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HIV Slide Set
Human Immunodeficiency Viruses
In 1981, the Centres for Disease Control reported cases of Pneumocystis carinii pneumonia and Kaposi's sarcoma in previously healthy young male homosexuals. Before then, pneumocystis carinii was mainly known to occur in immunodepressed patients following organ transplantation or suffering from congenital immunodeficiencies. Shortly afterwards, the same condition was seen in IV drug abusers, haemophilliacs and babies of IV drug abusing mothers. It was recognized that these patients had profound immunosuppression due to the depletion of T4 helper lymphocytes. The name acquired immunodeficiency was coined for this syndrome. Epidemiological studies soon established that this disease was infectious, transmitted by sexual intercourse, blood or blood products. There were initial difficulties in isolating the virus due to the fact that the lymphocytes of patients died early. Eventually the virus was isolated by Montagnier and Gallo in 1984. In 1986, HIV-2 was isolated from West Africa which did not cross-react serologically with HIV-1 in screening tests.
Belong to the lentivirus subfamily of the retroviridae
Enveloped RNA virus, 120nm in diameter, contains 2 copies of the RNA genome
Replication depends on reverse transcriptase
cytopathic viruses, in contrast to the oncornaviruses
HIV-2 less cytopathic for in cell culture than HIV-1, and shares 40% nucleotide homology with HIV-1
HIV-II is more related to SIV than to HIV-I
genome consists of 9200 nucleotides (HIV-1)
gag core proteins - p15, p17 and p24
pol - p16 (protease), p31 (integrase/endonuclease)
env - gp160 (gp120:outer membrane part, gp41:transmembrane part)
other regulatory genes are present in addition to the above ie. tat, rev, vif, nef, vpr and vpu
Electronmicrograph of HIV particles
Schematic representation of an HIV particle
The replication cycle is as follows; The first step of infection is the binding of gp120 to the CD4 receptor of the cell, which is followed by the fusion of the virus and cell membrane and is mediated by the gp41 molecule. The virus then penetrates into the cell and uncoating, reverse transcription, provirus synthesis and integration takes place. This is followed by the synthesis and maturation of virus progeny.
Source: Mims C, Playfair J, Roitt I, Wakelin D,
Williams R. 1993.
Medical Microbiology. p. 24.16
During 1996, members of the seven-transmembrane spanning receptor family of chemokine receptors were proved to be the necessary co-receptors for HIV-1 entry. These receptors were CXCR4 (fusin) for T-cell line (T)-tropic strains and (principally) CCR5 for macrophage (M)-tropic strains. It is important to note that both M- and T-tropic viruses replicate in primary CD4-positive T cells (Figure 1), despite the confusing nomenclature, which will soon be revised.
The profound immunosuppression seen in AIDS is due to the depletion of T4 helper lymphocytes. In the immediate period following exposure, HIV is present at a high level in the blood (as detected by HIV Antigen and HIV-RNA assays). It then settles down to a certain low level (set-point) during the incubation period. During the incubation period, there is a massive turnover of CD4 cells, whereby CD4 cells killed by HIV are replaced efficiently. Eventually, the immune system succumbs and AIDS develop when killed CD4 cells can no longer be replaced (witnessed by high HIV-RNA, HIV-Antigen, and low CD4 counts).
HIV-1 Half-Lives. Activated cells that become infected with
HIV produce virus immediately and die within one to two days.
Production of virus by short-lived, activated cells accounts for
the vast majority of virus present in the plasma. The time
required to complete a single HIV life-cycle is approximately 1.5
days. Resting cells that become infected produce virus only after
immune stimulation; these cells have a half-life of at least 5-6
months. Some cells are infected with defective virus that cannot
complete the virus life-cycle. Such cells are very long lived,
and have an estimated half-life of approximately three to six
months. For a more detailed article on HIV pathogenesis, go to
the Medscape article "HIV
Pathogenesis and Viral Markers" Daniel R. Kuritzkes
The natural course of HIV infection is as follows;
1. Seroconversion illness - this is seen in 10% of individuals a few weeks after inoculation and coincides with seroconversion. These patients present with a mononucleosis like illness which comprises of fever, sore throat, enlarged lymph nodes, skin rash, joint aches and general malaise.
