Why does convalescent plasma therapy for severe COVID-19 show mixed success?

Treatment modalities against the coronavirus disease 2019 (COVID-19) pandemic are limited in efficacy. This has led many centers to use convalescent plasma from recovered COVID-19 patients in the management of this condition.

A surprising new preprint, released on the medRxiv* server, suggests that the beneficial effects of convalescent plasma (CP) in this disease stem not only from the presence of neutralizing antibodies, but also the immunomodulatory effects of this plasma that shapes the host immune response.

Study: Functional Antibodies in COVID-19 Convalescent Plasma. Image Credit: SWP.93 / Shutterstock
Study: Functional Antibodies in COVID-19 Convalescent Plasma. Image Credit: SWP.93 / Shutterstock

Discrepancy in results from convalescent plasma trials

Randomized controlled trials have failed to yield evidence of significant benefit from CP when administered late in the course of illness. However, within three days of symptom onset, CP with high titers of antibodies led to a 73% reduction in the risk of progressive COVID-19.

Individual CP units are produced from unique collections of antibodies obtained from more than one convalescent COVID-19 patient. It may be administered to one or more patients. The factors involved include host genetics, the severity of illness, and the time since recovery.

As a result, the virus-specific antibodies in COVID-19 CP differ in subclass, isotype, Fc-receptor binding and Fc-effector functions. These also participate in pathogen clearance and modulating inflammation, along with binding antibodies and neutralization titers.

Study details

The current study focused on CP units administered to 19 patients, one unit being 200 mL of plasma. All patients received CP within three days of admission, and all were on supplemental oxygen. Half of them required ventilation, either non-invasive pressure or invasive mechanical ventilation.

Antibody profiles were conducted on the CP units given on day -1, day 1 and day 3 of CP administration. CP units from four donors were given to two unique patients each, and those from the remaining ten went to one patient each.

Heterogeneous CP profiles

The researchers found that there were marked differences in the antibody profiles between CP donors and between CP donors and recipients. Both antibodies to the virus and FcR binding were higher in severe COVID-19 before CP administration.

In particular, IgA and Fcα-receptor (FCAR) binding were markedly enhanced in severe COVID-19. This supports the concept that IgA is a marker of early COVID-19 and may cause increased severity.

Spike-specific IgG1 was found in all CP units but distributed differently across the antibody subclasses. Some units were dominated by IgG3, and some by IgA. The neutralizing antibody titer varied among donor units, as did the Fc-binding ratio (FcRA).

Functional antibodies in CP vs acute COVID-19

Most severely ill patients had globally increased specific antibody titers following CP administration. The titers of specific IgG1 antibodies targeting the spike protein increased rapidly and markedly over time, across subclasses and isotypes, but with lower S1-specific FcAR, and anti-nucleoprotein (N) antibody binding to neonatal FcR.

CP showed a higher level of functional antibodies to SARS-CoV-2 than recipient plasma. This was so despite lower IgA titers and Fcα-receptor (FCAR) binding.

Nonetheless, antibody-dependent cell phagocytosis (ADCP), antibody-dependent NK cell activation (ADNK), and antibody-dependent complement deposition (ADCD), as well as Fc effector functions, were enriched in CP units, indicating that the antibody profile is different and the functionality superior to that of acute severe COVID-19. The enhanced levels of highly functional FcR-binding IgG3 SARS-CoV-2 specific antibodies in CP support this hypothesis.

All functional IgG profiles were present in all units, as well as RBD IgM antibodies in all severe COVID-19 patients. The tight correlation between anti-spike IgG3 and increased IgM/IgA responses in the latter category may indicate that viral replication is continuing in these patients.

The researchers also found indications that complement-fixing antibodies in CP could change the profile of developing humoral immunity to SARS-CoV-2 spike in severe COVID-19.

CP effects depend on pre-existing spike-specific IgGs

The extent of change in humoral immunity was also a function of the pre-existing spike IgG levels. The effect of anti-spike antibodies in CP was to up- or down-regulate inflammatory and antibody-mediated responses in the host depending on the level of pre-existing specific antibodies.

With high antibody loads, the transferred antibodies in CP may be unable to compete with endogenous antibodies to reach their effector sites. However, by functional optimization, the antibody titers needed to achieve the same effect can be markedly reduced.

