Introduction
Multiple myeloma (MM) is a malignant clonal plasma cell dyscrasia [
1]. The prognosis varies greatly, and almost all patients face disease relapse. CD38 monoclonal antibody is the first monoclonal antibody drug approved for the treatment of MM and has shown efficacy in the treatment of relapsed and refractory MM [
2,
3,
4]. Dara induces MM cell death through several mechanisms, including complement-dependent cytotoxicity (CDC), antibody-dependent cellular cytotoxicity (ADCC), antibody-dependent cellular phagocytosis (ADCP), apoptosis induced by cross-linking of CD38 on the target cells, and immunomodulatory effects via elimination of CD38
+ immunosuppressive cells [
5,
6]. However, a simple and practical biomarker that can accurately predict the treatment response of MM patients undergoing Dara therapy is limited.
Natural killer (NK) cells are an important part of the innate immune system and play an important role in anti-infection and antitumour immunity. The cytotoxic activity of NK cells, particularly against MM cells, has been confirmed in many studies [
7,
8]. Nonetheless, significant changes in the quantity and quality of NK cells have been described in MM patients. Therefore, many studies have focused on enhancing the functional ability of NK cells, relieving the inhibitory effect of NK cells by MM niches, or the adoptive transfer of NK cells to increase their ability against MM cells. However, due to the highly expressed level of CD38 in NK cells, treatment with dara leads to rapid depletion of CD38
high NK cells in patients, which results in crippled ADCC and consequently increases the incidence of infection [
9]. Whether the subtype of NK cells before and after Dara treatment can predict the clinical response is currently unclear.
Previous studies have shown that surviving expanded CD38-knockout NK cells exhibit stronger cytotoxicity against MM cell lines and primary MM cells than CD38
WT NK cells during Dara treatment, and adoptive infusion of CD38
KO NK cells expanded in vitro could control MM.1S progression in mice [
10,
11]. Despite this, the prognostic role of CD38-negative NK cells in Dara treatment has not yet been determined. High-throughput spectral flow cytometry is a powerful tool to broadly assess surface markers on immune cells. Dimensional reduction and visualization with tSNE (t-distributed stochastic neighbour embedding) combined with machine learning-driven clustering with methods such as FlowSOM (Flow self-organizing maps) allow for discrimination of distinct immune cell clusters in an unbiased way. Therefore, we aimed to gain more insight into NK cell immunity in daratumumab-treated RRMM patients via spectral flow cytometry and found that KIR
-NKP46
+ NK subsets were resistant to Dara-induced apoptosis and correlated with clinical efficacy after Dara-treatment. Further functional analysis showed that KIR
-NKP46
+ NK subsets maintained strong proliferation ability and anti-MM effects during Dara treatment, indicating that higher KIR
-NKP46
+ NK cells post-Dara treatment may be a good indicator of a clinical response.
Materials and Methods
Patients
Paired peripheral blood samples from a total of 19 RRMM patients at Peking University of People’s Hospital before and after daratumumab treatment were collected. The clinical characteristics and outcomes of those patients are listed in
Table 1. Among these 19 patients, 13 were monitored using a 5-laser 25-color panel (Table S1) and the other 6 patients were monitored using a 3-laser 9-color panel (Table S2). Written informed consent was obtained from all patients. Peripheral blood mononuclear cells (PBMCs) from those 13 patients were isolated using Ficoll density gradient centrifugation and cryo-stored before the experiment and the other 6 cases were newly enrolled patients.
Cell Lines
MM cell lines (U266 and LP-1) were kindly obtained from Professor G.R. Ruan’s laboratory and K562 cells that were maintained in RPMI-1640 medium supplemented with 10% heat-inactivated foetal bovine serum (FBS).
Multiparametric Spectral Cytometry Immunophenotypic Studies
Frozen PBMCs from 13-paried patients before and after one-dose daratumumab treatment as well as from healthy donors were rapidly thawed in a 37 °C water bath and then rested in prewarmed growth cultured medium overnight. The spectral cytometry panel including 25 antibody-fluorophore conjugates was listed in Table S1. All antibodies were titrated to find their best concentration and the results of similarity index matrix (SIM) which measures how similar two spectra are to each other as well as stain index were shown in Table S3(A,B). Flow cytometry was performed with a 5-Laser Cytek Aurora flow cytometer for those 13-paired patients using 25-color panel and new enrolled 6 patients were test using 3-Laser Cytek Aurora (Cytek® Biosciences Inc., Fremont, California, United States) via 9-color panel.
