The immune system deploys different cell types to take out cancer cells. True to their name, one type of immune cell known as natural killer cells kills tumor target cells by releasing toxic proteins that kill the harmful cells. In humans, these immune cells are defined, among other things, by the presence of a protein called CD56 on their cell surface. This protein (which is also known as NCAM) is thought to help cells to stick to their surroundings and control their movements. However, it was not clear whether CD56 also plays a role in the destructive abilities of natural killer cells.

Gunesch et al. have now looked to see what would happen if natural killer cells lacked CD56 on their surface. The experiments included deleting the gene for CD56 from two kinds of human natural killer cell that are commonly grown in the laboratory (called NK92 and YTS). In both cases, the cells lacking CD56 killed fewer cancer cells than the unedited natural killer cells. The NK92 cells were much more affected by the loss of CD56 than the YTS cells, and after Gunesch et al. compared the two kinds of cell they identified another protein called Pyk2 as the potential reason behind the difference.

The Pyk2 protein is known to help a natural killer cell latch onto target cancer cells and release its toxic proteins. To do this, Pyk2 must first be activated with phosphate groups via a process known as phosphorylation. Gunesch et al. showed that Pyk2 protein in unedited NK92 cells was more highly phosphorylated than those of the YTS cells, and that Pyk2 activation by phosphorylation was greatly decreased in NK92 cells when the gene for CD56 was deleted. Together these and other results suggest that CD56 on natural killer cells helps to promote Pyk2 to activate the cells’ cancer-killing abilities through Pyk2 phosphorylation, especially in NK92 cells.

These findings open up new lines of investigation into the relationship between sticky surface proteins and the activation of immune cells. They may also have important implications for the use of the immune system to treat cancer via immunotherapy.

Natural killer (NK) cells are innate immune effectors that play an important role in the clearance of virally infected and tumorigenic cells and modulation of immune responses. Human NK cells represent approximately 10% of circulating lymphocytes and can be defined within this population as CD56⁺CD3⁻ cells. CD56^dim cells are the predominant subset in peripheral blood while the minority of the circulating NK cell population are CD56^bright cells. CD56^bright and CD56^dim NK cells have unique expression of cell surface receptors, transcription factors and intracellular effector molecules that contribute to their distinct phenotypic and functional capacities (Caligiuri, 2008; Freud and Caligiuri, 2006). Despite its conserved expression on human NK cells and its use as a phenotypic identifier, the role of CD56 in immune function is poorly defined.

CD56 is the cluster of differentiation nomenclature for neural cell adhesion molecule (NCAM). In addition to being abundantly expressed in cells of neuronal origin, NCAM is also expressed in other tissues including the heart, kidney, skeletal muscles, liver, and on hematopoietic-derived cells including dendritic cells and human natural killer (NK) and NKT cells (Galuska et al., 2017; Lanier et al., 1989; Roothans et al., 2013). NCAM is a member of the immunoglobulin superfamily of cell adhesion molecules and has multiple isoforms due to RNA splicing (Cunningham et al., 1987; Shapiro et al., 2007; Williams and Barclay, 1988).The most abundant isoforms are the glycophosphatidylinositol (GPI)-anchored 120 kDa form (NCAM120) and two isoforms that contain transmembrane endodomains, NCAM140 and NCAM180 (Cunningham et al., 1987; Kleene and Schachner, 2004). The extracellular portion of NCAM is highly conserved between the isoforms and contains 5 Ig-domains and two fibronectin-III domains that mediate homophilic (NCAM-NCAM) and heterophilic interactions between cells and the extracellular matrix (Soroka et al., 2003; Walmod et al., 2004). NCAM^−/− mice have impaired learning and memory and behavioral disorders that are attributed to impaired nervous system development and post-differentiation maintenance of signaling and plasticity (Brandewiede et al., 2014; Cremer et al., 1994). As such, NCAM plays an important role in non-immune cellular differentiation and function.

In addition to its role as an adhesion molecule, NCAM signaling in neuronal cells is important for neurite outgrowth and synaptic plasticity. NCAM140 is constitutively associated with the membrane-associated Src-family tyrosine kinase Fyn in axonal growth cones (Beggs et al., 1997). Signaling through NCAM140 by agonist antibodies induces the recruitment of the non-receptor tyrosine kinase focal adhesion kinase (FAK) in neuronal cells, leading to a rise in intracellular Ca++ and neurite outgrowth (Beggs et al., 1997; Ditlevsen et al., 2008; Schmid et al., 1999). In addition to this signaling pathway, NCAM activates fibroblast growth factor receptor −1 (FGFR1) following trans-homophilic binding or binding of soluble extracellular NCAM and activates a non-canonical signaling pathway via phospholipase Cγ (PLCγ) that activates Erk1/2 in a Src-kinase dependent manner (Francavilla et al., 2009).

