WERF Endometriosis Phenome and Biobanking Harmonisation Project for Experimental Models in Endometriosis Research (EPHect-EM-Heterologous): heterologous rodent models

Abstract

Endometriosis, defined as the growth of endometrial-like tissues outside the uterus, is a common disease among women. Numerous in vivo rodent models of endometriosis have been developed to explore multiple aspects of this poorly understood disease. Heterologous models utilize human endometrial tissues engrafted into immunocompromized mice, while homologous models engraft rodent endometrium into immunocompetent mice or rats. Heterologous models of endometriosis more closely replicate the human disease; however, the murine humoral immune response must be suppressed to prevent rejection of the xenograft tissue. Although the innate immune system remains intact, suppression of the humoral response leads to a markedly different local and systemic immune environments compared to humans. Despite this limitation, experiments using heterologous models have contributed significantly to our understanding of endometriosis establishment and progression, the pre-clinical effectiveness of various therapeutic strategies, and genetically modifiable host factors that contribute to disease. Unfortunately, a lack of harmonization of the models used by different laboratories has impeded the reproducibility and comparability of results between groups. Therefore, the World Endometriosis Research Foundation (WERF) formed an international working group of experts in heterologous models of endometriosis to develop guidelines and protocols that could contribute to unifying experimental approaches across laboratories. Nine critical variables were identified: (i) mouse strain; (ii) human tissue type; (iii) hormonal status of the human tissue donor; (iv) human tissue preparation; (v) method and location of tissue placement; (vi) hormonal status of the recipient animal; (vii) whether or not mice were engrafted with human immune cells; (viii) endpoint assessments; and (ix) number and type of replicates. Herein, we outline important considerations for each major variable and make recommendations for unification of approaches. Widespread adoption of harmonized protocols and implementation of standardized documentation and reporting should further improve the reproducibility and translation of experimental findings both within and between laboratories.

Keywords: collaboration; endometriosis; experimental models; heterologous; research; rodents.

Figure 1.

Decision tree. There are four major design variables that must be addressed prior to the initiation of a heterologous experimental endometriosis study. These variables are presented along with experimental considerations, which should aid in decision-making.

Figure 2.

Experimental human endometriosis established in immunocompromized mice. (A) Subcutaneous lesion observed through the skin of a nude mouse. (B) Lesion in (A) with skin removed (15×). (C): Haematoxylin and eosin (H&E) staining of lesion in A and B (40×). (D–F) Gross morphology of common types of intraperitoneal lesions. A thin adhesion is also visible in (E) (arrow). Insets: H&E of the same lesion, 100×. (G) Gross morphology of a lesion created using isolated cells embedded in a hydrogel matrix. (H and I) H&E staining of lesion in G. 100× and 200×, respectively.

Figure 3.

Endometriotic lesions: native human disease versus experimental model. Lesions of human endometriosis, photographed during a laparoscopic surgery (Courtesy of Dr Gil Wilshire) often appear as highly vascularized red nodules (A). Human tissues growing as ectopic lesions in nude mice frequently exhibit a similar phenotype (B). The histology of formalin fixed, paraffin-embedded human endometriotic lesions (C) and human tissues removed from mice as lesions (D) and stained with haematoxylin and eosin, also frequently exhibit marked similarities. Original magnification, 100× for both photomicrographs.