Evaluation of multiplex flow cytometric opsonophagocytic assays for determination of functional anticapsular antibodies to Streptococcus pneumoniae

Effect of trifluoroacetic acid on the antigenicity of capsular polysaccharides obtained from various Streptococcus pneumoniae serotypes

Determining the safety, antigenicity, and immunogenicity by in vitro and in vivo studies is a prerequisite for the development of new vaccines. And this study investigated it for a vaccine made from Streptococcus pneumoniae serotypes 2, 5, 12F, 18C, and 22F. The crude CPS was purified and partially depolymerized by conventional and trifluoroacetic acid methods. 1H NMR analysis confirmed the identity of the depolymerized CPS which gave similar profiles to reference polysaccharides, except for serotype 18C which was de-O-acetylated during TFA treatment. The antigenicity of the depolymerized CPS prepared by either method was comparable to that of the native CPS for serotypes 2, 5, 18C, and 22F based on multiplex bead based competitive inhibition assay. This study demonstrated a relationship between antigenicity and immunogenicity, which offers more suitable candidates for conjugation. It was found that after partial depolymerization process, the CPS with optimal molecular size resulted in higher antigenicity. The immunogenicity of S. pneumoniae serotype 2 conjugates in mice was evaluated by opsonophagocytic assay and a multiplex bead-based assay, wherein on day 42 after immunization, the total and functional IgG titer was found to be increased by 32-fold.

Pneumococcal serotype-specific cut-offs based on antibody responses to pneumococcal polysaccharide vaccination in healthy adults

Antibody responses to pneumococcal polysaccharide vaccination are frequently used as a diagnostic tool for humoral immunodeficiencies, part of the larger collection of inborn errors of immunity. Currently, arbitrary criteria, such as a serotype specific titer of >/= 1.3 µg/mL is most often used as a cut-off for interpretation of pneumococcal antibody responses. The magnitude of the antibody response to each of the 23 serotypes in Pneumovax®, and serotype-specific cut-offs in healthy pneumococcal vaccine-naïve adults has not been previously characterized. IgG antibody concentrations were measured prospectively for 23 pneumococcal serotypes pre and 4-6 weeks post-Pneumovax® vaccination in 100 healthy adults, using a multiplex bead-based assay. Antibodies to 19 of 23 serotypes were informative for distinguishing subjects who responded to vaccination, and the serotype threshold was determined to be 9 of 19 serotypes, which characterized an antibody response to pneumococcal vaccination. While this study may facilitate classification of IgG serotype-specific antibody responses post-pneumococcal vaccination in adult patients undergoing diagnostic immunological evaluation for antibody immunodeficiencies or other relevant contexts, additional studies in healthy children and S. pneumoniae protein-conjugate-vaccinated healthy adults will need to be undertaken in the future.

Prime-boost vaccination strategy enhances immunogenicity compared to single pneumococcal conjugate vaccination in patients receiving conventional DMARDs, to some extent in abatacept but not in rituximab-treated patients

OBJECTIVE

To explore whether a prime-boost vaccination strategy, i.e., a dose of pneumococcal conjugate vaccine (PCV) and a dose of 23-valent polysaccharide vaccine (PPV23), enhances antibody response compared to single PCV dose in patients with inflammatory rheumatic diseases treated with different immunosuppressive drugs and controls.

METHODS

Patients receiving rituximab (n = 30), abatacept (n = 23), monotherapy with conventional disease-modifying antirheumatic drugs (cDMARDs, methotrexate/azathioprine/mycophenolate mofetil, n = 27), and controls (n = 28) were immunized with a dose PCV followed by PPV23 after ≥ 8 weeks. Specific antibodies to 12 serotypes included in both vaccines were determined using a multiplex microsphere immunoassay in blood samples before and 4-8 weeks after each vaccination. Positive antibody response was defined as ≥ 2-fold increase from pre- to postvaccination serotype-specific IgG concentration and putative protective level as IgG ≥ 1.3 μg/mL. The number of serotypes with positive antibody response and IgG ≥ 1.3 μg/mL, respectively, after PCV and PCV + PPV23 were compared within each treatment group and to controls. Opsonophagocytic activity (OPA) assay was performed for serotypes 6B and 23F.

