Synergy among receptors on resting NK cells for the activation of natural cytotoxicity and cytokine secretion

Abstract

Freshly isolated, resting natural killer (NK) cells are generally less lytic against target cells than in vitro interleukin 2 (IL-2)-activated NK cells. To investigate the basis for this difference, the contribution of several receptors to activation of human NK cells was examined. Target-cell lysis by IL-2-activated NK cells in a redirected, antibody-dependent cytotoxicity assay was triggered by a number of receptors. In contrast, cytotoxicity by resting NK cells was induced only by CD16, and not by NKp46, NKG2D, 2B4 (CD244), DNAM-1 (CD226), or CD2. Calcium flux in resting NK cells was induced with antibodies to CD16 and, to a weaker extent, antibodies to NKp46 and 2B4. Although NKp46 did not enhance CD16-mediated calcium flux, it synergized with all other receptors. 2B4 synergized with 3 other receptors, NKG2D and DNAM-1 each synergized with 2 other receptors, and CD2 synergized with NKp46 only. Resting NK cells were induced to secrete tumor necrosis factor alpha (TNF-alpha) and interferon gamma (IFN-gamma), and to kill target cells by engagement of specific, pair-wise combinations of receptors. Therefore, natural cytotoxicity by resting NK cells is induced only by mutual costimulation of nonactivating receptors. These results reveal distinct and specific patterns of synergy among receptors on resting NK cells.

Figure 1.

Engagement of CD16, but not NK-cell receptors NKp46, NKG2D, 2B4, DNAM-1, or CD2, induces cytotoxicity by resting NK cells. Redirected lysis assay of P815 target cells with IL-2-activated NK cells (A) or resting NK cells (B) at indicated E/T cell ratios. P815 cells were preincubated with IgG1 mAbs to specific NK-cell receptors, as indicated. ⋄ indicates isotype control antibody; ♦, anti-CD16; □, anti-NKp46; ▪, anti-NKG2D;▵, anti-2B4; ▴, anti-CD2; ○, anti-DNAM-1; and ▪, anti-CD56. (C) Lysis of K562 cells by resting (○) and IL-2-activated (▪) NK cells. Experiments are representative of at least 5 independent experiments.

Figure 2.

Expression of activating receptors on resting and IL-2-activated NK cells. Freshly isolated, resting NK cells or long-term IL-2-cultured NK cells were stained with directly conjugated mAbs to the indicated receptors (x-axis) and anti-CD56 (y-axis).

Figure 3.

Synergistic activation of Ca²⁺ flux in resting NK cells by co-crosslinking pair-wise combinations of receptors. (A-B) NK cells were preincubated with mAbs to indicated receptors on ice, loaded with Fluo-4 and Fura Red, resuspended in HBSS 1% FBS, and prewarmed at 37°C. Cells were analyzed by flow cytometry. After 30 seconds, secondary F(ab′)₂ goat anti-mouse IgG was added to each sample. (A) FL-1/FL-3 ratios are plotted as a function of time. Black lines show activation with isotype control. Blue lines show activation by the single receptors, indicated in blue. Green lines show activation by the single receptors, indicated in green. Red lines show activation by the combination of both receptors. (B) The peak FL-1/FL-3 ratio after cross-linking of indicated receptor combinations was measured in several independent experiments. Bars indicate the SD (≥ 3 independent experiments).

Figure 4.

Single-cell Ca²⁺ flux analysis. (A-F) Resting NK cells were loaded with Fluo-4 and Fura Red, placed on coverslips, and stained with antibodies as indicated. Cells were washed, resuspended in HBSS 1% FBS, and prewarmed at 37°C. Fluorescence was measured by confocal microscopy. Thirty seconds after the beginning of each scan, secondary F(ab′)₂ goat anti-mouse IgG was added to each sample. Traces of the Fluo-4/Fura Red ratio of 2 or 3 representative NK cells are shown.

Figure 5.

Degranulation by resting NK cells induced by NKG2D and 2B4 synergism. NK cells were incubated for 2 hours either alone or with P815 cells and mAbs as specified. Cells were stained with anti-CD56 and isotype control or anti-CD107a mAbs. Plots were gated on a forward scatter/side scatter lymphocyte gate. The experiment is representative of 7 independent experiments.

Figure 6.

TNF-α secretion, IFN-γ secretion, and cytotoxicity induced by pair-wise combinations of NK-cell receptors. TNF-α (A) and IFN-γ (B) secretion by resting NK cells after 2 or 6 hours of stimulation, respectively, with beads coated with specific combinations of mAbs, as indicated. (C) Redirected lysis of P815 cells by resting NK cells at an E/T cell ratio of 10, for 3 hours at 37°C. P815 target cells were preincubated with combinations of mAbs to specific NK-cell receptors, as indicated.

Figure 7.

Schematic representation of synergies among receptors in resting NK cells. Solid lines represent strong synergies between pairs of receptors.