Microbiology Human Immunodeficiency Virus Case File

Microbiology Human Immunodeficiency Virus Case File

Eugene C.Toy, MD, Cynthia Debord, PHD, Audrey Wanger, PHD, Gilbert Castro, PHD, James D. Kettering, PHD, Donald Briscoe, MD

CASE 29

A 28-year-old man presents to the physician’s office for evaluation of a rash on his chest. He started with one oval-shaped purplish area that he thought was a bruise but has subsequently developed multiple new lesions. The growths don’t hurt, itch, or bleed, but he continues to get new ones, and the existing ones are getting larger. He has never had anything like this before, has no history of allergies and denies exposure to any new medications, foods, lotions, or soaps. His past medical and family histories are unremarkable. His review of systems is significant for a 15-lb weight loss in the past 2 months, approximately 6 weeks of diarrhea, and a 3-week history of a sore throat. On examination, he is a thin but generally well appearing male. His vital signs are normal. Examination of his pharynx shows thick white plaques on the posterior pharynx and soft palate. On the skin of his chest are multiple oval-shaped purple or brown macules. They are firm on palpation and vary in size from 0.5 to 4 cm in length. Several of them appear to be growing together into larger, confluent plaques. You perform a punch biopsy of one of the lesions. In 5 days you get the pathology report with the diagnosis of Kaposi sarcoma.

◆ With what virus is this patient likely infected?

◆ What specific cell types are most commonly infected with this virus? What cell surface receptor is the binding site of this virus?

◆ What serologic testing is most frequently performed to make this diagnosis?

ANSWERS TO CASE 29: HUMAN IMMUNODEFICIENCY VIRUS

Summary: A 29-year-old man has weight loss, white plaques on the pharynx, and purple lesions on the abdomen, which on biopsy reveals Kaposi sarcoma.

◆ Virus with which this patient is most likely infected: Human immunodeficiency virus (HIV)

◆ Specific cells infected by and binding site of HIV: CD4 surface receptor protein on macrophages and T lymphocytes

◆ Serologic testing to confirm diagnosis: HIV enzyme-linked immunosorbent assay (ELISA) and Western-blot analysis, or PCR

CLINICAL CORRELATION

The HIV is a human retrovirus in the Lentivirinae subfamily. It is a spherical, enveloped RNA virus with a cone-shaped capsid that contains two copies of a positive-strand RNA genome. HIV infects cells of macrophage lineage and helper T cells by binding to the CD4 surface receptor protein on these target cells, resulting in fusion of the viral envelope with the cellular plasma membrane to gain entry. On entry into the host cell cytoplasm, an RNA-dependent DNA polymerase enzyme (reverse transcriptase), which is present in the viral capsid, uses the viral RNA to synthesize viral DNA. The viral DNA is transported to the host nucleus, where it is spliced into the host genome. The integrated viral DNA acts as a host cellular gene and is transcribed by host RNA polymerase II to produce new copies of viral RNA and proteins, which assemble into new HIV virions. HIV initially infects cells of macrophage lineage, but quickly reaches the lymph nodes where CD4 T cells are infected. The immunosuppression caused by HIV is primarily caused by a reduction in the helper and delayed type hypersensitivity responses mediated by CD4 T cells. Infected macrophages probably serve as reservoirs and means of distribution of HIV. HIV avoids the host immune system in several ways. Infection of macrophages and helper T cells inactivates central components of the host immune system. Also, HIV has an intrinsic genetic instability as a result of errors caused by reverse transcriptase which may contribute to an antigenic drift in the virus, resulting in reduced host immune system recognition. Symptomatic disease caused by HIV is proportionate to the loss of CD4 T cells and the resulting immune dysfunction. Acquired Immune Deficiency Syndrome (AIDS) is defined by the presence of HIV, a reduction of CD4 T cells, and the acquisition of characteristic opportunistic infections. Serologic diagnosis of HIV infection is primarily made by ELISA testing and, when this is positive, confirmation by Western blot analysis. Current HIV treatment involves using medications, individually or in combinations, which interfere with the actions of reverse transcriptase and block the proteases that activate the virion.

