Tumor-Primed Human Natural Killer Cells Lyse NK-Resistant Tumor Targets: Evidence of a Two-Stage Process in Resting NK Cell Activation

Abstract

NK cells are defined as those cells that lyse tumor cells without priming. In this study, we show that the preincubation of resting human NK cells with the leukemia cell CTV-1 primes NK cells to lyse NK-resistant cell lines, primary leukemias, and solid tumors even when HLA-matched, allogeneic or autologous. The primed NK cells remained nonresponsive to HLA-C matched and mismatched normal mononuclear cells from multiple donors. CD69, a known NK trigger receptor, was shown to be the predominant trigger on the tumor-primed NK cells because lysis was blocked with the rCD69 protein. The lack of lytic activity against normal hemopoietic cells implied that the ligand for CD69 is tumor restricted, and this was confirmed by experiments using fluorochrome labeled rCD69. It has been recently shown that resting NK cells require prior stimulation with IL-2 before triggering by all known NK-triggering ligands. In this study, we show that a tumor cell can provide the NK priming signal independently of IL-2. These data provide evidence for two NK evasion strategies for tumor cells, namely the prevention of priming (type1 evasion) and failure to trigger (type 2 evasion). Most NK-resistant cell lines are type 1 and fail to prime resting NK cells but are lysed by IL-2-primed NK cells. In contrast, CTV-1 cells prime resting NK cells but fail to trigger (type 2), and coincubation with CTV-1 primes for triggering by type 1 NK-resistant tumor cells. These tumor-activated NK cells lyse a broad spectrum of tumor cells with a degree of specificity never previously reported.

Requirements for control of B‐cell lymphoma by NK cells

“…Therefore, we used a c-myc transgenic model of spontaneously arising lymphoma [29] and analyzed lack of inhibiting and presence of NKG2D-mediated activating signals in the light of the two-step NK-cell activation hypothesis. We show that the two-stage activation process that was previously described only in vitro [26] can adequately explain the situation in vivo. However, tumor escape seems to be more complex than might be suggested by definitions in terms of type 1 or type 2 resistance.…”

“…In this model, activation of resting NK cells required a priming signal that was provided by IL-2 or by unknown ligands of tumor cells independently of IL-2, and a subsequent triggering event that was mediated by CD69. MHC class I down-regulation was not needed for tumor-induced NK activity in this study [26]. Resistance of tumors might either arise through a lack of priming of NK cells (type 1 evasion) or by the inability of the tumor to deliver triggering signals to already primed NK cells (type 2 evasion) [26].…”

“…MHC class I down-regulation was not needed for tumor-induced NK activity in this study [26]. Resistance of tumors might either arise through a lack of priming of NK cells (type 1 evasion) or by the inability of the tumor to deliver triggering signals to already primed NK cells (type 2 evasion) [26].…”

“…In another study, NK-dependent lysis of some tumors was only dependent on NCR, whereas in other tumors, synergistic effects of NCR and NKG2D were found [25]. Recently, a sequential NK-cell activation process was proposed [26]. In this model, activation of resting NK cells required a priming signal that was provided by IL-2 or by unknown ligands of tumor cells independently of IL-2, and a subsequent triggering event that was mediated by CD69.…”

“…Reports suggesting a two-signal requirement for NK-cell activation were only based on in vitro studies, and the role of NKG2D that was described as an NCR in earlier studies [27,28] was not addressed in the context of the two-stage model [26]. We were therefore interested in the mechanisms of NK-cell activation in tumor surveillance in vivo and we specifically investigated the role of ''missing self'' and of NKG2D/ligand interactions as well as the mechanisms underlying tumor escape.…”