MICA, A NEW POLYMORPHIC HLA-RELATED ANTIGEN, IS EXPRESSED MAINLY BY KERATINOCYTES, ENDOTHELIAL-CELLS, AND MONOCYTES

MICA is a new polymorphic gene in the HLA region expressed in epitheli al cell lines and gastrointestinal epithelium. Little is yet known abo ut the MICA protein, and the pattern of its expression by freshly isol ated cells has not been established. In the present experiments, we us ed antibodies raised in rabbits against alpha 1 and alpha 2 domain-pep tides to study the expression of MICA. By western blot and immunopreci pitation, we detected a band of 62 000 M-r in various cell lines (THP- 1, U937, HeLa, A431, Raji, MOLT-4, and HUV-EC-C) and in freshly isolat ed keratinocytes, endothelial cells, and monocytes but not in CD4(+) a nd CD8(+) T cells, and CD19(+) cells (B lymphocytes). It was not possi ble to up-regulate the expression of MICA in different cells by stimul ation with gamma-interferon, but the expression of MICA was induced in phytohemagglutinin-stimulated T cells. We confirmed that MICA is expr essed at the cell surface by flow cytometry. Results of immunoprecipit ation studies of beta(2)-microglobulin(beta(2)m)- or MICA-depleted, me tabolically labeled HeLa cells indicated that MICA was not associated with beta(2)m. Although the function of MICA is still unknown, its res tricted pattern of tissue expression, the fact that it is expressed on the cell surface, and its polymorphic nature suggest that this new mo lecule, encoded close to HLA class I, may play a role in the interacti on between epithelial cells and cells of the immune system.