• autoantibodies
• autoimmune diseases
• autoimmunity
• B cells
• polymyositis

Immune checkpoint inhibitors enhance the immune response against tumours but may also trigger immune-related adverse events (IRAEs). Myositis is a rare IRAE. For example, creatine kinase (CK) elevations occurred in just 0.3% of those treated with avelumab, an antiprogrammed death-ligand 1 antibody.1

Thymomas are the most common anterior mediastinal masses in adults. Since effective systemic therapies for thymic epithelial tumours are lacking, we included seven patients with recurrent thymoma and one patient with recurrent thymic carcinoma in a phase I trial of avelumab (NCT01772004). Details regarding this trial have been published separately.2

Myasthenia gravis and myositis occur in up to 30% and 5% of patients with thymoma, respectively.3 Although no patient had a history of autoimmunity or weakness and each had normal baseline CK levels, four patients developed weakness and elevated CK levels, ranging from 762 IU/L to 16 037 IU/L, within 5 weeks of avelumab administration (see online supplementary text and table 1). CK levels normalised in patients within weeks of stopping avelumab and starting immunosuppressive therapy. Of note, one patient with myositis also had myocarditis and one patient without myositis developed autoimmune enteritis.

We tested for thymoma-associated autoantibodies in sera collected before and after avelumab treatment (table 1). Four patients had pre-existing muscle acetylcholine receptor (mAChR) autoantibodies and each developed CK elevations. No patient without mAChR autoantibodies developed myositis (100% vs 0%; p=0.029). Myositis and myasthenia have been reported to occur together as an IRAE.4 Although we cannot exclude the possibility that our patients could have had both myositis and myasthenia, electrophysiological studies revealed evidence of a neuromuscular junction defect in just one patient. Three patients had both mAChR and striational autoantibodies. Voltage-gated potassium channel autoantibodies were found in two patients and one of them developed myositis; neither patient developed manifestations of potassium-channel autoimmunity. Since approximately 70% of patients with myositis have a myositis-specific autoantibody (MSA), we screened preavelumab and postavelumab serum samples for 16 MSAs using Autoimmune Inflammatory Myopathies line blots (EUROIMMUN). Although no patient had an MSA on this panel, we cannot exclude the possibility that they may have had an unidentified, potentially pathogenic, autoantibody.

Flow cytometry performed on peripheral blood mononuclear cells collected prior to avelumab therapy revealed that patients who developed myositis had low B cell frequencies (Figure 1, online supplementary tables 2 and 3). A single patient without myositis, but who developed enteritis, also had low B cell levels. Taken together, patients with thymoma who developed myositis or enteritis had lower B cell frequencies (0.19%, 0.12%–0.73%; median, IQR) than patients with thymoma who did not (12.37%, 5.14%–16.5%), those with non-thymic malignancies (8.3%, 2.4%–11.7%) or healthy controls (16.3%, 11.9%–17.65%).

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Figure 1

Immune cell subsets in patients with thymoma prior to treatment with avelumab. Flow cytometry was performed on peripheral blood mononuclear cells (PBMCs) prior to avelumab treatment. Cell types were defined as follows: regulatory T cells are CD4+CD25+FoxP3+CD127−, natural killer cells are CD56+CD3−, and myeloid derived suppressor cells are CD11b+HLA-DR^low/−CD33+. The median and IQR are indicated by long and short horizontal bars, respectively. The patient without myositis who developed autoimmune enteritis is noted with an open square.

These observations suggest that testing for mAChR autoantibodies and/or B cell levels may identify patients with thymoma most at risk for developing myositis with avelumab. Since mAChR autoantibodies cause myasthenia but not myositis or elevated CK levels, and because mAChR autoantibody levels did not increase with myositis, we conclude that they are most likely a marker of pre-existing autoimmunity rather than the direct cause of muscle damage. B cell lymphopaenia, which occurs in half of patients with thymoma,5 has not been described in myositis. Interestingly, a recent study reported that declining B cells preceded the development of IRAEs in patients with melanoma following combination CTLA4 and PD1 checkpoint blockade; however, unlike the patients described here, these patients had normal B cell levels prior to checkpoint blockade.6 It remains unclear why declining B cell levels would be associated with IRAEs, including myositis.

Additional studies are needed to confirm these findings and to determine whether pre-existing autoantibodies or immune cell subset dysregulation predicts which non-thymic tumour patients are at increased risk for IRAEs.