DIFFERENTIAL EXPRESSION OF MTV LOCI IN MHC CLASS II-POSITIVE THYMIC STROMAL CELLS

Despite the important role played by endogenous superantigen in shapin g the T cell repertoire, little is known concerning the expression of the different Mtv loci in cells of the thymic microenvironment involve d in repertoire selection. Here we have examined the expression of a p anel of Mtv Ags by different MHC class II+ stromal cell types using re verse transcriptase-PCR and monitored the effects of these stromal cel ls on the development of cells expressing Mtv-reactive TCR V beta elem ents in closed thymic organ culture systems. Although Mtv-6 and Mtv-8/ 9 mRNAs are expressed in normal thymus robe organ cultures, no Mtv exp ression was detected in MHC class II+ thymic epithelial cells. In cont rast a striking pattern of differential expression was observed in den dritic cells of thymic origin that were devoid of Mtv-8/9 but expresse d readily detectable levels of Mtv-6. This pattern of Mtv gene express ion correlated well with TCR V beta repertoire development. TCRV beta 3(+) T cells, normally deleted in response to Mtv-6, were virtually ab sent from the single positive thymocyte compartment in thymic organ cu ltures where dendritic cells are present but were present in reaggrega te cultures where the only MHC class II-positive cells were thymic epi thelial cells. On the other hand, V beta 11(+) T-cells were not delete d in organ cultures, possibly reflecting the absence of Mtv-8/9 expres sion in dendritic cells. Our studies suggest that the influence Mtvs h ave on shaping the T cell repertoire not only depends on their express ion within a particular strain but also on their tissue specific expre ssion in relation to MHC class II, which is necessary for their presen tation.