Pax5 Activates Immunoglobulin Heavy Chain V to DJ Rearrangement in Transgenic Thymocytes

Mice deficient for the B cell–restricted transcription factor Pax5 show a defect in the V(H) to DJ(H) rearrangement step of immunoglobulin heavy chain gene assembly even though the expression of the V(D)J recombinase is not diminished in Pax5 (−/−) pro–B cells. To investigate whether Pax5 is limiting for V(H) to DJ(H) rearrangement, we generated transgenic mice which express Pax5 in developing thymocytes. We show that enforced expression of Pax5 in thymocytes results in a partial block in T cell development due to defective pre-TCR signaling in β-selection. Moreover, our results demonstrate that expression of Pax5 in early thymocytes is sufficient to induce V(H) to DJ(H) rearrangements in CD4(+)CD8(+) T cells and lead us to suggest that Pax5 may play a direct role in the lineage-specific regulation of immunoglobulin heavy chain gene rearrangement.