T-cells Found To Be Linked To Influenza Immunity – Could A Universal Vaccine Be On The Way?

Influenza, or more commonly referred to as the flu, is a highly contagious virus that affects the respiratory system. Because it is so contagious, it can affect thousands of people annually. In fact, somewhere between 5% and 20% of the United States population will contract the virus each year (2). However, despite its frequency of occurrence, the flu is not always a deadly disease. Of the people who are infected, there is an average of 200,000 hospital visits a year, and, depending on the strain, the number of flu-related deaths can range from 3,000 to 49,000 (2). The following fact is what makes the flu so dangerous though: the virus mutates every year. Every year doctors scramble to create a vaccine that will protect the public from the different flu strains that hit during flu season; they do not always make the appropriate vaccine, though, and that is when fatalities can accumulate. This leads to a pandemic.

The most recent influenza pandemic was the H1N1 virus that occurred in 2009. Also known as swine flu, this virus hospitalized thousands; everyone was caught off guard. On top of that, scientists found that symptoms varied drastically among similar individuals with the swine flu. Some people experienced no symptoms while some were hospitalized. This has shown with the other strains of influenza as well, and this was a question that scientists desperately were trying to find an answer to: What decides whether one is going to be asymptomatic? After the onset of the swine flu in 2009, a group of researchers at Imperial College London began a study hoping to answer this question, and the results that they found were groundbreaking.

Correlation between HLA alleles and pH1N1 mortality rates

            Many people have had the flu at last once in their life and see their doctors every year for a flu vaccination.  The vaccine is typically trivalent, which means that it protects against three different strains of the influenza virus: H1N1, H3N2, and an influenza B strain.  This vaccine is given as an intradermal shot meaning it is injected into the skin instead of the muscle².  Seeing as how flu season is fast approaching I though it fitting to bring to light an article relating the flu, or more specifically the pandemic influenza A virus strain (pH1N1) of the 2009-2010 flu season.  However, before delving into the article I would like to provide the basic, necessary background information.

            When an antigen is engulfed by an antigen presenting cell (APC), its proteins are broken down and a small peptide chain of about 10 base pairs is expressed so that activated cytotoxic T-cells can destroy the infected cell³.  The proteins that express these peptides are called MHC (major histocompatabiltiy complex) class I and MHC class II proteins which are encoded by the human leukocyte antigen (HLA) gene³.  For MHC class I gene on HLA there are three regions: HLA-A, HLA-B, and HLA-C for which many different alleles exist³.  Some MHC alleles display the peptides for T-cell recognition better than others which is why different people have different immune responses to the same pathogen.  

Additionally, CD8+ is the specific receptor that MHC class I binds to on T-cells³.  Infected cells also release interferons (INFs) that increases the expression of MHC molecules which helps to fight off infection by causing more interactions with T-cells³.

There May Be Hope for a Universal Flu Shot

A recent study suggests that the development of vaccines that protect against multiple strains of Influenza A Virus (IAV) on a long-term scale is possible.¹

If you have ever stayed home from school or work with a sudden fever, cough, sore throat, and generally achiness, you probably had the flu. You may have even gotten the flu shot, but influenza still got the best of you. Vaccination for the flu is difficult, because the virus mutates and develops different strains all the time.² The flu shot or nasal mist that we use to prevent influenza is typically trivalent, which means it has the ability to protect against three specific flu strains.³ These strains change depending on what scientists predict will be the most dangerous strains to the public that year.³ Obviously, an ideal vaccine would protect against any form of the flu. For this reason, research that shows how CD8 T cells can be cross-protective against heterosubtypic infections is especially promising.

The goal of the study was to better understand the effects of vaccination on the way the immune system responds to infection with IAV. Influenza A virus is found in both animals and humans. It is also generally responsible for seasonal flu epidemics in humans.⁴ The most at risk individuals are children, the elderly, and other people with weakened immune systems. Influenza is responsible for between three thousand and forty-nine thousand deaths per year, as well as up to two hundred thousand hospital visits.⁴

  

In order to understand the methods and

results of the experiment, one must have a basic understanding of certain components of the immune system, as well as specific terms and phrases that are important in the study.

