Development of a screening method to identify regulators of MICA shedding

The fatty-acid amide hydrolase inhibitor URB597 inhibits MICA/B shedding

MICA/B proteins are expressed on the surface of various types of stressed cells, including cancer cells. Cytotoxic lymphocytes expressing natural killer group 2D (NKG2D) receptor recognize MICA/B and eliminate the cells. However, cancer cells evade such immune recognition by inducing proteolytic shedding of MICA/B proteins. Therefore, preventing the shedding of MICA/B proteins could enhance antitumor immunity. Here, by screening a protease inhibitor library, we found that the fatty-acid amide hydrolase (FAAH) inhibitor, URB597, suppresses the shedding of MICA/B. URB597 significantly reduced the soluble MICA level in culture medium and increased the MICA level on the surface of cancer cells. The effect was indirect, being mediated by increased expression of tissue inhibitor of metalloproteinases 3 (TIMP3). Knockdown of TIMP3 expression reversed the effect of URB597, confirming that TIMP3 is required for the MICA shedding inhibition by URB597. In contrast, FAAH overexpression reduced TIMP3 expression and the cell-surface MICA level and increased the soluble MICA level. These results suggest that inhibition of FAAH could prevent human cancer cell evasion of immune-mediated clearance.

Viral Evasion of Natural Killer Cell Activation

Natural killer (NK) cells play a key role in antiviral innate defenses because of their abilities to kill infected cells and secrete regulatory cytokines. Additionally, NK cells exhibit adaptive memory-like antigen-specific responses, which represent a novel antiviral NK cell defense mechanism. Viruses have evolved various strategies to evade the recognition and destruction by NK cells through the downregulation of the NK cell activating receptors. Here, we review the recent findings on viral evasion of NK cells via the impairment of NK cell-activating receptors and ligands, which provide new insights on the relationship between NK cells and viral actions during persistent viral infections.