<PAGE>   1
                                                                   EXHIBIT 10.64

February 5, 1998




Alessandro Sette, Ph.D.
5551 Linda Rosa Avenue
La Jolla, California 92037

Re:     Severance Benefits Agreement

Dear Alex,

This letter will memorialize our agreement as to benefits to which you will be
entitled upon termination of your employment by Epimmune Inc. (the "Company")
under the circumstances described in this Agreement.

We agree that you are employed by the Company as an "at-will" employee and that
either you or the Company has the right at any time to terminate your employment
with the Company, with or without cause or advance notice, for any reason or for
no reason.

For purposes of this Agreement, the following terms will have the meanings set
forth below:

        "BASE SALARY" means your salary (excluding bonus, any other incentive or
        other payments and stock option exercises) paid by the Company in
        consideration for your service during the 12 months prior to the
        termination of your employment which is includable in your gross income
        for federal income tax purposes or which would have been includable in
        gross income except for an election either under Section 125 or
        402(e)(3) of the Internal Revenue Code of 1986, as amended (the "Code"),
        or under the terms of a nonqualified deferred compensation arrangement
        sponsored by the Company.

        "BENEFIT" means any payment received or to be received by you pursuant
        to this Agreement.

        "CAUSE" means (i) your material breach of any provision of this
        Agreement; or (ii) your engaging or in any manner participating in any
        activity which is directly competitive with or intentionally injurious
        to the Company or which violates any provision of any confidential
        information agreement or nonsolicitation agreement between you and the
        Company; or (iii) your commission any fraud against the Company; or (iv)
        your intentional improper use or appropriation for your personal use or
        benefit of any funds or properties of the Company not authorized by the
        Board of Directors of the Company or an authorized committee of such
        Board of Directors to be so used or appropriated; or (v) your conviction
        of any crime involving dishonesty or moral turpitude.

        "CHANGE IN CONTROL" of the Company will be deemed to have occurred if
        and when (i) any person or entity (other than the Company) or group of
        persons and/or entities acting in concert acquires, directly or
        indirectly, beneficial ownership of more than 50% of the outstanding
        shares of voting stock of the Company; or (ii) the Company is a
        participant in a merger or consolidation in which it does not survive as
        an independent company; or (iii) the business or businesses of the
        Company for which your services are principally performed are disposed
        of by the Company pursuant to a partial or complete liquidation of the
        Company.


<PAGE>   2
Alessandro Sette, Ph.D.
February 5, 1998
Page 2




        "COMPANY" for the purposes of this Agreement shall include the Company's
        successor if a Change in Control occurs.

        "EXCISE TAX" means the excise tax imposed by Section 4999 of the Code.

        "GOOD REASON" means (i) reduction of your rate of compensation as in
        effect immediately prior to the effective date of this Agreement; or
        (ii) the Company's failure to provide to you a package of welfare
        benefit plans which, taken as a whole, provide substantially similar
        benefits to those in which you are entitled to participate immediately
        prior to the occurrence of the termination of your employment (except
        that employee contributions may be raised to the extent of any cost
        increases imposed by third parties) or any action by the Company which
        would adversely affect your participation or reduce your benefits under
        any of such plans; or (iii) a change in your responsibilities,
        authority, title or office resulting in diminution of your position,
        excluding for this purpose an isolated, insubstantial and inadvertent
        action not taken in bad faith which is remedied by the Company promptly
        after written notice thereof is given by you; or (iv) a request that you
        relocate to a worksite that is more than 35 miles from your prior
        worksite, unless you accept such relocation opportunity; or (v) a
        material reduction in your duties; or (vi) a failure or refusal of a
        successor to the Company to assume the Company's obligations under this
        Agreement; or (vii) a material breach by the Company or any successor to
        the Company of any of the material provisions of this Agreement.

If your employment is involuntarily terminated without Cause by the Company or
you voluntarily terminate your employment for Good Reason, then upon your
furnishing to the Company an executed waiver and release of claims (a form of
which is attached hereto as Exhibit A), you will be entitled to the following
Benefits:

        If your employment is involuntarily terminated without Cause any time
        other than described below, then (1) within 30 days of such termination
        you will receive a lump-sum payment equal to six months of Base Salary
        subject to any applicable withholding of federal, state or local taxes
        and (2) the vesting of any stock option for Company stock and the time
        during which such option may be exercised immediately will be
        accelerated by six months.

        If, within 365 days following the occurrence of a Change in Control,
        your employment is involuntarily terminated without Cause or you
        voluntarily terminate your employment for Good Reason, then (1) within
        30 days of such termination you will receive a lump-sum payment equal to
        six months of base salary subject to any applicable withholding of
        federal, state or local taxes and (2) the vesting of any stock option
        for Company stock and the time during which such option may be exercised
        immediately will be accelerated as to 50% of the then unvested shares.