2. Incubation period - this is the period when the patient is completely asymptomatic and may vary from a few months to a more than 10 years. The median incubation period is 8-10 years. Children tend to have a shorter incubation period.
3. AIDS-related complex or persistent generalized lymphadenopathy - at the end of the incubation period, a number of signs and symptoms may appear which do not fulfill the definition of AIDS or other HIV-associated syndromes. These include slight immunological, dermatological, haematological and neurological signs. Constitutional symptoms, such as fever, weight loss, night sweats, and diarrhoea may develop. Generalized lymphadenopathy may be seen. Laboratory findings may show a decrease in the CD4 count, hyperimmunoglobulinaemia and cytopenias.
The lymphadenopathy syndrome is defined as the enlargement of lymph nodes to 1 cm or more at 2 or more body regions which persists for more than 3 months without any other recognizable cause other than HIV infection. AIDS-related complex is defined as fever, weight loss, night sweats, or chronic diarrhoea of more than 1 month's duration in the presence of disturbances of CMI and in the absence of any other recognizable cause other than HIV infection. These definitions may be in part overlapping and are not mutually exclusive and they can be described generally as pre-AIDS.
4. AIDS - The first manifestation of HIV infection may be noted at any disease stage and the different stages may not occur consecutively. The transition to full-blown AIDS may occur rapidly or slowly. The disease progression is probably influenced by cofactors, such as other virus infections, stress, genetic makeup of the individual etc. Poor prognostic factors include the serial decrease in the number of CD4 lymphocytes, the reappearance of HIV antigen in the blood, the decline or disappearance of anti-core antibodies, and increased levels of B2-microglobulin and neopterin. The diagnosis of AIDS is established with the appearance of opportunistic infections, or of certain neoplasms, such as Kaposi's sarcoma, primary lymphoma of the brain and other non- Hodgkin's lymphomas. In the US, the diagnosis of AIDS is also established by the finding of a CD4 count of less than 200 cells/mm3.
pneumocystis cariniI (now thought to be a fungi)
toxoplasmosis of the brain
crytosporidosis with diarrhoea
candidiasis (oesophagus, trachea, lungs)
crytococcosis, extrapulmonary histoplasmosis
Mycobacterium avium complex
atypical mycobacterial disease
multiple or recurrent pyogenic bacterial infection
Opportunistic infections by protozoa, fungi, bacteria and viruses are seen in patients with aids. Pneumonia caused by Pneumocystis carinii is the most frequent opportunistic infection in AIDS patients in N. America and Western Europe. The typical symptoms are shortness of breath on exertion, dry cough and fever. Diagnosis requires bronchoscopy and the demonstration of Pneumocystis in the lavage fluid. Pentamidine and co-trimoxazol are the treatment of choice. Toxoplasmosis is particularly relevant because it causes CNS disease. It can be treated by a combination of pyrimethamine and a sulfonamide. Cryptosporidium frequently causes intractable diarrhoea for which no therapy is available.
Candida Albicans is the second most frequent opportunistic infection in AIDS patients. Thrush of the oral cavity is frequently seen in patients with pre-AIDS. Candidiasis of the oesophagus is a CDC criterion defining AIDS. Therapy is relatively simple with amphomoronal solution or ketaconazol tablets. Of further clinical importance is infection with the fungus crptococcus neoformans which, if untreated, is fatal in all cases. Early diagnosis may be made by the demonstration of the fungus or fungal antigen in the CSF.
