Document:

EX-4.23

 Exhibit 4.23 

EMPLOYMENT AGREEMENT 

THIS AGREEMENT is made as of 30 July 2018, between Celyad Inc., a Delaware corporation (the “Corporation”), and Filippo Petti
(the “Employee”). 
 Introduction 

The Corporation is engaged in research and development of biological pharmaceutical products or medical devices, solely or in combination (the
“Business”). 
 The Corporation is a wholly owned subsidiary of Celyad SA, a publicly listed company on Euronext Brussels,
Euronext Paris and Nasdaq, with registered offices in Mont-Saint-Guibert, Rue Edouard Belin 2, Belgium (“Celyad”); 
 Celyad and
its subsidiaries and affiliates, including the Corporation, comprise the “Celyad Group”; 
 The Corporation wishes to retain the
services of Employee at its offices in Boston, Massachusetts (the “Corporate Office”). 
 NOW, THEREFORE, for good and valuable
consideration, the receipt and sufficiency of which are hereby acknowledged, the parties agree as follows: 
 1. Employment. As of
the Employment Date, defined below, the Employee will be employed by the Corporation. The Corporation and the Employee acknowledge and accept the Employee’s employment upon the terms and conditions hereinafter set forth. Notwithstanding
anything in this Agreement to the contrary, nothing in this Agreement shall be construed to alter the at-will nature of the Employee’s employment, nor shall anything in this Agreement or any benefit
program be construed as providing the Employee with a definite term of employment. 
 2. Term. Unless otherwise agreed by the parties
in writing, Employee’s employment shall commence on 3 Sept 2018 (the “Employment Date”). 
 3. Duties. 

(a) Employee initially shall serve as Chief Financial Officer (CFO) for the Corporation reporting directly to the Chief Executive Officer
(CEO) (the “Employee Supervisor”). The Employee’s reporting relationship may be changed by the Corporation. 
 (b) The
Employee shall be responsible for such duties as may be reasonably assigned from time to time by the Employee Supervisor or other authorized designee of the Corporation, inclusive of investor relations and business development (collectively, the
“Services”). 
 4. Compensation. 

(a) In consideration for the Services, the Corporation shall pay Employee a salary at the annual rate of three hundred twenty thousand dollars
($320,000.00) (“Base Salary”). The Base Salary may be adjusted from time to time by the Corporation. Payments of Base Salary shall be made in accordance with the Corporation’s payroll practices. 

(b) The Corporation will pay a sign on bonus of one hundred thousand dollars ($100,000.00) to the Employee (the “Signing Bonus”) on
the Corporation’s first payroll date applicable to the Employee’s position 

 
following the Employment Date, subject to the Employee’s continued employment with the Corporation through the date the Signing Bonus is paid. If the Employee resigns (i) during the
first four (4) months of employment (between 3rd Sep 2018 and 2nd Jan 2019), he will reimburse the Corporation for the full amount of the Signing Bonus; (ii) during the next four (4) months of employment (between 3rd Jan 2019 and 2nd
May 2019), he will reimburse the Corporation for 50% of the amount of the Signing Bonus and (iii) during the following four (4) months of employment (between 3rd May 2019 and 2nd Sept 2019), he will reimburse the Corporation for 25% of the
amount of the bonus (in any such case ((i), (ii), (iii)), the “Signing Bonus Repayment”). The Employee shall make the Signing Bonus Repayment within the 30-day period following the date of the
Employee’s termination of employment. 
 (c) Employee shall be eligible for a target annual bonus equal to 35% of the Base Salary if
Employee achieves performance milestones defined at or reasonably promptly after the beginning of each year by the Corporation in good faith and in its sole discretion. Notwithstanding anything to the contrary in this Section 4(c), whether a
bonus is awarded, the bonus target and the amount of any bonus shall be determined by the Corporation and/or Celyad in their sole discretion. To earn any bonus, Employee must be employed by the Corporation on the date the bonus is paid. For the
avoidance of doubt, Employee shall not be eligible for any pro rata bonus in connection with the Employee’s termination for any reason. 

(d) Subject to the approval of the Boards of Directors of Celyad and/or the Corporation as applicable (the “Boards”), the Employee
may be eligible to participate in Celyad’s warrant plans (or similar long term incentives) in effect from time to time. For the avoidance of doubt, Employee understands that under Belgian law, stock warrant plans are proposed by the Celyad
Board and approved by an Extraordinary General Meeting of Celyad’s shareholders and the amount of warrants allocated to employees under an approved warrant plan is determined exclusively by the Remuneration and Compensation Committee of the
Celyad Board. Subject to any applicable Board approval, a sign-up award of 20,000 (twenty thousand) warrants to Employee will be granted under the current plan, subject to vesting conditions and the other
terms and conditions of the applicable plans and option agreement(s). 
 (e) All compensation paid to Employee shall be subject to taxes and
other withholdings. 
 5. Expenses and Travel. 

Employee shall be entitled to receive prompt reimbursement for all reasonable, documented expenses incurred by Employee in performing the
Services hereunder, including all reasonable expenses of travel and living while away from home, provided that such expenses are incurred and accounted for in accordance with the policies and procedures established by the Corporation. 

6. Medical, Vacation and Other Benefits. 

Employee shall be entitled to receive certain benefits applicable to employees of Celyad Group, which currently include dental and health care
plans, in each case in accordance with the terms of such plans. In the event that Employee elects not to receive medical benefits from the Corporation, Employee may be eligible to receive a monthly cash payment, as determined by the Corporation in
its discretion, in an amount equal to the premium the Corporation would have contributed toward individual medical insurance coverage for Employee. 

Employee shall receive twenty (20) days of vacation annually, in addition to all legal U.S. Federal holidays, both paid at the expense of
the Corporation. Such vacation shall be subject to Corporation policy in all respects. In the absence of such policy, such vacation shall accrue ratably and shall not roll over from year to year. 

Employee shall be eligible for the Corporation’s 401(k) plan on the first day of the first month following Employee’s first day of
employment. Currently, on an annual basis, the Corporation pays, at no additional cost to the Employee, a contribution equivalent to five percent (5%) of the Employee’s Base Salary, subject to Employee’s employment with the Corporation on
the date such contribution is made. 

 Notwithstanding this foregoing Section 6, any member of the Celyad Group may alter the
terms and conditions of any employee benefit plan, program or agreement, or eliminate any such plan, program or agreement, at any time in such entity’s discretion. 

