The present invention provides treatment methods involving modulating a sirtuin activity and/or a sirtuin mRNA and/or a sirtuin polypeptide level. In some embodiments, the present invention provides treatment methods involving modulating SIRT1 activity and/or SIRT mRNA and/or polypeptide level. The present invention provides methods of inhibiting SIRT1 Tat deacetylase activity. Methods of inhibiting SIRT1 Tat deacetylase activity are useful for treating immunodeficiency virus infections, particularly human immunodeficiency virus (HIV) infection. Thus, the present invention provides methods of treating an immunodeficiency virus infection, generally involving inhibiting SIRT1 Tat deacetylase activity. The present invention further provides methods of identifying agents that modulate sirtuin activity (e.g., SIRT1 activity), particularly ability of sirtuins to interact with (e.g., bind and/or deacetylate) a substrate, e.g., a viral substrate such as a Tat polypeptide. The present invention further provides active agents that modulate sirtuin activity or expression; and compositions, including pharmaceutical compositions, comprising the active agents.