The purpose of this study is to intensively characterize the immune response, particularly the T-cell response, to a three-dose regimen of an adenovirus-based HIV-1 vaccine in HIV-uninfected adults.

This study will look for relationships among the immune responses induced by MRKAd5 HIV-1 gag/pol/nef vaccine. The study will also determine if the T cells that respond to different vaccine epitopes have correspondingly different functional profiles. The study will evaluate the safety and tolerability of the vaccine regimen as well.

This study will last 60 weeks. All enrolled participants will receive vaccinations at Weeks 0, 4, and 26. There will be between 8 and 20 study visits including the screening visit, depending on the site location. A physical exam, interview, and blood collection will occur at most or all visits. All participants will undergo leukapheresis approximately 4 weeks after their last vaccination and at Week 52. Medical history, an HIV test, a pregnancy test, and HIV and risk reduction counseling will occur at selected visits. Additional blood collection is now occurring in this study to collect more information about the relationship between the immune response and efficacy to the vaccine.

Note: As of 09/19/07, enrollment and vaccinations have been discontinued.

Inclusion Criteria:

• Good general health
• HIV uninfected
• Weight of 110 pounds or greater
• Have access to a participating HIV Vaccine Trials Unit (HVTU) and are willing to be followed during the study
• Willing to receive HIV test results
• Understand the vaccination procedure
• Willing to use acceptable methods of contraception for at least 21 days prior to study entry and until the last study visit

Exclusion Criteria:

• HIV vaccines or placebos in prior HIV trial. Participants who can provide documentation that they received a placebo in a prior HIV trial may be eligible.
• Immunosuppressive medications within 168 days prior to first study vaccination
• Blood products within 90 days prior to first study vaccination or within 14 days after the injection
• Immunoglobulin within 90 days prior to first study vaccination or within 14 days after the injection
• Live attenuated vaccines within 42 days prior to first study vaccination or within 14 days after the injection
• Investigational research agents within 30 days prior to first study vaccination
• Medically indicated subunit or killed vaccines within 5 days prior to first study vaccination or within 14 days after the injection
• Allergy treatment with antigen injections within 30 days prior to first study vaccination
• Clinically significant medical condition, abnormal physical exam findings, abnormal laboratory results, or past medical history that may affect current health
• Any medical, psychiatric, or social condition that, in the opinion of the investigator, would interfere with the study.
• History of anaphylaxis and/or allergy to vaccine components
• Autoimmune disease or immunodeficiency
• Uncontrolled hypertension
• Bleeding disorder
• Cancer. Participants with surgically removed cancer that is unlikely to recur are not excluded.
• Seizure disorder
• Absence of the spleen
• Abnormal laboratory values
• Mental illness that would interfere with the study
• Hysterectomy
• Pregnancy or breastfeeding