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In doing liver transplantation, it is important to know the effect of immunosuppressive agents on hepatic regeneration. In many such patients, livers are transplanted that are not the ideal size for a particular recipient. Accommodation of these organs to recipient size depends on poorly understood growth homeostasis which conceivably could be altered by the immunosuppressive agents. In fact, it has been reported that the regeneration response is inhibited by azathioprine and adrenal cortical steroids. 1,2
Largely because of such concerns, Makowka et al3 tested hepatic generation after partial hepatectomy in rats treated with CyA and reported that the regeneration response was actually greater than normal. This finding has been confirmed by several investigators.4–6
More recently, we have demonstrated an even more dramatic augmentation of hepatic regeneration by FK 506. CyA and FK 506 have completely different molecular structures (Fig 1), but they have in common the relatively specific inhibition of the T cell component of the immune system.7,8 Because of this similar mechanism. we have suggested that hepatic regeneration may be subject to immune modulation.6
The Immune modulation hypothesis was supported by the data summarized in Fig 2. In essence, the increase in DNA and mitotic activity which is characteristic of a 70% hepatic resection in rats was increased significantly with CyA and nearly doubled when FK 506 was administered for 3 days preoperatively and for a final time just after completing the hepatic resection.
What was the explanation for these curious results? The first step was to establish pure hepatocyte cultures9 and to determine if the resting rate of mitoses was altered by the direct addition of CyA or FK 506 to the medium. A wide range of doses of both drugs was completely without effect (Fig 3).
Hepatocyte cultures were now used for additional experiments under two different conditions that are known to increase the basal rate of mitoses. In the first experiment, epidermal growth factor was added to the hepatocyte culture which, as expected.10 increased the basal mitotic rate approximately 10-fold. The addition of CyA and FK 506 had no further effect (Fig 3).
In the second experiment, normal rat serum, which in tissue culture systems is mitogenic in low doses.9 was added to the hepatocyte culture in graded quantities (Table 1). The expected increase in mitoses was seen at first with an inhibition of this effect as larger volumes of normal rat serum were added (Table 1). As in the first experiment, there was no augmentation effect with CyA on FK 506.
From this we concluded that neither CyA nor FK 506 had a direct effect on hepatocyte mitoses, no matter whether the mitotic index was at a resting or heightened level at the time of addition of the drug to the medium.
In contrast, the serum from animals treated for 4 days with therapeutic doses of CyA and FK 506 had a pronounced effect on hepatocyte cultures (Table 1). These sera caused an increase in mitosis far exceeding that seen with comparable amounts of normal rat serum. The stimulatory potency of the rat serum was greater if the serum donors were treated with FK 506 than if therapy was with CyA.
We questioned whether CyA and FK 506 altered the regeneration of other organ systems. To test this possibility, DNA synthesis was measured in kidneys before and after right nephrectomy. The basal level of DNA synthesis was essentially doubled in the contralateral kidney after right nephrectomy, but this was not further increased by the treatment with FK 506 that enhanced liver regeneration (Table 2).
With resection of 50% of the small bowel, DNA synthesis in the residual small bowel tripled, as has been described by others.11 This powerful regeneration response was neither increased or decreased by treatment of these animals with FK 506 (Table 2).
These observations could have a deeper biologic significance than those practical questions which prompted us to begin these studies. It remains possible that the immune system has a “braking” effect on hepatocyte cell mitosis, and that this is unmasked by powerful T cell-specific immunosuppression. From the work presented here, it is probable that this control is at least relatively liver-specific: at least it could not be identified in regenerating kidney or intestine. The fact that CyA and FK 506 did not have a direct action on cultured hepatocytes. but that the serum of immunosuppressed animals was stimulatory to these same cell cultures, suggests that some circulating substance is induced by CyA and FK 506. CyA and FK 506 act by inhibiting IL2 production and binding. 7,8 It is conceivable that by inhibition of this second signal, both drugs could interrupt the secretion of liver regulatory factors, either by as yet unknown immune mechanisms or, possibly, by pathways that are not directly connected with the immune system.
Supported by research grants from the Veterans Administration and Project Grant No. DK 29961 from the National Institutes of Health, Bethesda, MD, and Grant 87/01291-44 from Consiglio Nazionale delle Ricerche, Italy.