A Human NK Cell Activation/Inhibition Threshold Allows Small Changes in the Target Cell Surface Phenotype To Dramatically Alter Susceptibility to NK Cells

Abstract

NK cell activation is negatively regulated by the expression of target cell MHC class I molecules. We show that this relationship is nonlinear due to an NK cell activation/inhibition threshold. Ewing’s sarcoma family tumor cell monolayers, which were highly susceptible to NK cells in vitro, developed a highly resistant phenotype when cultured as three-dimensional multicellular tumor spheroid structures. This suggested that tumor architecture is likely to influence the susceptibility to NK cells in vivo. Resistance of the multicellular tumor spheroid was associated with the increased expression of MHC class I molecules and greatly reduced NK cell activation, implying that a threshold of NK cell activation/inhibition had been crossed. Reducing MHC class I expression on Ewing’s sarcoma family tumor monolayers did not alter their susceptibility to NK cells, whereas increased expression of MHC class I rendered them resistant and allowed the threshold point to be identified. This threshold, as defined by MHC class I expression, was predictive of the number of NK-resistant target cells within a population. A threshold permits modest changes in the target cell surface phenotype to profoundly alter the susceptibility to NK cells. Whereas this allows for the efficient detection of target cells, it also provides a route for pathogens and tumors to evade NK cell attack.

Blocking oncogenic RAS enhances tumour cell surface MHC class I expression but does not alter susceptibility to cytotoxic lymphocytes

“…For IL-2 stimulation, NK cells were cultured for 5-7 days in 50 units/ml IL-2 (R&D systems). NK cell mediated killing of tumour cells and granule exocytosis assays were performed as we have described previously [24][25][26], including after siRNA transfection of target cells [26,27]. T cells restricted to the HLA-A2…”

“…Similar to the situation with CTL, it is likely that the relatively small inhibition in cell surface MHC class I expression is insufficient to alter NK cell recognition. The existence of a threshold of NK cell activation/inhibition regulated by MHC class I levels [26,45,46] In colorectal cancer, patients whose tumours expressed high levels of MHC class I showed similar survival times to those whose tumours had lost MHC class I [47]. However, patients with reduced MHC class I expression had the worst prognosis of these three groups; the authors of this study suggest that reduced MHC class I levels on these tumours might be insufficient to mediate CTL recognition but are nevertheless still capable of inhibiting NK cell activity, rendering the tumour resistant to both classes of cytotoxic lymphocytes [47].…”

Loss of DNAM-1 ligand expression by acute myeloid leukemia cells renders them resistant to NK cell killing

“…4D). Thus, we conclude that the DNAM-1-CD112/155 axis is required to breach a conjugation and subsequent activation threshold, 28 which is required for stable synapse formation and delivery of cytotoxic granules.…”

“…1D), this suggested that synapse-associated adhesion/activation interactions were not occurring efficiently in the pSMAC, resulting in a failure to breach the activation threshold necessary to trigger degranulation. 28 DNAM-1 is known to be involved in NK cell adhesion, activation and tumor immune surveillance, and is often downregulated on immune cells from AML patients. 12 The observed poor NK cell conjugation and killing of AML cells suggested that the DNAM-1-CD112/155 receptor-ligand axis might not be functioning optimally.…”

“…28,33 Given the heterogeneity of CD112 and CD155 expression within AML cultures (Fig. 2), we questioned whether poor NK-cell-mediated lysis of these populations is derived from clones that fail to express these ligands.…”