APPROACH TO HUMAN IMMUNODEFICIENCY VIRUS

Objectives

1. Know the structure and characteristics of the human immunodeficiency virus.
2. Know the mechanism by which HIV is transmitted, infects target cells, replicates, and causes immune deficiency.
3. Know the diagnosis, treatment, and opportunistic infections associated with AIDS.

Definitions

Retrovirus: An RNA virus. Retroviruses have an enzyme called reverse transcriptase that gives them the unique property of transcribing RNA into DNA. The retroviral DNA can then integrate into the chromosomal DNA of the host cell to be expressed there.

HAART: Highly active antiretroviral therapy. It is the treatment for human immunodeficiency virus (HIV) infection that uses a combination of several antiretroviral drugs. The drugs inhibit the ability of the virus to multiply in the body, and they slow down the development of AIDS.

AIDS: Acquired immunodeficiency syndrome. A cell-mediated immunity caused by infection by the HIV leading to depletion of CD4 helper T lymphocyte cells.

DISCUSSION

Characteristics of Human Immunodeficiency Virus

HIV appears to have been derived from primate (chimpanzee, especially) lentiviruses and are the etiologic cause of AIDS. AIDS was described in 1981, and the virus was isolated in 1983. AIDS is one of the most significant public health problems worldwide at the current time.

HIV is a retrovirus (reverse transcriptase or RNA-dependent DNA polymerase) in the lentivirus subgroup. It is a medium-sized virus (approximately 100 nm) with two copies of a positive-sense (same as messenger RNA [mRNA]) single-stranded RNA genome. This genome is the most complex of all retroviruses. The lipid envelope contains glycoproteins that undergo antigenic variation, making vaccine development difficult, if not impossible, at the present time. Protease enzymes are coded for by the viral genome, and these are required for the production of infectious viruses. The reverse transcriptase makes a double-stranded DNA copy (provirus) of the viral genomic RNA, which is incorporated into a host chromosome. The proviral DNA later serves as a template for viral mRNA’s and new virion genomes. Virions bud from the plasma membrane of the host cell. Heterogeneous populations of viral genomes are found in an infected individual, especially the env gene, which codes for envelope glycoproteins. The gp120 viral receptor contains binding domains responsible for viral attachment to the CD4 molecule (host receptor) and coreceptors and determines cell tropisms (lymphocytes versus macrophages). These glycoproteins cause antibodies to be formed by the host and are only weakly neutralizing to the virus. The gp41 product contains a transmembrane domain that anchors the glycoprotein in the viral envelope and a fusion domain that facilitates viral entry into the target (host) cells. The virus is inactivated by treatment at room temperature for 10 minutes by any of the following: 10 percent bleach, 50 percent ethanol, 35 percent isopropanol, 0.5 percent paraformaldehyde, or 0.3 percent hydrogen peroxide. HIV in blood in a needle or syringe, however, requires exposure to undiluted bleach for 30–60 seconds for inactivation. Heating at 56°C (132.8°F) for 10 minutes (same as for complement inactivation) will inactivate HIV in 10 percent serum, but HIV in dried protein-containing mixtures is protected. Lyophilized blood products need to be heated to 68°C (154.4°F) for 72 hours to ensure inactivation of contaminating viruses.

Diagnosis

HIV infection can be diagnosed by virus isolation, detection of antiviral antibodies, or measurement of viral nucleic acid or antigens. HIV may be cultured from lymphocytes in peripheral blood primarily. Virus numbers vary greatly in an individual. The magnitude of plasma viremia is an excellent correlate of the clinical stage of HIV infection compared to the presence of antibodies. The most sensitive viral isolation technique requires cocultivation of the test sample with uninfected mitogen-stimulated peripheral blood mononuclear cells. Virus growth is usually detected in 7–14 days by measuring viral reverse transcriptase activity or virus-specific antigens. Virus isolation of HIV is usually considered a research technique, and most medical center viral diagnostic laboratories will not offer this service.