7. Performance of Services. During the term of this Agreement, Employee shall use Employee’s best efforts to promote the interests
of the Celyad Group and shall devote Employee’s full time and efforts to its Business and affairs in an honest and ethical manner in compliance with this Agreement and all applicable laws, rules and regulations, promulgated from time to time,
applicable to the Business, including the federal, state and municipal non-discrimination laws in the United States, rules and regulations. Except for vacation, sick time and other Company-approved leaves of
absence subject to Company policies, the employee is expected to and shall work 40 hours per week at minimum. The Employee shall not engage in any other activity that could reasonably be expected to interfere with the performance of Employee’s
duties, responsibilities and services hereunder subject to Section 9 below. 
 8. Employee Representations. Employee represents
and warrants to the Celyad Group that Employee is qualified to perform the services under this Agreement and that neither Employee’s execution of the Agreement, nor Employee’s performance of such services is limited or prohibited by, and
will not cause a conflict of interest or breach of, any law, regulation, agreement, understanding, order, judgment, decree or other instrument, contract, or document to which Employee is a party or subject, including without limitation any
confidentiality or restrictive covenant agreement with any prior employer. 
 9. Conflicts of Interest. Employee confirms that
Employee has advised the Celyad Group in writing prior to the date of signing this Agreement of any current relationship with third parties, including competitors of Celyad Group. The Chief Executive Officer of Celyad and Employee will review each
of those relationships and determine together which ones need to be terminated due to a conflict of interest, or prohibition of Employee carrying out the terms of this Agreement, or which would present a significant risk of disclosure of
Confidential Information. 
 10. Exclusivity. For the duration of this Agreement, Employee shall provide services exclusively to
Celyad Group and Employee shall not seek, accept or perform any consulting or other services (whether or not for compensation) without the specific and written approval of the Chief Executive Officer of Celyad, or its designee. 

11. Restrictive Covenants. 

(a) During the term of this Agreement and for a period of twelve (12) months after the date of termination of Employee’s employment,
regardless of the reason for such termination, Employee will not, directly or indirectly, whether as an officer, director, employee, consultant, contractor, equity owner or agent of, or otherwise advise or participate in the ownership or operation
of (i) any cell therapy company developing CAR T therapies focused on NKG2D, B7H6 and NKp (such as NKp30, NKp40, NKp44), or (ii) any other business activity that competes with the business activity referred to above (in section 11. a (i))
of the Corporation or any subsidiary of the Corporation. Nothing in this Section 11(a) shall be deemed to prohibit Employee from investing in any company engaged in such business, the stock of which is available in a public securities market;
provided, however, that Employee shall not own in excess of five percent (5%) of the total issued and outstanding stock of such company. 

(b) During Employee’s employment and for a period of one (1) year after the termination of such employment, regardless of the reason
for such termination, Employee will not, directly or indirectly, solicit, recruit, endeavor to entice away, hire, attempt to hire, or otherwise materially interfere with the business relationship of any member of the Celyad Group, any person who is,
or was within the twelve (12) month period immediately prior to the termination of Employee’s employment with the Corporation, employed or engaged (whether as an employee, independent contractor or otherwise) by any member of the Celyad
Group. 
 (c) During Employee’s employment and for a period of one (1) year after the termination of such employment, regardless
of the reason for such termination, Employee will not, directly or indirectly, solicit, 

 
recruit endeavor to entice, do business with, or materially interfere with the business relationship of any member of the Celyad Group, any person or entity who is, or was within the twelve
(12) month period immediately prior to such termination, a customer, client or supplier to or of the Celyad Group. 
 (d) During and
after Employee’s employment, Employee agrees not to make any disparaging statements concerning the Corporation or any of its affiliates, products, services or current or former officers, directors, shareholders, employees or agents, except in
the context of performing Employee’s legitimate duties to the Corporation during Employee’s employment with the Corporation. During and after Employee’s employment, Corporation agrees not to make any disparaging statements concerning
the Employee. 
 12. Confidentiality of Information. Employee recognizes and acknowledges that the trade secrets of the Celyad Group
and all other confidential and proprietary information of a business, financial, personal or other nature, including without limitation, scientific and technical information and improvements thereon, data from or results of clinical trials, patient
information, lists of the Celyad Group’s actual and prospective customers, financial information and business and marketing plans, as they exist from time to time (collectively, the “Confidential Information”), are a valuable and
unique asset of the Celyad Group and therefore agrees that Employee will not, either during or after the term of Employee’s employment, disclose any Confidential Information concerning any entity in the Celyad Group, to any person, firm,
corporation, association or other entity, for any reason whatsoever, unless previously authorized in writing to do so by the Chief Executive Officer of Celyad. The term “Confidential Information” shall not include any information that
(i) is or becomes publicly available through no direct or indirect action of the Employee; or (ii) is required to be disclosed by a court of competent jurisdiction or pursuant to any arbitration, provided that Employee first gives
notice of such disclosure requirement to the Corporation. Employee shall not at any time, make any use whatsoever, directly or indirectly, of Confidential Information, except as required in connection with the performance of Services. 

13. Injunctive Relief. The Employee acknowledges that a breach of any of the provisions contained in Sections 11 or 12 would result in
irreparable injury to the Celyad Group for which there may be no adequate remedy at law and that, in the event of an actual or threatened breach by the Employee of the provisions of Sections 11 or 12, any member of the Celyad Group, shall be
entitled to pursue and obtain injunctive relief from a court of competent jurisdiction restraining Employee from doing any act prohibited thereunder. Nothing contained herein shall be construed as prohibiting Celyad Group or the Corporation, as
appropriate, from pursuing any other remedies available to it for such breach or threatened breach, including the recovery of any monetary damages to which it would be entitled under the law. In the event that any provision of Section 11 is
held to be unenforceable as a result of it being too broad, including in terms of time or geographical extent, Employee agrees that the court can adapt and limit this Section so as to make the provisions hereof enforceable to the fullest extent
permissible. The post-employment restricted periods in Section 11 shall be extended by each day that the Employee is in breach of any provision of Section 11. 

