It is often difficult to decouple the relative importance of different factors in regulating MSC differentiation. Genetically modified mice provide model systems whereby some variables can be manipulated while others are kept constant. Fracture repair in thrombospondin-2 (TSP2)-null mice is characterized by reduced endochondral ossification and enhanced intramembranous bone formation. The proposed mechanism for this shift in MSC fate is that increased vascular density and hence oxygen availability in TSP2-null mice regulates differentiation. However, TSP2 is multifunctional and regulates other aspects of the regenerative cascade, such as MSC proliferation. The objective of this study is to use a previously developed computational model of tissue differentiation, in which substrate stiffness and oxygen tension regulate stem cell differentiation, to simulate potential mechanisms which may drive alterations in MSC fate in TSP2-null mice. Four models (increased cell proliferation, increased numbers of MSCs in the marrow decreased cellular oxygen consumption, and an initially stiffer callus) were not predictive of experimental observations in TSP2-null mice. In contrast, increasing the rate of angiogenic progression led to a prediction of greater intramembranous ossification, diminished endochondral ossification, and a reduced region of hypoxia in the fracture callus similar to that quantified experimentally by the immunohistochemical detection of pimonidazole adducts that develop with hypoxia. This study therefore provides further support for the hypothesis that oxygen availability during early fracture healing is a key regulator of MSC bipotential differentiation, and furthermore, it highlights the advantages of integrating computational models with genetically modified mouse studies for further elucidating mechanisms regulating stem cell fate. ß

Thrombospondin-2 (TSP2) is a matricellular protein that is highly up-regulated during fracture healing. TSP2 negatively regulates vascularity, vascular reperfusion following ischemia, and cutaneous wound healing. As well, TSP2-null mice show increased endocortical bone formation due to an enhanced number of mesenchymal progenitor cells and show increased cortical thickness. Mice deficient in TSP2 (TSP2-null) show an alteration in fracture healing, that is unrelated to their cortical bone phenotype, which is characterized by enhanced vascularization with a shift towards an intramembranous healing phenotype; thus, we hypothesized that there would be enhanced ischemic fracture healing in the absence of TSP2. We investigated whether an absence of TSP2 would enhance ischemic fracture healing utilizing Laser doppler, mCT and histological analysis. Ischemic tibial fractures were created in wildtype (WT) and TSP2-null mice and harvested 10, 20, or 40 days post-fracture. TSP2-null mice show enhanced vascular perfusion following ischemic fracture. At day 10 post-fracture, TSP2-null mice have 115% greater bone volume than WT mice. This is associated with a 122% increase in vessel density, 20% increase in cell proliferation, and 15% decrease in apoptosis compared to WT. At day 20, TSP2-null mice have 34% more bone volume, 51% greater bone volume fraction, and 37% more bone tissue mineral density than WT. By 40 days after fracture the TSP2-null mice have a 24% increase in bone volume fraction, but other parameters show no significant differences. These findings indicate TSP2 is a negative regulator of ischemic fracture healing and that in the absence of TSP2 bone regeneration is enhanced. Keywords: thrombospondin-2; ischemia; bone fracture healing Blood supply during fracture healing is crucial for proper bone repair. Vasodilation following fracture promotes edema and inflammation at the site of injury, and contributes to the development of the fracture hematoma. Inflammatory cells that occupy the hematoma produce cytokines and growth factors that then drive neovascularization which is essential for the next phases of repair. Intact vessels are required for new vessel formation to occur during the normal healing process.1,2 However, with a fracture injury there is almost always some disruption of established blood vessels. Thus, fractures with severely disrupted vascularization (ischemia) present a complex medical challenge. Up to 46% of patients with impaired vascular flow have complications during fracture repair, leading to delayed healing or nonunion that necessitates multiple surgeries that typically include staged bone grafting or possibly even amputation. [3][4][5] An important, yet largely untapped, therapeutic strategy to improve ischemic bone regeneration is to enhance vascularization at the fracture site. This can be achieved by either activating angiogenic pathways or by blocking angiogenesis inhibitors. Blocking angiogenic inhibitors has received relatively little attention as a therapeutic target in f...

Facility-wide Corynebacterium bovis eradication was established using vaporized hydrogen peroxide (VHP) decontamination guided by C. bovis PCR surveillance. Prior attempts limited to culling PCR-positive mice and decontaminating affected rooms were ineffective in preventing recurrence. Because research aims often require trafficking to and use of procedural cores, a 12-mo facility-wide C. bovis PCR surveillance of 2064 specimens was performed and documented that, despite the presence of few clinically hyperkeratotic mice, 35% of the murine housing and use space was contaminated by C. bovis. The airways of IVC racks and air-handling units (AHU) provided a substantive niche for C. bovis survival, comparable to the primary enclosure, with 26% of murine and 22% of airway specimens PCR-positive for C. bovis. Equipment airway VHP sterilization in a 'flex room' required an 'active-closed' setting with the IVC rack connected to the AHU set to the VHP cycle, because 12% of specimens from 'static-open' VHP-exposed airways remained PCR-positive for C. bovis, whereas 0% of specimens from active-closed VHP exposures were positive. VHP decontamination of the 29,931-ft2 facility was completed in 2 mo. C. bovis PCR testing of IVC exhaust plenums for 200 d in previously C. bovis-affected rooms confirmed that none of the 259 specimens tested were PCR-positive for the organism. Monthly surveillance identified a single recurrence during June 2017 (month 9), ensuring rapid culling of C. bovis PCR-positive mice and acute VHP decontamination of equipment and rooms. Molecular persistence of C. bovis was resolved in procedural and personnel areas, and no murine or housing specimens tested C. bovis PCR-positive during study months 11 and 12. Furthermore, since the conclusion of the 12-mo study, none of the 452 additional murine, cell biologic, environmental, and monthly equipment surveillance specimens tested were C. bovis PCR-positive, documenting an 11-mo period of facility-wide C. bovis eradication to date. Study invalidation due to C. bovis can be avoided through PCR surveillance for the organism, immediate culling of PCR-positive mice, and acute VHP decontamination of affected areas.