Antibody detection is the most common way to diagnose HIV infection. Seroconversion in HIV infection is generally found to occur in approximately 4 weeks. Most individuals are seropositive within 6–12 weeks after infection, and essentially all will be antibody positive in 6 months. Commercially available enzyme-linked immunoassays (EIA, ELISA) are routinely used as screening tests. If done properly, the reported sensitivity and specificity are at least 98 percent. Two separate EIA tests need to be positive for antibodies in the usual screening situation, and a confirmation test (Western blot usually) will be done to rule out EIA false-positives. Western blot tests (also commercially available) will usually detect antibodies to viral core protein p24 or envelope glycoproteins gp41, gp120, or gp 160. An oral swab test is commonly used.

Amplification assays (RT-PCR, DNA PCR. or bDNA tests) are used to detect viral RNA in clinical specimens. These tests may be quantitative when reference standards are used in each test. These molecular-based tests are very sensitive and form the basis for plasma viral load measurements. HIV RNA levels are important predictive markers of disease progression and monitors of the effectiveness of antiviral therapies.

Treatment and Prevention

Treatment of HIV infection uses classes of drugs that inhibit the virally-coded reverse transcriptase and inhibitors of the viral protease enzymes. Unfortunately, current treatments are biostatic, not virucidal. Therapy with combinations of antiretroviral drugs is called highly active antiretroviral therapy (HAART). It appears to lower viral replication below the limits of laboratory detection but is not curative. The virus persists in reservoirs of longlived, latently infected cells. When HAART is discontinued, viral production rebounds. Monotherapy usually results in the rapid emergence of drugresistant mutants of HIV. HAART therapy has turned HIV infection into a chronic, treatable disease. Unfortunately, large numbers of HIV-infected persons worldwide do not have access to the drugs.

A safe and effective vaccine would be the best hope for controlling HIV infection. Currently, many candidate vaccines are under development and in clinical trials. We have seen that viral vaccines are best when used in a preventative manner. Uninfected individuals are given the vaccine and develop antibodies that prevent infection or disease if the wild-type virus is encountered. HIV vaccine development is difficult because HIV mutates so rapidly. There appears to be so much variation in immune responses in HIV infections that no vaccine has been able to be protective to all individuals in a population. Nothing being currently developed appears to be close to approval in this area, although many organizations are working to produce an effective vaccine. A big hurdle for this, in part, is the lack of an appropriate and cost-effective laboratory animal model for HIV. The SIV-macaque model of simian AIDS is only partially useful for the development of a human HIV vaccine.

COMPREHENSION QUESTIONS

[29.1] During a medical check-up for a new insurance policy, a 60-year-old grandmother is found to be positive in the ELISA screening test for antibodies against HIV-1. She has no known risk factors for exposure to the virus. Which of the following is the most appropriate next step?

A. Immediately begin therapy with azidothymidine.

B. Perform the screening test a second time.

C. Request that a blood culture be done by the laboratory.

D. Tell the patient that she is likely to develop AIDS.

E. Test the patient for Pneumocystis carinii infection.

[29.2] In a person with HIV-1 infection, which of the following is the most predictive of the patient’s prognosis?

A. CD4+ cell count

B. CD4:CD8 cell ratio

C. Degree of lymphadenopathy

D. Level of HIV-1 RNA in plasma

E. Rate of decline in anti-HIV antibody

[29.3] Highly active antiretroviral therapy against HIV infection includes one or more nucleoside analogue reverse transcriptase inhibitors in combination with representatives of which class of antiretroviral agents?

A. Inhibitors of viral binding

B. Inhibitors of viral protein processing

C. Inhibitors of viral release

D. Inhibitors of viral uncoating

E. Nonnucleoside antiretroviral agents

[29.4] Which of the following is the pathogen responsible for blindness in advanced HIV infections?

