No abstract

IntroductionNatural killer (NK) cells are a small lymphocyte subpopulation resident in peripheral blood and in some lymphoid and nonlymphoid organs, capable of rapidly migrating to peripheral sites in response to infections or to neoplastic transformation. 1 They represent an important component of innate immunity by exerting both a constitutive cytotoxic activity, directed against infected or transformed cells and immature hematopoietic precursors, and the antibody-dependent cytolytic activity (ADCC), thanks to the presence of CD16, the low-affinity Fc receptor for immunoglobulin G (IgG) (Fc␥RIII), on the vast majority of them. 2,3 Besides their cytolytic function, NK cells also rapidly secrete a variety of cytokines and chemokines in response to stimulation, by means of which they amplify the recruitment and activation of other effector cell populations. 1,4,5 NK cell activation is regulated by the fine balance of positive and negative signaling pathways initiated by multiple receptors displaying either activating, costimulatory, or inhibitory activity, whose expression and/or functional capability can be modulated during NK cell activation/differentiation. 6-8 NK cell functions are rapidly augmented by a vast array of both innate and adaptive cytokines and a number of other biologic response modifiers. 1,5 NK cells play a crucial role in the natural resistance against viral infections, both by exerting an effector function in the early containment of the infection and by participating in the instructive phase of the adaptive immune response. 9-11 NK cells rely on a variety of cues to precociously sense and respond to the presence of viral infections, such as the up-regulation of viral or virus-induced peptidic ligands for different activating receptors, the virusinduced down-modulation of major histocompatibility complex (MHC) class I ligands for inhibitory receptors, and the presence of proinflammatory cytokines, such as type I interferon (IFN), which promptly up-regulate NK cytolytic activity and cytokine production. 5,[9][10][11][12][13] In particular, the synthetic copolymer polyinosinicpolycytidylic acid (poly I:C), which mimics double-stranded (ds) RNA viral products, has been previously shown to rapidly promote NK cell responses in vivo through its capacity to induce type I IFN production. 1,[9][10][11]14 Innate immunity effector cells recognize the presence of different pathogens mainly through a recently identified family of 10 genetically invariant receptors, the Toll-like receptors (TLR1-10), capable of recognizing distinct molecular components of microbes. 15,16 In particular, TLR3 is the specific receptor for dsRNA, a common intermediate in the reproductive cycle of many viruses; indeed, responsiveness to viral dsRNA or poly I:C is severely compromised in TLR3 knock-out mice. 17 TLR3 expression in the human hematopoietic compartment has been initially reported to be restricted mainly to myeloid dendritic cells, where it induces cell maturation, and the production of inflammatory and antimicrobial...

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.

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 Fc 4 RI. 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.