The immune reaction to heterologous serum causes osteonecrosis in rabbits

Development of a mouse model of ischemic osteonecrosis

BACKGROUND

Availability of a reliable mouse model of ischemic osteonecrosis could accelerate the development of novel therapeutic strategies to stimulate bone healing after ischemic osteonecrosis; however, no mouse model of ischemic osteonecrosis is currently available.

QUESTIONS/PURPOSES

To develop a surgical mouse model of ischemic osteonecrosis, we asked, (1) if the blood vessels that contribute to the blood supply of the distal femoral epiphysis are cauterized, can we generate an osteonecrosis mouse model; (2) what are the histologic changes observed in this mouse model, and (3) what are the morphologic changes in the model.

METHODS

We performed microangiography to identify blood vessels supplying the distal femoral epiphysis in mice, and four vessels were cauterized using microsurgical techniques to induce ischemic osteonecrosis. Histologic assessment of cell death in the trabecular bone was performed using terminal deoxynucleotidyl transferase mediated dUTP nick-end labeling (TUNEL) and counting the empty lacunae in three serial sections. Quantitation of osteoclast and osteoblast numbers was performed using image analysis software. Morphologic assessments of the distal femoral epiphysis for deformity and for trabecular bone parameters were performed using micro-CT.

RESULTS

We identified four blood vessels about the knee that had to be cauterized to induce total ischemic osteonecrosis of the distal femoral epiphysis. Qualitative assessment of histologic sections of the epiphysis showed a loss of nuclear staining of marrow cells, disorganized marrow structure, and necrotic blood vessels at 1 week. By 2 weeks, vascular tissue invasion of the necrotic marrow space was observed with a progressive increase in infiltration of the necrotic marrow space with the vascular tissue at 4 and 6 weeks. TUNEL staining showed extensive cell death in the marrow and trabecular bone 24 hours after the induction of ischemia. The mean percent of TUNEL-positive osteocytes in the trabecular bone increased from 2% ± 1% in the control group to a peak of 98% ± 3% in the ischemic group 1 week after induction of ischemia (mean difference, 96%; 95% CI, 81%-111%; p < 0.0001). The mean percent of empty lacunae increased from 1% ± 1% in the control group to a peak of 78% ± 15% in the ischemic group at 4 weeks (mean difference, 77%; 95% CI, 56%-97%; p < 0.0001). Quantitative analysis showed that the mean number of osteoclasts per bone surface was decreased in the ischemic group at 1, 2, and 4 weeks (p < 0.0001, < 0.0001, and p = 0.02, respectively) compared with the control group. The mean number of osteoclasts increased to a level similar to that of the control group at 6 weeks (p = 0.23). The numbers of osteoblasts per bone surface were decreased in the ischemic group at 1, 2 and 4 weeks (p < 0.0001 for each) compared with the numbers in the control group. The mean number of osteoblasts also increased to a level similar to that of the control group at 6 weeks (p = 0.91). Mean bone volume percent assessed by micro-CT was lower in the ischemic group compared with the control group from 2 to 6 weeks. The mean differences in the percent bone volume between the control and ischemic groups at 2, 4, and 6 weeks were 5.5% (95% CI, 0.9%-10.2%; p = 0.01), 5.3% (95% CI, 0.6%-9.9%; p = 0.02), and 6.0% (95% CI, 1.1%-10.9%; p = 0.008), respectively. A deformity of the distal femoral epiphysis was observed at 6 weeks with the mean epiphyseal height to width ratio of 0.74 ± 0.03 in the control group compared with 0.66 ± 0.06 in the ischemic group (mean difference, 0.08; 95% CI, 0.00-0.16; p = 0.03).

CONCLUSION

We developed a novel mouse model of ischemic osteonecrosis that produced extensive cell death in the distal femoral epiphysis which developed a deformity with time.

CLINICAL RELEVANCE

The new mouse model may be a useful tool to test potential therapeutic strategies to improve bone healing after ischemic osteonecrosis.

