Novel mutant mice secreting soluble CD4 without expression of membrane-bound CD4

Mutant mice derived from C57BR/cdJ mice were found to have a novel genetic defect in CD4 expression. Flow-cytometric analysis demonstrated that there were no CD4+ cells in either the thymus or the peripheral lymphoid organs of the mutant mice. Thymocytes of the mutant mice expressed an amount of CD4 mRNA comparable to normal mouse thymocytes, but the mutant CD4 mRNA was slightly smaller in size than normal CD4 mRNA. The sequence analysis of the mutant CD4 cDNA obtained from thymic RNA revealed that the defect in the CD4 expression was attributable to the deletion of the entire exon VIII, encoding a transmembrane domain of the CD4 molecule. Moreover, soluble CD4 was detected both in the culture supernatant of thymocytes and sera from mutant mice. The analysis of the genomic DNA sequence elucidated that one thymine was substituted for 14 base pairs at the junction between exon VIII and intron VIII in the mutant mice, which could possibly account for the alternative splicing of CD4 mRNA. These mutant mice showed reduced delayed-type hypersensitivity reactions against sheep red blood cells and antibody production against T-dependent antigen but not against T-independent antigen. Thus, these mutant mice have a novel defect in CD4 expression where CD4 mRNA is alternatively spliced to delete a transmembrane domain, giving rise to secretion of soluble CD4 instead of expression of membrane-bound CD4.