Document:

EX-10.2

 Exhibit 10.2 

Option No.             

SPERO THERAPEUTICS, INC. 

Stock Option Grant Notice 

Stock Option Grant under the Company’s 2017 Stock Incentive Plan 

 

			
	 1.      Name and Address of Participant:
	 	  

		 	  

		 	  

		
	 2.      Grant Date:
	 	  

		
	 3.      Type of Grant:
	 	  

		
	 4.      Maximum Number of Shares for
	 	
	 which this Option is exercisable:
	 	  

		
	 5.      Exercise (purchase) price per share:
	 	  

		
	 6.      Option Expiration Date:
	 	  

		
	 7.      Vesting Start Date:
	 	  

  

	8.	Vesting Schedule: This Option shall become exercisable (and the Shares issued upon exercise shall be vested) as follows provided the Participant is an Employee, director or Consultant of the Company or of an Affiliate
on the applicable vesting date:1 

 [25% of the Shares shall be
vested on the first anniversary of the Vesting Start Date, and thereafter the remainder of the Shares not yet vested shall vest in equal monthly installments for 36 months beginning on the first anniversary of the Vesting Start Date.] 

OR 
 [100% of
the Shares shall be vested as of the Grant Date.] 
 OR 

[25% of the Shares shall be vested on the six-month anniversary of the Vesting Start Date, and
thereafter the remainder of the Shares not yet vested shall vest in equal 
  

1 Vesting Schedules to be customized for each optionholder. 

  

 
monthly installments for 42 months beginning on the six-month anniversary of the Vesting Start Date.] 

[Notwithstanding the foregoing, in the event the Participant’s employment is terminated by the Company without Cause (as
defined below) within the 30-day period prior to a Change of Control (as defined below) or within the twelve (12) month period following a Change of Control, (i) if the
Participant’s employment with the Company commenced at least 24 months prior to the Change of Control, then 100% of the Option shall accelerate and become vested and exercisable on the date of termination of employment,
(ii) if the Participant’s employment with the Company commenced less than 24 months but at least 12 months prior to the Change of Control, then 50% of the then unvested portion of the Option shall accelerate and become vested
on the date of termination of employment, and (iii) if the Participant’s employment with the Company commenced less than 12 months prior to the Change of Control, then 25% of the then unvested portion of the Option shall
accelerate and become vested on the date of termination of employment.  
 Cause” shall mean (i) dishonest statements or
acts by the Participant with respect to the Company or any Affiliate of the Company, or any of the Company’s current or prospective customers, suppliers vendors or other third parties with which such entity does business; (ii) the
Participant’s commission of (A) a felony or (B) any misdemeanor involving moral turpitude, deceit, dishonesty or fraud; (iii) the Participant’s failure to perform to the reasonable satisfaction of the Board of Directors the
duties and responsibilities assigned by the Board of Directors which failure continues, in the reasonable judgment of the Board of Directors, after written notice given to the Participant by the Board of Directors; (iv) gross negligence,
willful misconduct or insubordination by the Participant with respect to the Company or any Affiliate of the Company; or (v) a violation of any provision of any agreement(s) between the Participant and the Company relating to noncompetition,
nondisclosure and/or assignment of inventions. 
 “Change of Control” means the consummation of (i) the dissolution or
liquidation of the Company, (ii) the sale of all or substantially all of the assets of the Company on a consolidated basis to an unrelated person or entity, (iii) a merger, reorganization or consolidation involving the Company in which the
shares of Company voting equity outstanding immediately prior to such transaction represent or are converted into or exchanged for securities of the surviving or resulting entity immediately upon completion of such transaction which represent less
than fifty percent (50%) of the outstanding voting power of such surviving or resulting entity, (iv) the acquisition of all or a majority of the outstanding voting equity of the Company in a single transaction or a series of related
transactions by a person or group of persons, or (v) any other acquisition of the business of the Company, as determined by the Board of Directors; provided, however, that the Company’s initial public offering, any subsequent public
offering or another capital raising event, or a merger effected solely to change the Company’s domicile shall not constitute a “Change of Control.”]2 

 
  

2 Note: Double Trigger acceleration clause applicable only to certain executives of the Company. 

  
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 The foregoing rights are cumulative and are subject to the other terms and conditions of this
Agreement, the ROFR Agreement (as defined in the Stock Option Agreement attached hereto), the Voting Agreement (as defined in the Stock Option Agreement attached hereto) and the Company’s 2017 Stock Incentive Plan. 

[Remainder of page intentionally left blank] 

  
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 The Company and the Participant acknowledge receipt of this Stock Option Grant Notice and agree to the terms of
the Stock Option Agreement attached hereto and incorporated by reference herein, the Company’s 2017 Stock Incentive Plan and the terms of this Option Grant as set forth above. 

 

			
	SPERO THERAPEUTICS, INC.
		
	By:	 	  

		 	Name:                                     
                               
		 	Title:                                     
                                 
	
	  

	Participant

  

 SPERO THERAPEUTICS INC. 

STOCK OPTION AGREEMENT- INCORPORATED TERMS AND CONDITIONS 

AGREEMENT made as of the date of grant set forth in the Stock Option Grant Notice by and between Spero Therapeutics, Inc. (the
“Company”), a Delaware corporation, and the individual whose name appears on the Stock Option Grant Notice (the “Participant”). 

WHEREAS, the Company desires to grant to the Participant an Option to purchase shares of its common stock, $0.001 par value per share (the
“Shares”), under and for the purposes set forth in the Company’s 2017 Stock Incentive Plan (the “Plan”); 

WHEREAS, the Company and the Participant understand and agree that any terms used and not defined herein have the same meanings as in the
Plan; and 
 WHEREAS, the Company and the Participant each intend that the Option granted herein shall be of the type set forth in the Stock
Option Grant Notice. 
 NOW, THEREFORE, in consideration of the mutual covenants hereinafter set forth and for other good and valuable
consideration, the parties hereto agree as follows: 
  

	 	1.	GRANT OF OPTION. 

 The Company hereby grants to the Participant the right and option to
purchase all or any part of an aggregate of the number of Shares set forth in the Stock Option Grant Notice, on the terms and conditions and subject to all the limitations set forth herein, under United States securities and tax laws, and in the
Plan, which is incorporated herein by reference. The Participant acknowledges receipt of a copy of the Plan. 
  

	 	2.	EXERCISE PRICE. 

 The exercise price of the Shares covered by the Option shall be the
amount per Share set forth in the Stock Option Grant Notice, subject to adjustment, as provided in the Plan, in the event of a stock split, reverse stock split or other events affecting the holders of Shares after the date hereof (the “Exercise
Price”). Payment shall be made in accordance with Paragraph 9 of the Plan. 
  

	 	3.	EXERCISABILITY OF OPTION. 

 Subject to the terms and conditions set forth in this
Agreement and the Plan, the Option granted hereby shall become vested and exercisable as set forth in the Stock Option Grant Notice and is subject to the other terms and conditions of this Agreement and the Plan. 

	 	4.	TERM OF OPTION. 

 This Option shall terminate on the Option Expiration Date as specified
in the Stock Option Grant Notice and, if this Option is designated in the Stock Option Grant Notice as an ISO and the Participant owns as of the date hereof more than 10% of the total combined voting power of all classes of capital stock of the
Company or an Affiliate, such date may not be more than five years from the date of this Agreement, but shall be subject to earlier termination as provided herein or in the Plan. 

If the Participant ceases to be an Employee, director or Consultant of the Company or of an Affiliate for any reason other than the death or
Disability of the Participant, or termination of the Participant for Cause (the “Termination Date”), the Option to the extent then vested and exercisable pursuant to Section 3 hereof as of the Termination Date, and not previously
terminated in accordance with this Agreement, may be exercised within three (3) months after the Termination Date, or on or prior to the Option Expiration Date as specified in the Stock Option Grant Notice, whichever is earlier, but may not be
exercised thereafter except as set forth below. In such event, the unvested portion of the Option shall not be exercisable and shall expire and be cancelled on the Termination Date.  

If this Option is designated in the Stock Option Grant Notice as an ISO and the Participant ceases to be an Employee of the Company or of an
Affiliate but continues after termination of employment to provide service to the Company or an Affiliate as a director or Consultant, this Option shall continue to vest in accordance with Section 3 above as if this Option had not terminated
until the Participant is no longer providing services to the Company. In such case, this Option shall automatically convert and be deemed a Non-Qualified Option as of the date that is three (3) months
from termination of the Participant’s employment and this Option shall continue on the same terms and conditions set forth herein until such Participant is no longer providing service to the Company or an Affiliate. 

