SARS-CoV-2 Research Highlights the Importance of Human Milk Immunobiology

  • Milk researchers all over the world are focusing on how human milk responds when mothers have COVID-19, the disease caused by SARS-CoV-2.
  • Several research teams are investigating the use of purified milk antibodies directed at SARS-CoV-2 and possibly even whole milk collected from COVID-19-positive mothers as a therapy to treat critically ill COVID-19 patients.
  • Although milk may contain antibodies against SARS-CoV-2, it may also be a vertical mode of transmission by passing on the virus itself.
  • A large collaborative study is working to determine if viral RNA specific to SARS-CoV-2 is present in milk from COVID-19-positive mothers.

 

Over the last six months, scientists all over the world have put their planned research programs on hold and pivoted to study SARS-CoV-2 (severe acute respiratory syndrome coronavirus 2). Milk researchers are no exception. Milk from mothers that have COVID-19, the illness caused by SARS-CoV-2, could be a source of antibodies directed against the virus. Like convalescent plasma (i.e., blood from recovered COVID-19 patients), these maternally-derived antibodies offer potential as a therapeutic to help severely ill patients. But human milk could also contain RNA from SARS-CoV-2, and possibly even infectious viral material. Telling infected mothers to stop nursing “just in case” is not an option, particularly in populations without access to human milk alternatives. There is urgency in identifying both therapeutics to help those with the most severe infections and to establish informed public health policy for nursing mothers that are COVID-19 positive. The vast number of investigators tackling these questions across institutions and countries offers promise that answers will soon be available [1].

Passive (and Aggressive?) Immunity

Vaccines are an incredible scientific achievement, providing a way to develop pathogen-specific antibodies without having to suffer any of the ailments associated with the pathogen. But vaccines have nothing on passive immunity; mammalian mothers pass on their pathogen experience (aka antibodies) to their offspring in milk without the infant needing to make any energetic investment in mounting an immune response.

Human milk contains several types of antibodies (or immunoglobulins), including immunoglobulin M (IgM), immunoglobulin G (IgG), and secretory immunoglobulin A (sIgA), which is the most prevalent. The specificity of these immunoglobulins to particular viruses or pathogens allows for immature infants to mount a very mature, and aggressive, immune response that can attenuate or possibly even prevent infection and illness.

The spread of a novel coronavirus around the world means that no one has built up immunity to SARS-CoV-2. But could infants of COVID-19-positive mothers be the exception? And if so, can we take advantage of those passively obtained antibodies as a potential therapy for severe cases of COVID-19?

One researcher tackling these questions is Dr. Rebecca Powell from the Icahn School of Medicine at Mount Sinai in New York City. In early April 2020, Dr. Powell put out a request for milk samples from New York City mothers on social media. The response from mothers was overwhelming. “I was shocked by the response I got,” said Powell. “I recruit for milk studies all the time, but this was at such a high level and so unexpected.”

New York City was the epicenter of the SARS-CoV-2 pandemic in the United States, so Powell expected that there might be many mothers that had either tested positive or presumed they had COVID-19, including many healthcare workers that were on the front lines. But she had not expected that so many mothers would be interested and willing to share milk samples with her for her research.

Powell and her colleagues are analyzing milk samples for the presence of SARS-CoV-2- specific antibodies, including IgM, IgG, and sIgA. “The bulk of what I expect to be the longer lasting antibody is sIgA, but I want to measure the other immunoglobulins in order to fully understand the maternal response,” explained Powell. Despite decades of research on human milk immune factors, researchers are still testing hypotheses about the relationship between the maternal immune response and the types and quantities of immune factors in human milk. “We really don’t know enough to focus on only one type of immunoglobulin. And we can’t assume that someone who is infected and has antibodies in their blood would have them in their milk. We can’t take for granted that we would just know this as a given.”

In a pilot research project [1] (which has not yet undergone peer-review), Powell and colleagues report on findings from 15 milk samples from COVID-19-positive mothers compared with 10 milk samples collected prior to December 2019 (i.e., pre-COVID-19, to use as a control). All samples were analyzed for how well different antibody classes reacted to a Receptor Binding Domain on one of the virus’s spike proteins. The findings, although preliminary, suggest that SARS-CoV-2-specific antibodies are passed from mothers to infants in human milk. Eighty percent of samples from COVID-19-positive mothers had reactive IgA antibodies and reactive secretory antibodies, suggesting sIgA was specific to SARS-CoV-2 [1].

The next step is to verify these findings from a larger sample to demonstrate a typical pattern in the immune response in milk. Then, Powell hopes to purify SARS-CoV-2-specific antibodies to use as a treatment for severely ill patients. “This would be for people who have a high likelihood of dying and would probably be used in combination with other treatments,” said Powell. “[Purified milk antibodies] could mitigate the pathology enough to help survival.” But first, she will need to get approval to use milk antibodies as a treatment. If approved, this type of research could pave the way for using isolated milk antibodies to treat other diseases. “Milk immunobiology might finally get the recognition it deserves,” she explains. “We can really learn lessons that help with other diseases.”

Milk as Medicine

At the same time Dr. Powell was using social media to recruit participants, the Dutch Milk Bank put out a request for milk samples on Facebook to Dutch mothers that had tested positive or suspected that they had COVID-19.

