Trials of adoptive natural killer (NK)-cell immunotherapy for hematologic malignancies have thus far shown only marginal effects, despite the potent in vitro antitumor activity of these cells. Homing of infused cells to tumor microenvironments is critical for efficacy, but has not been well characterized. We established a novel method to track and quantify the distribution of adoptively transferred NK cells using rhesus macaques (RM) as a clinically relevant preclinical model.
RM NK cells were expanded ex vivo for 14–21 days, labeled with 89Zr-oxine complex, and assessed for phenotype, function, and survival. Trafficking of 89Zr-labeled ex vivo–expanded NK cells infused into RMs was monitored and quantitated by serial positron emission tomography (PET)/CT (n = 3, 2.05 ± 0.72 MBq, 23.5 ± 2.0 × 106 NK cells/kg) and compared with that of 89Zr-labeled nonexpanded NK cells, apoptotic NK cells, and hematopoietic stem and progenitor cells (HSPC).
NK cells retained sufficient levels of 89Zr for accurate in vivo tracking for 7 days. 89Zr labeling did not alter cellular phenotype, viability, or function. PET/CT showed NK cells initially localized in the lungs, followed by their migration to the liver, spleen, and, at low levels, bone marrow. One day following transfer, only 3.4% of infused NK cells localized to the BM versus 22.1% of HSPCs. No clinical side effects were observed, and dosimetry analysis indicated low organ radioexposures of 6.24 mSv/MBq (spleen) or lower.
These data support translation of this technique to humans to track the distribution of adoptively infused cells and to develop novel techniques to improve immune cell homing to tumor microenvironments.
The success of adoptive immunotherapy for hematologic malignancies may require infused cells to traffic to the bone marrow (BM), where malignant cells often reside. We developed a novel method to track and quantify organ distribution of adoptively transferred natural killer (NK) cells using 89Zr-oxine ex vivo cell labeling and positron emission tomography (PET). Low 89Zr-oxine doses required for imaging did not alter NK-cell viability or function. Following autologous infusion into rhesus macaques, labeled NK cells were tracked for 7 days, passing through the lungs, then localizing primarily to the liver, spleen, and, at low levels, the BM. No toxicity was observed, and dosimetry estimates extrapolated to humans suggested tracking could be achieved with very low radioexposure to organs. These data suggest that 89Zr-oxine–labeled NK-cell PET can be safely translated to humans for tracking distribution of adoptively infused cells and for assessing cell modification techniques to improve immune cell homing to tumor microenvironments.
Adoptive transfer of allogeneic and autologous NK cells is being investigated clinically for the treatment of malignancies. Despite their capacity to induce cellular cytotoxicity, NK cells maintain self-tolerance regulated by the interaction of activating and inhibitory receptors with self-MHC class I molecules and their derivatives (1–4). The lack of a requirement for prior sensitization and the ability to induce tumor cytotoxicity in an antigen-independent manner without causing graft-versus-host disease (5) may offer advantages to adoptive NK-cell immunotherapy compared with approaches using T cells. Although regression of malignancies in humans with advanced cancers has been observed, overall efficacy of NK-cell–based immunotherapy remains marginal (6–8). One potential limitation of adoptive NK-cell therapy for leukemia and other hematologic malignancies may be the inability of NK cells to efficiently traffic to bone marrow (BM) where these malignancies primarily reside (9, 10). Most preclinical adoptive NK-cell studies have used tissue harvesting to identify transferred cells. Such approaches preclude longitudinal analysis of cell fate and are problematic in humans.
Imaging adoptively transferred cells in vivo could provide a useful method to assess the capacity of NK-cell homing to desired locations, such as tumors. Among the cell tracking imaging methods clinically available, MRI with ex vivo superparamagnetic iron oxides (SPIO) cell labeling permits high resolution without ionizing radiation; however, interpretation and quantitation are difficult and the SPIOs remain in the tissue after labeled cell death (11). Indium-111 (111In)-oxine for single-photon emission CT imaging has been used to label hematopoietic cells, but the high 111In doses required can impair cellular viability and function (12, 13).