Notwithstanding the foregoing, in the event of the Participant’s Disability or death within three (3) months after the Termination
Date, the Participant or the Participant’s Survivors may exercise the Option within one (1) year after the Termination Date, but in no event after the Option Expiration Date as specified in the Stock Option Grant Notice, and this Option
shall thereupon terminate. 
 In the event the Participant’s service is terminated by the Company or an Affiliate for Cause, the
Participant’s right to exercise any unexercised portion of this Option even if vested shall cease immediately as of the time the Participant is notified his or her service is terminated for Cause, and this Option shall thereupon terminate.
Notwithstanding anything herein to the contrary, if subsequent to the Participant’s termination, but prior to the exercise of the Option, the Administrator determines that, either prior or subsequent to the Participant’s termination, the
Participant engaged in conduct which would constitute Cause, then the Participant shall immediately cease to have any right to exercise the Option and this Option shall thereupon terminate. 

  
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 In the event of the Disability of the Participant while an Employee, director or Consultant of
the Company or of an Affiliate, as determined in accordance with the Plan, the Option shall be exercisable within one (1) year after the Participant’s termination of service due to Disability or, if earlier, on or prior to the Option
Expiration Date as specified in the Stock Option Grant Notice. In such event, the Option shall be exercisable: 
  

	 	(a)	to the extent that the Option has become exercisable but has not been exercised as of the date of the Participant’s termination of service due to Disability; and 

 

	 	(b)	in the event rights to exercise the Option accrue periodically, to the extent of a pro rata portion through the date of the Participant’s termination of service due to Disability of any additional vesting rights
that would have accrued on the next vesting date had the Participant not become Disabled. The proration shall be based upon the number of days accrued in the current vesting period prior to the date of the Participant’s termination of service
due to Disability. 

 In the event of the death of the Participant while an Employee, director or Consultant of the Company or
of an Affiliate, the Option shall be exercisable by the Participant’s Survivors within one year after the date of death of the Participant or, if earlier, on or prior to the Option Expiration Date as specified in the Stock Option Grant Notice.
In such event, the Option shall be exercisable: 
  

	 	(x)	to the extent that the Option has become exercisable but has not been exercised as of the date of death; and 

  

	 	(y)	in the event rights to exercise the Option accrue periodically, to the extent of a pro rata portion through the date of death of any additional vesting rights that would have accrued on the next vesting date had the
Participant not died. The proration shall be based upon the number of days accrued in the current vesting period prior to the Participant’s date of death. 

  

	 	5.	METHOD OF EXERCISING OPTION. 

 Subject to the terms and conditions of this Agreement, the
Option may be exercised by written notice to the Company or its designee, in substantially the form of Exhibit A attached hereto (or in such other form acceptable to the Company, which may include electronic notice). Such
notice shall state the number of Shares with respect to which the Option is being exercised and shall be signed by the person exercising the Option (which signature may be provided electronically in a form acceptable to the Company). Payment of the
Exercise Price for such Shares shall be made in accordance with Paragraph 9 of the Plan. The Company shall deliver such Shares as soon as practicable after the notice shall be received, provided, however, that the Company may delay issuance of such
Shares until completion of any action or obtaining of any consent, which the Company deems necessary under any applicable law (including, without limitation, state securities or “blue sky” laws). The Shares as to which the Option shall

  
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have been so exercised shall be registered in the Company’s share register in the name of the person so exercising the Option (or, if the Option shall be exercised by the Participant and if
the Participant shall so request in the notice exercising the Option, shall be registered in the Company’s share register in the name of the Participant and another person jointly, with right of survivorship) and shall be delivered as provided
above to or upon the written order of the person exercising the Option. In the event the Option shall be exercised, pursuant to Section 4 hereof, by any person other than the Participant, such notice shall be accompanied by appropriate proof of
the right of such person to exercise the Option. All Shares that shall be purchased upon the exercise of the Option as provided herein shall be fully paid and nonassessable. 
  

	 	6.	PARTIAL EXERCISE. 

 Exercise of this Option to the extent above stated may be made in
part at any time and from time to time within the above limits, except that no fractional share shall be issued pursuant to this Option. 
  

	 	7.	NON-ASSIGNABILITY. 

 The Option shall not
be transferable by the Participant otherwise than by will or by the laws of descent and distribution. If this Option is a Non-Qualified Option then it may also be transferred (i) pursuant to a qualified
domestic relations order as defined by the Code or Title I of the Employee Retirement Income Security Act or the rules thereunder or (ii) for no consideration to or for the benefit of the Participant’s Immediate Family (including, without
limitation, to a trust for the benefit of the Participant’s Immediate Family or to a partnership or limited liability company for one or more members of the Participant’s Immediate Family), and the transferee shall remain subject to
all the terms and conditions applicable to the Option prior to such transfer and each such transferee shall so acknowledge in writing as a condition precedent to the effectiveness of such transfer. The term “Immediate Family” shall mean
the Participant’s spouse, former spouse, parents, children, stepchildren, adoptive relationships, sisters, brothers, nieces, nephews and grandchildren (and, for this purpose, shall also include the Participant.). Except as provided above in
this paragraph, the Option shall be exercisable, during the Participant’s lifetime, only by the Participant (or, in the event of legal incapacity or incompetency, by the Participant’s guardian or representative) and shall not be assigned,
pledged or hypothecated in any way (whether by operation of law or otherwise) and shall not be subject to execution, attachment or similar process. Any attempted transfer, assignment, pledge, hypothecation or other disposition of the Option or of
any rights granted hereunder contrary to the provisions of this Section 7, or the levy of any attachment or similar process upon the Option shall be null and void. 
  

	 	8.	NO RIGHTS AS STOCKHOLDER UNTIL EXERCISE. 

 The Participant shall have no rights as a
stockholder with respect to Shares subject to this Agreement until registration of the Shares in the Company’s share register in the name of the Participant. Except as is expressly provided in the Plan with respect to certain changes in the
capitalization of the Company, no adjustment shall be made for dividends or similar rights for which the record date is prior to the date of such registration. 

  
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	 	9.	ADJUSTMENTS. 

 The Plan contains provisions covering the treatment of Options in a number
of contingencies such as stock splits and mergers. Provisions in the Plan for adjustment with respect to stock subject to Options and the related provisions with respect to successors to the business of the Company are hereby made applicable
hereunder and are incorporated herein by reference. 
  

	 	10.	TAXES. 

 The Participant acknowledges and agrees that (i) any income or other taxes
due from the Participant with respect to this Option or the Shares issuable upon exercise of this Option shall be the Participant’s responsibility; (ii) the Participant was free to use professional advisors of his or her choice in
connection with this Agreement, has received advice from his or her professional advisors in connection with this Agreement, understands its meaning and import, and is entering into this Agreement freely and without coercion or duress;
(iii) the Participant has not received and is not relying upon any advice, representations or assurances made by or on behalf of the Company or any Affiliate or any Employee of or counsel to the Company or any Affiliate regarding any tax or
other effects or implications of the Option, the Shares or other matters contemplated by this Agreement and (iv) neither the Administrator, the Company, its Affiliates, nor any of its officers or directors, shall be held liable for any
applicable costs, taxes, or penalties associated with the Option if, in fact, the Internal Revenue Service were to determine that the Option constitutes deferred compensation under Section 409A of the Code. 

If this Option is designated in the Stock Option Grant Notice as a Non-Qualified Option or if the
Option is an ISO and is converted into a Non-Qualified Option and such Non-Qualified Option is exercised, the Participant agrees that the Company may withhold from the
Participant’s remuneration, if any, the minimum statutory amount of federal, state and local withholding taxes attributable to such amount that is considered compensation includable in such person’s gross income. At the Company’s
discretion, the amount required to be withheld may be withheld in cash from such remuneration, or in kind from the Shares otherwise deliverable to the Participant on exercise of the Option. The Participant further agrees that, if the Company does
not withhold an amount from the Participant’s remuneration sufficient to satisfy the Company’s income tax withholding obligation, the Participant will reimburse the Company on demand, in cash, for the amount under-withheld. 

 

	 	11.	PURCHASE FOR INVESTMENT. 

 Unless the offering and sale of the Shares to be issued upon
the particular exercise of the Option shall have been effectively registered under the Securities Act of 1933, as now in force or hereafter amended (the “1933 Act”), the Company shall be under no obligation to issue the Shares covered by
such exercise unless the Company has determined that such exercise and issuance would be exempt from the registration requirements of the 1933 Act and until the following conditions have been fulfilled: 

  
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	 	(a)	The person(s) who exercise the Option shall warrant to the Company, at the time of such exercise, that such person(s) are acquiring such Shares for their own respective accounts, for investment, and not with a view to,
or for sale in connection with, the distribution of any such Shares, in which event the person(s) acquiring such Shares shall be bound by the provisions of the following legend which shall be endorsed upon any certificate(s) evidencing the Shares
issued pursuant to such exercise: 

 “The shares represented by this certificate have been taken for investment and they
may not be sold or otherwise transferred by any person, including a pledgee, unless (1) either (a) a Registration Statement with respect to such shares shall be effective under the Securities Act of 1933, as amended, or (b) the Company
shall have received an opinion of counsel satisfactory to it that an exemption from registration under such Act is then available, and (2) there shall have been compliance with all applicable state securities laws;” and 

 

	 	(b)	If the Company so requires, the Company shall have received an opinion of its counsel that the Shares may be issued upon such particular exercise in compliance with the 1933 Act without registration thereunder. Without
limiting the generality of the foregoing, the Company may delay issuance of the Shares until completion of any action or obtaining of any consent, which the Company deems necessary under any applicable law (including without limitation state
securities or “blue sky” laws). 

