Development of an Opsonophagocytic Killing Assay Using HL-60 Cells for Detection of Functional Antibodies against Streptococcus pyogenes

A flow cytometry-based assay to determine the ability of anti-Streptococcus pyogenes antibodies to mediate monocytic phagocytosis in human sera

Streptococcus pyogenes, commonly referred to as Group A Streptococcus (Strep A), causes a spectrum of diseases, with the potential to progress into life-threatening illnesses and autoimmune complications. The escalating threat of antimicrobial resistance, stemming from the prevalent reliance on antibiotic therapies to manage Strep A infections, underscores the critical need for the development of disease control strategies centred around vaccination. Phagocytes play a critical role in controlling Strep A infections, and phagocytosis-replicating assays are essential for vaccine development. Traditionally, such assays have employed whole-blood killing or opsonophagocytic methods using HL-60 cells as neutrophil surrogates. However, assays mimicking Fcγ receptors- phagocytosis in clinical contexts are lacking. Therefore, here we introduce a flow cytometry-based method employing undifferentiated THP-1 cells as monocytic/macrophage model to swiftly evaluate the ability of human sera to induce phagocytosis of Strep A. We extensively characterize the assay's precision, linearity, and quantification limit, ensuring robustness. By testing human pooled serum, the assay proved to be suitable for the comparison of human sera's phagocytic capability against Strep A. This method offers a valuable complementary assay for clinical studies, addressing the gap in assessing FcγR-mediated phagocytosis. By facilitating efficient evaluation of Strep A -phagocyte interactions, it may contribute to elucidating the mechanisms required for the prevention of infections and inform the development of future vaccines and therapeutic advancements against Strep A infections.

A conserved 3D pattern in a Streptococcus pyogenes M protein immunogen elicits M-type crossreactivity

Coiled coil-forming M proteins of the widespread and potentially deadly bacterial pathogen Streptococcus pyogenes (strep A) are immunodominant targets of opsonizing antibodies. However, antigenic sequence variability of M proteins into >220 M types, as defined by their hypervariable regions (HVRs), is considered to limit M proteins as vaccine immunogens because of type specificity in the antibody response. Surprisingly, a multi-HVR immunogen in clinical vaccine trials was shown to elicit M-type crossreactivity. The basis for this crossreactivity is unknown but may be due in part to antibody recognition of a 3D pattern conserved in many M protein HVRs that confers binding to human complement C4b-binding protein (C4BP). To test this hypothesis, we investigated whether a single M protein immunogen carrying the 3D pattern would elicit crossreactivity against other M types carrying the 3D pattern. We found that a 34-amino acid sequence of S. pyogenes M2 protein bearing the 3D pattern retained full C4BP-binding capacity when fused to a coiled coil-stabilizing sequence from the protein GCN4. We show that this immunogen, called M2G, elicited cross-reactive antibodies against a number of M types that carry the 3D pattern but not against those that lack the 3D pattern. We further show that the M2G antiserum-recognized M proteins displayed natively on the strep A surface and promoted the opsonophagocytic killing of strep A strains expressing these M proteins. As C4BP binding is a conserved virulence trait of strep A, we propose that targeting the 3D pattern may prove advantageous in vaccine design.

Correlates of immunity to Group A Streptococcus: a pathway to vaccine development

Understanding immunity in humans to Group A Streptococcus (Strep A) is critical for the development of successful vaccines to prevent the morbidity and mortality attributed to Strep A infections. Despite decades of effort, no licensed vaccine against Strep A exists and immune correlates of protection are lacking; a major impediment to vaccine development. In the absence of a vaccine, we can take cues from the development of natural immunity to Strep A in humans to identify immune correlates of protection. The age stratification of incidence of acute Strep A infections, peaking in young children and waning in early adulthood, coincides with the development of specific immune responses. Therefore, understanding the immune mechanisms involved in natural protection from acute Strep A infection is critical to identifying immune correlates to inform vaccine development. This perspective summarises the findings from natural infection studies, existing assays of immunity to Strep A, and highlights the gaps in knowledge to guide the development of Strep A vaccines and associated correlates of protection.