If any Benefit received or to be received by you pursuant to this Agreement
would constitute a "parachute payment" within the meaning of Section 280G of the
Code and but for this paragraph be subject to the Excise Tax, then such Benefit
shall be reduced to the extent necessary so that no portion of the Benefit would
be subject to the Excise Tax, as determined in good faith by the Company;
provided, however, that if, in the absence of any such reduction (or after such
reduction), you believe that the Benefit or any portion thereof (as reduced, if
applicable) would be subject to the Excise Tax, the Benefit shall be reduced (or
further reduced) to the extent determined by you in your discretion so that the
Excise Tax would not apply. If, notwithstanding any such


<PAGE>   3
Alessandro Sette, Ph.D.
February 5, 1998
Page 3




reduction (or in the absence of such reduction), the Internal Revenue Service
determines that you are liable for the Excise Tax as a result of the Benefit,
then you shall be obligated to return to the Company, within 30 days of such
determination by the Internal Revenue Service, a portion of the Benefit
sufficient such that none of the Benefit retained by you constitutes a
"parachute payment" within the meaning of Code Section 280G that is subject to
the Excise Tax.

This Agreement may be changed or terminated only upon the mutual written consent
of the Company and you. Nothing in this Agreement will prevent or limit your
continuing or future participation in any benefit, bonus, incentive or other
plans, programs, policies or practices provided by the Company and for which you
may otherwise qualify, nor will anything herein limit or otherwise affect such
rights as you may have under any stock option or other agreements with the
Company.

If either party hereto brings any action to enforce such party's rights
hereunder, the prevailing party in any such action will be entitled to recover
such party's reasonable attorneys' fees and costs incurred in connection with
such action.

This Agreement constitutes the entire agreement between you and the Company. It
is entered into without reliance on any promise or representation other than
those expressly contained herein. All questions concerning the construction,
validity and interpretation of this Agreement will be governed by the law of the
State of California.

Very truly yours,

Epimmune Inc.


/s/ Deborah A. Schueren


Deborah A. Schueren
President

DAS/dw






Agreed and Accepted:

/s/ Alessandro Sette
-------------------------------
Alessandro Sette, Ph.D.


Dated: 2/12, 1998


<PAGE>   4
                                    EXHIBIT A
                          RELEASE AND WAIVER OF CLAIMS



        In exchange for consideration provided to me in the Agreement to which
this form is attached, I hereby furnish Epimmune Inc. (the "Company") with the
following release and waiver.

        I hereby release, and forever discharge the Company, its officers,
directors, agents, employees, stockholders, successors, assigns and affiliates,
of and from any and all claims, liabilities, demands, causes of action, costs,
expenses, attorneys fees, damages, indemnities and obligations of every kind and
nature, in law, equity, or otherwise, known and unknown, suspected and
unsuspected, disclosed and undisclosed, arising at any time prior to and
including my employment termination date with respect to any claims relating to
my employment and the termination of my employment, including but not limited
to, claims pursuant to any federal, state or local law relating to employment,
including, but not limited to, discrimination claims, claims under the
California Fair Employment and Housing Act, and the Federal Age Discrimination
in Employment Act of 1967, as amended (ADEA), or claims for wrongful
termination, breach of the covenant of good faith, contract claims, tort claims,
and wage or benefit claims, including but not limited to, claims for salary,
bonuses, commissions, stock, stock options, vacation pay, fringe benefits,
severance pay or any form of compensation (other than the obligations under the
Agreement.)

        I also acknowledge that I have read and understand Section 1542 of the
California Civil Code which reads as follows: "A general release does not extend
to claims which the creditor does not know or suspect to exist in his favor at
the time of executing the release, which if known by him must have materially
affected his settlement with the debtor." I hereby expressly waive and
relinquish all rights and benefits under that section and any law of any
jurisdiction of similar effect with respect to any claims I may have against the
Company.

        I acknowledge that, among other rights, I am waiving and releasing any
rights I may have under ADEA, that this waiver and release is knowing and
voluntary, and that the consideration given for this waiver and release is in
addition to anything of value to which I was already entitled as an employee of
the Company. I further acknowledge that I have been advised, as required by the
Older Workers Benefit Protection Act, that: (a) the waiver and release granted
herein does not relate to claims which may arise after this agreement is
executed; (b) I have the right to consult with an attorney prior to executing
this agreement (although I may choose voluntarily not to do so); (c) I have
twenty-one (21) days from the date I receive this agreement in which to consider
this agreement (although I may choose voluntarily to execute this agreement
earlier); (d) I have seven (7) days following the execution of this agreement to
revoke my consent to the agreement; and (e) this agreement shall not be
effective until the seven (7) day revocation period has expired.




Dated: _____________________  By:________________________________