  

	 	12.	RESTRICTIONS ON TRANSFER OF SHARES. 

 12.1 The Shares acquired by the Participant
pursuant to the exercise of the Option granted hereby shall not be transferred by the Participant except as permitted herein and in the Right of First Refusal and Co-Sale Agreement dated June
[    ], 2017, as amended, between the Company and its stockholders, as may be amended from time to time (the “ROFR Agreement”), and in the Voting Agreement dated June [    ], 2017,
between the Company and its stockholders, as may be amended from time to time (the “Voting Agreement”), both agreements to which the Participant agrees to become a party by executing signature pages thereto. If the terms of this Agreement
and the ROFR Agreement conflict, the terms contained in the ROFR Agreement shall govern and supersede any conflicting provision contained in this Agreement. If the terms of this Agreement and the Voting Agreement conflict, the terms contained in the
Voting Agreement shall govern and supersede any conflicting provision contained in this Agreement. 
 12.2 [Intentionally Omitted] 

12.3 It shall be a condition precedent to the validity of any sale or other transfer of any Shares by the Participant that the following
restrictions be complied with (except as otherwise provided in this Section 12): 

  
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	 	(i)	No Shares owned by the Participant may be sold, pledged or otherwise transferred (including by gift or devise) to any person or entity, voluntarily, or by operation of law, except in accordance with the terms and
conditions hereinafter set forth. 

  

	 	(ii)	Before selling or otherwise transferring all or part of the Shares, the Participant shall give written notice of such intention to the Company in accordance with Section 2.1(b) of the ROFR Agreement. Such notice
shall constitute a binding offer by the Participant to sell to the Company such number of the Shares as if such Shares were Key Holder Transfer Stock (as defined in the ROFR Agreement). If the Company shall fail to accept any such offer, such Shares
shall be subject to a Secondary Refusal Right, as defined in Section 1.24 of the ROFR Agreement. 

  

	 	(iii)	If the Company and any Senior Preferred Investor (as defined in the ROFR Agreement), as the case may be, shall fail to accept any such offer, the Participant shall be free to sell all, but not less than all, of the
Shares set forth in his or her notice to the designated transferee at the price and terms designated in the Participant’s notice, provided that (i) such sale is consummated within six (6) months after the giving of notice by the
Participant to the Company as aforesaid, and (ii) the transferee first agrees in writing to be bound by the provisions of this Section 12 so that such transferee (and all subsequent transferees) shall thereafter only be permitted to sell
or transfer the Shares in accordance with the terms hereof. After the expiration of such six (6) months, the provisions of this Section 12.3 shall again apply with respect to any proposed voluntary transfer of the Participant’s
Shares. 

  

	 	(iv)	The restrictions on transfer contained in this Section 12.3 shall not apply to (a) transfers by the Participant to his or her spouse or children or to a trust for the benefit of his or her spouse or
children, (b) transfers by the Participant to his or her guardian or conservator, and (c) transfers by the Participant, in the event of his or her death, to his or her executor(s) or administrator(s) or to trustee(s) under his or her will
(collectively, “Permitted Transferees”); provided however, that in any such event the Shares so transferred in the hands of each such Permitted Transferee shall remain subject to this Agreement, and each such Permitted Transferee shall so
acknowledge in writing as a condition precedent to the effectiveness of such transfer. 

  

	 	(v)	The provisions of this Section 12.3 may be waived by the Company. Any such waiver may be unconditional or based upon such conditions as the Company may impose. 

12.4 In the event that the Participant or his or her successor in interest fails to deliver the Shares to be repurchased by the Company under
this Agreement, the Company may elect (a) to establish a segregated account in the amount of the repurchase price, such account to be turned over to the Participant or his or her successor in interest upon delivery of such Shares, and
(b) immediately to take such action as is appropriate to transfer record title of such Shares from 

  
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the Participant to the Company and to treat the Participant and such Shares in all respects as if delivery of such Shares had been made as required by this Agreement. The Participant hereby
irrevocably grants the Company a power of attorney which shall be coupled with an interest for the purpose of effectuating the preceding sentence. 

12.5 If the Company shall pay a stock dividend or declare a stock split on or with respect to any of its Common Stock, or otherwise distribute
securities of the Company to the holders of its Common Stock, the number of shares of stock or other securities of the Company issued with respect to the shares then subject to the restrictions contained in this Agreement shall be added to the
Shares subject to the Company’s rights to repurchase pursuant to this Agreement. If the Company shall distribute to its stockholders shares of stock of another corporation or equity in another entity, the shares of stock of such other
corporation or equity in such other entity, distributed with respect to the Shares then subject to the restrictions contained in this Agreement, shall be added to the Shares subject to the Company’s rights to repurchase pursuant to this
Agreement. 
 12.6 If the outstanding shares of Common Stock of the Company shall be subdivided into a greater number of shares or combined
into a smaller number of shares, or in the event of a reclassification of the outstanding shares of Common Stock of the Company, or if the Company shall be a party to a merger, consolidation or capital reorganization, there shall be substituted for
the Shares then subject to the restrictions contained in this Agreement such amount and kind of securities as are issued in such subdivision, combination, reclassification, merger, consolidation or capital reorganization in respect of the Shares
subject immediately prior thereto to the Company’s rights to repurchase pursuant to this Agreement. 
 12.7 The Company shall not be
required to transfer any Shares on its books which shall have been sold, assigned or otherwise transferred in violation of this Agreement, or to treat as owner of such Shares, or to accord the right to vote as such owner or to pay dividends to, any
person or organization to which any such Shares shall have been so sold, assigned or otherwise transferred, in violation of this Agreement. 

12.8 The provisions of Sections 12.1 and 12.3 shall terminate upon the effective date of the registration of the Shares pursuant to the
Securities Exchange Act of 1934. 
 12.9 The Participant agrees that in the event the Company proposes to offer for sale to the public any of
its equity securities and such Participant is requested by the Company and any underwriter engaged by the Company in connection with such offering to sign an agreement restricting the sale or other transfer of Shares, then it will promptly sign such
agreement and will not transfer, whether in privately negotiated transactions or to the public in open market transactions or otherwise, any Shares or other securities of the Company held by him or her during such period as is determined by the
Company and the underwriters, not to exceed 180 days following the closing of the offering, plus such additional period of time as may be required to comply with FINRA Rules or similar rules promulgated by another regulatory authority (such period,
the “Lock-Up Period”). Such agreement shall be in writing and in form and substance reasonably satisfactory to the Company and such underwriter and pursuant to customary and prevailing terms and
conditions. Notwithstanding whether the Participant has signed such an 

  
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agreement, the Company may impose stop-transfer instructions with respect to the Shares or other securities of the Company subject to the foregoing restrictions until the end of the Lock-Up Period. 
 12.10 The Participant acknowledges and agrees that neither the Company, its
shareholders nor its directors and officers, has any duty or obligation to disclose to the Participant any material information regarding the business of the Company or affecting the value of the Shares before, at the time of, or following a
termination of the service of the Participant by the Company, including, without limitation, any information concerning plans for the Company to make a public offering of its securities or to be acquired by or merged with or into another firm or
entity. 
 12.11 All certificates representing the Shares to be issued to the Participant pursuant to this Agreement shall have endorsed
thereon a legend substantially as follows: “The shares represented by this certificate are subject to restrictions set forth in a Stock Option Agreement dated as of
                        , a copy of which Agreement is available for inspection at the offices of the Company or will be
made available upon request.” 
  

