FETAL LAMBS ARE DEPLETED OF IGM(-IGM ANTIBODY EARLY IN GESTATION() CELLS FOLLOWING A SINGLE INJECTION OF AN ANTI)

B-cell depleted fetal sheep were created following a single injection of an anti-IgM monoclonal antibody early in gestation. Six sheep fetus es were given a single intraperitoneal injection of a monoclonal antib ody directed against IgM at 63 days of gestation (gestation in sheep = 150 days). The fetuses were killed at 138-142 days of gestation and l ymphoid tissues were collected for subsequent light microscopy and imm unohistochemical examination. The ileal and jejunal Peyer's patch (PP) follicles in four of the six injected fetuses were markedly reduced i n size. Cells in the rudimentary follicles of the ileal PP of these an imals showed no reactivity for IgM and most were negative for CD45. Th e dome regions contained many T cells, which were predominantly CD8(+) cells and included gamma delta T cells. The interfollicular areas of the PP of the markedly affected fetuses contained large populations of T cells. The spleen and lymph nodes were also markedly depleted of Ig M(+) cells and these tissues contained only a small, scattered populat ion of weakly IgM(+) cells. Follicular accumulations of IgM(+) cells w ere absent. Large populations of T cells were present in the white pul p of the spleen and cortex of the lymph nodes. The liver did not conta in IgM(+) cells and the medulla of the thymus was depleted of IgM(+) c ells. The results of this study suggest that a surface IgM(+) B-cell p opulation is present in the sheep fetus at 63 days of gestation, which is essential for the colonization of the ileal PP and subsequent B-ce ll development.