Cellular+Treatment+of+Human+Immunodeficiency+Virus+(HIV-1)

= **Introduction** =

toc HIV-1 is a distressing subject because the virus can become a death sentence once Acquired Immunodeficiency Syndrome (AIDS) develops. Since the human body is a complex series of processes that all occur simultaneously, research on how the body functions is a primary area of concern. In attempting to counteract the virus, research has taken multiple paths to assess how HIV affects the human body. Trends in current treatment research include cell-to-cell transmission, reverse transcriptase modification, and the use of protease inhibitors. A review of these treatments and their translation into antiretroviral drugs, their ability to work effectively, their strengths and weaknesses in past studies, and how they can further be improved will be discussed. Areas in need of additional research may be identified.

=**Cell-to-cell transmission research** =

In order to best understand the cell-to-cell transmission research, it is important to explain how the HIV-1 virus is transmitted. The HIV-1 virus is able to replicate itself inside the T-cell and spread to other cells via cell-to-cell transmission and the formation of virological synapses that can contribute significantly to the spread of the virus (Wang et al. 2017). The virus is involved in an active process in which cells are being infected and dying at a very high rate and in large numbers - this results in an extremely large number of replication cycles that drives the pathogenic process and causes genetic variation (Coffin 1995). This poses a challenge to researchers when searching for ways to combat the virus as its high mutability means researchers are trying to hit a rapidly moving target. Mathematical modeling on cell-to-cell transmission demonstrated that if the reproductive numbers for viral infection ( R 0) are less than one, then the virus can be cleared from the system; however, if ( R 0) is greater than one, then the virus will replicate without inhibition (Wang et al. 2016). Due to this high rate of replication, it’s a target for vaccine production because successfully reducing the cycles will reduce the rate of infection of HIV-1 and therefore, it will lower the threshold needed to prevent AIDS. This is consistent with biological findings of HIV epidemics in vivo, where at the level of the lymph nodes, cell-to-cell transmission is crucial for the development of the disease. The cell-to-cell HIV pathway sets off a stronger immune response than that of the virus-to-cell transmission in lymphoid tissues (Pinto et al. 2017). Scientists are focusing on a way to strengthen the body’s immune system in order to keep (R0) less than one. Researchers began studying the use of antibodies to increase the immune system’s response and therefore reduce the spread of HIV-1 in the lymphatic system.

In order to slow down the rate of cell-to-cell infection, broadly neutralizing antibodies (bNAbs) were introduced as a treatment for the virus. These antibodies have been isolated from HIV-1 patients and they block a wide spectrum of HIV-1 subtypes (Li et al. 2017). These patients were identified via their improved recovery and reduced their status from terminal to chronic. Their ability block multiple subtypes is in part a key to their success because resistant strains of HIV-1 are a large problem. The cells that use bNAbs to produce antigens are referred to as dendritic cells and are often the infection point for the virus (Dutarte et al. 2016). Since these cells are located everywhere in the body and more densely in the lymphatic system, providing dendritic cells with antibodies staves off infection and progression into AIDS. By introducing these antibodies, it’s possible to fully inhibit cell-to-cell transmission of the virus, however there are barriers that arise with this treatment.

The challenge that researchers face with this method stems from the virus’ ability to mutate at a faster rate than vaccines can be designed to fight them. The “polymorphisms” in the domain of the virus that fuse to CD4+ lymphocytes will not only affect the virus replication capacity but also the ability to target the virions in the envelope as they vary across subtypes (Silva et al. 2016). Some possible barriers that may limit the success of bNAbs are the emergence of HIV-1 resistant mutants as well as the establishment of latently infected cells. Though mutant resistance is likely to occur with single monoclonal antibodies, using a combination of bNAbs has been successful in reducing the probability of a mutation occurring. Multiple bNAbs can be created through isolation of single monoclonal antibodies from donors (Walker et al. 2009; Boulle et al. 2016).

