LAUSR

To date there was no evidence of existence of the naturally circulating antibodies to checkpoint receptors in humans. LAG3 checkpoint is crucial in modulation of immune response that is altered in humans with inflammatory and malignant conditions. We have previously described novel methodology in identification of diagnostic and therapeutic antibodies (US Patent #9810694). Here we report identification of novel potentially therapeutic fully human anti-LAG3 antibodies in cancer patients. Anti-LAG3 antibody presence was evaluated in 25 individuals: 11 healthy donors (controls) and 14 cancer patients in two different laboratories blinded to clinical information. Surface plasmon resonance analysis was done using the optical biosensor BiacoreT200 where LAG3 protein was covalently immobilized on the optical chip and biosensor signals from human serum samples were analyzed. Western Blotting used recombinant LAG3 loaded on a 10% SDS PAGE followed by blotting with plasma samples followed by biotinylated anti-human IgA/IgG/IgM and IrDye 800 streptavidin. Plasma anti-LAG3 IP utilized recombinant LAG3 crosslinked to MagResyn Carboxyl beads followed by incubation with plasma, followed by a biotinylated anti human IgA/IgG/IgMand IrDye streptavidin. No anti-LAG3 antibodies were detected in control group. Among three assays there was complete correlation for presence of anti-LAG3 antibody in 3 cancer patients. One of these patients had sufficient plasma quantity and anti-LAG3 concentration (21 - 33 µg/ml) that should allow for the antibody purification and de novo sequencing. The novel fully human anti-LAG3 antibody will be identified, purified, and the sequence will be determined.To our knowledge this is the first report of checkpoint antibody presence in humans. Further study with cloning and evaluation of therapeutic properties of novel fully human anti-LAG3 antibody in xenograft models will be performed. Citation Format: Ilya Tsimafeyeu, Pavel Ershov, Leonid Kaluzhskiy, Jean-Philippe Couture, Hugo Gagnon, Vladimir Kuznetsov, Aleksei Kalachev, Alexis Ivanov, Gennady Bratslavsky. Naturally circulating anti-LAG3 antibodies exist in cancer patients [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 4115.