Summary

Bone Marrow Transplantation Procedures in Mice to Study Clonal Hematopoiesis

Published: May 26, 2021
doi:

Summary

We describe three methods of bone marrow transplantation (BMT): BMT with total-body irradiation, BMT with shielded irradiation, and BMT method with no pre-conditioning (adoptive BMT) for the study of clonal hematopoiesis in mouse models.

Abstract

Clonal hematopoiesis is a prevalent age-associated condition that results from the accumulation of somatic mutations in hematopoietic stem and progenitor cells (HSPCs). Mutations in driver genes, that confer cellular fitness, can lead to the development of expanding HSPC clones that increasingly give rise to progeny leukocytes harboring the somatic mutation. Because clonal hematopoiesis has been associated with heart disease, stroke, and mortality, the development of experimental systems that model these processes is key to understanding the mechanisms that underly this new risk factor. Bone marrow transplantation procedures involving myeloablative conditioning in mice, such as total-body irradiation (TBI), are commonly employed to study the role of immune cells in cardiovascular diseases. However, simultaneous damage to the bone marrow niche and other sites of interest, such as the heart and brain, is unavoidable with these procedures. Thus, our lab has developed two alternative methods to minimize or avoid possible side effects caused by TBI: 1) bone marrow transplantation with irradiation shielding and 2) adoptive BMT to non-conditioned mice. In shielded organs, the local environment is preserved allowing for the analysis of clonal hematopoiesis while the function of resident immune cells is unperturbed. In contrast, the adoptive BMT to non-conditioned mice has the additional advantage that both the local environments of the organs and the hematopoietic niche are preserved. Here, we compare three different hematopoietic cell reconstitution approaches and discuss their strengths and limitations for studies of clonal hematopoiesis in cardiovascular disease.

Introduction

Clonal hematopoiesis (CH) is a condition which is frequently observed in elderly individuals and occurs as a result of an expanded hematopoietic stem and progenitor cell (HSPC) clone carrying a genetic mutation1. It has been suggested that by the age of 50, most individuals will have acquired an average of five exonic mutations in each HSPC2, but most of these mutations will result in little or no phenotypic consequences to the individual. However, if by chance one of these mutations confers a competitive advantage to the HSPC—such as by promoting it’s proliferation, self-renewal, survival, or some combination of these—this may lead to the preferential expansion of the mutant clone relative to the other HSPCs. As a result, the mutation will increasingly spread through the hematopoietic system as the mutated HSPC gives rise to mature blood cells, leading to a distinct population of mutated cells within the peripheral blood. While mutations in dozens of different candidate driver genes have been associated with clonal events within the hematopoietic system, among these, mutations in DNA methyltransferase 3 alpha (DNMT3A) and ten eleven translocation 2 (TET2) are the most prevalent3. Several epidemiological studies have found that individuals who carry these genetic mutations have a significantly higher risk of cardiovascular disease (CVD), stroke, and all-causal mortality3,4,5,6,7. While these studies have identified that an association exists between CH and increased incidence of CVD and stroke, we do not know whether this relationship is causal or a shared epiphenomenon with the aging process. To gain a better understanding of this association, proper animal models that correctly recapitulate the human condition of CH are required.

Several CH animal models have been established by our group and others using zebrafish, mice, and non-human primates8,9,10,11,12,13,14. These models often use hematopoietic reconstitution methods by transplantation of genetically modified cells, sometimes using Cre-lox recombination or the CRISPR system. This approach allows for the analysis of a specific gene mutation in hematopoietic cells to assess how it contributes to disease development. In addition, these models often employ congenic or reporter cells to distinguish the effects of mutant cells from normal or wild-type cells. In many cases, a pre-conditioning regimen is required to successfully engraft donor hematopoietic stem cells.

Currently, the transplantation of bone marrow to recipient mice can be divided into two main categories: 1) myeloablative conditioning and 2) non-conditioned transplantation. Myeloablative conditioning can be achieved by one of two methods, namely, total body irradiation (TBI) or chemotherapy15. TBI is carried out by subjecting the recipient to a lethal dose of gamma or X-ray irradiation, generating DNA breaks or cross-links within rapidly dividing cells, rendering them irreparable16. Busulfan and cyclophosphamide are two commonly used chemotherapy drugs that disrupt the hematopoietic niche and similarly cause DNA damage to rapidly dividing cells. The net result of myeloablative preconditioning is apoptosis of hematopoietic cells, which destroys the recipient’s hematopoietic system. This strategy not only allows for the successful engraftment of the donor HSPCs, but can also prevent graft rejection by suppressing the recipient’s immune system. However, myeloablative preconditioning has severe side effects such as damage to tissues and organs and their resident immune cells as well as destruction of the native bone marrow niche17. Therefore, alternative methods have been proposed to overcome these undesirable side effects, particularly in regard to damage to the organs of interest. These methods include shielded irradiation of recipient mice and the adoptive BMT to non-conditioned mice9,17. Shielding the thorax, abdominal cavity, head or other regions from irradiation by the placement of a lead barriers keeps tissues of interest protected from the damaging effects of irradiation and maintains their resident immune cell population. On the other hand, the adoptive BMT of HSPCs to non-conditioned mice has an additional advantage because it preserves the native hematopoietic niche. In this manuscript, we describe the protocols and results of HSPC engraftment after several transplantation regimens in mice, specifically the delivery of HSPC to TBI mice, to mice partially shielded from irradiation, and to non-conditioned mice. The overall goal is to help researchers understand the different physiological effects of each method as well as how they affect experimental outcomes in the setting of CH and cardiovascular disease.