An increasing problem are infections with mycobacteria. In N. America and Western Europe infections with the atypical mycobacteria, M. avium-intracellulare and M. Kansaii etc. dominate. In the developing countries, particularly in Central Africa, the classic M tuberculosis is more frequent. Although classic TB is treatable, no therapy exists for atypical mycobacteriosis.The most frequent viral infections are caused by CMV, HSV and VZV. CMV causes retinitis, colitis, or less frequently pneumonitis and hepatitis. Treatment is possible with DHPG. HSV and VZV infections are best treated by acyclovir.
b. Opportunistic Tumours
The most frequent opportunistic tumour, Kaposi's sarcoma, is observed in 20% of patients with AIDS. For unknown reasons, KS is observed mostly in homosexuals and its relative incidence is declining. KS is a vascular tumour is in its characteristic form is readily identifiable. In 30 to 40% of patients, the mucosal membrane, most frequently the oral cavity and the larynx, and the internal organs, most frequently the oesophagus, the stomach and the intestines are affected. In most case, no treatment is required. Excision, local treatment with vincristine or laser irradiation are the mainstay of palliative therapy.
Malignant lymphomas of HIV-infected patients differ from other known lymphomas by their localization, degree of malignancy, and response to therapy. HIV-associated lymphomas are frequently found outside the lymphatic system, particularly in the brain, bone marrow, GI tract, and skin. Their response to therapy is much poorer than that of classical lymphomas. The pathogenesis of KS and HIV-associated lymphomas are uncertain. In the case of lymphomas, at least a certain proportion appears to be associated with EBV.
c. Neurological manifestations
A high percentage of HIV-infected patients shows neurological changes that are not explained by opportunistic infections or tumors. The spectrum of symptoms ranges from slight neuropsychological abnormalities (disturbances of memory, mood and behavior) to organic psychosis and complete dementia. In almost all cases a continuous deterioration is observed. The most frequent neurological disorder is subacute encephalitis (AIDS encephalopathy, AIDS dementia complex) which is seen in two thirds of cases. Other clinical manifestations may be seen, such as acute meningoencephalitis, aseptic meningitis and peripheral neuropathy.
d. Dermatological Manifestations
In all stages of HIV infection, characteristic skin manifestations may be observed which frequently may be the first symptoms which lead the patient to seek medical attention. HIV specific conditions include oral hairy leucoplakia and an itching maculopapular eruption. Seborrhoeic eczema occur in 70% of AIDS patients and allergic exanthemas and acne like eruptions are also very common. Viral induced skin eruptions are also frequently seen, such as herpes zoster, cobdylomata acuminatum, verruca vulgaris, molluscum contagiosum.
e. Gastrointestinal Manifestations
Persistent diarrhoea which may be copious in volume, is a frequent problem among these patients. Giardia lamblia, Entamoeba histolytica, Shigella, Salmonella and Campylobacter all cause symptomatic disease. However, appropriate treatment does not always eliminate the watery diarrhoea. Cryptosporadium, CMV, M. avium intracellulare or Kaposi's sarcoma may also be associated with diarrhoea. It had also been suggested that HIV itself may be enteropathic through infection of the mucosal cells and/or through attacking the autonomic nervous supply to the gut.
f. Manifestations in children and during pregnancy
In children, the clinical spectrum of HIV infection differs from that observed in adults. The typical opportunistic infections of adults are less frequent and bacterial infections predominate. In about 50% of children, a pneumonia is observed which is called lymphoid interstitial pneumonia. It had been suggested that this pneumonia is directly caused by HIV. In addition, failure to thrive, weight loss and neurological symptoms are seen.
HIV transmission to the fetus has been observed as early as
the 15th week of pregnancy. Prenatal infection may cause a HIV-
specific embryopathy in the majority of infected children. This
is characterized by a small forehead, short flat nose, pronounced
philtrum, microcephaly, thick lips and hypertelorism. The numbers
of newborns infected by their mothers have been rising steadily.
The transmission rate from mother to the newborn is estimated at
one-third. There is also evidence to suggest that pregnancy also
favours the progression of the HIV disease in the mother. The
maternal-fetal transmission rate varies geographically from
around 15% in Western Europe to up to 50% in Africa. It appears
the most important determinant is the virus load present in the
The earliest unambiguously identified HIV-antibody positive serum stems from Kinhasa, Zaire dating back to 1959. It is evident that HIV infection spread unrecognized in the 1960s and 1970s. Over 100,000 cases of AIDS have been reported to the WHO by June 1988. The WHO estimates of all HIV-infected persons ranges from 5 to 10 million. Reliable estimates about the incidence and prevalence of HIV infection do not exist because of the lack of representative data. It is still unclear to what extent the mode of transmission, virus dose, cofactors, and genetic predisposition influence the infection and the incubation period. It is now thought that up to 95-99% of all HIV-infected persons eventually develop AIDS.
