Background:  Lentivirus infections are characterized by a dramatic loss of mucosal CD4⁺ T cells and subsequent chronic immune activation and breakdown of the gut mucosa. In addition, multiple studies have shown that myeloid dendritic cells (mDC) and, in particular, IFN-α-producing plasmacytoid dendritic cells (pDC) are rapidly mobilized during acute lentivirus infection, but are subsequently depleted in the blood and lymph nodes during chronic infection. However, despite the importance of the gut mucosae in HIV/SIV pathogenesis, little is known about pDC in these tissues during infection.

Methods:  mDC and pDC, from naïve (n = 10) and chronically simian immunodeficiency virus (SIV)-infected macaques (n = 12), were enumerated in peripheral blood and rectal biopsies using a novel bead-based flow cytometry assay. Phenotypic analyses were performed using polychromatic flow cytometry and cells were evaluated functionally by intracellular cytokine staining after stimulation with PMA and/or polyI:C. Molecular analyses of cytokine mRNA in whole biopsy pieces were performed by RT-PCR.

Results:  In chronically SIV-infected macaques, numbers of circulating pDC were reduced 3-fold compared to naïve controls, correlating with increasing plasma viral load (R = 0.888, p = 0.0014). In contrast, numbers of mDC were unchanged. Both pDC and mDC expressed high levels of the tissue-trafficking marker, CCR9, but expression was unaltered during infection. Interestingly, expression of the gut-homing marker, a4b7, was negative-to-low on blood mDC and pDC in naïve macaques, but was up-regulated ~2.5-fold on pDC during SIV infection, correlating with increasing viral load (R = 0.699, p = 0.05). Absolute numbers of pDC in gut biopsies were 4-fold greater in infected compared to naïve macaques. Following stimulation, mucosal pDC from both naïve and SIV-infected macaques produced copious amounts of IFN-α, and in some infected animals whole biopsy quantification showed elevated levels of IFNA1 mRNA transcripts in some infected macaques.

Conclusions:  pDC are immune sentinels of viral replication and here we show increased trafficking to the gut mucosa during chronic SIV infection. While pDC accumulation in the mucosa could aid in virus control, over-production of IFN-α derived from these cells could also contribute to increase the immune activation, inflammation, and cell death in the gut mucosa commonly associated with progressive lentivirus infections.