Background/Purpose: Sjögren’s syndrome is characterized by exocrine gland dysfunction and autoantibody production.  Some data suggest autoantibodies binding the muscarinic 3 receptor (M3R) mediate poor function of the salvary gland.  We have previously produced recombinant human monoclonal antibodies (rhMab) from antibody-secreting B cells (ASC) infiltrating the salivary glands of Sjögren’s syndrome subjects.  We undertook this study to determine anti-M3R properties of these rhMab.

Methods: Minor salivary gland biopsies were obtained in a comprehensive sicca evaluation clinic.   ABCs were single cell purified with heavy and light genes amplified by PCR.  These genes were then expressed in 239 cells with production of monoclonal IgG or IgM.  The rhMab were assayed for anti-Ro, Ro peptide and M3R extracellular loop (ECL) peptides by ELISA.  Ability to act as an agonist or antagonist of M3R was assessed via GeneBlazer™ M3-NFAT-bla CHO-K1 (M3R) cell line (K1716). 

Results: We studied 47 rhMabs from SS subjects and 18 from non-SS sicca subjects.  There were 10/47 and 0/18 hrMab from the SS and non-SS sicca subjects, respectively, that were positive for binding to M3R ECL2 (p=0.05) and 5/47 SS vs. 0/18 non-SS sicca subjects that were positive for M3R ECL3 (p=0.311). Four of the rhMabs (1-G04k, 1-E06k, 2-D06k and 5-E04k) from one SS patient (pSS-3) bound both M3R ECL2 and ECL3.  When we considered positivity to either M3R ECL2 or ECL3, 11 of 47 rhMabs from SS subjects bound, while 0 of 18 rhMabs from non-SS sicca subjects bound (p=0.03 by 2-tailed Fisher’s Exact Test).  Several rhMab that bound M3R ECL2 showed high affinity binding (Kd values ranging from 10-7 to 10-9).  Among the Ro60-binding rhMab derived from SS subjects, 8/23 (34.8%) also bound M3R ECL2.  and 4/23 (17%) bound the M3R ECL3 by ELISA. None of the anti-Ro60 rhMab from the non-SS sicca subjects bound either M3R ECL2 or ECL3.  Of 65 rhMab, 4 from the SS group (all from pSS-3) and 2 from the non-SS sicca group (1 from NSS-1 and 1 from NSS-4) showed M3R antagonist activity.

Conclusion: Recombinant monoclonal antibodies derived from salivary gland infiltrating antibody secreting cells bind M3R extracellular loop.  Some of these antibodies show M3R inhibitory activity; and, thus, may be directly involved in salivary gland dysfunction.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/antibodies-binding-ro-ssa-and-muscarinic-3-receptor-in-sjogrens-syndrome/