Immunosuppressive cytotoxic T lymphocyte associated antigen-4 immunoglobulin fusion proteins (CTLA4-Ig) block the CD28:CD80/86 costimulatory pathway. On a cellular level, CTLA4-Ig is understood to dampen T cell responses. As a mechanism, CTLA4-Ig has been reported to affect dendritic cell (DC) function via inducing the immunosuppressive indoleamine 2,3 dioxygenase (IDO) pathway and promoting a DC regulatory phenotype. We here probed cellular mechanisms of CTLA4-Ig immunoregulation in an allogeneic setting using C57BL/6 splenic or bone marrow derived DCs (BMDCs) as stimulators of allogeneic Balb/c derived T cells. To address whether CTLA4-Ig immunosuppression affected DCs, we pre-exposed C57BL/6 splenic or BMDCs to CTLA4-Ig and removed unbound CTLA4-Ig before co-culture with allogeneic T cells. CTLA4-Ig disappeared rapidly (within 4 h) from the cell membrane by combined internalization and dissociation. These CTLA4-Ig pre-exposed DCs were fully capable of stimulating allogeneic T cell proliferation, suggesting that CTLA4-Ig does not impair the DC stimulatory capacity. Only the presence of CTLA4-Ig during DC/T cell co-culture resulted in the expected inhibition of proliferation. C57BL/6 splenic or BMDCs exposed to CTLA4-Ig did not display IDO activity. We conclude that CTLA4-Ig immunosuppressive activity does not depend on a DC regulatory phenotype but on its presence during DC/T cell interaction.
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