CD8 T cells are cytotoxic cells that specifically terminate the acute influenza infection and contribute to long-term memory of the virus.¹ Virus-like particles (VLP) are developed from influenza virus proteins, but they do not contain IAV genomic material.¹ This means that resemble the actual virus, but they are replication deficient. Because of this characteristic, VLPs have a greater potential for use in the vaccination of high influenza risk individuals.¹ Lung-draining lymph nodes (dLNs) are the part of the body in which naïve CD8 T cells are primed. The cells then leave the dLNs when they migrate to the lungs (the site of infection with IAV).

The study conducted by Hemann, Kang, and Legge made many comparisons between control mice and mice that were vaccinated with VLPs prior to IAV infection. The experiment was broken up into several different parts in order to show how different aspects of immunity contributed to their overall determination.

The Power of the MHC

 HLA MHC Complex on

 Human Chromosome 6

Infectious diseases threaten us and our animals on a daily basis. However, inside the human body on chromosome 6, there is a group of genes that is essential for your protection and health. A May 2013 paper by Kubinak et al. examined viral evolution in the context of the major histocompatibility complex (MHC). The MHC is one of the most gene-dense regions in the vertebrate genome (Kubinak et al. 2013). Although in humans, it is only approximately 3600 kb in length, it contains roughly 224 genes, 40% of which have been found to have immunological functions (Garrigan 2003). Therefore, despite making up only 0.1% of the genome, it contains a disproportionate number of genes associated with immunity. It is evident that the MHC is crucial to the immune system.

Infectious and autoimmune diseases have been correlated to polymorphisms within the MHC. Polymorphisms are defined as different alleles of a gene existing in a population. The high levels of polymorphisms in the MHC are suggested to be due to co-evolution between host genotypes and pathogens. In other words, the “pathogen bad guys” evolve to avoid detection by the “good guys” in our bodies. The “good guys” respond though a process by which MHC genes encode cell surface proteins which bind and present peptide antigens (in the form of peptide MHC complexes) to T cells. A T cell mediated immune response (“good guys” spring into action) is then triggered if the antigen is recognized as foreign. Specifically, pathogen adaptation and virulence evolution seem to be locked in a battle with polymorphic antigen-presenting MHC genes for control of the health of the human body. It is primarily the interaction effect between the virus genotype and the host genotype that dictates variation in viral fitness and virulence, not the individual effects of virus genotype or host genotype (Table 1). This leads to specialization between a particular pathogen and host genotype.

HIV and Neurocognitive Dysfunction

            HIV, human immunodeficiency virus, infection is one of the largest public health problems we face today. The WHO estimates that currently over 35 million people worldwide are infected with HIV. HIV primarily infects and kills helper T cells, cells that have the critical role of activating and supporting other cells of the immune system, thus impairing the immune response on a broad level. It is well known for causing AIDS, the acquired immunodeficiency syndrome, which severely weakens the body’s immune response and leaves it open to attack by foreign antigens. However, less widely known, HIV also causes neurocognitive impairment and encephalitis (swelling of the brain), estimated by Heaton et al to affect one third of individuals infected with HIV. This facet of HIV infection, which affects a whopping 10 million people worldwide, is an important concern and is now receiving more attention.

            Recently, the Section of Infectious Diseases of the Nervous System at NIH, led by Dr. Avindra Nath, and their colleagues at Johns Hopkins School of Medicine have discovered possible mechanisms for HIV-mediated neurocognitive impairment. In a recent paper in the Journal of Neurovirology, these researchers looked at the impairment of adult neurogenesis and neurite outgrowth in the hippocampus using a HIV mouse model. They believe that the growth patterns of neurons in this part of the brain, which deals with processes such as memory consolidation, may be disrupted during HIV infection and underlie the cognitive dysfunction seen in HIV infected patients. While previous research had found that HIV infects glial cells and not neurons, it was shown that HIV infection lead to synaptic pruning and apoptosis (Ellis et al 2007).