	 	13.	NO OBLIGATION TO MAINTAIN RELATIONSHIP. 

 The Participant acknowledges that: (i) the
Company is not by the Plan or this Option Agreement obligated to continue the Participant as an Employee, director or Consultant of the Company or an Affiliate; (ii) the Plan is discretionary in nature and may be suspended or terminated by the
Company at any time; (iii) the grant of the Option is a one-time benefit which does not create any contractual or other right to receive future grants of options, or benefits in lieu of options;
(iv) all determinations with respect to any such future grants, including, but not limited to, the times when options shall be granted, the number of shares subject to each option, the option price, and the time or times when each option shall
be exercisable, will be at the sole discretion of the Company; (v) the Participant’s participation in the Plan is voluntary; (vi) the value of the Option is an extraordinary item of compensation which is outside the scope of the
Participant’s employment or consulting contract, if any; and (vii) the Option is not part of normal or expected compensation for purposes of calculating any severance, resignation, redundancy, end of service payments, bonuses, long-service
awards, pension or retirement benefits or similar payments. 
  

	 	14.	IF OPTION IS INTENDED TO BE AN ISO. 

 If this Option is designated in the Stock Option
Grant Notice as an ISO so that the Participant (or the Participant’s Survivors) may qualify for the favorable tax treatment provided to holders of Options that meet the standards of Section 422 of the Code then any provision of this
Agreement or the Plan which conflicts with the Code so that this Option would not be deemed an ISO is null and void and any ambiguities shall be resolved so that the Option qualifies as an ISO. The Participant should consult with the
Participant’s own tax advisors regarding the tax effects of the Option and the requirements necessary to obtain favorable tax treatment under Section 422 of the Code, including, but not limited to, holding period requirements. 

  
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 Notwithstanding the foregoing, to the extent that the Option is designated in the Stock Option
Grant Notice as an ISO and is not deemed to be an ISO pursuant to Section 422(d) of the Code because the aggregate Fair Market Value (determined as of the Date of Option Grant) of any of the Shares with respect to which this ISO is granted
becomes exercisable for the first time during any calendar year in excess of $100,000, the portion of the Option representing such excess value shall be treated as a Non-Qualified Option and the Participant
shall be deemed to have taxable income measured by the difference between the then Fair Market Value of the Shares received upon exercise and the price paid for such Shares pursuant to this Agreement. 

Neither the Company nor any Affiliate shall have any liability to the Participant, or any other party, if the Option (or any part thereof)
that is intended to be an ISO is not an ISO or for any action taken by the Administrator, including without limitation the conversion of an ISO to a Non-Qualified Option. 

 

	 	15.	NOTICE TO COMPANY OF DISQUALIFYING DISPOSITION OF AN ISO. 

 If this Option is designated
in the Stock Option Grant Notice as an ISO then the Participant agrees to notify the Company in writing immediately after the Participant makes a Disqualifying Disposition of any of the Shares acquired pursuant to the exercise of the ISO. A
Disqualifying Disposition is defined in Section 424(c) of the Code and includes any disposition (including any sale) of such Shares before the later of (a) two years after the date the Participant was granted the ISO or (b) one year
after the date the Participant acquired Shares by exercising the ISO, except as otherwise provided in Section 424(c) of the Code. If the Participant has died before the Shares are sold, these holding period requirements do not apply and no
Disqualifying Disposition can occur thereafter. 
  

	 	16.	NOTICES. 

 Any notices required or permitted by the terms of this Agreement or the Plan
shall be given by recognized courier service, facsimile, registered or certified mail, return receipt requested, or by email addressed as follows: 
 If to
the Company: 
 Spero Therapeutics, Inc. 

675 Massachusetts Avenue 

Cambridge, MA 02139 
 Attention:
Chief Financial Officer 
 If to the Participant at the address set forth on the Stock Option Grant Notice or such address as the Company may then have in
its records for the Participant or to such other address or addresses of which notice in the same manner has previously been given. Any such notice shall be deemed to have been given upon the earlier of receipt, one business day following delivery
to a recognized courier service or three business days following mailing by registered or certified mail. 

  
 10 

	 	17.	GOVERNING LAW. 

 This Agreement shall be governed by and construed in accordance with the
laws of the State of Delaware, without giving effect to its internal principles governing the conflict of law. For the purpose of litigating any dispute that arises under this Agreement, the parties hereby consent to exclusive jurisdiction in the
Commonwealth of Massachusetts and agree that such litigation shall be conducted in the state courts of Suffolk County, Massachusetts or the federal courts of the United States for the District of Massachusetts. 

 

	 	18.	BENEFIT OF AGREEMENT. 

 Subject to the provisions of the Plan and the other provisions
hereof, this Agreement shall be for the benefit of and shall be binding upon the heirs, executors, administrators, successors and assigns of the parties hereto. 
  

	 	19.	ENTIRE AGREEMENT. 

 This Agreement, together with the Plan, the Voting Agreement, and the
ROFR Agreement, embodies the entire agreement and understanding between the parties hereto with respect to the subject matter hereof and supersedes all prior oral or written agreements and understandings relating to the subject matter hereof. [In
furtherance of the foregoing, you acknowledge that you no longer have any rights or interests in the Company’s former parent entity Spero Therapeutics, LLC.]3 No statement,
representation, warranty, covenant or agreement not expressly set forth in this Agreement shall affect or be used to interpret, change or restrict, the express terms and provisions of this Agreement, provided, however, in any event, this Agreement
shall be subject to and governed by the Plan. 
  

	 	20.	MODIFICATIONS AND AMENDMENTS. 

 The terms and provisions of this Agreement may be
modified or amended as provided in the Plan. 
  

	 	21.	WAIVERS AND CONSENTS. 

 Except as provided in the Plan, the terms and provisions of this
Agreement may be waived, or consent for the departure therefrom granted, only by written document executed by the party entitled to the benefits of such terms or provisions. No such waiver or consent shall be deemed to be or shall constitute a
waiver or consent with respect to any other terms or provisions of this Agreement, whether or not similar. Each such waiver or consent shall be effective only in the specific instance and for the purpose for which it was given, and shall not
constitute a continuing waiver or consent. 
  

	 	22.	DATA PRIVACY. 

  

 

3 NTD: Include for replacement option grants only. 

  
 11 

 By entering into this Agreement, the Participant: (i) authorizes the Company and each
Affiliate, and any agent of the Company or any Affiliate administering the Plan or providing Plan recordkeeping services, to disclose to the Company or any of its Affiliates such information and data as the Company or any such Affiliate shall
request in order to facilitate the grant of options and the administration of the Plan; (ii) to the extent permitted by law waives any data privacy rights he or she may have with respect to such information; and (iii) authorizes the
Company and each Affiliate to store and transmit such information in electronic form for the purposes set forth in this Agreement. 

[REMAINDER OF PAGE INTENTIONALLY LEFT BLANK] 

  
 12 

 Exhibit A 

NOTICE OF EXERCISE OF STOCK OPTION 

[Form for Unregistered Shares] 

To:        Spero Therapeutics, Inc. 

Ladies and Gentlemen: 
 I hereby exercise my
Stock Option to purchase                      shares (the “Shares”) of the common stock, $0.001 par value, of Spero Therapeutics,
Inc. (the “Company”), at the exercise price of $[                ] per share, pursuant to and subject to the terms of that certain Stock Option
Agreement between the undersigned and the Company dated [                ], 201[    ]. 

I am aware that the Shares have not been registered under the Securities Act of 1933, as amended (the “1933 Act”), or any state
securities laws. I understand that the reliance by the Company on exemptions under the 1933 Act is predicated in part upon the truth and accuracy of the statements by me in this Notice of Exercise. 

I hereby represent and warrant that (1) I have been furnished with all information which I deem necessary to evaluate the merits and
risks of the purchase of the Shares; (2) I have had the opportunity to ask questions concerning the Shares and the Company and all questions posed have been answered to my satisfaction; (3) I have been given the opportunity to obtain any
additional information I deem necessary to verify the accuracy of any information obtained concerning the Shares and the Company; and (4) I have such knowledge and experience in financial and business matters that I am able to evaluate the
merits and risks of purchasing the Shares and to make an informed investment decision relating thereto. 
 I hereby represent and warrant
that I am purchasing the Shares for my own personal account for investment and not with a view to the sale or distribution of all or any part of the Shares. 

I understand that because the Shares have not been registered under the 1933 Act, I must continue to bear the economic risk of the investment
for an indefinite time and the Shares cannot be sold unless the Shares are subsequently registered under applicable federal and state securities laws or an exemption from such registration requirements is available. 

I agree that I will in no event sell or distribute or otherwise dispose of all or any part of the Shares unless (1) there is an effective
registration statement under the 1933 Act and applicable state securities laws covering any such transaction involving the Shares or (2) the Company receives an opinion of my legal counsel (concurred in by legal counsel for the Company) stating
that such transaction is exempt from registration or the Company otherwise satisfies itself that such transaction is exempt from registration. 