A 89Zr-oxine complex was recently developed as a cell labeling agent for tracking cells utilizing positron emission tomography (PET; ref. 14). 89Zr has a half-life of 78.4 hours, ideal for tracking labeled cells over multiple days. With inherently higher sensitivity and spatial resolution of PET (15), studies in various murine models have established that only extremely low doses of 89Zr-oxine are necessary to track labeled cells for up to 7 days, with minimal to no deleterious radiotoxicity (14, 16, 17).
Here, we present a method to track adoptively transferred ex vivo–expanded NK cells using 89Zr-oxine cell labeling and PET imaging, validated in vivo in a clinically relevant large animal model using rhesus macaques (RM). Tissue 89Zr distribution accurately quantitated NK-cell trafficking with low radioexposure to organs, suggesting this method can be safely translated to humans.
Materials and Methods
All RM experiments were performed in accordance with a protocol approved by the Institutional Animal Care and Use Committee. RMs were monitored daily in an Association for Assessment and Accreditation of Laboratory Animal Care International–approved facility.
Collection and purification of NK cells and expansion
See Supplementary Methods for material sources and details. Briefly, RM or human NK cells were first enriched by either (i) T cell depletion of peripheral blood mononuclear cells (PBMC) alone or (ii) additional NK-cell selection with NKp80 for RM or CD56 for human NK cells. Enriched NK cells were then expanded for 14–21 days as per an ongoing clinical trial of adoptive NK-cell therapy (NCT00720785; refs. 18, 19), by coculturing with an irradiated (100 Gy) human Epstein–Barr virus transformed lymphoblastoid cell line (SMI-EBV-LCL, authenticated March 4, 2009, by The Center for Cell and Gene Therapy using HLA typing) in X-VIVO 20 medium with 10% RM or human serum, supplemented with 500 IU/mL recombinant human IL2. For experiments using nonexpanded RM NK cells, PBMCs obtained by apheresis and density-gradient centrifugation (Ficoll, GE Healthcare) were magnetically depleted of CD3+ (T cells), CD20+ (B cells), and CD14+ cells (monocytes), before positive selection of NK cells with anti-NKp80, a marker present on the vast majority of RM NK cells, in contrast to CD56, which is downregulated in a population of mature NK cells in RM (20).
Collection and purification of hematopoietic stem and progenitor cells
RM CD34+ hematopoietic stem and progenitor cells (HSPC) were purified as described following mobilization with granulocyte-colony stimulating factor and plerixafor, apheresis, and then immunomagnetic selection (refs. 21, 22; Supplementary Methods). Eight million CD34+ HSPCs with a purity of 92.3% were isolated and labeled with 89Zr-oxine prior to PET/CT studies.
Synthesis of 89Zr-oxine complex and cell labeling
89Zr-oxine complex was synthesized from 89ZrCl4 produced at the institutional cyclotron facility (23) and oxine, as reported previously (14, 16). RM or human NK cells in PBS were incubated with 89Zr-oxine solution (63–100 kBq/106 cells) at a 30:1 volume ratio for 15 minutes, washed twice in the culture medium, and transferred to a new tube in culture medium for in vitro assays or in PBS for PET/CT studies. CD34+ HSPCs were labeled in a similar manner.
Impact of 89Zr labeling on NK-cell surface phenotype, viability, function, and 89Zr activity retention
NK cells with and without 89Zr-oxine labeling were cultured in medium without IL2 and, at indicated time points, examined for CD16, CD56, NKG2A, annexin V, Ki67, and CD107a expression by flow cytometry (FACSCalibur or LSRII, BD Biosciences) or assessed for cell number and 89Zr activity retention (see Supplementary Methods for antibody and reagent information and details). Ki67 staining was performed using the cells fixed and permeabilized with −20°C 80% ethanol. To evaluate degranulation, a surrogate for cytotoxic function, at each time point, 5 × 106 NK cells were cocultured with 5 × 106 K562 target cells (human chronic myeloid leukemia in blast crisis) purchased from ATCC (authenticated on March 8, 2019, by ATCC using short-tandem repeat profiling) for 3 hours and NK-cell CD107a expression was analyzed. Viable cell quantitation was performed using 0.4% Trypan blue dye (Thermo Fisher Scientific). The cell suspension was spun and 89Zr activity of the pellet was measured by a γ-counter (γ-WIZARD2, Perkin Elmer) and corrected for 89Zr activity decay.
