Anti-idiotype-mediated epitope spreading and diminished phagocytosis by a human monoclonal antibody recognizing late-stage apoptotic cells

A pregnancy hormone-cell death link promotes enhanced lupus-specific immunological effects

Women of reproductive age demonstrate an increased incidence of systemic lupus erythematosus, and reproductive hormones have been implicated in disease progression. Additionally, pregnancy can be associated with disease "flares", the reasons for which remain obscure. While apoptotic bodies are believed to provide an autoantigenic trigger in lupus, whether autoantigenic constituents vary with varying cellular insults, and whether such variations can be immunologically consequential in the context of pregnancy, remains unknown. As assessed by antigenicity and mass spectrometry, apoptotic bodies elicited by different drugs demonstrated the differential presence of lupus-associated autoantigens, and varied in the ability to elicit lupus-associated cytokines from lupus splenocytes and alter the phenotype of lupus B cells. Immunization of tamoxifen-induced apoptotic bodies in lupus-prone mice generated higher humoral autoreactive responses than did immunization with cisplatin-induced apoptotic bodies, and both apoptotic bodies were poorly immunogenic in healthy mice. Incubation of lupus splenocytes (but not healthy splenocytes) with the pregnancy hormone human chorionic gonadotropin (hCG) along with tamoxifen-induced apoptotic bodies (but not cisplatin-induced apoptotic bodies) induced increases in the secretion of lupus-associated cytokines and in the up-modulation of B cell phenotypic markers. In addition, levels of secreted autoantibodies (including of specificities linked to lupus pathogenesis) were enhanced. These events were associated with the heightened phosphorylation of several signaling intermediates. Observations suggest that hCG is a potential disease-promoting co-stimulant in a lupus-milieu; when combined with specific apoptotic bodies, it enhances the intensity of multiple lupus-associated events. These findings deepen mechanistic insight into the hormone's links with autoreactive responses in lupus-prone mice and humans.

Innate and humoral recognition of the products of cell death: differential antigenicity and immunogenicity in lupus

While apoptotic debris is believed to constitute the original antigenic insult in lupus (which is characterized by a time-dependent diversification of autoreactivity), whether such debris and autoantibodies specifically recognizing its constituents mediate differential effects on innate and humoral responses in lupus-prone mice is currently unknown. Apoptotic blebs (as opposed to cellular lysate) enhanced preferentially the maturation of dendritic cells (DCs) from bone marrow precursors drawn from lupus-prone mice. Murine, somatically mutated, apoptotic cell-reactive immunoglobulin (Ig)G monoclonal antibodies demonstrated enhanced recognition of DCs and also displayed a prominent lupus strain-specific bias in mediating DC maturation. Further, immunization of such antibodies specifically in lupus-prone mice resulted in widespread humoral autoreactivity; hypergammaglobulinaemia (a hallmark of systemic autoimmunity) was observed, accompanied by enhanced antibody titres to cellular moieties. Induced antibodies recognized antigens distinct from those recognized by the antibodies employed for immunization; in particular, nephritis-associated anti-double stranded (ds) DNA antibodies and neonatal lupus-associated anti-Ro60 antibodies were elicited by a non-dsDNA, non-Ro60 reactive antibody, and Sm was a favoured target. Further, only in lupus-prone mice did such immunization enhance the kinetics of humoral anti-self responses, resulting in the advanced onset of glomerulosclerosis. These studies reveal that preferential innate and humoral recognition of the products of cell death in a lupus milieu influence the indices associated with autoimmune pathology.

Elucidation of the potential disease-promoting influence of IgM apoptotic cell-reactive antibodies in lupus

The undigested remnants of apoptosis are believed to stimulate the generation of autoantibodies in lupus. The biological properties of initiator, disease-specific IgM antibodies that specifically recognize apoptotic cells, readily detected in the sera of lupus patients, remain unclear. Apoptotic cell-reactive IgM monoclonal antibodies (generated from lupus-prone mice), as opposed to control IgM, preferentially stimulated maturation of bone marrow-derived dendritic cells (BMDCs) derived from such mice, relative to BMDCs derived from healthy mice. An influence of both antibody specificity and cell genotype was also apparent in the secretion of signature inflammatory cytokines. Immunization of such antibodies in lupus-prone animals induced increases in total serum IgG levels, with the elicited antibodies also preferentially recognizing moieties on dying cells. An expanded specificity was apparent both upon Western blot on cellular lysate and from the enhanced recognition of dsDNA, Ro60, RNP68 and Sm; the antibody most efficient in mediating autoreactive diversity, while being germline encoded, also induced the highest degree of phenotypic changes on BMDCs. Apoptotic cell-reactive IgM antibodies may therefore be potentially capable of influencing the course of systemic autoimmune disease by affecting both innate and adaptive immunity.

Macrophage differentiation and polarization via phosphatidylinositol 3-kinase/Akt-ERK signaling pathway conferred by serum amyloid P component

