Date: 23rd April 2019
The increasing use of CRISPRs for gene editing is an exciting development for potential gene therapy approaches in the healthcare industry. The FDA has recently approved CRISPR-Cas9 use in several clinical trials and the first patient has already been treated. With CRISPRs offering significant potential here, the leap from lab to clinic is moving closer to being realised and methods to circumvent its limitations to use as a therapeutic are becoming increasingly important.
With this in mind scientists from Arizona State University have been exploring the immunogenic response elicited by Cas9 protein. Derived from a common bacterium, in this case Streptococcus pyogenes, the concerns are that naturally occurring resistance in patients would provide an immunogenic barrier against treatments. The starting point for the group therefore was to collect blood samples to test for an immunogenic response to Cas9 protein. This initial analysis showed that antibodies to Streptococcus pyogenes Cas9 (SpCas9) could be detected in 5% of 143 healthy individuals. The group then used an enhanced predictive algorithm to identify two immunodominant SpCas9 T cell epitopes and, using T-cell assays with healthy donor peripheral blood mononuclear cells (PBMCs) the group implicated these epitopes as key targets in modulating the immunogenicity of SpCas9 protein. Furthermore, when the group modified MHC binding residues within these epitopes, whilst the crucial functional properties and specificity of the SpCas9 were retained, the molecule was rendered less immunogenic.
So what does this stealth mode of Cas9 offer? Potential CRISPR applications for human therapies are rapidly evolving and the level of exposure to CRISPRs may differ depending on the therapy and treatment regime. For example, epigenetic therapies may require weeks or months of treatment whilst gene editing may be achieved by a relatively quicker exposure. The stealth mode of CRISPR reported here provides a proof-of-concept that would enable the potential use of CRISPRs under the radar of the immune system. It may also give an insight into approaches to serial treatment regimes as immunity is not invoked and, for individuals with high levels of antibodies to the native protein, this offers a potential solution that bypasses high-dose systemic immunosuppression which may compromise the patient. Whilst the results presented here in vitro provide a useful proof of concept, further studies including more comprehensive epitope mapping and in vivo analysis will help to refine the technical approach.
Ferdosi, S. R., R. Ewaisha, F. Moghadam, S. Krishna, J. G. Park, M. R. Ebrahimkhani, S. Kiani and K. S. Anderson (2019). “Multifunctional CRISPR-Cas9 with engineered immunosilenced human T cell epitopes.” Nature Communications 10(1): 1842.