Ossified Particles in the Peripheral Circulation of Young and Old male Fischer‐344 Rats following Bone Marrow Ablation and Interleukin‐1 Receptor Antagonism

Abstract

BACKGROUND A pro‐inflammatory marrow microenvironment presumably augments ossification in bone marrow blood vessels. We speculate that ossifying blood vessels results in bone‐like particles (i.e., ossified particles, OSP) in the peripheral blood. By performing bone marrow ablation (BMA) and administering an interleukin‐1 receptor antagonist (IL‐1RA) during recovery, we sought to reduce the number of OSP in the circulation. METHODS Young (6‐mon) and old (24‐mon) male Fischer‐344 rats were assigned accordingly: Control (young CON, n=10; old CON, n=8), IL‐1RA (young, n=11; old, n=8), BMA (young, n=12; old, n=7) and combined treatment (young IL‐1RA+BMA, n=8; old IL‐1RA+BMA, n= 8). In the BMA and IL‐1RA+BMA rats, bone marrow in the right femoral shaft was ablated. During 3 weeks of recovery, CON and BMA rats received PBS (100 μL, 3 d/wk, i.p.), while IL‐1RA and IL‐1RA+BMA rats received the antagonist (3 μg/kg, 3 d/wk, i.p.). At sacrifice, left ventricular whole blood samples were collected. Plasma IL‐1α and IL‐1β were measured by ELISA. Five‐hundred microliters of blood were prepared for flow cytometry and OSP were sorted to obtain a count. The diameters of OSP were determined with a Cellometer. Some samples were pooled to meet the minimum detection threshold. These data are expressed as a percent of the total OSP count according to diameter, i.e., % OSP per 0‐14µm, 15‐30µm, 31‐45µm, and 46‐60µm. ELISA data were analyzed by two‐way ANOVA using SPSS (v. 25). The OSP count distributions were highly positively skewed with a mean and variance indicating overdispersed data. Therefore, negative binomial models were used to compare OSP count and % OSP using SAS GENMODE (v. 9.4). Data are presented as Means ± Standard Error. RESULTS Main effects for Age and Condition were detected in the ELISAs. Plasma IL‐1α did not differ; however, IL‐1β was higher (p<0.05) in Old (30.8±4.6 pg/mL) vs. Young (15.0±4.3 pg/mL). For Condition, IL‐1RA+BMA (49.6±14.1 pg/mL) had higher (p<0.05) plasma IL‐1α vs. CON (7.5E‐15±12.7 pg/mL) and IL‐1RA (9.4±13.2 pg/mL), and both CON (31.7±6.1 pg/mL) and IL‐1RA (41.2±6.3 pg/mL) had higher plasma IL‐1β vs. BMA (6.5±5.8 pg/mL) and IL‐1RA+BMA (12.3±6.8 pg/mL). For the OSP, Age x Condition interactions were detected. OSP count was higher (p<0.05) in Old IL‐1RA+BMA (3977±1631) vs. all other groups except Old IL‐1RA (1422±476). In addition, OSP count was higher (p<0.05) in Old IL‐1RA vs. Young IL‐1RA (212±67), Young BMA (276±80), and Young IL‐1RA+BMA (298±106). OSP count did not differ between Young CON (399±134) and Old CON (561±199). Analysis of % OSP revealed a main effect for diameter. Of the total in circulation, 38.4±7.8% were 15‐30µm, higher (p<0.05) than those 31‐45µm (5.7±1.2%) and 46‐60µm (2.2±0.5%). In addition, the % OSP 0‐14 µm (25.7±3.8%) were higher (p<0.05) vs. 31‐45µm and 45‐60µm. Further, the % OSP 31‐45µm was higher (p<0.05) vs. 46‐60µm. CONCLUSION Plasma concentrations of IL‐1α and IL‐1β varied with age and condition. Treatment with the IL‐1RA in old rats exacerbated the number of OSP in cir...