Antigen Reactivity and Clinical Significance of Autoantibodies Directed Against the Pyruvate Dehydrogenase Antigen Complex in Patients With Connective Tissue Disease

Mechanistic considerations linking SARS-CoV-2 infection, inflammation, and the loss of immune tolerance

The immune response to SARS-CoV-2 has been implicated in the onset of multiple, seemingly unrelated, autoimmune diseases. The immune response to SARS-CoV-2 has also been implicated in the unmasking and/or production of multiple autoantibodies, even in the absence of clinical disease. Despite such data, it remains unclear whether antibodies targeting antiviral signaling proteins and mitochondrial antigens reflect bystander activation or alternatively contribute to de novo viral immune escape mechanisms. With these comments in mind, a variety of professional antibody presenting cells and including lung resident macrophages of COVID-19 infected patients are impacted and dependent on the uptake of antibody-opsonized virus by Fcγ receptors; yet infection is aborted via antibody-dependent effector mechanisms or pyroptosis, possibly leading to autoantibody production, and autoinflammatory manifestations, respectively. TRIM21/Ro52, a cytosolic E3-ubiquitin ligase with an Fc-gamma receptor domain, functions as an intracytoplasmic antibody receptor, directs immune complexes binding virions but also autoantigens to autophagy. During autophagy, Ig-virions-TRIM21/Ro52-autoantigens complexes bind directly to class II human leukocyte antigen in lysosomal compartment, leading to subsequent presentation on the cell surface. This process favors the development of a specific humoral immune response but has the potential to lead to loss of tolerance. Interestingly, TRIM21/Ro52 can also contribute to pyroptosis. We propose that TRIM21/Ro52 is well-placed at the crossroad between the inflammatory response and clinical autoimmunity.

Clinical relevance and frequency of cytoplasmic patterns observed in ANA-Hep-2: experience of Cairo University Hospitals

Antinuclear antibodies (ANA) are the most common biomarkers observed in autoimmune diseases. Cytoplasmic staining patterns on ANA-Hep-2 are gaining recognition but with scanty information about their clinical and diagnostic role. The aim is to assess the frequency of cytoplasmic ANA patterns in autoimmune diseases, and to evaluate their possible associations with clinical diagnoses and autoantibodies. This observational cross-sectional study was conducted by examining and/or reviewing ANA by indirect immunofluorescence assay during a 13-month period. This was followed by testing the group of patients with a positive cytoplasmic staining pattern (n = 92) using the Microblot-Array ANA plus for the presence of 44 specific autoantibodies. Out of 2741 samples, 1791 (65.3%) tested negative, 845 (30.9%) tested positive nuclear staining patterns, 56 (2.0%) positive solitary cytoplasmic staining patterns, and 49 (1.8%) positive mixed nuclear and cytoplasmic patterns. Ninety-two cases (3.4% of the total cases) were analyzed using Microblot-Array ANA plus, with reticular as the most frequent cytoplasmic pattern, followed by dense fine speckled. The most frequently associated disease with reticular pattern was primary biliary cholangitis (28.9%), and the most frequently detected autoantibodies were against M2 (66.7%). The most frequently associated disease with dense fine speckled pattern was systemic lupus erythematosus (69.4%), and the most frequently detected autoantibodies were against nucleosome (57.7%) and ribosomal P0 (53.8%). This study highlights the significance of reporting cytoplasmic staining patterns and their importance in assessment of autoimmune diseases. Larger cohort studies on treatment naïve patients are recommended.

Evaluation strategy of anti-mitochondrial antibodies M2-negative: the role of multiplex rodent tissues and related clinical implications

Indirect immunofluorescence on HEp-2 cell line (HEp-2-IIF) remains "gold standard" method for the detection of antinuclear antibodies (ANA). ANA is an operative definition, showing the possibility of autoantibodies (Aab) to bind nuclear, and cytoplasmic antigens. One of the major examples is represented by anti-mitochondrial antibodies (AMAs), which target proteins of the inner and outer mitochondrial membranes, located into the cytoplasm. The standard IIF on rat kidney/stomach/liver tissue sections, with the combined use of other commercial assays, may all be used in ordinary lab life to validate the AC-21 pattern on Hep-2 cells. The routine lab experience teaches that commercial kits cannot always be detected and define specific AMAs. In these cases the literature proposes the use of other homemade assays to detect AMAs as immunoprecipitation (IP) and Western blot (IP-WB). However, using IP or IP-WB is difficult to apply in a routine laboratory, because of numerous cases to process and the related troubles. Where find confirmation of the AC-21 pattern if line-immunoblot and other routine methods (ELISA, CLIA/FEIA assays) fail? We review AC-21 AMA-like sera from our patients (year 2022) and propose a revised diagnostic algorithm based on the combined use of IIF on Hep-2 cells, line immunoblot and IIF on rodent tissue as a third line method. We demonstrated that, particularly in cases where the second level test was unsuccessful, the application of IFI on rodent tissues became indispensable to verify the existence of AMAs.

Systemic evaluation of lymphocyte-bound C4d and immunoglobulins for diagnosis and activity monitoring of Systemic Lupus Erythematosus

OBJECTIVE

To investigate the role of lymphocyte-bound C4d (LB-C4d: T-C4d, B-C4d) and immunoglobulins (LB-Igs: T-IgG, T-IgM, B-κ and B-λ) in the diagnosis and monitoring of SLE.