Cytotoxicity and Proliferation Assay
A flow-based cytotoxicity assay was performed as previously reported [
12]. Briefly, PBMCs from healthy donors or RRMM patients were cocultured with U-266 and LP-1 cell lines at 5:1 effector(2*10
5)/target(4*10
4) ratios in 96-well u-shaped plate at 37 °C with or without daratumumab (MCE, 10 µg/ml) for 4 h, and the cytotoxicity of NK cells was measured via
surface CD107a, followed by intracellular IFN-γ and TNF--α staining in another smaller panel as CD3,CD56,KIR,NKP46, IFN-γ and TNF--α.
To evaluate the fratricide of NK cell subsets, purified NK cells from PBMCs of RRMM patients were sorted first by negative selection (NK cell isolation Kit, Miltenyi, USA) and then further sorted into KIR+ and KIR- subpopulations via flow cytometry (ARIA SORP, BD, USA) via CD3-V500, CD56-APC and KIR-FITC. Purified NK subpopulations were cocultured with or without targets cells (U266, LP-1 and K562 cells, the K562 cells were treated as negative control to exclude education effect in NK cells) in the treatment of Dara at a 5:1 effector (1*105)/target (2*104) ratios for 5 hours. Before coculture, the targets were firstly labelled with CFSE(5μM). The cell death was monitored by flow cytometric analysis with an anti-Annexin V antibody and 7-ADD. In addition, KIR- NK cell subsets were blocked with or without NKP46 (5 µg/ml, BD).
For the cell proliferation assay, PBMCs from healthy donors or RRMM patients were labelled with Cell Trace CFSE Cell Proliferation Dye (Invitrogen) and placed in culture medium supplemented with 200 U IL-2 with or without daratumumab (10 ng/ml, MCE) for 7 days, followed by surface staining to test the proliferation of NK cells.
Data Analysis
Data analysis was performed on the unmixed FCS files using FlowJo software (FlowJo, LLC). The measurements and spectral unmixing were performed using Spectroflo® Software (Cytek Biosciences Inc). To check that the unmixing of the 25-color panel was accurate, data was cleaned up (singlets, live, scatter gate) and NxN plot permutations were screened as shown in Figure S1B.
For clustering analysis, data were manually compensated, and live single cells were gated based on forwards scatter, and side scatter (Figure S1A). To remove anomalous events, FlowAI was applied based on the flow rate and dynamic range of fluorochrome parameters. Next, NK cells were selected by gating for low side scatter CD45+ events and further separated into CD3-CD56+ subsets. For NK cells, only healthy control samples were down-sampled to equal numbers, while all NK cells from patients post daratumumab treatment were included due to their low numbers. Clustering of the NK cells was based on panel from Table S1.
Statistical Analysis
Visualizations and statistical analyses were performed in either GraphPad Prism v8.4.3 (GraphPad Software, California, United States) or R. Continuous outcome measures were compared between two groups using multiple unpaired t tests. The Holm‒Sidak correction was applied to correct for multiple testing. Statistical significance was set as an adjusted p value of <0.05.
Discussion
By broad cellular immunophenotyping profiling, we identified multiple changed immune cell populations in RRMM patients pre- and post-Dara treatment. We found that NK cells were significantly higher in RRMM patients, whether treated with Dara or not, than in healthy donors. In addition, NK cells in RRMM patients correlated with an activated phenotype. However, the percentages of CD107a and TNF
α expression by NK cells against MM cell lines were significantly lower compared with the healthy controls, indicating an inversion of the quality and quantity of NK cells in the MM tumour environment, which is in accordance with Souza et al.’s report [
13]. Interestingly, we found that the secretion of CD107a and TNF
α in NK cells of RRMM patients significantly decreased compared with that in NK cells of healthy donors after Dara treatment in vitro, but it had no effect on the secretion of IFN-γ. A previous study showed that IFN-γ production and degranulation are differentially regulated in response to stimulation in murine NK cells [
15], indicating that Dara mainly plays an anti-MM role by enhancing the NK cell degranulation pathway.
