Technical information

In a typical immunological response IgM antibodies are usually the first line of defence. Seroconversion to IgG then occurs later on and these high-affinity IgG antibodies will be responsible long term immunity and immunological memory (Li et al. 2020; Racine et al. 2009).

Figure 1: Schematic representation of typical adaptive immune response after viral infection during course of disease and convalescence.

Due to the late onset of antibody development during the course of the disease, molecular methods will be preferred for diagnostics of SARS-CoV-2. However, sensitivity limitations were observed in molecular testing and serology could aid to identify patients appearing false-negative in PCR despite showing symptoms (Farnsworth and Anderson 2020; Xu et al. 2020). Further, serology is strongly suggested to play a major role for screening populations to determine exposure as well as potential immunity, to identify convalescent individuals as plasma donors as well as for research on immune response and to help identify neutralizing antibodies (Amanat et al. 2020; Farnsworth and Anderson 2020; Okba et al. 2020).

A prerequisite of developing highly sensitive serological assays for diagnostic purposes is the use of high quality raw materials that reliably enable distinguishing healthy individuals from those infected. One of the core raw materials of such tests are antigens capable of capturing the patient’s antibodies of interest. DIARECT has newly developed SARS-CoV-2 Nucleocapsid (N) Protein and SARS-CoV-2 Spike (S) Glycoprotein outperforming reference antigens  (manufactured in different expression systems) in different serological assays. DIARECT’s antigens showed a higher reactivity towards patient serum with comparably low background in negative samples at the same time (Figure 2).

Figure 2: Comparison of DIARECT‘s Nucleocapsid (N) antigen and Spike (S) RBD antigen to respective reference products

A; B: Line Assay. The antigens and controls (IgG; IgM; IgGMA) were printed on a nitrocellulose membrane as indicated. Antigens were printed at the following concentrations: Line 5 (Reference N) 50 µg/ml; Line 6 (DIARECT N) 50 µg/ml; Line 7 (Reference N) 25 µg/ml; Line 8 (DIARECT N) 25 µg/ml; Line 9 (Reference RBD) 50 µg/ml; Line 10 (DIARECT RBD) 50 µg/ml; Line 11 (Reference RBD) 75 µg/ml; Line 12 (DIARECT RBD) 75 µg/ml; Line 13 (Reference RBD) 100 µg/ml; Line 14 (DIARECT RBD) 100 µg/ml. Each line assay was incubated with a patient serum (1-5), serum from a healthy blood donor (1-3) or PBS (negative control). Detection was made using anti-human IgG (Figure 2 A) or anti-human IgM (Figure 2 B) secondary antibodies. Each line was prepared in duplicates (only one shown per serum).

A‘; B‘: Graphic evaluation of the line assays. Mean of intensities of line assays (n=2) including standard deviations are displayed.

DIARECT’s SARS-CoV-2 antigens are produced in the baculovirus/insect cell expression system.

References:

Amanat et al. (2020) Nat Med. 26 (7): 1033 - 1036
Farnsworth and Anderson (2020) Clin Chem. hvaa107
Gorbalenya et al. (2020) Nat Microbiol. 5 (4): 536 - 544
Lake (2020) Clin Med (Lond). 20 (2):124 - 127
Li et al. (2020) J Med Virol. 92: 1518 - 1524
Lu et al. (2020) Lancet. 395 (10224): 565 - 574
McBride et al. (2014) Viruses. 6 (8): 2991 - 3018
Okba et al. (2020) Emerg Infect Dis. 26 (7)
Racine et al. (2009) Immunology Letters 125 (2): 79 - 85
Qiu et al. (2005) Microbes Infect. 7 (5-6): 882 - 889
Tortorici and Veesler (2019) Adv Virus Res. 105: 93 - 116
Xu et al. (2020) Emerg Microbes Infect. 9 (1): 924 - 927
Zhou et al. (2020) Nature. 579 (7798): 270 - 273

NB - In some countries the use of certain antigens in diagnostic tests may be protected by patents. DIARECT is not responsible for the determination of these issues and suggests clarification prior to use.