce a similar result. By using this type of vaccine, the immune system recognizes it as a foreign invader and it causes an immune response to occur. The immune system produces antibodies against the virus and memory cells are formed. As stated before, the memory cells can become more distinct in recognizing certain antigens, thus producing long-lasting immunity. A common live, attenuated virus is the oral polio vaccine, or OPV. The vaccine consists of the three common strains of the polio virus, each having substitution mutations in the viral genome.35) Because the only difference between this virus and the wild-type virus is the mutations, it’s recognized by the immune system in the same way as if it did not have those mutations. It causes B and T cell differentiation and memory cell formation. Because of this, when someone is exposed to the wild-type strain of polio, their immune system recognizes it and the memory cells become activated triggering response that can prevent or reduce infection. However, using live, attenuated viruses can be a problem. The virus is alive and it is possible that it can cause infections in immunocompromised individuals. Also it can mutate to become pathogenic. Thus, using a live attenuated virus has its advantages and disadvantages.
Another vaccine strategy is to use a killed version of the virus. This will allow the immune system to process the proteins within or on the surface of the virus, without the virus causing harm to the body by being able to infect cells. This would elicit an immune response, however it would be very weaker than then using a live virus. Because the virus cannot infect cells, the only way it will cause a response is to be phagocytosed. It also would not trigger cell mediated response, which is important for eliminating invasive microbes. The weaker response to a killed virus or bacterium, usually results in multiple vaccination in a lifetime to keep boosting immunity.
Thirdly, one can package an antigen from a bacterium or virus and use that to initiate an immune response. However, this would not be effective. The antigen would have to be phagocytosed, like before, and processing would have to occur. The catch for this is that the antigen needs to be specific enough to produce a particular immune response to the antigen. If mutations in the antigen occur in the wild-type virus or bacterium, then your immunity to the packaged antigen is useless in defending against the invader.
In general, a good vaccine strategy to produce memory cells would be to illicit an immune response that mimics a natural response to a particular foreign invader. This ensures that antibodies get produced by the plasma cells, the memory cells keep producing antibodies and are ready for the infection of the same invader again. However, with this strategy, there is room for improvement.
A vaccine to produce long lasting immunity against S. pyogenes could be produced. Ideally, the vaccine would be a live, attenuated bacterium of S. pyogenes, with mutations or knockouts of genes that render it harmless to us. These mutations could be within the hemolytic toxins or the exotoxins genes that would make S. pyogenes not able to produce these toxins. From this, our body would recognize the M protein on the surface, possibly, and initiate an immune response against the bacterium. This would help produce memory cells against the bacterium, producing specific antibodies against the M protein. Thus, when one actually contracts the harmful S. pyogenes, our bodies would be prepared to initiate an immune response against the bacterium.
The idea of producing memory cells is what helps us become immune to foreign invaders for a long period of time. Because we have a live bacterium within our bodies from the vaccine, our immune system acts as if the bacterium will cause harm to cells, even though we know it will not. However, our immune system does not know that, thus it will produce the necessary response to kill the bacterium. This response would be as if we were fighting off a normal infectious bacterium or virus, producing memory cells and a lot of antibodies. From this, we will be able to recognize the infectious type of S. pyogenes and neutralize it before it causes harm. A graph to the left shows colored lines to where an invader causes disease. The red, yellow, or blue line is the ideal situation for the vaccine. The dashed line is called the disease threshold, anything above the disease threshold would show symptoms of infection to the invader. Anything below the line of the disease threshold, would not show any symptoms of disease. The purple line would be like if the host received no vaccination or primary infection occurred. As one can see there is a great difference in the strength of infection because there is no previous exposure to the pathogen. The green line represents immunity triggered by memory against a pathogen that is very virulent. The green line crosses the disease threshold, however, it is much lower in strength of infection compared to the primary infection. This would indicated that either memory due to vaccination or previous infection would slow down a very pathogenic invader. To create an effective vaccine we would want to keep the strength of reoccurring invader below the disease threshold, so the host does not notice that they are infected. The blue line would produce a quality vaccine. It would prevent the host from developing symptoms, however, infection would occur and just be controlled by the immune system. Ideally, we would like results similar to what is shown by the red line. The invader comes in and is quickly neutralized or cleared. This immunity would be produced by the first line of defense of antibodies secreted by LLPCs. This would be ideal because LLPCs secreting antibodies can quickly and effectively eliminate the reoccurring pathogen. The next best results would be similar to the yellow line, which demonstrates sterilizing immunity. This means that the virus invades, however, over time the immune system is able to control it and eliminate it. This would trigger reactivation of MBCs, and memory T cells. This reactivation would produce high levels of high-affinity antibody and activate T cells to eliminate the pathogen as well. To create a good vaccine we would want to quickly eliminate the virus (red line) or control and eliminate the pathogen (yellow line). 36)
The delivery of the vaccine would also be crucial to prevent the infection of S. pyogenes. As stated before, the bacterium only infects the mucosal tissue of the throat, thus having an immune response in the stomach against S. pyogenes would be futile. The vaccine would need to trigger mucosal immunity. That is why the delivery of the vaccine would have to be intranasal. This technique provides the greatest amount of protection for the greatest amounts of mucosal tissues. It also is in direct contact with the throat. By using a nasal spray with live, attenuated bacteria, we would initiate the most desired immune response to produce memory cells in the correct places.
Recently, several new strategies were developed to produce a longer lasting, potent immunological response.
Interaction of CD40 and CD40L (the ligand that CD40 binds) causes APCs to secrete cytokines IL-12 and IL-15. It has been shown that IL-15 contributes to induction and maintenance of cytotoxic T lymphocytes (CTL) memory and longevity. Incorporation of IL-15 results in recruitment of more memory CTL cells. 37)
Another study found that vaccines containing costimulatory molecules such as B7-1, ICAM-1, and LFA-3 increased both maintenance and avidity of the CD8+ memory cells. It was also shown that the use of booster vaccines with costimulatory molecules had superior anti-tumor effects comparing to the currently used vaccination strategy. 38)
With regards to CD4+ memory cells, it was also shown that IL-7 plays a previously unknown role in the maintenance of memory in the CD4+ cell population as well as the survival of CD4+ cells with a capacity to become memory cells. 39)
A cell-based cancer vaccine was recently developed by an engineered expression of costimulatory molecules CD80 and CD137L on the surface of a neuroblastoma cell line. The tumor then becomes a cell-based vaccine which was shown to increase the number of activated CD8+ effector memory cells. 40)