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M(+) group a streptococci are phagocytized and killed in whole blood by C5a-activated polymorphonuclear leukocytes
Eric DeMaster1, Norbert Schnitzler, Qi Cheng
1Department of Microbiology, University of Minnesota, Minneapolis, Minnesota 55455, USA.
Abstract:
Historically, resistance to phagocytosis has been determined by incubating group A streptococci in human blood and comparing the numbers of CFU before and after incubation. Utilizing a flow cytometry-based technique, we have investigated the phagocytosis of M(+) group A streptococci by polymorphonuclear leukocytes (PMNs) in heparinized human peripheral whole blood. Intracellular labeling of streptococci with a nontoxic fluorescent dye allowed us to quantify the association and phagocytosis of M(+) streptococci by PMNs in whole blood in the presence or absence of C5a, a physiologically important chemotactic activator of PMNs. We found that wild-type strains of group A streptococci that are resistant to phagocytosis (determined by the classical Lancefield method) readily associate with C5a-activated whole-blood PMNs. In the absence of opsonizing M-type-specific antibodies, the M(+) streptococci associated with PMNs are phagocytized and killed. In addition, blockade of the beta(2) integrin, CD11b/CD18, with anti-human CD11b monoclonal antibody inhibited association between M(+) streptococci and C5a-activated PMNs. These findings establish a new relationship between M(+) streptococci and PMNs, in which C5a-activated PMNs have the capacity to kill M(+) streptococci in whole blood through a receptor-mediated phagocytic mechanism.
Insights
Group A streptococci resistant to phagocytosis can be killed by human polymorphonuclear leukocytes (PMNs) in whole blood. This process is mediated by C5a-activated PMNs and involves M(+) streptococci association via beta(2) integrin.
Area of Science:
- Immunology
- Microbiology
- Cell Biology
Background:
- Phagocytosis resistance in Group A Streptococcus (GAS) is traditionally assessed using colony-forming units (CFU).
- Polymorphonuclear leukocytes (PMNs) are critical immune cells involved in clearing bacterial infections.
Purpose of the Study:
- To investigate the phagocytosis of M-protein positive (M(+)) GAS by human PMNs using flow cytometry.
- To explore the role of C5a, a neutrophil chemoattractant, in GAS-PMN interactions.
Main Methods:
- Utilized flow cytometry to quantify M(+) GAS association and phagocytosis by PMNs in whole blood.
- Employed intracellular fluorescent labeling of GAS and C5a activation of PMNs.
- Investigated the effect of blocking the beta(2) integrin (CD11b/CD18) on GAS-PMN association.
Main Results:
- Wild-type M(+) GAS, typically resistant to phagocytosis, readily associated with C5a-activated PMNs.
- In the absence of specific antibodies, associated M(+) GAS were phagocytized and killed by PMNs.
- Inhibition of beta(2) integrin significantly reduced the association between M(+) GAS and C5a-activated PMNs.
Conclusions:
- C5a-activated PMNs can kill M(+) GAS in whole blood via a receptor-mediated phagocytic mechanism.
- This study establishes a novel interaction pathway between M(+) GAS and PMNs, highlighting the role of beta(2) integrin.
- The findings provide new insights into the innate immune response against Group A Streptococcus.