Assessing whether 89Zr transferred from 89Zr-oxine–labeled NK cells to neighboring cells
89Zr-oxine–labeled human NK cells from healthy donors were cultured alone or with different cell types (non-89Zr-labeled) for 24 to 36 hours. Thereafter, the two cell types were dissociated by magnetic separation and 89Zr activity was measured for each population (Supplementary Methods).
Tracking cells by a clinical PET/CT imager
Supplementary Table S1 summarizes the 89Zr-oxine–labeled cell doses infused to RMs in this study. All RMs (weight 6.4 ± 0.9 kg) received a continuous intravenous infusion of deferoxamine (Desferal) beginning immediately before 89Zr-oxine–labeled cell transfer until day 7, using first a 15 mg/kg/hour loading dose for 4 hours then 3 mg/kg/hour for the remainder of the study. RMs underwent serial PET/CT imaging for up to 7 days using a clinical PET/CT scanner (Supplementary Methods). In addition to autologous, ex vivo–expanded NK cells, 3 RMs served as controls to track 3 alternative autologous cell populations; (i) nonexpanded NK cells, (ii) apoptotic NK cells, and (iii) HSPCs. VivoQuant software (inviCRO LLC) was used to fuse the maximum intensity projection (MIP) PET images with CT.
Quantitation of PET images
To quantitate 89Zr distribution, contours were drawn on the acquired PET/CT images delineating organs and whole-body using MIMvista (MIM software). Standardized uptake value (SUV) was calculated by the formula: SUV = [Bq/ml (decay corrected) in tissue]/[Bq of the cells injected/body weight (g)]. Percentage of injected dose (% ID) was calculated by decay corrected activity in the tissue divided by the injected dose.
Dosimetry estimates were derived using the organs showing high 89Zr activity from which residence times were measured. The residence times were converted to human equivalent and used as input parameters into OLINDA 1.1 (Vanderbilt University, Nashville, TN). See Supplementary Methods.
Expanded autologous RM NK cells were labeled with 2 μmol/L CellTracker Orange CMRA (Thermo Fisher Scientific) according to the manufacturer's instructions, then intravenously infused into the RM (5 × 107 cells/kg). One day later, a percutaneous liver biopsy was performed under anesthesia. The fresh biopsy samples were analyzed using an inverted Leica SP5 five-channel confocal multiphoton microscope (Leica Microsystems), employing 561 nm excitation and 570–640-nm emission range (Supplementary Methods). Flow cytometry analysis (Fortessa, BD Biosciences) was also performed on liver tissue mashed into suspension and stained with aqua live/dead stain (Thermo Fisher Scientific).
NKG2A expression was analyzed by Wilcoxon test. CD56/CD16 and CD107a expression within each time point, and expression of annexin V and Ki67 as well as cellular viability over time, were analyzed by repeated measure ANOVA with Sidak test. Friedman test with Dunn correction was used to compare CD107a expression at multiple time points. 89Zr transfer from 89Zr-labeled NK cells to CD19+ cells or PBMCs was analyzed by Wilcoxon test. Prism ver.8.2.0 (GraphPad Software) was used for all statistical analyses. The results are indicated as mean ± SD and P values less than 0.05 were considered significant.
89Zr-oxine labeling does not alter the phenotype, viability, or function of expanded NK cells
We first examined whether labeling RM and human NK cells with 89Zr-oxine complex had any impact on NK-cell phenotype. RM and human NK cells labeled at 19.5 ± 4.0 kBq/106 cells and 20.2 ± 4.0 kBq/106 cells, respectively, were used in all in vitro experiments. Flow cytometry analysis indicated that expression of CD56 and CD16 (RM: n = 9, human: n = 7), and NKG2A (RM: n = 8), was not altered immediately (3 hours) after 89Zr-oxine labeling (Fig. 1A and B) or over the subsequent 24 hours of culture (n = 3, Supplementary Fig. S1). These conditions were chosen to mimic an in vivo environment in patients not receiving exogenous IL2.
