Background:  Late in the course of infection, in the absence of intact lymphoid tissue, HIV-1 replicates efficiently in extra-nodal, peripheral tissues. We hypothesized that in this microenvironment, as in lymph node,  local factors can enhance viral replication.

Methods and Results:   We have discovered that a likely constituent of such a microenvironment, human vascular endothelial cells, provide signals that enhance HIV-1 replication 50- to 100-fold in previously resting memory CD4⁺ T cells. The experiments revealed that the HIV-1-infected co-cultures contained a subpopulation of T cells that expressed a high concentration of p24^Gag. However, these cells did not evince proliferation as assessed by IL-2 secretion or CFSE dilution. In addition, neither class II MHC-expressing fibroblasts nor professional antigen presenting cells  could replace vascular endothelial cells  in this system. However, co-cultivation of T cells with vascular endothelial cells  and B lymphoblastoid cells substantially increased the amount of virus produced over that of T cells co-cultured with either B lymphoblastoid cells or vascular endothelial cells alone. This suggests that the vascular endothelial cell signals might be delivered in transit to T cells, and thereby, work in concert with signals derived from conventional antigen presenting cells. Also,  endothelial-cell-mediated enhancement of HIV-1 replication was observed for wild type HIV-1 (Nef⁺) but not for Nef-deleted (Nef^-) virus, thus revealing a new function for Nef protein. Furthermore, we observed that mutation of the putative SH3-binding domain of Nef did not affect its capacity to enhance HIV-1 replication in this assay. In addition to these observations in vitro, we observed that huPBL-SCID/bg mice bearing a human skin graft perfused by human endothelial cell-lined micro-vessels, but lacking lymphoid tissue supported 50-fold higher levels of Nef⁺ than Nef^- virus production within the skin graft.

Conclusions:  Based on these observations, we propose that as yet undetermined endothelial cell-derived signals, acting through Nef, are responsible for enhanced HIV-1 replication in extra-nodal T cells. We envision that endothelial cells at sites of inflammation, which can attract and activate memory T cells, contribute to the formation of a microenvironment that leads to efficient HIV-1 production in peripheral tissues. Thus, Nef-mediated endothelial cell signals to CD4⁺ T cells could contribute to the maintenance of high viral loads in the periphery in the extended late stage of infection, and to progression to AIDS.

Keywords: endothelium; nef; microenvironmet