Chemoprevention of retroviral infection: success is determined by virus inoculum strength and cellular immunity

Prevention of human immunodeficiency virus and AIDS: postexposure prophylaxis (including health care workers)

Postexposure prophylaxis (PEP), which is designed to prevent human immunodeficiency virus (HIV) infection after an exposure, is one of several strategies for HIV prevention. PEP was first used after occupational HIV exposures in the late 1980s, with the Centers for Disease Control and Prevention issuing the first set of guidelines that included considerations regarding the use of antiretroviral agents for PEP after occupational HIV exposures in 1990. Use of PEP has been extended to nonoccupational exposures, including after sexual contact or injection-drug use. This article provides a rationale for PEP, assessment of the need for PEP, and details of its implementation.

Preclinical assessment of HIV vaccines and microbicides by repeated low-dose virus challenges

BACKGROUND

Trials in macaque models play an essential role in the evaluation of biomedical interventions that aim to prevent HIV infection, such as vaccines, microbicides, and systemic chemoprophylaxis. These trials are usually conducted with very high virus challenge doses that result in infection with certainty. However, these high challenge doses do not realistically reflect the low probability of HIV transmission in humans, and thus may rule out preventive interventions that could protect against "real life" exposures. The belief that experiments involving realistically low challenge doses require large numbers of animals has so far prevented the development of alternatives to using high challenge doses.

METHODS AND FINDINGS

Using statistical power analysis, we investigate how many animals would be needed to conduct preclinical trials using low virus challenge doses. We show that experimental designs in which animals are repeatedly challenged with low doses do not require unfeasibly large numbers of animals to assess vaccine or microbicide success.

CONCLUSION

Preclinical trials using repeated low-dose challenges represent a promising alternative approach to identify potential preventive interventions.

Postexposure prophylaxis in children and adolescents for nonoccupational exposure to human immunodeficiency virus

Exposure to human immunodeficiency virus (HIV) can occur in a number of situations unique to, or more common among, children and adolescents. Guidelines for postexposure prophylaxis (PEP) for occupational and nonoccupational (eg, sexual, needle-sharing) exposures to HIV have been published by the US Public Health Service, but they do not directly address nonoccupational HIV exposures unique to children (such as accidental exposure to human milk from a woman infected with HIV or a puncture wound from a discarded needle on a playground), and they do not provide antiretroviral drug information relevant to PEP in children. This clinical report reviews issues of potential exposure of children and adolescents to HIV and gives recommendations for PEP in those situations. The risk of HIV transmission from nonoccupational, nonperinatal exposure is generally low. Transmission risk is modified by factors related to the source and extent of exposure. Determination of the HIV infection status of the exposure source may not be possible, and data on transmission risk by exposure type may not exist. Except in the setting of perinatal transmission, no studies have demonstrated the safety and efficacy of postexposure use of antiretroviral drugs for the prevention of HIV transmission in nonoccupational settings. Antiretroviral therapy used for PEP is associated with significant toxicity. The decision to initiate prophylaxis needs to be made in consultation with the patient, the family, and a clinician with experience in treatment of persons with HIV infection. If instituted, therapy should be started as soon as possible after an exposure-no later than 72 hours-and continued for 28 days. Many clinicians would use 3 drugs for PEP regimens, although 2 drugs may be considered in certain circumstances. Instruction for avoiding secondary transmission should be given. Careful follow-up is needed for psychologic support, encouragement of medication adherence, toxicity monitoring, and serial HIV antibody testing.

Emerging viral infections

The past decade has witnessed the emergence of several significant viral pathogens and the further evolution of additional viral pathogens. Transmitted by a variety of differing routes, these organisms have presented substantial intellectual challenges to medicine of the 20th and 21st centuries. As perhaps the benchmark pathogen of the past decade, HIV has provided medicine and society with a most formidable opponent, and one that has yet to be fully conquered. Nonetheless, a variety of additional viral pathogens have also perplexed medicine over the past 10-15 years.

Containment of simian immunodeficiency virus infection: cellular immune responses and protection from rechallenge following transient postinoculation antiretroviral treatment

To better understand the viral and host factors involved in the establishment of persistent productive infection by primate lentiviruses, we varied the time of initiation and duration of postinoculation antiretroviral treatment with tenofovir (9-[2-(R)-(phosphonomethoxy)propyl]adenine) while performing intensive virologic and immunologic monitoring in rhesus macaques, inoculated intravenously with simian immunodeficiency virus SIVsmE660. Postinoculation treatment did not block the initial infection, but we identified treatment regimens that prevented the establishment of persistent productive infection, as judged by the absence of measurable plasma viremia following drug discontinuation. While immune responses were heterogeneous, animals in which treatment resulted in prevention of persistent productive infection showed a higher frequency and higher levels of SIV-specific lymphocyte proliferative responses during the treatment period compared to control animals, despite the absence of either detectable plasma viremia or seroconversion. Animals protected from the initial establishment of persistent productive infection were also relatively or completely protected from subsequent homologous rechallenge. Even postinoculation treatment regimens that did not prevent establishment of persistent infection resulted in downmodulation of the level of plasma viremia following treatment cessation, compared to the viremia seen in untreated control animals, animals treated with regimens known to be ineffective, or the cumulative experience with the natural history of plasma viremia following infection with SIVsmE660. The results suggest that the host may be able to effectively control SIV infection if the initial exposure occurs under favorable conditions of low viral burden and in the absence of ongoing high level cytopathic infection of responding cells. These findings may be particularly important in relation to prospects for control of primate lentiviruses in the settings of both prophylactic and therapeutic vaccination for prevention of AIDS.