DESIGN & METHODS

The levels of C4d and Igs on peripheral lymphocytes were measured in 172 patients with SLE, 174 patients with other non-SLE inflammatory diseases and 100 healthy individuals. Immunobinding and blocking experiments were performed to characterize Igs from SLE patients to generate LB-C4d/Igs in vitro. Sixty-five patients with SLE were followed up longitudinally. Disease activity was assessed for each SLE patient.

RESULTS

Patients with SLE had the highest median LB-C4d/Igs levels. LB-C4d had a significant but weak positive association with LB-Igs, with correlation coefficients ranging from 0.008 to 0.316. Anti-cardiolipin IgG and anti-β2GP1 IgG, but not C3 and C4, were found to be closely associated with LB-C4d/Igs formation, with correlations as high as 0.337. Compared to anti-dsDNA, LB-C4d performed better in SLE diagnosis, while B-κ and B-λ performed better in disease activity monitoring.

CONCLUSIONS

Both autoantibodies and receptors on lymphocytes contribute to LB-C4d/Igs formation. LB-C4d/Igs could be used as reliable indicators for SLE diagnosis and activity monitoring.

Systemic sclerosis and primary biliary cholangitis share an antibody population with identical specificity

Anti-centromere (ACA) and antimitochondrial antibodies (AMA) are specific for limited-cutaneous systemic sclerosis (lcSSc) and primary biliary cholangitis (PBC), respectively, and can coexist in up to 25 and 30% of SSc and PBC patients. Here, we evaluated whether anti-centromeric protein A (CENP-A) antibodies cross-react with mitochondrial antigens. To this end, sera from two lcSSc patients (pt1 and pt4), one of them (pt4) also affected by PBC, were used as the source of ACA, previously shown to recognize different groups of amino acids (motifs) in the CENP-A region spanning amino acids 1-17 (Ap1-17). Pt1 and pt4 Ap1-17-specific IgG were purified by affinity-chromatography on insolubilized Ap1-17-peptide column and tested by western blotting with nuclear and cytoplasmic protein extract from HeLa cells. Immunoreactive proteins were identified by mass spectrometry and validated by immunodot. The results showed that affinity-purified SSc/PBC pt4 anti-Ap1-17 and not SSc pt1 anti-Ap1-17 Ab, specifically cross-reacted with the E2 component of the mitochondrial pyruvate dehydrogenase complex (PDC-E2), the major mitochondrial autoantigen in PBC. Sequence homology analysis indicated that the motif A-x-x-P-x-A-P recognized by pt4 anti-Ap1-17 IgG and shared by CENP-A and PDC-E2, is also expressed by some members of the Human Herpesvirus family, suggesting that they may trigger the production of these cross-reacting antibodies.

The pyruvate dehydrogenase complex: Life's essential, vulnerable and druggable energy homeostat

Found in all organisms, pyruvate dehydrogenase complexes (PDC) are the keystones of prokaryotic and eukaryotic energy metabolism. In eukaryotic organisms these multi-component megacomplexes provide a crucial mechanistic link between cytoplasmic glycolysis and the mitochondrial tricarboxylic acid (TCA) cycle. As a consequence, PDCs also influence the metabolism of branched chain amino acids, lipids and, ultimately, oxidative phosphorylation (OXPHOS). PDC activity is an essential determinant of the metabolic and bioenergetic flexibility of metazoan organisms in adapting to changes in development, nutrient availability and various stresses that challenge maintenance of homeostasis. This canonical role of the PDC has been extensively probed over the past decades by multidisciplinary investigations into its causal association with diverse physiological and pathological conditions, the latter making the PDC an increasingly viable therapeutic target. Here we review the biology of the remarkable PDC and its emerging importance in the pathobiology and treatment of diverse congenital and acquired disorders of metabolic integration.

Presence of anti-gp210 or anti-sp100 antibodies in AMA-positive patients may help support a diagnosis of primary biliary cholangitis

BACKGROUND

Anti-mitochondrial antibody (AMA) positivity is not always associated with primary biliary cholangitis (PBC). We aimed to determine the additional value of anti-sp100 or anti-gp210 antibody in AMA-positive patients for PBC.

METHODS

Patients (n = 190) and healthy donors (n = 50) were evaluated for AMA, anti-gp210 and anti-sp100 antibodies by ELISA. Antibody frequencies in cohorts and performance characteristics in some patients categorized as 'definitive-', 'probable-', and 'no PBC' were determined following review of their charts.

RESULTS

Of the patients (n = 190), 38.4% were AMA-positive (n = 73) and 61.6% AMA-negative (n = 117). Frequency of anti-sp100 or anti-gp210 antibody was 17.8%, 2.6%, and 0% in AMA-positive, AMA-negative and healthy controls, respectively. Clinical data was available for 63 of 73 AMA-positive patients with 28.6%, 22.2%, and 49.2% categorized as definite, probable, and no PBC, respectively. Patients with definite PBC had higher mean levels of AMA and frequencies of sp100 or gp210 antibody compared to other groups. Sensitivities were low (anti-sp100: 18.8% and anti-gp210: 16.7%) with specificities above 98.0% for both.

CONCLUSION

AMA-positive patients positive for anti-sp100 or anti-gp210 antibody were more likely to have a diagnosis of definite or probable PBC than those with AMA alone. Use of all tests is likely to improve characterization of patients at-risk for PBC.