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SARS-CoV-2 Neutralizing Antibody Titers in Convalescent Plasma and Recipients in New Mexico: An Open Treatment Study in COVID-19 Patients

Abstract Background Convalescent plasma (CP) is a potentially important therapy for coronavirus disease 2019 (COVID-19). However, knowledge regarding neutralizing antibody (NAb) titers in donor plasma and their impact in acute COVID-19 patients remains largely undetermined. We measured NAb titers in CP and in acute COVID-19 patients before and after transfusion through the traditional FDA IND pathway. Methods We performed a single-arm interventional trial measuring NAb and total antibody titers before and after CP transfusion over a 14-day period in hospitalized patients with laboratory-confirmed severe acute respiratory syndrome coronavirus (SARS-CoV-2) infection. Trial Registration: Clinicaltrials.gov identifier: NCT04434131 (https://clinicaltrials.gov/ct2/show/NCT04434131) Results NAb titers in the donor CP units were low (<1:40 to 1:160) and had no effect on recipient neutralizing activity one day after transfusion. NAb titers were detected in 6/12 patients upon enrollment and in 11/12 patients during at least two timepoints. Average titers peaked on day 7 and declined towards day 14 (P=0.004). NAb and IgG titers were correlated in donor plasma units (ρ=0.938, P<0.0001) and in the cumulative patient measures (ρ=0.781, P<0.0001). Conclusions CP infusion did not alter recipient NAb titers. Pre-screening of CP may be necessary for selecting donors with high levels of neutralizing activity for infusion into patients with COVID-19.

On March 24, 2020, the US Food and Drug Administration (FDA) issued guidance for CP donor eligibility and the investigational use of CP in hospitalized COVID-19 patients with severe or life-threatening disease, through emergency INDs, and the IND regulatory pathway (21 CFR 312). Initial guidance recommended NAb testing when available and using donors with NAb titers of at least 1:320 [12], was later revised to suggest titers >1:160, or 1:80 if a unit with a titer of 1:160 was not available [13]. However, SARS-CoV-2 NAb assays are not widely available [14], and CP treatment in the US has largely proceeded without measurement of NAb titers in donors or CP prior to transfusion [7].
The primary objective of the study was to provide CP and evaluate NAb titers following transfusion of CP to hospitalized patients with COVID-19 through the traditional FDA IND pathway. As such, we measured SARS-CoV-2 NAb titers in the donor CP units and in recipients before and after transfusion. We additionally measured IgG and IgM levels specific for S1 subunit of SARS-CoV-2 spike protein in CP and in recipients prior to and following transfusion and SARS-CoV-2 viral load in the upper respiratory system. Secondary endpoints were safety as measured by rapid deterioration in ordinal score within 4 hours of transfusion and clearance of viral shedding.       No study-related serious adverse events were attributed to the CP transfusion.
However, patient #004, a 91-year-old female, was transitioned to comfort measures by family and succumbed to respiratory failure on day 7, and patient #006, a 53-year-old male with morbid obesity (BMI ~65), developed shock and acute kidney injury and succumbed on day 7. Although planned target accrual was 30 subjects, the study was suspended after post-transfusion testing revealed unexpectedly low NAb titers in the donor CP.

Temporal Dynamics of Antibody Levels and SARS-CoV-2 Viral Load:
To explore the longitudinal dynamics of antibody levels, we measured NAb, IgG, and IgM levels on days 0, 1, 3, 7, and 14. NAb titers peaked at day 7, and then declined towards day 14 (P=0.004, Figure 3A). IgG levels increased until day 7 and were highest on day 14 (P=0.004, Figure 3B). Levels of IgM increased from day 0 to day 7 and then plateaued (P=0.003, Figure 3C).
Individual patient NAb, IgM, and IgG levels across the time course is shown in Supplemental Figure 1A, B, and C. SARS-CoV-2 viral loads from upper respiratory tract samples were highest on day 1 and gradually decreased until day 14 (P=0.006, Figure 3D). Viral load data for each patient is shown in Supplemental Figure 1D.

Relationship between Neutralizing Antibodies, IgG titers and viral load: NAb and IgG
measurements were used to establish the relationship between neutralizing activity and IgG titers. Linear regression analyses showed a significant positive correlation between NAb and IgG levels in the donor plasma (=0.938, P<0.0001, Figure 4A). A significant positive correlation was also observed between NAb and IgG titers in cumulative measures across all sampling days in CP recipients (=0.781, P<0.0001, Figure 4B). There was no correlation between donor Nab levels and viral loads in the recipients on day 1 (=0.244, P=0.440).
A c c e p t e d M a n u s c r i p t 10 Comparison of neutralizing titers and viral load across the sampling days showed a significant inverse relationship (=-0.370, P<0.01, Figure 4C). A significant inverse correlation was also present between IgG levels and viral load (=-0.485, P<0.001, Figure   4D).

