Influence of sample collection tube method, anticoagulant-containing plasma versus serum, on influenza virus hemagglutination inhibition titer and microneutralization titer serological assays

Influence of blood sample storage and different types of anticoagulants on results of NK cytotoxicity tests

Introduction

Natural killer (NK) cells are important players in the human immune response. Impaired NK function may lead to serious, life-threatening conditions. Defects may be consequences of genetic mutations or results of secondary factors such as infections, malignancies and autoimmune diseases. The cytotoxicity test is very useful, but its accessibility is limited to special immunological laboratories. Blood samples are often transported to remote centers, which takes time and requires special conditions.The aim of this study was to compare cytotoxicity assay results between samples preserved with three different anticoagulants to standardize the diagnostic procedure.

Material and methods

Peripheral blood from healthy donors was taken with three anticoagulants: heparin, K2EDTA and citrate. Peripheral blood mononuclear cells (PBMC) were isolated and tested directly after blood drawing and after 24-hour storage. Cytotoxic abilities of NK cells were tested in 4 h co-culture with K562. NK cytotoxicity was measured by flow cytometry.

Results

In most cases of analyzed healthy donors, cytotoxicity results were similar regardless of type of anticoagulant. However, the highest mean values were obtained in samples with citrate. There was a significant decrease in cytotoxicity after 24 hours of storage of the whole blood at ambient temperature. The mean drop in cytotoxicity results was substantial for all anticoagulants: 76% for heparin, 67% for citrate and 70% for EDTA.

Conclusions

Results of spontaneous NK cytotoxicity seem to be affected by the anticoagulants used for blood protection. Commercial instant cytotoxicity testing and delayed analysis after blood storage gave the highest results in blood with sodium citrate.

Agreement and error of titration assays

Titration assays can be used to define positivity either in terms of a change over time, i.e. seroconversion, or relative to a fixed threshold. The operating characteristics of these definitions depend on the precision of the assay. We present methods for estimating the distribution of errors, at the level of a single replicate, from the distribution of within-pair agreement. When the maximum replicate-level error is one dilution, a simple probability argument is used, with estimation by method of moments. For the more general case, a discretized Gaussian model is used, with maximumum likelihood estimation. These models fit well to eight published datasets. The discretized Gaussian model also allows the potential performance of alternative dilution factors to be assessed. For influenza hemagglutination-inhibition, the approach is compared to a previous Markov chain Monte Carlo data augmentation model. These methods allow the estimation of the underlying error distribution from observed between-replicate differences under repeatability conditions. The results can be used to guide the choice of the fold change necessary to infer seroconversion. Finer dilution factors, e.g. 1.5 rather than 2, could facilitate a better balance between the sensitivity and specificity of titration assays.

Antibody Responsiveness to Influenza: What Drives It?

The induction of a specific antibody response has long been accepted as a serological hallmark of recent infection or antigen exposure. Much of our understanding of the influenza antibody response has been derived from studying antibodies that target the hemagglutinin (HA) protein. However, growing evidence points to limitations associated with this approach. In this review, we aim to highlight the issue of antibody non-responsiveness after influenza virus infection and vaccination. We will then provide an overview of the major factors known to influence antibody responsiveness to influenza after infection and vaccination. We discuss the biological factors such as age, sex, influence of prior immunity, genetics, and some chronic infections that may affect the induction of influenza antibody responses. We also discuss the technical factors, such as assay choices, strain variations, and viral properties that may influence the sensitivity of the assays used to measure influenza antibodies. Understanding these factors will hopefully provide a more comprehensive picture of what influenza immunogenicity and protection means, which will be important in our effort to improve influenza vaccines.

Determination of the Concentration of IgG against the Spike Receptor-Binding Domain That Predicts the Viral Neutralizing Activity of Convalescent Plasma and Serum against SARS-CoV-2

Several hundred millions of people have been diagnosed of coronavirus disease 2019 (COVID-19), causing millions of deaths and a high socioeconomic burden. SARS-CoV-2, the causative agent of COVID-19, induces both specific T- and B-cell responses, being antibodies against the virus detected a few days after infection. Passive immunization with hyperimmune plasma from convalescent patients has been proposed as a potentially useful treatment for COVID-19. Using an in-house quantitative ELISA test, we found that plasma from 177 convalescent donors contained IgG antibodies specific to the spike receptor-binding domain (RBD) of SARS-CoV-2, although at very different concentrations which correlated with previous disease severity and gender. Anti-RBD IgG plasma concentrations significantly correlated with the plasma viral neutralizing activity (VN) against SARS-CoV-2 in vitro. Similar results were found using an independent cohort of serum from 168 convalescent health workers. These results validate an in-house RBD IgG ELISA test in a large cohort of COVID-19 convalescent patients and indicate that plasma from all convalescent donors does not contain a high enough amount of anti-SARS-CoV-2-RBD neutralizing IgG to prevent SARS-CoV-2 infection in vitro. The use of quantitative anti-RBD IgG detection systems might help to predict the efficacy of the passive immunization using plasma from patients recovered from SARS-CoV-2.