            The investigators used a transgenic mouse model that expressed the HIV envelope glycoprotein gp120 under the control of the GFAP promoter (glial fibrillary acidic protein, a protein expressed by glial cells in the brain). HIV gp120 is part of the HIV viral envelope “spike,” a device used to bind host cell receptors and enter the cell. Briefly, the spike consists of three gp120 units bound to three gp41 units; gp120 binds to the CD4 co-receptor on a helper T cell, allowing the HIV envelope glycoprotein gp41 to contact the host cell membrane and promote viral fusion. Thus, the investigators used this mouse model to simulate HIV infection, during which gp120 would be circulating in the body.

Do T Cells Have A Need for Speed?

A recent study focuses on the time required for T cells to identify, interact with, and mount a response against pathogens within the human body.

The life of an infectious pathogen is not an easy one. Should a virus, bacterium, prion, or fungus manage to make it past the body’s first line of defense – obstacles including anatomical barriers such as the skin and physiological barriers like the hostile low pH environment of the stomach – it still must face a barrage of innate and adaptive immune responses before successfully establishing itself.  Once a pathogen has entered the body, the innate immune system – defined as the cells and mechanisms that defend the host against foreign organisms in a non-specific manner - may recognize the non-self intruder and induce an immune response. This eventually leads to the activation of the adaptive immune system and, in turn, of white blood cells like T and B lymphocytes (or T and B cells) which quickly identify and eliminate the pathogen. It is the accuracy and speed of these cells’ responses that makes the adaptive immune system so effective in protecting the body against infection. So the short answer is, yes, T cells do have a need for speed. To contest to this, a recent study now suggests that that once a T cell senses an antigen – a task that can take only a matter of seconds – it can decide the fate of the invading microorganism within minutes.

This new study, released in The Journal of Immunology just a few months ago, tested T cells under various conditions to judge their aptitude for identifying invading pathogens. According to the authors, there are four factors that limit the ability of T cell to recognize pathogens. These include: 

1. T lymphocyte bears a single T cell receptor (TCR).  This means that any given T cell can only recognize a single antigen structure. Therefore T cells are very sensitive to interactions but are vastly outnumbered by antigen presenting cells (APCs)!
2.  Thousands of proteins can be used to generate the peptide - major histocompatibility complex (pMHC) on the invading cell surface. The T cell must be able to quickly recognize one to a few of these intricate complexes on the cell to identify it.  
3. Exposed cell pMHCs may differ by a few amino acids. Despite this, the TCR must still recognize them and be able to properly bind to trigger a T cell response.    
4. The TCR and pMHC complex is only ~ 14 nanometers in length. One nanometeris one-billionth of a meter, so these are miniscule complexes that must somehow closely interact in a stable way. This interaction is strongly dependent on the tiny movements made by the T cell membrane.

These limiting factors require that we know two important things about T cells if we want to understand the speed and specificity of the antigen detection process. Firstly, we need to know the how often a T cell and an antigen presenting cell contact each other and for how long. Secondly, we need to know if and how T cell membrane motions are altered as a result of antigen detection.

The Road to Unconventional Immune System Memory

            

One of the coolest parts of the human immune system is its ability to remember previous exposure to viruses, bacteria, or other foreign invaders.  This is crucial in creating effective vaccines that prevent humans from contracting potentially fatal diseases.  If an invader is remembered, the immune system can launch a highly specific attack and prevent the person from ever getting sick.  However, this memory is not well understood by scientists.  As more and more studies are completed, new information about new subsets of cell types involved in or capable of memory is being released.  For example, you may know how long it takes to drive from Philadelphia to Denver, but what good is that if you don’t know what roads to take?  This same idea can be applied to the memory function of the immune system.  It’s great that scientists know that once a person is infected, certain cell types can remember the invader, but it’s difficult to promote a memory response if it’s unknown how the response even occurs.