RESULTS

Compared to single-dose PCV, prime-boost vaccination increased the number of serotypes with positive antibody response in patients with abatacept, cDMARDs, and controls (p = 0.02, p = 0.01, and p = 0.01), but not in patients on rituximab. After PCV + PPV23, the number of serotypes with positive antibody response was significantly lower in all treatment groups compared to controls but lowest in rituximab, followed by the abatacept and cDMARD group (p < 0.001). Compared to PCV alone, the number of serotypes with putative protective levels after PCV + PPV23 increased significantly only in patients in cDMARDs (p = 0.03) and controls (p = 0.001). Rituximab treatment was associated with large reduction (coefficient - 8.6, p < 0.001) and abatacept or cDMARD with moderate reductions (coefficients - 1.9 and - 1.8, p = 0.005, and p < 0.001) in the number of serotypes with positive antibody response to PCV + PPV23 (multivariate linear regression model). OPA was reduced in rituximab (Pn6B and Pn23F, p < 0.001), abatacept (Pn23F, p = 0.02), and cDMARD groups (Pn6B, p = 0.02) compared to controls.

CONCLUSIONS

Prime-boost strategy enhances immunogenicity compared to single pneumococcal conjugate vaccination in patients with inflammatory rheumatic diseases receiving cDMARDs, to some extent in abatacept but not in patients on rituximab. Pneumococcal vaccination should be encouraged before the initiation of treatment with rituximab.

TRIAL REGISTRATION

ClinicalTrials.gov, NCT03762824. Registered on 4 December 2018, retrospectively registered.

Specific Antibody Deficiencies in Clinical Practice

Specific antibody deficiency (SAD) is defined as the inability to mount an antibody response to purified Streptococcus pneumoniae capsular polysaccharide antigens in the presence of normal immunoglobulin concentrations and normal antibody responses to protein antigens. In this review, we discuss the difficulties in using presently available testing methods to adequately define SAD. The fact that there are different forms of SADs to pneumococcal surface polysaccharides is detailed. The diagnostic and therapeutic implications of recognizing that, in addition to SAD, there are other forms of SAD in the response to S. pneumoniae polysaccharides are described in detail. The conclusion of this review is that assessment of immunity and therapeutic actions to deal with SADs need to be based on clinical evidence rather than solely on arbitrarily defined antibody responses.

Development of an opsonophagocytic killing assay for group a streptococcus

Group A Streptococcus (GAS) or Streptococcus pyogenes is responsible for an estimated 500,000 deaths worldwide each year. Protection against GAS infection is thought to be mediated by phagocytosis, enhanced by bacteria-specific antibody. There are no licenced GAS vaccines, despite many promising candidates in preclinical and early stage clinical development, the most advanced of which are based on the GAS M-protein. Vaccine progress has been hindered, in part, by the lack of a standardised functional assay suitable for vaccine evaluation. Current assays, developed over 50 years ago, rely on non-immune human whole blood as a source of neutrophils and complement. Variations in complement and neutrophil activity between donors result in variable data that is difficult to interpret. We have developed an opsonophagocytic killing assay (OPKA) for GAS that utilises dimethylformamide (DMF)-differentiated human promyelocytic leukemia cells (HL-60) as a source of neutrophils and baby rabbit complement, thus removing the major sources of variation in current assays. We have standardised the OPKA for several clinically relevant GAS strain types (emm1, emm6 and emm12) and have shown antibody-specific killing for each emm-type using M-protein specific rabbit antisera. Specificity was demonstrated by pre-incubation of the antisera with homologous M-protein antigens that blocked antibody-specific killing. Additional qualifications of the GAS OPKA, including the assessment of the accuracy, precision, linearity and the lower limit of quantification, were also performed. This GAS OPKA assay has the potential to provide a robust and reproducible platform to accelerate GAS vaccine development.