Serological Screening of Influenza A Virus Antibodies in Cats and Dogs Indicates Frequent Infection with Different Subtypes

Influenza A viruses (IAVs) infect humans and a variety of other animal species. Infections with some subtypes of IAV were also reported in domestic cats and dogs. In addition to animal health implications, close contact between companion animals and humans also poses a potential risk of zoonotic IAV infections. In this study, serum samples from different cat and dog cohorts were analyzed for IAV antibodies against seven IAV subtypes, using three distinctive IAV-specific assays differing in IAV subtype-specific discriminatory power and sensitivity. Enzyme-linked immunosorbent assays against the complete hemagglutinin (HA) ectodomain or the HA1 domain were used, as well as a novel nanoparticle-based, virus-free hemagglutination inhibition assay. Using these three assays, we found cat and dog sera from different cohorts to be positive for antibodies against one or more IAV subtypes and/or strains. Cat and dog serum samples collected after the 2009 pandemic H1N1 outbreak exhibit much higher seropositivity against H1 compared to samples from before 2009. Cat sera, furthermore, displayed higher reactivity for avian IAVs than dog sera. Our findings show the added value of using complementary serological assays, which are based on reactivity with different numbers of HA epitopes, to study IAV antibody responses and for improved serosurveillance of IAV infections. We conclude that infection of cats and dogs with both human and avian IAVs of different subtypes is prevalent. These observations highlight the role of cats and dogs in IAV ecology and indicate the potential of these companion animals to give rise to novel (reassorted) viruses with increased zoonotic potential.

Hemagglutination Inhibition Assay

The hemagglutination inhibition (HI) assay for influenza A virus has been used since the 1940s. The assay may be utilized to detect or quantify antibodies to influenza A viruses and can be used to characterize differences in antigenic reactivity between influenza isolates. In addition, data from HI assays are routinely used for antigenic cartography, influenza virus surveillance, epidemiology, and vaccine-seed strain selection. For antibody quantification, the HI assay is a fast and inexpensive method; other than a source of red blood cells, no expensive or unusual lab equipment is needed, and results can be obtained within a few hours. Historically, the HI assay has also served as a primary method of subtype identification and is still used widely. However, as gene sequencing technology has evolved to be cheaper and faster, it is replacing the HI assay for this purpose.

Impact of Maternal HIV Infection and Placental Malaria on the Transplacental Transfer of Influenza Antibodies in Mother-Infant Pairs in Malawi, 2013-2014

BACKGROUND

Maternal influenza vaccination protects infants against influenza virus infection. Impaired transplacental transfer of influenza antibodies may reduce this protection.

METHODS

We conducted a cross-sectional study of influenza vaccine-naïve pregnant women recruited at delivery from Blantyre (urban, low malaria transmission) and Chikwawa (rural, high malaria transmission) in Southern Malawi. HIV-infected mothers were excluded in Chikwawa. Maternal and cord blood antibodies against circulating influenza strains A/California/7/2009, A/Victoria/361/2011, B/Brisbane/60/2008, and B/Wisconsin/1/2010 were measured by hemagglutination inhibition (HAI). We studied the impact of maternal HIV infection and placental malaria on influenza antibody levels in mother-infant pairs in Blantyre and Chikwawa, respectively.

RESULTS

We included 454 mother-infant pairs (Blantyre, n = 253; Chikwawa, n = 201). HIV-infected mothers and their infants had lower seropositivity (HAI titer ≥1:40) against influenza A(H1N1)pdm09 (mothers, 24.3 vs 45.4%; P = .02; infants, 24.3 vs 50.5%; P = .003) and A(H3N2) (mothers, 37.8% vs 63.9%; P = .003; infants, 43.2 vs 64.8%; P = .01), whereas placental malaria had an inconsistent effect on maternal and infant seropositivity. In multivariable analyses, maternal HIV infection was associated with reduced infant seropositivity (A(H1N1)pdm09: adjusted odds ratio [aOR], 0.34; 95% confidence interval [CI], 0.15-0.79; A(H3N2): aOR, 0.43; 95% CI, 0.21-0.89). Transplacental transfer was not impaired by maternal HIV or placental malaria.

CONCLUSIONS

Maternal HIV infection influenced maternal antibody response to influenza A virus infection, and thereby antibody levels in newborns, but did not affect transplacental antibody transfer.