Abstract: The present invention provides a method of providing an individual with a biological factor or intermediate thereof which comprises introducing into the individual Sertoli cells genetically altered to produce the biological factor or intermediate thereof. The genetically altered Sertoli cells are administered in an amount effective to produce the desired effect. Aside from producing the biological factor or intermediate thereof, the engineered Sertoli cells also create an immunologically privileged site. Vectors comprising a promoter which functions in Sertoli cells operably linked to coding sequence for a desired biological factor are also provided as are Sertoli cells comprising such vectors. A pharmaceutical composition comprising Sertoli cells genetically altered to produce a biological factor is also provided.