Immunogenicity of CRISPR therapeutics-Critical considerations for clinical translation

Abstract

CRISPR offers new hope for many patients and promises to transform the way we think of future therapies. Ensuring safety of CRISPR therapeutics is a top priority for clinical translation and specific recommendations have been recently released by the FDA. Rapid progress in the preclinical and clinical development of CRISPR therapeutics leverages years of experience with gene therapy successes and failures. Adverse events due to immunogenicity have been a major setback that has impacted the field of gene therapy. As several in vivo CRISPR clinical trials make progress, the challenge of immunogenicity remains a significant roadblock to the clinical availability and utility of CRISPR therapeutics. In this review, we examine what is currently known about the immunogenicity of CRISPR therapeutics and discuss several considerations to mitigate immunogenicity for the design of safe and clinically translatable CRISPR therapeutics.

Keywords: CRISPR-Cas; Cas9; clinical translation; clinical trials; gene therapy; genome editing; immunogenicity.

FIGURE 1

Considerations to mitigate immunogenicity for clinical translation of CRISPR therapeutics. (A) Treatment approach: ex vivo gene editing poses a smaller risk of immunogenicity than in vivo administration but has limited utility for diseases requiring systemic delivery of CRISPR components. (B) Target tissue: CRISPR administration in immune-privileged sites may limit immunogenicity. (C) CRISPR effector protein: non-cross-reactive CRISPR effector protein orthologs from non-ubiquitous bacteria or archaea can be considered for repeated dosing or in individuals with pre-existing immunity. (D) Delivery vector: viral vectors are more efficient than non-viral vectors such as lipid nanoparticles but immunogenicity and ortholog cross-reactivity could limit repeated administration; viral vector tissue tropism can be used for targeting, which reduces the dose and systemic immune responses. (E) Patient-specific considerations: include immune status, pre-existing B and T cell immune responses, and MHC polymorphism which dictates immunodominant MHC class I and class II epitopes of various CRISPR effectors. (F) Intended CRISPR activity and duration: control of CRISPR duration of action could limit specific immune responses; the intended use of CRISPR for its nuclease or transcriptional modulation activity will dictate its persistence, duration, and immunogenicity; immunogenicity against the transgene product is possible in CRISPR therapeutics intended for targeted insertion of a therapeutic transgene. (G) Treatment regimen: co-administration of immunosuppressive drugs or adoptive transfer of CRISPR effector-specific T_reg cells can limit specific immune responses.

FIGURE 2

Assessment, mitigation, and monitoring of immune responses to CRISPR therapeutics. Strategies for mitigation of immune responses can be personalized to each patient’s needs, history, immune status, and treatment plan. Pre-treatment assessment protocols can include screening for immune responses to different CRISPR effector proteins, HLA typing, and other tests that are relevant to the treatment approach such as pre-existing immune responses to the viral vector if one is used for delivery. According to the assessment results, appropriate mitigation strategies can then be implemented. These can include selection of a CRISPR effector ortholog which the patient has no pre-existing immunity against and engineering the selected protein to silence the immunodominant epitope(s) based on the individual’s MHC haplotype. Following administration of the CRISPR therapeutic, the patient should be monitored for the development of innate or adaptive immune responses and appropriate intervention strategies can be implemented accordingly. The assessment process can be repeated for each subsequent dose or for future administration of a different CRISPR therapeutic.