HIV
HIV-1 is a retrovirus.
As stated above, HIV-1 is closely related to the HTLV-1 and HTLV-2, as all are human retroviruses. In fact, at first, Dr. Robert Gallo named the virus HTLV-3, but it quickly became clear that this virus was different. 19 The virus was renamed HIV-1, and it was categorized as a lentivirus, the prefix 'lenti' meaning slow. 20 They are a subset of the retroviruses that have the unique ability to infect non-dividing cells.
Nonetheless, HIV-1 and HTLV share many characteristics and mechanisms. They both infect the same immune system cells, which are cells that have CD4 surface proteins, such as T helper cells and dendritic cells. The life cycle of HIV with in the cell, is very similar to HTLV, and is described in the previous section. When the virus is released, infection spreads directly to the surrounding cells. Infected dendritic cells often carry the virus directly into the lymph nodes where these T helper cells congregate, providing large numbers of target cells for HIV to infect. The virus is then carried throughout the body by the blood and the lymph, and a systemic infection is established.
Another similarity between the viruses is that they both establish a latent infection. Their genome is integrated into the host genome, and, at certain times, produce such a low level of cellular activity that they are unable to be detected by the host immune system. These 'inactive' cells have come to be referred to as virus reservoirs, and can be thought of as a viral 'safe space'. This latent infection can take several days to set up, but that is usually enough time before the host adaptive immune system is ready to mount its response.
HIV also has some profound distinctions. Unlike HTLV, HIV usually kills the cells it infects because it produces far greater numbers of virus, which tear up the cellular membrane upon mass exit. For HTLV infection to occur, the target cell must be active. This is not true of the more aggressive HIV virus, which can infect some cells that are not proliferating at all. This strengthens the latent infection, but HIV really prefers a proliferating cell which has the ability to manufacture large amounts of new virus. When this happens, the immune response is powerful, but HIV even uses this to its advantage. It hijacks the entire system, infecting the now rapidly proliferating T helper cells, and other immune system cells which are designed to fight it. As more T helper cells are produced for the purpose of fighting the infection, they are infected and destroyed by the HIV virus, releasing ever more virus and producing a rapid and often devastating systemic infection.
In the early years of the AIDS epidemic, when little about HIV was understood, and there was really no treatment of any kind available, death from AIDS due to HIV infection was often swift and terrifying. However, even then, in many patients infected with HIV, this initial infection was often followed by a chronic phase, during which the patient remained either asymptomatic or demonstrated only mild symptoms. This is typical of HTLV, but as HIV is a much more aggressive virus, this phase does not last so long. As time wears on, the total number of T cells decreases as production of new ones cannot keep pace with the destruction of infected ones. After perhaps several years, there are not enough T helper cells to assist the killer T cells, and the virus eventually overwhelms the body's natural defenses, leaving the patient vulnerable to a host of pathogens, which, under normal circumstances, would rarely present a problem.
It is estimated that, today, thirty years after those frightening years at the beginning of the AIDS epidemic, there may be as little as a fraction of one percent of patients from that time who are still alive.
Anti-Retroviral Drug Therapy for Treatment of AIDS
Fortunately, treatment of HIV using anti-retroviral drug therapy has made it the chronic but manageable disease that it has become today. Azidothymidine (AZT) was the first, and for some time, the only approved drug available, but today, the number and variety of medications used to treat HIV has increased dramatically. Many have been fast tracked or have received accelerated approval by the FDA, especially during those early years when time was of the essence. There have been many drug and vaccine trials, with varying degrees of success and failure. Countless alternative therapies have come and gone, often representing the final glimmer of hope for AIDS patients desperate to survive.
One other notable aspect of HIV-1 not mentioned in the previous section is its extreme genetic variability. This produces mutant strains not only from host to host, but also in strains within a single individual. All retroviral reverse transcriptase (RT) is highly error prone when compared to cellular DNA, which has an error frequency of about one mistake per billion nucleotides copied, but HIV RT has one of the highest error frequencies ever seen in any virus. 21 This rapid genetic change over time is thought to contribute to the prolonged and progressive nature of the infection by allowing the mutating virus to avoid recognition by the host immune system. These mutations also have implications for current day drug therapies, because these variant unrecognizable strains are likely to produce drug resistance and subsequent treatment failure. AZT was shown to be resistant in some patients as early as 1989. 22
Today, combination therapy, meaning multiple drugs, is used when treating HIV. This is particularly true for long term survivors, whose virus has likely undergone many mutations. Regardless of the variety of names these drugs are given, all of them basically inhibit some mechanism of the virus at some point in its life cycle, as detailed in an earlier section. Most of these drugs, therefore, are referred to as 'inhibitors'.
The first point at which the virus can be attacked is at the entry level, thus the class of drugs which do this are called Entry Inhibitors (EI). Maraviroc binds to and distorts CCR5, a protein on the T cell surface to which HIV binds. 23 Fuzeon, also known as T20, is an injectable fusion inhibitor that binds to gp41, and interferes with process of bringing the virus and cell into direct contact. 24 There are also other receptors the virus can use, so as can be seen, there are multiple ways in which a virus can enter a cell, and research continues to develop multiple ways to stop that from happening.
