Nature of the Business and Basis of Presentation Selecta Biosciences, Inc. (the Company) was incorporated in Delaware on December 10, 2007, and is based in Watertown, Massachusetts. The Company is a biopharmaceutical company dedicated to developing the first generation of nanoparticle immunomodulatory drugs for the treatment and prevention of human diseases. Since inception, the Company has devoted its efforts principally to research and development of its technology and product candidates, recruiting management and technical staff, acquiring operating assets, and raising capital. The Company is subject to risks common to companies in the biotechnology industry including, but not limited to, new technological innovations, protection of proprietary technology, dependence on key personnel, compliance with government regulations and the need to obtain additional financing. Product candidates currently under development will require significant additional research and development efforts, including extensive preclinical and clinical testing and regulatory approval, prior to commercialization. These efforts require significant amounts of additional capital, adequate personnel infrastructure and extensive compliance-reporting capabilities. The Companys product candidates are in development. There can be no assurance that the Companys research and development will be successfully completed, that adequate protection for the Companys intellectual property will be obtained, that any products developed will obtain necessary government regulatory approval or that any approved products will be commercially viable. Even if the Companys product development efforts are successful, it is uncertain when, if ever, the Company will generate significant revenue from product sales. The Company operates in an environment of rapid change in technology and substantial competition from pharmaceutical and biotechnology companies. In addition, the Company is dependent upon the services of its employees and consultants. The accompanying financial statements have been prepared on a basis that assumes the Company is a going concern, and does not include any adjustments to reflect the possible future effects on the recoverability and classification of assets or the amounts and classifications of liabilities that may result from any uncertainty related to its ability to continue as a going concern. Unaudited Interim Financial Information The accompanying unaudited consolidated financial statements for the three months ended March 31, 2019 and 2018 have been prepared by the Company, pursuant to the rules and regulations of the Securities and Exchange Commission (the SEC) for interim financial statements. Certain information and footnote disclosures normally included in financial statements prepared in accordance with accounting principles generally accepted in the United States of America (GAAP) have been condensed or omitted pursuant to such rules and regulations. However, the Company believes that the disclosures are adequate to make the information presented not misleading. These consolidated financial statements should be read in conjunction with the Companys audited consolidated financial statements and the notes thereto for the year ended December 31, 2018 included in the Companys Annual Report on Form 10-K that was filed with the SEC on March 15, 2019 (the Annual Report on Form 10-K). The unaudited interim financial statements have been prepared on the same basis as the audited consolidated financial statements. In the opinion of management, the accompanying unaudited interim consolidated financial statements contain all adjustments that are necessary for a fair statement of the Companys financial position as of March 31, 2019 and consolidated results of operations and cash flows for the three months ended March 31, 2019 . Such adjustments are of a normal and recurring nature. The results of operations for the three months ended March 31, 2019 are not necessarily indicative of the results of operations that may be expected for the year ending December 31, 2019. Liquidity and Management's Plan The future success of the Company is dependent on its ability to develop its product candidates and ultimately upon its ability to attain profitable operations. The Company is subject to a number of risks similar to other early-stage life science companies, including, but not limited to, successful development of its product candidates, raising additional capital with favorable terms, protection of proprietary technology and market acceptance of any approved future products. The successful development of product candidates requires substantial working capital which may not be available to the Company on favorable terms. To date, the Company has financed its operations primarily through the initial public offering of its common stock, a private placement of its common stock, issuances of common and preferred stock, debt, research grants and research collaborations. The Company currently has no source of product revenue, and it does not expect to generate product revenue for the foreseeable future. All of its revenue to date has been collaboration and grant revenue. The Company has devoted substantially all of its financial resources and efforts to developing its ImmTOR technology, identifying potential product candidates and conducting preclinical studies and its clinical trials. The Company is in the early stages of development of its product candidates, and it has not completed development of any ImmTOR-enabled therapies. As of March 31, 2019 , the Companys short-term deposits and investments were $16.2 million , and cash, cash equivalents and restricted cash were $32.5 million , of which $0.4 million was held by its Russian subsidiary designated solely for use in its operations. The Company has incurred losses and negative cash flows from operating activities since inception. As of March 31, 2019 , and December 31, 2018 , the Company had an accumulated deficit of $292.5 million and $280.4 million , respectively. The Company anticipates operating losses to continue for the foreseeable future due to, among other things, costs related to research, development of its product candidates, conducting preclinical studies and clinical trials, and its administrative organization. The Company will require substantial additional financing to fund its operations and to continue to execute its strategy, and the Company will pursue a range of options to secure additional capital. These conditions raise substantial doubt about its ability to continue as a going concern within one year after the date that the financial statements are issued. Management is exploring various sources of funding such as strategic collaborations and the issuance of equity to fund its operations. If the Company raises additional funds through strategic collaborations and alliances, which may include existing collaboration partners, the Company may have to relinquish valuable rights to its technologies or product candidates, or grant licenses on terms that are not favorable to the Company. To the extent that the Company raises additional capital through the sale of equity, the ownership interest of its existing shareholders will be diluted and other preferences may be necessary that adversely affect the rights of existing shareholders. The current operating plan accounts for funding in preparation for the planned Phase 3 clinical program for SEL-212. The Company believes that it has the financial resources to complete the head-to-head Phase 2 trial against Krystexxa, but will require additional external sources of capital to conduct the planned Phase 3 clinical program. If the Company is unable to raise sufficient capital through the transactions discussed above, it intends to curtail expenses contemplated by the current operating plan, and the Company may be required to delay, limit, reduce or terminate its product development efforts or grant rights to develop and market product candidates that it would otherwise prefer to develop and market itself. Because of the uncertainty in securing additional capital and the insufficient amount of capital resources at March 31, 2019 , management has concluded that substantial doubt exists with respect to the Company's ability to continue as a going concern within one year after the date of the filing of this Quarterly Report on Form 10-Q . All amounts due under the 2017 Term Loan (Note 10) have been classified as a current liability as of March 31, 2019 due to the considerations discussed above and the assessment that the material adverse change clause under the 2017 Term Loan is not within the Company's control. The Company has not been notified of an event of default by the Lender as of the date of the filing of this Quarterly Report on Form 10-Q . Guarantees and Indemnifications As permitted under Delaware law, the Company indemnifies its officers, directors, consultants and employees for certain events or occurrences that happen by reason of the relationship with, or position held at, the Company. Through March 31, 2019 , the Company had not experienced any losses related to these indemnification obligations, and no claims were outstanding. The Company does not expect significant claims related to these indemnification obligations and, consequently, concluded that the fair value of these obligations is negligible, and no related reserves were established.