CD4 and HIV infectious disease: Simian immunodeficiency virus (SIV) and chimeric simian-human immunodeficiency virus (SHIV) infections of NHPs are well-characterized models of HIV infections, faithfully recapitulating key aspects of HIV infections, including the rapid seeding of viral reservoirs, sustained virus replication, the gradual loss of peripheral CD4 + T cells, and the development of simian acquired immunodeficiency syndrome (AIDS) [3–5].