Prenatally Educated Friendly NK Cells Suppress Allospecific T cell Responses and Expand Foxp3+CD4+Treg Cells

Abstract Prenatal allospecific tolerance hinges on the developmental selection of NK cells expressing a “friendly” phenotype that is not cytotoxic to allogeneic donor cells. Prenatally educated friendly NK cells (fNK’s) that express a tolerant alloreceptor phenotype toward the donor cells also exert suppressive effects on T cell alloimmunity in vivo and in vitro through direct cell contact. Little is known about the mechanisms regulating this suppression. Therefore, we hypothesized that fNK’s suppress induction of allospecific T cell responses through TGF-beta expression. To challenge this hypothesis, fNK cells were stimulated with PMA/Ionomycin (1ug/ml each) for 6h and supplemented with brefeldin A for the final 4 hours. TGF (LAP) was detected by intracellular flow cytometry. Following stimulation fNK’s from engrafter mice expressed significantly higher levels of TGF-beta when compared to fNK’s from naïve mice. Furthermore, when naïve splenocytes and allogeneic targets were co-cultured with fNK’s from engrafter mice, an expansion in the frequency of regulatory T cells was noted. Conversely, no change in Treg frequency was seen when co-cultured with fNK’s from naïve mice. From these results, we conclude that prenatally educated friendly NK cell exert antigen-specific suppressive effects on T cell immunity through TGF-b expression and Treg expansion.