            

General Memory B Cell Development

A recent paper published in The Journal of Immunology explores the road to a memory response.  The two big cell types involved in the pathway they explore are T and B cells.  The T cells involved help to activate B cells, which then produce antibodies.  These are small proteins that circulate through the body are responsible for “tagging” foreign bodies, or antigens, for destruction by other immune cells or neutralizing the effect of the foreign body.  Antibodies come in different structures with different functions, and can switch structures during the development and maturation of the B cell from a naïve cell to memory cell.  The two structures the researchers look at are IgM, which are usually produced by naïve cells that haven’t switched antibody type yet, and IgG, which are usually produced by B cells that have fully matured into memory B cells.  However, recent studies have shown that there are also memory IgM B cells, but their characteristics, purpose, and development are unclear.  Immune cells are often identified and characterized by the molecules expressed on their cell surface.  Called CD markers, or cluster of differentiation, they allow scientists to give a unique expression pattern to help identify and isolate new types of immune cells.  In the quest to understand the memory IgM B cells, one of the things the authors tried to undercover was a unique CD expression pattern on these cells.  They also looked at a mouse model of human ehrlichiosis, a bacterial infection from ticks, to explore the necessity of a T cell-B cell interaction for the activation of these IgM memory cells.  The also used this model to look at a secondary exposure to an antigen and determine the connection between these new memory cells and typical IgG memory 

Tiny Pests Causing Huge Problems

Why do people put on bug spray when they go outside?  The obvious answer is that having itchy mosquito bites all over your arms and legs is one of the most irritating feelings ever.  However, most people forget that these tiny insects can carry extremely dangerous viruses that can cause severe illness or even death.  West Nile virus (WNV), a well-known mosquito-borne virus, is the major cause of encephalitis (inflammation of the brain), which is a disease characterized by flu-like symptoms, and in severe cases, seizures and sensory/movement issues.  Dengue virus (DENV) is another dangerous mosquito-borne virus that is a major cause of viscerotropic disease, which is a disease that is associated with yellow fever and ultimately leads to organ failure and death.  These viruses cause significant problems all over the world, and currently there are no approved vaccines to prevent or combat WNV and DENV.  In order to develop effective vaccines, we need to understand how our bodies’ immune system recognizes the viruses and provide protection.  “Innate Immune Sensing of Flaviviruses” is a recently published paper in PLoS Pathogens, in which Suthar et al. (2013) discusses recent research findings on the way that the innate immune system interacts with WNV and DENV to trigger an antiviral response.¹ Also, understanding how these viruses can evade our immune response is equally important for developing new treatments.  A group mentioned in “Innate Immune Sensing of Flaviviruses,” Aguirre et al. (2012)², investigate one of the main mechanisms that DENV uses to inhibit inducing an immune response.

           

WNV and DENV are part of the flavivirus family, and they contain a single stranded RNA genome.  In order to recognize these viruses, we encode receptor proteins called pattern recognition receptors (PRRs), which recognize and bind to structures on pathogens that are not present in normal, human cells.  These structures are called pathogen-associated molecular patterns (PAMPs).  Once the virus is recognized, antiviral immune defenses are initiated which leads to inflammation.

  

The two PRRs that recognize the viral, non-self RNA structures are called retinoic-acid inducible gene-1 (RIG-1)-like receptors (RLRs) and myeloma differentiation factor 5 (MDA5).  RIG-1 recognizes and binds to double stranded RNA with a triphosphate group on the 5’ end.  MDA5 interacts with long double stranded RNA.  Double stranded RNA does not exist in normal, host cells, and it is a common viral PAMP.  Once the virus is recognized, RIG-1 and MDA5 induce the production of type 1 interferon (IFN) and proinflammatory cytokines that will migrate to the site of infection and recruit/help produce other factors needed to eradicate the virus.  A study done by Fredericksen et al. (2008) demonstrated that RIG-1 and MDA5 function cooperatively in creating an antiviral response to WNV.³ They found that RIG-1 is activated first by WNV, and then MDA5 is activated to sustain type 1 IFN and ISG expression (initiated by RIG-1).³   It is known that the PRRs RIG-1 and MDA5 are involved in pathogen recognition and initiate the immune response, but it is still unclear what the exact PAMP RNA substrate and nucleic acid sequence that RIG-1 and MDA5 interacts with.