Augmentation of antitumor effects by NK cell inhibitory receptor blockade in vitro and in vivo

Subsets of natural killer (NK) cells are characterized by the expression of inhibitory and/or stimulatory receptors specific for major histocompatibil ity complex (MHC) class I determinants. In mice, these include the Ly49 fam ily of molecules. One mechanism by which tumor cells may evade NK cell kill ing is by expressing the appropriate MHC class I and binding inhibitory Ly4 9 receptors. Therefore, the question of whether blocking the interaction be tween the Ly49 inhibitory receptors on NK and MHC class I cells on tumor ce lls augments antitumor activity was investigated. Blockade of Ly49C and I i nhibitory receptors using F(ab')(2) fragments of the 5E6 monoclonal antibod y (mAb) resulted in increased cytotoxicity against syngeneic tumors and dec reased tumor cell growth in vitro. The effect of 5E6 F(ab)(2) was specific for the MHC of the tumor, as the use of F(ab')(2) of the mAb against Ly49G2 failed to increase NK activity. Treatment of leukemia-bearing mice with 5E 6 F(ab')(2) fragments or adoptive transfer of NK cells treated ex vivo with the F(ab')(2) resulted in significant increases in survival. These results demonstrate that blockade of NK inhibitory receptors enhances antitumor ac tivity both in vitro and in vivo, suggesting that NK inhibitory receptors c an be responsible for diminishing antitumor responses. Therefore, strategie s to block inhibitory receptors may be of potential use in increasing the e fficacy of immunotherapy. (Blood. 2001;97:3132-3137) (C) 2001 by The Americ an Society of Hematology.