Exhibit 10.1

Confidential Materials omitted and filed separately with the
Securities and Exchange Commission. Double asterisks denote omissions.

June 6, 2017

Selecta Biosciences, Inc.
480 Arsenal Street, Building One
Watertown, MA 02472
Attention: General Counsel
Facsimile No.: 617-924-3454

RE:    Letter Agreement Regarding Selecta/Spark License and Option Agreement

Dear Sir or Madam:

This letter agreement (the “Letter Agreement”) confirms the understanding
between Selecta Biosciences, Inc. (“Selecta”) and Spark Therapeutics, Inc.
(“Spark” and, together with Selecta, the “Parties”) with respect to certain
rights, remedies and obligations of the Parties pursuant to that certain License
and Option Agreement between the Parties dated as of December 2, 2016, as such
agreement may be amended pursuant to its terms (the “License Agreement”) and
that certain Stock Purchase Agreement between the Parties dated as of December
2, 2016, as such agreement may be amended pursuant to its terms (the “Stock
Purchase Agreement”). Any capitalized terms not defined in this Letter Agreement
shall have the meanings ascribed to them in the License Agreement or the Stock
Purchase Agreement, as applicable.

1.
First Acquisition Right Shares. The Parties hereby agree that Spark is hereby
deemed to have timely delivered the FAR Notice and that May 31, 2017 shall be
considered the date on which Spark delivered such FAR Notice, each for purposes
of Section 2.4 of the Stock Purchase Agreement.  Selecta hereby waives any and
all rights it that it may have to terminate the Stock Purchase Agreement
pursuant to Section 8.2(a)(i) of the Stock Purchase Agreement. Subject to
Spark’s purchase of the First Acquisition Right Shares by 5 P.M. Eastern Time on
June 8, 2017 in accordance with the Stock Purchase Agreement, and Selecta hereby
waives any and all rights that it may have to terminate the License Agreement
pursuant to Section 9.2(f)(ii) and (iii) of the License Agreement with respect
to the purchase of the First Acquisition Right Shares.

2.
Second Scheduled Payment. The Parties hereby agree that Spark shall make the
Scheduled Payment described in Section 6.1(b) of the License Agreement no later
than June 6, 2017. Subject to Spark’s timely payment of such Scheduled Payment
pursuant to this Paragraph 2, Selecta hereby waives any and all rights that it
may have pursuant to Sections 9.2(e) of the License Agreement with respect to
such Scheduled Payment.

3.
Reimbursement. Selecta hereby agrees to reimburse Spark for all costs and
expenses, including, without limitation, the cost of materials provided by
Selecta, actually incurred by Spark (without markup or administrative fees
imposed by Spark) with respect to Spark’s pre-clinical research studies and/or
Development of any Licensed Product pursuant to the License Agreement, up to an
aggregate maximum of [**] and for up to [**] from the date of this Letter
Agreement. Spark’s FTE rate shall not exceed the Selecta FTE Rate in the License
Agreement. Spark shall not seek reimbursement for aggregate employee time in
excess of [**] for the preclinical work under this Letter Agreement. Spark shall
invoice Selecta after the end of each calendar quarter for all such detailed
costs and expenses for which Spark is seeking reimbursement and Selecta shall
pay the invoiced amount within [**] after receipt thereof. Spark agrees to
provide Selecta written reports on the results of any preclinical studies for
which Spark seeks reimbursement from Selecta under this paragraph 3 (which
reports shall exclude anything specific to [**] including the specific [**] used
in such activities and excluding specific [**]). Spark shall endeavor to answer
Selecta’s reasonable questions regarding such reports, provided that Spark shall
not be required to answer questions regarding anything specific to [**]
including the [**]. For clarity, such reimbursement for expenses shall be paid
by Selecta for pre-clinical work conducted during the term of the License
Agreement and such reimbursement obligation shall survive termination of the
License Agreement even if such invoice is delivered to Selecta after termination
of the License Agreement.

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4.
Severability. In case any one or more of the provisions contained in this Letter
Agreement will, for any reason be held to be invalid, illegal, or unenforceable
in any respect, such invalidity, illegality, or unenforceability, will not
affect any other provision of this Letter Agreement, and the Parties will
negotiate in good faith to modify this Letter Agreement to preserve (to the
extent possible) their original intent.

5.
Counterparts. This Letter Agreement, or any part thereof requiring signing by
the Parties, may be executed in separate counterparts, each of which will be an
original as against any Party whose signature appears thereon but all of which
together will constitute one and the same instrument. A facsimile transmission
of the signed Letter Agreement, and those parts thereof requiring signing by the
Parties, will be legal and binding on the Parties.

6.
Amendments. No amendment or modification of or supplement to the terms of this
Letter Agreement will be binding on a Party unless reduced to writing and signed
by all Parties.

7.
Entire Agreement. This Letter Agreement, the License Agreement and the Stock
Purchase Agreement set forth the entire agreement among the Parties as to the
subject matter hereof and supersede all prior and contemporaneous agreements,
understandings, negotiations and discussions, whether oral or written, between
the Parties as to the subject matter hereof. The terms of this Letter Agreement
shall control over any contrary terms of the License Agreement or Stock Purchase
Agreement. This Letter Agreement and all disputes arising out of or related to
this Letter Agreement, or the performance, enforcement, breach or termination
hereof, and any remedies relating thereto, will be construed, governed,
interpreted and applied in accordance with the laws of the State of New York,
without regard to conflict of laws principles.

[END OF PAGE. SIGNATURE PAGE FOLLOWS]

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This Letter Agreement is signed below by authorized representatives of Selecta
and Spark respectively indicating such Parties’ acceptance of the terms and
conditions of this Letter Agreement.

Spark Therapeutics, Inc.

 
/s/ Joseph W. La Barge
By:
 
Joseph W. La Barge
Title:
 
General Counsel

AGREED AND ACCEPTED:
Selecta Biosciences, Inc.

 
 
/s/ David Abraham
By:
 
David Abraham
Title:
 
General Counsel, CCO, Corp.Sec'y.