Myeloid Dendritic Cells Enhance HIV Latency in T Cells

Diagram of the HIV virus

The Human Immunodeficiency Virus commonly known as HIV in the last 30 years has become a worldwide epidemic affecting approximately 23 million people with an additional 4 million cases annually.  This virus systematically disrupts and destroys the host immune system and is the direct precursor to Acquired Immunodeficiency Syndrome or AIDS.  Despite years of intense scientific research there is no cure or vaccine for HIV.  The current form of treatment is continual use of anti-retroviral drugs which can help to keep HIV at bay but are expensive and have side effects.  The virus itself invades a number of different immune cells and inserts its DNA into the DNA of the cell causing it to produce more of the HIV virus.  The virus itself is also toxic to subsets of these cells known as CD4+ T cells which die after the virus has replicated within the cell.  The destruction of these CD4+ T cells mediates the inability of the immune system to fight off other pathogens. 

                A major problem in combating HIV is that after the virus infects a cell it sometimes lays dormant in the cells to be replicated later.  In this case the virus is not immediately killing the cell but it in the meantime escapes normal processes of destruction.  This viral latency allows the virus to remain in the immune system undetected and escape destruction by other immune cells or anti-retroviral therapy.  Despite this common occurrence, the mechanisms underlying this process are relatively unknown.  However, a recent paper has partially uncovered how some CD4+ T cells are latently infected with the HIV virus.  A paper published this month by Evans et al. has demonstrated the myeloid dendritic cells are responsible for the latent infection of inactivated CD4+ T cells.  In this study, researchers co-cultured inactive CD4+ T cells with different types of infected dendritic cells to see whether they would cause a latent infection and if so which dendritic cells specifically.  The resting CD4+ cells were given a green fluorescent protein which gave off a green light when the cells were actively infected with the HIV-like virus.  The researchers then separated out the non-glowing cells which were not actively infected and further examined those.  When these CD4+ T cells were stimulated a small percentage of them turned green when they had not before, indicating that they contained the virus previously but it was not active and thus the virus was in a latent state.  This process allowed scientists to culture the CD4+ T cells with other cells or chemicals to see what factors caused the cells to have an active or latent infection.  Given this, it was soon discovered that infected myeloid dendritic cells caused a latent infection but not other types of dendritic cells.  Myeloid DCs are cells which migrate in the body and encounter pathogens.  These pathogens are then engulfed, chopped up by enzymes within the cell, and represented on the surface of the cell so that T cells may encounter the antigen and form an immune response against it.   Intriguingly it was previously thought that chemicals released by DCs or the environment called cytokines might be playing a role in the process, however, when the chemicals from the DC-T cell interaction were removed and cultured with new CD4+ T cells they did not develop a latent infection indicating that the myeloid DCs must have a physical connection to the T cells for this process to occur.  The connection between the two is an immunological synapse which involves numerous molecular adhesion molecules such as LFA-1 or ICAM.  The same study found that when they blocked these adhesion proteins there was a reduced effect of infection latency among CD4+ T cells but it did not eliminate latent infection completely so more than these two adhesion molecules must be involved.  These would be new potential targets for anti-retrovirals or other drugs.

Depiction of HIV spread from a DC to a T cell

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?

Just Sitting Back, Biding Its Time. Waiting for the Moment to Pounce

     Human cytomegalovirus (HCMV) is a type of herpesvirus. HCMV infects the majority of individuals, but a healthy immune system is able to suppress a primary infection. Upon initial infection HCMV inhibits an antiviral response through the up-regulation of myeloid cell leukemia-1 protein (MCL-1). The induction of MCL-1 expression is dependent on the activation of the ERK-MAPK pathway rather than viral gene expression.(1) The ERK-MAPK pathway induces proteins such as mitogen-activated protein kinase and extracellular signal-regulated kinase. After the primary infection is controlled HCMV establishes a latent infection in undifferentiated bone marrow precursor cells and monocytes cells.(2) The activation of the ERK-MAPK pathway impacts long-term latency in progenitor cells by priming them from the initial virus encounter and enabling them for future reactivation.(1) When the individual with the latent infection becomes immunocompromised such as during transplants, however, the latent HCMV can become reactivated and have detrimental effects. During viral latency there is limited viral gene expression and no detectable virion production. One viral protein expressed during latency is latency unique natural antigen (LUNA) encoded by the latency associated transcript UL81-82ast. Previous studies have suggested that LUNA is involved in the regulation of HCMV reactivation by suppressing lytic transcription and without LUNA may not be able to enter into latency in order to be reactivated later.(2,3) Another viral protein present during latency is UL138. The UL133-138 is required for HCMV infection in endothelial cells.(4) The function of UL138 in productive and lytic infection is understood, but not much is known about the function of UL138 in latent infection.

     Using plasma membrane profiling the authors determined which plasma membrane protein levels were affected by the presence of UL138 during HCMV latency. Multidrug resistance-associated protein-1 (MRP1) was the most dramatically down-regulated of the three proteins affected by the presence of UL138. In fact, in the presence of UL138 MRP1 was undetectable in cells, suggesting that it might be getting degraded.  MRP1 is important in multidrug resistance and handling organic anions. It is found in bone marrow progenitor calls and monocytes, as well as mature leukocytes(5). In infected cells, the deletion of UL138 restored the expression of MRP1, indication that UL138 is necessary for MRP1 down-regulation. UL138 is also known to be present during lytic infection in addition to latent infection and high levels of UL138 48 hours after lysis coincides with decreased levels of MRP1. It was determined that MRP1 mRNA levels were not decreased from UL138 expression, so the down-regulation of MRP1 is probably post-transcriptional. MRP1 produces many cytotoxic agents including vincristine. When vincristine was added to HCMV-latent monocytes the number of latent cells decreased as well as the expression of UL138 RNA. This indicates that vincristine caused the death of latently infected cells. In addition to reducing the number of latently infected cells through killing, vincristine also reduced the number of cells in which reactivation of HCMV occurred after differentiation of monocytes to dendritic cells.

     What is not understood is why UL138 targets MRP1 for degradation in latent cells. One possible reason is that the reduction of MRP1 also reduced LTC4 which is a substrate of MRP1 and therefore inhibited migration of infected cells and impaired the activation of an immune response. The decreased expression of MRP1 could also inhibit the differentiation of precursor cells until the environment for reactivation was sufficient, therefore maintaining latent infection.