Peak protein expression of IL-2 and IFN-gamma correlate with the peak rejection episode in a spontaneously tolerant model of rat liver transplantation

Spontaneous orthotopic liver transplant (OLT) tolerance occurs uniformly be tween the inbred rat strains of DA (MHC haplotype RT1(a)) into PVG (RT1(c)) despite a fully allogeneic barrier. Animals transplanted in this combinati on do, however, undergo a rejection episode which appears to be self-limiti ng. In order to clarify this further we under took in situ measurements of the cytokines IL-2, IFN-gamma and TNF-alpha prior to, during and post rejec tion episode. The cytokine protein product was examined via immunoblotting assays and mRNA levels by RT-PCR. Comparisons were also made for syngeneic transplant combinations over the same time period. Peak protein expression of IL-2 and, to a lesser extent, IFN-gamma, occurred during the rejection e pisode between days 10 and 14. IFN-gamma was still present in syngeneic OLT on day 10 but was only present in allogeneic OLT on day 14, IL-2 was only detectable in allogeneic OLT on days 10 and 14. Transient increases in TNF- alpha occurred in allogeneic and syngeneic OLT with TNF-alpha levels fallin g by the peak rejection episode. Immunoblotting also confirmed the ability of hepatocytes to produce each of the cytokines studied. mRNA levels, by co ntrast, were maximal at days 1 and 10 for IL-2 and day 3 for IFN-gamma in a llogeneic OLT when compared with syngeneic and non-transplanted controls. E arlier increases in IL-2 and IFN-gamma mRNA and time of peak protein expres sion do not correlate in this model. We therefore conclude that the measure ment of peak mRNA levels alone are not enough to evaluate the rejection pro cess especially since it is the cytokine protein products which have potent ial biological activity. (C) 2001 Academic Press.