Latent cells that are inactive in the host’s immune system are a cause of concern especially in antiretroviral therapy as they can become viral during periods of stress and from a fatigued immune system from other illnesses. However, it was found that bNAbs are successful in reducing the rebound of HIV-1 in rats when combined with anti-latency agents. Finally, when comparing antibody-based therapy for HIV-1 infection with the conventional antiretroviral therapy, antibodies appear to work more effectively. This is mainly because of their ability to directly target both free HIV-1 and HIV-1-infected cells, unlike antiretroviral therapy which only targets infected cells, thus boosting immune responses via engagement with the effector immune cells; this therapy is well tolerated in patients (Yaseen et al. 2017). This may offer a new approach to vaccination by adapting natural immune strategies. Therefore, bNAbs could be a valuable addition to the “armamentarium” of drugs that work against HIV (Klein et al. 2013). Further research on this model of treatment could offer great promise of an HIV vaccine (Burton et al. 2012). The use of antibodies in conjunction with other antiretroviral treatments presents an additional area of research that could offer the best possible outcomes for HIV patients.

=**Reverse transcriptase** =

In order to combat this virus, reverse transcriptase inhibitors (RT) were developed to undermine HIV-1’s ability to replicate itself inside the host cell. These inhibitors are bioengineered to compete for binding sites on the reverse transcriptase enzyme that cause loss of function. Researchers were able to isolate the exact enzyme of reverse transcriptase that HIV-1 uses to attack the host’s DNA. In this way, they discovered that conformational changes that accompany the formation of the catalytic complex, or structure of the virus, to produce distinct residues that are altered and are not resistant to reverse transcriptase inhibitors (Huang et al. 1998). Other investigations of the HIV-1 RT were successful in describing its structure as the four individual subdomains of RT that make up the polymerase domains, and are named fingers, palm, thumb, and the connection (Jacobo-Molina et al. 1993). After these studies were completed, research began using RT inhibitors to combat HIV.

When testing the use of RT inhibitors, scientists first had to figure out the virus’ accuracy and replication rates. To determine the fidelity of purified HIV-1, researchers ran different polymerization assays on the virus and found the high error rate in replication suggests that the misincorporation by HIV-1 reverse transcriptase is responsible for the hypermutability of the AIDS virus (Preston et al. 1988). When the accuracy was analyzed for RT in HIV-1, it was found that it was the least accurate reverse transcriptase described to date and the data was consistent with the notion that the diversity of HIV-1 genome results from error prone reverse transcription and therefore an easy target to undermine the functionality of the virus (Roberts et al. 1988). Additional testing was done on HIV-1 in vivo versus purified HIV-1 to determine the forward mutation rate for HIV-1, which was approximately 3.4x1025 mutations per base pair per cycle. However, the in vivo mutation rate for HIV-1 is about 20 times lower than the error rate of purified HIV-1 reverse transcriptase. The same target sequence indicated that cell mutation is less likely to occur in the body than in the lab (Mansky et al. 1995). This would indicate that the mutation rate is lower inside a host body than outside and therefore reduces the risk of the virus becoming resistant to antiretroviral treatments.

Researchers have developed multiple antiretroviral treatments that are effective in different ways. Scientists were able to produce azidovudine (AZT), a popular antiretroviral treatment that had greater successful treatment rates when it was first introduced. However, a resistant population of the HIV virus developed and an infectious molecular clone that constructed four mutations in the RT yielded highly resistant HIV-1 strains, rendering the treatment somewhat obsolete (Larder et al. 1989). Further research on AZT determined that the same genome that conferred AZT resistance also increased its sensitivity to AZT, however, the patients that switched to dideoxyinosine (ddI) allowed for the suppressant effect of AZT resistance and was used as treatments in conjunction with ddI (St Clair et al. 1991). Scientists also engineered the drug BI-RG-587 (Ki of 200 nanomolar) for HIV-1 RT as an anti-viral treatment because it had no effect on feline and simian RT or any mammalian DNA polymerases, indicating that the drug only targeted HIV-1 function without affecting host processes (Merluzzi et al. 1990).

