Maternal Regulatory T Cells: One of Mother’s (and Baby’s) First Lines of Defense

Maternal regulatory T cells play a vital role in maintaining maternal-fetal tolerance and sustaining pregnancies.  By better understanding the mechanisms by which maternal regulatory T cells mediate immune reactions during pregnancy and how pathogens manipulate such immune reactions, it is possible to develop preventative measures against opportunistic pathogens that target mothers during this critical period of time.

In many ways, the immune system’s response to pregnancy is akin to that of organ transplantation; the maternal immune system must determine if the growing fetus poses a threat to the mother’s own survival and if it must be rejected (Gobert and Lafaille 2012). This problem results from the fetus being semiallogenic in nature, meaning that it expresses a combination of both maternal and paternal antigens (Gobert and Lafaille 2012). However, despite the fact that the paternal antigens are considered foreign by the maternal immune system, most maternal immune systems do not cause significant damage to the developing fetus.  This is made possible by the establishment of maternal-fetal tolerance, which increases the probability that the mother’s body will instead perceive the baby as “temporary self” (Trowsdale and Betz 2006).  However, establishing a maternal-fetal tolerance puts the mother at risk by limiting the capacity of her immune system to identify other foreign pathogens that could cause her harm.  Therefore, many non-overlapping mechanisms have evolved to maintain a delicate balance between immunosuppression – to protect the fetus from the maternal immune systems – and immune reactivity – to protect both mother and child from invading pathogens (Munoz-Suano, Hamilton and Betz 2011).   Recent research into maternal-fetal tolerance seeks to shed some light on these mechanisms, particularly those mediated by a subpopulation of T cells called regulatory T cells (T_regs) (Gobert and Lafaille 2012, Munoz-Suano, Hamilton and Betz 2011, Rowe, et al. 2013).

NK ALERT! New Subpopulation Linked in Restriction of B-Cell Transformation by EBV

EBV, or Epstein-Barr Virus, is a pretty common and current virus that affects approximately well over ninety percent of the human adult population. The virus belongs to the herpes family meaning after the bodies defenses respond to the initial viral lytic infection, the virus remains latent in B cells till further proliferation. The virus enters our tonsils after initial transmission via the saliva¹.

Usually, in the vast majority of infected individuals, virus induced CD8⁺ T cells are responsible for the main fight against EBV infection. However, Natural Killer (NK) cells play an important role for the initial innate immune response against EBV. This paper, A Distinct Subpopulation of Human NK Cells Restricts B Cell Transformation by EBV, further explores the abilities of a specific subpopulation of NK cells in preventing B cell transformation by EBV. The specific NK subpopulation, known as CD56brightNKG2A+CD94+CD54+CD62L2, was seen by researchers to produce an interestingly high amount of IFN-γ³. IFN-γ is an important cytokine that is essential for innate and adaptive immunity when dealing with viral and intracellular bacterial infections. The importance of IFN-γ in the immune system stems in part from its ability to inhibit viral replication directly, and most importantly for its immunostimulatory and immunomodulatory effects.³ So what does this mean?

SLAM Associated Protein (SAP) may Provide a Critical Role for Autoimmune Collagen Induced Arthritis through T Cell Development

             The formation of immune cells which attack one's own body (known collectively as autoimmune diseases) is still a widely studied topic which has yet to be fully understood. In normal immune function adaptive immune cells such as B cells and T cells are screened during development so that they do not recognize an antigen which is present in the body and therefore only attack invading pathogens; however when this process is disrupted these B and T cells will recognize cells of the body and attack them as if they were a pathogen. These diseases can be focused on the attack of specific organs or the body as a whole leading to devastating and often lethal consequences.   Recent studies have implicated a small protein (SAP) which is encoded by the SH2D1A gene in the manifestation of autoimmune diseases such as Systematic Lupus Erythematosus, Rheumatoid Arthritus, and Myasthenia Gravis.  This intracellular protein contains a single SH2 domain which binds to a family of receptors (known as the SLAM receptors) and is  implicated in a number of immune system functions including the differentiation and production of T cell subsets.  This protein acts in two ways: 1. It has the capacity to bind and activate the protein Fyn which is critical for the signaling pathways involved in cell growth and differentiation  2. It prevents the binding of inhibitory proteins SHIP1 and SHP1 which act to impede cell growth and differentiation.  Therefore the presence of the SAP protein presents a two fold mechanism for cell growth and differentiation which helps explain why it is a key regulator of immunce cell functions.

