INTRODUCTION OF EXOGENOUS CLASS-II TRANSACTIVATOR IN MHC CLASS II-DEFICIENT ABI FIBROBLASTS RESULTS IN INCOMPLETE RESCUE OF MHC CLASS-II ANTIGEN EXPRESSION

Previously, we have shown that fibroblasts established from type III b are lymphocyte syndrome patient ABI are characterized by the absence o f MHC class II gene expression and a strongly reduced amount of MHC cl ass I transcripts. Complementation analysis has suggested that the gen e defective in these ABI fibroblasts is different from that encoding t he class II trans-activator (CIITA), which has been attributed an esse ntial role in both constitutive and inducible expression of MHC class II genes. In the present study it is shown by reverse transcriptase-PC R analysis that the amount of CIITA transcripts in ABI fibroblasts is greatly reduced compared with that in fibroblasts derived from a healt hy individual, Transient cotransfection of a construct in which CIITA is under the control of a constitutive promoter with an HLA-DRA promot er-luciferase reporter plasmid resulted in enhanced luciferase express ion in ABI fibroblasts. Furthermore, ABI fibroblasts stably transfecte d with CIITA re-express functional HLA-DR Ags, but do not express HLA- DQ and DP Ags at the cell surface. Comparison of these data with those obtained for normal fibroblasts and fibroblasts defective for CIITA i ndicate that the gene defect and the resulting lack of MHC class II ex pression in ABI fibroblasts can only partly be corrected by the introd uction of CIITA. Furthermore, DNase I hypersensitivity analysis of ABI fibroblasts has revealed a closed chromatin structure in the promoter region of the MHC class II DRA gene. However, CIITA transfection resu lted in an open DNA configuration, which suggests a role for CIITA in provoking changes in the chromatin structure of the DRA gene.