Reduced expression of major histocompatibility complex class I free heavy chains and enhanced sensitivity to natural killer cells after incubation of human lymphoid lines with beta 2-microglobulin.

Enhancement of major histocompatibility complex (MHC) class I expression leads to protection from recognition by natural killer (NK) cells in several systems. MHC class I gene products can be expressed in different forms at the cell surface--for example as "empty" beta 2-microglobulin (beta 2m)-associated heterodimers or free heavy chains. To study the role of different class I heavy chain forms in NK target interactions, we have used lymphoblastoid target cell lines preincubated with beta 2m. This was found to shift the equilibrium between beta 2m-associated and non-associated--heavy chains in favor of the former. In parallel, there was a significant increase in NK sensitivity. The recognition of MHC class I-deficient cell lines was not affected by beta 2m, arguing against a general nonspecific effect of beta 2m on NK sensitivity. Our data indicate that protection against NK recognition correlates with target cell expression of free heavy chains (i.e. devoid of beta 2m) rather than with expression of complexes.