A. Cytomegalovirus

B. Epstein-Barr virus

C. Fungus

D. Toxoplasma

Answers

[29.1] B. Because HIV cannot be safely isolated and grown in the standard medical center diagnostic laboratory, diagnosis of HIV infections relies on detection of antibodies against the virus. The standard screening test is done by ELISA (enzyme-linked immunosorbent assay). ELISA test formats are quite reliable and accurate and can be used for antibody or antigen detection. By definition, however, screening tests are not 100 percent accurate for sensitivity and specificity. HIV infection, especially, is a tragic infection that requires utmost accuracy in laboratory diagnosis results to aid the physician in counseling the involved patient and family. Under the conditions described in Question 29.1, no known risk factors for HIV contact are claimed or identified. For this situation and any other requiring diagnostic laboratory testing for HIV infection, extra effort is taken to ensure accuracy and correct results. Because it is widely accepted that HIV ELISA screening is not 100 percent sensitive and specific (approximately 98 percent accurate, however), a second blood sample is collected for retesting by ELISA. If both ELISA results are positive, a second confirming test is done. This is usually a Western blot technique. If the Western blot test is positive, then HIV infection is confirmed and related to the patient.

[29.2] D. Amplification assays (RT-PCR, DNA PCR, and b DNA tests) are routinely used to detect viral RNA in clinical specimens. The tests can be quantitative when reference standards are used, and appropriate positive and negative controls must be included in each test. Because these molecular based tests are very sensitive, they form the basis for plasma viral load determinations. It is generally agreed that the amount of HIV in the blood (viral load) is of significant prognostic value. There are continual rounds of viral replication and cell killing in each patient, and the steady-state level of virus in the blood varies with individuals. A single measurement of plasma viral load approximately 6 months after infection can predict the risk of development of AIDS in men several years later. In women, viral load appears to be less predictive. The plasma viral load appears to be the best predictor of long-term clinical outcome, whereas CD4 lymphocyte counts are the best predictor of short-term risk of developing an opportunistic disease. Plasma viral load measurements are a critical element in assessing the effectiveness of antiretroviral drug therapy.

[29.3] B. A growing number of drugs have been approved for treatment of HIV infections. It must be remembered that all HIV drug treatments are only virostatic and not virucidal at this point in time. Classes of drugs include nucleoside and nonnucleoside inhibitors of the viral reverse transcriptase and inhibitors of the viral protease enzyme. The protease inhibitors are significant because protease activity is absolutely essential for production of infectious virus, and the viral enzyme is distinct from human cell proteases. These inhibitors (approved in 2003) block virus entry into host cells.

[29.4] A. The predominant causes of morbidity and mortality among patients with late-stage HIV infection are opportunistic infections. These are defined as severe infections induced by agents that rarely cause disease in immune-competent individuals. Opportunistic infections usually do not occur until CD4 T cell counts drop from normal (1000 cells per microliter) to less than 200 cells per microliter. The common opportunistic infections in untreated AIDS patients are caused by protozoa, fungi, bacteria, and other viruses. Coinfection with DNA viruses are reported to lead to enhanced expression of HIV in cells in vitro. Herpesvirus infections are common in AIDS patients, and cytomegalovirus (CMV) has been shown to produce a protein that acts as a chemokine receptor and is able to help HIV infect cells. CMV retinitis is the most common severe ocular complications of AIDS.

MICROBIOLOGY PEARLS ❖ HIV is a retrovirus and requires reverse transcriptase to make a double-stranded DNA copy of the viral genomic RNA. ❖ Antibody detection is the most common method of diagnosing an infection. ❖ The treatment of HIV largely depends on targeting viral reverse transcriptase and protease enzymes.

REFERENCES

Brooks GF, Butel JS, Morse SA, Jawetz, Melnick, & Adelberg’s Medical Microbiology, 23rd ed. New York: McGraw-Hill, 2004:605–22. 

Knipe DM, Howley PM. Fields Virology, 4th ed. Philadelphia, PA:Lippincott Williams and Wilkins, 2001:1971–2094. 

Murray PR, Rosenthal KS, Pfaller MA. Medical Microbiology, 5th ed. St. Louis, MO: Mosby, 2005:657–74. 

Ryan JR, Ray CG. Sherris Medical Microbiology, 4th ed. New York: McGraw-Hill, 2004:601–16.

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