Hilaire AJ, Gerson DF. Novel polysaccharide-protein conjugates provide an immunogenic 13-valent pneumococcal conjugate vaccine for S. pneumoniae

Pneumonia remains the single leading cause of childhood death worldwide. Despite the commercial availability of multiple pneumococcal conjugate vaccines (PCVs), high dosage cost and supply shortages prevent PCV delivery to much of the developing world. The current work presents high-yield pneumococcal conjugates that are immunogenic in animals and suitable for use in human vaccine development. The 13-valent pneumococcal conjugate vaccine (PCV-13) investigated in this research incorporated serotypes 1, 3, 4, 5, 6A, 6B, 7F, 9V, 14, 18C, 19A, 19F, and 23F. Pneumococcal polysaccharides (PnPSs) and CRM₁₉₇ carrier protein were produced and purified in-house, and used to prepare PnPS-CRM conjugates using unique, cyanide-free, in vacuo glycation conjugation methods. In vitro characterization confirmed the generation of higher molecular weight PnPS-CRM conjugates low in free protein. In vivo animal studies were performed to compare PnuVax's PCV-13 to the commercially available PCV-13, Prevnar^®13 (Pfizer, USA). A boost dose was provided to all groups post-dose 1 at t = 14 days. Post-dose 2 results at t = 28 days showed that all 13 serotypes in PnuVax's PCV-13 were boostable. Per serotype IgG GMCs demonstrated that PnuVax's PCV-13 is immunogenic for all 13 serotypes, with 10 of the 13 serotypes statistically the same or higher than Prevnar^®13 post-dose 2. As a result, the novel polysaccharide-protein conjugates developed in this work are highly promising for use in human PCV development. The in vacuo conjugation technique applied in this work could also be readily adapted to develop many other conjugate vaccines.

Correlates of Protection for M Protein-Based Vaccines against Group A Streptococcus

Group A streptococcus (GAS) is known to cause a broad spectrum of illness, from pharyngitis and impetigo, to autoimmune sequelae such as rheumatic heart disease, and invasive diseases. It is a significant cause of infectious disease morbidity and mortality worldwide, but no efficacious vaccine is currently available. Progress in GAS vaccine development has been hindered by a number of obstacles, including a lack of standardization in immunoassays and the need to define human correlates of protection. In this review, we have examined the current immunoassays used in both GAS and other organisms, and explored the various challenges in their implementation in order to propose potential future directions to identify a correlate of protection and facilitate the development of M protein-based vaccines, which are currently the main GAS vaccine candidates.

Pneumococcal vaccine and opsonic pneumococcal antibody

Streptococcus pneumoniae is a major human pathogen responsible for the majority of bacterial pneumonia cases as well as invasive pneumococcal diseases with high mortality and morbidity. Use of conjugate vaccines targeting the pneumococcal capsule has dramatically reduced the incidence of invasive diseases, and there are active efforts to further improve the conjugate vaccines. However, in children new pneumococcal vaccines can no longer be tested with placebo-based clinical trials because effective vaccines are currently available. Thus, vaccine studies must depend on surrogate markers of vaccine efficacy. Although traditional antibody levels (e.g., ELISA) are useful as a surrogate marker of protection, they have limitations, and a bioassay measuring the capacity of antibodies to opsonize pneumococci has been developed. This opsonophagocytosis assay (OPA) replicates the in vivo mechanism of antibody protection and should therefore better reflect protection by vaccine-induced antibodies. Technical improvements of OPA have made this bioassay rapid, multiplexed, and practical for analyzing small samples including those from children. Strong correlations between ELISA and OPA have been observed in many studies of young children. However, poor correlations have been found in some important clinical situations (such as determination of protection by cross-reactive antibodies) and populations (such as elderly adults and immunodeficient patients). In these settings, OPA has become a useful supplementary measure of pneumococcal vaccine immunogenicity. Current efforts to standardize OPA will further expand its uses.