Design of Broadly Cross-Reactive M Protein-Based Group A Streptococcal Vaccines

Group A streptococcal infections are a significant cause of global morbidity and mortality. A leading vaccine candidate is the surface M protein, a major virulence determinant and protective Ag. One obstacle to the development of M protein-based vaccines is the >200 different M types defined by the N-terminal sequences that contain protective epitopes. Despite sequence variability, M proteins share coiled-coil structural motifs that bind host proteins required for virulence. In this study, we exploit this potential Achilles heel of conserved structure to predict cross-reactive M peptides that could serve as broadly protective vaccine Ags. Combining sequences with structural predictions, six heterologous M peptides in a sequence-related cluster were predicted to elicit cross-reactive Abs with the remaining five nonvaccine M types in the cluster. The six-valent vaccine elicited Abs in rabbits that reacted with all 11 M peptides in the cluster and functional opsonic Abs against vaccine and nonvaccine M types in the cluster. We next immunized mice with four sequence-unrelated M peptides predicted to contain different coiled-coil propensities and tested the antisera for cross-reactivity against 41 heterologous M peptides. Based on these results, we developed an improved algorithm to select cross-reactive peptide pairs using additional parameters of coiled-coil length and propensity. The revised algorithm accurately predicted cross-reactive Ab binding, improving the Matthews correlation coefficient from 0.42 to 0.74. These results form the basis for selecting the minimum number of N-terminal M peptides to include in potentially broadly efficacious multivalent vaccines that could impact the overall global burden of group A streptococcal diseases.

Cross-reactive immunogenicity of group A streptococcal vaccines designed using a recurrent neural network to identify conserved M protein linear epitopes

The M protein of group A streptococci (Strep A) is a major virulence determinant and protective antigen. The N-terminal sequence of the protein defines the more than 200 M types of Strep A and also contains epitopes that elicit opsonic antibodies, some of which cross-react with heterologous M types. Current efforts to develop broadly protective M protein-based vaccines are directed at identifying potential cross-protective epitopes located in the N-terminal regions of cluster-related M proteins for use as vaccine antigens. In this study, we have used a comprehensive approach using the recurrent neural network ABCpred and IEDB epitope conservancy analysis tools to predict 16 residue linear B-cell epitopes from 117 clinically relevant M types of Strep A (~88% of global Strep A infections). To examine the immunogenicity of these epitope-based vaccines, nine peptides that together shared ≥60% sequence identity with 37 heterologous M proteins were incorporated into two recombinant hybrid protein vaccines, in which the epitopes were repeated 2 or 3 times, respectively. The combined immune responses of immunized rabbits showed that the vaccines elicited significant levels of antibodies against all nine vaccine epitopes present in homologous N-terminal 1-50 amino acid synthetic M peptides, as well as cross-reactive antibodies against 16 of 37 heterologous M peptides predicted to contain similar epitopes. The epitope-specificity of the cross-reactive antibodies was confirmed by ELISA inhibition assays and functional opsonic activity was assayed in HL-60-based bactericidal assays. The results provide important information for the future design of broadly protective M protein-based Strep A vaccines.

Studying Neutrophil Function in vitro: Cell Models and Environmental Factors

Neutrophils are the most abundant immune cell type in the blood and constitute the first line of defense against invading pathogens. Despite their important role in many diseases, they are challenging to study due to their short life span and the inability to cryopreserve or expand them in vitro. Thus, research into neutrophils has to rely on cells freshly isolated from peripheral blood of human donors, introducing donor-dependent variation in the experimental data. To counteract these problems, researchers tried to develop adequate cell models, such as cell lines. For those functional studies that cannot rely on cell models, a standardization of protocols regarding neutrophil purification and culturing could be a solution. In this review, we provide an overview of the most commonly used models for neutrophil function (HL-60, PLB-985, NB4, Kasumi-1 and induced pluripotent stem cells). In addition, we describe the effects of glucose concentration, pH, oxygen tension and temperature on neutrophil function.

Polyethylenimine: An Intranasal Adjuvant for Liposomal Peptide-Based Subunit Vaccine against Group A Streptococcus

Group A Streptococcus (GAS) and GAS-related infections are a worldwide challenge, with no commercial GAS vaccine available. Polyethylenimine (PEI) attaches to the cells' surface and delivers cargo into endosomal and cytosolic compartments. We hypothesized that this will confer mucosal adjuvant properties for peptide antigens against group A Streptococcus (GAS). In this study, we successfully demonstrated the development of PEI incorporated liposomes for the delivery of a lipopeptide-based vaccine (LCP-1) against GAS. Outbred mice were administrated with the vaccine formulations intranasally, and immunological investigation showed that the PEI liposomes elicited significant mucosal and systemic immunity with the production of IgA and IgG antibodies. Antibodies were shown to effectively opsonize multiple isolates of clinically isolated GAS. This proof-of-concept study showed the capability for PEI liposomes to act as a safe vehicle for the delivery of GAS peptide antigens to elicit immune responses against GAS infection, making PEI a promising addition to liposomal mucosal vaccines.