Different magnetic resonance imaging features in two types of nontraumatic rabbit osteonecrosis models

A single injection of high-dose steroid (20 mg/kg) has been reported to induce necrotic lesions in the proximal metaphysis and diaphysis of the rabbit femur. In the rabbit osteonecrosis (ON) model induced by two-dose horse serum injections, contrast-enhanced magnetic resonance imaging (MRI) and T2*-weighted dynamic MRI have been reported to detect necrotic lesions at 3 days after the second serum injection sensitively. The purpose of the present study was to determine whether contrast-enhanced MRI and T2*-weighted dynamic MRI could detect early development of necrotic lesions in the rabbit proximal femora after a single high-dose steroid injection and compare MRI features of the two types of nontraumatic rabbit ON models. We performed nonenhanced MRI, contrast-enhanced MRI and T2*-weighted dynamic MRI of bilateral proximal femora 3 days (10 femora), 1 week (10 femora), 3 weeks (10 femora), 6 weeks (18 femora) and 9 weeks (18 femora) after a single 20 mg/kg steroid injection. Femoral signal intensity of each T2*-weighted dynamic MRI was measured from a 1-cm(2) region of interest in the proximal metaphysis and diaphysis. As a control, MRI was performed in untreated animals (six femora). Histologically, no necrotic lesions were observed in the proximal femora at 3 days and 1 week. Bone marrow necrosis was observed in four (40%) femora at 3 weeks, two (11.1%) femora at 6 weeks and six (33.3%) femora at 9 weeks. Bone marrow lesion completely replaced by granulation tissue was observed in one femur at 6 weeks and one femur at 9 weeks. Histologic evidence of repair tissue surrounding bone marrow necrosis was seen after 6 weeks. Average lesion area including repair tissue was 4.40 mm(2) (range, 0.32 to 20.2 mm(2)). At 9 weeks, contrast-enhanced MRI could detect four (66.7%) femora with bone marrow necrosis of more than 4 mm(2) in the lesion area, while T2*-weighted dynamic images showed a finding of complete ischemia in only one of these four femora. In conclusion, neither contrast-enhanced MRI nor T2*-weighted dynamic MRI could detect early development of necrotic lesions in the single-dose steroid ON model. These results indicated that development of necrotic lesions in the single-dose steroid ON model was not accompanied by as diffuse a femoral hemodynamic change as the two-dose horse serum ON model.

Potential involvement of p53 in ischemia/reperfusion-induced osteonecrosis

Idiopathic osteonecrosis of the femoral head (ION) is a devastating pathological condition of unknown etiology. In this study, we developed a simple murine model of osteonecrosis and investigated the underlying molecular mechanisms. In this model, the central portion of the tails of male C57BL/6 mice were tightly ligated to produce ischemic regions at sites distal to the ligatures. The occlusive ligatures were maintained for the indicated periods and then removed to induce reperfusion. The tails were histologically examined, and gene expression was analyzed by PCR array. The effect of p53 expression on osteocytes apoptosis was examined using preosteocytic MLO-A5 cells. In addition, the expression of p53 was analyzed in the femoral head samples obtained from hip osteoarthritis (OA) patients and ION patients. Caudal vertebrae distal to the ligatures (distal region) exhibited histological changes mimicking those observed in ION. Expression of p53 was increased in the distal region, and overexpression of p53 induced apoptosis in MLO-A5 cells. Treatment with a p53 inhibitor suppressed osteocyte apoptosis in the distal region. Strong p53 immunostaining was observed in osteocytes, vascular endothelial cells, and bone marrow cells in the femoral heads from ION patients but not from OA patients. Ischemia/reperfusion of the caudal vertebrae is a useful murine model of osteonecrosis, mimicking the histological changes found in ION. Using this model, we found the possible involvement of p53 in the osteocyte apoptosis observed in ION. Therapeutics targeting p53 might be a useful approach to ameliorating or even preventing osteonecrosis in ION patients.

Osteonecrosis of the femoral head of laboratory animals: the lessons learned from a comparative study of osteonecrosis in man and experimental animals

Animal models of osteonecrosis of the femoral head are indispensable to the understanding of successful treatment modalities for avascular necrosis of the femoral head in adults and in children with Legg-Calvé-Perthes disease. Many of these models adequately reflect the current "vascular deprivation" theory regarding the etiology of the disease. In addition to spontaneous occurrence, surgical- and corticosteroid-induced models are suitable, common experimental ones. Osteonecrosis of spontaneously hypertensive rats appears to be due to defective bone formation and compression of the arteries entering the femoral head at its lateral facets by daily weight-bearing loads. Successful modeling of surgical-induced femoral capital necrosis can be a challenge in animals with a dual epiphyseal blood supply. High doses of corticosteroids are a pivotal risk factor in the development of osteonecrosis. The pathogenesis of corticosteroid-induced osteonecrosis likely resides in reduced blood flow. Steroids may reduce blood flow by numerous mechanisms, including marrow adipocytic hypertrophy leading to sinusoidal compression, venous stasis and, eventually, obstruction of the arteries, and arterial occlusion by fat emboli and lipid-loaded fibrin-platelet thrombi. Other, less common varieties of osteonecrosis include those secondary to endotoxin-induced disseminated intravascular coagulation, immune reactions, immoderately low or high temperatures, and high-impact-related injuries. Common to these diverse forms of osteonecrosis are fibrin thrombi clogging arterioles and small arteries.