  
 Exhibit A-1 

 I consent to the placing of a legend on my certificate for the Shares stating that the Shares
have not been registered and setting forth the restrictions on transfer contemplated hereby and to the placing of a stop transfer order on the books of the Company and with any transfer agents against the Shares until the Shares may be legally
resold or distributed without restriction. 
 I understand that at the present time Rule 144 of the Securities and Exchange Commission (the
“SEC”) may not be relied on for the resale or distribution of the Shares by me. I understand that the Company has no obligation to me to register the sale of the Shares with the SEC and has not represented to me that it will register the
sale of the Shares. 
 I understand the terms and restrictions on the right to dispose of the Shares set forth in the 2017 Stock Incentive
Plan and the Stock Option Agreement, both of which I have carefully reviewed. I consent to the placing of a legend on my certificate for the Shares referring to such restriction and the placing of stop transfer orders until the Shares may be
transferred in accordance with the terms of such restrictions. 
 I understand and agree that the Shares are subject to the Right of First
Refusal and Co-Sale Agreement dated June [    ], between the Company and its stockholders (the “ROFR Agreement”) and the Voting Agreement dated June
[    ], 2017 between the Company and its stockholders (the “Voting Agreement”). I acknowledge that I have read and understand the ROFR Agreement and the Voting Agreement which sets forth certain restrictions and
limitations on the Shares, including the ability to transfer or sell them in the future. I further acknowledge and agree that to the extent the terms of Section 12 of the Stock Option Agreement conflict with either the ROFR Agreement or the
Voting Agreement, the terms contained in the ROFR Agreement and Voting Agreement shall govern. 
 I have considered the Federal, state and
local income tax implications of the exercise of my Option and the purchase and subsequent sale of the Shares. 
 I am paying the option
exercise price for the Shares as follows: 
  

 
 Please issue
the Shares (check one): 
 ☐ to me; or 

☐ to me and
                                    , as joint tenants with
right of survivorship 
 and mail the certificate to me at the following address: 
  

	
	  

	  

	  

  
 Exhibit A-2 

 My mailing address for shareholder communications, if different from the address listed above is:

  

	
	  

	  

	  

  

	
	Very truly yours,
	
	  

	Participant (signature)
	
	  

	Print Name
	
	  

	Date
	
	  

	Social Security Number

  
 Exhibit A-3 

 Exhibit B 

NOTICE OF EXERCISE OF STOCK OPTION 

[Form for Shares Registered in the United States] 

To:        Spero Therapeutics, Inc. 

IMPORTANT NOTICE: This form of Notice of Exercise may only be used at such time as the Company has filed a Registration Statement with the Securities
and Exchange Commission under which the issuance of the Shares for which this exercise is being made is registered and such Registration Statement remains effective. 

Ladies and Gentlemen: 
 I hereby exercise my
Stock Option to purchase                      shares (the “Shares”) of the common stock, $0.001 par value, of Spero Therapeutics,
Inc. (the “Company”), at the exercise price of $                     per share, pursuant to and subject to the terms of that
Stock Option Grant Notice dated                             , 2017. 

I understand the nature of the investment I am making and the financial risks thereof. I am aware that it is my responsibility to have
consulted with competent tax and legal advisors about the relevant national, state and local income tax and securities laws affecting the exercise of the Option and the purchase and subsequent sale of the Shares. 

I am paying the option exercise price for the Shares as follows: 

 
  

Please issue the Shares (check one): 

☐ to me; or 
 ☐ to me
and
                                         
   , as joint tenants with right of survivorship, 
 at the following address: 

 
  

 
  

 
  

  
 Exhibit B-1 

 My mailing address for shareholder communications, if different from the address listed above,
is: 
  

	
	  

	  

	  

  

	
	Very truly yours,
	
	  

	 Participant (signature)

	
	  

	 Print Name

	
	  

	 Date

	
	  

	 Social Security Number

  

  
 Exhibit B-2EX-10.3

 Exhibit 10.3 

RESTRICTED STOCK AGREEMENT 

SPERO THERAPEUTICS, INC. 

AGREEMENT made as of the [            ] day of
[                        ], 2017 (the “Grant Date”), between Spero Therapeutics, Inc. (the
“Company”), a Delaware corporation, and [                            ] (the
“Participant”). 
 WHEREAS, the Company has adopted the 2017 Stock Incentive Plan (the “Plan”) to promote
the interests of the Company by providing an incentive for Employees, Directors and Consultants of the Company or its Affiliates; 

WHEREAS, pursuant to the provisions of the Plan, the Company desires to offer to the Participant shares of the Company’s common stock,
$0.001 par value per share (“Common Stock”), in accordance with the provisions of the Plan, all on the terms and conditions hereinafter set forth; 

WHEREAS, the Participant wishes to accept said offer; and 

WHEREAS, the parties hereto understand and agree that any terms used and not defined herein have the meanings ascribed to such terms in the
Plan. 
 NOW, THEREFORE, in consideration of the premises and the mutual covenants contained herein and for other good and valuable
consideration, the receipt and sufficiency of which are hereby acknowledged, the parties hereby agree as follows: 
 1. Terms of
Grant. The Participant hereby accepts the offer of the Company to issue to the Participant, in accordance with the terms of the Plan and this Agreement,
[                            
(                    )] shares of the Company’s Common Stock (such shares, subject to adjustment pursuant to Section 24 of the Plan
and Subsection 2.1(h) hereof, the “Granted Shares”) at a per share purchase price of $0.001 (the “Purchase Price”), receipt of which is hereby acknowledged by the Company [by the Participant’s prior service
to the Company and which amount will be reported as income on the Participant’s W-2 [or 1099] for this calendar year]. 

2. Restrictions on Granted Stock. 

2.1. Lapsing Forfeiture Right. 

(a) Lapsing Forfeiture Right. Except as set forth in Subsections 2.1(b), (c), and (d) hereof, in the event that for any reason the
Participant is no longer an Employee, Director or Consultant of the Company or an Affiliate (the “Termination”) prior to [INSERT THE DATE OF THE END OF THE VESTING PERIOD], the Participant (or the Participant’s Survivor) shall,
on the date of Termination, immediately forfeit to the Company (or its designee) all of the Granted Shares that are then unvested in accordance with the schedule set forth below (the “Lapsing Forfeiture Right”): 

 [Insert Lapsing Forfeiture Right (vesting schedule) – sample below.] 

(i) If the Participant’s Termination is prior to the one-year anniversary of the
Grant Date, all of the Granted Shares shall be forfeited to the Company. 
 (ii) If the Participant’s Termination is on
or after the one-year anniversary of the Grant Date, but prior to
[                                ,
20        ], [        ]% of the Granted Shares shall be forfeited to the Company (rounded up to the next highest whole number of shares). 

(b) Effect of Termination for Disability or upon Death. The following rules apply to the Company’s Lapsing Forfeiture Right if the
Participant’s Termination is by reason of Disability or death: to the extent the Company’s Lapsing Forfeiture Right has not lapsed as of the date of Termination due to Disability or death, as the case may be, the Participant shall forfeit
to the Company any or all of the Granted Shares subject to such Lapsing Forfeiture Right; provided, however, that the Company’s Lapsing Forfeiture Right shall be deemed to have lapsed to the extent of a pro rata portion of the Granted Shares
through the date of Termination due to Disability or death, as would have lapsed had the Participant not been terminated due to Disability or death, as the case may be. The proration shall be based upon the number of days accrued in such current
vesting period prior to the Participant’s date of Termination due to Disability or death, as the case may be. In the case of death all Granted Shares which are no longer subject to the Company’s Lapsing Forfeiture Right shall be issued to
the Participant’s Survivor. 
 (c) Effect of a For Cause Termination. Notwithstanding anything to the contrary contained in this
Agreement, in the event of the Participant’s Termination for Cause or in the event the Administrator determines, within one year after the Participant’s Termination, that either prior or subsequent to the Participant’s Termination the
Participant engaged in conduct that would constitute Cause, all of the unvested Shares then held by the Participant shall be forfeited to the Company immediately as of the time the Participant is notified that he or she has been terminated for Cause
or that he or she engaged in conduct which would constitute Cause. 
 [(d) Effect of Change of Control. In the event the
Participant’s employment is terminated by the Company without Cause within the 30-day period prior to a Change of Control (as defined below) or within the twelve (12) month period following a Change
of Control, (i) if the Participant’s employment with the Company commenced at least 24 months prior to the Change of Control, then the Participant’s ownership of 100% of the Granted Shares then owned by the Participant shall be deemed
vested and the Company’s Lapsing Forfeiture Right shall terminate with respect to all of the Granted Shares; (ii) if the Participant’s employment with the Company commenced less than 24 months but at least 12 months prior to the
Change of Control, then the Participant’s ownership of 50% of the Granted Shares then subject to the Company’s Lapsing Forfeiture Right shall become vested and the Company’s Lapsing Forfeiture Right shall terminate with respect to
such Granted Shares on the date of Termination and the Participant shall forfeit to the Company the remaining Granted Shares then subject to the Lapsing Forfeiture Right on the Termination; and (iii) if the Participant’s employment with
the Company commenced less 12 months prior to the Change of Control, then the Participant’s ownership of 25% of 

  
 2 

 
the Granted Shares then subject to the Company’s Lapsing Forfeiture Right shall become vested and the Company’s Lapsing Forfeiture Right shall terminate with respect to such Granted
Shares on the date of Termination and the Participant shall forfeit to the Company the remaining Granted Shares then subject to the Lapsing Forfeiture Right on the Termination. 

