Human natural killer cells are large granular lymphocytes that provide a critical component of the innate immune response. Lacking antigen specificity, they are the least well characterized of lymphocytes, yet the oldest lymphocyte subset in evolution and phylogeny. NK cells make up approximately 15% of all circulating lymphocytes and, through their early production of cytokines and chemokines, are responsible for monocyte/macrophage activation and actively involved in orienting the subsequent antigen specific immune response. Human NK cells can be divided into two subsets based on cell-surface density of CD56, CD56 bright and CD56 dim,each with distinct phenotypic and, as we will present in this project, functional profiles. We will provide preliminary evidence that human NK cell subsets can be distinguished on the basis of their cytokine profiles following activation, and that NK cell expansion and activation are optimized with co-stimulatory signals. This has broad clinical implications, as NK cells produce large amounts of IFN-gamma and promote antibody-dependent cellular cytotoxicity (ADCC). A better understanding of the biology of human NK cell subsets and the factors responsible for their activation will likely identify improved immunotherapeutic strategies to manipulate a more effective anti-tumor activity in clinical cancer trials. We propose to address the following issues relating to NK cell subset activation and cytekine production: 1) to characterize the mechanisms by which human NK cell subsets regulate cytokine production; 2) to investigate CD56 bright and CD56 dim NK receptor expression and recognition of autologous leukemic blasts; and 3) to examine the function of NK cell subsets following Fc activation.