Opsonic Activity of Conservative Versus Variable Regions of the Group A Streptococcus M Protein

Group A Streptococcus (GAS) and GAS-associated infections are a global challenge, with no licensed GAS vaccine on the market. The GAS M protein is a critical virulence factor in the fight against GAS infection, and it has been a primary target for GAS vaccine development. Measuring functional opsonic antibodies against GAS is an important component in the clinical development path for effective vaccines. In this study, we compared the opsonic activity of two synthetic, self-adjuvanting subunit vaccines containing either the J8- or 88/30-epitope in Swiss outbred mice using intranasal administration. Following primary immunization and three boosts, sera were assessed for IgG activity using ELISA, and opsonization activity against seven randomly selected clinical isolates of GAS was measured. Vaccine constructs containing the conservative J8-epitope showed significant opsonic activity against six out of the seven GAS clinical isolates, while the vaccine containing the variable 88/30-epitope did not show any significant opsonic activity.

Vaccine-Induced Th1-Type Response Protects against Invasive Group A Streptococcus Infection in the Absence of Opsonizing Antibodies

Availability of a group A Streptococcus vaccine remains an unmet public health need. Here, we tested different adjuvant formulations to improve the protective efficacy of non-M protein vaccine Combo5 in an invasive disease model. We show that novel adjuvants can dramatically shape the type of immune response developed following immunization with Combo5 and significantly improve protection. In addition, protection afforded by Combo5 is not mediated by opsonizing antibodies, believed to be the main correlate of protection against GAS infections. Overall, this report highlights the importance of adjuvant selection in raising protective immune responses against GAS invasive infection. Adjuvants that can provide a more balanced Th1/Th2-type response may be required to optimize protection of GAS vaccines, particularly those based on non-M protein antigens.

Recent global advocacy efforts have highlighted the importance of development of a vaccine against group A Streptococcus (GAS). Combo5 is a non-M protein-based vaccine that provides protection against GAS skin infection in mice and reduces the severity of pharyngitis in nonhuman primates. However, Combo5 with the addition of aluminum hydroxide (alum) as an adjuvant failed to protect against invasive GAS infection of mice. Here, we show that formulation of Combo5 with adjuvants containing saponin QS21 significantly improves protective efficacy, even though all 7 adjuvants tested generated high antigen-specific IgG antibody titers, including alum. Detailed characterization of Combo5 formulated with SMQ adjuvant, a squalene-in-water emulsion containing a TLR4 agonist and QS21, showed significant differences from the results obtained with alum in IgG subclasses generated following immunization, with an absence of GAS opsonizing antibodies. SMQ, but not alum, generated strong interleukin-6 (IL-6), gamma interferon (IFN-γ), and tumor necrosis alpha (TNF-α) responses. This work highlights the importance of adjuvant selection for non-M protein-based GAS vaccines to optimize immune responses and protective efficacy.

An Opsonophagocytic Killing Assay for the Evaluation of Group A Streptococcus Vaccine Antisera

Group A Streptococcus (GAS) is a major cause of global mortality, yet there are no licensed GAS vaccines. Vaccine progress has been hampered, in part, by a lack of standardized assays able to quantify antibody function in test antisera. The most widely used assay was developed over 50 years ago by Rebecca Lancefield and relies on human whole blood as a source of complement and neutrophils. Recently, an opsonophagocytic killing (OPK) assay has been developed for GAS by adapting the OPK methods utilized in Streptococcus pneumoniae vaccine testing. This assay uses dimethylformamide (DMF)-differentiated human promyelocytic leukemia cells (HL-60 cells) as a source of neutrophils and baby rabbit complement, thus removing the major sources of variation in the Lancefield assays. This protocol outlines methods for performing a GAS OPK assay including titering test sera to generate an opsonic index. This in vitro assay could aid in selecting vaccine candidates by demonstrating whether candidate-induced antibodies lead to complement deposition and opsonophagocytic killing.

Lancefield Whole Blood Killing Assay to Evaluate Vaccine Efficacy

While the Lancefield whole blood killing assay is named after the renowned streptococcal researcher Rebecca Lancefield, the protocol was first described by Todd in 1927 (Br J Exp Pathol 8:1-5, 1927). Initially, the assay was used to identify novel Group A Streptococcal (GAS) serotypes through the supplementation of non-immune human blood (often from infants) with type-specific antisera prepared in rabbits (Lancefield, J Exp Med 106:525-544, 1957; Maxted, Br J Exp Pathol 37:415-422, 1956) and to demonstrate the impressive longevity of type-specific immunity in patients following invasive GAS infection (Lancefield, J Exp Med 110:271-292, 1959). The modern assay is routinely used to screen defined GAS mutants (Wessels, Bronze, Proc Natl Acad Sci U S A 91:12238-12242, 1994; Zinkernagel et al., Cell Host Microbe 4:170-178, 2008) or transposon libraries (Le Breton et al., Infect Immun 81:862-875, 2013) for enhanced susceptibility to opsonophagocytic killing or to screen vaccine antisera (Salehi et al., mSphere 3:e00617-e00618, 2018) or other serological preparations (Reglinski et al., Sci Rep 5:15825, 2015) for anti-streptococcal activity.

