Cell line immortalisation biosafety
Maybe, cells should be contained under BSL-2 and agents (viral particles etc) under stricter BSL-3? That is, after immortalization you can keep cells under lower security.


  Biosafety Level 2 builds upon BSL-1. BSL-2 is suitable for work involving agents that pose moderate hazards to personnel and the environment.
  
  Biosafety Level 3 is applicable to clinical, diagnostic, teaching, research, or production facilities where work is performed with indigenous or exotic agents
  that may cause serious or potentially lethal disease through the inhalation route of exposure
  
  CDC Biosafety in Microbiological and Biomedical Laboratories, Laboratory Biosafety Level Criteria


As of HEK 293 cells, check this out:


  Biosafety level 2 practices and containment facilities for all activities involving HEK-293 cell lines. 
  
  HEK 293 Cell Line Risk Assessment - IBC