Natural killer (NK) cells are a component of the innate immunity against viral infections through their rapid cytotoxic activity and cytokine production. Although the synthetic double-stranded (ds) RNA polyinosinic-polycytidylic acid (poly I:C), a mimic of a common product of viral infections, is known to rapidly up-regulate their in vivo functions, NK cell ability to directly respond to dsRNA is still mostly unknown. Our results show that treatment with poly I:C significantly up-regulates both natural and CD16-mediated cytotoxicity of highly purified human NK cells. Poly I:C also induces the novel capability of producing CXCL10 chemokine in human NK cells and synergistically enhances interferon-γ (IFN-γ) production induced by either adaptive or innate cytokines. In accordance with the expression of Toll-like receptor-3 (TLR3) and of TRIF/TICAM-1 adaptor, poly I:C stimulation induces the activation of interferon regulatory factor-3 (IRF-3) transcription factor and of p38 mitogen-activated protein kinase (MAPK) in human NK cells. Finally, we demonstrate that p38 MAPK activity is required for the dsRNA-dependent enhancement of cytotoxicity and CXCL10 production. The occurrence of dsRNA-induced signaling and functional events closely correlates with the TLR3 mRNAprofile in different NK cell populations. Taken together, these data identify p38 as a central component of NK cell ability to directly respond to dsRNA pathogen-associated molecular pattern (PAMP).

It has been recently demonstrated that the CD94/NKG2-A killer inhibitory receptor (KIR) specifically recognizes the HLA-E class Ib molecule. Moreover, the apparent CD94-mediated specific recognition of different HLA class Ia allotypes, transfected into the HLA-defective cell line 721.221, indeed depends on their selective ability to concomitantly stabilize the surface expression of endogenous HLA-E molecules, which confer protection against CD94/NKG2-A⁺ effector cells. In the present study, we show that a selective engagement of the CD94/NKG2-A inhibitory receptor with a specific monoclonal antibody (mAb) (Z199) was sufficient to induce tyrosine phosphorylation of the NKG2-A subunit and SHP-1 recruitment. These early biochemical events, commonly related to negative signaling pathways, were also detected upon the specific interaction of NK cells with an HLA-E⁺ 721.221 transfectant (.221-AEH), and were prevented by pre-incubation of .221-AEH with an anti-HLA class I mAb. Furthermore, mAb cross-linking of the CD94/NKG2-A receptor, segregated from other NK-associated molecules by transfection into a rat basophilic leukemia cell line (RBL-2H3), promoted tyrosine phosphorylation of NKG2-A and co-precipitation of SHP-1, together with an inhibition of secretory events triggered via FcϵRI. Remarkably, interaction of CD94/NKG2-A⁺ RBL cells with the HLA-E⁺ .221-AEH transfectant specifically induced a detectable association of SHP-1 with NKG2-A, constituting a more formal evidence for the receptor-HLA class I interaction.

Tumor-targeting mAb are widely used in the treatment of a variety of solid and hematopoietic tumors and represent the first immunotherapeutic approach successfully arrived to the clinic. Nevertheless, the role of distinct immune mechanisms in contributing to their therapeutic efficacy is not completely understood and may vary depending on tumor- or antigen/antibody-dependent characteristics. Availability of next-generation, engineered, tumor-targeting mAb, optimized in their capability to recruit selected immune effectors, re-enforces the need for a deeper understanding of the mechanisms underlying anti-tumor mAb functionality. NK cells participate with a major role to innate anti-tumor responses, by exerting cytotoxic activity and producing a vast array of cytokines. As the CD16 (low-affinity FcγRIIIA)-activating receptor is expressed on the majority of NK cells, its effector functions can be ideally recruited against therapeutic mAb-opsonized tumor cells. The exact role of NK cells in determining therapeutic efficacy of tumor-targeting mAb is still unclear and much sought after. This knowledge will be instrumental to design innovative combination schemes with newly validated immunomodulatory agents. We will summarize what is known about the role of NK cells in therapeutic anti-tumor mAb therapy, with particular emphasis on RTX chimeric anti-CD20 mAb, the first one used in clinical practice for treating B cell malignancies.

Natural killer (NK) cells hold a pivotal role in tumor-targeting monoclonal antibody (mAb)-based activity due to the expression of CD16, the low-affinity receptor for IgG. Indeed, beyond exerting cytotoxic function, activated NK cells also produce an array of cytokines and chemokines, through which they interface with and potentiate adaptive immune responses. Thus, CD16-activated NK cells can concur to mAb-dependent “vaccinal effect”, i.e., the development of antigen-specific responses, which may be highly relevant in maintaining long-term protection of treated patients. On this basis, the review will focus on strategies aimed at potentiating NK cell-mediated antitumor functions in tumor-targeting mAb-based regimens, represented by (a) mAb manipulation strategies, aimed at augmenting recruitment and efficacy of NK cells, such as Fc-engineering, and the design of bi- or trispecific NK cell engagers and (b) the possible exploitation of memory NK cells, whose distinctive characteristics (enhanced responsiveness to CD16 engagement, longevity, and intrinsic resistance to the immunosuppressive microenvironment) may maximize therapeutic mAb antitumor efficacy.