Involvement of platelet activation in experimental osteonecrosis in rabbits

Osteonecrosis (ON) was produced experimentally in rabbits by intravenous injection of platelet activating factor (PAF) in combination with lipopolysaccharides (LPS) on two occasions separated by a week-long interval. Eleven of 15 rabbits (73%), with administration of both LPS (50 microg/kg) and PAF (10 microg/kg), exhibited microcirculatory injuries including extravasation of erythrocytes into sinusoidal spaces and formation of microthrombi in arterioles near regions of erythrocyte extravasation. Seven of 15 rabbits (47%), which received both LPS (50 microg/kg) and PAF (10 microg/kg), exhibited necrosis of trabeculae and 8 of 15 (53%) exhibited bone marrow necrosis. In addition, PAF receptor antagonist (0.3 and 3.0 mg/kg) significantly reduced the incidence of trabecular necrosis (0%) in this model. The findings of the present study suggest that platelet activation may play an important role in inducible ON, and that suppression of platelet activation may contribute prevention of ON.

Hemorrhage and thrombus formation in early experimental osteonecrosis

The aim of the current study was to test the hypothesis that the induction of an underlying immunologic condition in rabbits may enhance the development of steroid-induced osteonecrosis. Thirty-five adult rabbits were divided into four groups. Group I: 10 rabbits were immunized at 15-day intervals for 2 months by murine antibodies to deoxyribonucleic acid autoantibodies. Four weeks after the end of the immunization, the animals received injections of methylprednisolone for 7 days and then prednisolone per os for 8 months. Group II: 10 animals only received immunizations according to the protocol used in Group I. Group III: 10 animals only were treated with corticosteroids according to the protocol used in Group I. Group IV: five animals were used as controls. Various changes were observed in the proximal metaphysis and diaphysis of the femur in eight of 10 animals in Group I (80%) and in five of 10 animals in Group II (50%) when compared with the animals in Group III and Group IV. The most common feature was evidence of new and old hemorrhage through the sinusoids, exudative reaction and thrombus formation in veins and small arteries. Focal necrotic areas of bone marrow showed an accumulation of cell debris, residue of hemorrhage, and disappearance of marrow elements. These findings suggest that (1) corticosteroids may potentiate the effects of a preexisting condition to increase the risk of osteonecrosis; (2) immunologic reaction may be an important factor in the pathogenesis of necrotic lesions; and (3) repeated intramedullary hemorrhage and thrombus formation may represent early major pathologic findings in bone necrosis.

Inducible osteonecrosis in a rabbit serum sickness model: deposition of immune complexes in bone marrow

We established inducible osteonecrosis in a rabbit serum sickness model. Osteonecrosis with marrow necrosis could be induced by the intravenous injection of horse serum in two doses separated in time by a period of three weeks. In this model, osteonecrosis could be successfully produced in rabbit femoral metaphysis. The incidence of marrow necrosis was 45% (9 of 20 rabbits) and trabecular necrosis occurred in 6 of 20 rabbits (30%) at 7 days after the second injection of the horse serum. In bone marrow of the femoral metaphysis, extravasation of erythrocytes and the formation of micro-thrombi in arterioles were often observed in an early stage of the present model and both findings correlate well each other (p = 0.0001). Immune complexes could be demonstrated using immunohistochemistry in bone marrow of the femoral metaphysis as well as in glomeruli of the kidney. Extravasation of erythrocytes in bone marrow of the femoral metaphysis was observed in 8 of 12 (67%) cases with immune complex deposition in the sinusoidal space of the femoral metaphysis and in 12 of 21 (57%) cases with immune complex deposition in glomeruli of the kidney. Immune complex deposition both in the sinusoidal space of femoral bone marrow (p = 0.0385) and in glomeruli of the kidney (p = 0.0209) closely related to extravasation of erythrocytes and microthrombi in arterioles in the early stage of this model. Early microcirculatory injury (extravasation of erythrocytes and microthrombi in arterioles) adjacent to osteonecrosis could be induced by immune complex deposition in femoral bone marrow and might be predictable characteristics for the inducible osteonecrosis in the present serum sickness model. The important findings in this study were that early microcirculatory injury was closely related to the deposition of immune complexes in femoral bone marrow, and that early microcirculatory injury associated with immune complex deposition was located close to osteonecrotic regions.