A recent paper published in the November 1st addition of the Journal of Biochemistry examined the role of SAP in the regulation of the autoimmune disease Collagen-induced Arthritus (CIA).  This study induced murine CIA (similar to Rheumatoid Arthritis in humans) and altered the expression of SH2D1A in order to elucidate the role SAP plays in disease progression.  The authors of the paper found that when the SH2D1A gene was eliminated in the mice using a Cre-Lox system the mice did not develop CIA while the genetically normal mice did.  The implication of this initial test demonstrated that SAP must be playing  some kind of role in the progression of this autoimmune disease.  To further this investigation, researchers then used the same Cre-Lox system to eliminate the SH2D1A gene (and thus the formation of SAP) but utilized different promoter regions which were specific to B cell or T cells.  This meant that half of the mice had the gene deleted in their B cells only and the other half had the gene deleted in their T cells only.  The results of this demonstrated that when the gene was deleted from B cells there was no change in the disease course but when the gene was deleted in T cells the mice did not develop CIA.  So essentially SAP plays role in the development of T cells likely through its binding of SLAM receptors and concurrent phosphorylation and activation of the Fyn protein.  To test this idea, a final line of mice were induced with CIA but this time the mice expressed a different allele in which the SAP protein could bind to the SLAM receptors but not Fyn.  When the disease induction was given in a high dose the inability of SAP to bind and activate Fyn made no difference in the disease incidence and only a slight decrease in disease progression.  In contrast, when the disease was induced with a low dose of the antigen only half the mice developed CIA and the disease progression was even more reduced.  This would indicate that the binding of SAP to Fyn IS (in fact) necessary for the course of the disease.

Transforming Growth Factor-β Regulates Tissue Resident Memory T cells Retention in the Gut

Tissues such as the skin, lungs, and the gastrointestinal tract act as barriers against invading pathogens. To protect the individual host, invading pathogens must be efficiently and quickly controlled by the immune system. In order to control pathogens with efficiency, one of the hallmarks of the adaptive immune system known as memory provides long-term protection of lymphocytes that easily recognize pathogens to rapidly clear them from the tissues. Cells in the immune system that partake in memory are known as long-lived CD8+ memory T cells, which include central memory (TCM) and effector memory (TEM) cells. TCM cells circulate through secondary lymphoid organs while TEM cells circulate through non-lymphoid tissues. Some of these memory T cells migrate into peripheral tissues, including skin, lung, and gastrointestinal tract, and become resident long-lived memory T cells (TRM) that protect against infection by taking up residence within the peripheral tissues. The underlying mechanisms and signals that regulate the retention of TRM cells in the tissues are not completely understood so scientists in a recent study looked at the effects of the TGF-b signaling on TRM cell migration to the gut tissues as well as TGF-b signaling while TRM cells reside in the gut tissues.  

An important point to know is that the TEM cells develop a gut-homing capacity during local and systemic infections that upregulate a bunch of integrins for the T cells to settle into the gut. The T cells in the spleen upregulate the expression of a4b7 integrin that would allow the T cells to settle into the gut. When the T cells are inside the lamina propria or intraepithelial lymphocyte compartment (both are structural tissues of the gut), the T cells downregulate the integrin a4b7. As a result of residing in the gut, the T cells upregulate CD103 (integrin aE), CD69, and integrin a1 leading the T cells to being TRM cells. The scientists noted that a lack of CD103 (integrin aE) expression leads to less T cells residing in the gut. Given that, the scientists think the 3 proteins’ expression leads to the T cells staying in the gut. The scientists realized TGF-b can increase the expression of CD 103 in the gut on CD8+ T cells to a great extent. TGF-b (Transforming Growth Factor Beta) normally works to differentiate Th17 and Tregs, but it also has many other functions. The scientists believed TGF-b signaling is important for the retention of intestinal TRM cells so they used an experiment to show the regulation of TGF-b on TRM cells.   

