Stable RNA/DNA hybrids in the mammalian genome: Inducible intermediates inimmunoglobulin class switch recombination

Although it is well established that mammalian class switch recombination i s responsible for altering the class of immunoglobulins, the mechanistic de tails of the process have remained unclear, Here, we show that stable RNA/D NA hybrids form at class switch sequences in the mouse genome upon cytokine -specific stimulation of class switch in primary splenic B cells. The RNA h ybridized to the switch DNA is transcribed in the physiological orientation . Mice that constitutively express an Escherichia coli ribonuclease H trans gene show a marked reduction in RNA/DNA hybrid formation, an impaired abili ty to generate serum immunoglobulin G antibodies, and significant inhibitio n of class switch recombination in their splenic B cells. These data provid e evidence that stable RNA/DNA hybrids exist in the mammalian nuclear genom e, can serve as intermediates for physiologic: processes, and are mechanist ically important For efficient class switching in vivo.