A New Mechanism of Cell Death by HIV

            It’s a disease that is well known all over the world: human immunodeficiency virus, better known as HIV.  It is often talked about in tandem with acquired immunodeficiency syndrome, or AIDS, which develops in HIV patients over time and is the end stage of the disease.  HIV originated in chimpanzees as simian immunodeficiency virus (SIV) and transferred over into humans in the 1800s.  The first cases in the United States were reported in 1981.  Throughout the 1980s, cases increased dramatically, peaking in the early 1990s.  However, a breakthrough in drug treatment for people living with HIV and HIV prevention campaigns helped to bring the number of cases back down.  The drug therapy known as antiretroviral therapy (ART) is still used today as the main way to help people with HIV live normal lives, hopefully preventing/delaying the progression to AIDS.  They target different points in the HIV virus replication cycle to try and slow down its progression through the body.

Figure 1: HIV replication in a cell.  It is able to dump its
contents into the cell, reverse transcribe its RNA,
integrate it into host DNA, and use the host to create new viral copies.

HIV is a retrovirus, a form of RNA virus that can be inserted into the host DNA, and then uses host cells to replicate.  The virus comes with all sorts of proteins that let it do this.  For example, reverse transcriptase allows the viral RNA to be turned into viral DNA.  Integrase allows it to be inserted into the host DNA to then use the host’s own protein making mechanisms to make new viruses and viral proteins.  Figure 1 shows an outline of HIV replication.  It is possible to trace the viral RNA to viral DNA to host DNA and then back out to spread to other cells. 

HIV also specifically infects a certain class of immune cells known as CD4+ T cells.  CD4+ T cells are called this due to the presence of a cell surface receptor called CD4.  These cells can differentiate into all different subtypes of CD4+ T cells called T helper cells, or Th cells, whose name describes their function: they “help” other immune cells mount responses to pathogens.  CD4+ T cells and all of their progeny are crucial for providing immunity to all sorts of infections, pathogens, and the like.  HIV comes into play and infects the CD4+ T cells.  It creates a chain reaction, infecting, spreading, and slowly killing all the CD4+ T cells in the body.  In simple terms: HIV is slowly knocking out an entire branch of the immune system.  Any further immune function that would need a CD4+ cell to work won’t be able to work once the CD4+ T cells are gone.  Figure 2 lays this out in a graph showing CD4+ T cells in blue, viral RNA copies in read, and time in weeks on the x-axis.  It is possible to see that as the RNA copies go up, the CD4+ T cells go way down.  Many of the ART drugs target these proteins that prevent the HIV from infecting cells as easily or spreading once it has infected a cell.  However, it is still unknown how HIV actually kills CD4+ T cells.

Figure 2: Timeline of HIV infection.  As viral RNA increases,
CD4+ cells decrease.  A latent period exists where the
person may not know they are infected
until their cell count reaches a certain point.

Cooper et al demonstrate one potential way HIV could kill CD4+ T cells.  They first infected cells with HIV and stained for a particular viral protein called p24.  They noticed that the CD4+ T cells that were killed didn’t express this viral protein, while cells that weren’t killed did.  Next, they looked at whether these cells that were lacking expression of this viral protein had been infected with the virus before they died.  More T cells were infected with HIV that also encoded for green fluorescent protein (GFP), which fluoresces green.  GFP is often used as an indicator for protein production. The gene for GFP is placed within the HIV genome, so if HIV proteins are being produced, GFP will also be produced.  If the cells are dead, no GFP will be detectable.  This is a commonly used method to visualize and also quantify protein production.  They analyzed the cells for GFP expression and cell viability, as well as viral cDNA.  Non-viable GFP- cells were found to have copies of viral cDNA.  When viable GFP+ cells were watched over time, the researchers saw that many of these cells eventually died (therefore losing their GFP expression) but retained viral cDNA.  These data together suggest that the cells that were killed died after successful HIV gene expression.

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