Blocking LLT1 (CLEC2D, OCIL)-NKRP1A (CD161) interaction enhances natural killer cell-mediated lysis of triple-negative breast cancer cells.

Triple-negative breast cancer (TNBC) is the most invasive form of breast cancer due to an absence of estrogen (ER), progesterone (PR), and human epidermal growth factor-2 (HER2) receptors on the cell surface. TNBC accounts for approximately 12 to 20 percent of all breast cancer cases. The absence of ER, PR, and HER2 receptors on TNBCs and its ability to develop drug resistance renders it difficult to eradicate or retrogress tumor growth with hormonal therapy and chemotherapy. Triple-negative breast cancer is associated with poorer prognosis, increased chance of relapse, and lower chance of survival. Patients with TNBC have poorer outcome to conventional treatments than patients with other types of breast cancer. Natural killer cell-mediated immunotherapy is a promising therapeutic option for patients with TNBC. Natural killer cells contribute to the immune system by recognizing tumor cells through interactions between ligands on tumor cells and natural killer cell receptors. NK cell function is regulated by a net balance of signals from activating and inhibitory receptors interacting with ligands on target cells. Lectin-like Transcript-1 (LLT1, CLEC2D, OCIL) is a ligand that interacts with NK cell receptor NKRP1A (CD161) and inhibits NK cell activation. In this study, we have identified expression of LLT1 on TNBC cell lines MDA-MB-231 and MDA-MB-436 through flow cytometry, western blot, and confocal microscopy. We have demonstrated that blocking LLT1 on TNBCs with antibodies disrupts interaction with NKRP1A and enhances lysis of TNBCs by primary natural killer cells. We have also shown that a gene knockdown of LLT1 decreases cell surface expression of LLT1 on TNBCs and increases NK cell-mediated lysis of these TNBCs. The results suggest that LLT1 on TNBCs function as a method of evasion from immunosurveillance by NK cells. Blocking LLT1-NKRP1A interaction activates lysis by NK cells and will potentially open a new immunotherapeutic strategy for treatment of TNBC.