Antibody dependent cell-mediated cytotoxicity--an additional mechanism in human autoimmune myasthenia gravis

A Noncanonical CD56dimCD16dim/- NK Cell Subset Indicative of Prior Cytotoxic Activity Is Elevated in Patients with Autoantibody-Mediated Neurologic Diseases

Neuromyelitis optica spectrum disorder (NMOSD), myelin oligodendrocyte glycoprotein Ab disease, and autoimmune myasthenia gravis (MG) are autoantibody-mediated neurologic conditions where autoantibodies can induce Ab-dependent cellular cytotoxicity (ADCC), a NK cell-mediated effector function. However, whether ADCC is a pathogenic mechanism in patients with these conditions has not been confirmed. We sought to characterize circulatory NK cells using functional assays, phenotyping, and transcriptomics to elucidate their role in pathology. NK cells from NMOSD patients and MG patients with elevated disease burden exhibited reduced ADCC and CD56dimCD16hi NK cells, along with an elevated frequency of CD56dimCD16dim/- NK cells. We determined that ADCC induces a similar phenotypic shift in vitro. Bulk RNA sequencing distinguished the CD56dimCD16dim/- population from the canonical CD56dimCD16hi cytotoxic and CD56hiCD16- immunomodulatory subsets, as well as CD56hiCD16+ NK cells. Multiparameter immunophenotyping of NK cell markers, functional proteins, and receptors similarly showed that the CD56dimCD16dim/- subset exhibits a unique profile while still maintaining expression of characteristic NK markers CD56, CD94, and NKp44. Notably, expression of perforin and granzyme is reduced in comparison with CD56dimCD16hi NK cells. Moreover, they exhibit elevated trogocytosis capability, HLA-DR expression, and many chemokine receptors, including CCR7. In contrast with NMOSD and MG, myelin oligodendrocyte glycoprotein Ab disease NK cells did not exhibit functional, phenotypic, or transcriptomic perturbations. In summary, CD56dimCD16dim/- NK cells are a distinct peripheral blood immune cell population in humans elevated upon prior cytotoxic activity by the CD56dimCD16hi NK cell subset. The elevation of this subset in NMOSD and MG patients suggests prior ADCC activity.

Morphological effects of myasthenia gravis patient sera on human muscle cells

Myasthenia gravis (MG) is caused primarily by autoantibodies against the nicotinic acetylcholine receptor (AChR), but autoantibodies to other muscle proteins may be present. Many of these proteins have structural or signalling functions, the disruption of which may affect muscle cell morphology or viability. In order to investigate the role of such autoantibodies in MG, we examined the effect of MG patient sera, of different autoantibody composition and obtained at different stages of disease severity, on primary human muscle cells. Sera from MG patients induced changes in cell morphology from typical elongated cells to an irregular phenotype, caused the formation of inclusion bodies and intracellular vesicles, and led to a disordered arrangement of actin microfilaments. Sera from the most severely affected patients also induced cell death, which did not occur via classic apoptosis. The effects were not complement-mediated and were dose- and time-dependent. As the effects observed in the cell culture system correlated with disease severity, a greater understanding of the individual factors responsible for these effects may improve our understanding of MG pathogenesis and be of value in the assessment of disease in individual patients.

Complement activation by titin and ryanodine receptor autoantibodies in myasthenia gravis. A study of IgG subclasses and clinical correlations

To elucidate the mechanism of immune damage caused by titin and ryanodine receptor (RyR) autoantibodies in myasthenia gravis (MG), we studied the complement-activating capacity and the IgG subclass distribution of these autoantibodies in sera from 49 MG patients. Complement activation occurred in 38 out of 49 titin antibody positive sera, and in 14 out of 21 RyR antibody positive sera. The titin antibodies occurred only in the IgG 1 and IgG 4 subclasses, whereas the RyR antibodies occurred in all four IgG subclasses but with IgG 1 predominance. Complement-activating RyR antibodies occurred with higher frequency in sera of thymoma MG than of late-onset MG. RyR IgG 1 antibodies occurred more often in severe MG than in mild and moderate disease groups. Mean total IgG and IgG 1 titin and RyR antibody titers fell during long-time patient observation together with an improvement of the MG symptoms.

Different age-related effects of thymectomy in myasthenia gravis: role of thymoma, zinc, thymulin, IL-2 and IL-6

Different age-related immune pathogenetic mechanisms in myasthenia gravis (MG) have been suggested because of restoration after thymectomy (Tx) of altered zinc, thymulin (TH) and T-cell subsets exclusively in early-onset patients (younger <50 years), not in late-onset patients (older >50 years). In this context interleukin-2 (IL-2), interleukin-6 (IL-6) and thymoma are crucial because both involved in MG pathogenesis and correlated with acetylcholine receptors (AchRs) Ab production. Moreover, IL-2 and IL-6 are zinc-dependent, are altered in aging and related with zinc and TH age-dependent declines. Moreover, zinc is relevant for immune efficiency. In order to confirm these different age-related pathogenetic mechanisms further, the role of thymoma, zinc, TH, IL-2 and IL-6 is studied in MG patients with generalized MG with and without thymoma before and 1 month and 1 year after Tx. The high IL-2, IL-6, zinc, and AChR Ab levels observed before Tx are significantly correlated each other in younger MG patients (<50 years) independently by thymoma and in older MG patients (>50 years) with thymoma. No correlations exist in older MG patients without thymoma. Thymulin is not correlated with other parameters considered to be both in younger and older MG patients independently by the thymoma. Thymectomy restores zinc; immune parameters and AChR Ab are exclusively in the younger group, not in the older one. These findings suggest that IL-2 and IL-6, via zinc, rather than TH, may be involved in different age-related pathogenetic mechanisms mainly in early-onset MG. By contrast, thymoma may be involved in MG etiology in late-onset representing, as such, a useful discriminant tool for MG etiology between early and late-onset MG patients. Because autoimmune phenomena may rise in aging, a parallelism with altered immune functions during aging is discussed.