We next assessed the viability of labeled cells and retention of 89Zr. 89Zr-oxine–labeled and nonlabeled RM NK cells showed similar viability and similar decline in live cell number (Fig. 1C, n = 11) over time, as well as a similar increase in the percentage of the Annexin V+ apoptotic/necrotic cells (Fig. 1D, n = 3). Some cells proliferated at early time points, as indicated by Ki67 expression, which is likely due to lingering effect of IL2 used for ex vivo expansion (Fig. 1D, n = 3). As a result, 89Zr radioactivity per cell declined to approximately 50% of initial 89Zr activity by day 3 (Fig. 1E, n = 6). Similar findings were observed with human NK cells (Fig. 1F: n = 9, G: n = 3, H: n = 6). 89Zr labeling did not impair the cytotoxic function of RM (Fig. 1I, n = 3) or human (Fig. 1J, n = 3) NK cells; degranulation, assessed by measuring NK-cell surface CD107a expression up to 24 hours following coculture with K562 tumor cells, was similar between 89Zr-labeled and nonlabeled NK cells.
89Zr is not readily transferred from the labeled NK cells to neighboring cells
To exclude the possibility that 89Zr could transfer from labeled to nonlabeled cells in vivo, which would confound the interpretation of 89Zr signals on PET/CT, we assessed whether 89Zr could be transferred from labeled NK cells to adjacent cells ex vivo by passive diffusion. When 89Zr-oxine–labeled human NK cells were cocultured with Raji or Ramos cells for 24 to 36 hours, >96% of 89Zr-activity remained within the NK cells after repurification of the cocultured cells (Fig. 1K, n = 3). More importantly, the radioactivity detected in the CD19+ fraction (either Raji or Ramos cells) did not differ from background radioactivity measured when a sham CD19+ cell selection was performed on 89Zr-labeled NK cells alone. Similar results were found when NK cells from 3 HLA-A2–negative donors were labeled with 89Zr-oxine and cocultured with HLA-A2–positive PBMCs. The 89Zr-activity in the immunomagnetically selected HLA-A2–positive fraction was equivalent to the background radioactivity obtained from a sham HLA-A2–positive selection of 89Zr-labeled HLA-A2–negative NK cells alone (Fig. 1L, n = 3). These data suggest that 89Zr-oxine–labeled NK cells retain the radiolabel without significant transfer to neighboring cells.
PET/CT imaging provides clear visualization of the trafficking of adoptively transferred 89Zr-oxine–labeled NK cells in RMs
Sequential PET/CT imaging of autologously transferred 89Zr-oxine–labeled expanded NK cells (n = 3, 2.05 ± 0.72 MBq, 13.6 ± 5.3 kBq/106 cells, 23.5 ± 2.0 × 106 NK cells/kg; Supplementary Table S1) showed initial localization to the lungs, followed by migration to the liver and spleen (Fig. 2A). The majority of NK cells remained in these organs until day 7, when the experiment concluded. 89Zr accumulation in the bladder indicates effective renal clearance of free 89Zr by deferoxamine. Low levels of radioactivity were observed within the BM, as shown in an axial PET/CT section of the 4th lumbar vertebrae with 89Zr activity localized in the center of the spinal body, the location of vertebral marrow (Fig. 2B). In contrast, measurable labeled NK cells were not localized within lymph nodes. Quantitation of organ 89Zr activity on acquired images showed the SUV of the liver and spleen reached an average of 14.4 and 25.3, respectively, on day 1, in contrast to the BM which only reached an SUV of 0.64 (Fig. 2C and Supplementary Fig. S2A, which shows the same data with magnification of the x-axis up to day 1). The percentage of 89Zr activity measured in the liver corresponded to 31.3% ± 8.1% and 34.7% ± 11.8% of the infused 89Zr activity on days 1 and 2, respectively, indicating that approximately one third of the transferred cells had trafficked to the liver (Fig. 2D; Supplementary Fig. S2B for magnification up to day 1). In contrast, only approximately 3% of the cells homed to the spleen and BM at these time points. As expected, whole-body radioactivity declined over time (Fig. 2E), likely due to renal clearance of 89Zr released from dead cells and chelated by deferoxamine, suggesting that one third and one half of the transferred ex vivo–expanded NK cells had died by days 2 and 7, respectively.