Cause” shall mean (i) dishonest statements or acts by the Participant with respect to the Company or any Affiliate of the
Company, or any of the Company’s current or prospective customers, suppliers vendors or other third parties with which such entity does business; (ii) the Participant’s commission of (A) a felony or (B) any misdemeanor
involving moral turpitude, deceit, dishonesty or fraud; (iii) the Participant’s failure to perform to the reasonable satisfaction of the Board of Directors the duties and responsibilities assigned by the Board of Directors which failure
continues, in the reasonable judgment of the Board of Directors, after written notice given to the Participant by the Board of Directors; (iv) gross negligence, willful misconduct or insubordination by the Participant with respect to the
Company or any Affiliate of the Company; or (v) a violation of any provision of any agreement(s) between the Participant and the Company relating to noncompetition, nondisclosure and/or assignment of inventions. 

For purposes of this subsection 2.1(d), “Change of Control” means the consummation of (i) the dissolution or liquidation of
the Company, (ii) the sale of all or substantially all of the assets of the Company on a consolidated basis to an unrelated person or entity, (iii) a merger, reorganization or consolidation involving the Company in which the shares of
Company voting equity outstanding immediately prior to such transaction represent or are converted into or exchanged for securities of the surviving or resulting entity immediately upon completion of such transaction which represent less than fifty
percent (50%) of the outstanding voting power of such surviving or resulting entity, (iv) the acquisition of all or a majority of the outstanding voting equity of the Company in a single transaction or a series of related transactions by a
person or group of persons, or (v) any other acquisition of the business of the Company, as determined by the Board of Directors; provided, however, that the Company’s initial public offering, any subsequent public offering or another
capital raising event, or a merger effected solely to change the Company’s domicile shall not constitute a “Change of Control.”]1 

(e) Escrow. The Granted Shares issued to the Participant hereunder which from time to time are subject to the Lapsing Forfeiture Right
shall be held in escrow by the Company as provided in this Subsection 2.1(e). Promptly following receipt by the Company of a written request from the Participant, the Company shall release from escrow and deliver to the Participant the Granted
Shares, if any, as to which the Company’s Lapsing Forfeiture Right has lapsed. In the event of forfeiture to the Company of Granted Shares subject to the Lapsing Forfeiture Right, the Company shall release from escrow and cancel the number of
Granted Shares so forfeited. Any cash or securities distributed in respect of the Granted Shares held in escrow, including, without limitation, ordinary cash dividends or shares issued as a result of stock splits, stock dividends or other
recapitalizations (“Retained Distributions”), shall also be 
  

	1 	 Note: Double Trigger acceleration clause applicable only to certain executives of the Company.

  
 3 

 
held in escrow in the same manner as the Granted Shares and all Retained Distributions shall be forfeited to the Company or released from escrow and delivered to the Participant, as the case may
be, at such time and in such manner as the Granted Shares to which such Retained Distributions so relate. All ordinary cash dividends retained hereunder shall, during the period in which such dividends are retained by the Company, be deposited into
an account at a financial institution selected by the Company, which shall not be required to bear interest or be segregated in a separate account. 

(f) Prohibition on Transfer. The Participant recognizes and agrees that all Granted Shares and Retained Distributions which are subject
to the Lapsing Forfeiture Right may not be sold, transferred, assigned, hypothecated, pledged, encumbered or otherwise disposed of, whether voluntarily or by operation of law, other than to the Company (or its designee). However, the Participant,
with the approval of the Administrator, may transfer the Granted Shares and Retained Distributions for no consideration to or for the benefit of the Participant’s Immediate Family (including, without limitation, to a trust for the benefit of
the Participant’s Immediate Family or to a partnership or limited liability company for one or more members of the Participant’s Immediate Family), subject to such limits as the Administrator may establish, and the transferee shall remain
subject to all the terms and conditions applicable to this Agreement prior to such transfer and each such transferee shall so acknowledge in writing as a condition precedent to the effectiveness of such transfer. The term “Immediate
Family” shall mean the Participant’s spouse, former spouse, parents, children, stepchildren, adoptive relationships, sisters, brothers, nieces and nephews and grandchildren (and, for this purpose, shall also include the Participant). The Company shall not be required to transfer any Granted Shares or Retained Distributions on its books which shall have been sold, assigned or otherwise transferred in violation of this Subsection
2.1(f), or to treat as the owner of such Granted Shares or Retained Distributions, or to accord the right to vote as such owner or to pay dividends to, any person or organization to which any such Granted Shares or Retained Distributions shall have
been so sold, assigned or otherwise transferred, in violation of this Subsection 2.1(f). 
 (g) Failure to Deliver Granted Shares to be
Forfeited. In the event that the Granted Shares to be forfeited to the Company under this Agreement are not in the Company’s possession pursuant to Subsection 2.1(e) above or otherwise and the Participant or the Participant’s Survivor
fails to deliver such Granted Shares to the Company (or its designee), the Company may immediately take such action as is appropriate to transfer record title of such Granted Shares from the Participant to the Company (or its designee) and to treat
the Participant and such Granted Shares in all respects as if delivery of such Granted Shares had been made as required by this Agreement. The Participant hereby irrevocably grants the Company a power of attorney which shall be coupled with an
interest for the purpose of effectuating the preceding sentence. 
 (h) Adjustments. The Plan contains provisions covering the
treatment of Shares in a number of contingencies such as stock splits and mergers. Provisions in the Plan for adjustment with respect to the Shares and the related provisions with respect to successors to the business of the Company are hereby made
applicable hereunder and are incorporated herein by reference. 

  
 4 

 2.2 General Restrictions on Transfer of Granted Shares; Requirements for Vested Shares.

 (a) Limitations on Transfer and Requirements. In addition to the restrictions set forth above in Section 2.1, the Granted
Shares issued to the Participant hereunder and no longer subject to the Lapsing Forfeiture Right described in Section 2.1 (the “Vested Shares”) shall not be transferred by the Participant except as permitted herein, shall be
subject to the provisions of Sections 2.1(e), (f) and (g) above, and shall be subject to the repurchase rights and obligations described in this Section 2.2. With respect to the Vested Shares, Participant hereby agrees to the following:

 (i) Participant shall no later than the date hereof become a party to the Right of First Refusal and Co-Sale Agreement dated June [    ], 2017 between the Company and its stockholders, as may be amended from time to time (the “ROFR Agreement”), by execution of a signature
page thereto. If the terms of this Agreement and the ROFR Agreement conflict, the terms contained in the ROFR Agreement shall govern and supersede any conflicting provision contained in this Section 2.2. 

(ii) Participant shall no later than the date hereof become a party to a Voting Agreement dated June
[    ], 2017, between the Company and its stockholders, as may be amended from time to time (the “Voting Agreement”), by executing a signature page thereto. If the terms of this Agreement and the Voting
Agreement conflict, the terms contained in the Voting Agreement shall govern and supersede any conflicting provision contained in this Section 2.2. 

(b) Right to Repurchase following Termination of Service. In the event of the Participant’s Termination for any reason, including
due to death or Disability, then the Company shall have the option to repurchase the Vested Shares not previously forfeited to the Company in accordance with the provisions of Section 2.1 of this Agreement as follows: 

(i) The Company’s option to repurchase the Vested Shares in the event of Termination under this Section 2.2(b) shall
be valid for a period of one year commencing with the date of such Termination. 
 (ii) In the event the Company shall
be entitled to and shall elect to exercise its option to repurchase the Vested Shares under this Section 2.2(b), the Company shall notify the Participant, or in case of death, the Participant’s Survivor, in writing of its intent to
repurchase the Vested Shares. Such written notice may be mailed by the Company up to and including the last day of the time period provided for in Section 2.2(b)(i) for exercise of the Company’s option to repurchase. 