14. Rights in Celyad Group Property; Inventions. The Employee hereby recognizes the Celyad Group’s proprietary rights in the
tangible and intangible property of the Celyad Group and acknowledges that the Employee will not obtain or acquire through such employment any personal property rights in any of the property of any member of the Celyad Group, including but not
limited to, any writing, communications, manuals, documents, instruments, contracts, agreements, files, literature, data, technical information, know-how, secrets, formulas, products, methods, procedures,
processes, devices, apparatuses, trademarks, trade names, trade styles, service marks, logos, copyrights, patents, or other matters which are the property of any member of the Celyad Group. The Employee agrees that during his employment by the
Corporation, any and all discoveries, inventions, improvements and innovations (including all data and records pertaining thereto), whether or not patentable, copyrightable or reduced to writing, which the Employee may have conceived or made, or may
conceive or make, either alone or in conjunction with others and whether or not during working hours or by the use of the facilities of the Corporation, which are related or in any way connected with the Business of the Corporation or any of its
affiliates, are and shall be the sole and exclusive property of the Corporation. The Employee shall promptly disclose all inventions to the Corporation, shall execute at the request of the Corporation any assignments or other documents the
Corporation may deem necessary to protect or perfect its rights therein, and shall assist the Corporation, at the Corporation’s expense, in obtaining, defending and enforcing the 

 
Corporation’s rights therein. The Employee hereby assigns, sets over and transfers to the Corporation all of his right, title and interest in and to any inventions. The Employee hereby
appoints the Corporation as his attorney-in-fact to execute on his behalf any assignments or other documents reasonably necessary by the Corporation to protect or
perfect its rights to any inventions. 
 15. Protected Disclosures. Employee understands that nothing contained in this Agreement
limits Employee’s ability to communicate with any federal, state or local governmental agency or commission, including to provide documents or other information, without notice to the Corporation. Employee also understands that nothing in this
Agreement limits Employee’s ability to share compensation information concerning Employee or others, except that this does not permit Employee to disclose compensation information concerning others that Employee obtains because Employee’s
job responsibilities require or allow access to such information. 
 16. Defend Trade Secrets Act of 2016. Employee understands that
pursuant to the federal Defend Trade Secrets Act of 2016, Employee shall not be held criminally or civilly liable under any federal or state trade secret law for the disclosure of a trade secret that (a) is made (i) in confidence to a
federal, state, or local government official, either directly or indirectly, or to an attorney; and (ii) solely for the purpose of reporting or investigating a suspected violation of law; or (b) is made in a complaint or other document
filed in a lawsuit or other proceeding, if such filing is made under seal. 
 17. Compensation Upon Termination of Employment for Any
Reason. In connection with Employee’s termination for any reason, the Corporation shall pay the Employee any (i) base salary; (ii) unused vacation; and (iii) unreimbursed expenses (subject to the Corporation’s expense
policy), in each case ((i) through (iii)) accrued through the date of the Employee’s termination (the “Termination Date,”) and the obligations described in (i) through (iii), (the “Accrued Obligations”). 

18. Termination by Corporation without Cause or by Employee with Good Reason. 

(a) The Corporation shall have the right to terminate Employee’s employment without Cause. The Corporation will endeavor to, but is not
required to, provide the Employee with 30 days advance notice of such a termination. Notwithstanding any such 30-day notice period, the Corporation may subsequently accelerate the Employee’s date of
termination. 
 (b) If the Corporation terminates the Employee’s employment without Cause, or if the Employee terminates
Employee’s employment with Good Reason, and subject to the Release Requirement, the Corporation shall pay the Employee six (6) months of Employee’s final Base Salary rate in installments on the Corporation’s regular payroll dates
following the Termination Date if the Termination Date occurs after three (3) months after the Employment Date (the “Severance Pay”). To avoid doubt, if the Corporation terminates the Employee’s employment without Cause or the
Employee terminates his employment for Good Reason and the Termination Date occurs prior to the date that is three (3) months after the Employment Date, the Employee shall not be eligible for any severance pay. The Corporation shall not be
required to begin paying any Severance Pay until its first payroll date after the Release Requirement has been fulfilled. The “Release Requirement” means the Employee’s execution, return and nonrevocation, in each case with the time
periods required by the Release but in no event later than 30 days after the Termination Date (or 60 days in the event of a group layoff under the Older Workers’ Benefits Protection Act), of a separation agreement in a form provided by the
Celyad Group containing, among other terms, a release of claims against the Celyad Group and related persons and entities (the “Release”). In no event will the Release include any additional post-employment noncompetition or
nonsolicitation covenants that are not included in this Agreement. 
 (c) For purposes of this Agreement, “Good Reason” shall mean
(i) a material reduction in Employee’s Base Salary or target annual bonus other than a general reduction in Base Salary or target annual bonus that affects all similarly situated employees in substantially the same proportions; (ii) a
relocation of Employee’s principal place of employment by more than 60 miles; or (iii) a material, adverse change in Employee’s title, authority, duties, Services required, or responsibilities (other than temporarily while Employee is
physically or 

 
mentally incapacitated or as required by applicable law). Before a termination by the Employee for Good Reason, the Employee must (i) reasonably determine in good faith that a “Good
Reason” condition has occurred; (ii) notify the Company in writing of the first occurrence of the Good Reason condition within 60 days of the first occurrence of such condition; (iii) cooperate in good faith with the Company’s
efforts, for a period not less than 30 days following such notice (the “Cure Period”), to remedy the condition and notwithstanding such efforts, the Good Reason condition continues to exist; and (v) terminate his employment within 60
days after the end of the Cure Period. If the Company cures the Good Reason condition during the Cure Period, Good Reason shall be deemed not to have occurred. 

