GUM - HIV

Question 1

What is the difference between HIV-1 and HIV-2?

Answer 1

The acquired immunodeficiency syndrome (AIDS) is caused by the human immunodeficiency virus (HIV-1). HIV-2 causes a similar but less aggressive illness occurring mainly in West Africa. Immune deficiency arises from continuous HIV replication leading to virus- and immune-mediated destruction of CD4 lymphocytes.

Question 2

What is the structure of the HIV virus? What is present in the virion core? What are the two important surface glycoproteins in HIV?

Answer 2

Under the electron microscope, HIV appears as a spherical enveloped virion with a central cylindrical nucleocapsid.

At the virion core lie 2 identical SS RNA pieces (a dimer). Associated with these are nucleocapsid proteins bound to the RNA and the 3 essential retroviral enzymes; protease, reverse transcriptase and integrase. Surrounding the RNA dimer lies the capsid shell which has icosahedral symmetry. The proteins that make up this shell are called the capsid proteins. The major capsid protein is p24; this can be measured in the serum to detect early HIV infection.

The rest of the virus has the same structure as the influenza virus. Proteins under the envelope are called matrix proteins. These proteins serve to hold glycoprotein spikes that traverse the lipid bilayer. The surface glycoproteins are referred to as gp followed by a number: gp120 and gp41 (together making gp160).

Question 3

The HIV genome is complex. It contains two long terminal repeats (LTRs), as well as the gag, pol and env genes. What is the function of the long terminal repeats?

Answer 3

LTRs flank the whole viral genome and serve 2 important functions:

1) Sticky ends - these are sequences, recognised by integrase, that are involved in the insertion into the host DNA. Transposons, mobile genetic elements, have similar flanking DNA pieces
2) Promotor/ enhancer function - once incorporated into the host DNA, proteins bind to LTRs that can modify viral DNA transcription

Question 4

What is the function of the gag gene?

Answer 4

Gag (group antigen) sequences code for the proteins inside the envelope: nucleocapsid, capsid (p24) and matrix proteins. Thus gag codes for the virions major structural proteins that are antigenic.

Question 5

What is the pol gene?

Answer 5

Pol encodes the protease, integrase and reverse transcriptase enzymes. Protease is a vital HIV enzyme that cleaves gag and pol proteins from their larger precursor molecules (post translational modification).

Protease deficient HIV virions can not form their viral core and are non infectious.

Question 6

What is the function of the env gene?

Answer 6

Env codes for ENVelope proteins that, once glycosylated, form the glycoprotein spikes gp120 and gp41. Gp120 forms the head and gp40 forms the stalk. Together they are called gp160 and bind to CD4 receptors on T cells.

Question 7

Why is it difficult to develop a vaccine against HIV?

Answer 7

Genome heterogeneity.
HIV possesses the ability to change its genome in a critical area. Within the env gene, particularly the area encoding the gp120 glycoprotein, lie hypervariable regions, where point mutations occur. This changes the antigenic structure and protects the virus from vaccine induced antibodies. The gene encoding reverse transcriptase also undergoes point mutations.

Question 8

How is HIV transmitted?

Answer 8

HIV is transmitted via the parenteral route, like hepatitis B.

Transmission occurs:
By sexual contact
By exposure to blood or blood products (e.g. drug users, patients with haemophilia or occupationally in health- careworkers)
Vertically from mother to child in utero, during birth or by breastfeeding

In the Americas and Western Europe, the epidemic has until now predominantly affected men who have sex with men (MSM), whereas in Eastern Europe, Central Asia, the Middle East and South- east Asia, IV drug use also causes many infections. In sub-Saharan Africa, the Caribbean and Oceania, most transmission is heterosexual. Worldwide, the major route of transmission is heterosexual.

Question 9

What type of exposure carries the highest risk of transmission?

Answer 9

Transfusion of HIV infected blood carries a 90% risk of developing HIV.
Vaginal delivery by an infected mother carries a 15% risk to the child.

By comparison, receptive anal intercourse carries a 0.5% risk, and male to female vaginal intercourse has a risk of 0.1%. IVDU carries a risk of 0.67%.

Question 10

What cells are affected by HIV infection?

Answer 10

Once in the blood stream, HIV gp160 glycoproteins bind to the CD4 receptor on target cells. This CD4 receptor is present in high concentrations on T helper lymphocytes (CD4+). Other cells that possess CD4 receptors in lower concentrations and which can become infected are macrophages, monocytes and central nervous system dendritic cells. Following HIV binding to the CD4 receptor, the viral envelope fuses with the infected host cell, allowing capsid entry.