Inflammatory Bowel Disease: More Fun Than It Sounds

Inflammatory bowel disease (IBD) is a defined as inflammation of the intestines.  It comes in two forms: 1) Crohn’s disease (CD) which causes lesions in the entire wall of the bowel and 2) ulcerative colitis (UC) which is characterized as inflammation in the mucosal layer of the colon (hope this did not gross anyone out too much…).   Surprisingly though, the symptoms associated with IBD is NOT caused by a particular pathogen, but by the attack of one’s own immune system.  Therefore, IBS is characterized as an autoimmune disease.  Microbial translocation, where microbial products enter systematic circulation due to loss of barrier integrity of the intestines, has also been observed in IBD, as well as other disorders.  Although great strides have been made to the understanding of IBD, the roles of specific immune cells types are still not completely clear.  In order to better understand IBD, Funderburg et al. (2013) studied if the accompanying inflammation is linked to T-cell activation and thus microbial translocation. 

Funderburg et al. found several important results.  First, blood plasma samples were taken in individuals with IBD and soluble inflammatory markers were measured. CRP (a protein found in the blood during inflammation) and IL-6 (pro-inflammatory cytokine) levels both increased in IBD patients, which is not surprising because IBD is characterized by inflammation. 

Inflamed Colonic Tissue

"Ready or Not, Here I Come!" How Shigella flexneri Hides in our Bodies

            Dysentery is a GI disease with which not many in the industrialized world are familiar, but which posed a severe problem only a hundred years ago and still causes serious illness and death in the developing world. The bacterium that causes dysentery is called Shigella flexneri, which infects the mucosal surfaces of the colon and rectum. Shigella is a gram-negative, intracellular bacterium. This means that it has a cell membrane but no cell wall, and that when it infects a host it establishes itself and replicates inside the cell rather than in the spaces and fluid between cells. Intracellular bacteria can often be more difficult for the immune system to find, especially if they have ways to make a cell look healthy instead of showing the danger signs of infection.

            The innate immune system, or first responder to any infection of your body by a foreign pathogen, is not specific to particular pathogens and relies generally on compounds that are part of bacteria, viruses, or fungi, but are not present in humans¹. A distinguishing characteristic of gram-negative bacteria like Shigella is a compound called lipopolysaccharide, or LPS. LPS is a component of the bacterial cell membrane, and is highly toxic when separated from the membrane². LPS is strongly recognized by the innate immune system as a danger signal, also known as a pathogen-associated molecular pattern (PAMP)¹. LPS is made of a lipid called Lipid A, an oligosaccharide (a type of sugar polymer), and the O-polysaccharide, another sugar located on the bacterial surface³. Immune cells recognize the Lipid A part of bacterial LPS, and this causes the release of certain small molecules called cytokines. The specific cytokines released, particularly IL-1ß , TNF-alpha, IL-18, cause inflammation, which helps alert the rest of the immune system to start combating the infection, but also causes many of the symptoms associated with bacillary dysentery³.

1. Diagram of bacterial LPS

T Cell Exhaustion: A Normal Response to Viral Infections?

                T cell exhaustion is an interesting phenomenon that scientists have found to occur in chronic viral infections. This is a state in which T cell activity rapidly declines and enters a refractory period due to the presence of a virus. Initially, when a virus infects the body, specific CD8+ memory T cells (1) will produce cytokines (2) in the attempt to begin fighting off the infection. T cell exhaustion then occurs when T cells cannot function properly and cytokine production is occurring at a slower or non-existent rate (Wherry 2011). Thus, T cell exhaustion is often association with chronic viral infections such as cancer because chronic infections fatigue the immune system. Hindered T cell function affects the body’s ability to fight back against a disease, and it prevents the immune system from properly reacting to tumors and long lasting infections.

               

Yi, John S. et. al. T-Cell Exhaustion: Characteristics, Causes, and Conversion. April 2010. 

             Immunology. 129(4): 474-481.

Until recently, T cell exhaustion was considered to be a by-product of chronic infection, for scientists believed that it was an abnormal physiological and immunological response to the virus. However, in September an article was published by Martin Hosking and his other colleagues challenging this assumption and providing evidence for the claim that T cell exhaustion is a normal response to any type of infection: acute or chronic – or even latent. Infecting mice with one of two different types of acute viral infections, the scientists measured CD8+ T cell activity over time, looking at IFN- γ (3) cytokine production in relation to hours elapsed since primary and secondary infection. Their results indicated that acute viral infections induce T cell exhaustion as well; therefore they claimed that T cell exhaustion is a normal response to infection, and it should not only be associated with chronic viral diseases.

