Dear peers,
Recently we purchased some NSG mice from JAX lab (#005557) to establish an orthotopic lung tumor model by intratracheal injection. However, we came across some enigmatic issues in tumor development.
Our study has 2 cohorts (7 mice in each cohort), and those mice received luciferase+ A549 cells through intratracheal cell administration. After the cell administration, the mice were monitored under the IVIS system, and the bioluminescence signal indicated successful cell delivery. Subsequently, the mice underwent regular weekly IVIS imaging to monitor tumor development. As expected, some cells will die after the injection, and we confirmed the cell loss by reduced luciferase activity. Then, during the following 2 months, 5 mice kept showing negative signals all the time, but the rest mice developed tumors. So, we paused the IVIS monitoring and kept the mice for 1 more month, followed by an autopsy.
To our surprise, all 5 mice showing negative luciferase activities developed tumors, and some were quite large. We are confused why the tumors were not developed in the first 2 months but rapidly grew in the last month.
We are aware that if we take batch imaging, the mice with larger tumors will display strong signals, which can make the mice with sporadic tumors show negative signals. So we also took the images individually but still didn't see any luciferase activity.
Are there any possible reasons? NSG mice have functional neutrophils and partially functional monocytes. Could these cells suppress the tumor cell proliferation at the monitoring window?
I truly appreciate your answers.
I cannot answer your question directly, not having grown tumors in NSG mice, but I can share some information about a similarly immune deficient strain, scid/beige mice. I developed that strain by breeding C.B-17-scid mice with BALB/c-beige to make an NK-deficient scid strain. I was using them for allogeneic stem cell transplantation, but one of my co-workers was growing human tumor cell lines in scid mice and wanted to try growing the same tumors in scid-beige, figuring that the NK deficient would lead to faster engraftment. But much to our surprise, the tumors grew slower in the scid-beige recipients than they did in scid mice. I was able to document that scid mice actually had a high level of circulating IFN-gamma while IFN-gamma was almost undetectable in the scid-beige mice. So the working model was that the beige defect that caused the lack of NK activity also interfered with inflammatory cytokine production by the mice, leading to slower tumor growth. Those studies were funded by a biotech company that has since gone defunct and, as far as I know, were never published.
Dr. Gilmore, Gary Lee Gilmore Thank you so much for the valuable information.
It has been decades since I was involved in that work, and the human tumor studies were being conducted for a biotech company. I was working at an affiliated research institute. Unfortunately I do not remember the specific tumor cell lines that were being tested. I do know that the tumors grew faster in the standard C.B-17 scid mice than in the scid-beige double mutants. From my studies on comparing the scid-beige mice to scid mice, I found detectable circulating IFN-gamma in the scid mice, but not in the scid-beige mice. The point was not proven, but the working hypothesis was that the absence of IFN-gamma was delaying the growth of the human tumors - potentially by less vascularization of the tumor by the mouse circulatory system.
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