HIV REPLICATION

Human immunodeficiency viruses (HIVs) 1 and 2 are retroviruses of the Lentivirus group. Like other lentiviruses, they exhibit tropism for cells of the immune system and cause immunological disorders, particularly immunodeficiency. Infection of immune cells in the nervous system may also cause neurological disease. Entry of HIV into cells of the immune system is via cell surface receptors. The major receptor is the CD4 molecule but the chemokine receptors CCR5 and CXCR4 play an important role as co-receptors. Inside the cell, the viral RNA is reverse-transcribed into DNA by a viral reverse transcriptase enzyme, and the DNA is integrated into the DNA of the host cell as proviral DNA by a viral integrase enzyme. The proviral DNA remains there until the cell is activated, when it is transcribed into RNA, which provides the template for assembly of new HIVs under the control of viral enzymes such as proteases. Budding of new virus from the cell is followed by infection of new cells and a repeat of the replication cycle.

PRIMARY HIV INFECTION

Initial infection by HIV-1 is associated with a primary HIV infection syndrome (also known as a seroconversion illness) in 50–70% of patients. This syndrome is similar to infectious mononucleosis, being characterised by fever, lymphadenopathy, headache, photophobia, fatigue and myalgia. However, mucocutaneous lesions, neurological disease and even transient immunodeficiency may also occur and, when present, this differentiates it from infectious mononucleosis. It may take a number of weeks for HIV antibody (detected by enzyme-linked immunosorbent assay (ELISA)) to become positive during seroconversion; however the level of plasma HIV RNA is very high. The diagnostic test of choice during this period is the measurement of HIV viral load utilising HIV RNA RT/PCR (reverse transcription of HIV RNA and amplification by polymerase chain reaction). HIV RNA peaks at a median of 10 days.¹

CHRONIC HIV INFECTION

The viraemia associated with primary HIV infection is controlled by a cellular and antibody-mediated immune response, which results in resolution of symptoms. However, even though the great majority of people become asymptomatic, HIV replication continues to take place. This results in activation of the immune system, depletion of CD4+ T cells and immunodeficiency, especially cellular immunodeficiency. These abnormalities develop at different rates in different individuals. In untreated patients the median time to develop the acquired immunodeficiency syndrome (AIDS) after acquiring HIV infection is 9 years. However, about 5% of HIV-infected individuals have no abnormalities after 15 years and are referred to as long-term non-progressors.

Early in the course of chronic HIV infection, virus is present in lymphoid tissues where it is bound to follicular dendritic cells. There is a persistent immune response against the HIV but, as this fails, viral replication increases and the viral load increases and other cells become infected, including macrophages, microglial cells of the nervous system and CD4+ T cells.

Chronic HIV infection may cause weight loss, fevers and diarrhoea, though such symptoms are more likely to be caused by an opportunistic infection in immunodeficient patients. Effects of worsening HIV infection are an immunodeficiency syndrome, which results in the development of opportunistic infections, tumours and neurological disease. Neurological disease is a consequence of HIV infection of macrophages and microglial cells in the central and peripheral nervous system.

DIAGNOSIS

A diagnosis of HIV infection can usually be made by demonstrating anti-HIV antibodies in the patient’s serum. However, the serological diagnosis of HIV infection can sometimes be problematic. A small minority of individuals who are not infected by HIV have serum antibodies which are reactive with some HIV proteins, and give false-positive results with some ELISAs. Many laboratories use two different types of ELISA to identify such sera. To ensure that HIV infection is not incorrectly diagnosed, an antibody test should only be considered positive if antibody is also detected according to defined criteria, using a confirmatory antibody test such as a Western blot immunoassay.² Anti-HIV antibodies may be absent from the serum of patients with primary HIV infection. They are usually detectable by 2–6 weeks after infection, and almost always detectable by 12 weeks. After this time, absence of HIV antibodies excludes HIV infection in all but the most advanced cases of AIDS, or in a patient with an antibody deficiency disorder.

VIROLOGICAL MONITORING

It could be argued that HIV has been one of the major driving forces in the development of modern virology. For the first time HIV viral load monitoring has enabled the clinician to predict outcome, and monitor therapy during a viral infection.