      

Ikaros Plays an Important Role in the Th17 Response

Autoimmune diseases are a sections of diseases with which we are struggling to find a cures, and if we can learn more about them, then we are likely to solve resolve this issue.  That is why this article was so interesting.  It talks about a specific lineage of T-helper (Th) cells, Th17 cells, and a protein, Ikaros, that can lead the naïve Th cells towards a Th17 response.  Before we jump into the article, though, some important background information is necessary.

The exact function of Th17 cells are though to be involved in autoimmune and tumor cell response¹, so how are these Th17 produce? After a naïve Th cell is activated by signals 1, 2, and 3, it can choose to illicit one of three different responses based upon the cytokines present within its surrounding environment: Th1, Th2, or Th17¹.  (If you’re interested in learning more about Th1 or Th2 responses check out this link) In order to become a Th17 cell, the activated Th cell must be exposed to IL-6  and TGFβ to differentiate¹.  After differentiation is complete, the now Th17 cell releases its own cytokines, IL-17 and IL-6, to cause an immune response¹.  For the visual learners, here’s an image that depicts this as well:

 

Peterson, David. "Am I TH1 or TH2 or TH17?" Living Wellness. N.p., 12 Oct. 2012. Web. 05 Nov. 2013. <http://livingwellnessblog.wordpress.com/2012/10/12/am-i-th1-or-th2-or-th17/>.

Another type of immune system cell is regulatory T (Tregs) cells which help to control the ongoing immune responses¹.  Additionally, Foxp3 is a necessary transcription factor that allows for proper development and function of Tregs which, when functioning properly, can inhibit the Th1, Th2, and Th17 response¹. 

A Silent Killer Meets Its Match: Hepatitis C

HCV(Hepatitis C Virus) is contracted physically by sharing needles, contamination with another's blood, sexual intercourse, etc. Hep C is not a deadly virus but it can possibly have a cure which has yet to be found. When people are infected they don't show any symptoms right away and it usually doesn't affect their lives but some suffer serious liver issues. Hepatitis C had decreased over the years from it's abundance in the 80's but is beginning to become prevalent again as drugs with needles and tattooing has increased in the newer generations.

While reading the NY times there was this article that was about the immunological discoveries regarding Hepatitis C and a cure. According to Dr.Mitchell, "we are on the verge of wiping out Hepatitis C." A man at 63 was in a clinical trial and appeared to be cured once he took a concoction of IFN and Ribavirin. Ribavirin is a nucleotide inhibitor that ends RNA viral synthesis. This prevents the virus from replicating and then allows IFN to effectively fight off the remaining virus. Sofosbuvir is another inhibitor that acts as a building block of RNA and blocks off virus reproduction. All three of these are needed in order cure the majority of Hep C. These methods are not completely accessible to everyone or effective so they are looking at other methods such as a one-a-day pill and one that doesn't need IFN because that could be detrimental to people with unstable psyche. They are trying to figure out these mechanisms soon and hopefully Hepatitis C cure will be a step in immunological advances for years to come. 

http://www.nlm.nih.gov/medlineplus/ency/article/000284.htm

http://www.nlm.nih.gov/medlineplus/druginfo/meds/a605018.html

Pollack, Andrew. "A Silent Killer Meets Its Match." NY Times 5 Nov. 2013, sec. D: D1+. Print.

Immunosupression in HIV/AIDS Mediated by Myeloid Derived Supressor Cells

MDSCs: Gabrilovich & Nagaraj, Nat Rev Immunol 2009

Recently, researchers have found a new subset of immune cells that functions to suppress immunity in multiple diseases. Early research has identified them as myeloid-derived suppressor cells (MDSCs), a heterogenous group of immature myeloid cells, divided into granulocytic or monocytic subsets (CD15 or CD14). Recent research has focused on their role in tumor immunology; Hosoi et al showed that during adoptive CTL (cytotoxic T lymphocyte) treatment of cancer (melanoma), CTLs triggered expansion of MDSCs that eventually outnumbered CTLs, secreted reactive oxygen and nitrogen species, and prevented further proliferation of CTLs. This research in cancer biology shows that immunotherapy and immune response can trigger a counter-regulatory response that dampens the immune system. In a recent paper, Ankita Garg and Stephen Spector tested the roles of MDSCs in viral infection, specifically HIV/AIDS, and found a very similar suppressive effect. As not much is known about the mechanisms behind how MDSCs function in the immune response, these authors used HIV infection of PBMCs and co-culture techniques to explore how MDSCs suppress immunity, covering their expansion, function, and mechanisms of function.

