HIV and SIV infected CD4 T cells localize primarily to lymphoid and mucosal tissues, where they constitute >90% of infected cells in chronic infection. While largely assumed, it remains to be established if peripheral blood (PB) viremia originates from these tissues, or from infected cells directly within the vasculature. Here we assessed in rhesus macaques (RM) and humans the potential contribution of tissue-based virus production to plasma viremia (VL).
Four RM were infected i.v. with barcoded SIVmac239, and treated with the lymphocyte migration inhibitor FTY720 daily from day 7 or 28 until day 90. PB and lymphoid tissue (LT) samples were collected for cell and virus quantification. In parallel, we collected PB and thoracic duct lymph (TDL) from 11 HIV+ donors (3 viremic, 8 ART) and assessed VL in each compartment. Viral phylogeny was characterized by SGS gp160 env sequencing of plasma and TDL.
In the FTY720-treated RM we observed near complete redistribution of circulating CD4 T cells into tissues within 7 days of FTY720 treatment (pre-FTY720: 513±283 CD4 T cells/µl, post-FTY720: 5±2 CD4 T cells/µl). Despite the absence of PB CD4 T cells, all animals, regardless FTY720 administration, had peak and set point plasma VL similar to historical controls. Barcode sequencing of cell-associated virus from LT and plasma virus during FTY720 treatment revealed substantial overlap in the dominant virus populations replicating in the LT and circulating in plasma. Together, these results suggest that the circulating plasma virus originated from tissues. We next assessed paired TDL and plasma from HIV+ donors. HIV RNA copies were higher in TDL vs. PB (p=0.0137; up to 10-fold higher in viremic), and the virus populations were phylogenetically indistinguishable between the compartments. Based upon the differential VLs, and incorporating viral clearance rate, plasma volume, and lymph output we calculated that ~50% of plasma virus originates from thoracic duct output, in some individuals reaching a 100% contribution.
Our results indicate that HIV infected cells within LT and non-LT, rather than the vasculature, are the major source of PB viremia. A large proportion of this viremia is maintained through thoracic duct lymphatic efflux, indicating that virus released from infected cells in tissues travels through lymphatics into PB.