Cytotoxic T lymphocytes (CTL) can recognize antigenic peptides bound in a groove formed by the alpha 1 and alpha 2 domains of the heterodimeric major histocompatibility complex class I molecule. Proper assembly, transport, and stability of functional class I molecules is thought to require beta 2 microglobulin (beta 2m), the light chain of the class I heterodimer. We show here that the requirement for beta 2m is not absolute. beta 2m- cells can be stained by the Db alpha 1 domain-specific B22-249.1 monoclonal antibody, which detects a conformation-dependent epitope. Furthermore, beta 2m- Con A blast target cells can be lysed by alloreactive CTL, even in serum-free conditions. Contrary to previous reports, the expression of low levels of conformed Db heavy (H) chains is a property to both normal and transformed beta 2m- cells. Finally, we present evidence that a subset of properly conformed H chains, free of beta 2m, may have almost equal representation on beta 2m+ and beta 2m- cells.

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