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Secretion and function of the third component of complement (C3) by murine leukocytes
1Department of Immunology, Tohoku University, Sendai, Japan.
Abstract:
Our new finding of de novo synthesis and secretion of C3 by both murine peritoneal macrophages and polymorphonuclear leukocytes (PMN) was confirmed by the incorporation of [35S]methionine into C3 molecules and their complete inhibition by cycloheximide. The methods of secretion of C3 from these two types of cells were compared by examining the C3 contents in their culture supernatants. Completely different modes of secretion were observed, i.e. although macrophages synthesize and secrete C3 constitutively, PMN has a mechanism to store the already synthesized C3 in the cell and secrete it in response to stimuli. Protein kinase C (PKC) activators, e.g. 12-O-tetradecanoyl-phorbol 13-acetate, dioctanoyl glycerol, and mezerein, as well as calcium ionophore A23187 stimulate the secretion of C3 from PMN. These results suggest the involvement of PKC and the calmodulin pathway. A very sensitive method for measuring C3 activity was developed which enabled us to show for the first time that C3 secreted by PMN had opsonizing activity and that particles cultured with PMN were phagocytosed effectively.
Insights
Macrophages and neutrophils (PMN) synthesize and secrete complement C3. Macrophages secrete C3 constitutively, while PMN store and release C3 upon stimulation, demonstrating its opsonizing activity.
Area of Science:
- Immunology
- Cell Biology
Background:
- Complement component 3 (C3) is crucial for innate and adaptive immunity.
- The cellular sources and secretion mechanisms of C3, particularly from myeloid cells, require further elucidation.
Purpose of the Study:
- To investigate the de novo synthesis and secretion of C3 by murine peritoneal macrophages and polymorphonuclear leukocytes (PMN).
- To compare the distinct C3 secretion pathways employed by macrophages and PMN.
- To characterize the functional activity of C3 secreted by PMN.
Main Methods:
- Confirmation of C3 synthesis and secretion via [35S]methionine incorporation and cycloheximide inhibition.
- Quantification of C3 in cell culture supernatants to compare secretion modes.
- Stimulation of PMN C3 secretion using protein kinase C (PKC) activators and calcium ionophore.
- Development of a sensitive C3 activity assay.
Main Results:
- Both macrophages and PMN synthesize C3.
- Macrophages exhibit constitutive C3 secretion, whereas PMN store and release C3 in response to stimuli like PKC activators and calcium ionophore.
- A novel, sensitive assay demonstrated that PMN-secreted C3 possesses opsonizing activity, enhancing phagocytosis.
Conclusions:
- Macrophages and PMN are significant sources of C3, utilizing different secretion strategies.
- PMN C3 secretion involves PKC and calmodulin pathways.
- PMN-derived C3 actively participates in immune responses through opsonization and phagocytosis enhancement.