Seroconversion - the interval between infection and seroconversion was generally assumed to last about 3 to 12 weeks. Recently, serolatency of many months and up to several years have been recognized in some patients.These observations may have far- reaching implications.
Incubation period - the actual incubation period cannot be predicted in individual cases. The available evidence so far points to an median incubation period of 8 to 10 years (range: a few months to more than 14 years)
Manifestation rate - the percentage of infected persons who finally develop AIDS increases with the duration of the infection by an average of 5 to 7% per year.
Modes of transmission - HIV is transmitted by sexual contact, blood and blood products, or from the mother to the newborn infant. Other theoretical ways of transmission such as by saliva or insects do not play a role in the epidemiology of HIV infection. Evidence is increasing that the infectivity of HIV-infected individuals vary with the course of infection. It appears that infectivity is correlated with viraemia and is greater very early after infection before the appearance of antibodies and later in the course after the development of symptomatic immunodeficiency.
Risk groups - male homosexuals and bisexuals constitute the largest risk group in N. America and Western Europe. Anal intercourse and the multiplicity of partners in the case of homosexuals are the risk factors in the case of homosexuals.In contrast, heterosexual spread is the largest category in developing nations and may be associated with other sexually transmitted disease, particularly those causing genital ulcers. Prostitutes, particularly IV drug-abusing prostitutes are important multiplicators of the epidemic. The infection rate of prostitutes ranges from 0% in some European and American cities to 90% in some African cities.
HIV is transmitted by blood transfusions, clotting factors, and by contaminated needles and syringes. Blood transfusions as a cause of infection have been virtually eliminated in Western Europe and N. America since the introduction HIV antibody screening in 1985 but it continue to be a important cause of HIV transmission, particularly in Central Africa because of the high degree of infection of the donors and the lack of resources to implement the screening of the donor's blood. Infection transmitted via clotting factor have been eliminated since HIV inactivation steps have been included in their production process since 1985. IV drug abusers represent the largest AIDS patient groups in Italy (65%) and Spain (54%) and the second largest group in the US and the majority of Western European countries.
The increase in HIV-infected mothers, mostly drug-dependent is paralleled by an increase of HIV-infected newborns. The transmission rate from infected mothers to their newborn infants is about one-third. Medical personnel are not considered to be a risk group. By April 1988, only 22 cases of medical personnel infected via needle injuries and contact with the blood of HIV-infected persons have been described worldwide.
Incidence and prevalence of HIV infection - no reliable data exist on the incidence and prevalence of HIV infection in the general population. The prevalence of HIV infections in homosexuals in the US ranges from 20% to 70% in some urban centres. On the basis of studies of samples from HIV risk groups and from the general population, the total prevalence of HIV-infected persons has been estimated in several countries. The prevalence of HIV infected persons in the USA was estimated to be 1 to 1.5 million in June 1988.
The worldwide distribution of AIDS
Africa - HIV infection is not limited to certain risk groups. The male:female ratio is 1:1 in contrast to Western Europe and N. America where the ratio is approximately 13:1. The extent of infection in the general population is considerable in some regions, mainly in the large cities and in some rural areas. In Kinshasa (Zaire) the prevalence of HIV infection was 7% in pregnant women and blood donors in early 1988. The seropositivity rate amongst blood donors in Kigali (Uganda) was 18% in 1984. The highest infection rates were found in prostitutes and in patients with sexually transmitted diseases.
Caribbean and South America - In these countries, the transmission is homosexual and heterosexual. The male:female ratio in Haiti is 4:1, suggesting that heterosexual transmission is important.
