Cross-immunity: Can cow's milk help fight COVID-19?

SARS-CoV-2 shares many characteristics with other coronaviruses in the same family, to the extent that cross-immunity may be possible. Researchers want to evaluate the therapeutic potential of antibodies directed against a bovine coronavirus for treating the pandemic virus. Easily produced at low cost, these antibodies could be consumed in the form of milk from immunized cows. Amid the COVID-19 pandemic, treatment options for coronavirus infection remain limited. As with many viruses, there is still no specific, effective treatment for SARS-CoV-2. To treat severe cases, it is becoming urgent to identify immune and pharmacological mechanisms and strategies to control the disease. Vaccines that build preventive immune defenses have proven effective, and certain antibodies are already being used to neutralize the virus. Researchers are now proposing to rely on cross-immunity transmitted through the milk of immunized cows. SARS-CoV-2 is a coronavirus, but it is far from being the only one. Many others infect the animal kingdom, notably BCoV, a bovine coronavirus that causes gastroenteritis in young, unimmunized calves, with possible respiratory complications. SARS-CoV-2 and BCoV are phylogenetically very closely related β-coronaviruses that share common characteristics. Structurally, BCoV is enveloped by five structural proteins: the spike glycoprotein (S), the envelope protein (E), the membrane protein (M), the nucleocapsid protein (N), and the hemagglutinin-esterase (HE) protein. The structure of SARS-CoV-2 also contains the first four. Among these shared proteins is the spike (S) protein. It is this same protein, which is widely exposed on the viral surface, that triggers an immune response against the virus and constitutes an immunodominant structure against which the vaccines developed to date are directed. Various studies demonstrate that this similarity between the viruses gives rise to cross-immunity, since the viruses retain these shared epitopes. The researchers therefore propose leveraging the heterologous passive immunity conferred by BCoV—induced by the milk of immunized cows—as an immunostimulatory therapy to control SARS-CoV-2 infection. Cow’s milk contains more than 30 g/L of proteins, including nearly 1 g/L of immunoglobulins—and even more than 200 g/L in colostrum. These antibodies in bovine milk survive gastric exposure and resist proteolytic digestion in the stomach and intestine. Thus, colostrum confers passive immunity that provides remarkable protection against most infectious diseases in animals and has been well studied for nearly all coronaviruses of veterinary interest. The presence of antibodies specific to SARS-CoV-2 has also been detected in human milk. Logically, milk from a cow immunized against BCoV would activate the intestinal immune system against the virus and, at the same time, induce cross-immunity against SARS-CoV-2, thanks to antigenic recognition of certain highly conserved shared structures, notably M and S. Anti-BCoV antibodies present in the milk would lead to the total or partial inactivation of SARS-CoV-2, acting somewhat like a vaccine and supporting the specific immune response. Beyond these mechanisms, researchers hope to further investigate the efficacy of immunized milk and other immune-boosting compounds (derived from milk, egg yolk, etc.) in controlling COVID-19 in human patients. Due to their economic, medical, and epidemiological importance, various commercial vaccines against BCoV (including live and inactivated vaccines) are currently available and have demonstrated excellent protective efficacy. It would therefore be relatively easy and inexpensive to obtain immunized milk as quickly as possible and in large quantities, for the benefit of all.