ABSTRACT
Anti-ADAMTS13 autoantibodies are the main cause of acquired thrombotic thrombocytopenic purpura. Binding of these antibodies to ADAMTS13 eventually results in the formation of antigen-antibody immune complexes. Circulating ADAMTS13-specific immune complexes have been described in patients with acquired thrombotic thrombocytopenic purpura, although the prevalence and persistence of these immune complexes over time have hitherto remained elusive. Here, we analyzed a large cohort of patients with acquired thrombotic thrombocytopenic purpura for the presence of free and complexed anti-ADAMTS13 antibodies. In the acute phase (n=68), 100% of patients had free IgG antibodies and 97% had ADAMTS13-specific immune complexes. In remission (n=28), 75% of patients had free antibodies (mainly IgG) and 93% had ADAMTS13-specific immune complexes. Free antibodies were mainly of subclasses IgG1 and IgG4, whereas IgG4 was by far the most prevalent in ADAMTS13-specific immune complexes. Comparison of ADAMTS13 inhibitor and anti-ADAMTS13 IgG (total and subclasses) antibody titers in acute phase and in remission samples showed a statistically significant decrease in all parameters in remission. Although non-significant, a trend towards reduced or undetectable titers in remission was also observed for ADAMTS13-specific immune complexes of subclasses IgG1, IgG2 and IgG3. No such trend was discernible for IgG4; IgG4 immune complexes persisted over years, even in patients who had been treated with rituximab and who showed no features suggesting relapse.
Subject(s)
ADAM Proteins/immunology , Antigen-Antibody Complex/immunology , Autoantibodies/immunology , Purpura, Thrombotic Thrombocytopenic/immunology , ADAM Proteins/blood , ADAMTS13 Protein , Adult , Aged , Antibodies, Monoclonal, Murine-Derived/therapeutic use , Antigen-Antibody Complex/blood , Autoantibodies/blood , Disease Progression , Female , Humans , Immunoglobulin A/blood , Immunoglobulin A/immunology , Immunoglobulin G/blood , Immunoglobulin G/immunology , Immunoglobulin M/blood , Immunoglobulin M/immunology , Immunologic Factors/therapeutic use , Male , Middle Aged , Purpura, Thrombotic Thrombocytopenic/blood , Purpura, Thrombotic Thrombocytopenic/drug therapy , Rituximab , Time Factors , Treatment Outcome , Young AdultABSTRACT
The ribosome binding site of Escherichia coli rpoS mRNA, encoding the stationary sigma-factor RpoS, is sequestered by an inhibitory stem-loop structure (iss). Translational activation of rpoS mRNA at low temperature and during exponential growth includes Hfq-facilitated duplex formation between rpoS and the small regulatory RNA DsrA as well as a concomitant re-direction of RNAse III cleavage in the 5´-untranslated region of rpoS upon DsrA·rpoS annealing. In this way, DsrA-mediated regulation does not only activate rpoS translation by disrupting the inhibitory secondary structure but also stabilizes the rpoS transcript. Although minor structural changes by Hfq have been observed in rpoS mRNA, a prevailing question concerns unfolding of the iss in rpoS at low growth temperature. Here, we have identified the DEAD-box helicase CsdA as an ancillary factor required for low temperature activation of RpoS synthesis by DsrA. The lack of RpoS synthesis observed in the csdA mutant strain at low growth temperature could be attributed to a lack of duplex formation between rpoS and DsrA, showing that at low temperature the sole action of Hfq is not sufficient to permit DsrA·rpoS annealing. An interactome study has previously indicated an association between Hfq and CsdA. However, immunological assays did not reveal a physical interaction between Hfq and CsdA. These findings add to a model, wherein Hfq binds upstream of the rpoS iss and presents DsrA in a conformation receptive to annealing. Melting of the iss by CsdA may then permit DsrA·rpoS duplex formation, and consequently rpoS translation.