Asia and Australia - the prevalence of HIV infection in Asia is rising dramatically, especially in Thailand and India. It is predicted that HIV infection in Asia will outstrip that in Africa in due course. Most of the initial cases stem from Japan and Israel. Transmission was primarily by students from Africa and by contacts of Asian business people abroad. The epidemiology of AIDS in Australia and New Zealand is similar to that of other industrialized countries of N. America and Western Europe.
Several hypothesis regarding the origin of HIV have been developed (1) mutation of older viruses (2) transmission of animal viruses eg. SIV to man (3) transmission by an isolated remote section of the population eg. by change of sexual behavior and mobility. Available data at present favour the third possibility.
1. Serology - the diagnosis of HIV infection is usually based on serological tests.
(a) Antibody tests - ELISAs are the most frequently
used method for screening of blood samples for HIV antibody. The
sensitivity and specificity of the presently available commercial
systems approaches 100% but false positive and false negative
reactions occur. Other test systems available include passive
particle agglutination, immunofluorescence, Western blots and
RIPA bioassays. Western blots are regarded as the gold standard
and seropositivity is diagnosed when antibodies against both the
env and the gag proteins are detected. The sensitivity of the
test systems are currently being improved by the use of
Microplate ELISA: coloured wells indicate reactivity
Interpretation of Western blot results for HIV antibody
(b) Antigen tests - HIV antigen can be detected early in the course of HIV infection before the appearance of antibody. It is undetectable during the latent period (antigen-antibody complexes are present) but become detectable during the final stages of the infection. It was argued that the routine use of antigen screening tests in the blood transfusion service may result in earlier cases of HIV infection being identified. However a large scale study carried out in the US failed to show any benefit.
2. Virus isolation - virus isolation is accomplished by the cocultivation of the patient's lymphocytes with fresh peripheral blood cells of healthy donors or with suitable culture lines such as T-lymphomas. The presence of the virus can be confirmed by reverse transcriptase assays, serological tests, or by changes in growth pattern of the indicator cells. However virus isolation is tedious and time consuming (weeks) and is successful in only 70 to 90% of cases. Therefore virus isolation is mainly used for the characterization of the virus.
3. Demonstration of viral NA - this can be accomplished by probes or by PCR techniques. The latter may be useful because of its extremely high sensitivity.
4. Prognostic Tests - the following may be useful as prognostic tests; (1) HIV antigen (2) Serial CD4 counts (3) Neopterin (4) B2-microglobulin. (5) Viral load. Of these tests, only serial CD4 counts and HIV viral load are still routinely used.
a. HIV viral load - It appears that HIV viral load has the greatest prognostic value. HIV viral load in serum may be measured by assays which detect HIV-RNA e.g. RT-PCR, NASBA, or bDNA. HIV viral load has now been established as having good prognostic value, and in monitoring response to antiviral chemotherapy. Patients with a low viral load during the incubation period had a better prognosis than those with a high viral load. Patients whose viral load decreased significantly following the commencement of antiviral therapy had a better prognosis than those who did not respond.Among patients who responded to antiviral therapy, those who had a low pre-treatment viral load had a better prognosis than those who had a high pre-treatment viral load.
b. CD4 counts - despite the increasing use of HIV-RNA
assays, measurement of CD4 still has important value in
monitoring disease progression and response to antiviral
chemotherapy. whereas CD4 count gives an indication of the stage
of disease. The measurement of HIV viral load tells
us where the disease is going, whereas CD4 count tells us where
the disease is at this moment
Link to IAPAC article "Monitoring Antiretroviral Therapy with Plasma HIV RNA and CD4 Counts"
5. Antiviral susceptibility assays
Because of the increasing range of anti-HIV agents available, there is increasing pressure on the provision of antiviral susceptibility assays. There are two types of antiviral susceptibility assays: phenotypic and genotypic assays
Phenotypic assays define whether a particular strain of virus is sensitive or resistant to an antiviral agent by determining the concentration of the drug needed to inhibit the growth of virus in vitro. e.g. Plaque-reduction assay for HSV, plaque-reduction assay for HIV. However, phenotypic assays can only be used for viruses that can be cultivated. Moreover, in the case of HIV, plaque reduction assays may not be that appropriate since not all HIV strains produce plaques in cell culture.