This effect was observed in this study as N-specific antibodies in COVID-19 patients were replaced or displaced with CP antibodies to reduce the level of inflammatory cytokines.

We found multiple N-specific antibody features in CCP tracked with dampened inflammatory evolution of SARS-CoV-2 specific humoral immunity in patients.”

Limiting inflammatory responses

The rapid and diverse expansion of antibody production in acute severe COVID-19 is not in proportion to the functional nature of the immune response. In contrast, CP units had higher functional antibody levels, but also lower S-specific antibody levels.

Inflammatory antibody development was restricted in proportion to S- and N-specific IgG3 titers in the CP, in those who already had high IgG titers. This could imply the possibility of limiting inflammatory antibodies by administering highly functional antibodies.

In short, different types of antibodies produce different effects on the immunological response depending on the pre-existing levels of S-specific IgG1. That is, the roles played by N- and S-specific responses vary widely with the level of pre-existing S-specific IgG.

Functional enhancement of neutrophils

S-specific antibody-dependent neutrophil phagocytosis (ADNP) was associated with anti-inflammatory activity only if the initial S-specific IgG titer was low. This may mean that among these individuals, the humoral inflammatory antibody response can be limited by S-specific antibodies.

These could enhance the phagocytic capability of neutrophils. In turn, this would facilitate virus clearance by neutrophils, the first responders, before the virus can infiltrate monocytes to initiate the hyperinflammatory cascade.

What are the implications?

Overall CCP profiles were marked by enhanced antibody effector function despite having lower overall S-specific subclass titers, isotype titers, and FcR binding. These observations suggest units of CCP, collected following the resolution of moderate SARS-CoV-2 infection, have qualitatively different antibodies with superior function.”

The mystery of why CP is useful in some COVID-19 patients, but not others, may be partly answered by the findings of this study. These indicate that CP affects the immune response of the recipient in many and novel ways, beyond IgG titers and neutralization capacity

A recent CP trial in the Netherlands was cut short on finding that most of the patients already showed neutralizing antibodies before CP administration. This overlooks the lack of correlation between neutralizing antibodies and the resolution of infection. In fact, neutralizing activity is equally or more closely linked to the progression of COVID-19.

Secondly, the use of intravenous immunoglobulin (IVIG) in COVID-19 has shown therapeutic advantage, thus implying that other mechanisms may be operating to provide protection via antibodies.

These may not involve neutralization or even specificity. For instance, DC-SIGN receptors, or FcγR IIb, may result in lowered levels of inflammation following their bindng. Similarly, CP may reduce immunological activation of inflammatory pathways, resulting in a reduction in serum cytokines in severe COVID-19.

Another set of possibilities could be that antibodies speed up viral clearance, activate complement-mediated destruction of the virus or of infected cells, trigger cytotoxic activity or promote opsonization of the virus or of infected cells. This is all the more likely in view of the polyclonal nature of the antibodies, including pro- and anti-inflammatory antibodies.

This adds to the already known potential advantage of passive immunity via CP. The lower spike-specific antibody titers in CP may be because the patients had less severe disease or else the less inflammatory nature of the long-term immune response.

The efforts to enrich CP for neutralizing activity may reduce ADNP effector antibodies, thus reducing an important anti-inflammatory component of the CP that could prevent the patient's progression to severe or critical COVID-19.

The evaluation of the functional antibody parameters in CP is therefore likely to be crucial in determining its efficacy and in designing newer monoclonal therapeutic antibodies.

Though CCP was thought of a therapy of necessity in March 2020 that would be replaced by more refined monoclonals, emerging COVID-19 variants are creating new therapeutic gaps that an improved CCP may be needed to fill.”

*Important Notice

medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.

Journal reference:
Dr. Liji Thomas

Written by

Dr. Liji Thomas

Dr. Liji Thomas is an OB-GYN, who graduated from the Government Medical College, University of Calicut, Kerala, in 2001. Liji practiced as a full-time consultant in obstetrics/gynecology in a private hospital for a few years following her graduation. She has counseled hundreds of patients facing issues from pregnancy-related problems and infertility, and has been in charge of over 2,000 deliveries, striving always to achieve a normal delivery rather than operative.

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