Tissue dendritic cells (DCs) and macrophages (Macs) are major target cells for HIV. In both there is noncytopathic low level HIV infection. Furthermore macrophages act as reservoirs, particularly in brain, whereas DCs take up HIV and transfer it to T cells. HIV infection completely inhibits types I and III interferon production by monocyte derived DCs and Macs. It also stimulates them to produce specific subsets of interferon stimulated genes (ISGs) some of which have direct antiviral activity (Harman et al. Blood, 2011, Nasr et al. Blood, 2012). We are currently investigating the mechanisms of these two phenomena. In both cell types HIV inhibited phosphorylation of IRF3 and its translocation to the nucleus. In MDDCs the central upstream enzyme, TBK1, was ubiquitinated in a complex with the E3 ubiquitin ligase and IRF3 (detected by proximity ligation assay). However it was not phosphorylated, thus defining the stage at which HIV inhibits. Deletion of vpr restored IFNβ production so physical interaction of this and other accessory proteins with TBK1 is being examined.
In both these cells IRF1 (and possibly) IRF7 coordinated the ISG subset induction. In Macs the use of antiviral inhibitors of the HIV replication cycle demonstrated that the ISG subset was induced after integration and prior to HIV budding. This suggested that de novo produced, tat dependent HIV RNA is responsible, probably via RIG1, MAVS and then IRF1/7. This hypothesis is being examined by serial knockdown in Macs. So far Tat and IRF1/7 knockdown have ablated the ISG subset induction. Previously viperin was shown to have anti-HlV activity but other ISGs, IFT1, 2 and IFT3 are also inhibitory. Thus these ISGs act as inducible restriction factors for HIV replication, contributing to low level persistent infection in both cell types and to a reservoir function in macrophages in vivo.
Supported by NHMRC Project grant APP1028014.