NCAM is unique amongst other glycoproteins in that its fifth Ig-domain contains 2 N-linked glycosylation sites that can be highly polysialylated (Cunningham et al., 1987; Kleene and Schachner, 2004). PSA-NCAM is particularly important for synaptic plasticity and is highly expressed in embryonic development, with decreasing and restricted expression found in adults. The phenotype of olfactory bulb precursor cell migration in NCAM-deficient mice can be recapitulated by enzymatic removal of PSA, demonstrating the importance of PSA in NCAM signaling (Ono et al., 1994). The lack of expression of NCAM or a known NCAM homologue on murine NK cells precludes the use of mouse models to test the in vivo requirement for NCAM or PSA-NCAM on NK cells. However, while murine NK cells are not modified by PSA, a subset of murine myeloid precursors in bone marrow are PSA^high, and mice deficient for the ST8Sia IV sialyltransferase exhibit a sustained and severe contact hypersensitivity response, suggesting a role for PSA in the functional immune response (Drake et al., 2008). Further investigation demonstrated that C-C motif chemokine receptor 7 (CCR7) is a carrier of PSA on dendritic cells. Binding of PSA on CCR7 to the C-C motif chemokine ligand 21 (CCL21) relieves CCL21 autoinhibition, leading to DC migration and helping confer specificity for response to chemokine (Kiermaier et al., 2016). Together these studies underscore the significance of this unusual protein modification in both neuronal cells and certain immune contexts, however the role of PSA addition to NCAM on human NK cells has not been defined.

NK cell cytotoxic function is exerted through the directed secretion of perforin- and granzyme-containing lytic granules following formation of an immunological synapse (IS) that includes adhesion, effector, and termination stages (Orange, 2008). Cell adhesion is mediated by integrins, particularly LFA-1, that play a critical role in IS formation and actin polymerization. Prior to the polarization of the microtubule organizing center (MTOC) to the IS, lytic granules converge to the MTOC in a dynein-dependent minus-ended directed manner that is independent of actin polymerization and microtubule dynamics; as such, ligation of LFA-1 or CD28 alone is sufficient to induce convergence (James et al., 2013; Mentlik et al., 2010). LFA-1-mediated outside-in signaling leads to F-actin polymerization and reorganization, phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2) generation, and the activation of protein tyrosine kinases, including Src family kinases (Mace et al., 2009; Steblyanko et al., 2015). Activating signaling and sustained F-actin remodeling promotes the polarization of the MTOC, with converged lytic granules, to the lytic IS (Chen et al., 2007; Krzewski et al., 2008; Lagrue et al., 2013). Ultimately, lytic granules transverse a dynamic yet pervasive cortical actin network to access the plasma membrane, where they undergo SNARE-mediated exocytosis at the IS (Brown et al., 2011; Carisey et al., 2018; Krzewski and Coligan, 2012; Rak et al., 2011).

Previous studies have demonstrated that CD56 can promote human NK cell cytotoxic function against some CD56⁺ target cells (Jarahian et al., 2007; Nitta et al., 1989; Taouk et al., 2019; Valgardsdottir et al., 2014), although in some cases lysis is not enhanced by target cell CD56 expression (Lanier et al., 1991). As these studies focused on homotypic CD56-mediated interactions, the significance of CD56 binding to heterotypic ligands is unclear yet is likely relevant. In neuronal cells, CD56 binds FGFR1 in cis and in trans, and CD56 on NK cells binding to FGFR1 on T cells leads to IL-2 production (Kos and Chin, 2002). CD56 also mediates direct recognition of A. fumigatus leading to fungal-induced NK cell production of MIP-1α, MIP-1β and RANTES (Ziegler et al., 2017). This interaction is marked by accumulation of CD56 at the interface between the NK cell and A. fumigatus and is actin-dependent (Ziegler et al., 2017).

