Two distinct genetic loci regulating class II gene expression are defective in human mutant and patient cell lines.

Heterokaryons were prepared and analyzed shortly after cell fusion using two mutant class‐II‐negative human B cell lines (RJ 2.2.5 and 6.1.6) and a cell line (TF) from a patient with a class‐II‐negative Bare Lymphocyte Syndrome. The resulting transient heterokaryons were analyzed by using an anti‐HLA‐DR monoclonal antibody to assess the cell surface expression of HLA‐DR (the major subtype of class II antigens) by immunofluorescence microscopy and by using uniformly 32P‐labeled SP6 RNA probes in Northern blots and RNase protection assays to assess mRNA synthesis. We find that class II gene expression in a B cell line from a Bare Lymphocyte Syndrome patient (TF) is rescued by a B cell line which expresses class II antigens indicating that this disease, at least in part, is caused by a defect(s) in a genetic locus encoding a factor(s) necessary for class II gene expression. Secondly, reciprocal genetic complementation was demonstrated in the heterokaryons 6.1.6 x RJ 2.2.5 and TF x RJ 2.2.5 (but not in TF x 6.1.6) by detection of cell surface DR by immunofluorescence microscopy and by a novel class II mRNA typing technique which allows characterization of distinct class II alleles. Thus, the two mutants generated in vitro have defects at two different genetic loci encoding specific regulatory factors necessary for human class II gene expression. One of these mutant cell lines, but not the other, complements the defect in the patient cell line, TF.