The impact of B-cell perturbations on pneumococcal conjugate vaccine response in HIV-infected adults

Untreated HIV infection results in severe perturbations of the B-cell population and hyporesponsiveness to vaccination. We studied associations between circulating B-cell subsets and antibody response to pneumococcal conjugate vaccine in treated and untreated HIV patients.

Ninety-five HIV-infected adults were grouped according to antiretroviral therapy (ART) and CD4+ cell count as follows: 20 ART-naïve (no prior ART), 62 ART-responders (received ART, and CD4 count >500 cells/µl), and 13 impaired responders (received ART for more than 3 years, and CD4 count <500 cells/µl). All subjects were immunized twice with double-dose 7-valent pneumococcal conjugate vaccine with or without 1 mg CPG 7909 (toll-like receptor 9 agonist) at baseline and after three months. Pre-vaccination B-cell subpopulations were assessed by flow cytometry. Serum IgG concentrations for vaccine serotypes were quantified by ELISA at baseline and 3, 4, and 9 months post-vaccination. ART responders had more isotype-switched memory B cells and more marginal-zone (MZ)-like B cells compared with impaired responders. Furthermore, ART-naïve patients had higher concentration of transitional B cells and plasmablasts compared with B cells of other patient groups. The concentration of MZ-like, isotype switched memory cells and plasmablasts correlated positively with post-vaccination IgG concentration at 3, 4, and 9 months. Low concentrations of isotype-switched memory B cells was the strongest independent predictor of poor pneumococcal conjugate vaccine responsiveness, emphasizing that B-cell subset disturbances are associated with poor vaccine response among HIV-infected patients

The efficacy of pneumococcal capsular polysaccharide-specific antibodies to serotype 3 Streptococcus pneumoniae requires macrophages

The efficacy of antibody immunity against Streptococcus pneumoniae stems from the ability of opsonic, serotype (ST)-specific antibodies to pneumococcal capsular polysaccharide (PPS) to facilitate killing of the homologous ST by host phagocytes. However, PPS-specific antibodies have been identified that are protective in mice, but do not promote opsonic killing in vitro, raising the question of how they mediate protection in vivo. To probe this question, we investigated the dependence of antibody efficacy against lethal systemic (intraperitoneal, i.p.) infection with Streptococcus pneumoniae serotype 3 (ST3) on macrophages and neutrophils for the following PPS3-specific monoclonal antibodies (MAbs) in survival experiments in mice using a non-opsonic human IgM (A7), a non-opsonic mouse IgG1 (1E2) and an opsonic mouse IgG1 (5F6). The survival of A7- and PPS3-specific and isotype control MAb-treated neutrophil-depleted and neutrophil-sufficient and macrophage-depleted and macrophage-sufficient mice were determined after i.p. challenge with ST3 strains 6303 and WU2. Neutrophils were dispensable for A7 and the mouse MAbs to mediate protection in this model, but macrophages were required for the efficacy of A7 and optimal mouse MAb-mediated protection. For A7-treated mice, macrophage-depleted mice had higher blood CFU, cytokines and peripheral neutrophil levels than macrophage-sufficient mice, and macrophage-sufficient mice had lower tissue bacterial burdens than control MAb-treated mice. These findings demonstrate that macrophages contribute to opsonic and non-opsonic PPS3-specific MAb-mediated protection against ST3 infection by enhancing bacterial clearance and suggest that neutrophils do not compensate for the absence of macrophages in the model used in this study.

Pneumococcal conjugate vaccination in persons with HIV: the effect of highly active antiretroviral therapy

OBJECTIVE

Vaccination responses may be affected by concomitant use of highly active antiretroviral therapy (HAART). We aimed to determine HAART's impact on seven-valent pneumococcal conjugate (7vPnC) vaccine immunization with or without a Toll-like receptor 9 (TLR9) agonist adjuvant.

DESIGN

Observational cohort study.

