We present here the molecular characterization of a new activation-induced surface structure on human T lymphocytes, termed LA45, with high homology (93% at protein level) to MHC class I molecules. Antigen modulation and sequential immunoprecipitation experiments revealed that LA45 and HLA class I proteins do not crossreact with the corresponding antibodies. Furthermore, LA45 is not associated with beta 2-m. On the other hand, we could show that the separation of HLA-A,B,C and beta 2m molecules, induced by SDS-denaturation, leads to a conformational change in the heavy chain in such a way that it becomes reactive with LA45. The 90/45 kD LA45 proteins thus appear to be non-beta 2m-associated MHC class I alpha chains that are selectively expressed by activated but not by resting human T lymphocytes.

Monoclonal antibodies (mAbs) W6/32, HC10, and 4E were used to precipitate class I antigens from 21 selected individuals with at least one HLA-C "blank" allele. In 19 of these individuals, characteristic HLA-C banding patterns which could be precipitated by all three HLA class I mAbs were observed on one-dimensional isoelectric focusing gels--obviously the gene products of HLA-C "blank". At least four allelic HLA-C "blank" gene products with different isoelectric points could be discerned. All of them segregated with HLA-C "blank" haplotypes in informative families; two of them were associated with HLA-B51, one with HLA-B38, and one with HLA-B18. Reactivity of the HLA-C "blank" heavy chains with mAb W6/32 indicates that they are able to associate with beta-2 microglobulin, and hence are most probably expressed at the cell surface.

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