Elevated HLA-G Expression On Myeloid Dendritic Cells Correlates With Regulatory T Cell Foxp3 Expression In Operationally Tolerant Pediatric Liver Transplant Recipients

Abstract

Introduction: Human leukocyte antigen (HLA)-G is a non-classical HLA class I molecule expressed as membrane-bound and soluble isoforms. Interaction of HLA-G with its receptor, Ig-like transcript (ILT)4 on professional antigen (Ag)-presenting dendritic cells (DC) down-regulates their T cell costimulatory ability. We examined the expression of HLA-G and ILT4, together with other immune regulatory molecules, on circulating DC subsets in three groups of clinically stable, pediatric liver recipients: operationally tolerant (TOL: 26); prospective immunosuppressive drug weaning (PW: 28) and maintenance immunosuppression (MI: 24), and in healthy controls (HC: 28). Methods: PBMC were stained with a lineage monoclonal antibody (mAb) cocktail, andwith mAbs, directed against blood DC Ag (BDCA)-1 (CD1; monocytoid [m]DC),and BDCA-2 (CD303; plasmacytoid [p]DC), as well as with mAbs against HLA-G and ILT4 (CD85d) or CMRF-44, inducible costimulatory ligand (ICOSL) or glucocorticoid-induced tumor necrosis factor-related protein ligand (GITRL), then analyzed by flow cytometry. Serum HLA-G levels were determined by ELISA. Results: Both circulating mDC and pDC in HC expressed surface HLA-G and ILT4. The incidence of mDC expressing HLA-G (but not ILT4) was significantly higher than that of pDC, although fluorescence intensity (MFI) was lower for HLA-G and ILT4 on mDC, compared with pDC. In TOL patients, mDC but not pDC expressed higher HLA-G (both% positive cells and MFI) than in PW or MI patients or HC. By contrast, ILT4, CMRF-44, GITRL or ICOSL expression on either pDC or mDC did not differ significantly between the groups. In TOL patients, the incidence of regulatory T cells (Treg) and the intensity of Foxp3 expression by Treg were significantly higher than in MI or HC groups. In addition, the intensity of HLA-G expression on circulating mDC correlated significantly with that of Foxp3 in the TOL group. The incidence of patients with circulating HLA-G levels>100pg/ml was greatest in the TOL group. Conclusion: Higher HLA-G expression on circulating mDC,- the predominant DC subset, in TOL pediatric liver transplant recipients compared with MI or HC, suggests a possible role of HLA-G in immune regulation. This may be correlated with enhanced Foxp3 expression by host Treg. The findings also suggest the potential of HLA-G expression on mDC as a biomarker of operational pediatric liver transplant tolerance.