Origin of MDSCs; Gabrilovich & Nagaraj, Nat Rev Immunol 2009

To start, the authors treated PBMCs (peripheral blood mononuclear cells) with heat-inactivated or infectious HIV, or gp120, a HIV envelope glycoprotein responsible for binding to the CD4 co-receptor on a helper T cell and allowing the HIV envelope glycoprotein gp41 to contact the host cell membrane and promote viral fusion (see HIV and Neurocognitive Dysfunction). Both infectious HIV and inactivated HIV/gp120 were able to induce expansion of MDSCs (in this paper gated as CD11b⁺CD33⁺CD14⁺HLA-DR^-/lo; see note), suggesting that viral replication is not necessary of induction of MDSCs. Next, they looked that how gp120 was able to induced MDSC expansion, and found that PBMCs cultured with gp120 conditioned media showed increased an MDSC population, and found increased levels of IL-6, a major inflammatory cytokine, in gp120-treated PBMC supernatants, leading them to hypothesize that IL-6 was responsible for inducing MDSCs. They confirmed this by showing IL-6 neutralization inhibited MDSC expansion. Furthermore, they surmised that IL-6 would be acting through its STAT3 pathway (a major pathway for IL-6 signaling), and found increased levels of phosphorylated STAT3 (pSTAT3) in gp120-treated PBMCs. They showed that IL-6 neutralization completely abolished pSTAT3 expression in gp120-treated PBMCs, showing that IL-6 may cause MDSC expansion via signaling through the STAT3 pathway.

Role of Mucosal Immunity in HSV Infection

Herpes Simplex Virus (HSV) is one a virus that plagues many people as it is one of the most common viruses. It is transmitted by physical contact such as drinking, sexual contact, kissing, etc. There has yet to be a vaccine or cure to HSV which is interesting as it has been around so long and no cure has yet to be found. We have many defenses such as mucosal immunity but somehow this seems to make it through. We learned about many systems that play a role in mucosal immunity but for HSV the most pertinent would be vaginal muscosa. Mucosa has IgA secretory antibodies that can help defend from pathogens as well as other several cytokines.

In the paper, Role of Mucosal Immunity in HSV Infection, by Kuklin et. al. they were examining the purpose of mucosal immunity in HSV infection. Mice were immunized through the nose with 3 different variables: glycoprotein B, glycoprotein D, or VV. The mice were challenged with different concentrations of plaque forming units (pfu), a unit of particles capable of forming plaques. The mice were examined on a scale of 1-5 based on their severity. In order to measure monoclonal antibodies, they used anti-CD8+ and anti-CD4+ mAbs. This was analyszed with flow cytometry analysis (the use of an electronic detection to detect small particles in fluid such as mAb). This experiment lasted for 30 days.

After the initial immunization experiment they wanted to measure T cell depletion in vivo so they injected them with 2mg/mouse DP (DepoProvera). DP is a progesterone shot and it aims to regulate the period cycle of the female mice; similar to the DP shots women take as birth control. They challenged the mice with 5 X 10 ⁶ pfu of HSV. Anti CD8+ and anti CD4+ mAbs were given to the mice on day 2 and day 4. The mice were sacrificed after 10 days of this second experiment and there cells were combined and analyzed using two-color flow cytometry. Using samples of serum and vaginal secretion, they were able to use the same process as before to measure Ab production. 

Viruses achieve latency by immune-suppressing mechanisms

The majority of people will be exposed to a virus in the Herpes family at some point that will remain in their body for the rest of their life.  The Herpes family is an example of viruses that can establish latency, which is when the virus remain dormant within the host cell and are no longer proliferating, but their viral genome is still present and is being replicated along with the host genome.  In their publication, Human Cytomegalovirus Latency-Associated Proteins Elicit Immune-Suppressive IL-10 Producing CD4 T Cells, Mason et al. examined the mechanism of how these viruses establish latency.  The group focused on a member of the Herpes family, Human cytomegalovirus (HCMV).

HCMV infection is typically asymptomatic, unless the infected person has a compromised immune system.  During the initial infection, there is an extensive CD4+ and CD8+ T cell response (general information on T cells), which controls the active virus.  However, despite the initial immune response, the virus is unable to be cleared and it establishes latency within the host.  The virus establishing latency within the host is problematic because the virus is able to become active (lytic) again, and if this occurs at a time when the immune system is compromised, the individual will experience disease-like symptoms.  In order to clear these viruses from our body, it is important to understand how they are evading our immune system during latency. 

