Cells in murine fetal liver and in lymphoid colonies grown from fetal liver can suppress generation of cytotoxic T lymphocytes directed against their self antigens.

Fetal liver (FL) cells or, in much smaller numbers, cells in lymphoid colonies grown from FL of mouse strain A can suppress the development of cytotoxic activity in a B anti-A mixed lymphocyte reaction (MLR) but not a B anti-C MLR. A, B, and C represent three H-2 different mouse strains. Suppressor cells with similar properties have been described in thymus, bone marrow, lymphoid colonies grown from bone marrow, and athymic nude spleen. They have been referred to collectively as "veto cells". From the point of view of the suppressor cells, a response against self is being suppressed. The FL colony suppressor cells differ from those described in that they can sometimes also suppress an A anti-B response. The FL colony suppressor cells are Thy-1- and resistant to 1500 rad irradiation. These cells may play a role in maintenance of self tolerance. That they are found in FL as early as 13 days of gestation implies that potentially self-reactive cells may develop before experiencing the thymus environment.