Inclusion Criteria:

• * Male or female, age ≥ 18 and ≤ 65 years
• * HIV-1 infection
• * On continuous antiretroviral therapy for at least 12 months without any interruptions of greater than 14 consecutive days, and on a stable regimen that does not include a non-nucleoside reverse transcriptase inhibitor (NNRTI) for at least 4 weeks or any long-acting ART drug that may be active in the participant after ART interruption for up to one year, without plans to modify ART during the study period
• * Screening plasma HIV RNA levels below the limit of quantification on all available determinations in past 12 months (isolated single values ≥ 40 but \< 200 copies/mL will be allowed if they were preceded and followed by undetectable viral load determinations)
• * CD4+ T cell count nadir \> 300 cells/mm3
• * Screening CD4+ T-cell count ≥ 500 cells/mm3
• * Available ART treatment history and the capacity to construct an effective antiretroviral treatment regimen
• * Willing to pause ART as part of the study

Exclusion Criteria:

• * Pregnant, breastfeeding, or unwilling to practice birth control during participation in the study
• * ART regimen that includes a long-acting anti-HIV drug and/or NNRTI that may be active in the participant for up to one year after ART interruption
• * ART regimen that includes protease inhibitor(s) and/or AZT. These drugs may increase the toxicity of cyclophosphamide.
• * Any history of an HIV-associated malignancy, including Kaposi's sarcoma and any type of lymphoma, or virus-associated cancers
• * History of or current active hepatitis B (HBV) infection defined as positive HBV surface antigen test. A positive anti-HBc regardless of HBsAg status.
• * Active hepatitis C (HCV) infection
• * Active or latent tuberculosis infection
• * Chronic liver disease
• * Active and poorly controlled atherosclerotic cardiovascular disease
• * Unwillingness to abstain from sex or use barrier protection for any sexual activity during the treatment interruption.