HIV This Week

Phillips EJ, Mallal SA. Pharmacogenetics and the potential for the individualization of antiretroviral therapy. Curr Opin Infect Dis. 2008;21(1):16-24.

Genetic associations highlighting differences in the response to HIV infection and treatment have significantly furthered our understanding of the pathogenesis, pharmacokinetics and pharmacodynamics of antiretroviral drug action and toxicities and HIV disease itself. This review focuses on the current knowledge of associations between polymorphisms and treatment outcomes in HIV with particular emphasis on clinically relevant relationships likely to lead to the individualization of antiretroviral therapy. Our understanding of the immunogenetic basis of drug toxicity has been furthered by human leukocyte antigen associations with hypersensitivities for the antiretroviral drugs abacavir and nevirapine. For abacavir in particular, the use of HLA-B*5701 as a screening test appears to be generalizable across racially diverse populations and has been supported by both observational, and blinded randomized controlled trials. Differences in HIV acquisition and progression as well as antiretroviral efficacy and toxicity will continue to provide the basis for research to define the genetic basis of such diversity. Despite the plethora of research in this area, numerous barriers exist to the successful operationalization of genetic testing to the clinic. HLA-B*5701 screening to prevent abacavir hypersensitivity is currently the most relevant to clinical practice and highlights that the promise of cost-effective testing can be facilitated by robust laboratory methodology and quality assurance programs that can be applied to diverse treatment settings.

Editors´note: Drug hypersensitivity syndromes are not directly related to the dose of a drug but are thought to be related to a combination of factors and associated with specific human leukocyte antigen (HLA) alleles within the major histocompatibility complex. In the case of patients starting nevirapine, 16% develop rash, 5% have hypersensitivity reactions with fever and hepatitis, and 0.3% can have severe skin syndromes. Knowledge of HIV pharmacogenetics is growing but unfortunately it will be some time before antiretroviral treatment in the real world can be tailored to patient genetics to avoid hypersensitivity reactions.

During the evolution, the immune system has developed several strategies to fight viral infections. Apoptosis, autophagy and necrosis are different types of cell death that play a main role in the interactions between infective agents and the host, since they are often important defence mechanisms that have to avoid the spreading of the infection. In turn, viruses have evolved numerous ways to evade the host immune system by influencing the behaviour and functionality of several components. HIV infects and kills CD4+ T helper lymphocytes, preferentially those that are antigen-specific, but also encodes proteins with apoptotic capacities, including gp120, gp160, Tat, Nef, Vpr, Vpu, Vif and, last but not least, the viral protease. This latter protein can kill infected and uninfected lymphocytes through the action of several host molecules, mainly members of the tumour necrosis factor family, or via the mitochondrial apoptotic pathway. The proinflammatory state that is characteristic of both the acute and chronic phase of HIV infection facilitates cell death, and is an additional cause of immune damage. Potent antiretroviral drugs that are largely used in therapy can reduce apoptosis by different mechanisms, that not only include the diminished production of the virus by infected cells and the subsequent reduction of inflammation, but also a direct action on the viral protease. The role of the host genetic background is finally crucial in understanding the process of cell death in HIV infection.

Editors´note: Programmed cell death or apoptosis is one of our most ancestral and potent cell defence mechanisms. HIV manipulates the apoptotic machinery to its advantage. For example, some viral proteins cause down- regulation of CD4 preventing gp120-CD4-mediated cell death. If HIV caused massive cell death, it couldn’t survive itself. On our side, we trigger activation of non-infected cells to help cope with the virus but then these innocent bystanders are induced to die by the virus. Research on immunogenetics can provide insights into why some people control HIV better, which may help with the design of antiretroviral therapy for immune reconstitution.