Antibodies show differences
Analyzing the antibodies in blood samples taken from study participants on the day of their coronavirus test and 28 days later, the researchers discovered some notable differences between those who developed severe symptoms and those who didn’t.
Antibodies are proteins roughly shaped like two branched trees. They are produced by immune cells and secreted in response to things the body perceives as foreign, such as microbial pathogens. One amazing property of antibodies is that their ramifications can take on a variety of shapes. The resulting spatial and electrochemical diversity of the regions defined by the branches of the antibodies and their intersections is so great that the antibodies as a whole occupy all the corners.
When the shape and electrochemistry of an antibody is complementary to a characteristic of a pathogen, things get tight. Sometimes liability is not in the right place to prevent the pathogen from doing its nefarious business. Antibodies that bind pathogens in exactly the right places to prevent infection are called neutralizing antibodies.
In both cases, the resulting adhesion creates what is known as an immune complex, which attracts immune cells to the site.
The researchers found that while many participants whose symptoms remained mild initially had healthy levels of neutralizing antibodies to SARS-CoV-2, participants who were hospitalized initially had minimal or undetectable levels of neutralizing antibodies, although their immune cells began to pump out later in the course of the infection.
A second finding concerned an often-neglected structural aspect of antibody “skeletons”: they are decorated with chains of different types of sugar molecules that are linked together. The composition of these sugar chains has an impact on how inflammatory an immune complex will be.
We wondered why a minority of people develop this excessive inflammatory response when most people don’t.
Many types of immune cells have receptors for this sugar-coated antibody strain. These receptors distinguish between the sugar molecules on the antibodies and help determine how vigorously the immune cells respond. A key finding of the new study was that participants with severe COVID-19 had sugar chains on certain antibodies that target SARS-CoV2 that were deficient in a type of sugar called fucose. This deficiency was apparent the day these “progressors” first tested positive. So it was not a consequence of a serious infection, but preceded it.
In addition, the immune cells of these patients had excess receptors for these fucose-deficient types of antibodies. Such receptors, called CD16a, are known to increase the inflammatory activity of immune cells.
“Some inflammation is absolutely necessary for an effective immune response,” Wang said. “But too much can cause problems, like the massive inflammation we see in the lungs of people whose immune systems didn’t block SARS-CoV-2 quickly after infection” – for example because their early immune response wasn’t generated enough neutralizing antibodies against the virus.
A look at the vaccine response
The scientists also looked at the antibodies raised in 29 adults after they received the first and second doses of Pfizer’s mRNA vaccine. They compared these antibodies to those found in adults who didn’t develop serious illness about a month after being vaccinated or infected; They also compared them to antibodies from people hospitalized with COVID-19. Two doses of vaccine resulted in high levels of neutralizing antibodies overall. In addition, the antibody-fucose content was high in the vaccinated and mildly symptomatic groups but low in the hospitalized subjects.
Wang and her collaborators tested their results on mice that had been bioengineered so that their immune cells had human receptors for antibodies on their surfaces. They applied immune complexes — extracted differently from patients with high levels of fucose-deficient antibodies, patients with normal levels, or vaccinated adults — to the tracheae of the mice. The researchers observed four hours later that the fucose-deficient immune complex extracts triggered a massive inflammatory response in the mice’s lungs. Neither normal fucose extracts nor extracts from vaccinated subjects had this effect.
When the experiment was repeated in similar mice lacking CD16a, there was no such hyperinflammatory response in their lungs.
Wang said the immunological factors the researchers identified — a sluggish neutralizing antibody response, deficient levels of fucose on antibody-bound sugar chains, and excess fucose-deficient antibody receptors — each individually provide a modest predictor of the severity of COVID-19 were . But taken together, they allowed scientists to guess the course of the disease with about 80% accuracy.
Wang speculates that the abundance of CD16a on immune cells and the relative absence of fucose on the sugar chains of antibodies may not be entirely independent phenomena in some people, and that either alone is insufficient to consistently produce severe inflammatory symptoms after SARS-CoV-2 infection leads the combination into a devastating inflammatory overdrive.
Additional Stanford co-authors on the study are postdoctoral researchers Vamsee Mallajosyula, PhD, Megha Dubey, PhD, Usama Ashraf, PhD, Bowie Cheng, PhD, Nimish Kathale, PhD, Fei Gao, PhD, and Prabhu Arunachalam, PhD; life science researchers Kim Tran and Courtney Scallan; Genomics Manager Xuhuai Ji, MD, PhD; Scott Boyd, MD, PhD, associate professor of pathology; Mark Davis, PhD, director of the Stanford Institute for Immunity, Transplantation and Infection and professor of microbiology and immunology; Marisa Holubar, MD, clinical associate professor of infectious diseases; Chaitan Khosla, PhD, Professor of Chemical Engineering and Chemistry; Holden Maecker, PhD, Professor of Microbiology and Immunology; Yvonne Maldonado, MD, Professor of Pediatric Infectious Diseases and of Epidemiology and Public Health; Elizabeth Mellins, MD, professor of pediatric human gene therapy; Kari Nadeau, MD, PhD, Professor of Medicine and Pediatrics; Bali Pulendran, PhD, Professor of Pathology and Microbiology and Immunology; Upinder Singh, MD, Professor of Infectious Diseases and Geographical Medicine and of Microbiology and Immunology; Aruna Subramanian, MD, clinical professor of infectious diseases; PJ Utz, MD, professor of immunology and rheumatology; and Prasanna Jagannathan, MD, assistant professor of infectious diseases and microbiology and immunology.
researchers at San Jose State University; the University of California, San Francisco; the Icahn School of Medicine; and Cornell University contributed to the work.
Research was funded by the National Institutes of Health (Grants U19AI111825, U54CA260517, R01AI139119, U01AI150741-02S1, 5T32AI007290, U24CA224319, and U01DK124165), Fast Grants, CEND COVID Catalyst Fund, the Crown Foundation, the Sunshine Foundation, and the Marino Family Foundation.