Short-term pIFN-α 2 a treatment does not significantly reduce the viral reservoir of 1 SIV-infected , ART-treated rhesus macaques 2 3

24 The major obstacle to HIV-1 eradication is a reservoir of latently infected cells that 25 persists despite long-term antiretroviral therapy (ART) and causes rapid viral rebound if 26 treatment is interrupted. Type I interferons are immunomodulatory cytokines that induce 27 antiviral factors and have been evaluated for the treatment of HIV-infected individuals, 28 resulting in moderate reduction of viremia and inconclusive data about their effect on 29 reservoir size. Here, we assessed the potential of pegylated IFN-α2a (pIFN-α2a) to 30 reduce the viral reservoir in SIV-infected, ART-treated rhesus macaques (RMs). We 31 found that pIFN-α2a treatment of animals in which virus replication is effectively 32 suppressed with ART is safe and well-tolerated as no major clinical side effects were 33 observed. By monitoring the cellular immune response during this intervention, we 34 established that pIFN-α2a administration is not associated with either CD4 T cell 35 depletion or increased immune activation. Importantly, we found that Interferon 36 Stimulated Genes (ISGs) were significantly up-regulated in IFN-treated RMs when 37 compared to control animals, confirming that pIFN-α2a is bioactive in vivo. To evaluate 38 the effect of pIFN-α2a administration on the viral reservoir in CD4 T cells, we 39 performed cell-associated proviral SIV DNA measurements in multiple tissues and 40 assessed levels of replication-competent virus by a quantitative viral outgrowth assay 41 (QVOA). These analyses failed to reveal any significant difference in reservoir size 42 between IFN-treated and control animals. In summary, our data suggest that short-term 43 type I interferon treatment in combination with suppressive ART is not sufficient to 44 induce a significant reduction of the viral reservoir in SIV-infected RMs. 45