In this study, 12 patients received CP therapy with no immediate adverse effects.
This is similar to what others have observed in the treatment of COVID-19 with CP [20]. The two individuals who died during the study had known comorbidities, and their deaths were determined not related to CP therapy.
To the best of our knowledge, this is the first report of NAb and IgG titers in donor CP and in recipients before and one day after transfusion. We were concerned to find that the median NAb titer in donor CP units was 1:40 (range: undetectable to 1:160) and that NAb titers increased in only 2/12 recipients one day following transfusion. The patient with the greatest increase in NAb levels one day after transfusion received CP with undetectable NAb levels, so this increase as well as peak NAb levels on day 7 likely represent endogenous NAb production.
In contrast, IgG antibody specific for the S1 subunit of SARS-CoV-2 spike protein was detected in all 12 donor units. Six of 12 recipients had detectable levels of IgG before and 1 day following transfusion. In general, IgG titers increased in most patients towards day 14 Other studies showed low to moderate PRNT90 neutralizing titers in convalescent patients and little or no IgG in acute patients within 6 days of symptom onset [23]. Measurement of neutralizing activity at a median of 22 days after symptom onset with ELISA-positivity revealed that 59.2% had NAb titers >1:40, with 66.7% having neutralizing titers by 40 days [24]. Another investigation showed that at 21-30 days post-symptom onset, 26.1% of patients had undetectable NAb titers (<1:64), 52.2% showed titers between 1:64 and 1:512, and 21.7% at 1:512 or higher [25]. Duan et al. reported neutralizing titers ≥1:160 in 39/40 convalescent patients and 10/10 acute patients receiving CP [11]. In a randomized, open trial with CP treatment plus standard clinical care (n=52) versus standard clinical care alone (n=51), no significant impact on time to clinical improvement or 28-day mortality was noted [9]. Similar to the significant positive correlation we found between NAb and IgG (against S1 A c c e p t e d M a n u s c r i p t 12 subunit) titers, their study revealed a positive correlation (r=0.622, P=0.03) between NAb titers and IgG levels, and that a NAb titer of 1:80 was equivalent to an IgG titer of 1:1280 [9].
While the median dose of CP was 200 mL, weight-based dosing ranged from 4-13 mL/kg, and only plasma units with IgG titers ≥1:640 were transfused [9].
Neutralizing activity can be measured using live SARS-CoV-2 in a BSL3 facility, while viral pseudotypes carrying the SARS-CoV-2 spike protein can be utilized in standard BSL2 facilities. However, results can very between different assays based on the virus and methods used and should be compared in parallel for standardization [14]. ELISAs currently used for detection of SARS-CoV-2 antibodies are based on a variety of antigens, including nucleoprotein, whole spike protein (S1 and S2 subunits), S1 subunit alone, or RBD of the S1 subunit. For our experiments, ELISA was performed with the S1 subunit of the spike protein as an antigen, and PRNT80 neutralizing assays were conducted using live SARS-CoV-2.
Therefore, when comparing published assays on NAb and IgG levels for SARS-CoV-2, it is important to consider the measurement platforms utilized in different studies.
Our cohort was too small to reliably evaluate a relationship between NAb levels in CP or in recipients and clinical outcomes. NAb titers varied at the time of enrollment but were detected for at least 2 timepoints in 11/12 patients. The 91-year-old patient had no NAb titers detected at any of the temporal measurements. Patients #001 and #002 with moderate NAb titers (1:640) had favorable clinical outcomes, whereas patient #006 had high neutralizing activity (1:2560) at admission but died within 7 days.
In our cohort, NAb titers peaked on day 7 and declined by day 14 ( Figure 3A Our results raise concern over the lack of clinical efficacy data in randomized controlled trials and administration of CP to large numbers of patients with COVID-19 without first screening donors or plasma for NAb and IgG activity [7,20]. Consideration should also be given to CP volume and antibody levels and the weight or plasma volume of the recipient. Although recipients' BMIs ranged from 21 to 65, all received 200 mL of CP.
Experience treating Argentine hemorrhagic fever and hantavirus has demonstrated the importance of NAb dosing in relation to body size [3,4]. Interpretation of clinical outcomes following CP transfusion may be difficult without measurement of NAb titers in donor plasma and evaluation of CP NAb titers and CP volume in relation to recipient size.
Limitations of this pilot study include small sample size, NAb assay measurements at a single site, and lack of a control group. Other limitations include the lack of a standardized assay for NAb titers. Our NAb assay correlated well with the ELISA results and is based on standard PRNT80 assay procedures but we acknowledge that additional replicates and direct comparisons with other methods are important considerations that are not addressed here. Another limitation is the use of a single dilution of samples for ELISA experiments.
Due to the high cost of reagents, endpoint titers were not calculated. In this study, the time of symptom onset to CP infusion was 6-16 days (median 8.5 days), as listed in Table 1. We chose day of CP infusion as the starting point for the analysis in this study, as day of symptom onset is a less precise measurement based on patient recall and individual patient's subjective interpretation of which symptoms are due to COVID-19.
A c c e p t e d M a n u s c r i p t 14 We conclude that CP administered in the study had low NAb titers with no impact on titers in transfused recipients. As such, the study was suspended, and measures of doseeffect or benefit could not be determined. Findings here corroborate the heterogeneity of CP therapy for COVID-19. Clinical implications are the importance of measuring NAb titers in donor plasma prior to infusion. Future studies will benefit by defining donor CP neutralizing activity to determine the impact of therapy on clinical improvement for COVID-19 patients.    was determined using Spearman's rank correlation with significance set at P≤0.05. Similar analysis were used to evaluate the correlation between viral load and donor NAb titers (C) and IgG levels (D).