In the case of genotypic assays, mutations that are associated with resistance are assayed for by molecular biology methods such as PCR and LCR. However, these assays are tedious and are not suitable for a routine diagnostic laboratory. Moreover, he results of genotypic assays may prove very difficult to interpret since HIV mutates at a furious pace, and it is also possible that resistant strains are present right at the beginning of infection.
Link to IAPAC article "Antiretroviral
Drug Resistance and the Role of Resistance Monitoring"
The huge resources that had gone into HIV research had resulted in the development of a large number of anti-HIV agents. The speed of advance in this area is unprecedented in the history of medicine. Therapy of HIV is complicated by the fact that the HIV genome is incorporated into the host cell genome and can remain there in a dormant state for prolonged periods until it is reactivated. However although it may or not be possible to actually eradicate the virus completely, it seems possible that the infection can be indefinitely contained so that an infected will die with HIV infection rather than from it.
Zidovudine (AZT) was the first anti-viral agent used for the treatment of HIV and was introduced in 1987. However, it became clear with Concorde study in 1994 that monotherapy with AZT did not provide durable efficiency and hardly made any dent in the mortality rate. In 1995, results of the European DELTA and the American ACTG 175 studies became available and showed that combination therapy with two nucleoside analogues were better than monotherapy with one alone. A further breakthrough occurred with the introduction of HIV protease inhibitors which were specifically designed against HIV protease and were shown to be the most potent anti-HIV effect to date. An early clinical trial reported that the use of oral ritonavir decreased HIV mortality from 38% to 22%. Combination therapy, otherwise known as HAART (highly active antiretroviral therapy) using two or three agents became available. The rationale for this approach is that by combining drugs that are synergistic, non-cross-resistant and no overlapping toxicity, it may be possible to reduce toxicity, improve efficacy and prevent resistance from arising. The final breakthrough occurred when David HO (Time Magazine Man of the Year 1996) finally elucidated the pathogenesis of HIV infection. He showed that far from being latent during the "latent phase" as previously thought, there is actually massive replication during this period. David Ho had coined the slogan "hit hard and early". The results of the new approach was seen quickly where within Within four years, between 1994 and 1998, the incidence of AIDS in Europe sank from 30.7 to 2.5/100 patient years i.e. to less than a tenth.
What is less hopeful is the possibility of ever eradicating HIV from the body i.e. complete cure. In the beginning, it is thought that continuous treatment for 3 years would be sufficient to eradicate all the remaining latently infected cells. However, the period of treatment required kept on being revised upwards as new data became available. The most recent estimate of eradication of all latently infected cells is 73.3 years. Therefore, it is clear it will not be possible to achieve a complete cure in the short term. Compliance is a major issue when therapy is expected to be life-long. There is clearly a great need to have formulations whereby the number of tablets to be taken per day is kept to a minimum. The development of side-effects with long term use is another issue.
As knowledge builds up on the risks and efficacy on various agents and regimens, recommendations on HIV are being continually revised. So now, instead of "hit hard and early", there is a shift towards "hit hard, but only when necessary". There is still a lot of debate on when to actually commence therapy. Two measures are used for determining whether to start HIV therapy: CD4 counts and viral loads. It is generally agreed that HIV therapy should be given when the CD4 count is below 200. Some experts would recommend treatment for any patient whose CD4 count is below 350. What is less clear are patients with CD4 counts of 300-500 and a modest viral loads. A decision to start therapy must be taken on an individual basis with the patient after thorough discussion and counseling.
1. Anti-Retroviral Agents
A. Nucleoside Reverse Transcriptase Inhibitor
B. Non-Nucleoside Reverse Transcriptase Inhibitor
C. HIV Protease Inhibitors
D. HIV Fusion Inhibitors
There are a number of combination preparations on the market e.g. CBV (AZT+3TC), TZV (AZT+3TC+ABC), TVD (FTC+TDF), Kaletra (Lopinavir/ritonavir). The use of combination preparations will reduce the numbed of tablets that need to be taken each time.
