Material Event (Form 8-K)

On September 19, 2025, Equillium, Inc. (the "Company") filed a prospectus supplement (the "Prospectus Supplement") with the Securities and Exchange Commission (the "SEC") for the offer and sale of shares of its common stock, par value $0.0001 per share, having an aggregate offering price of up to $75,000,000 (the "Shares"), pursuant to that certain Open Market Sale Agreement, dated October 5, 2023, as amended August 3, 2025, by and between the Company and LifeSci Capital LLC (the "Sales Agreement"). The Prospectus Supplement amends and supplements the information in the prospectus dated January 18, 2023 (the "Prior Prospectus"), as previously amended and supplemented by the prospectus supplements, dated October 5, 2023 and February 23, 2024 (together with the Prior Prospectus, the "ATM Prospectus"), relating to the offer and sale of up to $21,950,000 of shares of the Company's common stock pursuant to the Sales Agreement, filed with the SEC as a supplement to the Company's Registration Statement on Form S-3(File No. 333-269153).The Prospectus Supplement should be read in conjunction with the ATM Prospectus, and is qualified by reference thereto, except to the extent that the information therein amends or supersedes the information contained in the ATM Prospectus. The Prospectus Supplement is not complete without and may only be delivered or utilized in connection with, the ATM Prospectus and any future amendments or supplements thereto. The Company has previously sold 1,719,485 shares of its common stock for aggregate gross proceeds of $0.96 million under the Sales Agreement. A copy of the opinion of Cooley LLP relating to the validity of the Shares is attached as Exhibit 5.1 hereto.