Analysis of ligand-independent cap formation induced in hypertonic medium

Ligand-independent (LI-) cap formation induced in hypertonic medium was investigated particularly in comparison with ligand-dependent (LD-) cap formation. Major observations presented in this report are: 1. 1. Two species of receptors moved differentially on individual lymphocytes during LI-cap formation. 2. 2. Bipolar cap was formed in a minor population of the total cells after incubation in 2 x PBS in the absence of ligand whereas no bipolar cap was obtained in the presence of ligand in 2 x PBS. 3. 3. LI-cap formation was induced repeatedly on the cells that had formed cap when exposed to 2 x PBS for the first time and had regained diffuse distribution of receptors by lowering the osmolarity of medium to PBS. 4. 4. Maintenance of the cap-state in 2 x PBS in the absence of ligand requires cellular functions which are inhibitable either by sodium azide or by cytochalasin B (CB). 5. 5. Specific binding of antibodies directed against immunoglobulin to B lymphocytes stimulates LI-cap formation of unrelated receptors on these cells. Although some essential features of LI-cap formation (e.g. independency of ligand and accumulation of microvilli in the region of cap) are clearly distinguishable from those of LD-cap formation, it can be pointed out from the present results that LI-cap formation appears to be similar in part to LD-cap formation of concanavalin A (ConA) receptors rather than to LD-cap formation of surface Ig in regard to its features such as the reversibility of cap formation. These results are discussed in terms of interactions between surface receptors and cytoplasmic structures.