Adhesion Molecule ICAM-4 Participates in Erythroblastic Island Formation.

Erythroblasts proliferate, differentiate and enucleate within erythroblastic islands, three dimensional structures comprised of developing erythroblasts surrounding a central macrophage. Collective evidence suggests that erythroblastic islands are highly specialized bone marrow subcompartments where adhesion events, in concert with cytokines, play critical roles in regulating erythropoiesis and apoptosis. ICAM-4, a recently characterized member of the immunoglobulin superfamily, is expressed early in erythroid differentiation. This adhesion molecule interacts with multiple integrin binding partners, including alpha4beta1 and alphaV integrins (alphaVbeta1, alphaVbeta3 and alphaVbeta5). Since erythroblasts express alpha4beta1 and ICAM-4 and macrophages exhibit alphaV, ICAM-4 is an attractive candidate for mediating erythroblast-erythroblast and erythroblast-macrophage attachments. A molecular model of ICAM-4 derived from the crystal structure of closely related ICAM-2 presents the extracellular region of ICAM-4 as two Ig-like domains comprised of A,B,C,D,E,F, and G strands. Employing targeted mutagenesis of surface-exposed amino acid residues, we earlier identified a patch or footprint that mediates adhesion to alphaV integrins comprised of three A strand residues and five G strand residues on N-terminal domain 1. To explore whether ICAM-4 attachments are active in erythroblastic islands we first developed a quantitative live cell assay for reforming islands from single cell suspensions of freshly harvested mouse bone marrow. Islands and their cellular components were identified and quantitated by three-color immunofluorescent microscopy employing fluoresceinated erythroid-specific TER119 antibody, macrophage-specific F4/80 antibody and a DNA probe. To determine the amount of variation in number of islands reformed from a single cell suspension of 1 x 10⁵ cells, we counted islands at the beginning and conclusion of experiments on five different mice. The island numbers were very reproducible and equaled 1000 +/− 158. We then tested the effects of two synthetic peptides that we have previously shown block ICAM-4/alphaV adhesion: peptides FWV and ATSR, corresponding to sequences of the A and G strands of ICAM-4 domain 1, respectively. Both peptides caused a marked, concentration dependent decrease in the percentage of islands formed. 2mM ATSR inhibited island formation by 75% while 2mM FWV inhibited island formation by 70%. In marked contrast, a strand D control peptide had minimal to no effect on island formation. Our data strongly suggest that erythroblast ICAM-4 binding to macrophage alphaV is critical for erythroblastic island formation. We postulate that this newly identified receptor-counterreceptor interaction may be important not only for adhesive integrity of the island structure but also for initiating intracellular signaling essential for normal erythroid terminal differentiation.