Elucidation of the Plasmacytoid Dendritic Cell Response to HIV-1

Abstract

Plasmacytoid dendritic cells (pDCs) are a specialized type I interferon (TI-IFN) producing cell of the innate immune system; they are critical in the control of HIV-1 infection. pDCs isolated from the blood during HIV-1 infection demonstrate profound dysfunction in their ability to produce the prototypic TI-IFN, IFN-α. In this thesis, I explore the factors that lead to pDC dysfunction during HIV-1 infection. Tim-3 (CD366), typically associated with activated T cells and of current interest as a target for immunotherapy, was identified on pDCs and shown to be upregulated during HIV-1 infection. Tim-3 expression in HIV-1 infection correlated with pDC dysfunction. Similarly, in healthy donors, Tim-3 expression was PI3K-dependent and induced in response to both TLR activation and in response to IFN-directly. Strong toll-like receptor (TLR) agonists, such as CpG, imiquimod and Sendai virus were potent inducers of Tim-3, leading to subsequent dysfunction. The mechanism of action of Tim-3 was also explored. Anti-Tim-3 antibodies did not affect pDC function. However, surface Tim-3 associated with submembrane disruption of TLR after activation. Tim-3 staining intensity inversely correlated with decreased IRF7 staining and intracellular Tim-3 co-localized with p85 and IRF7 within LAMP1+ lysosomes. Most surprisingly, we found that HIV-1 itself was a poor inducer of Tim-3 expression. Thus, we investigated the agonistic capacity of HIV-1 on pDCs. When the amount of TLR7 triggering viral RNA was taken into consideration, influenza virus was a vastly superior pDC agonist. Efficient uptake of HIV-1 was observed, but failed to initiate a response, however, if a second agonist was added to the culture, HIV-1 augmented that response in a TLR7 dependent manner. Thus, we investigated the possibility of HIV-1 structural proteins in the evasion of the pDC response. We found that chemical agents that target the zinc-finger nuclease region of the nucleocapsid increased IFN-α production in response to HIV-1 stimulation. Thus, I propose a putative model of pDC dysfunction during HIV-1 infection where HIV-1 is endocytosed and remains dormant within the pDC endosome with the capacity to augment the response to secondary pathogens leading to disproportionate responses and Tim-3 upregulation.