While CD56 has been implicated in NK cell development, migration, and cytotoxicity (Nitta et al., 1989; Taouk et al., 2019; Lanier et al., 1991; Chen et al., 2018; Mace et al., 2016), the signaling pathways that regulate its function in immune cells have not been described. Given signaling downstream of CD56 that is mediated by FAK in neuronal cells, one potential link between CD56 and IS formation is the closely related non-receptor tyrosine kinase 2 (Pyk2), which is highly expressed in NK cells (Gismondi et al., 1997). FAK and Pyk2, with its expression more restricted to cells of hematopoietic origin, play critical roles in cell adhesion, cell migration and actin remodeling. Stimulation or engagement through multiple receptors, including T cell receptors, integrins and G protein coupled receptors, leads to Pyk2 phosphorylation and activation. As has been reported for Fyn-dependent activation of FAK in neuronal cells, tyrosine 402 (Y402) of Pyk2 is a substrate for Fyn-dependent signaling downstream of TCR ligation (Qian et al., 1997). In addition, Pyk2 clustering leads to rapid autophosphorylation on Y402 by trans-acting intermolecular interactions (Eide et al., 1995; Park et al., 2004). Phosphorylation on Pyk2 Y402, which is equivalent to Y397 of FAK, enables binding and activation of SH2 domain-containing proteins, including Src kinases, and downstream activation of multiple signaling pathways that mediate cell adhesion and migration (Parsons, 2003). In NK cells, Pyk2 is phosphorylated downstream of integrin β2 ligation as part of an ILK-PINCH-PARVIN signaling cascade that leads to activation of Cdc42, which can control microtubule dependent polarity through CLIP-170 and actin remodeling through WASp and the Arp2/3 complex (Zhang et al., 2014). Pyk2 colocalizes with the MTOC in the uropod of migrating NK cells, however following activation it is translocated to the IS and is required for MTOC polarization in IL-2 activated primary NK cells (Sancho et al., 2000). Expression of dominant negative Pyk2 disrupts cytotoxicity in this system, and its interactions with β1 integrin, paxillin, and other protein tyrosine kinases suggests that Pyk2 plays a role as a scaffolding protein that helps orchestrate NK cell cytotoxicity (Gismondi et al., 1997; Zhang et al., 2014; Sancho et al., 2000).

Here, we describe a requirement for CD56 in human NK cell function and show that deletion of CD56 in two human NK cell lines leads to impaired secretion and accompanying lytic dysfunction. Furthermore, we identify Pyk2 as a critical signaling intermediate downstream of CD56. These data demonstrate a direct role for CD56 in the NK cell-mediated lysis of CD56-negative target cells and describe a novel activation pathway for cytotoxicity that is unique to human NK cells.

While CD56 is the prototypical identifier of human NK cells in peripheral blood, its function has been poorly defined. Early studies of its role in cytotoxicity largely focused on CD56 homotypic interactions and found that cytotoxicity of IL-2 expanded primary NK cells was diminished against CD56⁺ target cells in the presence of anti-CD56 blocking antibodies (Nitta et al., 1989). In contrast, Lanier et al. found no significant difference in NK cell-mediated lysis of CD56^– KG1a and CD56⁺ KG1a target cells (Lanier et al., 1991). In both cases, CD56 functionality was tested with IL-2 expanded primary NK cells using antibody to inhibit CD56 homotypic interactions, through the manipulation of CD56 expression on target cells, or a combination of both. CD56 was also implicated in alloantigen-specific recognition by human NK cells, and the cytotoxic activity of the NS2 cell line against its LCL stimulator cell line was diminished in the presence of monoclonal antibodies specific for CD56 (Suzuki et al., 1991). Conversely, the expression of NCAM on some target cell lines inhibits NK cell-mediated cytotoxicity (Jarahian et al., 2007). Here, using CD56-deficient NK92 and YTS human NK cell lines, we demonstrate a requirement for CD56 in killing of CD56-negative targets by human NK cell lines and define a mechanism for the requirement for CD56 in human NK cell cytotoxicity.

Our previous study defined a role for CD56 in human NK cell migration, and we had previously modeled this requirement using the NK92 cell line (Mace et al., 2016). Here we extended our studies to the use of YTS cells as a second human NK cell line. Despite a significant and consistent effect of CD56 deletion on the cytotoxic function of NK92 cells, this effect was not conserved between the two human NK cell lines that we tested. The profound deficiency in lytic function in the NK92 cell line was observed even when effector cells were incubated with targets for just one hour in a ⁵¹Cr release assay, a timescale which likely only permits a single target cell killing to occur (Gwalani and Orange, 2018). As this incubation time led to significant killing of target cells by WT NK92 cells, these data suggest that the observed decrease in killing by the NK92 CD56-KO cells is not due to an inability to mediate serial killing against multiple targets despite the previously demonstrated requirement for CD56 in NK92 cell migration (Mace et al., 2016; Bhat and Watzl, 2007). Furthermore, our subsequent recapitulation of cytotoxicity using antibody-coated glass to activate NK cells uncoupled the defect that we describe in exocytosis from one of cell migration.

