Coexpression of NKR-P1 and alpha beta-TCR on lymphoid cells in fully xenogeneic (rat-->mouse) chimeras and syngeneically reconstituted (A-->A) rats.

We have identified three populations of cells among rat splenocytes expressing NKR-P1, including cells of NKR-P1bright/alpha beta-TCR-, NKR-P1dim/alpha beta-TCR+, and NKR-P1dim/alpha beta-TCR- phenotypes. To study the phenotypic characteristics and development of these various cell populations, we have made use of transplanting untreated rat bone marrow into recipient rats (syngeneic) or mice (fully xenogeneic) conditioned with total body irradiation. Rat NK cells exhibit normal phenotypic markers (NKR-P1+, CD8+) and are normally functional by 28 days after reconstitution. We have found that the various populations of NKR-P1+ cells are enriched significantly in the spleen and follow a characteristic pattern of development in the first mo after reconstitution. After syngeneic bone marrow reconstitution (rat-->rat), NKR-P1dim and NKR-P1bright cells (3-15%) can be demonstrated among splenocytes as early as day 3 after bone marrow transplantation. By day 7, the NKR-P1+ cells reach peak levels and comprise as much as 80% of splenic lymphoid cells, with 35% being NKR-P1bright and 45% being NKR-P1dim. The percentage of NKR-P1+ cells decreases over the next several wk until they constitute "normal rat" levels with 8 to 20% being NKR-P1bright and only 1 to 5% being NKR-P1dim. These same populations are also present in fully xenogeneic chimeras. In both models, approximately 80 to 90% of the NKR-P1dim cells were found to coexpress alpha beta-TCR at all time points. These NKR-P1dim/alpha beta-TCR+ cells are not large granular lymphocytes and lack NK cell lytic activity against YAC-1 target cells. Additional analyses of cells derived from spleen, bone marrow, and thymus indicated that NKR-P1+ cells develop, for the most part, in a thymic-independent manner in our fully xenogeneic chimeras and syngeneically reconstituted rats. At present the developmental pathway of these NKR-P1dim cells remains speculative.