Human Cancer Biology Immunodeficient Mouse Strains Display Marked Variability in Growth of Human Melanoma Lung Metastases

Purpose: Immunodeficient mice serve as critical hosts for transplantation of xenogeneic cells for in vivo analysis of various biological processes. Because investigators typically select one or two immunodeficient mouse strains as recipients, no comprehensive study has been published documenting differences in human tumor engraftment. Taking advantage of the increased metastatic potential of RhoC-expressing human (A375) melanoma cells, we evaluate four immunodeficient mouse strains: severe combined immunodeficiency (scid), nonobese diabetic (NOD)-scid, NOD-scid β2m, and NOD-scid IL2Rγ as xenograft tumor recipients. Experimental Design: Bioluminescence, magnetic resonance imaging, and histopathology were used to monitor serial tumor growth. Natural killer (NK) cell function was examined in each mouse strain using standard Chromium release assays. Results: Melanoma metastases growth is delayed and variable in scid and NOD-scid mice. In contrast, NOD-scid β2m and NOD-scid IL2Rγ mice show rapid tumor engraftment, although tumor growth is variable in NOD-scid β2m mice. NK cells were detected in all strains except NOD-scid IL2Rγ, and in vitro activated scid, NOD-scid, and NOD-scid β2m NK cells kill human melanoma lines and primary melanoma cells. Expression of human NKG2D ligands MHC class I chain–related A and B molecules renders melanoma susceptible to murine NK cell–mediated cytotoxicity and killing is inhibited by antibody blockade of murine NKG2D. Conclusions: Murine NKG2D recognition of MICA/B is an important receptor-ligand interaction used by NK cells in immunodeficient strains to limit engraftment of human tumors. The absolute NK deficiency in NOD-scid IL2Rγ animals makes this strain an excellent recipient of melanoma and potentially other human malignancies. Mouse models of human cancer serve as essential experimental systems, and genetically defined immunodeficient mouse strains constitute a valuable tool for studying tumorigenesis. Athymic (nude) mice have been the standard for establishing in vivo models of human malignancies (1). However, the presence of residual adaptive and innate immunity can interfere with the establishment of tumor xenografts. Athymic mice develop small numbers of mature αβTCR lymphocytes with age; in addition, robust natural killer (NK) cell activity is present and increases with age (2, 3). Numerous reports confirm variable rates of human tumor growth in athymic animals (4–6); for example, of 200 human breast cancer samples tested in nude mice, just 25 (12.5%) grew as xenografts at the site of s.c. implantation (7). Severe combined immunodeficiency (scid) mice have relative Band T-cell deficiencies and are often used as recipients of human xenografts. Improved tumor engraftment rates have been reported in the nonobese diabetic (NOD)-scid strain, where introduction of the scid mutation into the NOD background results in reduced macrophage and NK function, as well as an absence of complement-dependent hemolytic activity (8, 9). Recently, two additional immunodeficient strains have been described as follows: NOD-scid β2m and NOD-scid IL2Rγ (10). The NOD-scid β2m strain was developed by backcrossing the β2m mutation to the NOD-scid strain resulting in mice deficient in MHC class I expression (NOD-scid β2m). Accumulating data suggest that NK cells that develop in a MHC class I–deficient background are unlicensed and, hence, unable to kill susceptible targets Authors' Affiliations: Departments of Medicine, Developmental Biology, Pathology-Immunology, Molecular Imaging Center, and Biomedical Magnetic Resonance Laboratory, Mallinckrodt Institute of Radiology, and Siteman Cancer Center, Washington University School of Medicine, St Louis, Missouri Received 9/27/08; revised 1/14/09; accepted 2/11/09; published online 5/15/09. Grant support: ACS IRG-58-010-49 (G. Linette), NIH P50 CA94056 (D.P. Worms), NIH/National Cancer Institute R24 CA 83060 and National Cancer Institute P30 CA91842 (J. Garbow), and NIH/NIAID K08 AI57361 (L.N.