For YTS cells, killing was only impaired at the 1 hr time point, signifying an initial delay in their ability to kill which could be rescued at later time points. The milder effect on cytotoxicity in the YTS cell line when compared to the NK92 cell line suggests fundamental differences in the requirements of the two cell lines for cytolytic function, and it remains possible that the effect on YTS cell cytotoxicity could be due to impaired cell migration affecting serial killing of targets at later time points given the requirement for CD56 in human NK cell migration (Mace et al., 2016). However, impaired IFNγ production by YTS CD56-KO cells following contact-dependent co-incubation with 721.221 target cells further underscores underlying differences in the requirement for CD56 in these cell lines and also demonstrates a functional relevance for CD56 in the YTS cell line. Previously demonstrated phenotypic and genotypic differences between the cell lines (Gunesch et al., 2019) supports this hypothesis and suggests that further investigation is required to fully delineate the relative role of CD56 in these contexts. Here, we have chosen to focus on the mechanism underlying the requirement for CD56 in the cytotoxic function of NK92 cells.

Our investigations into the mechanism of CD56 function in human NK cells were informed by extensive investigations into the functional role of NCAM in neural cells, where NCAM-mediated signaling plays a critical role in axonal growth, survival and proliferation (Ditlevsen et al., 2008). While there are multiple pathways by which this can occur that include homotypic and heterotypic NCAM interactions, NCAM signaling includes the formation of complexes that signal through Ras-MAPK-ERK pathways via interactions with p59^fyn and FAK. Specifically, NCAM140 constitutively interacts with p59^fyn, whereas FAK is recruited upon cross-linking of the NCAM extracellular domain (Beggs et al., 1997). Recruitment of FAK leads to its phosphorylation and kinase function, and ultimately growth cone migration. The demonstrated requirement for Src family kinases, including Fyn, in NK cell cytotoxicity suggested that a common pathway could be functioning in NK cells (Dong et al., 2012). Furthermore, while FAK is not known to be expressed in human NK cells, Pyk2 is expressed and is required for polarization of the MTOC during IS formation and resultant cytotoxic function (Sancho et al., 2000). Here, we define a requirement for CD56 in phosphorylation of Pyk2 on Y402, the primary autophosphorylation site that serves as a docking site for the SH2 domain of Src family kinases (Eide et al., 1995). This requirement was most significant in the NK92 cell line, which we found had a significantly greater amount of Pyk2 Y402 phosphorylation both at baseline and upon activation. The functional implication of this finding was the significantly greater impairment in NK92 cytotoxicity following pre-treatment of both cell lines with a Pyk2 inhibitor.

Despite showing that Pyk2 is required for NCAM-mediated function, it is still unclear precisely the mechanism by which this signaling occurs. We demonstrate that loss of CD56 leads to abolishment of lytic granule exocytosis, as surface CD107a was decreased on NK92 CD56-KO cells compared WT NK92 after activation with either plate-bound antibodies or with target cells. In addition, the maximum activity of granzyme A in the supernatant, an indirect readout of exocytosis, was decreased for NK92 CD56-KO cells. Given that the exocytosis defects we observed in the NK92 cell line were demonstrated using antibody cross-linking, and thus independently of target cell activation, it is unlikely that CD56 is binding directly to unknown target cell ligands in this context. Furthermore, our use of CD56-negative target cells defines independence from homotypic NCAM interactions. NCAM can also bind both in cis and in trans to FGFR, and interactions between CD56 on NK cells and FGFR1 on T cells is sufficient to induce T cell IL-2 production (Kos and Chin, 2002). Despite this indication that productive interactions occur within immune cells upon CD56-FGFR1 interactions, we have repeatedly failed to detect FGFR1 on human NK cells or the target cells used in this study (data not shown), suggesting that FGFR1 is not the mechanism by which NK cell function is mediated by CD56.

