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.