Antibody Response against SARS-CoV-2 Infection: Implications for Diagnosis, Treatment and Vaccine Development

Figures & data

Figure 1. Humoral response against SARS-CoV-2 and its implication: key messages. A. SARS-CoV-2 is a single-stranded positive-sense RNA virus which infects human cells expressing ACE2 receptor through its trimeric surface spike (S). Glycoprotein Nucleocapsid (N) protein is a highly immunogenic structural protein which participate in RNA package and virus particle release. The most common observations regarding the humoral response mounted by SARS-CoV-2 infected patients are summarized. B. Different serologic tests have been promptly implemented for the diagnosis of SARS-CoV-2 infection diagnosis. At present, serum antibodies against S and N proteins are the ones mainly detected. Potentialities and drawbacks of this kind of assay are summarized. C. Antibody response against SARS-CoV-2 has been used for the implementation of convalescent plasma therapy (up) and the isolation and characterization of neutralizing mAbs for therapy (down). Salient points concerning these two treatment options are reported. D. Both analysis of neutralizing response against SARS-CoV-2 and epitope characterization of isolated monoclonal Nabs are useful for vaccine design. Focal points of the design of a vaccine aimed to induce an effective antibody response are highlighted.

Figure 2. Antibody structure and function. A. Protective functions of antibodies are summarized. Antibodies are able to deploy a plethora of effector functions over the course of an infection: the antigen binding site of a neutralizing mAb is specifically directed against the viral surface antigen and directly interferes with virus-cell receptor interaction (on the top); human IgGs, particularly IgG1 and IgG3, bound to to the viral antigen exposed on the target cell, can subsequently interact through their Fc region with FcγRs expressed by effector cells or with complement component 1q, potentially supporting the destruction of target cells through ADCC or CDC, respectively. In addition, the Fc region of IgG can bind the salvage receptor FcRn after fluid-phase uptake by vascular endothelial cells and other cells, an interaction that contributes to the long (∼21 day) half-life of human IgG. B. The ADE phenomenon is illustrated as a possible adverse event that could occur with some antibodies. ADE has been documented to occur through two distinct mechanisms in viral infections: by enhanced antibody-mediated virus uptake into FcγR-expressing phagocytic cells leading to increased viral infection and replication, or by excessive antibody Fc-mediated effector functions or immune complex formation causing enhanced inflammation and immunopathology. Both ADE pathways can occur when non-neutralizing antibodies or antibodies at sub-neutralizing levels bind to viral antigens without blocking or clearing the infection. Identification and exclusion of ADE-associated epitopes in vaccine design or modification of the amino acid sequence of IgG reducing the interaction with one or more binding partners (for example by inserting LALA mutations) could be promising strategies to improve the clinical potential of antibodies.

Table 1. Clinical studies on convalescent plasma therapy: characteristics and results.

Reference and trial characteristic	Participants characteristics and treatment groups	Administration time	Doses and CP nAbs titers	Primary outcome (PO) data	Secondary outcome data
Ling et al. [49] Randomized trial	103 hospitalized adults > 18y with severe disease. Randomization: 52 patients with CP + SOC treatment + and 51 patients with SOC treatment	Not specified	CP ≥ 1:640 of anti S-RBD IgG titers. Two doses of 200 mL, 24 hrs apart	Clinical improvement within 28 days observed in 51.9% CP group versus 43.1% in the ctr group, no statistical significance If stratified for disease severity: PO reached in 91.3% in CP group and 68.2% in ctr group p > 0,03 excluding critically ill patients	No differences in mortality rate at 28 days and time of discharge Rate of negative PCR in CP group 87.2% vs 37.5% in ctr group, P < .001 at 72 hours
Agarwal et al. [50] PLACID trial, randomized	464 hospitalized adults > 18y with moderate ill patients Randomization: 235 patients with CP + SOC treatment and 229 patients with SOC treatment	Not specified	nAbs titer not known at the time of transfusion; Two doses of 200 mL, 24 hrs apart	Disease progression and mortality at 28 days, no statistical difference between group. Same results after nAbs titers stratification	Higher proportion of patients in CP group showed resolution of shortness of breath and fatigue at day 7; No differences in resolution of fever and cough Negative conversion of SARS-CoV-2 RNA at day 7 was significantly higher in CP arm compared with ctr arm
Joyner M et al. [51]. EAP three months results, open label study	35,322 hospitalized adults > 18y with severe acute infection with high risk of severe disease progression all receiving CP treatment	Variable. Administration time was used for stratified analysis.	nAbs titer not known at the time of transfusion 200 ml, at least one dose.	7 days mortality rate was 8.7% in patients transfused within 3 days of COVID-19 diagnosis but 11.9% in patients transfused 4 or more days after diagnosis (p < 0.001). Same observations for 30 days mortality with 21.6% vs. 26.7%, p < 0.0001 7-days mortality was 8.9% recipients of high IgG plasma, 11,6% for recipient of medium IgG plasma, 13,7% for recipients of low IgG plasma(p = 0.048).
Libster R. et al. [51]. Randomized, double-blind trial	160 older patients > 75 y Or >65 and < 74y with at least one defined coexisting condition	72 hours after the onset of mild Covid-19 symptoms	Anti-S IgG titer ≥ 1:1000. One 250 ml dose	Severe respiratory disease developed in 16% cases in CP group versus 31% in ctr group, p = 0,03. In CP group, 73% patients treated with CP with SARS-CoV-2 S IgG titer ≥ 1:3200 showed a reduced risk of COVID worsening versus 31,4% of patients treated with CP with SARS-CoV-2 S IgG titer < 1:3200	Combined secondary end-point event (life-threatening respiratory disease, critical systemic illness, and death, or any of these outcomes) occurred in 9% of patients in CP group and in 15% of patients in ctr group.
Shenoy A et al. [52]. Retrospective matched control study	576 hospitalized adults > 18y with severe acute infection. 263 treated with CP + SOC and 263 controls receiving SOC matched by gender, age and clinical characteristics	Not specified	nAbs titer not known at the time of transfusion One or two doses of 200 ml of CP according the FDA guidelines	No statistical difference in 28-day mortality was seen in CP group (25,5%) compared to controls (27%, P = 0,06). 7-day mortality was statistically better for CP group (9,1%) than controls (19,8%, P < 0,001) and continued at 14 days (14,8% vs. 23,6%, P = 0,01).	After 72 h since transfusion transition from nasal cannula to room air was superior in CP group (median 4 days vs. 1 day, P = 0,02). The length of stay was longer in CP group than controls (14,3 days vs. 11,4 days, P < 0,001).