METHODS

Adults with HIV were immunized with double doses of 7vPnC +/-1 mg CPG 7909, a TLR9 agonist and vaccine adjuvant, at 0 and 3 months, and 23-valent pneumococcal polysaccharide vaccine at 9 months. We measured IgG levels (ELISA) and opsonophagocytic activity (OPA) at months 0, 3, 4, 9, and 10. Persistent 7vPnC vaccine responders were defined as individuals with two-fold IgG increases to 1 microg/ml or more for at least five of the 7vPnC serotypes at 9 months.

RESULTS

We included 75 participants on HAART and 20 HAART-naive. Forty-one received CPG 7909 and 48 received placebo adjuvant. More persistent 7vPnC vaccine responders were found among HAART-treated than among HAART-naive (42.3 vs. 15.0%, P = 0.03). Mean loss of vaccine-specific IgG from month 4 to 9 was greater among HAART-naive than among HAART-treated (54.8 vs. 38.1%, P = 0.001). Functional activity (OPA) was higher among HAART-treated than among HAART-naive at 4, 9, and 10 months. In a logistic regression analysis (adjusted for baseline CD4 cell count, CPG 7909, smoking status, BMI, AIDS diagnosis, and age), HAART use was significantly associated with being persistent 7vPnC vaccine responder at month 9 [odds ratio = 4.65, 95% confidence interval (CI) 1.07-20.2].

CONCLUSIONS

HIV-infected adults on HAART achieved a more durable antibody response of higher functional activity following pneumococcal conjugate vaccination than HAART-naive individuals, independently of baseline CD4 cell count.

The effect of age on the response to the pneumococcal polysaccharide vaccine

BACKGROUND

Streptococcus pneumoniae is a leading cause of morbidity and mortality in the elderly. To prevent invasive pneumococcal diseases, the 23-valent pneumococcal polysaccharide vaccine (PPV) is recommended in subjects over 65 years of age. Although it has been reported to provide approximately 50-80% protection against invasive disease in the general elderly population, there is still controversy as to the effectiveness of the PPV in the elderly.

METHODS

To evaluate the immune response to the pneumococcal polysaccharide vaccine in the elderly, samples from young adults and elderly were obtained before and one month after vaccination. The quantitative and qualitative response to the vaccine were measured by the ELISA and opsonophagocytic killing assay for eight vaccine type serotypes (4, 6B, 9V, 14, 18C, 19A, 19F, 23F) and one vaccine-related serotype (6A).

RESULTS

The response to the pneumococcal polysaccharide vaccine showed a similar response between adults and elderly when evaluated by the ELISA, however the functional activity of the antibodies elicited after vaccination were lower in the elderly group for more than half of the serotypes evaluated. In comparison of the antibody needed for 1:8 opsonic titer, more antibodies were needed in the elderly for serotypes Pn 4, 19F, 23F and 6A, suggesting the functional activity of antibody detected by the ELISA was lower in the elderly compared with the adult group for these serotypes. As for subjects with an opsonic titer <8 after vaccination, only one subject each for serotypes Pn 4, 9V and 6A were found in the adult group. However, up to 10 (30.3%) of the subjects did not show opsonic activity after vaccination in the elderly group for serotypes Pn 4, 9V, 14, 19A and 6A.

CONCLUSIONS

Although the amount of antibodies elicited were similar between the two age groups, distinct differences in function were noted. This report highlights the importance of a quantitative and qualitative evaluation of the immunogenic response to the PPV in the elderly age group.

TRIAL REGISTRATION

This trial is registered with Clinicaltrials.gov. Registration number NCT00964769.

