Assay development for the qualitative assessment of SARS-CoV-2-specific immune responses

Test systems that reliably indicate the SARS-CoV-2 immune status play a crucial role in the targeted relaxation of control measures. The production of such immunoassays primarily relies on recombinantly produced variants of the SARS-CoV-2 spike protein (S-protein). Currently, there are severe bottlenecks in the supply of these reagents. Production capacities in animal cell culture systems were fully exhausted, and the quality of available reagents was sometimes insufficient.

Fraunhofer IME has the expertise and capacity for the rapid production of complex glycoproteins in plants. As part of the Fraunhofer Anti-Corona Program, the project “Corona diagnostics in plants” (CDP) focused on the rapid establishment and scaling of a plant-based transient production process for S-protein variants as specific assay reagents.

The human angiotensin-converting enzyme 2 (ACE2) is another important molecule for the development and improvement of SARS-CoV-2 serum assays. The receptor-binding domain (RBD) of the S-protein binds to ACE2, enabling the virus to enter host cells.

The ARRay project aimed to establish an efficient process for the recombinant production of ACE2 protein variants in the transient plant system. These assays would then be used to develop assays for the qualitative and quantitative characterization and quality control of S protein variants from the CDP project. The development of new assays for the in vitro quantification of the virus-neutralizing activity of immune sera via inhibition of the RBD-ACE2 interaction in a competitive setup was a second key project goal. This would contribute to defining a functionally based, quantifiable "correlate of protection" by correlating it with epidemiological data. With its contribution of reagents for quality control and the development of refined assays for the characterization of immune sera, ARRay represented an optimal and complementary addition to the CDP project. The project thus supported the goal of expanding diagnostic options for a better understanding of serology and thus the course of infection.