Fetal lambs are depleted of IgM+ cells following a single injection of an anti-IgM antibody early in gestation.

B-cell depleted fetal sheep were created following a single injection of an anti-IgM monoclonal antibody early in gestation. Six sheep fetuses were given a single intraperitoneal injection of a monoclonal antibody directed against IgM at 63 days of gestation (gestation in sheep = 150 days). The fetuses were killed at 138-142 days of gestation and lymphoid tissues were collected for subsequent light microscopy and immunohistochemical examination. The ileal and jejunal Peyer's patch (PP) follicles in four of the six injected fetuses were markedly reduced in size. Cells in the rudimentary follicles of the ileal PP of these animals showed no reactivity for IgM and most were negative for CD45. The 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 IgM+ cells and these tissues contained only a small, scattered population of weakly IgM+ cells. Follicular accumulations of IgM+ cells were absent. Large populations of T cells were present in the white pulp of the spleen and cortex of the lymph nodes. The liver did not contain IgM+ cells and the medulla of the thymus was depleted of IgM+ cells. The results of this study suggest that a surface IgM+ B-cell population is present in the sheep fetus at 63 days of gestation, which is essential for the colonization of the ileal PP and subsequent B-cell development.