Multiple Mechanisms Downstream of TLR-4 Stimulation Allow Expression of NKG2D Ligands To Facilitate Macrophage/NK Cell Crosstalk

Abstract

The activating receptor NKG2D recognizes proteins that are not normally expressed at the surface of most cells but are expressed during a cellular “stress” response (e.g., upon induction of the DNA damage pathway). This establishes recognition of “induced self” as an important strategy for surveillance of infections or tumor transformation. However, NKG2D ligands can also be induced on human macrophages by TLR stimulation, which has been far less studied. In this paper, we clarify that LPS, which ligates TLR-4, preferentially upregulated MICA and not MICB; CL097, which ligates TLR-7/8, upregulated both MICA and MICB; and polyinosinic-polycytidylic acid, which ligates TLR-3, upregulated neither. To probe how LPS stimulation triggers MICA expression, we determined that the stability of MICA mRNA was much longer than that of MICB mRNA, but neither was changed by LPS stimulation. This finding suggests that increased levels of MICA mRNA following LPS stimulation resulted from increased transcription. However, it was not sufficient for surface protein expression, which was controlled posttranscriptionally via a separate pathway involving the ataxia telangiectasia mutated/ataxia telangiectasia and Rad3 related kinases. Moreover, LPS stimulation decreased expression of microRNAs (miRNA)—miR-17-5, miR-20a, and miR-93—which target MICA, implicating a novel role for miRNAs in NKG2D ligand expression. Thus, TLR stimulation allows expression of NKG2D ligands through multiple pathways, including downmodulation of specific miRNAs.