Deferoxamine infusion results in rapid renal clearance of 89Zr released from apoptotic NK cells
It is well documented that free 89Zr can be taken up by bone (24). Our previous study in mice using 89Zr-labeled cells demonstrated that deferoxamine infusion can prevent bone uptake of free 89Zr (16). To confirm that continuous infusion of deferoxamine efficiently bound and then excreted free 89Zr released from dead cells in RM and that the 89Zr-signals observed in Fig. 2A are from live cells, we performed a cell tracking study using ex vivo–expanded NK cells that had become apoptotic as a consequence of removing IL2 from the medium used to culture cells ex vivo (276 kBq, 15 kBq/106 cells, 3.5 × 106 cells/kg, Supplementary Table S1). In contrast to RMs receiving viable NK cells, we observed significant radioactivity in the kidneys and bladder when the initial scan (20-minute duration) was conducted (Fig. 3A) following the apoptotic NK-cell transfer, consistent with rapid renal clearance of 89Zr. Although some 89Zr-labeled apoptotic NK cells were initially observed in the lungs and liver, by 4 hours, virtually all the 89Zr had moved to the bladder. On day 1, only 8.3 % of the total administered 89Zr remained in the body. These data show that deferoxamine chelation is effective in preventing transfer of free 89Zr from dying and/or dead cells into bone or other cellular populations with phagocytic capacity. Of note, following the infusion of apoptotic cells, a small amount of 89Zr accumulated in the gallbladder (1.5% ID at 4 hours), suggesting low-level clearance of 89Zr via the liver into the gallbladder.
Both freshly isolated and ex vivo–expanded NK cells showed limited BM homing compared with HSPCs
To assess whether ex vivo expansion may have altered the homing profile of NK cells, the same 89Zr-oxine labeling and PET/CT approach was employed using nonexpanded RM NK cells (185 kBq, 6.9 kBq/106 cells, 4.0 × 106 cells/kg; Supplementary Table S1). The cell number (and therefore 89Zr activity) infused was low because the number of NK cells obtainable by purification from blood, without expansion, was limited. Compared with expanded NK cells (Fig. 4A), nonexpanded NK cells distributed slightly less to liver and more to spleen, but their relative homing to the BM appeared similarly low (Fig. 4B). For comparison, we tracked 89Zr-oxine–labeled CD34+ HSPCs, a cell population known to efficiently localize and engraft in the BM (355 kBq, 44.4 kBq/106 cells, 1.2 × 106 cells/kg, Supplementary Table S1). In contrast to NK cells, HSPCs showed robust trafficking to the BM, although infused cell number and 89Zr activity were also low due to the limitation imposed by the number of these rare cells available for labeling (Fig. 4C; Supplementary S3A). The BM %ID on day 1 for expanded NK cells, nonexpanded NK cells, and HSPCs were 3.4%, 2.4%, and 22.1%, respectively (Fig. 4D; Supplementary Fig. S3B). These results show labeled HSPCs can still home to BM and validate the sensitivity of this approach for tracking cell populations of interest to the BM. Importantly, they show that only a small fraction of adoptively transferred NK cells trafficked to BM, suggesting the capability of infused NK cells to home to normal BM is limited.
Liver biopsy shows presence of transferred NK cells in the liver
To confirm that the radioactivity detected in the liver by PET/CT following the infusion of 89Zr-oxine–labeled NK cells (Fig. 2A) reflected viable NK cells that had homed to this organ, a liver biopsy was performed 24 hours following the infusion of CMRA-labeled NK cells (Fig. 5A). Microscopic analysis of the collected samples showed CMRA-labeled NK cells residing in the liver (Fig. 5B). Furthermore, flow cytometry analysis of the biopsy sample confirmed the presence of viable CMRA-positive NK cells within the liver (CD56+ NK cells represented 0.29% of the liver lymphocyte population; Fig. 5C).
