Although the gene-editing platform CRISPR-Cas9 holds great therapeutic potential, the platform is far from perfect. Because CRISPR-Cas9 has bacterial origins, most individuals’ immune systems have already been exposed to the Cas9 protein and, as studies are increasingly showing, are ready to attack it should it appear again.

Published in Nature Medicine, the latest evidence to warn the gene therapy community about preexisting adaptive immunity to CRISPR-Cas9 found that 95% of healthy human donors had T cells that reacted to Streptococcus pyogenes Cas9.1 Despite low frequencies of reactive T cells, the authors found that when the cells were enriched and expanded, the cells could recognize and kill CRISPR-Cas9–modified cells.

“Although this was a very artificial experiment, it is a warning that T cells could impact the efficacy and safety of CRISPR gene therapy,” the study’s first author Dimitrios Laurin Wagner, told Cancer Therapy Advisor, regarding the cell enrichment and expansion steps. Wagner is a doctoral student at the Charité’s Berlin-Brandenburg Center for Regenerative Therapies, Germany.

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Previous studies have shown that some people have preexisting antibodies against Cas9, although the prevalence of such an immunity was unclear. In a preprint article, researchers reported that 79% of human blood samples had antibodies to Staphylococcus aureus Cas9 and 65% had antibodies to Streptococcus pyogenes Cas9.2 By contrast, other investigators reported a much lower prevalence, showing 10% of human blood samples had antibodies to Staphylococcus aureus Cas9 and 2.5% had antibodies to Streptococcus pyogenes Cas9.3

The most recent study by Wagner and colleagues investigated the prevalence of Cas9-reactive T cells instead of antibodies directed against Cas9, as most CRISPR-based therapies in development circumvent antibody detection by either using a vector to deliver Cas9 directly into target cells or by modifying cells with Cas9 outside of the body. The CRISPR-based therapies meant to be delivered in vivo, however, typically cannot evade T-cell recognition.

In cancer, several clinical trials are underway to evaluate CRISPR-based therapies, and most of the investigational therapies use an ex vivo approach, in which T cells are edited outside of the body and then infused back into the patient. Indications currently being assessed in trials include esophageal cancer ( Identifier: NCT03081715); prostate cancer ( Identifier: NCT02867345); bladder cancer ( Identifier: NCT02863913); non-small cell lung cancer ( Identifier: NCT02793856); leukemia and lymphoma ( Identifiers: NCT03166878; NCT03398967); renal cell carcinoma ( Identifier: NCT02867332); multiple myeloma; melanoma; synovial sarcoma and myxoid/round cell liposarcoma ( Identifier: NCT03399448); Epstein-Barr virus–associated malignancies ( Identifier: NCT03044743); and others.