The next step at which the life cycle can be disrupted is the RNA to cDNA replication, and this class of drugs is referred to as Reverse Transcriptase Inhibitors (RTI). AZT is an early nucleoside RTI, and it works by posing as a nucleotide, and then interrupting the growing DNA chain, because its 3' carbon end is defective. 25 Truvada is a newer nucleoside RTI drug, and has less toxic side effects than AZT. There are also variations of RTIs available called nucleotide RTIs, such as Tenofovir. 26 Both of these are known as competitive substrate inhibitors because they substitute into the viral replication. Non-nucleoside RTIs, such as Intellence 27, use a completely different mechanism. They inhibit the movement of several proteins which are necessary to synthesize the cDNA molecule, and are thus called non-competitive inhibitors. Like the entry inhibitors, there are a variety of inhibitor options.
The integration of the viral genome into the host DNA can be prevented by using an Integrase Inhibitor. This class of drugs is relatively new, and Isentress, the first of its kind, was approved by the FDA in late 2007, after a very successful trial. 28 There are also variations in the way these drugs prevent the host DNA from being cleaved, facilitating the insertion of the viral genome.
During the final assembly stage of the new viral particles, an enzyme called HIV protease cuts the long polypeptide chain into shorter pieces that become the functional proteins that travel with the virus when it exits the cell. Because there is no equivalent enzyme in the cell that the virus could use instead, this is an ideal point at which to interrupt the virus. There are now many Protease Inhibitors on the market, and they have become increasingly effective since their introduction on December 6, 1995. The first was named Saquinavir, and was released through the FDA's accelerated approval program, marking the turning point in the war on AIDS. 29
HIV and Gene Therapy
The purpose of gene therapy is to modify the genetic material of living cells to produce a positive health outcome. It involves the insertion of a functional gene that contains some kind of information sequence. There are two types of gene therapy, somatic cell and germ line. A procedure using the somatic cell method can potentially eliminate the effects of a disease, without passing the inserted gene to the patient's offspring. Germ line therapy passes the inserted gene, because the reproductive cells will carry the new gene.
Prior to 1996, scientists mainly used modified retroviruses when gene transfer into the chromosomes of target cells was needed, and adenovirus vectors when such integration was not needed. A vector is simply a name given to genetic material that carries information into a cell. However, there has been little success in gene transfer with such virus vectors because even though the vectors can enter into their target cells, the cells need to be dividing to break the nuclear membrane so the gene can enter and integrate into the chromosome.
Scientists have realized that because members of the lentivirus subfamily, to which HIV belongs, also have the ability to transfer genetic material into the genomes of non-dividing cells, they might be a good choice to use as a vector. As it turns out, lentiviruses do provide highly effective gene therapy, and can change the expression of a target cell's gene for up to six months. 30 Gene therapy using lentiviruses and HIV-1 has been the focus of a number of labs during the past few years.
HIV has a unique 'matrix' protein that contains a localization sequence which is recognized by the import machinery of the nucleus of many cells. Once inside the cytoplasm of the cell, the nuclear import machinery docks the complex at a nuclear membrane pore, and then lets it pass into the nucleus. Lentiviruses can be used with differentiated cells such as neurons, macrophages, hematopoietic stem cells, retinal photoreceptors, and muscle and liver cells, cell types for which previous gene therapy methods could not be used. 31
HIV contains a single stranded RNA-genome that is approximately 10 kb long, flanked with long terminal repeats (LTR) that aid in the initiation of the transcription of a particular gene. Being a very aggressive virus, it is also a strong promoter, which means that it is highly successful at being taken up by the target cells. It has been shown that the HIV vector has a higher rate of expression in target cells than any other retrovirus The vector is designed so that a genetic sequence is cloned into a region that is flanked by LTRs, promoting integration of that gene into the genome of the target cell, just as the LTRs in HIV integrate the cDNA into the host chromosome. Certain proteins are not included, and the virus particles produced are replication deficient, and are therefore unable to continue to infect their host after the therapeutic gene is delivered.
The vectors are produced ex vivo, meaning they are manufactured outside of the host. The cells carrying the vector are then reintroduced in the patient for the therapeutic effect, whether that be halting protein production or enhancing it.
To date, three children with Metachromatic Leukodystrophy and three others with Wiskott-Aldrich syndrome, both of which are inherited mutations, have been treated with HIV vectors. Their diseases have stopped progressing and some of the children have stopped showing symptoms for up to 32 months following therapy. 32
Another disease involving HIV vectors and showing promising results in the clinic is called X-linked adrenoleukodystrophy (X-ALD). It is a rare central nervous system disorder, most common in males, and often affects the brain, eventually producing a vegetative state. A lentiviral vector based on HIV has been used with positive benefits for two patients with this disease, while other vectors were not able to produce these results. X-ALD was first brought to the public eye in a 1992 movie called Lorenzo's Oil. 33
Gene therapy trials involving lentiviruses now account for about 3 percent of gene therapy trials worldwide. 34
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