Associated complement C3b. Towards an understanding of its intracellular modifications

Cyclic AMP plays a critical role in C3a-receptor-mediated regulation of dendritic cells in antigen uptake and T-cell stimulation

The biochemical basis for complement acting directly on antigen-presenting cells to enhance their function in T-cell stimulation has been unclear. Here we present evidence that engagement of C3a receptor (C3aR) on the surface of dendritic cells (DCs) leads to alterations in the level of intracellular cyclic adenosine monophosphate (cAMP), a potent negative regulator of inflammatory cytokines. C3aR activation-induced depression of cAMP was associated with enhanced capacity of DCs for antigen uptake and T-cell stimulation. Conversely, C3aR-deficient DCs showed elevation of cAMP and impaired properties for antigen uptake and immune stimulation. Similarities in the phenotype of C3-deficient and C3aR-deficient DCs suggest that local production of C3 with extracellular metabolism to C3a is an important driver of DC alterations in cAMP. The finding of a link between complement and adaptive immune stimulation through cAMP offers new insight into how innate and adaptive immunity combine to generate efficient effector and memory responses.

A new role for complement C3: regulation of antigen processing through an inhibitory activity

Increasing evidence underlines the involvement of complement component C3 in the establishment of acquired immunity which appears to play a complex role and to act at different levels. As antigen proteolysis by antigen presenting cells is a key event in the control of antigen presentation efficiency, and consequently in the quality of the immune response, we investigated whether C3 could modulate this step. Our results demonstrate for the first time that C3 can interfere with antigen proteolysis: (i) proteolysis of tetanus toxin (TT) by the lysosomal fraction from a human monocytic cell line (U937) is impaired in the presence of C3, (ii) this effect is C3-specific and involves the C3c fragment of the protein, (iii) C3c is effective even after disulfide disruption, but none of its three constitutive peptides is individually accountable for this inhibitory effect and (iv) the target-protease(s) exhibit(s) a serine-protease activity. The physiological relevance of our results is demonstrated by experiments showing a subcellular colocalisation of TT and C3 after their uptake by U937 and the reduction of TT proteolysis once internalised together with C3. These results highlight a novel role for C3 that broadens its capacity to modulate acquired immune response.

Macrophages from C3-deficient mice have impaired potency to stimulate alloreactive T cells

Impaired T-cell reactivity is a feature of C3-deficient mice in several disease models. The mechanism behind the reduced T-cell response is, however, poorly understood. We explored the hypothesis that antigen-presenting cells (APCs) from C3-/- mice have impaired potency to stimulate antigen-specific T cells, in an alloantigen-dependent model. Our results show that C3-/- macrophages have reduced ability to elicit alloreactive T-cell responses in vitro and in vivo, affecting both the primary and secondary responses. The C3 status of donor macrophages had a major impact on the CD4 T-cell response. The impaired CD4 T-cell response was associated with reduced expression of MHC class II on the surface of C3-/- macrophages, without loss of class II gene expression. Furthermore, inhibition of C3 gene expression in C3+/+ macrophages reduced their ability to stimulate alloreactive T cells, suggesting that endogenous production of C3 could in part contribute to the potency of APCs. Our data provide compelling evidence that C3 deficiency modulates the potency of APCs to stimulate the T-cell response, suggesting a critical role for complement in the maintenance of APC function. This could offer a partial explanation as to why the T-cell response is impaired in C3-/- mice. (Blood. 2006;107:2461-2469)

Allograft rejection: effect of local synthesis of complement

The complement system is known for its ability to participate in non-specific inflammation and membrane injury as well as contributing to antigen-specific immune stimulation. In renal transplantation, the complement cascade behaves true to form in that both non-immune- and immune-mediated destruction of the renal tubules are complement dependent. What is remarkable, however, is the extent of involvement of local synthesis of complement in both of these injuries, suggesting that the extravascular tissue compartment is the domain of local synthesis, whereas the effect of circulating complement is much less. This creates a new paradigm for studying the influence of local synthesis of complement in other organ-based diseases and underlines the need for tissue-targeting strategies in successful therapeutic development.

