**10. Methods to enhance natural killer cell function and future perspectives**

Monoclonal antibodies are now increasingly being studied for their effect on enhancement of NK cell activity. The isotype IgG1 subclass is often used to induce ADCC through stimulating activating Fc receptors on NK cells. The most effective monoclonal antibody used in hematological malignancy is Rituximab, which targets antigen CD20 on B cells [174]. Notably, it was proved that such antibodies should be administered at appropriate time point as NK cells reconstituted early after HSCT are immature with lower expression of CD16 [95]. Interactions between inhibitory KIRs and HLA molecules transmit a potent inhibitory signal to the NK cell, thus monoclonal antibodies blocking inhibitory KIR receptors are used to enhance NK cell activity. The preclinical results demonstrated these antibodies appear safe and do not induce autoimmunity [175]. Additionally, a disintegrin and metalloprotease-17 (ADAM17) was demonstrated to induce activated NK cells to lose expression of CD16 and CD62L [176]. Therefore, pharmacologic inhibition of ADAM17 is used to enhance the cytotoxicity of NK cells toward Rituximab bound lymphoma cells. Following the improvement of antibody production, bispecific killer engagers (BiKE) and trispecific killer engagers (TriKE) are used to crosslink specific tumor antigens such as CD19, CD20 with a potent stimulator on NK cell such as CD16 [177–179]. CD16 × 19 BiKEs and CD16 × 19 × 22 TriKEs have been constructed and shown to significantly activate CD16 signaling of NK cells [180, 181]. Furthermore, a CD16 × 33 BiKEs was constructed recently and proved to result in potent cytotoxicity of NK cells in patients of refractory AML [182]. However, the very short half-life of bi- and trispecific antibodies might limit their therapeutic effect [179].

Another important focus is to enhance the immune response of NK cells through sensitizing target cells. Bortezomib is a drug classically used to treat multiple myeloma and mantle cell lymphoma. As a proteosome inhibitor, Bortezomib can sensitize tumor cells to TRAILmediated apoptosis [14]. Moreover, drugs such as doxorubicin and depsipeptide have been proved to upregulate the expression of death receptors including Fas, TRAIL-R1, TNFR1, thereby sensitize tumor cells to the cytotoxicity of NK cells [183, 184].

Recently, NK cells are also genetic engineered to express CARs which represent an effective therapy. The preclinical studies demonstrated that primary NK cells expressed CAR constructs can result in potent killing of B cell tumors [185, 186]. However, it is difficult to express exogenous genes in primary NK cells and to reach enough number for immunotherapy. Therefore, human NK-like cell line, NK-92 becomes another candidate for genetic engineering. Based on the results of phase I clinical trials of NK-92 cells [187], CAR-expressing NK-92 cells may be easily expanded *ex vivo* and well tolerated in patients.