2. Monitoring anti-HIV therapy
a. Viral Load
Initiation - viral load is now the preferred method of monitoring therapy. There should be >= 1 log reduction in viral load, preferably to less than 10,000 copies/ml HIV-RNA within 2-4 weeks after the commencement of treatment. If <0.5 log reduction in viral, or HIV-RNA stays above 100,000, then the treatment should be adjusted by either adding or switching drugs.
Monitoring - viral load measurement should be repeated every 4-6 months if patient is clinically stable. If viral load returns to 0.3-0.5 log of pre-treatment levels, then the therapy is no longer working and should be changed.
b. CD4 count
Initiation - within 2-4 weeks of starting treatment, CD4 count should be increased by at least 30 cells/mm3. If this is not achieved, then the therapy should be changed.
Monitoring - CD4 counts should be obtained every 3-6 months during periods of clinical stability, and more frequently should symptomatic disease occurs. If CD4 count drops to baseline (or below 50% of increase from pre-treatment), then the therapy should be changed.
c. Anti-HIV Drug Resistance Testing
Anti-retroviral drug resistance testing has become part and parcel of patient management in N. America and W. Europe. Many studies in treatment experienced patients have shown strong associations between the presence of drug resistance and failure of the antiretroviral treatment regimen to suppress HIV replication.
Genotypic Assays - genotypic assays detect drug
resistance mutations that are present in the relevant
viral genes (i.e. RT and protease). Some genotyping
assays involve sequencing of the entire RT and protease
genes, while others utilize oligonucleotide probes to
detect selected mutations that are known to confer drug
resistance. Genotyping assays can be performed relatively
rapidly, such that results can be reported within 1-2
weeks of sample collection. Interpretation of test
results requires an appreciation of the range of
mutations that are selected for by various antiretroviral
drugs, as well as the potential for cross-resistance to
other drugs conferred by some of these mutations.
Phenotypic Assays - phenotypic assays measure the
ability of viruses to grow in various concentrations of
antiretroviral drugs. Automated, recombinant phenotyping
assays have recently become commercially available with
turn-around times of 2-3 weeks; however, phenotyping
assays are generally more costly to perform compared with
genotypic assays. Recombinant phenotyping assays involve
insertion of the RT and protease gene sequences derived
from patient plasma HIV RNA into a laboratory clone of
HIV. Replication of the recombinant virus at various drug
concentrations is monitored by expression of a reporter
gene and is compared with replication of a reference
strain of HIV. The concentrations of drugs that inhibit
50% and 90% of viral replication (i.e. the IC50 and IC90)
are calculated, and the ratio of the IC50s of the test
and reference viruses is reported as the fold increase in
IC50, or fold resistance. Interpretation of phenotyping
assay results is complicated by the paucity of data on
the specific level of resistance (fold increase in IC50)
that is associated with failure of different drugs.
Use in clinical setting - resistance assays may be useful in the setting of virological failure on antiretroviral therapy. Recent prospective data supporting the use of resistance testing in clinical practice come from trials in which the utility of resistance tests were assessed in the setting of virological failure. The VIRADAPT and GART studies compared virological responses to antiretroviral treatment regimens when genotyping resistance tests were available to help guide therapy with those observed when changes in therapy were guided solely by clinical judgment. The results of both studies indicated that the short-term virological response to therapy was significantly greater when results of resistance testing were available. Similarly, a recent prospective, randomized, multicenter trial has shown that therapy selected on the basis of phenotypic resistance testing significantly improves the virological response to antiretroviral therapy, compared with therapy selected without the aid of phenotypic testing. Thus, resistance testing appears to be a useful tool in selecting active drugs when changing antiretroviral regimens in the setting of virological failure.
The rapidity of developments in anti-HIV therapy makes it virtually impossible for this site to keep up. For the latest information on HIV and anti-retroviral therapy, I recommend the HIV page at Medscape.com
The risk of contracting HIV increases with the number of sexual partners. It is interesting to note that the first cases of AIDS which were reported in homosexuals admitted to having more than 100 sexual partners per year. A change in the lifestyle would obviously reduce the risk. Paediatric AIDS generally occur early in life although some children have survived congenital infection for many years. Infected infants may serve as worldwide reservoirs of AIDS if they survive infancy, as in the case of hepatitis B.
