Comment on Miao et al, page 19
A potent immune response to gene therapy vectors, including naked DNA, creates a daunting problem. Miao and colleagues now present data showing that a combination of therapies that disrupt costimulation may obviate this immune response and induce tolerance to the newly expressed protein.
Gene therapy holds great promise for the treatment of a multitude of diseases. However, successes in expressing genes in vitro are often confounded when moved in vivo. Not only are many of the vectors used highly immunogenic, but a newly expressed gene product may often engender an immune response in a recipient lacking the desired protein, due to a lack of tolerance, as in hemophilia A. Moreover, naked DNA can promote an immune response to the expressed gene product via Toll-like receptor activation, specifically TLR9. In essence, it becomes its own adjuvant!
In 2005, at a meeting of the American Society for Gene Therapy, the importance of understanding the role of the immune system in attempting gene therapy was emphasized.1 Despite resistance by hard-core molecular biologists to learn immunology, it is clear that many groups have paid a great deal of attention to understanding and controlling the immune response under conditions of gene therapy. This is exemplified by the approach taken by Miao and colleagues at the University of Washington in the current issue of Blood.
Using a high-expressing B-domain deleted factor VIII plasmid, the authors have previously found short-lived expression and clear evidence of an antibody response to the newly expressed FVIII protein. To control the immune response to FVIII, they tried a series of standard immunosuppressive drugs either alone or in certain combinations, namely cyclosporine A (CSA); rapamycin (RAP); mycophenylate mofetil (MMF); CSA and MMF; or RAP and MMF. While moderate effects on gene expression were seen with these drugs, none led to long-term expression and tolerance to FVIII. In the past, efforts to block costimulation with monoclonal antibody against CD40 ligand or Ctla4-Ig have been used in a number of experimental models, including attempts to induce tolerance to therapeutic doses of FVIII.2 These too have not led to sustained tolerance despite some efficacy, particularly in transplantation models.2,3 The twist used by Miao and coworkers was to combine the antibody against CD40L (MR-1) with Ctla4-Ig. This combined treatment led to both long-term expression of naked DNA-expressed FVIII in all recipients, and a low titer of anti-FVIII in only 1 of 9 recipients (and this animal still had persistent FVIII expression). Because there was persistent expression of FVIII, it was assumed that the animals were tolerant, although formal proof that these mice are tolerant to FVIII via challenge or adoptive transfer is lacking. Further experiments are planned to examine this process.
The use of costimulatory blockade to block an immune response is not novel; the combination approach is an exciting result and offers promise for both gene therapy and protein therapy with FVIII. Indeed, Koenen et al4 have suggested using a bispecific antibody approach to block both CD40 and CD86; this also has therapeutic promise, as do other efforts with fusion immunoglobulins,5-7 which may act via Fc receptor pathways. Finally, concerns over the thrombotic events observed with anti-CD40L trials (as well as recent adverse events with an anti-CD28 trial) suggest caution with broad antibody-mediated therapies. Nonetheless, the success in the current animal model is highly encouraging for both gene therapy approaches and therapy for FVIII inhibitors. ▪