Safety profile of 89Zr-oxine–labeled NK-cell PET suggests this study approach is translatable to humans
RMs did not have any changes in their vital signs, weight, or clinical exam following the infusion of 89Zr-oxine–labeled cells. Dosimetry in humans, extrapolated from distribution of 89Zr-labeled NK cells in RMs, indicated that the highest radioexposure was observed in the spleen and liver (6.24 and 3.88 mSv/MBq, respectively, Table 1). When considering the U.S. Nuclear Regulatory Commission safety guideline limits of 500 mSv per organ or 50 mSv total body, per year, our data suggest that 89Zr-labeled NK cells below 80.1 MBq (2.17 mCi)/70 kg doses [i.e., 1.14 MBq (30.9 μCi)/kg doses] per year could be infused safely into humans while allowing for tracking of the cells to locations of interest.
|Organs .||mSv/MBq (mean ± SD) .|
|Liver||3.880 ± 1.228|
|Lungs||1.400 ± 0.323|
|Red marrow||0.319 ± 0.045|
|Spleen||6.240 ± 3.606|
|Total body||0.317 ± 0.048|
|Effective dose (mSv/MBq)||0.675 ± 0.135|
|1. n = 3|
|2. Extrapolated to 70 kg adult humans|
|Organs .||mSv/MBq (mean ± SD) .|
|Liver||3.880 ± 1.228|
|Lungs||1.400 ± 0.323|
|Red marrow||0.319 ± 0.045|
|Spleen||6.240 ± 3.606|
|Total body||0.317 ± 0.048|
|Effective dose (mSv/MBq)||0.675 ± 0.135|
|1. n = 3|
|2. Extrapolated to 70 kg adult humans|
Although pilot trials have established that adoptive NK-cell transfer can induce antitumor effects, the overall efficacy to date of NK cell–based immunotherapy has been limited (6–8). One reason may be that infused NK cells are simply not migrating through the blood stream to relevant target organs where the malignancies reside, such as the BM or lymph nodes for hematologic malignancies. In this study, we developed a method of tracking NK cells in vivo using 89Zr-oxine ex vivo cell labeling and PET/CT imaging in RM. RM NK cells have similar phenotypic and functional properties to human NK cells and can be expanded ex vivo using methods currently utilized in clinical trials (20). In contrast, mouse NK cells have significant phenotypic and functional divergence from human NK cells. For instance, most mouse NK cells lack Fas and TNF-related apoptosis-inducing ligand used to kill tumor targets (25, 26), and they do not possess killer-cell immunoglobulin-like receptors (27, 28), which play a critical role in regulating NK-cell cytotoxicity. Furthermore, most conditions used for expanding human NK cells ex vivo are ineffective in expanding murine NK cells. Taken altogether, these factors favor the use of RMs over mice for studies exploring NK-cell trafficking.
The BM is the site of origin and proliferation of many hematologic malignancies. NK cells are known to express BM homing receptors such as CXCR4 (29) and CD62L (30). However, CXCR4 may be downregulated during ex vivo NK-cell expansion (29, 30) and E-selectin ligands expressed on expanded NK cells are primarily nonglycosylated, making them inactive for binding E-selectin which is expressed on BM endothelial cells (31). These, as well as other factors, may, in part, account for the limited BM trafficking of ex vivo–expanded NK cells that we observed in our study. However, it is important to consider that we also observed that nonexpanded NK cells had poor homing to the BM. Migration of NK cells to lymph nodes was also not observed, consistent with low expression and/or downregulation of CCR7 reported during culture (32). In contrast, as would be expected, 89Zr-oxine–labeled HSPCs showed robust trafficking to the BM. Collectively, the low-level trafficking observed with adoptively transferred NK cells to the BM of RMs may in part explain the limited efficacy observed in clinical trials exploring adoptive NK-cell therapy for BM malignancies. It is possible that NK cells would show superior homing or may only home to the BM infiltrated by malignant cells. Unfortunately, we lack primate models for hematologic malignancies to address this interesting question.