There are different proteins expressed at different stages of HCMV infection.  During the lytic (active) phase, one of the major viral proteins that are recognized by the T cells is gB which is considered one of the immediate early (IE) genes.  When IE genes are absent, this indicates that the virus is latent.  A small amount of HCMV viral genes are expressed during latency, namely UL138 and LUNA.  Interestingly, the viral genes expressed during latency are also expressed during lytic infection.  If there is a T cell response against UL138 and LUNA in both lytic and latent infections, why is it that the infection unable to be cleared in latency?

Mycobacterium tuberculosis and Immune Evasion

Many of you have likely heard of tuberculosis (TB), a potentially lethal, infectious disease caused by intracellular bacterium called Mycobacterium tuberculosis. Although this bacterium usually attacks the lungs, it can also affect other parts of the body including the brain, the spine, or the kidneys. Tuberculosis germs are spread through the air, particularly when an infected person coughs, sneezes, or vocalizes. Symptoms of tuberculosis include weakness, a chronic cough, night sweats, fever, and weight loss. Tuberculosis is one of the world’s deadliest diseases. In 2012, there were 1.3 million tuberculosis related deaths worldwide (World Health Organization 2013). In fact, one-third of the world’s population is infected (Centers for Disease Control and Prevention 2013).

It is important to distinguish between two tuberculosis conditions: latent TB and active TB (also known as TB disease). Latent TB is defined by the presence of a TB infection, but the bacteria remain in the body remain inactive and do not cause symptoms. This is because the immune system acts to “wall off” the bacteria using a cellular structure called a granuloma which forms around the invaders. Active TB occurs when the immune system cannot prevent the TB bacteria from multiplying in the body. A person with active TB will be contagious and exhibit symptoms. People most susceptible to developing active TB disease are those who have been recently infected by TB bacteria or those with medical conditions that weaken the immune system such as HIV infection, tobacco use, or diabetes mellitus. Specifically, 90% of infected patients will have latent TB, while only 10% will progress to disease (World Health Organization 2013). TB is generally curable with antimicrobial drugs, but in many underdeveloped nations, access to such health care is unattainable. If untreated, 50% of active TB disease cases are fatal (World Health Organization 2013). Additionally, resistance to the medicines is increasing. This is evidenced by the emergence of multi-drug resistant TB, which is the result of bacteria that do not respond to standard anti-TB drugs (Centers for Disease Control and Prevention 2013).

Clearly, TB is a relevant concern in today’s world. An October 2013 study by Heuer et al. (http://www.biomedcentral.com/1471-2172/14/48) examined the effects of two Mycobacterium tuberculosis antigens on human dendritic cell maturation and how this affects immune response. This study was investigating the viability of these antigens as vaccine candidates, while also examining how these antigens may be involved in immune reaction or immune evasion. Antigens are entities (in this case, an element of the Mycobacterium tuberculosis) that can bind to the antigen receptor of a T or B cell (leukocytes that mediate adaptive immunity). Dendritic cells are phagocytic leukocytes that process antigen material and function as antigen presenting cells. Specifically, infection of these dendritic cells by Mycobacterium tuberculosis leads to the downregulation of the expression of MHC class I and II and CD1 (cell surface proteins which present antigens to T cells). Therefore, if there are less of these antigen presentation molecules, antigens will not be effectively presented to T cells and NKT cells so effective immune responses won’t be promoted. This suboptimal immune response is considered to be a cause of susceptibility to Mycobacterium tuberculosis.

A Few Short Term Interactions May Have Long Term Implications for Memory Cell Fate Decisions

Most people understand that when people become infected with a pathogen, like a virus or bacterium, their immune system provides a rapid and robust response to clear the pathogen. However, did you know that there are many different types of immune cells? Some of our immune cells act immediately during infection and then die, and some are long lasting and become memory cells. Vaccines mediate protection against pathogens by stimulating the production of immune cells, some of which eventually become memory cells. When we get a vaccine of an attenuated or inactivated pathogen our memory cells “remember” the pathogen, so if it attempts to invade our bodies again the memory cells can tell the rest of the immune system how to respond; this is called immunological memory. Because memory cells confer long-term protection against pathogens, much effort goes in to understanding how a cell becomes destined to be a memory cell. And while much progress has been made in this field, the complicated signals and interactions involved in deeming a cell a memory cell are still unknown (Kaech et al., 2002).