Structure and functions of proteases which cleave human C3 and are expressed on normal or tumor human cells: some are involved in tumorigenic and metastatic properties of human melanoma cells

Human C3 is a multipotent molecule which participates to different events involved in immune response as complement activation, antigen presentation, cell-cell interactions and cell proliferation. Thus, proteinases which cleave C3 may modify C3-dependent cellular functions. This led us to identify two membrane-associated proteinases which cleave human C3: (a) A p57 serine proteinase expressed on human erythrocyte membranes--This p57 proteinase shared antigenic determinants with ankyrine and may be involved in clearance of immune complexes; (b) A p41 cysteine proteinase, which shares antigenic determinants, amino-acid sequence and specific activity with procathepsin-L--This p41 C3-cleaving cyteine proteinase is also involved in tumorigenic and metastatic properties of human melanoma in nude mice. Indeed, pretreatment of highly tumorigenic and metastatic melanoma cells with anti-p39 Ab totally abolished their tumorigenicity and significantly decreased the number of experimental lung metastases in nude mice. Furthermore, overexpression of procathepsin-L in nonmetastatic melanoma cells increased their tumorigenicity and switched their phenotype to highly metastatic cells in nude mice. Altogether, these data support that expression and secretion of procathepsin-L, which cleaves human C3, might be one of the multiple mechanisms by which tumor cells escape the immune surveillance.

Complement C3b fragment covalently linked to tetanus toxin increases lysosomal sodium dodecyl sulfate-stable HLA-DR dimer production

Processing and presentation of covalently linked C3b-tetanus toxin (TT) complexes, as compared to unlinked C3b + TT, lead to increased T cell proliferation. The aim of this study was to analyze the effect of coupling C3b to TT on the efficiency of TT peptide loading on HLA-DR1 molecules. In the Epstein-Barr virus-transformed B cell line HOM 2, we detected a significant increase of sodium dodecyl sulfate (SDS)-stable major histocompatibility complex (MHC) class II molecules after exposure to C3b-TT as compared to unlinked C3b and TT. The ratio of compact form/unbound form (C/U ratio) obtained with C3b-TT as antigen (Ag) is about twice that obtained with uncomplexed TT + C3b as Ag. Similar results were obtained using HLA-DR1-transfected fibroblasts that do not express C3b complement receptors, indicating that the SDS-stable HLA-DR1 increase did not result simply from C3b opsonization but rather from a direct effect of C3b-TT linkage on peptide generation. Exposure of HOM 2 cells to C3b-TT resulted in an increase in concentration of SDS-stable HLA-DR molecules in lysosomes but not in endosomes. Thus, C3b attachment to Ag induces a redistribution of peptide/MHC complex which results in a higher efficiency of Ag presentation by MHC class II molecules.

Complement in acute and chronic arthritides: assessment of C3c, C9, and protectin (CD59) in synovial membrane

OBJECTIVES

To investigate the role of complement cascade induced damage and protection against it in acute arthritides compared to rheumatoid arthritis and other chronic joint derangements.

METHODS

C3c, C9, and protectin (CD59) were examined by avidin-biotin-peroxidase complex staining.

RESULTS

Marked deposits of C3c and C9 were found in synovial vasculature and intercellular matrix of the lining in rheumatoid arthritis and in acute arthritides (including bacterial, reactive, and osteoarthritis flare up). Furthermore, protectin was not visible in synovial lining cells and was relatively weakly expressed in stromal and endothelial cells in rheumatoid arthritis; also in acute arthritides protectin expression was weak. In contrast, C3c and C9 deposits were not found in chronic conditions associated with degenerative diseases (osteoarthritis and osteochondritis dissecans) or mechanical causes (patellar luxation and a ruptured meniscus), in which also the protectin expression was prominent in synovial lining, endothelial and some stromal cells.

CONCLUSIONS

Activation of the complement in rheumatoid arthritis and in acute arthritides seems to be associated with a decreased protection of synovial cells against cellular effects and lysis mediated by membrane attack complex.

