Cell Surface Expression and Function of the Macromolecular C1 Complex on the Surface of Human Monocytes

Abstract

The synthesis of the subunits of the C1 complex (C1q, C1s, C1r), and its regulator C1 inhibitor (C1-Inh) by human monocytes has been previously established. However, surface expression of these molecules by monocytes has not been shown. Using flow cytometry and antigen-capture enzyme-linked immunosorbent assay, we show here for the first time that, in addition to C1q, peripheral blood monocytes, and the monocyte-derived U937 cells express C1s and C1r, as well as Factor B and C1-Inh on their surface. C1s and C1r immunoprecipitated with C1q, suggesting that at least some of the C1q on these cells is part of the C1 complex. Furthermore, the C1 complex on U937 cells was able to trigger complement activation via the classical pathway. The presence of C1-Inh may ensure that an unwarranted autoactivation of the C1 complex does not take place. Since C1-Inh closely monitors the activation of the C1 complex in a sterile or infectious inflammatory environment, further elucidation of the role of C1 complex is crucial to dissect its function in monocyte, dendritic cell, and T cell activities, and its implications in host defense and tolerance.

C1q limits dendritic cell differentiation and activation by engaging LAIR-1

“…Approximately 80% of C1q present in plasma occurs as a C1qrs complex, and the remaining 20% occurs as uncomplexed C1q (26). We showed that both C1q and the C1 (C1qrs) complex engaged LAIR-1 on the cell surface and suppressed the production of proinflammatory cytokines.…”

“…Another important remaining question is whether C1q attached to apoptotic debris or present in immune complexes yields a distinct response. Given that a variety of cells in a range of tissues may secrete large quantities of C1q and C1 (26), and that C1q, r and s are distinctly regulated (23), control of local concentrations of C1q and C1 may be an important factor in determining LAIR-1 activation.…”

C1q and HMGB1 reciprocally regulate human macrophage polarization

“…This interpretation agrees well with the SPR results, which are also consistent with distinct binding sites for HMGB1 and C1q on RAGE. Because ;80% of C1q forms a complex with C1r and C1s in blood in the presence of calcium, 41,42 we performed a dot blot in the presence of calcium and demonstrated that C1 detected with an anti-C1r antibody bound to LAIR-1 (supplemental Figure 1D). Moreover, C1 complex bridged RAGE and LAIR-1 ( Figure 3H).…”

C1q as an autocrine and paracrine regulator of cellular functions

“…These include regulation of immune cells, inhibition of viral replication, and inhibition of cancer through induction of apoptosis (Black et al, 1997). Similarly, although a wide range of cell types are able to synthesize C1q, the major sources of C1q also appear to be the cells of the myeloid lineage such as monocytes, monocytes (Bensa et al 1983; Randazzo et al 1985; Tenner and Volkin, 1986; Drouet and Reboul, 1989; Gulati et al 1993; Hosszu et al, 2012) macrophages (Kaul and Loos, 1993a, 1993b), dendritic cells (Vegh et al, 2003; Castellano et al, 2004). Moreover, C1q is also synthesized as a type-II transmembrane protein, which remains membrane-anchored via a transmembrane sequence located in its A-chain until it is enzymatically cleaved off from its anchor domain to generate the soluble form that is found in plasma or tissues (Kaul and Loos 1993b).…”