One important type of immune cell is the T cell. There are many types of T cells, and one of these essential T cells is the CD8+ T cell, sometimes called a cytotoxic or killer T cell. These CD8+ T cells effectively “kill” regular cells in the body that have been infected with a pathogen; this helps stop the pathogen from multiplying and infecting more cells nearby. Because CD8+ T cells play such a central role in preventing infection, a subset of activated CD8+ T cells during infection become memory cells, to provide a swift response upon pathogen re-exposure. But how do these activated CD8+ T cells know if they should be a short-term effector cell, or to become a memory cell to stick around for the long term? A group of researchers thinks that they have found a piece to this puzzle. A recent study shows that the memory fate decision may be a function of the type of activation the T cell receives.

CD8+ T cells, hereon referred to as cytotoxic T cells or just T cells, are specific for a certain antigen. An antigen in this context is a peptide, a piece of protein, from a pathogen that the immune system recognizes as foreign, and serves as an activator for the immune response. A cytotoxic T cell is activated when a foreign peptide for which it is specific is presented to it by another immune cell called a dendritic cell (DC) (Fig. 1). The DC encounters the pathogen, takes up the peptide, and presents it to the naïve (inactivated) cytotoxic T cell. There are two phases to this activation process. In the first, the DC and cytotoxic T cell make many serial brief contacts. In the second phase a long-lasting interaction, about 30 minutes, is made between the cytotoxic T cell and DC cell. It was previously thought that both phases were necessary to activate the cytotoxic T cell to begin to divide, and make effector cells to go out and kill cells infected with pathogen (Hugues et al., 2004). However, this study has found that the interactions in phase I and II determine if a cell will become a short-term or memory cytotoxic T cell.

Fig. 1. DC:T Cell Interaction

The authors of this paper used a mouse model to study T and DC cell interactions because mice have a similar immune system to humans. The scientists isolated DC cells in culture, and treated them with either high (100C) or low (1C) amounts of an antigenic peptide. The DC cells that were 100C are considered to have a high concentration of antigenic peptide on their surface to be presented to their cognate cytotoxic T cell, and the 1C are considered low concentration. The scientists then fluorescently labeled the DCs, and injected 1C or 100C DC cell populations in to mice. Then 18 hours later, the researchers injected the mice with fluorescently labeled cytotoxic T cells that had a receptor specific for the antigenic peptide presented by the injected DCs. The scientists then waited a few hours to allow the T cells to travel to the lymph node where the injected DC cells reside. Then, to visualize the interactions between the T and DC cells, the group used a method called multiphoton intravital microscopy. Using this method, scientists can look through a microscope in to a tissue of a living mouse, and watch interactions between cells. The group from this paper used this technique to count the interactions, and length of interactions, between T cells and DC cells in the lymph node. (**If you would like to know more about multiphoton intravital microscopy, a link is provided at the end of this blog post. Note it is slightly graphic.)

Pathogen Mode of Entry Has a Significant Effect On Host Adaptation

In order for us to properly understand the diseases that make us sick we must examine the entire mechanism the pathogen (disease causing agent) uses to invade our bodies. Much experimental work has been done to understand the pathogen mechanism once it has generated an immune response in the body, however one of the most important steps in the mechanism, which hasn’t really been experimentally studied, is the mode of entry. There are two major ways that a pathogen can enter the body, either through ingestion (orally) or systematically (a prick or needle). When a pathogen enters the body systematically it bypasses much of the innate physiological and anatomical barriers that prevent pathogen entry, like our skin, sweat, oils and mucus we secrete. In contrast, a pathogen must overcome many more barriers when entering the body orally, for instance saliva.

It has been shown in a recent experiment that the path of pathogen entry affects the rate at which the pathogen is eliminated and the way that it responds to the same type of pathogen. In the experiment equal amounts of both male and female Drosophila melanogaster flies were administered with one of two treatments of P. entomophila bacterial strain. Half the male and female flies were introduced to P. entomophila orally (BactOral), whereby food plates were coated in the bacterial strain. The other half of male and female flies was injected in the thoracic (area where the legs and wings attach) region with P. entomophilia (BactSys). Controls for both the injection (ContSys) and food treatments (ContOral) were also observed in the experiment. The mortality of the flies was measured for at least 10 days. It was found that the survival rate of the flies was much higher when the bacterial strain was introduced orally. The evolution of resistance to the particular bacterial strain also developed much faster when P. entomophilia was introduced orally.