Table 2. SARS-CoV cross-reactive monoclonal antibodies.

Reference	Name	Isolation	Epitope information	Functionality	Clinical study level
Pinto et al. [63]	S309 (VIR-7831 or GSK4182136 was obtained engineering S309 to improve its pharmacokinetics properties)	SARS-CoV infected patient	RBD-not competing with ACE2 receptor-highly conserved, glycan containing epitope accessible in both open and closed S states	Neutralization ADCC ADCP	Phase 2/3
Wang et al. [64]	47D11	SARS-CoV –S immunized transgenic mouse expressing human VH and VL domains and rat constant regions	Conserved epitope on S1 subunity, external to RBD	Neutralization	Phase I (fully human version of the antibody)

Color of lines distinguishes mAbs for the region of S-protein recognized.

Table 3. SARS-CoV-2 specific monoclonal antibodies.

Reference of isolation	Name	Isolation	Epitope information	Functionality	Clinical study level
Jones et al. [65]	Ly-CoV555	SARS-CoV-2 convalescent patients	RBD- competing with ACE2 receptor	Neutralization	Phase 3
Shi et al. [66]	CB-6 CB-6-LALA version is also known as JS016 or Ly-CoV016 or Etesevimab	SARS-CoV-2 convalescent patients	RBD-epitope partially overlapping with ACE2 receptor	Neutralization	Phase 2/3 (Etesevimab) in combination therapy
Hansen et al. [67]	REGN10933	Joined approach using both immunized humanized mice and B cells from convalescent patients	RBD- competing with ACE2 receptor	Neutralization ADCC ADCP	Phase 3
Rogers et al. [25]	CC12.1, CC6.29, CC6.30	SARS-CoV-2 convalescent patients	RBD- competing with ACE2 receptor	Neutralization	Preclinical for therapeutic efficacy (for CC12.1)
Zost et al. [26]	COV2196, COV-2130	SARS-CoV-2 convalescent patients	RBD- competing with ACE2 receptor-two different epitopes	Neutralization, synergistic activity	Preclinical for therapeutic and prophylactic efficacy
Cao et al. [68]	BD-368-2	SARS-CoV-2 convalescent patients	RBD- competing with ACE2 receptor	Neutralization	Preclinical for therapeutic and prophylactic efficacy
Miersh et al. [69]	IgG 15033	Human Fab phage library	RBD- competing with ACE2 receptor	Neutralization	NA
Gai et al. [70]	Nb11-59	Phage library of camelid nanobodies	RBD- competing with ACE2 receptor	Neutralization, stable after nebulization	NA
Barnes et al. [71]	C121 and others	SARS-CoV-2 convalescent patients	RBD- competing with ACE2 receptor	Neutralization	NA
Hansen et al. [67]	REGN10987	Joined approach using both immunized humanized mice and B cells from convalescent patients	RBD- not competing with ACE2 receptor	Neutralization ADCC ADCP	Phase 3
Wu et al. [72]	n3088, n3031 and others	naive library of human 3 66*01 human VH genes	RBD- not competing with ACE2 receptor	Neutralization	NA
Barnes et al. [71]	C135 and others	SARS-CoV-2 convalescent patients	RBD- not competing with ACE2 receptor	Neutralization	NA
Chi et al. [73]	4A8	SARS-CoV-2 convalescent patients	S1 NTD	Neutralization	NA
Liu et al. [74]	5-7, 5-24, 2-17 and others	SARS-CoV-2 convalescent patients	S1 NTD	Neutralization	NA
Liu et al. [74]	2-43	SARS-CoV-2 convalescent patients	Quaternary epitope of S1 trimer	Neutralization	NA

Color of lines distinguishes mAbs for the region of S-protein that is recognized.

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