HIV-infected mothers are not recommended to have children at present and pregnancy itself would appear to be a risk factor for seropositive mothers. A recent clinical trial demonstrated the efficacy of AZT in preventing transmission of HIV from the mother to the fetus. The incidence of HIV infection in the baby was reduced by two-thirds. The regimen used in this trial included antenatal oral administration of AZT beginning at 14-34 weeks of gestation and continuing throughout pregnancy, followed by intrapartum IV AZT, and postnatal oral AZT to the infant for 6 weeks after delivery.
The spread of HIV through blood transfusion had virtually been
eliminated since the introduction of blood donor screening in
many countries. It must be borne in mind that recently infected
donors who have yet to develop antibodies will escape detection.
There had also been recent reports of new virus isolates from
patients serologically negative for HIV, yet who may have AIDS-
related symptoms. Blood products such as factor VIII now undergo
routine treatment which appears to inactivate any HIV present
An effective vaccine against HIV would have to elicit protection against both free virus and virus-infected cells. HIV neutralizing antibodies. Several examples of retroviral infections eg. FeLV, suggest that neutralizing antibodies on its own may be protective. The observation that HIV-neutralizing antibodies do not prevent the progression of the disease and the induction of neutralizing antibodies in chimpanzees by HIV gp160 do not prevent infection by HIV argues against the possibility that neutralizing antibodies are sufficient for protection against infection by HIV. Individuals with high neutralizing antibody titres, however, seem to have a longer disease free interval. Cellular immune response may be important for protection and may take place through Tc cells, NK cells and ADCC mechanisms. The role of these mechanisms in the protection against HIV infection and their relevance for anti-HIV vaccination are still unclear. Another obstacle to the development of an effective HIV vaccine may be the high genetic variability of HIV, which resides mainly in the env gene. In certain regions of the envelope gene, the variability may be as high as 50%. However, conserved regions exist in the gp120 and the gp41 region which can be recognized by HIV sera and neutralizing antibodies can be induced with peptides representing these regions. The main types of approaches to an AIDS vaccine is as follows:
Live attenuated virus
Live recombinant viruses
Recombinant DNA products (gp120, gp160)
Native envelope and/or core proteins
Because of the serious nature of the disease and the ability of retroviruses to induce latency, it is unlikely that live attenuated or inactivated virus will ever be acceptable for human use. Live recombinant viruses such as vaccinia or adenovirus recombinant have been developed for HIV vaccination. They carry a high immunogenicity since virus replication result in large quantities of virus antigen. Both humoral and cellular immunity can be induced. A disadvantage of this approach is possible adverse effects observed after inoculation of vaccinia virus. Synthetic peptides can be designed such that neutralizing and T cell epitopes can be included. The problems with synthetic peptides are related to the tertiary structures which may be different from those of native proteins. Another promising approach is the production of recombinant viral antigens, especially the gene products of the env region, and the membrane associated p17 core protein. An alternative approach would be the use of anti-idiotype antibodies whose usefulness had been demonstrated against several infectious agents. Passive immunization may eventually be available for postexposure protection.
As of 2008, despite 25 years of intense unprecedented research, the development of an effective HIV vaccine appears to be as far off as ever. There is a prevailing mood of pessimism that an HIV vaccine will not be available for at least another 10 years or more, if ever. The reason for this pessimism is the failure of the Phase IIb trial of the V520 vaccine in humans. Although a large number of animal trials have been carried out with different candidate vaccine. V520 was the first candidate to be tested in humans. V520 uses a recombinant adenovirus vector with 3 HIV genes and had shown great promise in previous animal trials. It is thought that besides offering protection against infection, it may offer therapeutic effects to those who had been infected. However, it did not provide any protection against infection to those who had been given the vaccine and those infected did not have a reduced viral load. Why a candidate vaccine that had worked so well in an animal model but failed in humans will be the subject of vigorous investigation.
HIV Slide Set