Elsevier

Molecular Immunology

Volume 44, Issue 13, July 2007, Pages 3389-3397
Molecular Immunology

Complement protein C1q induces maturation of human dendritic cells

https://doi.org/10.1016/j.molimm.2007.02.014Get rights and content

Abstract

Maturation of dendritic cells (DCs) is known to be induced by several stimuli, including microbial products, inflammatory cytokines and immobilized IgG, as demonstrated recently. Since immune complexes formed in vivo also contain C1q, moreover apoptotic cells and several pathogens fix C1q in the absence of antibodies, we undertook to investigate whether this complement protein has an impact on various functions of human DCs. Maturation of monocyte-derived immature DCs (imMDCs) cultured on immobilized C1q was followed by monitoring expression of CD80, CD83, CD86, MHCII and CCR7. The functional activity of the cells was assessed by measuring cytokine secretion and their ability to activate allogeneic T lymphocytes. Cytokine production by T cells co-cultured with C1q-matured DCs was also investigated. C1q, but not the structurally related mannose-binding lectin was found to bind to imMDC in a dose-dependent manner and induced NF-κB translocation to the nucleus. Immobilized C1q induced maturation of MDCs and enhanced secretion of IL-12 and TNF-α, moreover, elevated their T-cell stimulating capacity. As IFN-γ levels were increased in supernatants of MDC–T cell co-cultures, our data suggest that C1q-induced DC maturation generates a Th1-type response. Interestingly, IL-10 levels were elevated by C1q-treated MDCs but not in the supernatant of their co-cultures with allogeneic T cells. Taken together, these results indicate that C1q-opsonized antigens may play a role in the induction and regulation of immune response. Moreover our data are relevant in view of the role of C1q in removal of apoptotic cells and the association between C1q-deficiency and autoimmunity.

Introduction

Dendritic cells (DCs) are professional antigen presenting cells (APCs) playing a crucial role in the initiation of adaptive immunity (Banchereau et al., 2000). In their immature form (imDCs), they can capture and take up self and foreign antigens through different cell surface molecules, e.g. mannose-receptor, Toll-like receptors, scavenger receptors. After antigen uptake, DCs migrate to the draining lymph nodes whilst imDCs differentiate to phenotypically and functionally mature DCs (maDCs). DC maturation is known to be induced by several stimuli, including bacterial components (e.g. LPS), heat shock proteins, viral products (e.g. ds RNA) and endogenous factors such as inflammatory cytokines (Banchereau et al., 2000).

MaDCs are the most powerful APCs able to induce and direct the differentiation of naive T cells (Banchereau et al., 2000). Depending on the maturation signal and the cytokine environment, DCs may stimulate CD4+ helper, CD8+ cytotoxic or regulatory T cells, inducing tolerance. The level of costimulatory molecules expressed by DCs and the local cytokine environment at the time of T cell priming are all important factors, which define whether a Th1 or a Th2 response will develop—e.g. IL-12 generates Th1-, whereas IL-4 induces Th2 differentiation (Shortman and Liu, 2002). Thus, DCs are key players in the initiation and shaping of adaptive responses.

It is well known that most antigen–antibody complexes can initiate the classical complement pathway via activating the C1-complex, on the other hand, several pathogens and apoptotic cells bind the C1q subunit in an antibody independent manner and trigger complement activation (Nauta et al., 2002, Thielens et al., 2002). Thus these complexes contain bound IgG and complement proteins such as C1q and C3-derived fragments. As a result of these in vivo processes, opsonized antigens have the capacity to bind to several cell types expressing various Fc- and complement-receptors. All these interactions may in turn mediate phagocytosis and may additionally influence the maturation of DCs and their various functions (Bajtay et al., 2006). Recently it has been shown that immobilized IgG induces DC maturation ivolving FcγRII (Banki et al., 2003), but the role of complement components is not clarified yet.

