Immunological consequences of C1q and anti-c1q immune complexes in secondary cellular inflammation

The complement system comprises numerous plasma proteins that are finely orchestrated in a network of at least three separate pathways, namely the classical, lectin, and alternative pathway. This system represents a cornerstone of the innate immune system and plays a crucial role in first-line defense against invading pathogens. Additionally, the complement system displays other functions, including self-tolerance.
Systemic lupus erythematosus (SLE) is a prototypic systemic autoimmune disease with heterogeneous clinical manifestations and pathogenic mechanisms that are not fully understood. Although genetic deficiencies in the early components of the classical pathway (i.e., C1q, C1r, C1s, C4, and C2) are rare in humans, they are strongly associated with the development of monogenic SLE, particularly hereditary homozygous C1q deficiency. For instance, studies with mice deficient in C1q have strengthened this observation because affected animals developed an SLE-like disease. However, primary deficiency for C1q is uncommon in humans. Instead, most patients with SLE suffer from secondary hypocomplementemia associated with C1q autoantibodies (anti-C1q) present in 20–50% of patients. Notably, anti-C1q appear to play a role particularly in lupus nephritis (LN) since individuals with renal involvement have displayed increasing levels of anti-C1q before exacerbation and pronounced anti-C1q deposition in the glomeruli. However, exactly how anti-C1q contribute to disease activity and LN remains unclear.
Therefore, this dissertation investigates the immunological consequences of C1q and anti-C1q forming immune complexes in secondary cellular inflammation in the context of SLE and examines the following questions:
Part I – Do anti-C1q induce pro-inflammatory cytokine secretion in peripheral blood mononuclear cells (PBMCs) in the presence of activated T cells?
Although C1q alone has anti-inflammatory effects on human immune cells (i.e., monocytes, macrophages, dendritic cells [DC], and T cells), our group previously demonstrated a pro-inflammatory phenotype in human monocyte-derived macrophages (HMDMs) induced by C1q/anti-C1q complexes in vitro. However, the immunological consequences of C1q and C1q/anti-C1q complexes on other immune cells is unknown. Thus, in the first part of my thesis, I investigated the immunological effects of C1q and C1q/anti-C1q complexes in PBMCs with concomitant T cell activation. In an in vitro model for anti-C1q-mediated autoimmunity, I demonstrated that C1q/anti-C1q complexes produced an upregulation of tumor necrosis factor (TNF), interleukin 10 (IL-10), and interferon-γ (IFNγ) secretion in PMBCs. Specifically, activated T cells elicited a cell–cell contact-mediated increase in TNF and IFNγ secretion in monocytes. Moreover, the co-stimulatory pair cluster of differentiation (CD)40–CD154 was essential for the release of TNF in C1q/anti-C1q-conditioned monocytes. The latter depended on the tumor necrosis factor receptor-associated factors (TRAF) 6 and Janus family kinase (JAK) 3-signal transducer and activator of transcription (STAT) 5 signaling pathways.
Part II – What are the phenotypical characteristics of low and high C1q-producing HMDMs and what are the autocrine and paracrine effects of de novo synthesized C1q?
Unlike most proteins in the complement system, the majority of C1q is of non-hepatic origin. Instead, C1q synthesis predominantly occurs locally in tissue-resident myeloid cells, such as macrophages and DCs. Regarding the regulation of C1q synthesis, our group previously reported a continued de novo synthesis of C1q in HMDMs, mediated by C1q and C1q/anti-C1q. Although the overall concentration of C1q increased, not all HMDMs were equally involved in the production of new C1q. In the second part, I sought to define the heterogenous phenotypes of HMDMs and explore the potential autocrine and paracrine effects of the newly secreted C1q on immune cells. Combining transcriptional analysis of C1q mRNA and experiments inhibiting protein synthesis and secretion produced contradictory data that do not suggest de novo synthesis of C1q. Beyond this, coating of biotin-labeled C1q (C1q-biotin) revealed a considerable amount of the molecule in the cell culture medium to be derived from the plate. Together, the findings presented in this part of this thesis do not support the proposed notion of de novo synthesis of C1q in HMDMs triggered by C1q and C1q/anti-C1q complexes.
Part III – Do epitope-specific anti-C1q associate with specific SLE disease manifestations?
Anti-C1q are high-affinity polyclonal autoantibodies that predominantly recognize epitopes located in the collagen-like region (CLR) of C1q. There is weak to no binding of anti-C1q to soluble C1q, whereas attachment of C1q to a target allows anti-C1q to access neo-epitopes. However, the specificity of the autoantibodies has yet to be determined. In the last part of this thesis, we explored epitopes of C1q and investigated whether epitope-specific anti-C1q were associated with specific clinical presentations. In the first step, we investigated the epitope-specificity of patient-derived anti-C1q using a high-resolution epitope mapping approach. By using peptide microarrays to map the epitopes of anti-C1q, we identified three peptides of the C1q A-chain and three of the C1q B-chain with increased immunoglobulin (Ig) G binding. Next, screening a large SLE patient cohort by a newly established peptide-based enzyme-linked immunosorbent assay (ELISA) revealed that certain peptide-specific antibodies associated with selected disease manifestations. Notably, anti-C1q directed against the N-terminal C1q A-chain improved discrimination between controls and SLE beyond the conventional determination of anti-C1q.