Given the defect in exocytosis in CD56-deficient NK92 cells, we sought to define earlier steps in NK cell cytotoxicity and found that actin accumulation at the synapse was decreased in CD56-KO cells, although not significantly, and that MTOC polarization was impaired. While these features could be attributed to impaired integrin-mediated adhesion, conjugation of effectors to targets was not impaired in CD56-KO cells. Given the previously described requirement for Pyk2 in MTOC polarization during NK cell cytotoxicity (Zhang et al., 2014; Sancho et al., 2000), these data further suggest that the requirement for CD56 is in Pyk2-mediated function during cytotoxicity. This was further defined by impaired Pyk2 phosphorylation and recruitment to the IS in NK92 CD56-KO cells. Taken together, we propose a mechanism by which CD56 helps to localize or retain Pyk2 at sites of potential activation, where it could be available for subsequent autophosphorylation and activation of downstream signaling pathways required for polarization. Such a model has been proposed as the mechanism by which NCAM signals through FAK (Beggs et al., 1997). In addition, the well-defined requirement for activating microcluster formation in activation at the immune synapse is mediated by adhesion receptors, and integrin engagement promotes microcluster formation, mobility and signaling at the NK cell IS (Steblyanko et al., 2015). Inhibition of Pyk2 phosphorylation abrogates this effect, underscoring the importance of Pyk2 in amplifying activating signaling that is promoted by integrin engagement (Steblyanko et al., 2015). Here, we show another example of how an adhesion molecule may be playing a similar role; whether this is through direct binding of Pyk2 to CD56 through Fyn, or through indirect mechanisms, such as regulation of integrin spatial localization, remains to be defined. There are other mechanisms that could be at play, including the previously described role for NCAM in regulating lipid raft inclusion and exclusion of signaling receptors (Niethammer et al., 2002). Finally, given the significant degree of polysialation of CD56 on NK cells, particularly NK92 cells, the steric hindrance and negative charge of polysialated CD56 may be modulating membrane dynamics by physically influencing the organization and activity of integrins and activating receptors (Paszek et al., 2014). While the mechanisms of the regulation of this signaling remain to be elucidated, our data speak to CD56 being an important player in the orchestration of adhesion and cellular signaling at the NK cell membrane. Delineating the role that CD56 is playing in directly mediating signaling, as opposed to spatially modulating other players such as integrins and activating receptors, represents an exciting direction for NK cell biology.

While demonstrating a requirement for CD56 in cytotoxicity mediated by NK92 cells, we found that primary NK cells mediated target cell killing when CD56 was deleted from either total NK cells or specifically from the CD56^bright subset. Given the challenges inherent in manipulating gene expression in primary cells, we were reliant upon the use of cytokines, in this case IL-15, to promote NK cell survival and expansion following gene editing. It remains unclear as to whether this expansion bypasses a requirement for CD56 function in primary NK cells, although we do demonstrate that IL-15 stimulation of ex vivo NK cells significantly reduces reliance on Pyk2 Y402 phosphorylation for NK cell lytic function based upon the effect of a Pyk2 inhibitor. Despite the apparent reduced dependence on Pyk2 phosphorylation in IL-15 activated primary cells and the robust phosphorylation of Pyk2 Y402 in NK92 cells, activation of the NK92 cell line with IL-15 failed to rescue the defect in cytotoxicity in NK92 CD56-KO cells. This, combined with the observation that Pyk2 inhibition does not abrogate NK92 lytic function to the same extent as CD56 deficiency, suggests that there may be other mechanisms by which CD56 is exerting function in human NK cells. Ultimately, we show that despite the requirement for CD56 function in cell lines, ex vivo human NK cells are independent of this requirement, a finding that has relevance for clinical applications of NK cell-mediated adoptive therapies.

