Atara Biotherapeutics Inc. has priced its secondary stock offering to raise nearly $128 million, the company announced late Wednesday. Atara is headquartered in South San Francisco but its research and development operation is in Westlake Village. The company researches treatments in T-cell immunotherapy. The secondary offering involves 7 million shares at $18.25 a share. It is expected to close on Jan. 8. Atara has granted the underwriters a 30-day option to sell an extra 1 million shares. J.P. Morgan and Cowen Group are joint book-running managers for the proposed offering. William Blair & Co. and Canaccord Genuity Inc. are co-managers. Shares of Atara (ATRA) closed Thursday down $1.25 or 6.5 percent to $18 on the Nasdaq.