Macrophages and dendritic cells play key roles in viral infections, providing virus reservoirs that frequently resist anti-viral therapies and linking innate virus detection to anti-viral adaptive immune responses 1, 2 . HIV-1 fails to transduce dendritic cells and has a reduced ability to transduce macrophages, due to an as yet uncharacterized mechanism that inhibits infection by interfering with efficient synthesis of viral cDNA 3, 4 . In contrast, HIV-2 and related simian immunodeficiency viruses (SIVsm/mac) transduce myeloid cells efficiently owing to their virion-associated Vpx accessory proteins, which counteract the restrictive mechanism 5, 6 . Here we show that the inhibition of HIV-1 infection in macrophages involves the cellular SAM domain HD domain-containing protein 1 (SAMHD1). Vpx relieves the inhibition of lentivirus infection in macrophages by loading SAMHD1 onto the CRL4 DCAF1 E3 ubiquitin ligase, leading to highly efficient proteasome-dependent degradation of the protein. Mutations in SAMHD1 cause Aicardi-Goutieres syndrome (AGS), a disease that produces a phenotype that mimics the effects of a congenital viral infection 7, 8 . Failure to dispose of endogenous nucleic acid debris in AGS results in inappropriate triggering of innate immune responses via cytosolic nucleic acids sensors 9, 10 . Thus, our findings reveal that macrophages are defended from HIV-1 infection by a mechanism that prevents an unwanted interferon response triggered by self nucleic acids, and uncover an intricate relationship between innate immune mechanisms that control response to self and to retroviral pathogens.