These results reaffirm the theoretical notion that oral pathogens have to overcome more physical barriers in order for them to enter the body cavity. This is in contrast to a systematic infection because the physical barriers, like the skin have already been bypassed and the bacteria have been directly injected into the body cavity. So when a pathogen enters systematically the body must rely on other, more active ways of identifying and eliminating the invader, namely through members of the immune system that belong to the adaptive (more-specific) response like plasmatocytes. These more complex and specific ways of eliminating the invader are slower to act because B cells must be activated to produce antibodies, that than must find the pathogen and kill it, whereas much of the pathogen that was introduced to the mice orally didn’t really require the activation of the adaptive immune response. It only had to rely on the innate (more general) immune response and physiologic barriers that were already in place like the mucosa layers of the respiratory and gastrointestinal tracts to prevent pathogen entry into the body cavity. Since these defenses are already localized to the site of pathogen entry and need no further activation, the immune response is much faster when the pathogen is administered orally rather than systematically.

Th17 Cells Play a Role in Alzheimer’s Disease

Neuroinflammation mediated by Th17 cells in the brain has been linked to the neurodegeneration characterized in Alzheimer’s disease.¹

Alzheimer’s disease (AD) is the number one cause of dementia, which is classified as a loss of cognitive functioning due to neurodegeneration.^♯ AD is most common in people over the age of 60, and over 5 million Americans are estimated to have the disease.^♯ With the “baby boomer” generation entering this age group, increased understanding of AD is even more important in order to treat an aging population.

Healthy Brain vs. Alzheimer's Disease Brain

Many changes occur in the brains of people with Alzheimer’s disease. For an interactive tour of these changes, click here. Changes to the hippocampus and cortex are responsible for increased memory loss and decreased cognitive function. An overall shrinking of the brain and increase in the size of the ventricles occurs. These changes increase over time and are irreversible.

Currently, the most well understood cause of Alzheimer’s is due to a buildup of amyloid-β (Aβ) plaques.² These plaques develop when a protein called APP is cleaved by enzymes to create Aβ. The hippocampus and neocortex of the brain are most vulnerable to the plaques, which are responsible for behavioral and functional deficits of AD.² However, new research has targeted neuroinflammation as an important component in AD progression.³ Some experiments have shown that inflammatory mediators stimulate APP breakdown to further contribute to the disease.³


In a recent experiment conducted by Ju Zhang et al., the effects of Th17 immune cells in the brain of AD-model rats were studied.¹ Th17 cells are a type of differentiated helper T leukocyte. Helper T cells are characterized by the presence of the coreceptor CD4 and are important for adaptive immune function. Th17 cells specifically are responsible for inflammation through the release of cytokine “danger signals,” such as IL-17 and IL-22 and for autoimmune response.¹

Based on the knowledge that Th17 cells are involved in neuroinflammation and in Alzheimer’s brains, Zhang et al. hypothesized that Th17 cells are directly responsible for neuronal cell death through the interaction of transmembrane proteins Fas and FasL.¹ Fas and FasL are well-known receptors and ligands, respectively, which are involved in a pathway for apoptosis (programmed cell death). Fas exists on neurons, and FasL is located on the surface of Th17 cells.¹ The binding of these proteins is able to occur in AD brains, because a faulty blood-brain barrier (BBB) allows T cells to cross it, leading to elevated levels of Th17 cells in brain parenchyma.⁴ This does not occur in healthy brains, and, as we can see, has negative effects.

Sprague-Dawley rat

To test the hypothesis, it was necessary to induce rats to develop brain changes that would imitate AD. In this experiment, 4-month-old Sprague-Dawley rats were used.¹ Aβ was injected into the hippocampi to induce neurodegenerative changes that have been shown to imitate both pathological and behavioral characteristics of AD.⁵ Rats were studied 7 or 14 days following the injection. APP expression was upregulated and neuron loss occurred and the changes showed greater progression in AD-like changes from 7 to 14 days.¹

The study also demonstrated the effect of AD on the BBB of the rats. The BBB was shown to be impaired through the presence of immunoreactive cells for RORγ in the hippocampus of AD-model rats compared with control and saline-injected rats.¹ RORγ is a transcriptional factor specific to Th17 cells, and RORγ-positive cells indicate that the BBB in AD brains is faulty and allows Th17 to leak into the hippocampus.