X0306 4 2018-06-05 0 0001501697 Arsanis, Inc. ASNS 0001355415 Clark William D C/O ARSANIS, INC. 890 WINTER STREET, SUITE 230 WALTHAM MA 02451 1 0 0 0 Stock Option (right to buy) 16.99 2018-06-05 4 A 0 10000 0.00 A 2028-06-04 Common Stock 10000 10000 D The option becomes exercisable as to 1/12th of the shares underlying the award on the one-month anniversary of the date of grant, with the remainder vesting in equal monthly installments until the first anniversary of the date of grant, subject to the Reporting Person's continued service as a director through each applicable vesting date. /s/ Michael Gray, attorney-in-fact 2018-06-07