Ly49Q Positively Regulates Type I IFN Production by Plasmacytoid Dendritic Cells in an Immunoreceptor Tyrosine–Based Inhibitory Motif–Dependent Manner

Abstract

Plasmacytoid dendritic cells (pDC) are the major producers of type I IFN during the initial immune response to viral infection. Ly49Q, a C-type lectin-like receptor specific for MHC-I, possesses a cytoplasmic ITIM and is highly expressed on murine pDC. Using Ly49Q-deficient mice, we show that, regardless of strain background, this receptor is required for maximum IFN-α production by pDC. Furthermore, Ly49Q expression on pDC, but not myeloid dendritic cells, is necessary for optimal IL-12 secretion, MHC-II expression, activation of CD4+ T cell proliferation, and nuclear translocation of the master IFN-α regulator IFN regulatory factor 7 in response to TLR9 agonists. In contrast, the absence of Ly49Q did not affect plasmacytoid dendritic cell–triggering receptor expressed on myeloid cells expression or pDC viability. Genetic complementation revealed that IFN-α production by pDC is dependent on an intact tyrosine residue in the Ly49Q cytoplasmic ITIM. However, pharmacological inhibitors and phosphatase-deficient mice indicate that Src homology 2 domain-containing phosphatase 1 (SHP)-1, SHP-2, and SHIP phosphatase activity is dispensable for this function. Finally, we observed that Ly49Q itself is downregulated on pDC in response to CpG exposure in an ITIM-independent manner. In conclusion, Ly49Q enhances TLR9-mediated signaling events, leading to IFN regulatory factor 7 nuclear translocation and expression of IFN-I genes in an ITIM-dependent manner that can proceed without the involvement of SHP-1, SHP-2, and SHIP.

Ly49 Receptors: Innate and Adaptive Immune Paradigms

“…pDCs are specialized in direct virus recognition and secretion of large amounts of IFN-I early during infection (111, 112). Using a gene knock-out mouse model, we have shown that Ly49Q, despite being an inhibitory receptor, positively regulates Toll-like receptor (TLR)-mediated IFN-I production by pDCs (113, 114). In mice lacking Ly49Q, both TLR7 and TLR9-mediated IFN-I production by pDCs is attenuated (113).…”

“…In mice lacking Ly49Q, both TLR7 and TLR9-mediated IFN-I production by pDCs is attenuated (113). Although the precise mechanism of this positive regulation is not known, it is Ly49Q ITIM-dependent but may not involve interaction with the SHP-1/2 phosphatases (114). One of the possible mechanisms could be a cooperative activation of interferon regulatory factors (IRF) in conjunction with TLR-mediated downstream signals, leading to enhanced IFN-I production.…”

“…Alternatively, Ly49Q may aid in the uptake of microbial macromolecules, such as unmethylated CpG oligodeoxynucleotides (ODN), and their trafficking to endosomal compartments where TLRs are located. We have demonstrated a specific down-regulation of Ly49Q from the pDC surface when treated with TLR9 agonist CpG ODN (114). This is similar to the down-regulation and co-internalization of human KIR3DL2 with CpG ODN in NK cells (115).…”

Plasmacytoid dendritic cells: development, functions, and role in atherosclerotic inflammation

“…On the other hand, Ly49Q is also involved in the regulation of IFN-I production by enhancing Toll-like receptor-9 (Tlr9)-mediated signaling in pDCs (Rahim et al, 2013). On pDCs, Ly49Q was shown to bind H-2K b , a classical MHC class I molecule (Tai et al, 2007), which enhances pDC function (Tai et al, 2008).…”

Adaptations of Natural Killer Cells to Self-MHC Class I

“…This receptor enhanced, rather than inhibited, TLR9-mediated signaling events in plasmacytoid dendritic cells. The positive effect depended on the Ly49Q ITIM, but did not require SHP-1, SHP-2, or SHIP phosphatase activity (34). Thus ITIM-bearing Ly49 receptors can improve cellular activation.…”