Anti-human ICAM-1 monoclonal antibody binding cells in germfree neonatal piglets

Unique immunoiogically "virgin" germfree (GF), colostrum-deprived piglet model was used to examine which cells express ICAM-1 molecule, development of ICAM-1 positive cells, and distribution of these cells in the porcine lymphoid organs. Alkaline phosphatase-anti-alkaline phosphatase (APAAP) method using anti-human ICAM-1 mAb (R6.5) allowed visualization of the ICAM-1 positive cells in frozen sections of spleen and thymus, and bone marrow (BM) imprints. The BM imprints and spleen sections of GF piglets showed many positive cells while thymus was totally negative. In spleen sections of GF piglets the positive cells were always present as a cluster over the red pulp. These large, multinucleated cells were most abundant in 0 day-old GF piglets, decreasing with age and disappearing from 2 wks after birth. In BM imprints of GF piglets the positive cells had a similar shape and showed stronger positive staining than those observed in spleens of GF piglets and increased with age till 8 wks after birth. They showed abundant azurophilic granules in cytoplasm with Giemsa staining. Positive staining for human ICAM-1 mAb was completely blocked by excess of soluble ICAM-1 molecules in both spleens and BM. These results together with previous studies on B cell response suggest that ICAM-1 positive cells may be involved in the early development of B lymphocytes.