Discrepant effects of Chlamydia trachomatis infection on MICA expression of HeLa and U373 cells

Natural killer (NK) cells are an important component of the host immune defense against Chlamydia trachomatis (CT). In this study we investigated the effects of CT on the expression of two forms of MHC class I-related chain A (MICA). One form, a truncated variant that has a partial transmembrane region and no cytoplasmic tail, was expressed in HeLa cells. The other, a full-length variant, was expressed in astrocytoma U373 cells. CT infection had little effect on the expression of the truncated form, which was constitutively expressed in HeLa cells, but expression of the full-length MICA was down-regulated in CT-infected U373 cells. Down-regulation of MICA after CT infection protected U373 cells from NK cell lysis, whereas HeLa cells expressing the truncated form were killed. This study shows that the constitutive expression of the truncated from of MICA prevents CT from evading immune recognition by NK cells.