Question 11

Outline the HIV replication cycle?

Answer 11

1) Fusion of HIV to the host cell surface
2) HIV RNA reverse transcriptase, integrase, protease and other viral proteins enter the host cell
3) Viral DNA is formed by reverse transcriptase
4) Viral DNA is transported across the nucleus and integrates
5) New viral RNA is used as genomic RNA to make viral proteins
6) New viral RNA and proteins move to the cell surface and a new immature, HIV forms
7) The virus matures by protease releasing individual HIV proteins

Question 12

How is HIV diagnosed?

Answer 12

HIV seroconversion is symptomatic in 60-80% of individuals. HIV infection is detected by testing for host antibodies; most tests are sensitive to antibodies to both HIV-1 and HIV-2.

There are two types of test for HIV:

1) HIV antibody test (although these may not yet be present)
2) HIV PCR and p24 antigen tests can confirm the diagnosis

HIV antibody test:

• most common and accurate test
• usually consists of both a screening ELISA (Enzyme Linked Immuno-Sorbent Assay) test and a confirmatory Western Blot Assay
• most people develop antibodies to HIV at 4-6 weeks but 99% do by 3 months

p24 antigen test (“insti” test):

• usually positive from about 1 week to 3 - 4 weeks after infection with HIV
• sometimes used as an additional screening test in blood banks

Question 13

What test should be done following diagnosis of HIV?

Answer 13

Following diagnosis, the CD4 lymphocyte count should be deter- mined. This indicates the degree of immune suppression and is used to guide treatment. Counts between 200 and 500/mm3 have a low risk of major opportunistic infection; below 200/mm3 there is a high risk of AIDS-defining conditions. Quantitative PCR of HIV-RNA, known as viral load, is used to monitor the response to ART.

Question 14

What infections are common in a CD4 count of between 200-500?

Answer 14

In HIV infected persons the count declines by about 60 cells/ml blood/ year. Therefore, a CD4 count of between 200-500 is normally seen after about 7 years. Patients have constitutional symptoms (weight loss, fever, night sweats, lymphadenopathy) as well as irritating skin infections such as severe athletes foot, oral thrush and herpes zoster (shingles). Bacterial infections, especially H.inlfuenza, S.pneumoniae, Salmonella spp, S.aureus, S.epidermidis, and TB are common when the CD4 count drops below 400.

Hairy leukoplakia (secondary to EBV) and Kaposi sarcoma (secondary to HHV-8) are also common.

Question 15

A patient has a CD4 count of less than 200. What infections are they predisposed to?

Answer 15

This may be expected to occur after 8 years of diagnosis. As the immune system fails there is increased risk of serious opportunistic infections such as Pneumocystis carinii (pneumonia), Cryptococcus neoformans (meningitis) and Toxoplasma gondii (brain mass). HIV dementia and progressive multifocal leukoencephalopathy (secondary to the JC virus) can also occur in this range.

Question 16

What infection are patients with a CD4 count of <50 at an increased risk of?

Answer 16

Mycobacterium avium-intracellulare. This normally only causes infection in birds, but can cause disseminated disease in the AIDS patient. Cytomegalovirus infections (e.g. CMV colitis, retinitis, oesophagitis) also rise as the count moves from 50 to zero.

Question 17

What is meant by the term viral load?

Answer 17

CD4 counts are used to determine the severity of HIV infection, risk of opportunistic infection, prognosis and response to anti-viral therapy. Plasma measurements of HIV RNA by PCR have been shown to correlate with the risk of opportunistic infections, progression to AIDS and risk of death. A good analogy is the train analogy to explain the predictive value of CD4 count and viral load. Viral load tells you the speed at which the train is heading for the cliff (low CD4, development of opportunistic infections, and death), while the CD4 count tells you where the train currently is. For example, if a patient has a CD4 count of 450cells/mL and a viral load of >106 copies per micro litre, that patient is at particular risk of developing Pneumocystis carinii pneumonia today (CD4>200) but is at great risk in the future for rapid CD4 decline, opportunistic infection and death if not treated.

Question 18

What is HIV seroconversion?