Flow cytometry and the future of vaccine development

Vaccine research increasingly aims to understand the fundamental mechanisms of protection afforded by licensed and candidate vaccines. Historically, nearly all licensed vaccines have relied on measures of humoral immunity to provide correlates of protection, but cellular immunity is important for protection afforded by some vaccines and will be required for vaccines against TB and malaria. Common means of assessing vaccine-induced immune responses include measuring the frequency and functions of antigen-specific lymphocytes. While diverse assays can provide this information, flow cytometry is unique in its ability to simultaneously report other features of antigen-specific cellular responses. Here, we review the application of flow cytometry to characterizing three areas of immune responses to vaccines or diseases. First, analysis of cellular (T-cell) responses is more mature: polychromatic flow cytometric analysis of T-cell function has already yielded important insight into correlates of protection. Second, antibody and antigen-specific B-cell detection by flow cytometry are being actively developed; to date, these assays are not yet widely used. Finally, flow cytometry can also be used to analyze the contribution of innate immunity to vaccine efficacy and disease pathogenesis.

Pneumococcal capsular polysaccharide vaccine-mediated protection against serotype 3 Streptococcus pneumoniae in immunodeficient mice

Pneumococcal capsular polysaccharide (PPS) vaccines are less immunogenic in immunocompromised than immunocompetent individuals. However, neither the efficacy of PPS vaccines in immunocompromised individuals nor the host cellular subsets required for vaccine efficacy against pneumococcal disease have been directly investigated. In this study, we vaccinated CD4-deficient (CD4(-/-)), CD8-deficient (CD8(-/-)), and secretory immunoglobulin M-deficient (sIgM(-/-)) mice and wild-type C57BL/6 (Wt) mice with a conjugate of PPS of serotype 3 and tetanus toxoid (PPS3-TT) and determined the antibody response and efficacy of vaccination against systemic and pulmonary challenge with serotype 3 pneumococcus in immunized and control mice. Our results showed that the isotype and predominant IgG subclass of the PPS3 response differed between immunodeficient mouse strains and between immunodeficient and Wt mice, with CD8(-/-) mice having the most robust response. Vaccination protected Wt, CD4(-/-), and sIgM(-/-) mice from death resulting from both systemic and pulmonary challenge, whereas CD8(-/-) mice were protected only from systemic and not from pulmonary challenge. Passive vaccination with PPS3-TT-induced sera from Wt, CD4(-/-), CD8(-/-), and sIgM(-/-) mice protected naïve Wt mice from death due to pulmonary challenge; however, CD8(-/-) mice were not protected by sera from Wt or CD8(-/-) mice. Our findings suggest that PPS-based vaccines can be effective in the setting of CD4 T-cell deficiency but that CD8 T cells could be required for vaccine-mediated protection against pulmonary challenge with serotype 3 pneumococcus.

Development and validation of a fourfold multiplexed opsonization assay (MOPA4) for pneumococcal antibodies

Opsonophagocytic killing assays (OPAs) are essential for developing and improving pneumococcal vaccines. There is a need for a high-throughput, reliable, standardized, and fully characterized OPA for pneumococcal antibodies. To meet the need, we have developed and characterized a fourfold multiplexed OPA (MOPA4) against 13 serotypes (1, 3, 4, 5, 6A, 6B, 7F, 9V, 14, 18C, 19A, 19F, and 23F) of pneumococci. Thirteen target bacteria were made resistant to only one of the following antibiotics: optochin, streptomycin, spectinomycin, and trimethoprim. Following optimization of assay conditions, accuracy of MOPA4 was determined by testing 30 sera from old adults in the MOPA4 and the single-serotype assays. The opsonization titers obtained with both assays agreed well (r(2) > 0.95). Although 22 (out of 390; approximately 6%) results differed more than twofold, the differences were not reproducible. The assay was specific: preabsorbing test sera with homologous polysaccharide (PS) completely abrogated opsonic activity, but a pool of unrelated PS (5 mug/ml of each) had no effect. Intra- and interassay coefficients of variation were 10 and 22%, respectively. MOPA4 results were unaffected by having different target pneumococcal serotypes in each assay group. Also, HL60 cell-to-bacteria ratios could be varied twofold without affecting the results. We conclude that MOPA4 is sensitive, accurate, specific, precise, and robust enough for large-scale clinical studies. Furthermore, MOPA4 should allow evaluation of multivalent pneumococcal vaccines with the limited volume of serum typically available from young children.