Postexposure to HIV prophylaxis

Since the reports of an international case-control study during the period 1995-1997, further evidence has accumulated supporting the use of postexposure therapy with antiretrovirals. This includes evidence from vertical transmission studies with both zidovudine and nevirapine, and animal model data. Guidelines have been developed and revised for occupational postexposure prophylaxis provision, but there remains considerable debate over postexposure prophylaxis for nonoccupational (predominantly sexual) exposures.

The extent of early viral replication is a critical determinant of the natural history of simian immunodeficiency virus infection

Different patterns of viral replication correlate with the natural history of disease progression in humans and macaques infected with human immunodeficiency virus type 1 (HIV-1) and simian immunodeficiency virus (SIV), respectively. However, the viral and host factors influencing these patterns of viral replication in vivo are poorly understood. We intensively studied viral replication in macaques receiving identical inocula of SIV. Marked differences in viral replication patterns were apparent within the first week following inoculation, a time prior to the development of measurable specific immune effector responses to viral antigens. Plasma viral RNA levels measured on day 7 postinoculation correlated with levels measured in the postacute phase of infection. Differences in the susceptibility of host cells from different animals to in vitro SIV infection correlated with the permissiveness of the animals for early in vivo viral replication and hence with the postacute set point level of plasma viremia. These results suggest that host factors that exert their effects prior to full development of specific immune responses are critical in establishing the in vivo viral replication pattern and associated clinical course in subjects infected with SIV and, by extension, with HIV-1.

Early postinfection antiviral treatment reduces viral load and prevents CD4+ cell decline in HIV type 2-infected macaques

Reports of significant reductions in plasma viral load by anti-HIV drugs have raised the possibility that antiviral therapy, if initiated sufficiently early, may result in sustained control of infection and prolonged clinical benefits. We evaluated the effects of intervention coincident with infection using an antiviral nucleoside, d4T, in Macaca nemestrina infected with a highly pathogenic isolate of HIV-2 (HIV-2[287]). Infection with this virus reproducibly results in high viremia and rapid CD4+ cell depletion, allowing a sensitive measurement of the treatment effect on viral load and clinical outcome. Compared to the control group, d4T-treated macaques showed significantly lower (2-3 log10) plasma- and cell-associated viral load. No CD4+ cell decline was observed in the treatment group while on therapy with d4T whereas CD4+ cells of control macaques declined from a preinfection mean of 32% of PBMCs to below 10%. Notably, when d4T treatment was withdrawn after 16 weeks, five of the six macaques continued to control viral load and have maintained normal CD4+ cell level for more than a year. These results demonstrate that early antiviral intervention, even of a limited duration, may constitute an important strategy against lentiviral-induced disease.

Animal studies of prophylaxis

Antiviral and immune-based intervention strategies have been evaluated in vivo in animal models based on infections with lentiviruses and other retroviruses to explore their effect on preventing or limiting de novo virus infection. Variables that can be experimentally manipulated to address specific questions relevant to postexposure prophylaxis (PEP) include timing of initiation of treatment, duration of treatment, dosing regimen, virus strain, and virus inoculum. Prevention of the establishment of infection by PEP has been documented under defined experimental conditions. However, in the event that early intervention is not effective in preventing infection, evidence supporting control of viral replication and disease progression mediated by early intervention has also been reported. Thus, PEP may have a significant impact in preventing or altering viral replication in vivo and disease course.

Pathogenicity of live, attenuated SIV after mucosal infection of neonatal macaques

Adult macaques do not develop disease after infection with a nef deletion mutant of the simian immunodeficiency virus (SIV) and are protected against challenge with pathogenic virus. This finding led to the proposal to use nef-deleted viruses as live, attenuated vaccines to prevent human acquired immunodeficiency syndrome (AIDS). In contrast, neonatal macaques developed persistently high levels of viremia after oral exposure to and SIV nef, vpr, and negative regulatory element (NRE) deletion mutant. Severe hemolytic anemia, thrombocytopenia, and CD4+ T cell depletion were observed, indicating that neither nef nor vpr determine pathogenicity in neonates. Because such constructs have retained their pathogenic potential, they should not be used as candidate live, attenuated virus vaccines against human AIDS.