MHC antigen expression in vascularized organ allografts: Clinical correlations and significance

Human T cell responses to human and porcine endothelial cells are highly sensitive to cyclosporin A and FK506 in vitro

BACKGROUND

Human T cells proliferate in response to both human umbilical vein endothelial cells (HUVEC) and porcine aortic endothelial cells (PAEC) via the second signals LFA-3/CD2 and B7-2 (CD86), respectively. Previous studies have shown that stimulation of T cells via CD28 or phorbol myristate acetate (PMA) activation is highly resistant to inhibition by cyclosporine A (CsA) and tacrolimus (FK506), as is the response of T cells to phytohemmaglutinin in the presence of endothelial cells. We have investigated the inhibitory effects of CsA and FK506 on the direct response of human CD4+ T cells to HUVEC and PAEC and the effect of adding B7-1 transfectants.

METHODS

T cell proliferation, interleukin-2 release bioassays and a multiple cytokine bioassay employing the TF-1 cell line were used as indicators of T cell responses to HUVEC and PAEC either in the presence or absence of CsA and FK506. In some experiments, B7-1 transfectants were also added.

RESULTS

Proliferative responses and interleukin-2 release were highly sensitive to CsA, the ID50 being significantly less for HUVEC (6.5 ng/ml) than PAEC (15 ng/ml). The ID50 of CsA for the mixed lymphocyte response (MLR) was similar to PAEC (18.6 ng/ml), all these values being significantly less than the T cell activation by phytohemmaglutinin (PHA) (227 ng/ml). Addition of B7-1 transfectants significantly increased interleukin-2 production by T cells/HUVEC and resistance to CsA was greatly increased to an ID50 of > 1000 ng/ml. In contrast, addition of B7-1 transfectants to T cells/PAEC had no effect either on T cell proliferation, IL-2 production, or CsA resistance. Similar results were obtained with FK506. Using the TF-1 cell line, it was determined that cytokines other than IL-2 are released during CD4+ T cell/EC interactions, with similar sensitivity to CsA and FK506.

CONCLUSIONS

It is concluded that both allogeneic and xenogeneic T cell/endothelial responses should be inhibited by therapeutic levels of CsA in vivo, assuming the absence of trans-stimulation by B7 molecules.

Role of endothelial cells in allograft rejection

The immunological properties of endothelial cells suggest they perform a pivotal role in acute and chronic rejection following solid organ transplantation. Their constitutive expression of MHC class II molecules (which initiate allograft rejection by activating CD4 T cells) and accessory molecules allows them to present foreign antigen by both the direct and indirect route to the recipient's immune system. The costimulatory molecules used by endothelial cells appear to differ from those used by traditional antigen-presenting cells such as B cells and dendritic cells. Release of non-HLA antigens from damaged endothelial cells results in a chronic antibody response--possibly contributing to graft vasculopathy and chronic rejection. Further understanding of the factors that regulate MHC class II and accessory molecule expression on endothelial cells could lead to novel strategies of therapeutic intervention.

MHC class II expression on human heart microvascular endothelial cells: exquisite sensitivity to interferon-gamma and natural killer cells

BACKGROUND

Immunocytochemical analysis of human organs in situ reveals differential expression of MHC class II antigens by microvascular endothelial cells (MVEC) and endothelial cells (EC) from large vessels. In view of the role of EC as initiators of allograft rejection, it is of interest to understand the regulation of MHC class II regulation by human MVEC. We have previously isolated, cultured, and characterized MVEC from the human heart, showing that although these cells were initially MHC class II positive, the antigens were lost after about 14 days in culture. These results suggest that basal expression in vivo is maintained by circulating factors.

METHODS

Here we have compared the sensitivity of human heart MVEC, human umbilical vein endothelial cells (HUVEC), and adult large vessel EC (aorta, coronary artery, and pulmonary artery) to interferon (IFN)-gamma and natural killer (NK) cell-mediated induction of MHC class II antigens. MVEC and HUVEC were cultured with 1, 5, 10, 50, 100, and 500 U/ml of IFN-gamma for 4 days, the cells were washed, and flow cytometry was used to examine HLA-DR expression at days 1, 2, 4, 7, 10, 14, and 21. EC were also cultured with purified NK cells in the presence and absence of neutralizing antibodies to IFN-gamma, and MHC class II expression was analyzed.

RESULTS

As little as 5 U/ml of IFN-gamma produced 98% positive cells in heart MVEC compared with 100-500 U/ml needed for the same effect in HUVEC or other large vessel EC (coronary, aorta, pulmonary). Class II expression was maintained longer by MVEC (for 17 days) compared with HUVEC (for 10 days). NK cells and supernatant from MVEC/NK cultures induced MHC class II antigens on MVEC and HUVEC in a dose-dependent fashion; the MVEC showed an enhanced sensitivity compared with the HUVEC. The NK effects were inhibited by neutralizing antibodies to IFN-gamma. The allostimulatory ability of MHC class II-positive EC was shown to be proportional to the amount of MHC class II on the cell surface.

CONCLUSIONS

The results suggest that basal expression of MHC class II on human MVEC is maintained by circulating IFN-gamma and NK cells. This conclusion has implications for therapeutic interventions.