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Characterization of C1q receptor expression on human phagocytic cells: effects of PDBu and fMLP
Insights
This study investigated C1q receptor (C1qR) binding on human monocytes and neutrophils. Unlike CR1, C1qR expression on these cells is not significantly increased by fMLP or PDBu, suggesting distinct regulatory mechanisms.
Area of Science:
- Immunology
- Cell Biology
Background:
- Complement system component C1q plays a crucial role in innate immunity.
- Receptor-mediated interactions are key to immune cell function.
Purpose of the Study:
- To characterize C1q receptor (C1qR) binding on human monocytes and polymorphonuclear leukocytes (PMN).
- To compare C1qR expression and regulation with the C3b receptor (CR1).
Main Methods:
- C1q binding assays were used to quantify C1qR on purified monocytes and PMN.
- Fluorescent flow cytometry analyzed C1q binding distribution.
- Cells were pretreated with N-formylmethionylleucylphenylalanine (fMLP) and phorbol dibutyrate (PDBu) to assess receptor modulation.
Main Results:
- Monocytes and PMN express similar numbers of C1qR, but C1q binding is more homogeneous on monocytes.
- fMLP and PDBu significantly increased CR1 expression on both cell types.
- Neither fMLP nor PDBu treatment significantly increased C1qR expression; some doses even reduced C1q binding.
Conclusions:
- C1qR expression and regulation differ significantly from CR1.
- Monocytes and PMN exhibit distinct patterns of C1q binding.
- The regulation of C1qR is not modulated by common immune cell activators like fMLP and PDBu.
Abstract:
The receptor-mediated binding of C1q to human phagocytic cells was investigated in this study. By using a C1q binding assay, we determined that purified, elutriated monocytes expressed an average of 4.6 X 10(5) C1q receptors (C1qR) per cell, with an equilibrium binding constant (Keq) of 0.91 X 10(7) (M-1). When analyzed in an identical manner, the polymorphonuclear leukocytes (PMN) expressed an average of 4.2 X 10(5) C1qR per cell, with a Keq for C1q of 1 X 10(7) (M-1). Fluorescent flow cytometric analysis showed that C1q was bound by 98% of the monocytes studied. Further, the pattern formed by these cells was consistent with a normal log distribution, indicating that this was a homogeneous population of cells. When PMN were assayed with flow cytometry, however, we found that C1q was bound by an average of only 45% of the PMN analyzed. Further, these PMN were not dispersed in a normal log distribution, indicating some heterogeneity among the cells that bind C1q. We examined the abilities of the chemoattractant N-formylmethionylleucylphenylalanine (fMLP) and the phorbol ester phorbol dibutyrate (PDBu) to modulate expression of C1qR as compared to the receptor for C3b (CR1). Pretreatment of the monocytes and the PMN with either 10(-6)M fMLP or 10 ng/ml of PDBu significantly increased cell surface CR1 expression, as reported previously by other investigators. In contrast, no significant increase in the number of C1qR on the monocytes or the PMN was observed with any of the concentrations of fMLP or PDBu used during pretreatment. However, with certain pretreatment doses of these agents, some reduction was noted in the amount of 125I-C1q bound to the monocytes and the PMN. This study characterizes the binding of C1q to purified monocytes and confirms previously published values for PMN. The distribution of cells expressing C1qR is shown to be significantly different between identically treated populations of monocytes and PMN. Finally, the abilities of fMLP and PDBu to modulate the binding of C1qR are examined. Our results indicate that the control of C1qR expression differs markedly from that of CR1.