UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM 10-Q QUARTERLY REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES EXCHANGE ACT OF þ 1934 For the quarterly period ended March 31, 2023 OR TRANSITION REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES EXCHANGE ACT OF ☐ 1934 For the transition period from _______ to _______ Commission File Number: 001-37798 Selecta Biosciences, Inc. (Exact name of registrant as specified in its charter) Delaware 26-1622110 (State or other ...