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This opsonophagocytic killing assay is used to compare the ability of phagocytic immune cells to respond to and kill bacteria based on different treatments and/or conditions. Classically, this assay serves as the gold standard for assessing effector functions of antibodies raised against a bacterium as opsonin.
A key aspect of the immune response to bacterial colonization of the host is phagocytosis. An opsonophagocytic killing assay (OPKA) is an experimental procedure in which phagocytic cells are co-cultured with bacterial units. The immune cells will phagocytose and kill the bacterial cultures in a complement-dependent manner. The efficiency of the immune-mediated cell killing is dependent on a number of factors and can be used to determine how different bacterial cultures compare with regard to resistance to cell death. In this way, the efficacy of potential immune-based therapeutics can be assessed against specific bacterial strains and/or serotypes. In this protocol, we describe a simplified OPKA that utilizes basic culture conditions and cell counting to determine bacterial cell viability after co-culture with treatment conditions and HL-60 immune cells. This method has been successfully utilized with a number of different pneumococcal serotypes, capsular and acapsular strains, and other bacterial species. The advantages of this OPKA protocol are its simplicity, versatility (as this assay is not limited to antibody treatments as opsonins), and minimization of time and reagents to assess basic experimental groups.
The opsonophagocytic killing assay (OPKA) is a critical tool for linking alterations in bacterial structure or function to subsequent changes in immune response and function. As such, it is frequently used as a complementary assay to determine immune-based efficacy of antibody treatments, vaccine candidates, enzyme optimization, etc. While in vivo assays are necessary to determine effective clearance or protection in a bacterial infection model, the OPKA can be used to assess immune contribution to bacterial cell death at the most basic components: bacteria, immune cells, and experimental treatments. Previous studies have shown that OPKAs can be modified and used for a variety of bacteria and serotypes, including Streptococcus pneumoniae1, Staphylococcus aureus2, Pseudomonas aeruginosa3. Furthermore, these optimized assays can be used to assess different experimental treatments, including the ability of an enzyme to make the bacterium more accessible to complement-mediated immune cells4 and antibody treatments to improve opsonization5. Classically, OPKA assay has been successfully used in basic and clinical research settings as a powerful indicator for protection induced by pathogen-specific antibodies6,7,8,9.
Different types of immune cells may be used for assessment of opsonophagocytic killing. One commonly used phagocytic population is the HL-60 human leukemic cell line. This cell line can be kept as inactivated promyelocytes in culture; however, they can be differentiated into various activated states via different drug treatments10,11. Treatment of HL60 with N,N-dimethylformamide differentiates the cell line into activated neutrophils with strong phagocytic activity11. While HL-60 cells have been optimized and are frequently used for these phagocytosis assays10, other primary polymorphonuclear leukocytes can be used as the immune arm of the experiment12.
Additionally, these assays can be simplified13 or multiplexed14 to look at multiple antibiotic-resistant strains of the bacteria to be tested. The multiplexed method has been made more feasible through the development of software that can efficiently count bacterial colony forming units (CFUs) per spot on an agar plate15. Here, we describe a streamlined method using one bacterial strain, HL-60 cells, baby rabbit complement, and blood agar plates. With this method, multiple treatments can be assessed quickly to address specific research questions on how the innate immune response to bacterial infection can be modulated.
1. Culture, Differentiation, and Validation of HL-60 Cells
2. Preparation of OPKA Buffers and Reagents
3. Preparation of Bacterial Stock Samples
4. Bacterial Treatment and Culture
5. HL-60 bacterial Co-culture
6. Sample Plating and Overnight Incubation
Validation of HL-60 differentiation should be performed before starting the OPKA. This can be accomplished using flow cytometry to determine the extracellular expression of CD11b, CD35, CD71, and annexin V (Figure 1). Propidium iodide can also be used as a viability marker. After being treated with DMF for 3 days, expression of CD35 should be increased (≥55% of all cells) and expression of CD71 should be decreased (≤20% of all cells). The percentage of annexin V+ and propidium io...
OPKAs serve essential roles in assessing antibody mediated immune responses induced by vaccinations6,8. The main significance of this simplified OPKA is the adaptability in the conditions to be tested (i.e., antibodies, enzyme treatments, etc.). In this sense, while this assay can be used to test the contribution of opsonins (i.e., antibodies) in phagocytosis, it can also be used to assess ways to overcome virulence factors (i.e., capsular polysaccharides) that n...
The authors have nothing to disclose.
We thank Dr. Moon Nahm (University of Alabama Birmingham) for his invaluable assistance in establishing OPKA assays in our laboratory. This work was supported by National Institutes of Health Grant 1R01AI123383-01A1 to FYA.
Name | Company | Catalog Number | Comments |
Annexin V (APC conjugated) | BioLegend | 640919 | |
anti-CD35, human (PE conjugated) | BioLegend | 333405 | |
anti-CD71, human (PE conjugated) | BioLegend | 334105 | |
bacterial strain to be used (ie, Streptococcus pneumoniae, WU2) | Bacterial Respiratory Reference Laboratory (Dr. Moon Nahm) | ||
blood agar plates | Hardy Diagnostic | A10 | |
Fetal Clone serum | HyClone | SH30080.03 | |
glycerol | Sigma | G9012-1L | |
HL-60 cells | ATCC | CCL-240 | |
IgG Isotype Control (PE conjugated) | BioLegend | 400907 | |
N,N-dimethylformamide (DMF) | Fisher Chemical | UN2265 | |
propidium iodide | Sigma | P4864 | |
RPMI media with L-glutamine | Corning | 10-040-CV |
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