Building on Merluzzi’s study results, researchers were able to develop a drug for the HIV-1 RT called abacavir, which is a potent HIV-1 nucleoside analogue reverse transcriptase inhibitor. However, the drug is not a completely successful treatment for HIV as about 5% of patients develop a hypersensitivity reaction characterized by multisystem involvement that can be fatal in rare cases (Mallal et al. 2002). Because the proportion of new HIV infections that involve a drug-resistant version of the virus is increasing in North America, an initial antiretroviral therapy is more likely to fail in patients who are infected with a drug-resistant virus. Testing for resistance to drugs before therapy begins is now indicated even for recently infected patients (Little et al. 2002). Other drugs have attempted to circumvent fatal reactions, and a tenovir gel was created for women to apply externally. The tenovir gel was successful in reducing HIV acquisition by 39% overall and 54% in women with high gel adherence (Karim et al. 2010). Though successful in reducing the transmittance of HIV, the tenovir gel did not have a high margin of success so further studies on application via injection or through an intermediate, such as other topical gels, present an area of additional research. Other scientists focused on another trend in HIV research, protease inhibitors.

=**Protease inhibitors** =

Protease inhibitors block the activity of the protease enzyme, which HIV uses to break up large polyproteins into the smaller pieces required for assembly of new viral particles. While HIV can still replicate in the presence of protease inhibitors, the resulting virions are immature and unable to infect new cells (Staszewski et al. 1999). When targeting the protease enzyme, scientists were able to diagram a structure at 2A resolution, which is the highest resolution seen so far (Clark et al. 2016). These results demonstrated the power of a combined docking/QSAR approach to explore the probable binding conformations of compounds at the active sites of the protein target, and can also help to design and screen new compounds to obtain new HIV protease inhibitors with high activities (Tong et al. 2017). Researchers were able to show that protease inhibitors used in antiretroviral therapy directly inhibited the purified HIV enzyme. From this, they were able to develop atazanavir and lopinavir, however, resistant strains rose to the surface.

Five variants of the virus emerged and exhibited cross resistance to all six structurally diverse protease inhibitors, which suggests that combination of multiple inhibitors may not reduce viral activity and that prior use of a different inhibitor could affect the potency of a new inhibitor (Condra et al. 1995). Multiple complications from medication also arose from antiretroviral treatments such as cardiovascular disease, diabetes, and chronic kidney disease. After several years of research, darunavir was developed and has proved to be the strongest protease inhibitor yet with improved kidney function and reduced viral load by 79% (Jose et al. 2017). However, long term effects of darunavir have yet to be studied. Using protease inhibitors in combination with antibodies might demonstrate a greater impact and should be researched further.

=**Conclusion** =

In this review, three treatment options for HIV infection were identified and discussed in regards to their effectiveness, as well as strengths and weaknesses. Cell-to-cell transmission has been the most recent focus of research efforts and has yielded successful results. Despite the challenges that arise from viral mutations, bNAbs have proven to improve host immune function and effectively changed HIV diagnosis from terminal to chronic. It was also shown that bNAbs used in conjunction with other antiretroviral drugs could yield the best results. The most common antiretroviral treatment are reverse transcriptase inhibitors. Their affinity to undermine the virus’ ability to replicate has proven to be effective. However, there is an increase in resistant strains to this inhibitor and the effectiveness of treatment options are low and some result in multi-organ failure that can be fatal in some cases. Improving the effectiveness of the treatments for protease inhibitors should be a strong topic for further research. Protease inhibitors were another option for HIV vaccine research, but have serious side effects that outweigh the benefits. These side effect include cardiovascular disease, diabetes, and chronic kidney disease. Another issue is that the virus can be resistant to multiple inhibitors, therefore lowering their effectiveness. These studies suggest that research in the field of antibodies and immune system strengthening should be increased in order to slow down the development of AIDS and decrease mortality rates. They can be further researched in conjunction with antiretroviral treatments.

=**Discussion**=

Based on the results of the review, research on HIV treatments has not slowed despite less media coverage on the subject. Even though a “vaccine” has not been produced to date, the intricacies of developing an effective treatment have proved to be more challenging than first realized. Cell-to-cell transmission research has picked up significantly and seems to be the most promising treatment for two reasons: (1) it’s not producing severe side effects and (2) it improves the immune system function, which is more effective than targeting the virus itself. Further research in this field could lead to HIV simply being a treatable condition such as the flu vs. a lifelong disease, and should be pursued until that goal is reached.

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