Summary The complement system represents an evolutionary old and critical component of innate immunity where it forms the first line of defence against invading pathogens. Originally described as a heat-labile fraction of the serum responsible for the opsonisation and subsequent lytic killing of bacteria, work over the last century firmly established complement as a key mediator of the general inflammatory response but also as an acknowledged vital bridge between innate and adaptive immunity. However, recent studies particularly spanning the last decade have provided new insights into the novel modes and locations of complement activation and highlighted unexpected additional biological functions for this ancient system, for example in regulating basic processes of the cell. In this review, we will cover the current knowledge about complement’s established and novel roles in innate and adaptive immunity with a focus on the functional differences between serum-circulating versus intracellularly active complement and will describe and discuss the newly discovered cross-talks of complement with other cell effector systems particularly during T cell induction and contraction.

Autocrine activation of the complement receptors C3aR and CD46 by complement activation components C3a and C3b produced through C3 cleavage by the protease cathepsin L (CTSL) during T cell stimulation is a requirement for IFN-γ production and Th1 induction in human CD4+ T cells. Thus, lack of autocrine CD46 activation, such as in CD46-deficient patients, is associated with defective Th1 responses and recurrent infections. We have identified LGMN [the gene coding for legumain, also known as asparaginyl endopeptidase (AEP)] as one of the key genes induced by CD46 co-stimulation during human CD4+ T cell activation. AEP processes and activates a range of proteins, among those α1-thymosin and CTSL, which both drive intrinsically Th1 activity—but has so far not been described to be functionally active in human T cells. Here we found that pharmacological inhibition of AEP during activation of human CD4+ T cells reduced CTSL activation and the CTSL-mediated generation of intracellular C3a. This translated into a specific reduction of IFN-γ production without affecting cell proliferation or survival. In line with these findings, CD4+ T cells isolated from Lgmn−/− mice also displayed a specific defect in IFN-γ secretion and Th1 induction. Furthermore, we did not observe a role for AEP-driven autocrine α1-thymosin activation in T cell-derived IFN-γ production. These data suggest that AEP is an “upstream” activator of the CTSL-C3-IFN-γ axis in human CD4+ T cells and hence an important supporter of human Th1 induction.

These data suggest that GCSF, which is raised in patient serum, may play an important role in exacerbating disease in ANCA vasculitis. In addition, GCSF therapy for neutropenia should be used with caution in these patients.

SummaryAntibodies against neutrophil proteins myeloperoxidase (MPO) and proteinase 3 are thought to cause disease in anti-neutrophil cytoplasmic antibody (ANCA) vasculitis. There have been a number of recent developments in the animal models of ANCA vasculitis in both mice and rats. These include models based on an immune response to MPO generated in MPO-deficient mice, with other models using MPO-sufficient mice and rats. In addition, there is a report of the use of humanized mice where immunodeficient mice have been engrafted with human haematopoietic stem cells and injected with patient ANCA. Antibodies to another protein lysosomal-associated protein-2 have been found in patients with ANCA vasculitis, and evidence from a rat model suggests that they are also pathogenic. These models all have advantages and disadvantages, which are discussed. We also consider what these models have taught us about the pathogenesis of ANCA vasculitis. Experiments using genetically modified mice and pharmacological inhibition have given insights into disease mechanisms and have identified potential therapeutic targets. Toll-like receptor stimulation modifies disease by acting both at the level of tissue injury and in the generation of the autoimmune response. Complement is also potentially important with data to support the role of the alternative pathway and C5a in particular. Intracellular pathways have been examined, with a role showing p38 mitogen-activated protein kinase and phosphatidylinositol 3-kinase g. Serine proteases are now known to contribute to disease by release of interleukin-1b in ANCA-activated neutrophils and monocytes. Other potential therapies studied in these models include the use of bortezemib and strategies to modify antibody glycosylation.

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To explore the role of antigen-specific CD4؉ T cells in glomerulonephritis, we administered ovalbumin 323-339 peptide conjugated to glomerular-binding polyclonal antibody and induced disease in RAG1 ؊/؊ mice with CD4 ؉ T cells from OT2 ؋ RAG1 ؊/؊ mice. These OT2 ؋ RAG1 ؊/؊ mice have a transgenic T-cell receptor specific for this peptide. When CD4؉ T cells were primed in vivo, crescentic glomerulonephritis developed after 21 days in mice given peptide-conjugated glomerular-binding antibody but not unconjugated antibody control. We then investigated the relative roles of T H 1 and T H 17 cells, using Fab 2 fragments of glomerular-binding antibody to exclude a role for antibody in this model. T cells from OT2 ؋ RAG1 ؊/؊ mice were polarized in vitro, and T H 1 or T H 17 cell lines were injected into mice that were also given peptideconjugated Fab 2 or unconjugated Fab 2 control, giving four experimental groups. After 21 days crescentic glomerulonephritis was seen in mice receiving T H 17 cells and peptide-conjugated Fab 2 but in none of the other three groups. These results suggest that T H 17 but not T H 1 cells can induce crescentic glomerulonephritis.

Anti‐neutrophil cytoplasmic antibody vasculitis is a systemic autoimmune disease with glomerulonephritis and pulmonary haemorrhage as major clinical manifestations. The name reflects the presence of autoantibodies to myeloperoxidase and proteinase‐3, which bind to both neutrophils and monocytes. Evidence of the pathogenicity of these autoantibodies is provided by the observation that injection of anti‐myeloperoxidase antibodies into mice causes a pauci‐immune focal segmental necrotizing glomerulonephritis which is histologically similar to the changes seen on renal biopsy in patients. Previous studies in this model have implicated the alternative pathway of complement activation and the anaphylatoxin C5a. Despite this progress, the factors that initiate complement activation have not been defined. In addition, the relative importance of bone marrow‐derived and circulating C5 is not known. This is of interest given the recently identified roles for complement within leukocytes. We induced anti‐myeloperoxidase vasculitis in mice and confirmed a role for complement activation by demonstrating protection in C3‐deficient mice. We showed that neither MASP‐2‐ nor properdin‐deficient mice were protected, suggesting that alternative pathway activation does not require properdin or the lectin pathway. We induced disease in bone marrow chimaeric mice and found that circulating and not bone marrow‐derived C5 was required for disease. We have therefore excluded properdin and the lectin pathway as initiators of complement activation and this means that future work should be directed at other potential factors within diseased tissue. In addition, in view of our finding that circulating and not bone marrow‐derived C5 mediates disease, therapies that decrease hepatic C5 secretion may be considered as an alternative to those that target C5 and C5a. © 2016 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of Pathological Society of Great Britain and Ireland.

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