ACQUISITION OF LY49 RECEPTOR EXPRESSION BY DEVELOPING NATURAL-KILLER-CELLS

The formation of the repertoire of mouse natural killer (NK) cell rece ptors for major histocompatibility complex (MHC) class I molecules was investigated by determining the developmental pattern of Ly49 recepto r expression. During the first days after birth, few or Ilo splenic NK cells express Ly49A, Ly49C, Ly49G2, or Ly49I receptors. The proportio n of Ly49(+) splenic NK cells gradually rises to adult levels during t he first 6-8 wk of life. The appearance of appreciable numbers of sple nic Ly49(+) NK cells coincides with the appearance of NK activity at 3 -4 wk. After in vivo transfer, NK cells not expressing specific Ly49 r eceptors can give rise to NK cells that do, and cells expressing one o f these four Ly49 receptors can give rise to cells expressing others. Once initiated, expression of a Ly49 receptor is stable for at least 1 0 d after in vivo transfer. Hence, initiation of Ly49 receptor express ion occurs successively. Interestingly, expression of one of the recep tors tested, Ly49A, did not occur after in vivo transfer of Ly49A(-) c ells. One possible explanation for these data is that the order of Ly4 9 receptor expression by NK cells is nonrandom. The results provide a framework for evaluating models of NK cell repertoire formation, and h ow the repertoire is molded by host class I MHC molecules.