C1q, the recognition subunit of C1, the trimolecular complex of the classical complement pathway, is a hexameric protein consisting of six heterotrimeric collagen-like fibers prolonged by globular recognition domains at their C-terminal end (Gaboriaud et al., 2004). Ig-dependent or Ig-independent activation of the C1 complex leads to activation and dissociation of C1r and C1s, leaving C1q bound to the activator. C1q is known to bind to a wide variety of cell types including DCs, polymorphonuclear cells, monocytes, lymphocytes, endothelial cells, and platelets (Sim et al., 1998). Identification of the C1q domain(s) responsible for these interactions is still controversial. For a functional C1q receptor at present, the most widely accepted candidate is calreticulin (cC1qR), in association with CD91 on the cell surface (Sim et al., 1998). As for the interaction of C1q with human MDCs, Vegh et al. have shown that these cells express two types of C1q receptors; gC1qR and cC1qR, reacting with the globular and collagenous regions of the molecule, respectively (Vegh et al., 2003). These authors demonstrated that imMDCs express higher levels of both receptors than mature DCs. In contrast to this observation, Nauta and co-workers found that imMDCs are negative for cC1qR and CD91 (Nauta et al., 2004, Nauta et al., 2002). C1q and mannose-binding lectin (MBL) are structurally related proteins, both containing collagenous fibers prolonged by globular domains (Thiel et al., 1997). Based on this feature, it had been postulated that they bind to similar sites on the cell membrane. However, contrary to this expectation, the lack of cross-inhibition in binding to human monocytes suggests that these proteins interact with different sites on these cells (Bajtay et al., 2000).

In the search of possible functions for the C1q receptors on DCs, Nauta et al. demonstrated that opsonization of apoptotic cells with C1q targets them to DCs (Nauta et al., 2002). Moreover, binding of C1q to human imMDCs was recently found to induce directed locomotion, a process possibly involved in the recruitment of these cells to inflammatory sites (Vegh et al., 2005). Here we demonstrate that immobilized C1q, a major opsonin of antigens and apoptotic cells in vivo can induce maturation of human MDCs, whereas MBL, although structurally similar to C1q, shows no interaction with these cells. Based on our results, we propose that the effect of C1q is mediated by NF-κB translocation, influencing cytokine production by DCs, as well. Our results also demonstrate that MDCs matured on immobilized C1q activate allogeneic T-cells, pointing to a novel role for C1q and emphasizing the biological relevance of these findings in vivo.

Section snippets

Reagents

C1q was purified from human serum as described earlier by Arlaud et al. (1979). The presence of endotoxin in the purified C1q preparation was less than 0.1 EU/ml as measured by the limulus amoebocyte lysate (LAL) assay (Cambrex Bio Science Walkersville Inc., MD, USA). The collagenous fragment of C1q was obtained by pepsin digestion, essentially according to Sasaki and Yonemasu (1983), except that uncleaved C1q molecules were removed by an additional affinity chromatography step on

ImDC bind C1q but not MBL

Since apoptotic cells and several pathogens fix C1q in the absence of antibodies (Thielens et al., 2002, Nauta et al., 2002), and may interact with imDCs in peripheral tissues, we undertook to investigate whether this complement protein has an impact on various functions of human DCs. We have demonstrated previously that human monocytes bind C1q strongly but do not react with MBL (Bajtay et al., 2000). Now we set out to investigate how MDCs of various differentiation stages react with these

Discussion

DCs represent a major link between innate and adaptive immunity through their ability to present antigens to naive T lymphocytes. Maturation of DCs, a decisive step to initiate this process, is known to be induced by several stimuli including different microbes, microbial products, and inflammatory cytokines. In addition to these factors, some serum proteins have also been recently demonstrated to influence DC development and function. Immobilized IgG was shown to induce maturation of human

Acknowledgements

The work was supported by grants from Hungarian National Science Fund (OTKA) T047151, K63038 and T043729, and that of NKFP 1A/040/04.

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