Reactivation of class II antigen expression in murine macrophage hybrids

The induction of class II antigen on primary macrophages and on several macrophage cell lines has been demonstrated after exposure to IFN-γ. The murine macrophage cell line PU5 (H-2(d) haplotype) does not express class II antigen constitutively and only trace levels are detected after induction with IFN-γ. In an attempt to understand the nature of the defect in PU5, stable macrophage hybrids were derived by fusing a thymidine kinase-negative variant of PU5 with thioglycollate elicited peritoneal macrophages from CBA/J mice (H-2(k) haplotype). Hybrid clones isolated after HAT selection had approximately 35% more DNA than the PU5 parent suggesting that chromosomal loss had occurred post-fusion. While none of the hybrids expressed class II antigens of the k haplotype, constitutive expression of class II antigens of the d haplotype was detected by both micro-ELISA and by flow cytometry. Incubation of these hybrids with IFN-γ resulted in a further increase in class II expression. Co-ordinate reactivation of both I-A and I-E antigens was observed in the PU5-macrophage hybrids. In contrast, although the IFN-γ receptor on PU5 and the macrophage hybrids was indistinguishable by Scatchard analysis, neither intracellular class II antigen nor transcription of class II mRNA was detected in IFN-γ-treated PU5. Reactivation was dependent on the PU5 fusion partner and was not related to a general post-fusion event as hybrids formed between PU5 and the murine fibroblast line A9 did not express class II antigen constitutively or after IFN-γ induction.