Suppression of the immune response to alloantigen by factors released from ultraviolet-irradiated keratinocytes

The immune response to allogeneic histocompatibility Ag can be suppressed by injecting allogeneic spleen cells into mice that have been previously exposed to UV radiation. The suppression is associated with Ag-specific suppressor T cells found in the spleens of the UV-irradiated mice. An intriguing and as yet unanswered question is how the irradiation of the animal's dorsal skin leads to the induction of splenic Ag-specific suppressor cells. Our data suggest that soluble factors released by UV-irradiated keratinocytes are involved in the induction of Ag-specific suppressor cells. Injecting culture supernatants from UV-irradiated keratinocytes into normal mice produced the same effect as whole-body UV irradiation and suppressed the induction of delayed hypersensitivity to alloantigen. Spleen cells from these mice were unable to respond to the alloantigen in the MLR. Radiation-resistant, suppressor T cells (CD3⁺, CD4⁺, CD8^-) were found in the spleens of the mice injected with suppressive supernatants. Treating the keratinocytes with cycloheximide or treating the supernatants from the UV-irradiated keratinocytes with trypsin removed all suppressive activity, suggesting the active material is a protein. The suppressive activity bound to agarose beads coupled with Con A, and was eluted with α-methyl-D-mannoside, further suggesting the suppressive material is a glycoprotein. Because the suppression of the immune response to alloantigen induced by this suppressive cytokine mimicked the suppression found after exposure to UV radiation, these findings support the concept that the induction of systemic suppression by UV-irradiation results from the release of suppressive substances by UV-irradiated keratinocytes. In addition, these data suggest that the induction of Ag-specific suppressor cells by this factor may provide a novel method of suppressing allograft rejection.