Background. Serine proteinase inhibitor (PI)-9 with a reactive center P1 (Glu)-P1′ is a natural antagonist of granzyme B and is expressed in high levels in cytotoxic T lymphocytes (CTL). In view of the role of CTL in acute rejection, we explored the hypothesis that PI-9 would be hyperexpressed during acute rejection. Because PI-9 can protect CTL from its own fatal arsenal and potentially enhance the vitality of CTL, we examined whether PI-9 levels correlate with the severity of rejection as well as predict subsequent graft function.
Methods. We obtained 95 urine specimens from 87 renal allograft recipients. RNA was isolated from the urinary cells and mRNA encoding PI-9, granzyme B, or perforin and a constitutively expressed 18S rRNA was measured with the use of real-time quantitative polymerase chain reaction assay, and the level of expression was correlated with allograft status.
Results. The levels of PI-9 (P =0.001), granzyme B (P <0.0001), and perforin mRNAs (P <0.0001), but not the levels of 18S rRNA (P =0.54), were higher in the urinary cells from the 29 patients with a biopsy-confirmed acute rejection than in the 58 recipients without acute rejection. PI-9 levels were significantly higher in patients with type II or higher acute rejection changes compared with those with less than type II changes (P =0.01). Furthermore, PI-9 levels predicted subsequent graft function (r =0.43, P =0.01).
Conclusions. PI-9 mRNA levels in urinary cells are diagnostic of acute rejection, predict renal allograft histology grade, and predict functional outcome following an acute rejection episode.
Cytotoxic T lymphocytes (CTLs) induce target cell death by at least two effector pathways: the granule exocytosis pathway in which the granzyme B/perforin collaborate to induce target cell apoptosis and the Fas-Fas ligand pathway in which the cross linking of Fas by the Fas ligand displayed by CTLs results in target-cell demise (1).
A major biological puzzle is how the CTLs protect themselves from their own fatal arsenal. One potential explanation is that there are endogenous protective mechanisms that are up-regulated, pari passu, with cytotoxic effector mechanisms, and these endogenous blockers serve to limit the lytic activity. In this regard, Fas-Fas ligand associated apoptosis is constrained by a cellular protein FLIP (c-FLIP), and hyperexpression of c-FLIP has been hypothesized as a mechanism for the escape of tumors from CTL attack (2,3). With respect to granzyme B/perforin lytic pathway, a serine proteinase inhibitor (PI)-9 has been shown to irreversibly inactivate granzyme B and block CTL killing by way of the pathway of granzyme B/perforin (4-6).
PI-9 is expressed in high levels in CTLs (5). In view of the role of CTL in acute rejection (7), we explored the hypothesis that PI-9 would be hyperexpressed during acute rejection. Because PI-9 can protect CTL from its own fatal arsenal and potentially enhance the vitality of CTL, we examined whether PI-9 levels correlate with the severity of acute rejection as well as predict subsequent graft function.