We observed high levels of liver homing of transferred NK cells. Recent studies have revealed that high numbers of NK cells, consisting of 30%–40% lymphocytes, reside in the liver (33). Attraction and retention of NK cells in the liver are considered to be mediated by chemokines such as CCL3, CCL5, and CXCL16, but only the CD56bright hepatic NK cells have been characterized to express receptors for these ligands (34, 35). In the context of our study's findings, a characterization of liver homing receptors on NK cells expanded using a variety of different approaches would be of interest. Modification of expansion conditions (9, 10) or genetic modification of NK cells to overexpress BM homing receptors (29) or to underexpress liver homing receptors may improve trafficking of these cells to the BM. 89Zr-oxine cell tracking could be a very useful tool to assess the impact of such maneuvers to alter NK-cell homing to the BM, lymph nodes, and other target tissues.
Previous studies suggest that, following 89Zr-oxine cell membrane permeabilization, 89Zr is retained in viable cells (14, 16). Cell survival in culture showed substantial donor-to-donor variation, likely accounting for the large standard deviations observed in the calculated 89Zr activity per cell. Furthermore, following harvesting of ex vivo–expanded NK cells, some degree of proliferation continued to occur even after cells were labeled with 89Zr-oxine. We believe this persisting NK-cell proliferation likely caused the decline of 89Zr activity/cell observed at earlier time points. In this context, 89Zr-oxine–labeled cells that are damaged or die following adoptive transfer would be expected to release free 89Zr. Whole-body 89Zr retention after labeled NK-cell transfer was higher than expected from extrapolation of in vitro retention over time. Better in vivo survival of NK cells could perhaps be explained by support from endogenous cytokines such as IL15, or microenvironmental survival signals provided by tissues.
Although the majority of released 89Zr is excreted via the kidneys (14), it is possible that a fraction of free 89Zr could bind to hydroxyapatite in bone resulting in high 89Zr signals in the bones (24), potentially confounding the accurate quantitation of cellular homing to the BM. To preclude this possibility in our experiments, RMs received a continuous infusion of deferoxamine, the most commonly used and clinically applicable chelator for 89Zr (36), to chelate and promote the renal excretion of any free 89Zr. In our previous murine studies, multiple intramuscular injections of deferoxamine reduced bone uptake of free 89Zr (14, 16). As RM studies are limited by regulations and availability, it was not feasible to perform control 89Zr-oxine–labeled NK-cell tracking in the absence of deferoxamine. Although bone uptake of 89Zr would spuriously inflate calculated marrow localization, the very low BM localization observed with 89Zr-labeled NK cells in our study does not confound our conclusions.
The experiment infusing 89Zr-labeled apoptotic NK cells, confirmed the efficiency of deferoxamine chelation and suggests that deferoxamine minimizes the direct transfer of 89Zr from dead/dying cells to phagocytes. Of note, in vitro experiments did not detect 89Zr transfer from live 89Zr-oxine–labeled NK cells to nonlabeled cells when two cell populations were cocultured, including PBMCs containing cells with phagocytic properties. These characteristics of 89Zr retention within live cells and release and rapid clearance of 89Zr from the circulation after cell death are essential for accurate interpretation of imaging studies using radiolabeled agents for in vivo cell tracking. Importantly, liver biopsy revealed that viable CMRA-labeled NK cells were present in a RM following adoptive infusion of NK cells, confirming the findings of the PET studies. Furthermore, the deferoxamine doses we used were the same as those administered to humans treated with this agent for iron overload, establishing the clinical applicability of this strategy for patients receiving 89Zr-oxine–labeled NK cells.
