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Hematopoietic stem cells (HSCs) are the apical cells of the hematopoietic system, giving rise to cells of the blood and lymph lineages. HSCs reside primarily within bone marrow niches that contain matrix and cell‐derived signals that help inform stem cell fate. Aspects of the bone marrow microenvironment have been captured in vitro by encapsulating cells within hydrogel matrices that mimic native mechanical and biochemical properties. Hydrogel microparticles, or microgels, are increasingly being used to assemble granular biomaterials for cell culture and noninvasive delivery applications. Here, we report the optimization of a gelatin maleimide hydrogel system to create monodisperse gelatin microgels via a flow‐focusing microfluidic process. We report characteristic hydrogel stiffness, stability, and swelling characteristics as well as encapsulation of murine hematopoietic stem and progenitor cells, and mesenchymal stem cells within microgels. Microgels support cell viability, confirming compatibility of the microfluidic encapsulation process with these sensitive bone marrow cell populations. Overall, this work presents a microgel‐based gelatin maleimide hydrogel as a foundation for future development of a multicellular artificial bone marrow culture system. [ABSTRACT FROM AUTHOR]

Biomaterial platforms are an integral part of stem cell biomanufacturing protocols. The collective biophysical, biochemical, and cellular cues of the stem cell niche microenvironment play an important role in regulating stem cell fate decisions. Three-dimensional (3D) culture of stem cells within biomaterials provides a route to present biophysical and biochemical stimuli through cell-matrix interactions and cell-cell interactions via secreted biomolecules. Herein, we describe a maleimide-functionalized gelatin (GelMAL) hydrogel that can be crosslinked via thiol-Michael addition click reaction for the encapsulation of sensitive stem cell populations. The maleimide functional units along the gelatin backbone enables gelation via the addition of a dithiol crosslinker without requiring external stimuli (e.g. , UV light, photoinitiator), thereby reducing reactive oxide species generation. Additionally, the versatility of crosslinker selection enables easy insertion of thiol-containing bioactive or bioinert motifs. Hematopoietic stem cells (HSCs) and mesenchymal stem cells (MSCs) were encapsulated in GelMAL, with mechanical properties tuned to mimic the in vivo bone marrow niche. We report the insertion of a cleavable peptide crosslinker that can be degraded by the proteolytic action of Sortase A, a mammalian-inert enzyme. Notably, Sortase A exposure preserves stem cell surface markers, which are an essential metric of hematopoietic activity used in immunophenotyping. This novel GelMAL system enables a route to produce artificial stem cell niches with tunable biophysical properties, intrinsic cell-interaction motifs, and orthogonal addition of bioactive crosslinks. We describe a maleimide-functionalized gelatin hydrogel that can be crosslinked via a thiol-maleimide mediated click reaction to form a stable hydrogel without the production of reactive oxygen species typical in light-based crosslinking. The mechanical properties can be tuned to match the in vivo bone marrow microenvironment for hematopoietic stem cell culture. Additionally, we report inclusion of a peptide crosslinker that can be cleaved via the proteolytic action of Sortase A and show that Sortase A exposure does not degrade sensitive surface marker expression patterns. Together, this approach reduces stem cell exposure to reactive oxygen species during hydrogel gelation and enables post-culture quantitative assessment of stem cell phenotype. [Display omitted] [ABSTRACT FROM AUTHOR]

Hematopoietic stem cells (HSCs) primarily reside in the bone marrow, where they receive external cues from their local microenvironment. The complex milieu of biophysical cues, cellular components and cell-secreted factors regulates the process by which HSC produce the blood and immune system. We previously showed direct coculture of primary murine hematopoietic stem and progenitor cells with a population of marrow-derived mesenchymal stromal and progenitor cells (MSPCs) in a methacrylamide-functionalized gelatin (GelMA) hydrogel improves hematopoietic progenitor maintenance. However, the mechanism by which MSPCs influenced HSC fate decisions remained unknown. Herein, we report the use of proteomic analysis to correlate HSC phenotype to a broad candidate pool of 200 soluble factors produced by combined mesenchymal and hematopoietic progeny. Partial least squares regression (PLSR), along with an iterative filter method, identified TGFβ-1, MMP-3, c-RP and TROY as positively correlated with HSC maintenance. Experimentally, we then observe exogenous stimulation of HSC monocultures in GelMA hydrogels with these combined cytokines increases the ratio of hematopoietic progenitors to committed progeny after a 7-day culture 7.52 ± 3.65-fold compared to non-stimulated monocultures. Findings suggest a cocktail of the downselected cytokines amplifies hematopoietic maintenance potential of HSCs beyond that of MSPC-secreted factors alone. This work integrates empirical and computation methods to identify cytokine combinations to improve HSC maintenance within an engineered HSC niche, suggesting a route toward identifying feeder-free culture platforms for HSC expansion. Insight Hematopoietic stem cells within an artificial niche receive maintenance cues in the form of soluble factors from hematopoietic and mesenchymal progeny. Applying a proteomic regression analysis, we identify a reduced set of soluble factors correlated to maintenance of a hematopoietic phenotype during culture in a biomaterial model of the bone marrow niche. We identify a minimum factor cocktail that promotes hematopoietic maintenance potential in a gelatin-based culture, regardless of the presence of mesenchymal feeder cells. By combining empirical and computational methods, we report an experimentally feasible number of factors from a large dataset, enabling exogenous integration of soluble factors into an engineered hematopoietic stem cell for enhanced maintenance potential of a quiescent stem cell population. [ABSTRACT FROM AUTHOR]