Safety and immunogenicity of a 30-valent M protein-based group a streptococcal vaccine in healthy adult volunteers: A randomized, controlled phase I study

BACKGROUND

Streptococcus pyogenes (group A Streptococcus, Strep A) is a widespread pathogen that continues to pose a significant threat to human health. The development of a Strep A vaccine remains an unmet global health need. One of the major vaccine strategies is the use of M protein, which is a primary virulence determinant and protective antigen. Multivalent recombinant M protein vaccines are being developed with N-terminal M peptides that contain opsonic epitopes but do not contain human tissue cross-reactive epitopes.

METHODS

We completed a Phase I trial of a recombinant 30-valent M protein-based Strep A vaccine (Strep A vaccine, StreptAnova™) comprised of four recombinant proteins containing N-terminal peptides from 30 M proteins of common pharyngitis and invasive and/or rheumatogenic serotypes, adjuvanted with aluminum hydroxide. The trial was observer-blinded and randomized in a 2:1 ratio for intramuscular administration of Strep A vaccine or an alum-based comparator in healthy adult volunteers, at 0, 30 and 180 days. Primary outcome measures were assessments of safety, including assays for antibodies that cross-reacted with host tissues, and immunogenicity assessed by ELISA with the individual vaccine peptides and by opsonophagocytic killing (OPK) assays in human blood.

RESULTS

Twenty-three Strep A-vaccinated participants and 13 controls completed the study. The Strep A vaccine was well-tolerated and there was no clinical evidence of autoimmunity and no laboratory evidence of tissue cross-reactive antibodies. The vaccine was immunogenic and elicited significant increases in geometric mean antibody levels to 24 of the 30 component M antigens by ELISA. Vaccine-induced OPK activity was observed against selected M types of Strep A in vaccinated participants that seroconverted to specific M peptides.

CONCLUSION

The Strep A vaccine was well tolerated and immunogenic in healthy adults, providing strong support for further clinical development. [ClinicalTrials.gov NCT02564237].

A Multicomponent Vaccine Provides Immunity against Local and Systemic Infections by Group A Streptococcus across Serotypes

GAS is among the most common human pathogens and causes a wide variety of diseases, likely more than any other microorganism. The diverse clinical manifestations of GAS may be attributable to its large repertoire of virulence factors that are selectively and synergistically involved in streptococcal pathogenesis. To date, GAS vaccines have not been successful due to multiple serotypes and postinfection sequelae associated with autoimmunity. In this study, five conserved virulence factors that are involved in GAS pathogenesis were used as a combined vaccine. Intranasal immunization with this vaccine induced humoral and cellular immune responses across GAS serotypes and protected against mucosal, systemic, and skin infections. The significance of this work is to demonstrate that the efficacy of GAS vaccines can be achieved by including multiple nonredundant critical virulence factors and inducing local and systemic immunity. The strategy also provides valuable insights for vaccine development against other pathogens.

Group A streptococcus (GAS) species are responsible for a broad spectrum of human diseases, ranging from superficial to invasive infections, and are associated with autoimmune disorders. There is no commercial vaccine against GAS. The clinical manifestations of GAS infection may be attributable to the large repertoire of virulence factors used selectively in different types of GAS disease. Here, we selected five molecules, highly conserved among GAS serotypes, and involved in different pathogenic mechanisms, as a multicomponent vaccine, 5CP. Intranasal (i.n.) immunization with 5CP protected mice against both mucosal and systemic GAS infection across serotypes; the protection lasted at least 6 months. Immunization of mice with 5CP constrained skin lesion development and accelerated lesion recovery. Flow cytometry and enzyme-linked immunosorbent assay analyses revealed that 5CP induced Th17 and antibody responses locally and systemically; however, the Th17 response induced by 5CP resolved more quickly than that to GAS when challenge bacteria were cleared, suggesting that 5CP is less likely to cause autoimmune responses. These findings support that immunization through the i.n. route targeting multiple nonredundant virulence factors can induce immunity against different types of GAS disease and represents an alternative strategy for GAS vaccine development, with favorable efficacy, coverage, duration, and safety.