19. Termination Upon Death and Disability. 

(a) This Agreement may be terminated immediately due to Employee’s death or Disability without any Severance Pay 

(b) “Disability” shall mean a physical or mental impairment that substantially prevents Employee from performing Employee’s
duties hereunder and that has continued for either (i) one hundred eighty (180) consecutive days or (ii) any one hundred eighty (180) days within a consecutive three hundred sixty (360) day period. Any dispute as to whether
or not Employee is disabled within the meaning of the preceding sentence shall be resolved by a physician reasonably satisfactory to the Corporation, and the determination of such physician shall be final and binding upon both Employee and the
Corporation. Notwithstanding anything to the contrary in this Section, the inability of Employee to perform the Services, with or without a reasonable accommodation, upon completion of a medical leave of absence of one hundred eighty
(180) consecutive days shall constitute Employee’s Disability. 
 20. Termination by Corporation for Cause: Termination by
Employee without Good Reason. 
 (a) Corporation shall have the right to terminate Employee’s employment for Cause immediately upon
written notice, with the Termination Date occurring as specified in such notice from the Corporation. For purposes of this Agreement, “Cause” shall mean (i) conviction, commission of or entering a plea of guilty or nolo
contendere to any felony, or a crime involving dishonesty or moral turpitude; (ii) willfully engaging in conduct materially injurious, or reasonably likely to cause material injury, to any member of the Celyad Group; (iii) the material
breach of this Agreement by Employee or the Employee’s breach of any other restrictive covenant obligation Employee has to any member of the Celyad Group; (iv) Employee’s gross negligence, or willful and deliberate failure to perform
Employee’s duties, or (v) Employee’s failure to adhere to or comply with any material written policies or procedures of the Celyad Group, including but not limited to the code of conduct or those pertaining to expense reimbursement,
harassment, discrimination or retaliation, conflict of interest, or the prohibition of insider trading. Before a termination for Cause under (iii) – (v) above, and if the Employee’s breach or violation is curable, the Corporation shall
provide Employee with written notice and thirty (30) days from the delivery of such notice to cure the conduct, breach or violation (the “Cure Period”), provided that Employee shall not be entitled to more than two Cure Periods
in any twelve-month period. 
 (b) Employee shall have the right to terminate Employee’s employment without Good Reason upon thirty
days’ written notice to the Corporation and Celyad. If Employee provides such notice, the Corporation may accelerate the date of Employee’s termination without such acceleration itself constituting a termination by the Corporation under
this Agreement. 
 (c) For the avoidance of doubt, in the event of termination of employment by Corporation for Cause, a termination due to
death or Disability or a termination by Employee without Good Reason, Employee will be entitled only to the Accrued Obligations, and will not be entitled to any Severance Pay. 

21. Enforceability; Severability. This Agreement shall be interpreted in such a manner as to be effective and valid under applicable
law, but if any provision hereof shall be prohibited or invalid under any such law, such provision shall be ineffective to the extent of such prohibition or invalidity, without invalidating or nullifying the remainder of such provision or any other
provisions of this Agreement. If any one or more of the provisions 

 
contained in this Agreement shall for any reason be held to be excessively broad as to duration, geographical scope, activity or subject, such provisions shall be construed by limiting and
reducing it or them so as to be enforceable to the maximum extent permitted by applicable law. 
 22. Governing Law; Jurisdiction.
This Agreement shall be construed in accordance with and governed by the laws of the State of Delaware without giving effect to principles of conflicts of laws. To the extent permitted by Section 23 (Arbitration), including without limitation
the enforcement by the Corporation of any of Employee’s restrictive covenant obligations, the state and federal courts located in Boston, Massachusetts shall have exclusive jurisdiction and exclusive venue over any controversy or claim arising
out of the Employee’s employment or the termination of that employment. 
 23. Arbitration. Any controversy or claim arising out
of or relating to this Agreement or the breach thereof or otherwise arising out of the Employee’s employment or the termination of that employment (including, without limitation, any claims of unlawful employment discrimination whether based on
age or otherwise) shall, to the fullest extent permitted by law, be settled by arbitration in any forum and form agreed upon by the parties or, in the absence of such an agreement, under the auspices of the American Arbitration Association
(“AAA”) in Boston, Massachusetts in accordance with the Employment Arbitration Rules of the AAA, including, but not limited to, the rules and procedures applicable to the selection of arbitrators. In the event that any person or entity
other than the Employee or the Corporation may be a party with regard to any such controversy or claim, such controversy or claim shall be submitted to arbitration subject to such other person or entity’s agreement. Judgment upon the award
rendered by the arbitrator may be entered in any court having jurisdiction thereof. This Section shall be specifically enforceable. Notwithstanding the foregoing, this Section shall not preclude either party from pursuing a court action for the sole
purpose of obtaining a temporary restraining order or a preliminary injunction in circumstances in which such relief is appropriate; provided that any other relief shall be pursued through an arbitration proceeding pursuant to this Section. 

24. Section 409A. It is intended that the benefits provided under this Agreement shall comply with the provisions of Section 409A
of the Internal Revenue Code (“Section 409A”) or qualify for an exemption to Section 409A, and this Agreement shall be construed and interpreted in accordance with such intent. Any payments that qualify for the “short term
deferral” exception or another exception under Section 409A shall be paid under the applicable exception. Each payment provided under this Agreement shall be treated as a separate payment for Section 409A purposes. No member of the
Celyad Group (or its affiliates), the Board, or any employee, officer or director of the Celyad Group (or its affiliates) shall be held liable for any taxes, interest, penalties or other monetary amounts owed by the Employee as a result of this
Agreement. 
 25. Notices. Any notice or other communication given pursuant to this Agreement shall be in writing and shall be
personally delivered, sent by overnight courier or express mail, or mailed by first class certified or registered mail, postage prepaid, return receipt requested to the parties at their respective addresses set forth on the signature page hereof, or
to such other address as the parties shall have designated by notice to the other parties. 
 26. Amendment; Waiver. No provision of
this Agreement may be amended, modified, waived or discharged unless such amendment, modification, waiver or discharge is agreed to in writing and signed by the parties. No waiver by either party hereto at any time of any breach by the other party
hereto of, or compliance with, any condition or provision of this Agreement to be performed by such other party shall be deemed a waiver of similar or dissimilar provisions or conditions at the same or at any prior or subsequent time. 

27. Successors and Assigns. This Agreement shall inure to the benefit of and be binding upon the Corporation, its successors and
assigns, and the Employee, Employee’s heirs and legal representatives. Employee acknowledges that the Services are personal and that Employee may not assign this Agreement. 

28. Entire Agreement. This Agreement and any other confidentiality or restrictive covenant obligations Employee has to any member of
the Celyad Group constitutes the entire agreement between the parties with respect to the subject matter hereof and supersedes any and all prior agreements, arrangements and understandings, written or oral, relating to the same subjects covered by
this Agreement. 

 29. Counterparts. This Agreement may be executed in multiple counterparts, each of
which shall be deemed an original, but all of which together shall constitute one and the same agreement. The facsimile or electronic signature of either party to this Agreement for purposes of execution or otherwise, is to be considered as an
original signature, and the document transmitted is to be considered to have the same binding effect as an original signature on an original document. 

