TLR4 signaling in presence of HIV-induced activation enhances programmed death ligand-1 expression on human plasmacytoid dendritic cells and modulates their function

Human Plasmacytoid dendritic cells (pDC) only comprise a minute fraction of human mononuclear leukocytes, but are important anti-viral responders that mediate both innate as well as adaptive immune responses. Persistent activation of pDC enhances HIV pathogenesis by promoting immune suppressive mechanisms such as T regulatory cells. It is therefore important to identify the sources of pDC activation in the context of HIV infection. HIV-associated disruption of gut mucosa associated lymphoid tissue introduces normal flora-lipopolysaccharide (LPS) into systemic circulation, which exacerbates HIV-induced immune activation. We report here that pDC are capable of mediating functional TLR4 signaling upon LPS stimulation, and that pDC of HIV-infected individuals have enhanced TLR4 expression compared to healthy individuals. How TLR4 signaling affects pDC function in HIV infection has not been examined before. Hence we examined the influence of TLR4 signaling in presence of HIVstimulation on pDC and found that it not only potentiated HIV-induced activation but also strongly up-regulated Programmed death ligand-1 (PD-L1) expression and Interleukin-6 synthesis. TLR4 signaling specifically up-regulated PD-L1 expression on activated pDC in presence of HIV stimulation. LPS and HIV co-stimulated pDC demonstrated enhanced migratory potential and repressed T cell proliferation. Together, these results suggest that in the setting of HIV infection enhancement of pDC immune suppressive mechanisms such as PD-L1 may be an outcome of HIV-associated immune activation potentiated by TLR4 signaling.