Pulmonary Thromboembolism Due to Intravenous Fibrocyte Administration in C57BL/6J Mice (Mus musculus) and Recommendations for Refinement.

Cell therapy is a promising field of study for a range of diseases for which traditional therapies have failed. A primary problem for this therapy type in human medicine is failure of treatment efficacy due to the first pass trapping effect of the lung. Consequently, in rodents where these therapies are trialed, animals can be significantly affected by pulmonary thromboembolism due to the inherently smaller vessel sizes. Several different mechanisms for this have been demonstrated for rodent models, and several pharmacologic treatments have been proposed. In a C57BL/6J mouse model of cecal ligation and puncture, significant mortality was observed immediately after intravenous adoptive transfer of cultured fibrocytes. Antemortem clinical signs consisted of peracute dyspnea and lateral recumbency. Necropsy was performed on 3 affected mice, and heart and lung tissue were evaluated histologically. Cause of death was identified as acute thromboembolism of pulmonary arterioles. Special staining supported that the thromboemboli were caused by reticular fibers produced in culture by the transferred fibrocytes that were not removed by pretransfer cell isolation techniques. Prior literature describes either de novo thrombus formation or mechanical occlusion of pulmonary capillaries by transferred cells as the primary causes for pulmonary thromboembolism in rodent models of intravenous cell transfer. We present a novel mechanism in this case series and describe refinement steps such as the use of enhanced filtration steps during cell isolation and decreased cell concentration for administration. These refinements significantly reduced mortality in follow-up intravenous adoptive transfer experiments with C57BL/6J mice, improving animal welfare and enhancing research productivity.