Type III collagen (Col3) has been proposed to play a key role in tissue repair based upon its temporospatial expression during the healing process of many tissues, including bone. Given our previous finding that Col3 regulates the quality of cutaneous repair, as well as our recent data supporting its role in regulating osteoblast differentiation and trabecular bone quantity, we hypothesized that mice with diminished Col3 expression would exhibit altered long-bone fracture healing. To determine the role of Col3 in bone repair, young adult wild-type (Col3+/+) and haploinsufficent (Col3 +/−) mice underwent bilateral tibial fractures. Healing was assessed 7, 14, 21, and 28 days following fracture utilizing microcomputed tomography (microCT), immunohistochemistry and histomorphometry. MicroCT analysis revealed a small but significant increase in bone volume fraction in Col3+/− mice at day 21. However, histological analysis revealed that Col3+/− mice have less bone within the callus at days 21 and 28, which is consistent with the established role for Col3 in osteogenesis. Finally, a reduction in fracture callus osteoclastic activity in Col3+/− mice suggests Col3 also modulates callus remodeling. Although Col3 haploinsufficiency affected biological aspects of bone repair, it did not affect the regain of mechanical function in the young mice that were evaluated in this study. These findings provide evidence for a modulatory role for Col3 in fracture repair and support further investigations into its role in impaired bone healing.

Exposing immunodeficient mice to opportunistic microbes introduces risks of data variability, morbidity, mortality, and the invalidation of studies involving unique human reagents, including the loss of primary human hematopoietic cells, patient-derived xenografts, and experimental therapeutics. The prevalence of 15 opportunistic microbes in a murine research facility was determined by yearlong PCR-based murine and IVC equipment surveillance comprising 1738 specimens. Of the 8 microbes detected, 3 organisms-Staphylococcus xylosus, Proteus mirabilis, and Pasteurella pneumotropica biotype Heyl-were most prevalent in both murine and IVC exhaust plenum specimens. Overall, the 8 detectable microbes were more readily PCR-detectable in IVC exhaust airways than in murine specimens, supporting the utility of PCR testing of IVC exhaust airways as a component of immunodeficient murine health surveillance. Vaporized hydrogen peroxide (VHP) exposure of IVC equipment left unassembled (that is, in a 'static-open' configuration) did not eliminate PCR detectable evidence of microbes. In contrast, VHP exposure of IVC equipment assembled 'active-closed' eliminated PCR-detectable evidence of all microbes. Ensuring data integrity and maintaining a topographically complex immunodeficient murine research environment is facilitated by knowing the prevalent opportunistic microbes to be monitored and by implementing a PCR-validated method of facility decontamination that mitigates opportunistic microbes and the risk of invalidation of studies involving immunodeficient mice.Abbreviations: AHU, air handling unit; BI, biological indicator; CI, chemical indicator; VHP, vaporized hydrogen peroxide

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Modeling chronic myelomonocytic leukemia (CMML) in immunodeficient NSGS mice relies on unique human CMML specimens and consistent murine engraftment. Only anecdotal comments have thus far supported the notion that research data may be altered by Corynebacterium bovis, an opportunistic cutaneous pathogen of immunodeficient mice. C. bovis disseminated by asymptomatic and clinically affected mice with hyperkeratotic dermatitis, resulting in resilient facility contamination and infectious recurrence. Herein we report that, compared with C. bovis PCR-negative counterparts, C. bovis PCR-positive NSGS mice developed periocular and facial hyperkeratosis and alopecia and had reduced metrics indicative of ineffective human CMML engraftment, including less thrombocytopenia, less splenomegaly, fewer CMML infiltrates in histopathologic sections of murine organs, and fewer human CD45 + cells in samples from murine spleen, bone marrow, and peripheral blood that were analyzed by flow cytometry. All CMML model metrics of engraftment were significantly reduced in the C. bovis PCR-positive cohort compared with the -negative cohort. In addition, a survey of comprehensive cancer center practices revealed that most murine facilities do not routinely test for C. bovis or broadly decontaminate the facility or its equipment after a C. bovis outbreak, thus increasing the likelihood of recurrence of invalidated studies. Our findings document that CMML engraftment of NSGS mice is diminished-and the integrity of murine research data jeopardized-by C. bovis infection of immunodeficient mice. In addition, our results indicate that C. bovis should be excluded from and not tolerated in murine facilities housing immunodeficient strains.

During a previously reported program-wide Corynebacterium bovis outbreak, both immunocompetent depilated ( dep/dep) mutant mice and transgenic mice that express the papillomavirus E6 oncoprotein became persistently infected with C. bovis. An orthokeratotic, hyperkeratotic, acanthotic dermatitis developed in the C. bovis–infected dep/dep mice, which remained C. bovis PCR-positive for >45 days prior to euthanasia as part of the program-wide C. bovis eradication effort. Since both affected strains of mice have altered skin homeostasis, immune status or the presence of hair may not alone be sufficient to explain strain susceptibility to C. bovis–related cutaneous disease. In order to avoid invalidation of preclinical studies due to C. bovis infection, it may be necessary to isolate immunodeficient mouse strains, implement facililty-wide surveillance for C. bovis, and sterilize equipment with vaporized hydrogen peroxide.