HIV VIRAL LOAD MEASUREMENT

The major advance in viral diagnostics has been the development of polymerase chain reaction (PCR) and the application of this to the detection and quantification of viral nucleic acid in body fluids. Measurement of the blood ‘HIV load’ can be done by quantitating HIV RNA in plasma. Knowledge of the HIV load is useful in prognostication and essential when making decisions to commence and optimise therapy³ (Figure 60.1).

Figure 60.1 Correlation of viral load with disease progression. AIDS, acquired immunodeficiency syndrome.

(From Mellors JW, Rinaldo CR, Gupta P et al. Prognosis in HIV-1 infection predicted by the quantity of virus in plasma. Science 1996; 272: 1167–70. Copyright: American Association for the Advancement of Science; reproduced with permission.)

GENOTYPING HIV TO DETECT DRUG RESISTANCE MUTATIONS

Drug susceptibility testing of HIV from a clinical specimen is performed in order to detect mutations in the genome of HIV that predict failure of individual antiviral therapeutic agents. The predominant genotype from an individual’s plasma or cerebrospinal fluid sample is determined by sequencing the reverse transcriptase and protease genes of complementary DNA (cDNA) obtained following RT/PCR. This information is used to assist the clinician and patient in the choice of an antiretroviral drug combination with the best chance of successful suppression of HIV replication, especially where resistance is suspected, i.e. in a failing regimen.⁴

IMMUNOLOGICAL MONITORING

Immunodeficiency and neurological disease caused by HIV infection usually develop gradually over months to years. It is important to monitor their severity to determine when to commence therapy, and when the patient is susceptible to various disease manifestations.

The blood CD4+ T-cell count or percentage is the best indicator of the severity of HIV-induced immunodeficiency and, therefore, of the patient’s susceptibility to opportunistic infections. Measurement of the blood CD4+ T-cell count or percentage is therefore critical in determining if the symptoms of an HIV-infected patient are likely to be caused by an opportunistic infection, and if so, what type of infection (Figure 60.2; and see text below).⁵

Figure 60.2 Chronological framework for understanding human immunodeficiency virus (HIV) disease and its management.

(From Stewart G. Managing HIV. Med J Aust 1997; 5, reproduced with permission.)

MANAGEMENT OF THE HIV-INFECTED PATIENT

The impact of combination antiretroviral therapy on the morbidity and mortality associated with HIV infection has been dramatic. The HIV Outpatient Study demonstrated a stepwise reduction in opportunistic infections and mortality with increasing intensity of antiretroviral therapy.⁶ In Australia, a comparison of cohorts before and after the introduction of combination antiretroviral therapy documented the effectiveness of these agents in reducing the risk of progression to AIDS and death.⁷ This refiects similar epidemiological studies conducted in Switzerland,⁸ France⁹ and the USA.¹⁰ A 70–80% reduction in mortality over 5 years has been the norm. AIDS-defining illnesses in Australia now occur predominantly in those without a past diagnosis of HIV infection.

Six classes of antiretroviral drugs are currently in use (Table 60.1). Fusion inhibitors block fusion of the virus with the cell membrane and CCR5 inhibitors block binding of CCR5-tropic strains of virus with CCR5, a co-receptor for HIV. Reverse transcriptase inhibitors are of three types. Firstly, nucleoside analogues and nucleotide analogues act by substituting for natural nucleosides or nucleotides during HIV replication, thereby inhibiting DNA chain elongation and the effects of the reverse transcriptase enzyme. Secondly, non-nucleoside reverse transcriptase inhibitors inhibit the reverse transcriptase enzyme by a different mechanism. Thirdly, integrase inhibitors block integration of viral DNA into host DNA and protease inhibitors inhibit the viral protease. All antiretroviral drugs have a limited duration of efficacy if used alone because HIV eventually develops resistance to them. The use of drug combinations is much more effective than single drugs, partly because drug resistance develops more slowly. Adherence to antiretroviral therapy is critical for the success of treatment.

Table 60.1 Antiretroviral drugs used to treat human immunodeficiency virus (HIV) infection

* Administered with low-dose ritonavir to increase serum levels (lopinavir + low-dose ritonavir are co-formulated as Kaletra).