HEMCAM, AN ADHESION MOLECULE EXPRESSED BY C-KIT(+) HEMATOPOIETIC PROGENITORS

We have characterized the adhesion molecule HEMCAM, which is expressed by hemopoietic progenitors of embryonic bone marrow. HEMCAM belongs t o the immunoglobulin superfamily and consists of the V-V-C2-C2-C2 Ig d omains. There are three mRNA splice variants. One has a short cytoplas mic tail; another has a long tail; while the third seems to lack trans membrane and cytoplasmic regions. Except for the NH2-terminal sequence , HEMCAM is identical to gicerin, a molecule involved in neurite outgr owth and Wilm's kidney tumor progression in the chicken and it is sign ificantly homologous with MUC18 a molecule involved in melanoma progre ssion and metastasis in human beings. In the bone marrow the HEMCAM(+) cell population contains c-kit(+) subsets. HEMCAM(+) cells coexpressi ng the receptor tyrosine kinase c-kit give rise to T cells at a freque ncy of 0.17 when injected intrathymically in congenic animals. As HEMC AM(+), c-kit(+) cells differentiate into myeloid and erythroid CFU's t he double-positive cell population seems to contain precursors for mul tiple lineages. HEMCAM promotes cell-cell adhesion of transfected cell s, Cross-linking of murine HEMCAM leads to cell spreading of T-lymphoc yte progenitors adhering to the vascular adhesion molecules, PECAM-1 a nd VCAM-1. Thus, HEMCAM is likely to be involved in cellular adhesion and homing processes.