(iii) The written notice to the Participant shall specify the address at, and the time and date on, which payment of the
Repurchase Price (as hereinafter defined) is to be made (the “Post-Termination Repurchase Closing”). The date specified shall not be less than ten days nor more than 60 days from the date of the mailing of the notice, and the
Participant or the Participant’s Survivor with respect to the Vested Shares shall 

  
 5 

 
have no further rights as the owner thereof from and after the date specified in the notice. At the Post-Termination Repurchase Closing, the Repurchase Price shall be delivered to the Participant
or the Participant’s Survivor and the Vested Shares being purchased, duly endorsed for transfer, shall, to the extent that they are not then in the possession of the Company, be delivered to the Company by the Participant or the
Participant’s Survivor. 
 (iv) The price paid per share for any Vested Shares repurchased hereunder (the
“Repurchase Price”) shall equal the Fair Market Value of such Vested Shares determined in accordance with the Plan as of the date of Termination. 

(c) Right to Repurchase on Proposed Transfer. It shall be a condition precedent to the validity of any sale or other transfer of any
Vested Shares by the Participant that the following restrictions be complied with (except as hereinafter otherwise provided): 

(i) No Vested Shares owned by the Participant may be sold, pledged or otherwise transferred (including by gift or devise) to
any person or entity, voluntarily, or by operation of law, except in accordance with the terms and conditions hereinafter set forth. 

(ii) Before selling or otherwise transferring all or part of the Vested Shares, the Participant shall give written notice of
such intention to the Company in accordance with Section 2.1(b) of the ROFR Agreement. Such notice shall constitute a binding offer by the Participant to sell to the Company such number of the Vested Shares as if such Vested Shares were Key
Holder Transfer Stock (as defined in the ROFR Agreement). If the Company shall fail to accept any such offer, such Vested Shares shall be subject to a Secondary Refusal Right, as defined in Section 1.24 of the ROFR Agreement. 

(iv) If the Company and any Senior Preferred Investor (as defined in the ROFR Agreement), as the case may be, shall fail to
accept any such offer, the Participant shall be free to sell all, but not less than all, of the Vested Shares set forth in his or her notice to the designated transferee at the price and terms designated in the Participant’s notice, provided
that (a) such sale is consummated within six months after the giving of notice by the Participant to the Company as aforesaid, and (b) the transferee first agrees in writing to be bound by the provisions of this Agreement so that he or she
(and all subsequent transferees) shall thereafter only be permitted to sell or transfer the Vested Shares in accordance with the terms of this Agreement. After the expiration of such six months, the provisions of this Section 2.2(c) shall again
apply with respect to any proposed voluntary transfer of the Vested Shares. 
 (iv) The provisions of this
Section 2.2(c) may be waived by the Company. Any such waiver may be unconditional or based upon such conditions as the Company may impose. 

(v) The restrictions on transfer contained in this Section 2.2(c) shall not apply to (a) transfers by the Participant
to his or her spouse or children or to a trust 

  
 6 

 
for the benefit of his or her spouse or children, (b) transfers by the Participant to his or her guardian or conservator, or (c) transfers by the Participant, in the event of his or her
death, to his or her executor(s) or administrator(s) or to trustee(s) under his or her will (collectively, “Permitted Transferees”); provided however, that in any such event the Vested Shares so transferred in the hands of each such
Permitted Transferee shall remain subject to this Agreement, and each such Permitted Transferee shall so acknowledge in writing as a condition precedent to the effectiveness of such transfer. 

(d) The provisions of Sections 2.2(a) through (c) shall terminate upon the effective date of the registration of the Common Stock pursuant
to the Securities and Exchange Act of 1934, as amended. 
 (e) The Participant agrees that in the event the Company proposes to offer for
sale to the public any of its equity securities and such Participant is requested by the Company and any underwriter engaged by the Company in connection with such offering to sign an agreement restricting the sale or other transfer of Shares, then
it will promptly sign such agreement and will not transfer, whether in privately negotiated transactions or to the public in open market transactions or otherwise, any Shares or other securities of the Company held by him or her during such period
as is determined by the Company and the underwriters, not to exceed 180 days following the closing of the offering, plus such additional period of time as may be required to comply FINRA rules or similar rules thereto promulgated by another
regulatory authority (such period, the “Lock-Up Period”). Such agreement shall be in writing and in form and substance reasonably satisfactory to the Company and such underwriter and pursuant
to customary and prevailing terms and conditions. Notwithstanding whether the Participant has signed such an agreement, the Company may impose stop-transfer instructions with respect to the Shares or other securities of the Company subject to the
foregoing restrictions until the end of the Lock-Up Period. 
 (f) The Participant acknowledges and
agrees that neither the Company, its shareholders nor its directors and officers, has any duty or obligation to disclose to the Participant any material information regarding the business of the Company or affecting the value of the Shares before,
at the time of, or following the Participant’s Termination, including, without limitation, any information concerning plans for the Company to make a public offering of its securities or to be acquired by or merged with or into another firm or
entity. 
 3. Purchase for Investment; Securities Law Compliance. The Participant hereby represents and warrants that he or she is
acquiring the Granted Shares for his or her own account, for investment, and not with a view to, or for sale in connection with, the distribution of any such Granted Shares. The Participant specifically acknowledges and agrees that any sales of
Granted Shares shall be made in accordance with the requirements of the Securities Act, in a transaction as to which the Company shall have received an opinion of counsel satisfactory to it confirming such compliance. The Participant shall be bound
by the provisions of the following legend which shall be endorsed upon the certificate(s) evidencing the Granted Shares: 
 “THE SHARES
REPRESENTED BY THIS CERTIFICATE HAVE BEEN TAKEN FOR INVESTMENT AND THEY MAY NOT BE SOLD OR OTHERWISE 

  
 7 

 
TRANSFERRED BY ANY PERSON, INCLUDING A PLEDGEE, UNLESS (1) EITHER (A) A REGISTRATION STATEMENT WITH RESPECT TO SUCH SHARES SHALL BE EFFECTIVE UNDER THE SECURITIES ACT OF 1933, AS AMENDED, OR
(B) THE COMPANY SHALL HAVE RECEIVED AN OPINION OF COUNSEL SATISFACTORY TO IT THAT AN EXEMPTION FROM REGISTRATION UNDER SUCH ACT IS THEN AVAILABLE, AND (2) THERE SHALL HAVE BEEN COMPLIANCE WITH ALL APPLICABLE STATE SECURITIES LAWS.”

 4. Legend. In addition to any legend required pursuant to the Plan, all certificates representing the Granted Shares issued to the
Participant pursuant to this Agreement shall have endorsed thereon a legend substantially as follows: 
 “THE SHARES REPRESENTED BY THIS
CERTIFICATE ARE SUBJECT TO RESTRICTIONS SET FORTH IN A RESTRICTED STOCK AGREEMENT DATED AS OF
[                            ], 2017 WITH THIS COMPANY, A COPY OF WHICH AGREEMENT IS AVAILABLE
FOR INSPECTION AT THE OFFICES OF THE COMPANY OR WILL BE MADE AVAILABLE UPON REQUEST.” 
 5. Rights as a Stockholder. The
Participant shall have all the rights of a stockholder with respect to the Granted Shares, including voting and dividend rights, subject to the transfer and other restrictions set forth herein, including pursuant to Section 2.1(f) hereof and in
the Plan. 
 6. Incorporation of the Plan. The Participant specifically understands and agrees that the Granted Shares issued under
the Plan are being sold to the Participant pursuant to the Plan, a copy of which Plan the Participant acknowledges he or she has read and understands and by which Plan he or she agrees to be bound. The provisions of the Plan are incorporated herein
by reference. 
 7. Tax Liability of the Participant and Payment of Taxes. The Participant acknowledges and agrees that any income or
other taxes due from the Participant with respect to the Granted Shares issued pursuant to this Agreement, including, without limitation, the Lapsing Forfeiture Right, shall be the Participant’s responsibility. Without limiting the foregoing,
the Participant agrees that, to the extent that the lapsing of restrictions on disposition of any of the Granted Shares or the declaration of dividends on any such shares before the lapse of such restrictions on disposition results in the
Participant’s being deemed to be in receipt of earned income under the provisions of the Code, the Company shall be entitled to immediate payment from the Participant of the amount of any tax required to be withheld by the Company. 

Upon execution of this Agreement, the Participant may file an election under Section 83 of the Code in substantially the form attached as
Exhibit A. The Participant acknowledges that if he or she does not file such an election, as the Granted Shares are released from the Lapsing Forfeiture Right in accordance with Section 2.1, the Participant will have income for tax
purposes equal to the Fair Market Value of the Granted Shares at such date, less the price paid for the Granted Shares by the Participant. The Participant has been given the opportunity to 

  
 8 

 
obtain the advice of his or her tax advisors with respect to the tax consequences of the purchase of the Granted Shares and the provisions of this Agreement. 

If the Participant has not filed an election under Section 83 of the Code, the Participant shall be required to deposit with the Company
an amount of cash equal to the amount determined by the Company to be required with respect to the statutory minimum of the Participant’s estimated total federal, state and local tax obligations associated with the Granted Shares. 

