Activated Tissue-Resident Mesenchymal Stromal Cells Regulate Natural Killer Cell Immune and Tissue-Regenerative Function

Abstract

The interaction of mesenchymal stromal cells (MSCs) with natural killer (NK) cells is traditionally thought of as a static inhibitory model, whereby resting MSCs inhibit NK cell effector function. Here, we use a dynamic in vitro system of poly(I:C) stimulation to model the interaction of NK cells and tissue-resident MSCs in the context of infection or tissue injury. The experiments suggest a time-dependent system of regulation and feedback, where, at early time points, activated MSCs secrete type I interferon to enhance NK cell effector function, while at later time points TGF-β and IL-6 limit NK cell effector function and terminate inflammatory responses by induction of a regulatory senescent-like NK cell phenotype. Importantly, feedback of these regulatory NK cells to MSCs promotes survival, proliferation, and pro-angiogenic properties. Our data provide additional insight into the interaction of stromal cells and innate immune cells and suggest a model of time-dependent MSC polarization and licensing.

Keywords: Poly(I:C); TGF-β; TLR 3; mesenchymal stromal cell polarization; mucosal tissue; natural killer cell activation; senescence; type I interferon; viral infection.

Graphical abstract

Figure 1

Poly(I:C)-Stimulated MSCs Show a Time-Dependent Response that Modulates NK Cell Phenotype (A) MSCs were stimulated with poly(I:C) or left resting in medium, and SN was collected at 2, 4, 8, and 24 hr. Cytokines were measured by ELISA and bioassay (n = 4 MSC donors). MSC, unstimulated MSCs; pMSC, poly (I:C)-stimulated MSCs. The statistical analysis and p value shown refers to the comparison of the unstimulated and poly(I:C)-stimulated MSC groups. (B) The mean concentration of regulatory factors IL-6, IL-8, TGF-β, and MIF relative to the mean concentration of IFN-α/β in poly(I:C)-stimulated MSC SN at time points 4 hr (P4h) and 24 hr (P24h). (C) Diagram showing the principle of NK cell stimulation using poly(I:C)-stimulated MSC SN (pMSC). (D) NK cells were incubated overnight (18 hr) with SN from poly(I:C)-stimulated MSCs (P4h and P24h), unstimulated MSC SN (M4h and M24h), poly(I:C) alone, and medium. The expression of NK cell surface markers was measured for both CD56^brightCD16⁻ and CD56^dimCD16⁺ NK subsets. The delta median (marker expression minus the isotype) is shown (n = 3–5 donors). Two-way ANOVA and paired t test were used to test statistical significance (p < 0.05 considered as significant). Data are depicted as line graphs (each individual donor) (A), vertical scatter plot (mean ± SE) (B), or bar graphs (mean ± SE) (D). ^∗p < 0.05, ^∗∗p < 0.01, ^∗∗∗∗p < 0.0001; n.s., not significant.

Figure 2

NK Cell Effector Functions Are Promoted by the Inflammatory Early Response from Poly(I:C)-Stimulated MSCs (A) NK cells were incubated overnight with P4h, P24h, M4h, and M24h SN, with poly(I:C) alone and medium. The cytotoxicity of the NK cells was tested against K562 targets at a 1:2 effector/target (E/T) ratio for 3 hr (n = 5 donors). (B) NK cells were incubated overnight with the indicated conditioned medium. Degranulation of CD56^dimCD16⁺ NK cells against K562 was assessed using the CD107a assay (n = 4). (C) NK cells were incubated overnight with conditioned medium and expression of surface receptors was measured by flow cytometry. Data are shown as the delta median (marker expression minus the isotype) relative to medium control (n = 3). (D) NK cells were stimulated overnight with conditioned medium (as in A and B). CD107a assay and flow cytometry for CD69 were performed (n = 4). (E) NK cells were incubated with conditioned medium in the presence of monensine, IL-12, and IL-18. After 6 hr NK cells were stained for intracellular IFN-γ and the percentage of IFN-γ-positive NK cells was determined (n = 5 donors). (F) NK cells were incubated overnight with the indicated conditioned medium in the presence of neutralizing antibodies to IFN-α (1 μg/mL) and IFN-β (1 μg/mL) or isotypes as indicated. Degranulation of CD56^dimCD16⁺ NK cells against K562 was assessed using the CD107a assay (n = 4). (G) NK cells were incubated with conditioned medium in the presence of monensine, IL-12, and IL-18; additionally neutralizing IFN-α (1 μg/mL) and IFN-β (1 μg/mL) or isotypes were added where indicated. After 6 hr NK cells were stained for intracellular IFN-γ and the percentage of IFN-γ-positive NK cells was determined (n = 4). Two-way paired t test and linear regression model was used for statistical analysis, A ^∗p < 0.05 was considered significant and ^∗∗p < 0.01 or ^∗∗∗∗p < 0.0001 very significant. Data are depicted as bar graphs (mean ± SE) (A and E), vertical scatter graphs (mean ± SE) (B, C, F, G), and scatter graph (D).

