Will tolerance become a clinical reality?

Apoptotic death of infiltrating cells in human cardiac allografts is regulated by IL-2, FASL, and FLIP

INTRODUCTION

In vitro studies have shown that apoptotic cell death is triggered by a IL-2-dependent activation of the Fas-FasL pathway and that this pathway can be inhibited by FLIP.

METHODS

To define whether FLIP regulates apoptotic death of graft infiltrating T-cells during IL-2-mediated rejection, we analyzed endomyocardial biopsies (EMB) from cardiac allograft recipients for CD3, DNA strand breaks (TUNEL assay), FLIP (mRNA and protein), and FasL mRNA expression.

RESULTS

Apoptosis was present in CD3+ T-cell infiltrates. The number of TUNEL-stained mononuclear cells was inversely correlated with FLIP mRNA expression levels (P=.09). FLIP protein was present in 5% to 10% of the infiltrating cells and was constitutively produced by cardiomyocytes irrespective of the rejection grade. Rejection biopsies had elevated IL-2 and FasL mRNA expression levels compared to the expression levels before and after acute rejection (P=.03 and P=.11), while FLIP mRNA expression levels were significantly decreased during rejection (P=.05).

CONCLUSION

Our results indicate that during the IL-2-induced rejection process, infiltrated T cells become more sensitive to apoptosis.

Acceptance reaction: intragraft events associated with tolerance to renal allografts in miniature swine

Inbred miniature swine that are treated for 12 d with a high dose of cyclosporin A develop tolerance to MHC class II matched, class I-mismatched renal allografts. The aim of this study was to clarify the intrarenal allograft events associated with the development of tolerance in this protocol. Morphologic and immunologic studies were performed in serial biopsies from accepting grafts after 12 d of cyclosporin A treatment (n = 4) and were compared with those from untreated control rejecting grafts (n = 4). In accepting grafts with stable function, a transient interstitial infiltrate developed. The cellular infiltrate had many similarities to that in rejecting grafts; both had T cells and macrophages, similar proportions of T-cell subsets, and a similar frequency of in situ nick end labeling (TUNEL)+ apoptotic infiltrating cells. However, the cellular infiltrate in the acceptance reaction was distinguished by less T-cell activation (interleukin-2 receptor+), less proliferation (proliferating cell nuclear antigen+) of infiltrating cells, and less graft cell apoptosis in arteries, tubules, glomeruli, and peritubular capillaries. Thereafter, the infiltrate in the accepting grafts progressively resolved with decreased cell proliferation, activation, and apoptotic graft parenchymal cell injury, but the high frequency of apoptosis persisted in graft-infiltrating cells. In parallel to the intragraft events, donor-specific unresponsiveness developed as assessed by cell-mediated cytotoxicity by blood mononuclear cells in vitro. In conclusion, the acceptance reaction in transplanted grafts is characterized by progressive resolution of T-cell proliferation and activation and of cell-mediated graft injury, as well as prolonged T-cell apoptosis. These intragraft events suggest that both T-cell anergy and T-cell deletion occur in the graft during the development of tolerance. Some of the described immunopathologic findings (activation, proliferation, apoptosis) may be useful in distinguishing acceptance from rejection, as well as in predicting later graft acceptance in tolerance induction protocols.