The PET studies provided a good dosimetry model to estimate potential radioexposure in humans. The spleen and liver received the highest exposure among organs; however, the doses were still well within safety guidelines when no more than 157.7 MBq (4.26 mCi)/70 kg doses (i.e., 1.14 MBq (30.9 μCi)/kg doses) are administered to humans. This corresponds to 83.8 × 106 NK cells/kg when the cells were labeled with a specific activity of 13.6 kBq/106 cells used in this study. We observed high background noise with the PET/CT images of nonexpanded NK cells when only 185 kBq (27.2 kBq/kg) was infused, due to the small number of NK cells we were able to purify from PBMCs, suggesting this dose is at the detection limit of PET/CT. On the other hand, the dose used for HSPCs (355 kBq, 54.6 kBq/kg) generated high quality images with low background noise. On the basis of these results, we estimate that an approximately 55 kBq (1.5 μCi)/kg dose of 89Zr-oxine–labeled cells would be sufficient to provide good quality images by PET/CT to accurately track these cells in vivo. These data indicate that in future clinical trials evaluating adoptive NK-cell infusions in humans, where cell numbers in the order of 5–10 × 108 cells/kg are typically infused, only a small fraction of the cells being infused would need to be labeled to provide robust qualitative and quantitative information on cellular trafficking.
In conclusion, 89Zr-oxine labeling of NK cells followed by PET/CT imaging represents a promising new tool to track the in vivo fate of adoptively transferred NK cells. The RM model presented here provides a method to evaluate and optimize various strategies to improve NK-cell trafficking and homing to the BM, lymph nodes, and other locations where hematologic malignancies and tumors reside. These preclinical in vitro and in vivo data suggest this technology could be safely extended to humans and could be applied to other cellular populations besides NK cells.
Disclosure of Potential Conflicts of Interest
N. Sato and P.L. Choyke have filed a U.S. patent application on Zr-89 oxine complex generation and application used in this manuscript. No potential conflicts of interest were disclosed by the other authors.
The content of this publication does not necessarily reflect the views or policies of the Department of Health and Human Services, nor does mention of trade names, commercial products, or organizations imply endorsement by the U.S. Government.
Conception and design: N. Sato, K. Stringaris, J.K. Davidson-Moncada, R. Reger, C. Dunbar, P.L. Choyke, R.W. Childs
Development of methodology: N. Sato, K. Stringaris, J.K. Davidson-Moncada, P.L. Choyke, R.W. Childs
Acquisition of data (provided animals, acquired and managed patients, provided facilities, etc.): N. Sato, K. Stringaris, J.K. Davidson-Moncada
Analysis and interpretation of data (e.g., statistical analysis, biostatistics, computational analysis): N. Sato, K. Stringaris, J.K. Davidson-Moncada, S.S. Adler, P.L. Choyke, R.W. Childs
Writing, review, and/or revision of the manuscript: N. Sato, K. Stringaris, J.K. Davidson-Moncada, S.S. Adler, C. Dunbar, P.L. Choyke, R.W. Childs
Administrative, technical, or material support (i.e., reporting or organizing data, constructing databases): N. Sato, K. Stringaris, R. Reger, C. Dunbar, P.L. Choyke
Study supervision: K. Stringaris, C. Dunbar, P.L. Choyke, R.W. Childs
This research was supported by the Intramural Research Program of the NIH, NCI, Center for Cancer Research and the National Heart, Lung, and Blood Institute, Division of Intramural Research. This project has been funded in whole or in part with federal funds from the NCI, NIH, under contract no. HHSN261200800001E (to S.S. Adler). The authors thank Marvin L. Thomas, Theresa M. Engels, Randall R. Clevenger, Timothy Hunt, Kenneth R. Jeffreys, Mark Metzger, Allen E. Krouse, Sandra D. Price, Robert E. Donahue, Robert F. Hoyt Jr, Karen J. Keeran, Arthur Zetts, and Shawn Kozlov for animal care; Lawrence P. Szajek for generating 89ZrCl4; Gideon K. Kwarteng, Philip C. Eclarinal, and Alicia Forest for acquiring PET/CT; and Daniela A. Malide for microscopic analysis of the biopsy samples. The authors also acknowledge the NHLBI and NCI intramural research programs and the Dean R. O’Neill and Edward Rancic Cancer Fellowship for their support of this research.
The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.