COVID-19: Antibody-mediated Immunity and Antibody Testing (20 April 2020)

I was recently asked about the role of antibodies — specifically, antibody testing, vis-à-vis the current pandemic, and decided to make this the topic of my sixth COVID-focused post. Here, I provide a quick tutorial on antibodies followed by a discussion of the practical application of antibody-mediated immunity as it pertains to COVID-19.

By way of background, antibodies, also known as immunoglobulins, are Y-shaped proteins found in the serum and plasma portions of blood. They are mainly produced by plasma cells which, along with complement and antimicrobial peptides, comprise the “humoral” component of immunity. This is in contradistinction to “cellular” immunity, which is mediated by cells such as phagocytes and lymphocytes. Although there are different types of antibodies (which I’ll discuss in a bit), they share a similar Y-shaped structure (Figure A) consisting of “heavy” and “light” peptide chains. The diagonal fragments, contain both heavy and light chains, vary widely, and are tipped with a fragment antigen-binding (Fab) variable region, part of which (the paratope) recognizes and binds to a particular portion (epitope) of a marker (antigen) on a pathogen such as a bacterium or virus. The longer stem of the antibody (Fc) consists solely of heavy chains, is highly conserved within antibody classes, and enables the antibody to interact with other components of the immune system. Once an antibody binds to a pathogen, it triggers a sequence of events that clears the pathogen from circulation. These clearance mechanisms include 1) blocking the pathogen from binding to some receptor (neutralization); 2) causing the pathogens to clump (agglutination and precipitation), which marks them as a target for phagocytes; and 3) complement activation, which either lyses the pathogen (in the case of bacteria) or attracts inflammatory cells, which then destroy the pathogen.

Within humans and other mammals, there are five classes or “isotypes” of antibodies: Ig (for immunoglobulin) A, IgD, IgE, IgG, and IgM. Here, I will only discuss IgG and IgM. IgG provides the majority of antibody-based immunity against invading pathogens, and its presence in circulation simply indicates prior exposure to a particular pathogen. In contrast, IgM is produced in the early stages of infection, before sufficient IgG is produced, and is a marker of recent infection. By way of example, whereas the presence of anti-SARS-CoV-2 IgG in circulation indicates prior infection with the virus that causes COVID-19, the presence of anti-SARS-CoV-2 IgM indicates recent infection with the virus. Followed longitudinally, IgM titers appear earlier and then wane, as IgG titers rise later and persist. (Figure B)

With our immunology synopsis complete, what then is the clinical relevance of antibodies to SARS-CoV-2, the virus that causes COVID-19? We can broadly think of the answer to this question in both therapeutic and diagnostic terms. With respect to the former, recall that I mentioned in prior posts that the presence of antibodies (assuming they are protective) has implications for both the development of an effective COVID-19 vaccine as well as “herd immunity” and the duration of the pandemic (For more on this topic, see my post dated 23 March). In a subsequent post (6 April 2020), I introduced the topic of “passive immunization”, in which the antibody-containing sera from patients who have been infected with SARS-CoV-2 is used as a therapeutic for individuals with active infection. Another consideration is the use of serology (i.e. looking for the presence of antibodies in serum) to diagnose COVID-19. The current methodology for diagnosing COVID-19, PCR (polymerase chain reaction amplification), is unquestionably sensitive and specific. However, like all assays, PCR is imperfect and both false positive and false negative results occur. Consider, for example, that specimens for COVID-19 testing by PCR are obtained by inserting a swab, first through one nostril and then through the other, into the nasopharynx. This procedure is uncomfortable, and patients sometimes resist, making the quality of sampling questionable. In principle, the diagnostic yield could be increased by combining PCR with an antibody test, such as an ELISA or Enzyme-Linked Immunosorbent Assay (Think pregnancy test), with the presence of IgM confirming recent infection. (Figure C) Another practical application of antibody testing would be to determine who is (presumably) immune to SARS-CoV-2 and who is not. In those situations, in which critical personal protective equipment (PPE) such as N95 respirators and Powered Air-Purifying Respirator (PAPR) devices are in short supply, they could perhaps be reserved for those healthcare workers who are “seronegative” and thus susceptible to infection, with “seropositive” individuals wearing a surgical mask instead. Additionally, the results of antibody testing could theoretically be used to determine when individuals could return to work or school sans masks.

Despite the promise offered by antibody testing, these assays have limitations. Foremost among these is the uncertainty of the quality of the assay being used. Although commercial manufacturers claim their tests are highly sensitive and specific, few have published their data, making the reliability of any particular test questionable; and only one is currently approved by the Food and Drug Administration. Another limitation is the fact that individuals with immune compromising conditions or those on immune dampening medications might not mount a detectable antibody response. This could pose a problem for some people if antibody test results were to be used as “immunity certificates”, required before permitting people to return to the community. Another caveat is that individuals may still be shedding virus, as detected by PCR, even after they’ve begun making antibodies. Whether or not this represents contagiousness remains to be determined (Recall in a prior post that I explained that PCR detects genetic material — be it from a viable or nonviable source). Lastly, serosorting presupposes that SARS-CoV-2 antibodies are protective against reinfection. While this is likely the case, we don’t yet know for sure. Although some infections (e.g. measles) confer long-lived immunity, others (e.g. syphilis) do not. Generally speaking, coronaviruses appear to fall somewhere in between. Consider the fact that we all get the common cold (which is caused by an assortment of viruses including coronaviruses) time and again. Whether this is because we are reinfected with different strains or because immunity to any particular strain wanes over time, we simply don’t know. Nonetheless, despite these limitations, there will no doubt be a role for SARS-CoV-2 antibody testing, even if only for epidemiological purposes.

To date, I’ve given a general overview of coronaviruses, including SARS-CoV-2 (1 and 13 March 2020), a review of investigational therapeutics and vaccines currently being studied (23 March and 6 April), a discussion about the role of masks in providing protection from COVID-19 (6 April 2020), as well as a discussion on the virus’ transmissibility, viability on inanimate surfaces, and associated case fatality rate (14 April 2020); and here, I focused solely on the topic of SARS-CoV-2 antibodies. As with my previous posts, my intention here is not to politicize, sensationalize, or trivialize the pandemic, but simply to provide information and, whenever possible, to alleviate anxiety.

Until my next update — regards.

Michael Zapor, MD, PhD, CTropMed, FACP, FIDSA
(20 April 2020)