Protocol

All procedures involving animal subjects have been approved by the Institutional Animal Care and Use Committee (IACUC) at the University of Virginia. 1. Prior to preconditioning Place the recipient mice on antibiotic-supplemented water (5 mM sulfamethoxazole, 0.86 mM trimethoprim) ~24 h prior to irradiation. This is necessary to prevent infection, as the immune system will be suppressed following irradiation, and maintained for 2 weeks following irradiation. At this point, suppleme…

Representative Results

To compare the effect of three BMT/pre-conditioning methods on donor cell engraftment, the fractions of donor cells in peripheral blood and heart tissue were analyzed by flow cytometry at 1-month post-BMT. Isolated cells were stained for specific leukocyte markers to identify the different subsets of leukocytes. In these experiments, wild-type (WT) C57BL/6 (CD45.2) donor bone marrow cells were delivered to WT B6.SJL-PtprcaPrpcb/BoyJ (CD45.1) reci…

Discussion

For studies of clonal hematopoiesis, we described three methods of BMT: BMT with total-body irradiation, BMT with irradiation with partial shielding, and a less commonly used BMT method that involves no pre-conditioning (adoptive BMT). These methods have been used to assess the impact of clonal hematopoiesis on cardiovascular disease. Researchers can modify these methods accordingly to suit the specific purpose of their study.

Clonal hematopoiesis models
Clonal hematopoi…

Disclosures

The authors have nothing to disclose.

Acknowledgements

This work was supported by US National Institutes of Health grants to K. Walsh (HL131006, HL138014, and HL132564), to S. Sano (HL152174), American Heart Association grant to M. A. Evans (20POST35210098), and a Japan Heart Foundation grant to H. Ogawa.

Materials

0.5ml microcentrifuge Fisher Scientific 05-408-121 general supply
1.5ml microcentrifuge Fisher Scientific 05-408-129 general supply
1/2 cc LO-DOSE INSULIN SYRINGE EXELINT 26028 general supply
Absolute Ethanol (200 prfof) Fisher chemical 200559 general supply
BD 1mL Tuberculin Syringes 25G 5/8 Inch Needle Becton Dickinson 309626 general supply
BD PrecisionGlide Needle 18G (1.22mm X 25mm) Becton Dickinson 395195 general supply
Cesium-137 Irradiator J. L. Shepherd  Mark IV equipment
DietGel 76A Clear H2O 70-01-5022 general supply
Falcon 100 mm TC-Treated Cell Culture Dish Life Sciences 353003 general supply
Falcon 50 mL Conical Centrifuge Tubes Fisher Scientific 352098 general supply
Fisherbrand sterile cell strainers, 70 μm Fisher Scientific 22363548 general supply
Graefe Forceps Fine Science Tools 11051-10 general supply
Hardened Fine Scissors Fine Science Tools 14090-09 general supply
Isothesia (Isoflurane) solution Henry Schein 29404 Solution
Ketamine Zoetis 043-304 injection
Kimwipes Delicate Task Wipers Kimtech Science KCC34155 general supply
PBS pH7.4 (1X) Gibco 10010023 Solution
RadDisk – Rodent Irradiator Disk Braintree Scientific IRD-P M general supply
RPMI Medium 1640 (1X) Gibco 11875-093 Medium
Sulfamethoxazole and Trimethoprim TEVA 0703-9526-01 injection
Xylazine Akorn 139-236 injection
X-ray irradiator Rad source RS-2000 equipment

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Cite This Article
Park, E., Evans, M. A., Doviak, H., Horitani, K., Ogawa, H., Yura, Y., Wang, Y., Sano, S., Walsh, K. Bone Marrow Transplantation Procedures in Mice to Study Clonal Hematopoiesis. J. Vis. Exp. (171), e61875, doi:10.3791/61875 (2021).

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