Dr. Kasper Hettinga, Dr. Hans van Goudoever, and their colleagues at the University Medical Center, Amsterdam and several other institutions throughout The Netherlands are performing similar milk antibody research to Powell’s research. Hettinga and colleagues also hope to quantify the immune response in milk to SARS-CoV-2 and are asking for milk samples from a week after initial infection (or positive test result) through two months after infection. Dutch mothers have also shown strong interest in participating, with many providing daily samples that allow the researchers to see how antibody levels change over the course of the infection.

The Dutch researchers agree that these antibodies could be used as a therapeutic for the most critical COVID-19 patients. But instead of isolating antibodies, they plan to use whole milk as a therapeutic.

“The initial focus of our research is whether anti-SARS-CoV-2 antibodies in milk have a neutralizing function,” explains Hettinga. “But by looking at milk as a whole, we may see additional anti-viral activity as well.” These additional anti-viral milk factors include lactoferrin, lysozyme, and even free fatty acids. “We want to look at how milk works in modulating the immune system. It has such a wide range of proteins with antiviral functions that could benefit elderly vulnerable COVID-19 patients,” says Hettinga.

Giving severely ill patients human milk would allow them to take advantage of the synergistic activities of milk immune factors that evolved to enhance survival in vulnerable infants and young children. But because raw milk is not sterile, Hettinga and colleagues are also investigating various pasteurization techniques that make the milk safe to drink but also retain all of the important immune components.

Like Powell’s purified antibodies, the research from Hettinga and colleagues could pave the way for using pasteurized milk as a treatment for critically ill patients suffering from illnesses other than COVID-19.

Wolf in Sheep’s Clothing?

Dr. Michelle McGuire, Director and Professor in the School of Family and Consumer Sciences at the University of Idaho, is another scientist interested in the milk immune response to SARS-CoV-2—the entire immune response. This includes the presence of the virus itself.

McGuire is a nutritionist who started studying human milk because “it is,” as she describes “the ultimate food.” Her research usually focuses on identifying bacterial populations in human milk, more commonly known as the milk microbiome. But she is now part of a large collaboration across numerous institutions and one of several principal investigators looking at all of the components present in milk from mothers with COVID-19, including SARS-CoV-2.

“There are currently a lot of conflicting recommendations on breastfeeding for infected women,” explains McGuire. “We really need to be able to tell mothers what to do now. But we really need to tell moms when this virus hits Africa and low-income countries where there is no safe alternative to mother’s milk.”

Her role in this project, funded by both The Gates Foundation and National Science Foundation, includes the establishment of a standardized milk collection procedure for studying SARS-CoV-2 and determining whether or not viral RNA from SARS-CoV-2 can be detected in milk from mothers during the first weeks after diagnosis. These are not mutually exclusive; if you are looking to see if there is viral material in milk, you need to be able to eliminate other potential contaminants, including breast tissue.

There is an important difference between looking for viral RNA and looking for infectious viral material. “Even if the virus is detectable in milk, it doesn’t mean that milk is a mode of transmission,” McGuire explains. “We are looking at viral RNA only, not infectious particles. You need more than genetic material to be an active and infectious virus.”

McGuire and her team are not the only investigators looking for viral RNA in milk samples. To date there have been over 20 publications on the topic, with conflicting results. McGuire believes that although the quality of papers improved over time, the results should not be used to inform public policy. For starters, the methods of milk collection did not follow a standardized procedure that would eliminate other methods of contamination. Second, many of the studies used milk collected weeks after infection where viral material was less likely to be detected. And finally, only one of these papers discussed validating their method for identification of viral RNA in milk (as opposed to plasma).

McGuire and colleagues are recruiting COVID-19-positive mothers in various ways, including social media, through the American Academy of Pediatrics (AAP) website, and through UCLA and UCSF hospitals that refer mothers that test positive when they arrive in labor.

“We want to get 50 women enrolled during their first week of diagnosis,” explains McGuire. In addition to collecting milk samples, they will ask mothers about their specific symptoms and the severity of their symptoms to understand the relationship between the infection and the maternal response in milk.

“Within the next three months we hope to have solid data on whether we can detect viral RNA in milk produced in the first week after diagnosis using an assay validated for human milk.” If no viral RNA is present, then there is no need to look for infectious particles.

The results of McGuire’s work will be of great interest to organizations that disseminate public health recommendations, including the CDC, WHO, and AAP. “We are communicating directly with policy makers and they will have the data before they are published to have time to respond and dictate public policy.” As of June 2020, the WHO recommends COVID positive mothers continue to nurse as the benefits outweigh any potential risks.

McGuire has been encouraged by the collaborative nature of her research team. “Rather than a competition to be the first, the collaboration is really moving the science forward more quickly than usually happens when it is lab versus lab.”

If research does indeed proceed quickly for McGuire, as well as for Hettinga and Powell, look to SPLASH! in the fall for an update on all of these ongoing research projects on milk and SARS-CoV-2.

 

1. Fox A, Marino J, Amanat F, Krammer F, Hahn-Holbrook J, Zolla-Pazner, Powell RL. 2020. Evidence of a significant secretory-IgA-dominant SARS-CoV-2 immune response in human milk following recovery from COVID-19. medRxiv. 2020 Jan 1.

 

Contributed by
Dr. Lauren Milligan Newmark
Researcher, Science Writer