Figure 3

NK Cells Are Functionally Impaired and Undergo Differentiation to a Senescent-like Phenotype, Following Treatment with Regulatory Late-Response SN from Poly(I:C)-Stimulated MSCs (A) NK cells were incubated for 5 days with poly(I:C)-stimulated MSC SN (P4h and P24h), unstimulated MSC SN (M4h and M24h), poly(I:C) alone, and medium. At days 0, 1, 3, and 5, NK cells were collected and stained for CXCR4; data are shown as the delta median (marker expression minus the isotype). The asterisks refer to the significance between the P24h sample and the poly(I:C) sample at the given time point (n = 3 donors). (B) NK cells were incubated for 5 days in conditioned medium, and were then stained for CD56 and CD16 (scatter plots show staining and NK subsets, 1 representative experiment of 4). (C) Plots of the percentage of CD56^dimCD16⁺ and CD56^brightCD16⁻ against the CD56^dimCD16⁻ NK subsets (n = 4). (D) NK cells were treated for 3 days with conditioned medium after which the cells were stimulated with PMA/ionomycin in the presence of monensine for 6 hr. NK cells were then stained for intracellular IFN-γ and perforin. Bar graph shows delta median relative to M24h SN-treated NK cells (n = 3). (E) NK cells were treated for 3 days with conditioned medium and their ability to form conjugates, and degranulate and kill K562 target at a 1:2 E/T ratio was assessed (n = 3). (F) NK cells were treated for 3 days with conditioned medium; cells were then lysed and qPCR performed. uPAR, SOCS1, and SOCS3 expression was determined. The bar graph shows gene expression relative to M24h-treated NK cells. (G) NK cells were treated for 3 days with conditioned medium in the presence of IL-12 (10 ng/mL) and IL-18 (10 ng/mL). The production of NK cell-derived IL-6 and IL-8 was then measured (note that the MSC SN already contained IL-6/IL-8 produced by MSC, so to ascertain the NK cell-derived cytokine levels, the concentration of cytokines in MSC SN without NK cells was subtracted from the concentration of cytokines in MSC SN after NK cell incubation, thus the production of NK cell IL-6 and IL8 could be calculated) (n = 3). Data analysis was performed using two-way ANOVA with an additional Bonferroni post test, paired two-tailed t test, and linear regression model. ^∗p < 0.05 was considered significant and ^∗∗p < 0.01, ^∗∗∗p < 0.001, or ^∗∗∗∗p < 0.0001 very significant; n.s., not significant. Data are depicted as vertical scatter graphs (mean ± SE) (D, E, F, and G), line graph (mean ± SE) (A), and scatter plots (B and C).

Figure 4

Increased NK Cell Death Following Prolonged Treatment with Regulatory Late-Response SN from Poly(I:C)-Stimulated MSCs (A) NK cells were incubated for 5 days with (P24h), (M24h), poly(I:C) alone, and medium. At days 1, 3, and 5 NK cells were collected and stained for annexin V and 7AAD, and the percentage of NK cells showing late-stage apoptosis (i.e., annexin V and 7AAD double-positive) evaluated (n = 5 donors). (B) NK cells were treated for 1 day with conditioned medium and expression of CDKN2A/p16INK4a (P16) was determined by flow cytometry (n = 3). (C) NK cells were incubated for 9 days in the presence of conditioned medium, and bright-field micrographs were taken at a magnification of ×200; the arrows indicate an intact cell and the arrowhead indicates apoptotic bodies (1 representative experiment of 4 NK donors). (D) NK cells were incubated for 9 days in the presence of conditioned medium and were then analyzed using flow cytometry for granularity (SSC) and size (FSC) (1 representative experiment of 4 NK donors). (E) NK cells were incubated with conditioned medium and also in the presence of activating IL-15 with medium, rapamycin (mTOR inhibitor), and P24h SN. After 2 days the cells were stained for phosphorylated mTOR by flow cytometry (n = 4). (F) NK cells were incubated in conditioned medium for 9 days in the presence of activating IL-15. Percentage of living cells was determined by annexin V and staining (n = 4). (G) NK cells were labeled with CFSE and then incubated for 5 days with conditioned medium in the presence of IL-15 (200 U/mL) and IL-18 (10 ng/mL). Proliferated cells were quantified as CFSE low cells (n = 3). Data analysis was performed using a paired two-tailed t test and linear regression model. a ^∗p < 0.05 was considered significant and ^∗∗p < 0.01 or ^∗∗∗p<0.005 very significant. Data are depicted as vertical scatter graphs (mean ± SE) (B, E, F, and G), line graph (mean ± SE) (A), and scatter plot (D).