8. Equitable Relief. The Participant specifically acknowledges and agrees that in the event of a breach or threatened breach of the
provisions of this Agreement or the Plan, including the attempted transfer of the Granted Shares by the Participant in violation of this Agreement, monetary damages may not be adequate to compensate the Company, and, therefore, in the event of such
a breach or threatened breach, in addition to any right to damages, the Company shall be entitled to equitable relief in any court having competent jurisdiction. Nothing herein shall be construed as prohibiting the Company from pursuing any other
remedies available to it for any such breach or threatened breach. 
 9. No Obligation to Maintain Relationship. The Participant
acknowledges that: (i) the Company is not by the Plan or this Agreement obligated to continue the Participant as an Employee, Director or Consultant of the Company or an Affiliate; (ii) the Plan is discretionary in nature and may be
suspended or terminated by the Company at any time; (iii) the grant of the Granted Shares is a one-time benefit which does not create any contractual or other right to receive future grants of Shares, or
benefits in lieu of Shares; (iv) all determinations with respect to any such future grants, including, but not limited to, the times when Shares shall be granted, the number of Shares to be granted, the purchase price, and the time or times
when each Share shall be free from a lapsing repurchase or forfeiture right, will be at the sole discretion of the Company; (v) the Participant’s participation in the Plan is voluntary; (vi) the value of the Granted Shares is an
extraordinary item of compensation which is outside the scope of the Participant’s employment contract, if any; and (vii) the Granted Shares are not part of normal or expected compensation for purposes of calculating any severance,
resignation, redundancy, end of service payments, bonuses, long-service awards, pension or retirement benefits or similar payments. 
 10.
Notices. Any notices required or permitted by the terms of this Agreement or the Plan shall be given by recognized courier service, facsimile, registered or certified mail, return receipt requested, addressed as follows: 

If to the Company: 
 Spero
Therapeutics, Inc. 
 675 Massachusetts Avenue 

Cambridge, MA 02139 
 Attn:
[                        ] 

  
 9 

 If to the Participant: 

[                       
             ] 

[                       
             ] 

[                       
             ] 
 Attn:
[                           ] 

or to such other address or addresses of which notice in the same manner has previously been given. Any such notice shall be deemed to have been given on the
earliest of receipt, one business day following delivery by the sender to a recognized courier service, or three business days following mailing by registered or certified mail. 

11. Benefit of Agreement. Subject to the provisions of the Plan and the other provisions hereof, this Agreement shall be for the benefit
of and shall be binding upon the heirs, executors, administrators, successors and assigns of the parties hereto. 
 12. Governing Law.
This Agreement shall be construed and enforced in accordance with the laws of the State of Delaware, without giving effect to the conflict of law principles thereof. For the purpose of litigating any dispute that arises under this Agreement, whether
at law or in equity, the parties hereby consent to exclusive jurisdiction in the Commonwealth of Massachusetts and agree that such litigation shall be conducted in the state courts of Suffolk County, Massachusetts, or the federal courts of the
United States for the District of Massachusetts. 
 13. Severability. If any provision of this Agreement is held to be invalid or
unenforceable by a court of competent jurisdiction, then such provision or provisions shall be modified to the extent necessary to make such provision valid and enforceable, and to the extent that this is impossible, then such provision shall be
deemed to be excised from this Agreement, and the validity, legality and enforceability of the rest of this Agreement shall not be affected thereby. 

14. Entire Agreement. This Agreement, together with the Plan, the ROFR Agreement and the Voting Agreement, constitutes the entire
agreement and understanding between the parties hereto with respect to the subject matter hereof and supersedes all prior oral or written agreements and understandings relating to the subject matter hereof. No statement, representation, warranty,
covenant or agreement not expressly set forth in this Agreement shall affect or be used to interpret, change or restrict the express terms and provisions of this Agreement provided, however, in any event, this Agreement shall be subject to and
governed by the Plan. 
 15. Modifications and Amendments; Waivers and Consents. The terms and provisions of this Agreement may be
modified or amended as provided in the Plan. Except as provided in the Plan, the terms and provisions of this Agreement may be waived, or consent for the departure therefrom granted, only by written document executed by the party entitled to the
benefits of such terms or provisions. No such waiver or consent shall be deemed to be or shall constitute a waiver or consent with respect to any other terms or provisions of this Agreement, 

  
 10 

 
whether or not similar. Each such waiver or consent shall be effective only in the specific instance and for the purpose for which it was given, and shall not constitute a continuing waiver or
consent. 
 16. Counterparts. This Agreement may be executed in one or more counterparts, and by different parties hereto on separate
counterparts, each of which shall be deemed an original, but all of which together shall constitute one and the same instrument. 
 17.
Data Privacy. By entering into this Agreement, the Participant: (i) authorizes the Company and each Affiliate, and any agent of the Company or any Affiliate administering the Plan or providing Plan recordkeeping services, to disclose to
the Company or any of its Affiliates such information and data as the Company or any such Affiliate shall request in order to facilitate the grant of Shares and the administration of the Plan; and (ii) authorizes the Company and each Affiliate
to store and transmit such information in electronic form for the purposes set forth in this Agreement. 
 [THE NEXT PAGE IS THE SIGNATURE
PAGE] 

  
 11 

 IN WITNESS WHEREOF, the parties hereto have executed this Agreement as of the day and year first
above written. 
  

			
	SPERO THERAPEUTICS, INC.
		
	By:	 	                                      
                                         
 
	Name:
	Title:
	
	PARTICIPANT:
	
	  

	Print name:

 IF YOU WISH TO MAKE A SECTION 83(B) ELECTION, THE FILING OF SUCH ELECTION IS
YOUR RESPONSIBILITY. 
 THE FORM FOR MAKING THIS SECTION 83(B) ELECTION IS ATTACHED TO THIS AGREEMENT AS
EXHIBIT A. 
 YOU MUST FILE THIS FORM WITHIN 30 DAYS OF PURCHASING THE SHARES. 

IT IS YOUR RESPONSIBILITY TO FILE THIS FORM WITH THE IRS, REGARDLESS OF WHETHER YOU ASK THE COMPANY OR ITS AGENTS TO MAKE THIS FILING ON YOUR BEHALF, EVEN
IF THE COMPANY OR ITS AGENTS HAVE PREVIOUSLY MADE THIS FILING ON YOUR BEHALF. 
 The election should be filed by mailing a signed election form by
certified mail, return receipt requested to the IRS Service Center where you file your tax returns. See https://www.irs.gov/uac/where-to-file-your-tax-returns-with-or-without-a-payment 

  
 B-1 

 EXHIBIT A 

Election to Include Gross Income in Year 

of Transfer Pursuant to Section 83(b) 

of the Internal Revenue Code of 1986, as amended 

In accordance with Section 83(b) of the Internal Revenue Code of 1986, as amended (the “Code”), the undersigned hereby elects to include in his
gross income as compensation for services the excess, if any, of the fair market value of the property (described below) at the time of transfer over the amount paid for such property. 

The following sets forth the information required in accordance with the Code and the regulations promulgated thereunder: 

 

	1.	The name, address and social security number of the undersigned and the taxable year for this this election is being made are: 

  

											
		 	Name:	 	  
	 		 		  	
		 	Address:	 	  
	 		 		  	
		 		 	  
	 		 		  	
		 	Social Security No.:	 	  
	 		 		  	
		 	Taxable Year:	 	  
	 		 		  	

  

	2.	The description of the property with respect to which the election is being made is as follows: 

                    
(            ) shares (the “Shares”) of Common Stock, $0.001 par value per share, of Spero Therapeutics, Inc., a Delaware corporation (the “Company”). 

 

	3.	The property was transferred to the taxpayer on                     , 2017. 

 

	4.	The property is subject to the following restrictions: 

 In the event the taxpayer’s
service with the Company or an Affiliate is terminated (the “Termination”), the taxpayer shall forfeit all or a portion of the Shares. 
  

	5.	The fair market value of the property at the time of transfer (determined without regard to any restrictions other than a non-lapse restriction as defined in § 1.83-3(h) of the Income Tax Regulations) is $                    
($[                    ] per Share multiplied by
[                    ] Shares). 

  

	6.	The amount paid by taxpayer for said property was $                    
($[                    ] per Share multiplied by
[                    ] Shares). 

  

	7.	The amount to include in gross income is $[                    ] [The amount reported in Item 5 minus the
amount reported in Item 6]. 

 The undersigned taxpayer will file this election with the Internal Revenue Service office
with which taxpayer files his or her annual income tax return not later than 30 days after the date of transfer of the property. A copy of the election also will be furnished to the person for whom the services were performed. The undersigned is the
person performing the services in connection with which the property was transferred. 
 Signed this
                     day of
                    , 2017. 
  

	
	  
 Print Name:

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