Monocyte Expressed Macromolecular C1 and C1q Receptors as Molecular Sensors of Danger: Implications in SLE

Abstract

The ability of circulating blood monocytes to express C1q receptors (cC1qR and gC1qR) as well as to synthesize and secrete the classical pathway proteins C1q, C1r, and C1s and their regulator, C1-INH is very well established. What is intriguing, however, is that, in addition to secretion of the individual C1 proteins monocytes are also able to display macromolecular C1 on their surface in a manner that is stable and functional. The cell surface C1 complex is presumably formed by a Ca(2+)-dependent association of the C1r2⋅C1s2 tetramer to C1q, which in turn is anchored via a membrane-binding domain located in the N-terminus of its A-chain as shown previously. Monocytes, which circulate in the blood for 1-3 days before they move into tissues throughout the body, not only serve as precursors of macrophages and dendritic cells (DCs), but also fulfill three main functions in the immune system: phagocytosis, antigen presentation, and cytokine production. Since the globular heads of C1q within the membrane associated C1 are displayed outwardly, we hypothesize that their main function - especially in circulating monocytes - is to recognize and capture circulating immune complexes or pathogen-associated molecular patterns in the blood. This in turn may give crucial signal, which drives the monocytes to migrate into tissues, differentiate into macrophages or DCs, and initiate the process of antigen elimination. Unoccupied C1q on the other hand may serve to keep monocytes in a pre-dendritic phenotype by silencing key molecular players thus ensuring that unwarranted DC-driven immune response does not occur. In this paper, we will discuss the role of monocyte/DC-associated C1q receptors, macromolecular C1 as well as secreted C1q in both innate and acquired immune responses.

Keywords: C1q and C1q receptors; C1q in SLE; C1q in autoimmunity; DC and C1; c1q; monocyte C1.

Figure 1

Circulating blood monocytes express C1 and C1qRs. (1) Membrane anchored C1q displays its globular heads free to interact with antigens or immune complexes. (2) Some of the membrane anchored C1q is cleaved off and secreted into the pericellular milieu and is able to bind to the cell surface via gC1qR. The C1q figure was obtained from Gérard Arlaud.

Figure 2

Expression of C1q and C1qRs is maturation dependent. Circulating monocytes express both C1 and C1qRs, which increase during the monocyte to iDC transition in the presence of self- or foreign-danger signals. Fully mature DCs, however, express little or no C1q, low gC1qR but high cC1qR.

Figure 3

Maturation dependent expression of C1qRs. C1q receptors show varied expression on mono-DC precursors. Mononuclear cells were cultured in the presence of GM-CSF + IL-4, and collected and analyzed on days 0–4 for the expression of cC1qR (A,D), gC1qR (B,E), and CD91 (C,F) for both percent expression (A–C) and MFI (D–F). (A) The percent of cC1qR expression was variable on monocytes, but by day 2 nearly all monocyte-DCs had the receptor on their surface. (B) On day 0, gC1qR was present on almost all the cells, and its expression was only slightly reduced by day 4. (C) Monocytes expressed CD91 on their surface, but the percentage of CD91+ cells was significantly reduced by day 3 and 4. (D) Mean fluorescence analysis revealed that cC1qR expression was dramatically amplified by day 3 and 4. (E,F) gC1qR and CD91 MFIs remained at relatively steady levels throughout the days. Experiments were gated on DR+ cells, *p < 0.05, **p < 0.01 (n ≥ 4) [adapted from Ref. (35)].

Figure 4

Schematic representation of a fine-tuned balancing act. The transition from steady state to immunogenic response depends on the encounter between an antigen and C1q.

Figure 5

C1q delays GM-CSF + IL-4 induced DC maturation. Monocyte-DCs were isolated and cultured in the presence of GM-CSF + IL-4 (G4) and with or without 25 μg/ml C1q (A–C). For the dose response experiments several concentrations of C1q were added as indicated (D). Cells were collected on the days indicated and analyzed for the expression of CD86, CD83, and CD11c for each condition (A–C), or alternatively, on day 2 HLA-DR analyses were performed for all C1q doses (D). (A) C1q significantly decreased CD86 expression in monocyte-DCs compared to G4, *p < 0.05, **p < 0.01 (n = 4); (B) monocyte-DCs cultured in the presence of C1q showed a decrease in the percentage of CD83+ cells in comparison with cells cultured in G4 alone. While CD83 expression was detected on the surface of these cells, their MFI remained low throughout the days with or without the addition of C1q (data not shown), *p < 0.05 (n = 4); (C) CD11c expression was increased by day 2 with the addition of C1q compared to day 0. There was no significant difference in CD11c expression levels on cells cultured with or without C1q (n = 6); (D) dose response analysis revealed that the MFI of HLA-DR+ cells correlates negatively with increasing doses of C1q on day 2. While there was little or no difference in the distribution of HLA-DR on the cells at lower doses (10–25 μg/ml), at higher doses decreased expression was noted (MFI) (n = 3). Cells were gated on DR+ cells for all experiments [adapted from Ref. (35)].