T regulatory cells (Tregs) may play a critical role in formation of the latent reservoir, as they are susceptible to HIV/SIV infection and resting Tregs harbor higher levels of HIV/SIV than the general quiescent CD4+ T cell population. During acute infection, Tregs decisively contribute to the establishment of HIV reservoir by reversing CD4+ T cell immune activation status. During chronic infection, they contribute to the impairment of CTL responses, as Treg expansion correlates with loss of CTL function and their ex vivo depletion enhances T cell responses to HIV/SIV antigens. HLAB27+and B57+ HIV-specific CD8+ T cells from elite controllers evade Treg suppression. As such, we hypothesized that Treg depletion is a valid approach for HIV cure, in which a single intervention reduces the size of the reservoir, reactivates the virus and boosts cell-mediated immune responses.
Two SIVsab-infected RMs, in which spontaneous supercontrol of virus replication (<3 copies/ml plasma) associates complete control of immune activation, were depleted of Tregs by administration of Ontak (Denileukin diftitox), an engineered protein combining IL-2 and diphtheria toxin. Treg depletion was monitored by flow cytometry and immunohistochemistry; plasma viral load was measured by single copy assay; specific cellular immune responses to SIV antigens were monitored flow-cytometrically by intracellular cytokine staining after stimulation with SIVsab peptides.
Ontak administration to SIVsab-infected RMs resulted in significant depletion (>75%) of the circulating Fox-P3+ CD25+ CD4+ T cells. Up to 60% and >50% of Tregs were depleted from gut and the lymph nodes, respectively. Ontak impact on overall CD8+ T cell counts was minimal. Treg depletion resulted in a major increase of the levels of CD4+ T cell activation (Ki-67). In the absence of antiretroviral therapy, virus rebound to 103 vRNA copies/ml of plasma occurred after Ontak administration. Importantly, Treg depletion resulted in a significant boost of the SIV-specific CD8+ T cells and rapid clearance of the reactivated virus.
Treg depletion in chronically SIV-infected superelite controller RMs resulted in both reactivation of latent virus and a boost of CTL responses. The overall Treg ability to control immune responses was significantly impaired despite the fact that Treg depletion was incomplete. As no latency reversing agent in development has such a dual activity, our strategy holds great promises for cure research.