IN WITNESS WHEREOF, the parties have executed this Agreement as of the date first above written. 

 

			
		  	CELYAD INC.
		
		  	 /s/ Christian Homsy

	  	By: Christian Homsy
	  	Title: Chief Executive Officer
	  	Address:
	  	 c/o Christian Homsy, M.D.
 Celyad S.A.

Axisparc Business Center
 Rue Edouard Belin, 2

B-1435 Mont-Saint-Guibert, Belgium

	  	Employee
	  	 /s/ Filippo
Petti                           30/7/2018

	  	Filippo Petti
	  	Address:
	  	 3 Woodoak Drive
 Westbury, NY 11590,
USAEX-4.24

 Exhibit 4.24 
  

 
  
 CELYAD SA 

 
  

WARRANTS PLAN 
 26 October
2018 
  
  

Incentive plan by way of grant of subscription rights (Warrants) established in accordance with the 

Companies Code and articles 41 to 47 of the Act of 26 March 1999 relating to the Belgian action plan for 

employment 1998 and having various provisions 
  

 
  

 PREAMBLE 

This Warrant Plan of Celyad SA (the “Plan”) aims to motivate and inspire loyalty among the Beneficiaries. Well aware of the fact that their
contribution is essential to the development of its activities and the growth of its results, the Company wishes to give the Beneficiaries the opportunity to become shareholder or to increase their participation, hoping to make a financial gain in
the event of a positive evolution of the results and, consequently, the Company’s value. 
 The Plan’s principles have been determined by the
Board of Directors and have been approved by the general shareholders’ meeting in accordance with the principles of the Corporate Governance Code. 

In addition, the list of beneficiaries, as well as the exercise price of the Warrants are determined by the Board of Directors prior to any offer. 

The Plan is drawn up in accordance with the applicable provisions of the Belgian act of 26 March 1999 (and more precisely section VII hereof) governing
the shares with a discount and stock-options (articles 41 to 47). 
 The conditions governing the exercise of the Warrants must also be read in the light of
the provisions of the “Dealing Code” which is applicable within the Company and available on the Company’s website (www.celyad.com). 

  
 2 

 1. DEFINITIONS 
  

					
	 Share
	  	:	  	A new share of the Company, granting the same rights and advantages as the existing shares of the Company.
			
	 Allocation
	  	:	  	The allocation of Warrants following the acceptance of an Offer.
			
	 Bad Leaver
	  	:	  	Has the meaning given in article 8.6 of the Plan.
			
	 Beneficiary
	  	:	  	A current member of the staff, the Board of Directors or other beneficiaries of the Company to whom at least (1) Warrant has been allotted.
			
	Conditions for Exercice	  	:	  	The conditions under which the Beneficiaries are entitled to exercise a Warrant during the Exercise Periods.
			
	 Board of Directors
	  	:	  	The board of directors of the Company.
			
	 Offer Letter
	  	:	  	The template attached hereto in Annex 1 – offer letter.
			
	 Reply Form
	  	:	  	The template attached hereto in Annex 2 – Reply form.
			
	 Exercise Form
	  	:	  	The template attached hereto in Annex 3 – Exercise form.
			
	 Good Leaver
	  	:	  	Has the meaning given in article 8.6 of the Plan
			
	Act relating to Stock Options	  	:	  	The Act of 26 March 1999 relating to the Belgian action plan for employment of 1998 and having various provisions.
			
	 Offer
	  	:	  	The offer of at least one (1) Warrant to one or more Beneficiaries, in accordance with the provisions of the Plan.
			
	 Warrant
	  	:	  	A subscription right issued by the Company, granting the Beneficiaries the right to subscribe, in accordance with the terms and conditions as set out in the Plan, during the Exercise periods, to a number of Shares determined by
the Plan, against payment of the Price of Exercise.
			
	 Exercice Period
	  	:	  	The period during which the Warrants may be exercised in accordance with the Plan.
			
	 Plan
	  	:	  	This incentive plan.
			
	 Exercise Price
	  	:	  	The amount payable for the exercise of a Warrant pursuant to the Plan.
			
	 Company
	  	:	  	Celyad SA, a public limited liability company, registered with the register of legal entities kept at the Crossroads Bank for Enterprises under number 0891.118.115 (RLE Brabant Wallon) and of which its shares are listed on
EURONEXT Brussels and EURONEXT Paris.
			
	 Affiliated Companies
	  	:	  	Has the meaning given in article 11 of the Belgian Companies Code.

  
 3 

	2.	 GENERAL MECHANISME OF THE OFFER OF WARRANTS 

Pursuant to the Plan, the Company allots to the Beneficiaries a certain number of Warrants. These Warrants are issued by decision of the
general shareholders’ meeting or by decision of the Board of Directors within the framework of the authorized capital. The Warrants are then allotted upon decision of the Board of Directors resolving on the recommendation of the Remuneration
Committee. 
 Each Warrant gives its holder the right (but not the obligation) to subscribe, under the Exercise Conditions, during the
Exercise Periods and against payment of the Exercise Price, to one Share. 
  

	3.	 BENEFICIARIES 

The Warrants may be offered to any individual performing professional services, whether in principal or secondary, for the direct or indirect
benefit of the Company or an affiliated Company, in his capacity of an employee or future employee, in his capacity of a current or future consultant or in his capacity of director. 

The Warrants Offer does not create any right, on the part of the Beneficiaries, to receive (additional) Warrants in the future. 

The Warrants Offer and the right to exercise these are not part of the employment agreement or service agreement concluded with the Company and
therefore can not be considered as an acquired right. In addition, the Beneficiaries expressly accept that the decisions relating to the Warrants fall within the exclusive and discretionary competence of the Company. This grant shall not be taken
into account in the calculation of any indemnity whatsoever which may be due to the Beneficiaries. 
  

	4.	 CLOSED PERIOD 

The Beneficiaries shall have to comply, if need be, with the provisions of the Dealing of the Company. The Warrants cannot be exercised during
the “closed periods”. 
  

	5.	 WARRANTS 

  

	5.1	 Number of Warrants per Beneficiary 

The number of Warrants offered to each of the Beneficiaries is freely determined by the Board of Directors, acting upon the recommendation of
the Remuneration Committee. 
  

