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

Spontaneous orthotopic liver transplant (OLT) tolerance occurs uniformly between the inbred rat strains of DA (MHC haplotype RT1^a) into PVG (RT1^c) despite a fully allogeneic barrier. Animals transplanted in this combination do, however, undergo a rejection episode which appears to be self-limiting. In order to clarify this further we under took in situ measurements of the cytokines IL-2, IFN-γ and TNF-α prior to, during and post rejection 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-γ, occurred during the rejection episode between days 10 and 14. IFN-γ 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-α occurred in allogeneic and syngeneic OLT with TNF-α levels falling by the peak rejection episode. Immunoblotting also confirmed the ability of hepatocytes to produce each of the cytokines studied. mRNA levels, by contrast, were maximal at days 1 and 10 for IL-2 and day 3 for IFN-γ in allogeneic OLT when compared with syngeneic and non-transplanted controls. Earlier increases in IL-2 and IFN-γ mRNA and time of peak protein expression do not correlate in this model. We therefore conclude that the measurement of peak mRNA levels alone are not enough to evaluate the rejection process especially since it is the cytokine protein products which have potential biological activity.