Sec Form 4 Filing - MPM SunStates Fund, L.P. @ TCR2 THERAPEUTICS INC. - 2019-02-19

FORM 4 Check this box if no longer subject to Section 16. Form 4 or Form 5 obligations may continue. See Instruction 1(b).

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 STATEMENT OF CHANGES IN BENEFICIAL OWNERSHIP Filed pursuant to Section 16(a) of the Securities Exchange Act of 1934, Section 17(a) of the Public Utility Holding Company Act of 1935 or Section 30(h) of the Investment Company Act of 1940

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1. Name and Address of Reporting Person MPM SunStates Fund, L.P. 2. Issuer Name and Ticker or Trading Symbol TCR2 THERAPEUTICS INC. [ TCRR] 5. Relationship of Reporting Person(s) to Issuer (Check all applicable) _____ Director X __ 10% Owner _____ Officer (give title below) _____ Other (specify below) (Last) (First) (Middle) C/O MPM CAPITAL, 450 KENDALL STREET 3. Date of Earliest Transaction (MM/DD/YY) 02/19/2019 (Street) CAMBRIDGE, MA02142 4. If Amendment, Date Original Filed (MM/DD/YY) 6. Individual or Joint/Group Filing (Check Applicable Line) _____ Form filed by One Reporting Person __ X __ Form filed by More than One Reporting Person (City) (State) (Zip)

Table I - Non-Derivative Securities Acquired, Disposed of, or Beneficially Owned 1.Title of Security (Instr. 3) 2. Transaction Date (MM/DD/YY) 2A. Deemed Execution Date, if any (MM/DD/YY) 3. Transaction Code (Instr. 8) 4. Securities Acquired (A) or Disposed of (D) (Instr. 3, 4 and 5) 5. Amount of Securities Beneficially Owned Following Reported Transaction(s) (Instr. 3 and 4) 6. Ownership Form: Direct (D) or Indirect (I) (Instr. 4) 7. Nature of Indirect Beneficial Ownership (Instr. 4) Code V Amount (A) or (D) Price Common Stock 02/19/2019 C 2,825,396 A 3,021,298 I See Footnote ( 2 ) ( 3 ) Common Stock 02/19/2019 C 322,901 A 3,344,199 I See Footnote ( 2 ) ( 4 ) Common Stock 02/19/2019 P 706,666 A $ 15 4,050,865 I See Footnote ( 2 ) ( 5 )

Table II - Derivative Securities Acquired, Disposed of, or Beneficially Owned ( e.g. , puts, calls, warrants, options, convertible securities) 1. Title of Derivative Security (Instr. 3) 2. Conversion or Exercise Price of Derivative Security 3. Transaction Date (MM/DD/YY) 3A. Deemed Execution Date, if any (MM/DD/YY) 4. Transaction Code (Instr. 8) 5. Number of Derivative Securities Acquired (A) or Disposed of (D) (Instr. 3, 4, and 5) 6. Date Exercisable and Expiration Date (MM/DD/YY) 7. Title and Amount of Underlying Securities (Instr. 3 and 4) 8. Price of Derivative Security (Instr. 5) 9. Number of Derivative Securities Beneficially Owned Following Reported Transaction(s) (Instr. 4) 10. Ownership Form of Derivative Security: Direct (D) or Indirect (I) (Instr. 4) 11. Nature of Indirect Beneficial Ownership (Instr. 4) Code V (A) (D) Date Exercisable Expiration Date Title Amount or Number of Shares Series A Preferred Stock ( 1 ) 02/19/2019 C 17,500,000 ( 1 ) ( 1 ) Common Stock 2,825,396 $ 0 0 I See Footnote ( 6 ) Series B Preferred Stock ( 1 ) 02/19/2019 C 2,000,000 ( 1 ) ( 1 ) Common Stock 322,901 $ 0 0 I See Footnote ( 6 )

Reporting Owners Reporting Owner Name / Address Relationships Director 10% Owner Officer Other MPM SunStates Fund, L.P. C/O MPM CAPITAL 450 KENDALL STREET CAMBRIDGE, MA02142 X MPM Asset Management Investors SunStates Fund LLC C/O MPM CAPITAL 450 KENDALL STREET CAMBRIDGE, MA02142 X MPM SunStates Fund GP LLC C/O MPM CAPITAL 450 KENDALL STREET CAMBRIDGE, MA02142 X MPM SunStates GP Managing Member LLC MPM CAPITAL 450 KENDALL STREET CAMBRIDGE, MA02142 X Signatures /s/ Ansbert Gadicke, member of MPM Sunstates GP Managing Member LLC, the managing member of MPM SunStates Fund GP LLC, the general partner of MPM SunStates Fund, L.P. 02/21/2019 Signature of Reporting Person Date /s/ Howard Rubin, director of MPM Asset Management Investors SunStates Fund LLC 02/21/2019 Signature of Reporting Person Date /s/ Ansbert Gadicke, member of MPM SunStates GP Managing Member LLC, the managing member of MPM SunStates Fund GP LLC 02/21/2019 Signature of Reporting Person Date /s/ Ansbert Gadicke, member of MPM SunStates GP Managing Member LLC 02/21/2019 Signature of Reporting Person Date

Explanation of Responses:

( 1 )All shares of Series A Preferred Stock and Series B Preferred Stock automatically converted into the number of shares of the Issuer's common stock on a 6.1938:1 basis, for no additional consideration, immediately prior to the closing of the Issuer's initial public offering and had no expiration date. ( 2 )The Reporting Persons disclaim beneficial ownership of the securities except to the extent of their respective pecuniary interests therein. ( 3 )The shares are held as follows: 2,195,681 by MPM BioVentures 2014, L.P. ("BV 2014"), 146,447 by MPM BioVentures 2014(B), L.P. ("BV 2014(B)"), 79,644 by MPM Asset Management Investors BV2014 LLC ("AM BV2014"), 195,902 by MPM Asset Management LLC ("AM LLC"), 351,155 by MPM SunStates Fund, L.P. ("MPM SunStates") and 52,469 by MPM Asset Management Investors Sunstates Fund LLC ("AM SunStates"). MPM BioVentures 2014 GP LLC and MPM BioVentures 2014 LLC ("BV LLC") are the direct and indirect general partners of BV 2014 and BV 2014(B). MPM SunStates Fund GP LLC and MPM SunStates GP Managing Member LLC ("SunStates GP") are the direct and indirect general partners of MPM SunStates. Luke Evnin, Ansbert Gadicke and Todd Foley are the managing directors of BV LLC, Ansbert Gadicke is a member of SunStates GP and Luke Evnin and Anbsert Gadicke are the members of AM LLC. ( 4 )The shares are held as follows: 2,451,844 by BV 2014, 163,532 by BV 2014(B), 88,935 by AM BV2014, 195,902 by AM LLC, 386,270 by MPM SunStates and 57,716 by AM SunStates. ( 5 )The shares are held as follows: 3,056,272 by BV 2014, 203,846 by BV 2014(B), 110,859 by AM BV2014, 195,902 by AM LLC, 421,070 by MPM SunStates and 62,916 by AM SunStates. ( 6 )No securities held by the Reporting Persons.

Remarks: See Form 4 for MPM BioVentures 2014, L.P. for additional members of this joint filing.

Reminder: Report on a separate line for each class of securities beneficially owned directly or indirectly.

* If the form is filed by more than one reporting person, see Instruction 4(b)(v).

** Intentional misstatements or omissions of facts constitute Federal Criminal Violations. See 18 U.S.C. 1001 and 15 U.S.C. 78ff(a).

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