Growth Arrest-Specific Gene 6 (Gas6) Deficiency in Mice Alleviates Graft-versus-Host Disease (GVHD) in the Liver.

GVHD is the single most important complication of allogeneic bone marrow transplantation (BMT), mediated by donor-derived alloreactive T cells. Gas6, a vitamin K-dependent protein, interacts with phospholipid membranes via its γ-carboxyglutamic acid-containing tail and binds to the receptor tyrosine kinases Tyro3, Axl and Mer via its carboxy-terminal globular domain. Gas6 has been implicated in reversible growth arrest, survival, proliferation, cell adhesion, hemostasis and erythropoiesis. Preliminary experiments have shown that Gas6 deficient (Gas6-/-) mice have a leukocyte trafficking defect into inflamed tissues. We hypothesized that donor Gas6-/- T cells might induce less GVHD than wild-type (WT) T cells in MHC-mismatched BMT. We therefore compared the ability of Gas6-/- FVB (H-2^q) bone marrow cells to produce GVHD in lethally irradiated MHC haplo-mismatched Gas6-/- and WT B6 mice (H-2^b). Gas6-/- and WT B6 received 9 Gy total body irradiation and 10⁷ cells from bone marrow of FVB mice (H-2^q) of the same genotype. (Gas6-/- B6 mice received Gas6-/- FVB cells and WT B6 mice were injected with WT FVB cells.) On Day 28, recipients were sacrificed and chimerism was confirmed by expression of H-2^q haplotype in recipient bone marrow. Neither group had clinical evidence of GVHD. However, histologic studies revealed lesions in the livers of both genotypes, which were notably less severe in Gas6-/- mice. The size and number of portal space leukocyte infiltrates were reduced in Gas6-/- mice (40 ± 6.6 % of portal spaces infiltrated), compared to WT mice (76.8 ± 6.5 % of portal spaces infiltrated, P < 0.01). Apoptosis of liver cells was also reduced in Gas6-/- mice. We hypothesized that diminished leukocyte trafficking in the Gas6-/- mice may be due to defects in cytokine-induced expression of adhesion molecules. We therefore isolated primary capillary endothelial cells from Gas6-/- and WT mice, and assessed their response to TNF-α. In contrast to WT endothelial cells, Gas6-/- endothelial cells responded without increases in VCAM-1, ECAM-1, E-selectin, IL-1 or IL-6. This resistance to TNF-α was reversed when the Gas6-/- endothelial cells were co-treated with recombinant Gas6, suggesting that the effect of Gas6 on vascular endothelial function is critical for normal leukocyte trafficking in GVHD. Overall, these results provide strong rationale for considering Gas6 and/or its receptors as a potential therapeutic target to minimize GVHD.