About four months after starting its search for strategic alternatives, Sesen Bio Inc. disclosed an all-stock merger with privately held cell-therapy specialist Carisma Therapeutics Inc., a move that sent shares (NASDAQ:SESN) down 22 cents, or 32.7%, to close at 45 cents. Sesen stockholders are expected to own about 41.7% of the combined firm, and Carisma the remainder.