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Demonstration of a specific receptor for human C5a anaphylatoxin on murine macrophages
Abstract:
Human C5a anaphylatoxin is known to be a potent mediator of the acute inflammatory response. It serves to trigger a wide variety of neutrophil responses after binding to a specific cellular receptor. We have now demonstrated that this bioactive glycopolypeptide is also bound to a specific receptor found on murine resident peritoneal macrophages, thioglycollate-induced exudate macrophages, and the murine cell line P388D1. The apparent Kd of the C5a receptors. Resident macrophages appear to have an average of 2 x 10(5) binding sites per cell, whereas thioglycollate- induced cells have only 4-5 x 10(4) binding sites. The continuous cell line P388D1 is intermediate between these two cell types, exhibiting 8-10 x 10(4) C5a receptors per cell. Neither murine lymphocytes nor the parent cell line P388 displays a measurable number of C5a receptors. Macrophage receptor-C5a binding interactions are followed by cellular uptake and degradation of 125I-C5a, much as is observed with neutrophils. As demonstrated in another paper, binding of C5a to macrophages results in augmentation of the primary humoral immune response as well as enhancement of mixed lymphocyte reactions. These observations suggest that C5a should not only be considered as an acute inflammatory mediator but as an immunopotentiating modulator as well, thus serving as a critical link between complement activation and subsequent immune responses.
Insights
Human C5a anaphylatoxin, a potent inflammatory mediator, binds to receptors on macrophages, influencing immune responses. This study reveals C5a
Area of Science:
- Immunology
- Complement System
- Cellular Biology
Background:
- Human C5a anaphylatoxin is a key mediator of acute inflammation, primarily acting on neutrophils via specific receptors.
- The role of C5a in modulating immune cell functions beyond neutrophils is an area of ongoing investigation.
Purpose of the Study:
- To investigate the presence and characteristics of C5a receptors on murine macrophages.
- To explore the functional consequences of C5a binding to macrophages in the context of immune modulation.
Main Methods:
- Radioligand binding assays using iodinated C5a (125I-C5a) to quantify receptor expression on different macrophage populations.
- Characterization of C5a receptor binding affinity (Kd) and density on resident peritoneal macrophages, thioglycollate-induced macrophages, and the P388D1 cell line.
- Assessment of C5a uptake and degradation by macrophages post-receptor binding.
Main Results:
- Specific C5a receptors were identified on murine resident peritoneal macrophages, thioglycollate-induced macrophages, and the P388D1 cell line.
- Quantification revealed varying receptor densities: resident macrophages (2 x 10^5 sites/cell), thioglycollate-induced macrophages (4-5 x 10^4 sites/cell), and P388D1 cells (8-10 x 10^4 sites/cell).
- Murine lymphocytes and the parent P388 cell line did not exhibit measurable C5a receptors. Macrophage-C5a interactions led to cellular uptake and degradation of C5a.
Conclusions:
- C5a receptors are present on murine macrophages, indicating a role beyond neutrophil activation.
- C5a binding to macrophages enhances humoral immune responses and mixed lymphocyte reactions, suggesting immunomodulatory functions.
- C5a acts as a critical link between complement activation and immune responses, functioning as both an inflammatory mediator and an immunopotentiator.