	5.2	 Nature of the Warrants 

The Warrants are exclusively in registered form. As soon as they are offered and accepted, the Warrants will be numbered and recorded in a
special register, which will be kept up to date as regards the amount of Warrants held by each Beneficiary. 
  

	5.3	 Price of the Warrants 

The Warrants are allotted free of charge to the Beneficiaries. 
  

	5.4	 Term of the Warrants 

Warrants are allotted for a limited term of maximum five years. This term is determined by the Board of Directors, in compliance with the
provisions of the Corporate Governance Code and the Companies Code. 
 Any Warrant that has not been exercised on its date of maturity may no
longer be exercised without the Beneficiary being able to invoke any right to compensation. 

  
 4 

	5.5	 Non-transferability and securities 

The Warrants are strictly personal and may not be transferred after the Offer, except in the event of death as provided in article 9 below.

 Warrants may not be pledged or used as security, of any kind, as principal or accessory. 

Warrants that may have been transferred, pledged or used as a security of any kind, whether as a principal or accessory, in violation of the
provisions of this article 5.5, shall not be exercisable. 
  

	6.	 OFFER OF WARRANTS 

 

	6.1	 Date of the Offer 

The Company sends each Beneficiary a personalized Offer Letter for a number of Warrants. 

The Warrants are deemed to be offered to the Beneficiaries as from the date of dispatch of the Offer Letter. 

 

	6.2	 Acceptance or rejection of the Offer 

The Beneficiary is free to accept the Offer, either in whole or in part, or to reject it. 

A Reply Form is sent to each Beneficiary together with the Offer Letter, by which the Beneficiary notifies his decision as regards the Offer:
acceptance (either in whole or in part) or rejection. 
 The Reply Form is delivered, completed and signed, at the latest on the date
mentioned on the Reply Form, at the address mentioned therein. 
 The Offer of Warrants will be considered as altogether rejected if the
Beneficiary did not accept the Offer in writing within sixty (60) days as from the date of the Offer, without the Beneficiary being able to claim any right to indemnification. 

In case of absence of a signature, or if the Reply Form is not returned or is returned belatedly, the Offer will be considered as rejected as a
whole. 
 From a Belgian tax point of view, the Stock Option Law considers that the Warrants are deemed to have been allotted on the sixtieth
(60th) day following the date of the Offer, provided that the Beneficiary has notified in writing his Acceptance of the Offer before expiry of this period. The acceptance of the Offer must be notified to the Company prior to the expiry of the
sixty (60) day period referred to above, in accordance with this article 6.2, otherwise the Offer is deemed to be altogether rejected. 
  

	6.3	 Acceptance of the Plan 

Acceptance of the Offer by the Beneficiary entails the unconditional acceptance of the Plan. 

 

	7.	 ACQUISITION (VESTING) OF WARRANTS 

Notwithstanding the Allocation of the Warrants to the Beneficiaries, the Warrants are acquired by the Beneficiaries, subject to compliance with
the Conditions for Exercise provided for in article 8 and without prejudice to an eventual acceleration as provided under article 8.9, in accordance with the following terms: 
  

	 	-	 If the Beneficiary stops exercising his professional activities for the benefit of the Company before the first
anniversary of the Offer, the Warrants awarded to him shall be qualified as void and they cannot be exercised anymore; 

  

	 	-	 If the Beneficiary stops exercising his professional activities for the benefit of the Company during the
second year after the Offer, 33% of the Warrants awarded to him shall be considered as vested; 

  
 5 

	 	-	 If the Beneficiary stops exercising his professional activities for the benefit of the Company during the third
year after the Offer, 66% of the Warrants awarded to him shall be considered as vested; 

  

	 	-	 If the Beneficiary still exercises his professional activities for the benefit of the Company after the third
anniversary of the Offer, 100% of the Warrants awarded to him shall be considered as vested. 

 For the purposes of this
article the Beneficiary shall no longer be deemed to be carrying on his professional activity for the benefit of the Company as from the date on which he issued or received a notice of termination of his employment or co-operation agreement. 

 

	8.	 THE EXERCICE OF WARRANTS 

 

	8.1	 Conditions for Exercice 

The exercise of Warrant is subject to Conditions for the Exercise provided for in the Plan. 

 

	8.2	 Exercice Price 

The Exercise Price is equal to the fair market value of the Company’s shares at the time of the Offer. This value is determined by the
Board of Directors and corresponds to: 
  

	 	-	 either the closing price of the Company’s Share on the day before the date of the Offer;

  

	 	-	 or the average of the thirty (30) calendar days preceding the date of the Offer of the closing price of
the Company’s Share. 

 Furthermore, regarding the Warrants allotted to beneficiaries who are no employees, the
Exercise Price shall not be below the average of the thirty (30) calendar days preceding the date of issuance of the Warrants. 
 The
exercise price of each Warrant is stipulated in the Letter of Offer to each Beneficiary. 
  

	8.3	 Consequences of the Exercice 

In the event of exercise of the Warrants, the Shares issued in consideration for the exercise will be in registered or dematerialized form
according to the decision of the Beneficiaries. Such Shares shall have the same characteristics as the existing Shares of the Company. 
  

	8.4	 Exercise Period 

Without prejudice to article 8.9 of the Plan or a different decision of the Board of Directors to extend the Exercise Period, the Warrants will
be exercisable between the first day of the fourth calendar year following the Offer and the last day of the fifth year following the Offer. 

In order to streamline the exercise of the Warrants and to limit the costs associated with their exercise, the exercise of the Warrants and the
corresponding capital increases may take place during the first month of each quarter during the Exercise Period. 
 Where relevant, the
exercise of the Warrants will be recorded by notary deed within a maximum of 30 days following the closing of each exercise window. 
  

	8.5	 Number of Shares per Warrant 

One (1) Warrant gives right to subscribe to (1) Share. 

  
 6 

	8.6	 Attendance – Good Leaver and Bad Leaver 

 

	8.6.1	 In the event that the employment agreement or service agreement between the Company (or one of its Affiliated
Companies) and a Beneficiary (or management company of a Beneficiary) comes to an end: 

  

	 	(a)	 as a result of death, incapacity, retirement, termination of the employment agreement or service agreement
without any serious misconduct of the Beneficiary, resignation of the Beneficiary or unilateral breach by the Beneficiary of his employment agreement or service agreement, the Beneficiary shall be referred to as “Good Leaver”;

  

	 	(b)	 as a result of termination of the employment agreement or service agreement for serious misconduct of the
Beneficiary, the Beneficiary will be referred to as “Bad Leaver”. 

