Study can facilitate screening of COVID-19 convalescent plasma donors

Brazilian researchers show that commercial serological tests available in laboratories nationwide can be used to screen donated plasma in order to see if it contains enough antibodies capable of neutralizing the novel coronavirus

By Karina Toledo  |  Agência FAPESP – A study conducted by researchers in Brazil shows that two commercial tests widely used to detect antibodies against the novel coronavirus in blood samples and available in laboratories nationwide can also be used to screen plasma from convalescent COVID-19 patients for use in treating the disease.

According to the researchers, who are affiliated with the University of São Paulo (USP) and Fundação Pró-Sangue, which maintains the blood bank at the hospital complex run by USP’s Medical School (“Hospital das Clínicas”), the discovery can significantly increase access to this type of treatment, a possible safe alternative for moderate and severe COVID-19 patients in the absence of a specific drug to treat the disease. The results of the study are reported in an article available on medRxiv and still undergoing peer review.

“The efficacy of this therapy relates directly to the quantity of neutralizing antibodies in the donor’s blood plasma. Some European studies, as well as the FDA [US Food and Drug Administration], have set the minimum titer at one to 160 [1:160]. In other words, after diluting the plasma 160 times it must still be possible to find at least one neutralizing antibody. But the method currently used to measure the ratio is complex, time-consuming and limited to large research centers,” Alfredo Mendrone-Junior, Technical and Scientific Director of Fundação Pró-Sangue and first author of the article, told Agência FAPESP.

According to Mendrone-Junior, the main convalescent plasma screening technique is the viral neutralization test (VNT), which involves culturing SARS-CoV-2 in a laboratory certified for Biosafety Level III (BSL-3) and staffed by highly trained professionals. Moreover, it cannot be used on a massive scale because the methodology involved is manual.

“Our goal was to design an automated method with a high predictive value that could easily be performed in a laboratory,” he said. “We decided to see if there was a correlation between total antibodies measured by commercial serological tests and plasma levels of neutralizing antibodies. We found a linear correlation.”

Methodology

The study used plasma from 214 donors who had contracted COVID-19 and recovered. The samples were analyzed by VNT and two other commercial serological tests – the chemiluminescence test developed by Abbott Laboratories, and Euroimmun AG’s ELISA test – and the results were compared. 

According to Ester Sabino, a professor at the University of São Paulo’s Medical School (FM-USP), the tests evaluated in the study can detect immunoglobulin G (IgG) antibodies. While these are useful to indicate previous contact with the virus, they cannot identify the viral proteins against which the subject’s immune system has produced defenses.

SARS-CoV-2 contains four structural proteins: the spike, envelope, membrane, and nucleocapsid proteins. The spike protein binds to the angiotensin-converting enzyme 2 (ACE2) receptor in order to invade human cells.

“Neutralizing antibodies [nAbs] recognize and neutralize a specific part of the spike protein that binds to the ACE2 receptor. So nAbs are only a fraction of IgG antibodies,” Sabino said.

Mendrone-Junior explained that the purpose of the study was to see if the total values of IgG measured by commercial serological tests correlated with the quantity of nAbs measured by VNT. In other words, the researchers aimed to find out whether the serological test result could provide a cutoff value above which convalescent plasma very probably would contain nAbs at a minimum titer of 1:160 or higher.

“Both the ELISA test and the chemiluminescence test measure the level of IgG in terms of optical density [OD]. OD values higher than one correspond to a positive result, indicating that the subject has been infected by SARS-CoV-2 and has produced an immune response,” Mendrone-Junior said. “We showed that values higher than 4.57 have significant predictive power regarding the presence of neutralizing antibodies at the desired concentration of 1:160.”

According to the authors, the main advantage of these two techniques is that they are automated and can process 400-500 samples per day. In addition, they are available virtually everywhere in Brazil and Latin America, facilitating wider use of convalescent plasma to treat COVID-19.

Also known as passive immunity transfer (or passive antibody therapy), the technique was first developed in 1891 to treat diphtheria, which is caused by Corynebacterium diphtheriae. More recently it has been used during outbreaks of respiratory infections such as H1N1 influenza (2009-10), and to treat the respiratory syndromes caused by coronaviruses SARS-CoV-1 (2003) and MERS-CoV (2012).

According to Mendrone-Junior, the study described in the article is part of a longstanding partnership between researchers at Fundação Pró-Sangue and the FM-USP group led by Sabino. “We’ve investigated the prevalence of sexually transmitted diseases and arboviral diseases such as dengue and zika using material collected by blood banks. This year we also began to study COVID-19 seroprevalence with FAPESP’s support,” he said.

The article “Correlation between SARS-CoV-2 antibody screening by immunoassay and neutralizing antibody testing” can be read at: www.medrxiv.org/content/10.1101/2020.10.11.20210005v1.