NITRIC OXIDE PRODUCTION BY PIG ENDOTHELIAL CELLS IN RESPONSE TO HUMAN-DERIVED INJURY1

Background. The hyperacute rejection induced by natural antibodies is the first barrier to the success of pig to human organ xenotransplantation. When this barrier is overcome, an infiltrate of mainly monocytes and natural killer cells can be observed. Nitric oxide (NO) has been described to be involved in allo- and xenorejection, and to participate in the regulation of monocyte infiltration in other models. Methods. We have studied the capacity of human monocytes and natural antibodies to induce the production of NO by pig endothelial cells, by measuring NO2, a stable end product of NO. Results. Human monocytes can induce HuProVim (HUP), a pig endothelial line, and “in situ, ex vivo” pig endothelial cells to produce NO2. This NO2 production was inhibited by N G -nitro-L-arginine-methylester and N G -monomethyl-L-arginine, inhibitors of NO production. This induction can be observed even if cells are separated by a semipermeable membrane, which indicates that it is a result of soluble factors. Activated cells continue producing NO after triggering for 1 hr. No NO2 production was observed after activation of HUP cells with recombinant human interleukin (IL)1a, IL-1b, IL-6, IL-10, transforming growth factor-b1, IL-2, IL-4, interferon-g, or recombinant human tumor necrosis factor-a (rhTNF-a) alone. Only the combination of rhTNF-a1rIL-1a or rIL-1b, and rhTNF-a1rIL1a1IFN-g induces some NO production. Human natural anti-pig antibodies, which had been described to induce cytoskeleton changes on endothelial cells, do not induce NO production. Conclusions. Our data show that human monocytes induce the production of NO by pig endothelial cells. The inducing signal is soluble and cannot be provided by human anti-pig natural antibodies. The possible use of pigs as an organ source for transplantation into humans is now widely considered. For this reason, the interaction of the cells of the human immune system with pig cells is raising increasing interest. The first barrier to the success of transplantation of pig organs to humans is the hyperacute rejection (1). In some cases in which the barrier of the naturally occurring antibodies was overcome in pig to primates model, an important percentage of cells infiltrating the pig organs were monocytes and natural killer (NK*) cells (2). Similar results have been found in a human-rat ex vivo