The role of NK–DC interactions and IFNαβ in regulating both innate and adaptive immune defenses to CMV infection. (a) In mice with Ly49H+ NK cells (top left), binding of this activating receptor by MCMV m157 promotes NK proliferation and acquistion of effector functions, which in turn limit viral replication. Whilst pDC are a major source of IFNαβ at 36h when infecting with ‘normal’ doses of MCMV, their relative production at 44–48 h is marginal compared to cDC (depicted by arrow thickness). However, if low dose MCMV infection is performed, Ly49H+ NK cells restrict the 36 h IFNαβ production by pDC, resulting in enhanced survival of cDC which promotes increased numbers of MCMV-specific CD8T-cells at day 4 in the spleen. Cross-talk between Ly49H+ NK cells and CD8α DC maintains the numbers of both these cells, and involves DC-derived IL-12 and IL-18 (blue inset) [136]. Infection of Perforin-deficient mice (blue inset) results in lethal immunopathology mediated by CD11b+ F4/80+ and CD4+ cells secreting TNFα. IL-10, likely secreted by Ly49H+ NK cells, dampens immunopathogenic MCMV-specific CD8 T-cell responses that develop in Perforin^−/− mice [123]. NK cells are also activated in Ly49H− mice at 36 h (bottom), but in spite of producing IFNγ and TNFα they control viral spread considerably worse than Ly49H+ NK cells. Consequently, substantially higher levels of IFNαβ is produced by pDC at 36h in Ly49H− mice (low MCMV dose), promoting enhanced death of cDC and a 1 day delay in the priming/expansion of MCMV-specific CD8 T-cells [116]. (b) MCMV m157 interactswith the activating receptor Ly49H on NK cells, resulting in their proliferation and acquisition of effector functions. The cell surface expression of the NKG2D ligands RAE-1, H60 and MULT-1 are restricted by theMCMV proteins m138, m152 and m145 which induce their degradation in infected cells. The mechanism by which the MHCI-homologous protein m144 dampens NK control is unknown. The MCMV m04 protein complexes with H-2D^k and binds the Ly49P NK-activating receptor. MCMV m166 binds TRAIL-R2 and restricts its cell surface expression. Cross-talk between NK cells and CD11b⁺ DC occurs via NKG2D-NKG2D-L interactions, and in concert with IFNαβ induces enhanced NK cell cytotoxicity. DC-derived IL-12 and IL-18 promote IFNγ production by the NK cells (blue inset).

Regulation of the splenic IFNαβ response: both during initial MCMV infection, and in response to the first round of viral spread. At 8 h postinfection (pi), MCMV virus enters the spleen largely via the marginalzone (MZ) sinus where it predominantly infects MZ stromal cells expressing ER-TR7 and LTβR. The stromal cells require LTα1β2-expressing B cells to maintain their differentiation state, as well as to promote activation of noncanonical NFκB signalling, and these stromal cells secrete the first detectable wave of IFNαβ mRNA peaking at ~8h after infection. The stroma is also the source of the majority of systemic IFNαβ peaking at ~8–12 h. At 36 h pi, pDC preferentially localize to the MZ from the red pulp and produce the vast majority of the splenic and systemic IFNαβ detectable when MCMV is initiating its first round of spread in vivo. pDC utilize a TLR9/MyD88 dependent mechanism to produce IFNαβ, whilst production by the stroma is TLR-independent. In mice containing Ly49H-expressing NK cells, the white pulp is protected from MCMV infection at day 2–3 of infection, and this is dependent upon IFNαβ production as well.

Innate MCMV defenses in the liver promote the development of adaptive immunity. IFN-α/β signalling is required for the induction of CCR2 ligands in the bone marrow(BM) (MCP-1/CCL2 and MCP-5/CCL12). Upon CCR2 signalling, inflammatory macrophages egress from the bone marrow into the blood and infiltrate the liver. MIP1α/CCL3 produced by these inflammatory monocytes/macrophages, potentially in combination with production by liver-resident F4/80+ macrophages, recruits NK cells from the blood into the liver. NK cells are then activated in response to IFN-α/b and IL-12, potentially produced via a TLR2-dependent mechanism by inflammatory monocytes or by a currently unknown cell ‘X’ via a TLR9-independent, MyD88-dependent mechanism. Activated NK cells can then function to control MCMV spreadin the liver. In turn, NK-produced IFN-γ induces expression ofCXCR3-ligands (CXCL9 and CXCL10) by cell ‘X’, promoting the recruitment of MCMV-specific CD8T-cells from the blood, which function to further restrict MCMV replication and may also promote immune pathology in some settings. Effect or CD8T-cells induced by these cytokines and chemokines secrete IFN-γ and TNF. Furthermore, they are loaded with cytolytic granules containing granzyme and perforin.