mAb were used in an immunocytochemical assay to examine beta 2-microglobulin (b2-m) and class I MHC expression in human neuroblastoma cell lines. In lines with weak class I expression among the whole population, under ordinary assay conditions, strong b2-m and class I expression were concentrated in a small subpopulation. In positive cells, Ag was not restricted to any part of the cell body or processes. Strong expression was not required for establishment of any morphologic form or any type of cell contact. These findings complement studies in other experimental systems, where a nonimmunologic role for class I or b2-m in neural cell growth was not revealed. When the microscopic assay was modified to reveal Ag within the internal membrane system, b2-m was detected in every neuroblastoma cell. Most often, the Ag appeared as a ring around the nucleus, or in a punctate distribution in the juxtanuclear area. Internal expression of HLA chains and class I molecules was more difficult to detect, possibly reflecting a normal excess of b2-m. These findings increase understanding of MHC regulation in neural cell lines. They provide the technical and conceptual background for examination of internal MHC Ag in neural tissue.

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