Identification on melanoma cells of p39, a cysteine proteinase that cleaves C3, the third component of complement: amino-acid-sequence identities with procathepsin L

We previously identified, on normal or tumour cells, two membrane proteinases, p57 and p65, that cleave human C3, the third component of complement, thus regulating C3's biological properties. Whereas p57 was purified from human erythrocytes, p65 was identified using polyclonal anti-p57 antibodies on a human melanoma cell line resistant to complement lysis. Analysis of cell distribution of C3-cleaving proteinases established that DSm, a murine melanoma cell line, expressed a C3-cleaving proteinase distinct from p57 and p65 proteinases. Thus we purified the C3-cleaving proteinase solubilized from membranes of DSm cells. The purified proteinase, termed 'p39' on the basis of its molecular mass of 39 kDa, was identified, using specific proteinase inhibitors, as a cysteine proteinase. Anti-p39 antibodies, prepared against highly purified p39, localized the p39 C3-cleaving proteinase mainly at the cell surface and demonstrated that p39 is also secreted. Anti-p39 antibodies inhibited solubilized C3-cleaving activity. Preincubation of DSm cells with anti-p39 F(ab')2 fragments increased up to 60% complement cell susceptibility. Amino acid analysis of N-terminal and three other regions of p39 demonstrated that this C3-cleaving proteinase carries 100% identity within four regions of procathepsin L. This is the first demonstration that a melanoma cell line expresses on its surface and secretes a p39 C3-cleaving cysteine proteinase that shares sequence identities with procathepsin L. Thus the p39 cysteine proteinase represents a new member of the C3-cleaving proteinase family associated with, and/or expressed on, the cell surface.

C3b covalently associated to tetanus toxin modulates TT processing and presentation by U937 cells

Complement protein C3, like C4 and alpha 2-macroglobulin (alpha 2M), is a potentially bivalent ligand: (1) its proteolytic fragment, C3b, is able to interact covalently with antigens, and (2) this bound fragment is able to interact non-covalently with specific complement receptors of antigen presenting cells (APC). The formation of antigen-C3b complexes frequently occurs in vivo at inflammatory sites during the early stages of an immune response. Tetanus toxin (TT)-C3b covalent complexes, prepared from purified proteins, were used to study how C3b association influences the handling of TT by U937 cells used as APC. TT-specific T cell proliferation following TT-C3b processing was observed at a concentration when TT alone was inefficient. Whereas TT pinocytic uptake was low, TT-C3b uptake, through the help of complement receptor CR1, was three times higher. Free TT was rapidly transported to the lysosomes where it was proteolysed, whereas TT-C3b complexes were first retained in the endosomes and underwent only limited proteolysis. While the ester link of the complexes was fairly stable in the endosomes, it was gradually hydrolysed in the lysosomes with ensuing efficient proteolysis of the two proteins. This reflects the fact that associated C3b escorts TT during intracellular trafficking in the APC, and influences antigen processing. A triple role of C3b escorting antigen residues at the level of antigen uptake, routing, and proteolysis inside U937 cells, thus modulating antigen-dependent T cell proliferation.

Covalent binding of C3b to monoclonal antibodies selectively up-regulates heavy chain epitope recognition by T cells

Protein C3 of the complement system is known for its role in the nonspecific immune response. Covalent binding of C3b to antigen upon complement activation also plays a significant role in specific T cell immune response. C3b-antigen complexes can bind to complement receptors on the antigen-presenting cell, and the C3b antigen link (most often an ester link) remains fairly stable inside the cells. In this study, IgG1,kappa and IgG2a,kappa murine monoclonal antibodies (mAb) were used as antigens; covalent complexes between mAb and C3b were produced and purified in vitro from purified proteins; human B cell lines and T cell clones were raised from tumor patients who received mAb injections for cancer therapy or diagnosis. Recognition of epitopes of these mAb by T cell clones when the mAb were processed alone or bound to C3b was compared. IgG or IgG-C3b complexes presented by B cell lines were able to stimulate proliferation of kappa light chain-specific T cell clones at similar concentrations. In contrast, IgG-C3b complex recognition by heavy chain-specific T cell clones required 100-fold less IgG-C3b than uncomplexed IgG. As C3b was shown to be covalently bound only to the IgG heavy chains in the complexes, C3b chaperoning is restricted to only the IgG heavy chain and selectively influences intracellular steps of IgG heavy chain processing. This differential modulation of C3b suggests an early dissociation of IgG heavy and light chains in antigen-presenting cells.