Consistent with previous reports (Lanier et al., 1991), our data demonstrate that the transmembrane-containing 140 kDa NCAM (CD56) isoform is primarily expressed on human NK cells. Western blot analyses demonstrated a molecular weight range of ~130–160 kDa, which was significantly decreased following treatment of NK cell lines with PNGase F to remove polysialic acid. While these experiments, which led to the generation of a band that appeared close to 120 kDa, suggested that NCAM-120 could be expressed intracellularly (Drake et al., 2008), treatment with PI-PLC, which we demonstrated cleaved GPI-anchored proteins, did not affect the detection of CD56 on the cell surface. The single band that we detected was also consistent with the predicted weight of the NCAM140 isoform containing 858 amino acids (Lanier et al., 1989). Furthermore, expression of the extracellular domain (ΔICD) or the intracellular domain (ΔECD) of CD56 failed to restore cytotoxic function to the NK92 CD56-KO, whereas expression of full-length 140 kDa NCAM was sufficient. These experiments also concur with previous evaluations of transcript level expression that also describe the predominant isoform in human PBMCs as the 140 kDa isoform (Lanier et al., 1989). While other reports have suggested that NCAM120 is the primarily expressed isoform on freshly isolated NK cells based upon qPCR (Van Acker et al., 2019), the use of Western blotting in our study allows us to directly assess protein expression. The degree of polysialation in human NK cells is notable, however, particularly in NK92 cells and primary NK cells, and this high level of polysialation in human NK cells suggests that polysialic acid itself may be playing a role in CD56 function through its steric properties.

In summary, here we describe a novel, functional role for CD56, the canonical identifier of human NK cells. We build upon previous studies describing a role for CD56 in NK cell cytotoxicity and those that define its role in cell migration and NK cell maturation to elucidate, for the first time, its role in the lysis of CD56-negative targets. In doing so we describe an unexpected role for CD56 in the function of commonly used NK cell lines, including NK92 cells, but demonstrate that primary NK cells can execute cytotoxicity independently of CD56. Finally, we link the cytotoxicity defect in CD56-KO NK92 cells to impaired signaling through the non-receptor tyrosine kinase Pyk2, and thus define novel roles for CD56 and Pyk2 in human NK cell cytotoxic function.

Additional details on key reagents and resources are included in the key resources table (Supplementary file 1).

All data generated or analysed during this study are included in the manuscript and supporting files.

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Author details

1. Justin T Gunesch

   Baylor College of Medicine, Houston, United States

   Present address

   Immatics, Houston, United States

   Contribution

   Data curation, Formal analysis, Investigation, Writing - original draft, Writing - review and editing

   Competing interests

   No competing interests declared

2. Amera L Dixon

   1. Baylor College of Medicine, Houston, United States
   2. Rice University, Houston, United States
   3. Department of Pediatrics, Vagelos College of Physicians and Surgeons, Columbia University Irving Medical Center, New York, United States

   Contribution

   Formal analysis, Investigation, Writing - original draft, Writing - review and editing

   Competing interests

   No competing interests declared

3. Tasneem AM Ebrahim

   1. Department of Pediatrics, Vagelos College of Physicians and Surgeons, Columbia University Irving Medical Center, New York, United States
   2. Barnard College, New York, United States

   Contribution

   Formal analysis, Investigation, Writing - original draft, Writing - review and editing

   Competing interests

   No competing interests declared

4. Melissa M Berrien-Elliott

   Washington University School of Medicine, St. Louis, United States

   Contribution

   Formal analysis, Investigation, Writing - original draft, Writing - review and editing

   Competing interests

   No competing interests declared

5. Swetha Tatineni

   Rice University, Houston, United States

   Present address

   University of Chicago Pritzker School of Medicine, Chicago, United States

   Contribution

   Formal analysis, Investigation, Writing - original draft, Writing - review and editing

   Competing interests

   No competing interests declared

6. Tejas Kumar

   Rice University, Houston, United States

   Present address

   Baylor College of Medicine, Houston, United States

   Contribution

   Formal analysis, Investigation

   Competing interests

   No competing interests declared

7. Everardo Hegewisch-Solloa

   Department of Pediatrics, Vagelos College of Physicians and Surgeons, Columbia University Irving Medical Center, New York, United States

   Contribution

   Formal analysis, Investigation, Writing - original draft, Writing - review and editing

   Competing interests

   No competing interests declared

8. Todd A Fehniger

   Washington University School of Medicine, St. Louis, United States

   Contribution

   Conceptualization, Supervision, Writing - original draft, Writing - review and editing

   Competing interests

   No competing interests declared

   "This ORCID iD identifies the author of this article:" 0000-0002-8705-2887

9. Emily M Mace

   Department of Pediatrics, Vagelos College of Physicians and Surgeons, Columbia University Irving Medical Center, New York, United States

   Contribution

   Conceptualization, Formal analysis, Supervision, Funding acquisition, Investigation, Writing - original draft, Writing - review and editing

   For correspondence

   em3375@cumc.columbia.edu

   Competing interests

   No competing interests declared

   "This ORCID iD identifies the author of this article:" 0000-0003-0226-7393

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