CASE 13
You have been Anthony’s general practitioner since childhood. Other than the usual run of minor childhood disorders, there has been nothing of note in his medical history. Both parents and a younger sibling are well. He is currently 22 years of age and is home for the summer, on vacation from college. Anthony had called your office, requesting a complete physical examination, and, as a precursor to that visit, you have ordered a series of routine tests, including serum blood cell counts and electrolyte determinations, chest radiograph, and urinalysis. The day before the appointment the laboratory results become available and show, somewhat surprisingly to you, a marked increase in some liver enzymes (transaminases) and an elevated bilirubin level. Physical examination of this young man when he did come in revealed, not surprisingly, a mild jaundice, with some tenderness at the liver edge. There were no other significant abnormalities, although you also note some healing bruises and pinpricks in the forearm. He seems to be sniffing a lot, as though he has a perpetual postnasal drip, and he seems to be somewhat more edgy in your presence than usual. How are you going to open up a conversation? What do you want to address? Do you have significant concerns?
QUESTIONS FOR GROUP DISCUSSION
RECOMMENDED APPROACH
Implication/Analysis of Family History
There is nothing remarkable about the family history and because you have been Anthony’s physician since childhood you are more concerned about recent lifestyle changes now that he is attending college.
DIAGNOSIS
On the basis of the nucleic acid amplification tests, Anthony is informed that he is infected with HIV, the causative agent of acquired immunodeficiency disease (AIDS). Anthony’s CD4+ T cell count will need to be monitored. As the number of CD4+ T cells decreases, the number of opportunistic infections increases (Fig. 13-1).
THERAPY
Highly active antiretroviral therapy (HAART) is the treatment of choice for HIV-infected patients. This drug regimen includes two antinucleoside analogue inhibitors and a protease inhibitor or a non-nucleoside reverse transcriptase inhibitor. Even though the use of HAART is not a controversial issue, the “when” to initiate therapy is highly debated. This controversy follows the realization that the “hit hard, hit early” slogan of the mid 1990s does not lead to the eradication of the virus in patients despite long term, and early, drug therapy. HIV is detectable in latent infected cells even after prolonged therapy, and so the approach to drug therapy has had to be modified.
Advantages and Disadvantages of HAART
There are advantages and disadvantages to HAART. In patients receiving HAART, plasma HIV RNA levels fall to below the level of detection within 2 to 6 months. There is an increase in CD4+ T cell count and therefore delayed progression to AIDS. As well, in some cases HAART is accompanied by enlargement of the thymus. Whether this enlargement is due to regeneration of the thymus and active thymopoiesis or from the migration of peripheral blood cells into the thymus is currently being addressed using in vivo TREC (T cell receptor excision circles) assay of thymic function (see Case 2). On the down side, hepatotoxicity is a serious consideration, as is the development of viral variants that are resistant to the drugs. Additionally, HAART therapy does not eliminate HIV from resting memory CD4+ T cells carrying an integrated copy of the viral genome. Therefore, on discontinuation of HAART, viral load measures (plasma HIV-1 RNA) become significant when the latently infected cells are activated even by stimulatory molecules normally present in lymphoid tissues.
Role of Cytokines in Therapy
Those who advocate the use of IL-2 as therapy to increase the number of CD4+ T cells in conjunction with HAART suggest that the IL-2 would activate latent cells with the net effect being their eradication, while the patient is protected by HAART. Implicit assumptions in this approach are that the virus strains are not resistant to HAART and that all the latent cells are CD4+ T cells. Recall that some cells that do not express CD4 can be infected after binding to galactosyl ceramide.
Vaccine Failures: “Original Antigenic Sin”
Because mutations can occur early in the course of the disease, the activated cytotoxic T cells are no longer effective, yet immune response to escape mutants is much weaker than that to the primary HIV isolate (“original antigenic sin”). Antibody responses to the initial epitopes have also been reported to dominate. In addition to a blunted response to the escape mutants, the decrease in CD4+ T cells hampers cytotoxic T cell development (and antibody responses) because both require cognate interaction with, and cytokines derived from, CD4+ T cells.
ETIOLOGY OF AIDS: HUMAN IMMUNODEFICIENCY VIRUS
Cell Infection
Cell Surface Receptors
Productive infection requires that HIV binds to both a primary receptor, CD4, and a co-receptor CCR5 or CXCR4 on the target cell. CCR5 and CXCR4 are chemokine receptors that are targeted by different HIV strains (Fig. 13-2). CCR5 is used primarily by macrophage (M) tropic strains, whereas CXCR4 is used primarily by T cell (T) tropic strains. HIV interacts first with the primary receptor, CD4, via its envelope glycoprotein, gp120. Binding to CD4 creates a complex that allows gp120 to bind to CCR5 or CXCR4. After this binding, gp41, which is a fusion protein, enters the cell membrane to initiate fusion of the viral envelope and subsequent delivery of the viral proteins and genome into the cell. Under some circumstances, HIV can infect cells that do not express CD4 (e.g., endothelial cells, microglial cells) by binding galactosyl ceramide.
HIV Evasive Strategies
Downregulation of Class I Expression
Downregulation of class I MHC expression is a relatively common viral evasive strategy, but the mechanisms by which this is achieved vary from virus to virus. In HIV, downregulation of class I MHC is attributed to the action of the HIV regulatory protein, Nef, because its presence, or absence, in the HIV genome affects expression of class I MHC on the cell surface, a requirement for activation of cytotoxic T cells. At least two different mechanisms have been reported to explain this effect; the first is that Nef increases the endocytosis of surface class I MHC; the other is that Nef redirects class I MHC from the Golgi to the cytosol, or sequesters the complex in the Golgi. Irrespective of the mechanism, a decrease in the class I MHC-HIV peptides expressed on the infected cell surface will hinder recognition and targeting by cytotoxic T cells. Interestingly, the amount of class I MHC expressed on a cell surface has been shown to depend on the particular Nef allele encoded by a particular viral variant.
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