The Retrovirus group received its name from its manner of reproducing. “Retro” means ‘backwards,’ indicating that the viruses have a backwards mode of nucleic acid replication. The retroviruses are RNA viruses, but they replicate by means of a DNA intermediate. When DNA is produced, it readily integrates into the host cell chromosome. In such a state, the viral DNA is able to provide genetic messages that transform the host cell – which is also characteristic of tumor cells. However, the DNA transcribed from the HIV RNA does not easily integrate, but proceeds to destroy the host cell.
Retroviruses have unusually high incidents of mutations because the enzyme they use to transcribe the RNA to DNA, reverse transcriptase, has no error-correcting function, nor does it have an editing enzyme to change the code frequently in order to copy without making mistakes. Therefore, the rate of error of one change becomes 2,000 to 4,000 nucleotides for every generation of the virus.
Retroviruses are of interest for several reasons:
- They were the first viruses shown to cause cancer and have been extensively studied for their carcinogenic characteristics.
- The one causing AIDS has only been known since the early 1980s.
- The retrovirus genome can become specifically integrated into the host genome, paving the way for studies in gene therapy.
- The retroviruses have an exzyme called “reverse transcriptase’ which copies RNA sequences into DNA becoming a major tool in genetic engineering.
Like other retroviruses, HIV is a single-stranded RNA virus that infects and destroys helper T cells – basically weakening the immune system. To be even more specific, it is a class of retroviruses known as lentiviruses, associated with chronic arthritis and anemia, and known to cause slowly progressive fatal diseases. HIV needs a combination of at least forteen proteins of its own, plus the chemical combinations of the host cell in order to complete its replication process.
HIV stands for ‘human immunodeficiency virus.’ It is responsible for the disease known as AIDS (acquired immunodeficiency syndrome). This virus was first known as GRID (Gay-related immuno deficiency) until it started to cross those barriers into other sectors of society. The earliest documented case of a HIV infection was found in a 1959 vintage blood sample from central Africa. HIV is the third virus to be discovered since 1980.
HIV, like other retroviruses, causes latent infections, destroying a few cells in the beginning. The virus is a lymphotropic (preferring lymphoid cells) virus that prefers to infect helper T cells that have the CD4 marker. Once this infection has occurred, HIV can lurk unnoticed for as long as twelve years, while the virus is passed from one cell to another. This method of cell-cell transmission prevents the virus from being exposed to antibodies in the blood that might destroy it. While most antibodies are fighting off invaders, some antibody B cells are sent to the lymph nodes, which are able to produce specific antibodies on demand. However, these antibodies are only for viruses and bacteria that have previously been introduced into the body and not for the likes of HIV. As the infected cells begin to divide, the hidden virus bursts through the envelope, destroying the host cells and spewing new viruses into the bloodstream. The virus can also infect such other lymphoid cells such as B cells, and those of the brain, colon, Langerhan’s cells, and testes. During the late stages of the infection, the body may lose and replace two billion CD4 lymphocyte cells a day, with new viruses appearing at about 100 million to 680 million a day.
There are two different HIV isolates, HIV-1 and HIV-2. Both are antigenically different. HIV-1 is the original isolate made in 1983, and HIV-2, first isolated in 1986. It is found mostly in West Africa, Portugal, and Brazil. Even though AIDS was first recognized in the US in 1981, the etiology of the disease was not known. What was known was that each patient developed a rare form of cancer known as Kaposi’s sarcoma, suffered unexplained weight loss, had a fever and an enlargement of the lymph nodes (lymphadenopathy), and had an unusual type of pneumonia caused by the parasite Pneumocystis carinii. The defence system in these patients is so affected that numerous other diseases develop without any resistance to them. Serious life-threatening diseases caused by viruses, bacteria, fungi, and parasites are common. This basic immune deficiency coupled with complicating factors of associated diseases has resulted in a condition with a mortality rate that is essentially 100%. In addition to lymphotropism, HIV is also neurotropic, which results in the development of psychiatric abnormalities in some patients, which includesthe following: behavioral changes, slow thinking, decreased memory, and difficulties with speaking and walking. The virus also seems to speed up and work synergistically with other disease-causing agents. For example, tertiary syphilis normally takes years to develop, but in those who have AIDS, this phase may surface in just a few months.
HIV has five key strategies that make it one of the most successful disease-causing viruses known to date:
- its ability to remain latent for long periods;
- its slow mutatation;
- its capacity for unorthodox reproduction;
- its subversive capabilities;
- its being transmissable by a variety of means.
Through much lobbying and incredible pressure from certain groups, the mass media is rapidly replacing the concern for cancer and other diseases with a concern for AIDS. In terms of human life lost, AIDS does not exceed the devastation of such other infectious diseases as smallpox, plague, or typhus; but, in terms of economic demand, social change, and commitment of medical resources, it may eclipse all previously known maladies.