Internalization of anti-Hu IgG is not Fc gamma receptor mediated

DNA-histone complexes as ligands amplify cell penetration and nuclear targeting of anti-DNA antibodies via energy-independent mechanisms

We have generated three monoclonal cell-penetrating antibodies (CPAbs) from a non-immunized lupus-prone (NZB × NZW)F1 mouse that exhibited high anti-DNA serum titres. These CPAbs are polyreactive because they bind to DNA and other cellular components, and localize mainly in the nucleus of HeLa cells, albeit with a distinct nuclear labelling profile. Herein, we have examined whether DNA-histone complexes (DHC) binding to CPAbs, before cell entry, could modify the cell penetration of CPAbs or their nuclear staining properties. By applying confocal microscopy and image analysis, we found that extracellular binding of purified CPAbs to DHC significantly enhanced their subsequent cell-entry, both in terms of percentages of positively labelled cells and fluorescence intensity (internalized CPAb amount), whereas there was a variable effect on their nuclear staining profile. Internalization of CPAbs, either alone or bound to DHC, remained unaltered after the addition of endocytosis-specific inhibitors at 37° or assay performance at 4°, suggesting the involvement of energy-independent mechanisms in the internalization process. These findings assign to CPAbs a more complex pathogenetic role in systemic lupus erythematosus where both CPAbs and nuclear components are abundant.

In vivo cell penetration and intracellular transport of anti-Sm and anti-La autoantibodies

Anti-nuclear autoantibodies (ANA) are the hallmark of systemic autoimmune diseases. Yet, the in vivo function of ANA remains controversial to a large extent due to the intracellular nature of their antigenic targets. It has been reported that a subset of autoantibodies can penetrate live cells and translocate into the subcellular compartments containing the corresponding antigens. The studies presented herein show that murine anti-Sm and anti-La monoclonal autoantibodies can also enter a variety of cell types from different animal species and that the cell penetration activity is not isotype-restricted. Interestingly, only mAb with cross-reactivity against double-stranded DNA did enter cells. Both these autoantibodies rapidly accumulate in the nucleus of viable cells but display different penetration kinetics. In co-localization experiments, monoclonal autoantibodies did not accumulate significantly within endocytic vesicles containing dextran, suggesting that they are internalized by mechanisms distinct from conventional receptor-mediated endocytosis. This report represents the first evidence that anti-La and anti-Sm autoantibodies are capable of entering live cells. Our observations support the notion that the phenomenon of intracellular autoantibodies may have a larger scope than previously reported and are consistent with a potential pathogenic role for ANA.

Paraneoplastic anti-Hu serum: studies on human tumor cell lines

Patients with low titers of anti-Hu, the paraneoplastic encephalomyelitis/sensory neuronopathy (PEM/PSN) antibody, have a better tumor prognosis that those who do not harbor these antibodies. Accordingly, we examined the effects of serum from patients with anti-Hu antibodies on human tumor cell lines, in order to determine: (1) if the serum was toxic (growth inhibition or cytolysis) to tumor cells with or without complement, and (2) if anti-Hu antibodies contributed to tumor toxicity. The serum of 14 patients with anti-Hu associated PEM/PSN, 22 patients with small-cell lung cancer (SCLC) without anti-Hu antibodies, and 20 normal individuals were studied. Three cell lines (NT-2, BE(2)-C, and SH.SY5Y) that express Hu proteins, and one cell line (SAOS-2) that does not, were studied. We examined the effects of whole serum, IgG-depleted serum, and purified IgG in the presence or absence of complement. A higher percentage of anti-Hu sera were toxic (71%) compared with sera from anti-Hu negative SCLC patients (23%) (p < 0.0001). No correlation existed between the titer of anti-Hu antibodies and toxicity. The toxic effects were observed in all tumor cell lines including the cell line that does not express Hu antigens. Toxicity persisted in serum depleted of IgG. Purified anti-Hu IgG in the presence and absence of complement, was not toxic. Our findings indicate that anti-Hu serum is toxic for human tumor cell lines, but this toxicity does not appear to be mediated by anti-Hu antibodies.