REGULATED EXPRESSION OF IMMUNOGLOBULIN TRANS-MESSENGER RNA CONSISTING OF THE VARIABLE REGION OF A TRANSGENIC-MU CHAIN AND CONSTANT REGIONS OF ENDOGENOUS ISOTYPES

Our previous work demonstrated that chimeric immunoglobulin mRNAs (trans-mRNAs) composed of a transgenic V(H)DJ(H) region and endogenous C(H) sequences could be synthesized, most likely by a trans-splicing mechanism, in a transgenic line carrying a rearranged human membrane-type mu chain gene. In this study we further investigated regulation of trans-mRNA expression. Regulated expression of different gamma subclasses of trans-mRNA was similar to that of class switching: IL-4 together with lipopolysaccharide (LPS) predominantly increased the amount of gamma-1 trans-mRNA whereas LPS alone mainly induced gamma-3 and gamma-2b trans-mRNAs. Expression of the gamma class trans-mRNAs was preceded by germline transcription from the corresponding C(H) genes, but the co-existence of such germline transcripts and transgene transcripts was not sufficient for trans-mRNA production. Transforming growth factor-beta induced germline transcripts of the alpha chain C(H) gene but had no obvious effects on alpha trans-mRNA induction. Both C-alpha gene alleles were used in trans-mRNA but in different frequencies. We could also detect trans-mRNA expression in another transgenic mouse line which carries a rearranged mouse V(H)DJ(H)-C-mu gene. These results indicate that trans-mRNA synthesis is not restricted to either a particular transgenic line or an isotype, but is a general mechanism to express a second isotype with the V(H) regions of rearranged mu chain transgenes.