 The qualification as Good Leaver or
Bad Leaver will take place on the date of the determination of the above situation, namely on the date on which the event is brought to the attention of the parties. In this regard, the Beneficiary is referred to as Good / Bad Leaver on the date of
notification of termination of his contract, even if he must then provide a notice period. 
 With regards to the people enjoying the status
of Beneficiary because they are Director or provide products or services to the Company as a self-employed but on a regular basis (or, when appropriate, via a management or services company), the words “dismissal or revocation” and
“voluntary termination” refer to the various hypotheses in which a contract for the delivery of these products or services is being terminated permanently either by the Company or by the Beneficiary or the management or services company.
The words “serious misconduct” refer to the hypothesis in which this termination is based on a serious breach by the Beneficiary or the management or services company of their contractual obligations. An interruption of more than six
months in the delivery of the products or the services is considered as a permanent termination. 
 In case the labor contract is suspended
for more than six months in total, the consequences of said suspension on the rights related to the Warrants granted by the Company will be determined individually by the Company. 

 

	8.6.2	 Notwithstanding the realization of the vesting provided for in article 7 of the Plan, Warrants can no longer be
exercised in the event that the Beneficiary is considered to be a Bad Leaver prior to the exercise of the Warrants. 

  

	8.6.3	 Provided that the conditions of article 7 are met, the Beneficiary whose employment or service contract has
ended, without being regarded as a Bad Leaver, can only exercise its Warrants during the first exercise window (as per article 8.4 of the Plan) of the Exercise Period following the termination date of its employment or service contract. If its
Warrants are not exercised during this window, its Warrants can no longer be exercised. 

  

	8.7	 Terms of Exercise 

A Beneficiary willing to exercise its Warrants will specify, upon their exercise, the numbers of the Warrants that he intends to exercise. In
situations where the Beneficiary does not specify the numbers, the Beneficiary will be deemed to have exercised its Warrants in the chronological order in which they were allocated, from the oldest to the most recent. 

The Warrants can be exercised upon delivering an Exercise Form to the Company, for the attention of the Board of Directors. The Exercise Form
can be (i) delivered in person with delivery receipt, (ii) sent by registered mail or (ii) faxed with immediate confirmation by registered mail. 

The Exercise Form must be completed in full and signed by the Beneficiary, and must mention the number of Warrants that the Beneficiary intends
to exercise. 

  
 7 

	8.8	 Terms of payment 

The payment shall be made by bank transfer of the Price of Exercise of all exercised 

Warrants to the Company’s account as indicated by the latter in the Exercise Form. 

The Beneficiary shall have a period of ten (10) days as from the sending of the Exercise Form to proceed with the payment. 

 

	8.9	 Acceleration of the vesting and exercise of the Warrants 

Notwithstanding the delays and periods provided under articles 7 and 8.4 of the Plan, the Warrants can be immediately exercised by the
Beneficiaries in the following situations: 
  

	 	(a)	 Event provided under article 501 second paragraph of the Company Code, being a share capital increase in cash
without suspension of the preferential rights of the existing shareholders; 

  

	 	(b)	 Takeover bid on the Shares of the Company as of the announcement of the public offer by the FSMA;

  

	 	(c)	 Change of control on the Company; 

 

	 	(d)	 Conclusion of a “Strategic Partnership” with an important industrial actor, active in the
life-science sector, and if the “Strategic Partnership” is qualified as such by the Board of Directors. 

 As
from the occurrence of one of these events, and regarding events (a) and (d) minimum ten (10) days before their occurrence, the Company shall notify the Beneficiaries in order to allow them to exercise their Warrants for a ten
(10) days period. If the Warrants are not exercised during this ten (10) days period they will only be exercised under the conditions provided by articles 7 and 8.4 of the Plan. 

The Shares issued further to the exercise of the Warrants under the present article 8.9 can be, upon decision of their holder, immediately
dematerialised, listed and traded on the market. 
 The eventual tax consequences of the acceleration of the vesting and exercise will be
borne by the concerned Beneficiaries. 
  

	9.	 DEATH OF THE BENEFICIARY 

In the event of the death of the Beneficiary, its Warrants can be exercised by its legal successors. Successors and assigns are subject to the
same rules than the Beneficiaries. 
 In the event of the death of the Beneficiary prior to the exercise of Warrants, the provisions of
article 7 (vesting) will not be applicable. Legal heirs will therefore be able to exercise 100% of the Warrants that were allocated to the deceased Beneficiary. 

The rules of succession will be followed. However, where they are several legal heirs or where bare property rights/usufruct have been
separated, a sole representative of the succession will be appointed by the successors and assigns for the purpose of exercising the Warrants. 

The Company reserves the right to suspend the right to exercise the Warrants as long as this appointment has not taken place and as long as it
has not been duly notified. 
  

	10.	 NATURE OF THE SHARES ISSUED UPON THE EXERCISE OF THE WARRANTS 

 

	10.1	 Nature of the Shares 

The Shares are shares identical to the other shares issued by the Company. 

  
 8 

	10.2	 Rights attached to the Shares 

The Shares issued upon the exercise of the Warrants will benefit from the same rights and advantages (including voting rights) than the
existing Shares of the Company. 
  

	10.3	 Transferability of the Shares 

The transfer of Shares is subject to the terms and conditions defined in the articles of association of the Company. 

 

	11.	 OPERATIONS AUTHORISED 

By way of derogation from article 501 of the Company code, and without prejudice to the legally prescribed exceptions, the Company may pass all
resolutions that it deems necessary in relation to its capital, its articles of association or its management. Such resolutions may include, amongst others, capital reduction, with or without reimbursement for the shareholders, a capital increase by
way of incorporation of reserves whether or not with the issue of new shares, a capital increase in kind, a capital increase in cash with or without restriction or cancellation of the preferential subscription rights of the shareholders, the
issuance of profit shares, convertible bonds, preferred shares, bonds cum warrants or conventional bonds or warrants, an amendment the provisions of the articles of associations with regards to the distribution of the profits or the (net)
liquidation proceeds or other rights attached to the common shares, a splitting of shares, a payment of dividend in shares, the dissolution of the Company, a legal merger, a legal demerger or a contribution or transfer of a totality or a branch of
activity whether or not combined with the exchange of shares. The Company may pass such resolutions even if these implied or may imply that the benefits for the Warrant Holder arising from the issuance and the Warrant exercise provisions or the law
may be reduced unless such reduction is, in an obvious way, the sole objective of such a resolution. 
 However, in the event of a merger or
demerger, the Board of Directors has an obligation of means to ensure that the Warrants outstanding at the date of these transactions are adjusted in accordance with the exchange ratio applied to the Company’s existing shares. 