Figure 5

TGF-β and IL-6 Limit NK Cell Cytotoxicity and Induce CXCR4 Expression as well as NK Cell Apoptosis (A) NK cells were incubated for 1 day with 16 independently generated P24h SN. Ratio of IL-6 and TGF-β in the SN was calculated and plotted against the capacity of the SN to induce NK CXCR4. The data allowed a non-linear regression sigmoidal dose-response curve to be fitted, which predicted an EC₅₀ of 12.5. The predicted EC₅₀ was used as a cutoff to divide low IL-6/TGF-β ratio SN (below EC₅₀) and high IL-6/TGF-β ratio SN. The CXCR4 expression of the NK cells was sorted according to this criterion and a vertical scatter plot generated (the IL-6 and TGF-β high/low ratio relationship to CXCR4 can also be seen in the dose-response curves of D). (B) Histograms of NK CXCR4 following overnight incubation with recombinant IL-6 (50 ng/mL) alone and TGF-β (16 ng/mL) alone and in combination (1 representative experiment of 4 NK donors). (C) NK cells were cultured in P24h SN for 2 days in the presence of inhibitory antibodies to IL-6 (1 μg/mL) and TGF-β (1 μg/mL) or isotypes as indicated. Expression of CXCR4 on NK was measured (n = 3 donors). (D) Recombinant IL-6 and TGF-β were incubated with NK cells for 2 days. The expression of CXCR4 was then measured and a non-linear regression sigmoidal dose-response curve was plotted (n = 2). (E) NK cells were treated for 2 days with a combination of recombinant IL-6 (50 ng/mL) and TGF-β (16 ng/mL), the cells were then stained for annexin V-positive NK cells (representing early apoptosis) and annexin V-positive 7AAD-positive NK cells (representing late apoptosis) (n = 4). (F) NK cells were treated for 2 days with a combination of recombinant IL-6 (50 ng/mL) and TGF-β (16 ng/mL) and then stained for CD56 and CD16. Percentage of the CD56^dimCD16⁺ subset from the entire NK cell population is shown (1 representative experiment of 4 NK donors). (G) NK cells were treated for 2 days with TGF-β and IL-6 at low and high concentration ratios. Cytotoxicity of the NK cells was then tested against K562 targets. Gate indicates percentage of killed target cells (1 representative experiment of 2 NK donors). Data analysis was performed using a paired two-tailed t test and non-linear regression dose-response model. ^∗p < 0.05 was considered significant and ^∗∗p < 0.01 or ^∗∗∗∗p < 0.0001 very significant. Data are depicted as histograms (y axis is count) (B), line graph (mean ± SE) (D), vertical scatter plot (mean ± SE) (A, C, and E), and scatter plots (F and G).

Figure 6

Senescent-like NK Cells Favor MSC Survival and Proliferation (A) NK cells were incubated for 3 days with poly(I:C)-stimulated MSC SN (P24h) to generate senescent-like NK cells or were incubated with unstimulated MSC SN (M24h) as a control (NK). Killing of MSC by NK cells or senescent-like NK cells was assessed using annexin V/7AAD staining (n = 12). Percentage of double-negative living MSCs is shown. (B) Senescent-like NK cells or control NK cells (as in A) were labeled with CFSE. Conjugate formation with MSC and CD107a degranulation against MSC targets was assessed. Percentage of NK cells forming conjugates and percentage of CD107a⁺ NK cells was determined (n = 3). (C and D) Senescent-like NK cells or control NK cells (as in A and B) were incubated for 24 hr with a monolayer of MSCc (at a ratio of 1 NK cell to 5 MSCs). The SN of the co-cultures was then collected and IL-6 (C) or TGF-β (D) concentration measured (n = 4 donors). (E) Gene expression of MSCs following 24-hr incubation with medium only (without [W/O] NK cells) or SN generated from conditioned NK cells (n = 3). (F) Pooled SN from senescent-like or control NK cells was generated from three NK cell donors. MSCs were stimulated for 24 hr with medium only (MSC) or SN generated from conditioned NK cells. The MSC-NK-SN was collected and incubated overnight with a human microvascular endothelial cell line. Tubule formation was analyzed and average tube length recorded (n = 3). (G) An invasion assay was performed using MSCs incubated in the presence of medium only (without NK cells) or SN generated from conditioned NK cells. The rate of invasion was analyzed over 3 days and assessed by measuring the area of uninvaded space (n = 3 MSC donors and 3 NK cell donors). (H) Equal numbers of MSC were incubated in the presence of medium only (without NK cells) or SN generated from conditioned NK cells for 5 days. MSC proliferation was analyzed by MTT assay (n = 3 MSC donors and 3 NK donors). Data analysis was performed using a paired two-tailed t test and two-way ANOVA was performed with an additional Bonferroni post test. ^∗p < 0.05 was considered significant and ^∗∗p < 0.01, ^∗∗∗p < 0.001, or ^∗∗∗∗p < 0.0001 very significant. Data are depicted as bar graphs (mean ± SE) (C, D, G, and H) and vertical scatter plots (mean ± SE) (A, B, E, and F).

Figure 7

Time-Dependent Polarization of MSCs Differentially Modulates NK Cell Activation and Differentiation (A) Short-term activation of MSCs by poly(I:C) promotes NK cell activation and pathogen immunity (Figures 1 and 2). (B) TGF-β and IL-6 produced by activated MSCs limit NK cell effector function and promote NK cell differentiation (Figure 5). (C) In the context of prolonged MSC stimulation and NK cell interaction, NK cells undergo a senescence-like process and display reduced effector function (Figures 3 and 4). (D) Senescent-like NK cell feedback on MSCs to promote their survival, proliferation, and pro-angiogenic properties (Figure 6).