Further analysis of NK cells (CD3
-CD56
+) using tSNE showed diverse distributions of NK cell subsets among the healthy donors and paired RRMM patients, indicating a huge impact of the MM tumour environment on the phenotype of NK cells [
16], and Dara treatment may function greatly on the quantity of NK depletion. Flowsome clustering and manual gating analysis found that KIR
-NKP46
+ significantly increased post Dara treatment, and a higher frequency of KIR
-NKP46
+ subsets post Dara treatment correlated with a better response to Dara treatment. Although in vitro studies found that the addition of a clinical anti-KIR antibody (IPH2101) increased NK cell cytotoxicity against HLA-C-positive acute myeloid leukaemia and lymphoma cells [
17], less clear clinical responses were found in relapsed/refractory MM patients in a phase I clinical study [
18] and smouldering MM patients in a phase II trial [
19]. However, IPH2101 combined with an immunomodulatory augmented NK cell function and resulted in an objective response [
20,
21], indicating that the combination effect of anti-KIR and Dara treatment is involved in the anti-MM effect, which suggests the importance of the KIR
- NK subset against MM in the setting of Dara treatment.
In addition, a previous report showed that the activating receptor NKP46 in NK cells plays an important role against MM cells in vitro; therefore, we speculated that the anti-MM effect of the KIR
-NKP46
+ subpopulation is stronger under Dara stimulation. We found that the percentages of CD107a in KIR-NKP46+ NK cells were comparable to those in KIR+ and KIR- NK cells, suggesting that the overall anti-MM ability of NK cells was enhanced after Dara treatment. Although Dara can significantly enhance the anti-MM effect of NK cells, it can also cause NK cell suicide and therefore reduce the number of NK cells. Previous studies have found that CD38-negative NK cells can resist NK cell fratricide and therefore continue to play a sustained anti-MM role. We also found that KIR
-NKP46
+ NK cells had less fratricide compared with other subsets and sustained a higher proliferation ability among the NK cells, which can explain its predictive effect on Dara efficacy. Interestingly, we found higher expression of CD38 in KIR
-NKP46
+ NK subsets with less fratricide. This may be due to the impaired expression of CD16 in the KIR
-NKP46
+ NK subset, as fratricide of NK cells induced by Dara treatment is mainly caused by the ADCC effect [
10]. Therefore, the mechanism of KIR
-NKP46
+ NK cell resistance to NK cell fratricide needs to be determined in the future.
Compared to KIR-NK cells, KIR+ NK cells are considered licensed and therefore exhibit lower levels of robust activation. However, the functional differences between KIR
- and KIR
+ NK cells, resulting from educational variances, are also influenced by the cytokine environment. For instance, the presence of IL-12, IL-15, and IL-18 has been shown to decrease KIR downregulation, thereby enhancing the killing of tumour cells by NK cells [
22]. In our study, we have demonstrated that sorted healthy KIR- NK cells exhibit comparable anti-multiple myeloma (MM) effects to KIR
+ NK cells when stimulated with Dara treatment. This phenomenon may be attributed to the higher level of fratricide (NK cells killing each other) observed in KIR
+ NK cells. Furthermore, the fratricide level is significantly higher in KIR
+ NK cells compared to KIR
- NK cells, suggesting that a relatively higher proportion of KIR
- NK cells may survive in the body and exert a sustained anti-MM effect.
Our study has several limitations. The relatively small cohort of RRMM patients treated with Dara limits the ability to identify correlated immune features and precludes exact evaluation of clinical correlates of the cellular features identified. A larger cohort will be required in subsequent studies to determine the immune features detected here. In addition, our study focuses only on immune changes in peripheral blood samples and does not contain bone marrow samples. As there are few MM cells in peripheral blood, the determination of NK-related receptor ligands cannot be performed. Nonetheless, the substantial alterations demonstrated in circulating immune cells from RRMM patients’ response to Dara treatment support the idea that clinically relevant signals may be detectable in NK cells from PB samples.
In conclusion, this study highlighted that the expansion of KIR-NKP46+ NK subsets in RRMM patients post Dara treatment correlated with a better clinical response to Dara treatment, which may be attributed to the decreased apoptosis and relatively increased proliferation ability under Dara treatment compared with other subsets. In the future, the KIR-NKP46+ subsets might be superior at acting cooperatively with daratumumab to sustainably eradicate MM cells.