Macrophage differentiation and polarization is influenced by, and act on, many processes associated with autoimmunity. However, the molecular mechanisms underlying macrophage polarization in systemic lupus erythematosus (SLE) remain largely debated. We previously demonstrated that macrophage M2b polarization conferred by activated lymphocyte-derived (ALD)-DNA immunization could initiate and propagate murine lupus nephritis. Serum amyloid P component (SAP), a conserved acute-phase protein in mice, has been reported to bind to DNA and modulate immune responses. In this study, murine SAP was shown to promote macrophage-mediated ALD-DNA uptake through binding to ALD-DNA (SAP/ALD-DNA). Moreover, macrophage phenotypic switch from a proinflammatory M2b phenotype induced by ALD-DNA alone to an anti-inflammatory M2a phenotype stimulated with SAP/ALD-DNA were found because of PI3K/Akt-ERK signaling activation. Both in vivo SAP supplements and adoptive transfer of ex vivo programmed M2a macrophages induced by SAP/ALD-DNA into SLE mice could efficiently alleviate lupus nephritis. Importantly, increased IL-10 secretion, accompanied by anti-inflammatory effect exerted by M2a macrophages, was found to predominantly impede macrophage M2b polarization. Furthermore, neutralization of IL-10 notably reduced the suppressive effect of M2a macrophages. Our results demonstrate that binding of SAP to ALD-DNA could switch macrophage phenotypic polarization from proinflammatory M2b to anti-inflammatory M2a via PI3K/Akt-ERK signaling activation, thus exerting protective and therapeutic interventions on murine lupus nephritis. These data provide a possible molecular mechanism responsible for modulation of macrophage polarization in the context of lupus nephritis and open a new potential therapeutic avenue for SLE.

Serum and organ-associated anti-hemoglobin humoral autoreactivity: association with anti-Sm responses and inflammation

The release of hemoglobin (Hb) occurs in some infectious and autoimmune diseases characterized by inflammation. As levels of haptoglobin (Hp) fall, free Hb can cause pathology. Humoral autoreactivity to human Hb was demonstrated in the sera of systemic lupus erythematosus (SLE), leishmania and malaria patients. Serum anti-murine Hb antibody levels in lupus-prone mice also exhibited an age-dependent increase, with progressive organ sequestration; significant isotypic correlation was observed with anti-dsDNA antibodies. A suggestive link between anti-Hb and anti-Sm responses was observed: Human lupus sera expressing anti-Sm antibody reactivity preferentially contained heightened levels of anti-Hb autoantibodies, and immunization of lupus-prone mice with Sm led to enhanced anti-murine Hb reactivity. Human and murine anti-Hb monoclonal antibodies were generated, some of which were preferentially reactive toward disease-associated methemoglobin. Epitope-mapping studies revealed evidence of intra-molecular cross-reactivity. One such autoantibody synergized with Hb to enhance the secretion of pro-inflammatory cytokines while eliciting the increased production of monocyte migratory signals from endothelial cells. Preferential usage of specific variable region gene segments was not observed, although somatic mutations were documented. These studies reveal that, while the etiology, specificity and sequences of anti-Hb autoreactive antibodies can vary, they occur quite frequently and can have inflammatory consequences.

Autoantibodies from mice exposed to Libby amphibole asbestos bind SSA/Ro52-enriched apoptotic blebs of murine macrophages

Asbestos exposure is associated with increased autoimmune responses in humans. For example, in Libby, MT where significant asbestos exposure has occurred due to an asbestos-contaminated vermiculite mine near the community, residents have developed increased autoimmune responses compared to an unexposed population. However, the exact mechanism by which Libby amphibole asbestos generates autoimmune responses is unclear. A murine model of amphibole asbestos-induced autoimmunity was recently established, and one of the targets of the autoantibodies (AAs) was the SSA/Ro52 autoantigen. The purpose of this study was to determine whether the SSA/Ro52 autoantigen is exposed at the surface of cells as a result of asbestos exposure as a possible mechanism leading to antigenicity. Our results indicate that Libby asbestos induces apoptosis in murine macrophages as determined by phosphatidylserine exposure, cleavage of poly(ADP-ribose) polymerase and morphological changes such as nuclear condensation. Moreover, asbestos-induced apoptosis results in the formation of apoptotic cell surface blebs enriched in SSA/Ro52 as determined by confocal microscopy. Most importantly, apoptotic cell surface blebs are recognized by AAs from mice exposed to amphibole asbestos suggesting that these cell surface structures may be antigenic when presented in a pro-inflammatory context. This study supports the hypothesis that the induction of apoptosis plays a key role in environmentally induced autoimmunity through cell surface exposure of a known autoantigen.

Impaired clearance of apoptotic cardiocytes is linked to anti-SSA/Ro and -SSB/La antibodies in the pathogenesis of congenital heart block

The role of cardiocytes in physiologic removal of apoptotic cells and the subsequent effect of surface binding by anti-SSA/Ro and -SSB/La antibodies was addressed. Initial experiments evaluated induction of apoptosis by extrinsic and intrinsic pathways. Nuclear injury and the translocation of SSA/Ro and SSB/La antigens to the fetal cardiocyte plasma membrane were common downstream events of Fas and TNF receptor ligation, requiring caspase activation. As assessed by phase-contrast and confirmed by confocal microscopy, coculturing of healthy cardiocytes with cardiocytes rendered apoptotic via extrinsic pathways revealed a clearance mechanism that to our knowledge has not previously been described. Cultured fetal cardiocytes expressed phosphatidylserine receptors (PSRs), as did cardiac tissue from a fetus with congenital heart block (CHB) and an age-matched control. Phagocytic uptake was blocked by anti-PSR antibodies and was significantly inhibited following preincubation of apoptotic cardiocytes with chicken and murine anti-SSA/Ro and -SSB/La antibodies, with IgG from an anti-SSA/Ro- and -SSB/La-positive mother of a CHB child, but not with anti-HLA class I antibody. In a murine model, anti-Ro60 bound and inhibited uptake of apoptotic cardiocytes from wild-type but not Ro60-knockout mice. Our results suggest that resident cardiocytes participate in physiologic clearance of apoptotic cardiocytes but that clearance is inhibited by opsonization via maternal autoantibodies, resulting in accumulation of apoptotic cells, promoting inflammation and subsequent scarring.