Potent farnesyltransferase inhibitor ABT-100 abrogates acute allograft rejection

Inhibition of protein geranylgeranylation and farnesylation protects against graft-versus-host disease via effects on CD4 effector T cells

Despite advances in immunosuppressive regimens, acute graft-versus-host disease remains a frequent complication of allogeneic hematopoietic cell transplantation. Pathogenic donor T cells are dependent on correct attachment of small GTPases to the cell membrane, mediated by farnesyl- or geranylgeranyl residues, which, therefore, constitute potential targets for graft-versus-host disease prophylaxis. A mouse model was used to study the impact of a farnesyl-transferase inhibitor and a geranylgeranyl-transferase inhibitor on acute graft-versus-host disease, anti-cytomegalovirus T-cell responses and graft-versus-leukemia activity. Treatment of mice undergoing allogeneic hematopoietic cell transplantation with farnesyl-transferase inhibitor and geranylgeranyl-transferase inhibitor reduced the histological severity of graft-versus-host disease and prolonged survival significantly. Mechanistically, farnesyl-transferase inhibitor and geranylgeranyl-transferase inhibitor treatment resulted in reduced alloantigen-driven expansion of CD4 T cells. In vivo treatment led to increased thymic cellularity and polyclonality of the T-cell receptor repertoire by reducing thymic graft-versus-host disease. These effects were absent when squalene production was blocked. The farnesyl-transferase inhibitor and geranylgeranyl-transferase inhibitor did not compromise CD8 function against leukemia cells or reconstitution of T cells that were subsequently responsible for anti-murine cytomegalovirus responses. In summary, we observed an immunomodulatory effect of inhibitors of farnesyl-transferase and geranylgeranyl-transferase on graft-versus-host disease, with enhanced functional immune reconstitution. In the light of the modest toxicity of farnesyl-transferase inhibitors such as tipifarnib in patients and the potent reduction of graft-versus-host disease in mice, farnesyl-transferase and geranylgeranyl-transferase inhibitors could help to reduce graft-versus-host disease significantly without having a negative impact on immune reconstitution.

Delayed rejection of MHC class II-disparate skin allografts in mice treated with farnesyltransferase inhibitors

Farnesyltransferase inhibitors (FTIs), developed as anti-cancer drugs, have the potential to modulate immune responses without causing nonspecific immune suppression. We have investigated the possibility that FTIs, by affecting T cell cytokine secretion, can attenuate alloreactive immune responses. The effects of FTIs on murine alloreactive T cells were determined both in vitro, by measuring cytokine secretion or cell proliferation in mixed lymphocyte cultures, and in vivo, by performing skin allografts from H-2(bm12) mice to MHC class II-disparate B6 mice. We found that two different FTIs, ABT-100 and L-744,832, blocked secretion of IFN-gamma, IL-2, IL-4, and TNF-alpha from naïve T cells in vitro. ABT-100 and L-744,832 blocked cytokine production from both CD4(+) and CD8(+) naïve T cells stimulated with CD3 and CD28 antibodies, but only if the cells were pretreated with the FTIs for 48 h. Proliferation of alloreactive T cells in mixed lymphocyte cultures was blocked by either FTI. We also found that the proliferation of enriched T cells stimulated with IL-2 was blocked by ABT-100 treatment. In mice with an MHC class II-disparate skin graft, rejection of primary allografts was significantly delayed by treatment with either ABT-100 or L-744,832. Secondary rejection in mice previously primed to the alloantigen was found to be unaffected by L-744,832 treatment. We have shown that FTIs can block T cell cytokine secretion and attenuate alloreactive immune responses. The ability of FTIs to block secretion of cytokines, including IFN-gamma and IL-4, from naïve T cells provides a likely biological mechanism for the specific suppression of class II MHC-mediated allorejection. These results demonstrate that FTIs may have useful immunomodulatory activity due to their ability to delay priming to alloantigens.

Survival of skin allografts is prolonged in mice with a dominant-negative H-Ras

Ras is a guanine nucleotide-binding protein that plays a major role in regulating the proliferation of T cells. To investigate the mechanism of the Ras/mitogen-activated protein kinase pathway, one of the downstream signal-transduction pathways of T-cell receptors, in the response to alloantigen, we performed full-thickness skin grafting in the major histocompatibility complex (MHC) incompatible strain BALB/c (H-2Kd) (donor) and T-cell-specific H-Ras dominant-negative (dnRas) transgenic (tg) C57BL/6 (H-2Kb) (recipient) male mice. In vitro and in vivo dnRas tg mouse T-cell proliferation and cytotoxic T lymphocyte (CTL) activity assay were also performed. The median graft survival time in control B6/wild type (wt) mouse allografts was seven days. Conversely, the dnRas tg mouse group exhibited a significant (p<0.01) prolongation of graft survival to 15 days. However, all grafts were eventually rejected after one month. Mixed lymphocyte reaction and popliteal lymph node assay revealed that T-cell proliferation was decreased in response to alloantigen, but CTL activity was not changed in the dnRas tg mice. These results suggested that Ras is essential for peripheral T lymphocytes to respond to allo-MHC antigens, and Ras may be a molecular target for controlling transplant rejection.

Farnesyltransferase inhibitors inhibit T-cell cytokine production at the posttranscriptional level

Several cytoplasmic proteins, such as GTPases of the Ras family, containing a C-terminal CAAX motif are prenylated by farnesyltransferase to facilitate localization to cellular membranes where activation occurs. Farnesyltransferase inhibitors (FTIs) interfere with this farnesylation process, thereby preventing proper membrane localization and rendering the proteins unavailable for activation. Currently, FTIs are being explored as antineoplastic agents for the treatment of several malignancies. However, since farnesylated proteins like Ras are also involved in intracellular signaling in lymphocytes, FTIs might interfere with T-cell activation. Based on this hypothesis we examined the effect of several FTIs on cytokine production in response to anti-CD3 + anti-CD28 monoclonal antibodies or PMA + ionomycin. Murine Th1 and Th2 clones, stimulated in the presence of FTIs, showed a dose-dependent reduction of lineage-specific cytokine secretion (IFN-gamma, IL-2, IL-4, IL-5). However, no inhibition of ERK or JNK MAP kinases was observed, nor was induction of cytokine mRNA affected. Rather, intracellular cytokine protein synthesis was blocked. Inhibition of human T-cell INF-gamma production also was observed, correlating with reduced phosphorylation of p70S6K. These results indicate that FTIs inhibit T-cell activation at the posttranscriptional level and also suggest that they may have potential as novel immunosuppressive agents.