Faculty Bibliography

OBJECTIVE: Naturally arising IgM antibodies (NAb) to apoptotic cell (AC) determinants are present from birth and can be further induced by AC challenge. In systemic lupus erythematosus, lower anti-AC NAb levels have been associated with higher disease activity. We have recently shown that a prototypical AC-specific IgM NAb can suppress proinflammatory responses to purified agonists of Toll-like receptors and block the in vivo induction of IgG immune complex (IC)-induced arthritis. Nuclear antigens, which activate dendritic cells (DCs), form complexes with IgG autoantibody, and these have been implicated in the pathogenesis of autoimmune disease. In this study, we sought to investigate potential roles of such NAb for regulating IC-mediated activation of DCs, which is believed to be involved in disease initiation and perpetuation. METHODS: Bone marrow-derived myeloid DCs were stimulated with ICs composed of IgG autoantibody and chromatin or IgG autoantibody and RNA. Outcome was evaluated according to the production of inflammatory cytokines, as determined by enzyme-linked immunosorbent assay, and the expression of costimulatory molecules (markers of DC activation), as determined by flow cytometry. MAPK activation was evaluated by phospho-flow analysis and immunofluorescence microscopy. RESULTS: IgM anti-AC NAb dose-dependently suppressed the production of DNA IC- and RNA IC-induced interleukin-6 and DNA IC-induced tumor necrosis factor alpha, as well as the RNA IC-induced up-regulation of CD86 and CD40 on DCs. IgM NAb-mediated inhibition was associated with suppression of IC-mediated p38 MAPK activation and nuclear localization. CONCLUSION: We demonstrated a direct in vitro inhibitory effect of IgM NAb on inflammatory responses induced by IgG-nucleic acid ICs. These findings contribute to emerging evidence that regulatory NAb to AC determinants may oppose the influence of pathogenic lupus autoantibody ICs and thereby play roles in the maintenance of immune homeostasis.

To subvert host defenses, some microbial pathogens produce proteins that interact with conserved motifs in V regions of B cell Ag receptor shared by large sets of lymphocytes, which define the properties of a superantigen. Because the clonal composition of the lymphocyte pool is a major determinant of immune responsiveness, this study was undertaken to examine the in vivo effect on the host immune system of exposure to a B cell superantigen, protein L (PpL), a product of the common commensal bacterial species, Finegoldia magna, which is one of the most common pathogenic species among Gram-positive anaerobic cocci. Libraries of Vkappa L chain transcripts were generated from the spleens of control and PpL-exposed mice, and the expressed Vkappa rearrangements were characterized by high-throughput sequencing. A total of 120,855 sequencing reads could be assigned to a germline Vkappa gene, with all 20 known Vkappa subgroups represented. In control mice, we found a recurrent and consistent hierarchy of Vkappa gene usage, as well as patterns of preferential Vkappa-Jkappa pairing. PpL exposure induced significant targeted global shifts in repertoire with reduction of Vkappa that contain the superantigen binding motif in all exposed mice. We found significant targeted reductions in the expression of clonotypes encoded by 14 specific Vkappa genes with the predicted PpL binding motif. These rigorous surveys document the capacity of a microbial protein to modulate the composition of the expressed lymphocyte repertoire, which also has broad potential implications for host-microbiome and host-pathogen relationships.

Antibodies are a vital part of the armamentarium of the adaptive immune system for the fine-tuning of the recognition and response to foreign threats. However, in health there are some types of antibodies that instead recognize self-antigens and these contribute to the enhancement of primitive innate functions. This repertoire of natural IgM antibodies is postulated to have been selected during immune evolution for their contributions to critical immunoregulatory and housekeeping properties. The clearance of dying cells is one of the most essential responsibilities of the immune system, which is required to prevent uncontrolled inflammation and autoimmunity. In the murine immune system, natural IgM antibodies that recognize apoptotic cells have been shown to enhance the phagocytic clearance of dead and dying cells and to suppress innate immune signaling pathways. In the mouse, natural IgM are often the products of B-1 cell clones that arise during immune development without an absolute requirement for exogenous antigenic stimulation. In patients with systemic lupus erythematosus, IgM autoantibodies, which bind to neo-epitopes on apoptotic cells, have been demonstrated to be present at significantly higher levels in patients with lower disease activity and with less severe organ damage. While certain specificities of IgM autoantibodies correlate with protection from lupus renal disease, others may convey protective properties from lupus-associated atherosclerotic cardiovascular disease. New and unexpected insights into the functional roles of IgM antibodies are still emerging, especially regarding the functions of natural antibodies. Herein, we review recent progress in our understanding of the potential roles of natural IgM autoantibodies in the regulation of immune homeostasis and for protection from autoimmune and inflammatory diseases.

Emerging evidence suggests that there are IgM-autoantibodies that may play protective roles in SLE. While IgM are often considered polyreactive, we postulate that there are distinct sets of IgM-autoantibodies of defined autoreactive specificities relevant to different features of SLE. We examined the relationships between levels of IgM natural autoantibodies (NAbs) to apoptosis-associated phosphorylcholine (PC) or malondialdehyde (MDA) antigens, with lupus-associated autoantibodies and features of disease, in 120 SLE patients. IgM anti-PC was significantly higher in patients with low disease activity and less organ damage determined by the SELENA-SLEDAI, the physician's evaluation and the SLICC damage score. Furthermore, IgM anti-PC was significantly higher in patients without cardiovascular events. In contrast, IgM anti-cardiolipin and IgM anti-dsDNA were significantly higher in patients without renal disease. These results support the hypothesis that some IgM autoantibodies are part of a natural immune repertoire that provide homeostatic functions and protection from certain clinical lupus features.

Natural IgM Abs are the constitutively secreted products of B1 cells (CD5(+) in mice and CD20(+)CD27(+)CD43(+)CD70(-) in humans) that have important and diverse roles in health and disease. Whereas the role of natural IgM as the first line of defense for protection against invading microbes has been extensively investigated, more recent reports have highlighted their potential roles in the maintenance of tissue homeostasis via clearance of apoptotic and altered cells through complement-dependent mechanisms, inhibition of inflammation, removal of misfolded proteins, and regulation of pathogenic autoreactive IgG Abs and autoantibody-producing B cells. These observations have provided the theoretical underpinnings for efforts that currently seek to harness the untapped therapeutic potential of natural IgM either by boosting in vivo natural IgM production or via therapeutic infusions of monoclonal and polyclonal IgM preparations.