This is a revised competing continuation application that proposes to study the development of natural killer (NK) cells which are important constituents of the early host innate immune response to tumors and infections. They possess potent capacities to kill cells and secrete pro-inflammatory cytokines that must be controlled in order to prevent inadvertent tissue destruction, i.e., NK cells must display tolerance to self. In the prior funding period, the applicant's laboratory made several important contributions. The most important is related to how NK cells acquire self-tolerance. They showed that NK cells acquire the functional capacity to be triggered through their activation receptors to kill and produce cytokines. This process, termed "licensing," is mediated by Ly49 receptors on NK cells that recognize self-major histocompatibility complex (MHC) class I molecules. Ironically these receptors were originally identified as inhibitory receptors in NK cell effector responses. There are thus two types of self-tolerant NK cells, those that are licensed and inhibited by self-MHC through the same Ly49 receptors that conferred licensing and unlicensed NK cells that are tolerant because they are not functionally competent. However, unlicensed NK cells can derive enhanced function when pre-activated by other stimuli to bypass MHC-dependent licensing. Herein, the applicant proposes to study licensing in greater detail. The specific aims of the proposal are to study: 1) Ly49 specificities for MHC alleles in licensing;2) MHC class I epitope recognized by Ly49 in licensing;3) Signal strength in licensing;and 4) Bypassing MHC-dependent licensing by toll-like receptor stimulation. Thus, these studies will provide significant new insight into how NK cells achieve self-tolerance.