Follicular dendritic cell mediated augmentation of human immunodeficiency virus type-1 transcription

Throughout the course of disease, productive HIV infection occurs primarily in the lymphoid follicles surrounding follicular dendritic cells (FDCs) suggesting a highly permissive microenvironment for viral infection and replication. Because FDCs stimulate lymphocyte activation, I hypothesized that FDCs may provide signals that augment HIV replication. To test this hypothesis, I cultured HIV-1 infected peripheral blood lymphocytes (PBLs) with and without FDCs after which virus protein production and transcription were measured by p24 ELISA, RT-PCR and nuclear run-on analysis. Cultures with FDCs produced up to 480 ng/ml of p24, whereas infected T-cells alone produced only 133 ng/ml. When the levels of HIV RNA were assessed by RT-PCR, they were increased in cultures containing FDCs and this was due to an increase in the rate of transcription. FDCs also augmented transcription when cells were infected with clinical isolates of HIV-1. Separation of FDCs from infected cells using a semi-permeable membrane did not reduce the augmented level of virus transcription, indicating that the factor mediating the increase was soluble. This result was confirmed using cell-free supernatant fluid obtained from cultures of FDCs. Electromobility shift assays revealed that the levels of the transcription factor NF-kappaB were increased when infected cells were co-cultured with FDCs. The quintessential activator of NF-kappaB is tumor necrosis factor-alpha (TNF-alpha). Blocking soluble TNF-alpha with a TNF receptor-fusion protein prevented FDC mediated augmentation of transcription and increased HIV-1 p24 production suggesting that TNF-alpha mediated these events. In culture, FACS-purified FDCs produced between 75 and 200 pg/ml of TNF-alpha. Furthermore, the addition of TNF-alpha (50 and 100 pg/ml) to cultures of infected cells resulted in a dose-dependent increase in transcription indicating that the small amount of TNF-alpha produced by FDCs was sufficient to augment HIV transcription. Hence, FDCs not only serve as a reservoir of infectious virus but also contribute to increased local production of virus in the lymphoid follicles.