Impaired recognition of apoptotic neutrophils by the C1q/calreticulin and CD91 pathway in systemic lupus erythematosus

Abstract

Objective. A deficiency in a subcomponent of C1q can result in increased susceptibility to autoimmune diseases such as systemic lupus erythematosus (SLE). The monocyte endocytic receptor CD91 is implicated in the endocytosis of apoptotic neutrophils via interactions with C1q and calreticulin. In this clinical study, we studied the binding of C1q to leukocytes and determined whether C1q bound specifically to calreticulin and CD91 on cells undergoing apoptosis in SLE.Methods. Proximal antibody phage display, calreticulin-transfected cells, and immunocytochemical and confocal techniques were used in a comprehensive analysis of direct binding of C1q to apoptotic neutrophils that were obtained from healthy individuals and from patients with SLE. In addition, apoptotic cellular systems were assessed in vitro.Results. C1q appeared to colocalize to apoptotic blebs on the surface of leukocytes in association with both calreticulin and CD91, as determined by phage display and transfected cell studies. However, C1q did not bind to apoptotic cells isolated from SLE patients, despite the positivity of the cells for both calreticulin and CD91. Surface expression of calreticulin decreased on neutrophils as they aged, but increased on monocytes. In an apoptotic phagocytic assay, the addition of C1q and calreticulin significantly enhanced the phagocytosis of apoptotic cell debris by monocyte-derived cells.Conclusion. These observations indicate that neutrophils from SLE patients have a reduced ability to be recognized and removed by the C1q/calreticulin/CD91-mediated apoptotic pathway, despite the presence of main apoptotic recognition partners. This suggests that an additional component, as yet unidentified, acts as a C1q binding partner on apoptotic cells, and this component may be lacking in cells isolated from SLE patients.

Arginylated Calreticulin at Plasma Membrane Increases Susceptibility of Cells to Apoptosis

“…CRT is a multifunctional multicompartmental protein (36), is mainly located in the ER, but also has been implicated in a variety of processes that occur outside this organelle such as in the cytoplasm (3)(4)(5)(6), in the nucleus (7,8), on the cell plasma membrane, and in other extracellular compartments (9 -17). CRT coordinates a number of cellular events from the cell surface where it has been suggested to participate in: wound healing (9, 10), thrombospondin signaling (13)(14)(15)(16), antigen presentation and complement activation (11,12), clearance of apoptotic cells (17), and immunogenicity of cancer cell death (37,38). It is not surprising that post-translational modification of CRT plays a key role in its regulation, under several conditions, including stress.…”

“…CRT resides mainly in the ER and is a Ca 2ϩ -binding protein that function as a Ca 2ϩ buffer and molecular chaperone, although it has also been linked to a number of processes occurring outside the ER lumen (3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17), which implies other subcellular localizations for CRT other than the ER. However, the subcellular origin, biochemical features, and the precise molecular mechanism responsible for targeting CRT to the plasma membrane is not fully understood.…”

The Rheumatoid Arthritis Shared Epitope Triggers Innate Immune Signaling via Cell Surface Calreticulin

“…Cell surface CRT plays a pivotal role in the removal of apoptotic cells and, depending on the cosignaling context, ligation of CRT can lead to activation of either pro-or anti-inflammatory pathways (23). Impairments, which upset this multifactorial balance, could conceivably result in autoimmunity (24). Indeed, CRT has long been considered potential culprit in autoimmune diseases, including RA (13).…”

The consequences of apoptosis in autoimmunity

“…In recent years, C1q has taken on greater importance, since it has been implicated in the effective removal of apoptotic products [63]. Several experimental reports demonstrate the relationship between impaired clearance of apoptotic cells and SLE, and especially the involvement of C1q defects in SLE [64]. Kalaaji et al demonstrated that large chromatin fragments, derived from apoptotic cells, localize in the intraglomerular membrane in murine and human SLE [65], and that those intra-glomerular membrane-associated nucleosomes are targeted by anti-dsDNA autoantibodies in human lupus nephritis [66].…”