IMMUNE RECONSTITUTION FOLLOWING UNRELATED CORD BLOOD TRANSPLANTATION: ROLE OF THE FAS LYTIC PATHWAY
Z. Brahmi+, G. Hommel-Berrey+, H. Chua+, R. Barr+, K. Robertson*, F. Smith* and B. Thomson* Department of Medicine+ and Department of Pediatrics*, Indiana University School of Medicine, Indianapolis, IN.

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Umbilical cord blood (UCB) is widely accepted as a source of stem cells in pediatric cancer patients. We have recently shown that the results obtained in 30 UCB recipients compare most favorably to those obtained with other unrelated stem cell sources. In UCB recipients, NK cells were detected at about one month post-transplantation, in sharp contrast to other stem cell transplants, especially growth factor-mobilized peripheral blood stem cells where NK cell function is low for up to one year.

The ability of these early NK cells in UCB recipients to mediate Fas lytic activity has not been previously ascertained. We have reported that fresh cord blood NK do not mediate lytic activity in-vitro and that this hypo-responsiveness is only transient and can be boosted to normal adult levels after incubation with IL-2 or IL-12, for 18 hours. The increased activity was accompanied by an increase in perforin, granzyme A and granzyme B mRNAs, indicating that it was mediated by the granule exocytosis pathway. Surprisingly, cord blood NK cells expressed high levels of CD 28. Because NK cells can also destroy target cells via the Fas lytic pathway, we examined the expression of Fas ligand (FasL) in NK cells isolated from fresh cord blood. The expression of FasL mRNA in unstimulated, freshly isolated NK cells was barely detectable, but the level was markedly increased if treated with PMA and ionophore. We also showed that although fresh NK cells do not lyse tumor targets in-vitro by either the perforin or the Fas lytic pathway, both pathways are fully functional in-vivo as early as 30 days post-transplantation in UCB recipients. In addition, because the cumulative probability of acute GVH of grade III-IV averaged only 9% in our UCB recipients, we propose that the low incidence of GVH is most likely not due to NK hypo-responsiveness.