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Human Anti-CD38 Autoantibodies Raise Intracellular Calcium and Stimulate Insulin Release in Human Pancreatic Islets

¹ Metabolism Unit, Consiglio Nazionale delle Richerche Institute of Clinical Physiology and Department of Internal Medicine, University of Pisa, Pisa
² Laboratory of Immunogenetics, Department of Genetics, Biology, and Biochemistry, University of Torino
³ Division of Obstetrics and Gynecology, Department of Medical Sciences, University A. Avogadro of Eastern Piedmont, Novara
⁴ Department of Endocrinology and Metabolism, University of Pisa, Pisa
⁵ Endocrinology Unit, Department of Clinical Pathophysiology, University of Florence, Florence, Italy

CD38 is involved in transmembrane signaling in many cell types; anti-CD38 autoantibodies have been described in diabetic patients. We tested whether human anti-CD38 antibodies possess signaling properties by measuring their ability to raise intracellular calcium ([Ca²⁺]_i) using the fluo-3-acetoxymethyl ester method in a human-derived T-cell line (Jurkat T-cells, expressing high levels of surface CD38) and in dispersed human islet cells from normal donors. In Jurkat T-cells, 11 of 19 anti–CD38-positive sera raised [Ca²⁺]_i (by 20% of baseline), whereas no [Ca²⁺]_i-mobilizing activity was found in 27 anti–CD38-negative sera (² = 20.5, P < 0.0001). In dispersed human islet cells, 5 of 11 anti–CD38-positive sera (and none of three anti–CD38-negative sera) raised [Ca²⁺]_i significantly. When preincubated with Staphylococcus aureus protein A to remove IgG, anti–CD38-positive sera showed a 70 ± 5% reduction in [Ca²⁺]_i-mobilizing activity. Preincubation with CD38-transfected NIH-3T3 fibroblasts, but not with mock-transfected NIH-3T3 cells, abolished [Ca²⁺]_i mobilization. In blocking experiments, preincubation with nonagonistic anti–CD38 monoclonal antibodies also prevented [Ca²⁺]_i mobilization. In cultured human islets, anti–CD38-positive sera exhibiting [Ca²⁺]_i-mobilizing activity in Jurkat T-cells (n = 6) significantly stimulated insulin release at 3.3 mmol/l glucose (median [interquartile range] 738 µU/ml [234], P = 0.0001 vs. 320 [52] µU/ml of control), whereas 6 anti-CD38-positive sera without [Ca²⁺]_i-mobilizing activity and 10 anti–CD38-negative did not. In further incubations, the five anti–CD38-positive sera displaying [Ca²⁺]_i-mobilizing activity in dispersed islet cells significantly stimulated insulin release at both 3.3 mmol/l glucose (2.2 ± 0.3% of insulin islet content, P < 0.002 vs. 1.2 ± 0.1% of control) and 16.7 mmol/l glucose (3.7 ± 0.3 vs. 2.3 ± 0.3%, P < 0.002). We conclude that human anti–CD38 autoantibodies with agonistic properties on the CD38 effector system occur in nature; in human islets, their [Ca²⁺]_i-mobilizing activity is coupled with the ability to stimulate insulin release.

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