Moreover, in case of a capital reduction or any similar transaction resulting into a decrease of the Company’s equity as a result of a
decision of the shareholders taken by the general assembly, the exercise price of the Warrants may be modified by decision of the Board of Directors notified to the Beneficiaries in order to compensate for the loss of value resulting from the equity
decrease. The possible amendment will be applicable as soon as the Beneficiaries have been notified, without them having to formally accept it. 

The number of shares corresponding to the Warrants will be adjusted to reflect and take into account any increase or decrease in the number of
shares of the Company resulting from a demerger or regrouping, as the case may be. 
  

	12.	 COSTS 

  

	12.1	 The Company 

All costs associated with the issue of Warrants will be borne by the Company. 

If the underlying Shares are delivered on a securities account, the subscribed Shares will be delivered free of charge insofar as the account
is being held at a financial institution in Belgium. 
  

	12.2	 The Beneficiaries 

Nihil 

  
 9 

	13.	 INTERPRETATION OF THE PLAN 

The Board of Directors is competent for making any decision deemed useful or necessary in order to interpret or implement the Plan in
compliance with all applicable laws. Any decision having legal effect will be communicated in writing to the Beneficiaries concerned. 
  

	14.	 INFORMATION OF BENEFICIARIES 

The Allocation of Warrants is not, on the part of the Company, an incentive or a recommendation to subscribe to the Warrants, nor to exercise
them subsequently. The Beneficiaries are consequently invited to inform themselves and, as the case may be, to be advised to make decisions likely to have a significant effect on their assets. 

The Company cannot be held liable for any damage or losses possibly incurred by the Beneficiaries on account of their participation to the
Plan. 
  

	15.	 INVALIDITY OF A PROVISION 

The invalidity or unenforceability of one of the provisions of the present Plan does not affect in any manner the validity or enforceability of
the other provisions of the Plan. In such cases, the invalid or unenforceable provision will be replaced by another equivalent provision, valid and enforceable, with a similar economic effect for the parties concerned. 

 

	16.	 NOTIFICATIONS 

Any notification to the holders of Warrants will be made to the address mentioned in the subscription rights register of the Company. Any
notification to the Company or Board of Directors will be duly carried out to the address of the registered office of the Company. 
 Any
changes of address must be notified in compliance with the present provision. 
  

	17.	 APPLICABLE LAW AND JURISDICTION 

 

	17.1	 Applicable law 

The Plan and the Warrants are governed by Belgian law. 
  

	17.2	 Jurisdiction 

Any dispute arising out of the interpretation, execution, application, validity or resolution of the Plan shall be subject exclusively to the
court of the judicial district of the registered office of the Company. 

  
 10 

 Annex 1 – Offer Letter 

[Date] 
 Re : Warrants Plan Celyad SA 

Dear                     , 

I am delighted to offer you to participate to Celyad’s warrants plan up to
             warrants. 
 With this offer, the Board of Directors of Celyad wants to
promote the long term commitment and motivation of his collaborators. 
 You will find in attachment: 

 

	 	-	 the terms and conditions of the offer (the « Conditions ») and 

 

	 	-	 an acceptance form. 

Please read the Conditions carefully. Note also that Celyad cannot be hold responsible for any change in the tax treatment of the warrants, due for instance
to a legislative change or a modification of your personal situation. 
 The exercise price of the warrants, fixed in accordance with the article 8.2 of
the Conditions, is EUR XXX [for employees] / EUR XXX [for Directors and consultants] per warrant and the subscription period extends from XXXX until XXXX (60 days). 

Please sign and complete the acceptance form with your name and address, and the number of warrants that you have decided to accept (by multiple of 50). Each
warrant gives right to one Celyad share. 
 Please keep one copy for your record, and send the original copy to Philippe Dechamps, Chief Legal Officer. 

If we do not receive your acceptance form by XXXX, you will be deemed to have refused the offer. 

If you have any question regarding this offer, please contact the Chief Legal Officer. 

Regards, 
  

 
 Christian HOMSY 

Chief Executive Officer 

  
 11 

 Annex 2 – Reply Form 

Allocation 
 Celyad SA 

To: Monsieur Philippe Dechamps, Chief Legal Officer 
 Rue Edouard
Belin 2 
 1435 Mont-Saint-Guibert 
 Celyad SA –
Warrant plan approved by EGM of October 26, 2018 
 I hereby, 
  

 
 (name and address), 

Have received the offer dated 26 October 2018 to participate to the issuance of warrants to the benefit of collaborators of Celyad SA (the
“Warrants”). I accept the terms and conditions attached to the offer (the “Conditions”). I understand that the acceptation of the offer may constitute a taxable event and that I will be responsible for the payment
of those taxes, as the case may be. 
 My decision is as follows : 
  

	☐	 I accept a total of: 

 
  

Warrants 
 (I therefore refuse 

 
  

Warrants) 
  

	☐	 I do not wish to participate tot his offer. 

Made in two originals on              2018 

 
 Name and Signature 

  
 12 

 Annex 3 – Exercise Form 

INCENTIVE PLAN – CELYAD SA 
 The
undersigned
                                         
                                         
               (NAME) 
 Domiciled in : 

 

                          
                                         
                                         
                                        
(FULL ADDRESS) 
 Hereby declares that he/she exercises the following Warrants, pursuant to the terms provided for in the incentive Plan dated
                         (DATE): 

                        
(NUMBER) Warrants, allocated on                             (DATE) at a
Price of Exercise of                      (AMOUNT) EUR =
                             (AMOUNT) EUR 

Total amount:
                             EUR 

This amount will be paid to the bank account N° [●] (NUMBER) with value dated at the latest on 

[●] (DATE). 
 I hereby acknowledge that if
the above mentioned amount is overdue or is not paid in full on [●] (DATE), the exercise will not take place. 
 Done
in                    
on                         
  

Signature 

  
 13

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