Heterogeneity of non-lymphoid cells expressing HLA-D region antigens in human fetal gut.

Human fetal ileum contains an abundance of cells expressing HLA-D region (HLA-DR) antigens. In this study we characterized the HLA-DR positive cellular infiltrate in fetal ileum using a panel of monoclonal antibodies against cells of the macrophage/monocyte lineage. As well as anti-HLA-DR, the whole infiltrate was recognized by three of the antibodies in the panel studied: RFD1, reported to be an antibody to 'dendritic cells', leu3a which recognizes CD4 (an antigen expressed on helper/inducer T cells and macrophages), and PD7/26 which recognizes the leukocyte common antigen. The macrophage specific antibodies RFD7 and 3.9 stained fewer cells than the anti-HLA-DR. By sequential staining it was clear that most of the RFD7 positive macrophages in the fetal gut lamina propria were not recognized by 3.9 which is a broad specificity antimacrophage antibody in adult tissue. In contrast to this, more of the macrophages within the Peyer's patches of the fetal gut were recognized by 3.9 than by RFD7. In fetal lamina propria not all of the HLA-D region positive cells expressed macrophage markers and some expressed both markers associated with macrophages and 'dendritic' cells. Macrophages with different surface phenotypes were differentially distributed between the lamina propria and the primitive Peyer's patches; RFD7+, 3.9- macrophages were concentrated in the lamina propria whereas RFD7-, 3.9+ macrophages were abundant in Peyer's patches. Within the Peyer's patch lymphoid tissue RFD7+, 3.9- cells were present in the T cell zone whereas RFD7-, 3.9+ cells were concentrated in the dome region as they are in the adult. This suggests that functional heterogeneity of organized macrophage populations may occur in fetal ileum which is free of dietary and bacterial antigens.