Answer 18

Following initial infection, HIV can begin replication immediately, resulting in rapid progression to AIDS, or there can be a chronic latent course. The former, most common pattern occurs in 3 stages starting with initial infection, marked by an acute mononucleosis-like viral illness. This is called HIV seroconversion.

HIV seroconversion is symptomatic in 60-80% of patients and typically presents as a glandular fever type illness. Increased symptomatic severity is associated with poorer long term prognosis. It typically occurs 3-12 weeks after infection.

Features include:
sore throat
lymphadenopathy
malaise, myalgia, arthralgia
diarrhoea
maculopapular rash
mouth ulcers
rarely meningoencephalitis

There are high levels of blood-borne HIV (viremia) at this stage, and the viruses spread to infect lymph nodes and macrophages. An HIV-specific immune response arises, resulting in decreased viremia and resolution of the above symptoms. However, HIV replication continues in lymph nodes and peripheral blood.

Question 19

What are the differential diagnoses to consider in HIV seroconversion illness?

Answer 19

Acute EBV
Cytomegalovirus 
Streptococcal pharyngitis 
Toxoplasmosis
Secondary syphilis 

The presence of a maculopapular rash or mucosal ulceration suggests HIV rather than the other causes of infectious mononucleosis.

Question 20

What is the clinical latency period?

Answer 20

A clinical latency follows for a median of 8 years during which there are no symptoms of AIDS, although some patients develop a dramatic generalized lym- phadenopathy (possibly secondary to an aggressive im- mune attack against HIV harbored in the lymph nodes). This is not a true viral latency without viral replication; HIV continues to replicate in the lymphoid tissue and there is a steady gradual destruction of CD4 T-lym- phocytes (helper) cells.

A high viral load predicts a more rapid rate of decline in CD4 cells.

Question 21

What minor HIV associated conditions are important indicators that a patient may go on to develop AIDS?

Answer 21

A wide range of disorders indicating some impairment of cellular immunity occurs in most patients before they develop AIDS. Careful examination of the mouth is important, as oral candidiasis and oral hairy leucoplakia are common and important conditions that require the initiation of ART and prophylaxis against opportunistic infections, irrespective of the CD4 count.

Question 22

What is the definition of AIDS?

Answer 22

AIDS is defined by the development of specified opportunistic infections, tumours and other clinical features. This is accompanied by a fall in CD4 count to <200 cells/mm3, and a change in the spectrum of associated infections.

Question 23

What is the mechanism of T cell death in HIV?

Answer 23

The CD4 receptor appears to be involved in T cell death. Monocytes and macrophages, which possess lower CD4 receptor concentrations, are not destroyed as extensively as T cells.

When helper T cells are infected and the virus produces its structural proteins, gp160 is integrated into the T helper cell cytoplasmic membrane. The virion will bud at the site of gp160 insertion, taking this portion of the membrane to form its envelope.

3 mechanisms of T cell death are possible:
1) When the virion is budding, the gp160 may bind to adjacent CD4 receptors on the same T helper cell membrane, tearing the T cell membrane and destroying the cell

2) Gp160 in the T cell membrane may mark the T cell as non self, resulting in autoimmune destruction by cytotoxic CD8 T lymphocytes
3) A second phenomenon occurs between infected cells and noninfected CD4 cells. The gp160 in the infected cells binds to other CD4 T-helper cells, resulting in cell-to-cell fusion. One infected cell can fuse with as many as 500 uninfected CD4+ T-helper cells, forming multinucleated giant cells.

Question 24

How are B cells affected by HIV?

Answer 24

HIV does not actually infect the B- cells; however, B-cell dysfunction does occur with HIV infection. There is a polyclonal activation of B-cells, re- sulting in an outpouring of immunoglobulins. This hypergammaglobulinemia results in immune-com- plex formation and autoantibody production. The most important dysfunction that occurs is a diminished abil- ity to produce antibodies in response to new antigens or immunization. This is very serious in infants with AIDS because they cannot develop humoral immunity to the vast number of new antigens they are exposed to.

Question 25

How are monocytes and macrophages affected in HIV?

Answer 25

HIV infects these cells and actively divides within them. However, these cells are not destroyed by HIV. This is clinically significant in 2 ways:

1) Monocytes and macrophages serve as reservoirs for HIV as it replicates, protected within these cells from the immune system.
2) These cells migrate across the blood-brain barrier, carrying HIV to the central nervous system. HIV causes brain disease, and the predominant cell type harboring HIV in the central nervous system is the monocyte- macrophage line.