Rapamycin downregulates the inhibitory receptors ILT2, ILT3, ILT4 on human dendritic cells and yet induces T cell hyporesponsiveness independent of FoxP3 induction.

Dendritic cells (DC) mediate potent alloimmune responses through the positive costimulatory molecules CD40, CD80 and CD86 while the negative costimulatory molecules, immunoglobulin-like transcript 2 (ILT2), ILT3 and ILT4 have been associated with tolerogenic DC function. Due to the pivotal role played by DC in immunity the effect of the immunosuppressive agent rapamycin (RAPA) on the expression profile of both positive and negative costimulatory molecules during human DC differentiation and maturation was investigated. During monocyte differentiation to DC an increase in the mean fluorescence intensity (MFI) for CD40, CD80, CD83 and CD86 was observed in association with a concomitant downregulation of ILT2, ILT3, ILT4 and HLA-G expression. While DC differentiation in the presence of RAPA (10nM) showed a reduction in the MFI for CD40, CD80 and CD86 expressing cells, treatment after differentiation had no effect on the expression of these costimulatory molecules. The inhibitory receptors were also downregulated by RAPA only when added during differentiation. In comparison to RAPA, Cyclosporin A (CsA) had relatively minor effects on DC phenotype whereas IFN-alpha showed induction of CD80, CD86 and HLA-G when added prior to differentiation. Functionally, RAPA-treated DC used as stimulators in a DC-T cell mixed lymphocyte reaction (MLR) showed 40% inhibition of T cell proliferation relative to untreated DC (P=0.001) whereas CsA-treated DC showed no difference, and IFN-alpha-treated DC stimulated T cell proliferation. Nevertheless, the induction of T cell hyporesponsiveness by coculture of RAPA-treated DC with T cells was not associated with the generation of increased numbers of FoxP3 positive T regulatory cells. In conclusion, although RAPA downregulated ILT2, ILT3 and ILT4 expression in DC, the inhibition of T cell proliferation by RAPA-treated DC is predominantly due to the reduction of CD40, CD80 and CD86 expression rather than the propensity to generate FoxP3 expressing regulatory cells.